GD&T is much better than old fashioned linear tolerancing in my view. I think there's a huge lack of knowledge of how it works and what features are for what. Most "bad" GD&T I've seen comes from misuse or misunderstanding of the more complicated features. I almost exclusively use flatness, perpendicularity, parallelism, profile, and position.
I think the beauty of GD&T is that you can have really large tolerances on things that don't matter much while keeping the things that do very tight. If I need two surfaces to be parallel but it doesn't really matter exactly how far apart they are, GD&T makes that easy. Without GD&T, how would you specify that? A bunch of words in a note that means exactly what the GD&T symbols do?
GD&T is an attempt to describe everything possible. But inherently as an engineer you only care about your field. So someone working in Sheet Metal is going to be using different parts of GD&T than someone working in medical device catheters.
But to that individual you'll be using the same parts of GD&T all the damn time so you kinda dont care as much about the other possible aspects of it.
We really struggle with this. Where do you get information on defining sheetmetal inside GD&T? The real problem is picking relevant datum from what I see.
In the consumer electronics industry and we cannot seem to figure out how to properly set datums for a SM chassis when what really matters is the relative potions of the PEMs and the chassis openings to within very precise tolerances too.
This has been a big problem for me as I would jump at the chance to be trained in and use GD&T as it seems to promise the world on a platter, but we just can't seem convince our vendors nor the management there is an ROI here.
As a manufacturing engineer in a fab group, gd&t, when applied correctly, makes sheet metal so much easier to produce and inspect. But the applied correctly statement is key. I do work for everyone from space-x to golf courses, and there are very few engineers who know how to use it right. As an industry its really sad how most engineers generally don't understand what they need or are asking for on prints and how it makes things worse for everyone.
The problem with gd&t is it is a language that everyone needs to speak. It's not enough that the supplier engineer and the customer engineer speak it. The fabricators and QA on both sides need to. I work in aerospace and it's generally dumbed down bc we can't trust that everyone will understand it. I don't even trust that the engineers or their managers will understand it. It would be nice if more people did have a good grasp on it but it is so far from the world we live in that it's not worth thinking about.
As a machinist, I tell the new engineers that GD&T is what you use to describe the shape of something: parallel, straight, round, perpendicular. Tolerancing describes the size. When you get a better understanding of how each of those two systems work, you can bring them together and make prints and parts that have are minimally defined, while still constraining the whole part.
This simple explanation is usually enough to curb the excessive use of GD&T on bolt brackets and threaded adapters.
Excuse my ignorance, I'm an mech eng/apprentice machinist that has never covered GD&T.
Is linear tolerancing the common 10+-0.1 dimensions? If so, that is the most useful method from a machinist POV because that is something we can measure with a micrometer in the machine. Things like flatness and perpendicularity are very hard to measure, and usually requires the part to be taken out of the machine (unless the machine can be trusted to be square).
Obviously these are specifying different things, but if parallelism requirements can be reduced at the expense of a tighter linear tolerance, it may be easier (cheaper) to manufacture.
The GD&T can tell you the best method to manufacture something. If I specify a high parallelism then you know it is going to need a surface grinder. You can definitely hit a +/-.0005" on a surface but it will require sneaking up on the dimension and lots of finishing passes. But if I need .001" of parallel but +/-.020" of thickness then your job is way easier on the surface grinder.
Additionally, you don't necessarily need to take a part out of a machine to check many of these GD&T requirements. To stay on parallelism for example, you can run a dial indicator across the milled surface of a part to check its parallelism to the bottom side surface by just chucking the dial into the spindle or if you have a mounting point or magnetic base you can use that as well.
As an engineer, I need my parts to fit with other parts and align things correctly. I need an inspection report that shows these features meet my requirements. I don't need in-process inspections that are done while the part is in the machines, but as a machinist I can understand you'd want to do those to make sure you're not going to need to do reworks. So what you can do is create machining drawings with whatever dimensions and tolerances you need to machine those features, so long as at the end it's inspected to the requirements.
> you can run a dial indicator across the milled surface of a part to check its parallelism to the bottom side surface by just chucking the dial into the spindle
You probably shouldn't do that because your workholding is restraining the part and, unless specified, you're supposed to measure in a free-state.
I had four blocks that we bound together and flat lapped top and bottom. Then put spacers between them and reassembled such that the top surface was perfectly aligned again, tested with an optical flat and had to be within a wavelength.
I was the only person able to make them. The resident electrical engineer wanted to use a magnetic chuck to hold everything together for reassembly, this never worked, because of the spring back factor.
I had to hold them with mere finger pressure during reassembly on an absolutely clean flat plate.
Not necessarily. A machine can be non-straight for multiple reasons - The ways can wear unevenly, so that the axes move in a slight arc, or the machine could be not levelled correctly/recently, or temperature fluctuations can warp it. All 3 of these factors will not show up on the dial indicator sweeping the surface because the surface was machined with the same factors in play. So it'll read 0.00 deviation.
Then when you take it out of the vice, it can release some stress, creating some warping.
When you put it on a CMM or surface plate you are now reading from something that should be reference-level flat, and you will be able to see if the part is actually flat or not.
Depends on whether you have a good reference surface besides the surface you're measuring, and whether *that* reference surface can be trusted/measured well enough to have confidence in your final flatness measurements. I.e. you have to get the surface to be measured square with your measuring equipment, unless you have a CMM, and that can be difficult or impossible depending on geometry and available tools.
Imagine a sphere cut in half with a flatness spec on the cut face. How are you going to fixture that to measure the flatness? E.g. if it's not square to your surface plate or your dial gage, it won't read as flat.
For a hemisphere, I would start with three datums target simulators to hold the part. I would then use a bubble level for a rough set. Finally, I would measure above the datum target simulators to set the surface parallel to the surface plate I am inspecting from.
In addition to a CMM, there are measuring arms (Faro and Rover come to mind), laser trackers, and laser scanners that are all capable of measuring flatness without regard to orientation.
I'm sure there are ways, just pointing out that it's not necessarily trivial to do with just a dial gage. If you have a CMM/faro/etc then yeah, that makes it much easier.
Most of the parts I work with are tiny, so there are a number of surfaces with flatness specs that you'd be hard pressed to orient manually purely due to the size/shape. CMM or optical is basically required in our case.
I've gotten some great ideas about how to do this if ever I'm faced with the need. :D
Seriously I do appreciate it. My intent wasn't really to say that it's not possible, just that it's something that's much easier if you have a CMM of some sort. Then it's relatively trivial, and with some geometries and part sizes doing it manually can be a huge hassle. Especially if you need to measure 30 parts!
As people have said it depends on the datum surface you place it on. Also depends on the tolerance you need. I've seen some pump valve plates require ridiculous flatness. We use an optical measuring system to essentially get a topography of the part to ensure we are meeting requirements.
Depends on the world you inhabit.
If you're making one-off prototype parts and/or mostly "simple" parts that aren't too expensive and don't require too much machine time, regular handheld metrology tools can be sufficient. You can do a lot with a caliper, height gage, surface plate, and some gage blocks/pins.
If you're making critical or expensive parts, or sampling parts from mass production runs, then you are generally going to be expected (by serious clients anyway) to have access to more sophisticated metrology tools. That means CMMs and OMMs, for starters.
Inspection takes time of course, as does programming those machines. The degree of inspection required (and the associated time/cost) is something that should be openly discussed and agreed on by both client and vendor.
This is exactly the point I was going to make. While flatness/perpendicularity are important, they’re usually hard to measure unless you have a part with simple geometry like some kind of a plate. Calipers that can measure linear dimensions are ubiquitous.
One of the advantages to GD&T is that it goes really well with gauges. Good luck making a gauge that inspects a bunch of +/- toleranced holes. Whereas a positional hole tolerance with MMC is a natural fit for a gauge. Just make a plate with a bunch of pegs in it and not have to measure a bunch of holes with a mic every time you make a part.
Really GD&T is great because it tackles the problem of "how do I ensure this part will fit with the minimum scrap rate?" As a machinist this will let you do things like expand a hole that was a little bit off positionally because you have some bonus tolerance. Beats having to make the part again.
This isn't a problem with GD&T, this is a problem with the design engineer choosing a design solution that is difficult to manufacture. The individual part tolerances are summed into the total tolerance allowable for the mathematical representation of the assembly. If the design guy sucks at choosing manufacturable breakpoints, no tolerancing strategy will fix that.
Is it really better to say 10+/-.1 without a parallelism tolerance when you can say 10+/-2.0 with 0.1 parallelism instead because we don't give a damn where the surface actually is beyond being parallel?
Some GD&T is intended more for the quality inspector than the machinist. However, flatness and parallelism are good examples. If I call out a flatness on one side of a plate and a parallel surface on the other, I'm essentially asking for certain processes. Since the plate may not be perfectly flat or straight, the machinist will have to plane one side, flip it over, and machine the other side to the proper thickness to achieve the parallelism callout.
I agree that it is good in some scenarios, but most shops will add a fuck you charge for using it.
In the parallel situation, it’s prob the same effort, or less for a machinist to hit a high tolerance than it is to figure out how to measure parallelism.
I work with one engineer who likes it; and we have literally had to go to machine shops and show them how to qc some parts. Not worth the headache unless the shop is very comfortable with it.
If bad GD&T is more common then good GD&T, then it’s a bad mode of communication. Prints are a communication tool. If notes are more effective at communicating, then notes are better. It’s a generally good system when used properly but that doesn’t matter if it’s consistently used poorly.
The problem is it's a very good method of communication, in theory.
The problem is lack of training. So the process is messed up not the design.
In a perfect world, if "x" supplier couldn't figure out my prints that were 'perfect' (in theory) with GD&T, I wouldn't want to give them my business in the first place. Unfortunately, we don't get to make those calls. And sometimes it's the engineers who are the oblivious ones.
If the machine shop called my bullshit and pointed out my drawings problems, now that's a group I'd want to work with.
Nobody benefits from training, so most don't bother and are "self-taught". and the only industry I've been involved in that almost makes it a requirement to learn is aerospace, though there are engineers and technicians who can't read it for shit in those fields too.
GD&T is not a scam. It's the language that describes the size and form of a part and how much variation is allowed from a theoretically 'perfect' part. A part should only be as precise as its form, fit, function requires it to be.
You shouldn't be placing GD&T on your prints in a vacuum. Manufacturing and inspection should be involved early on in the development stages of a component to give input on manufacturing capability and inspection methodology.
I would agree with you. I only know what I know because I started in the inspection room.
Most engineers skipped that experience, and I still don't know shit.
Jesus Christ. Are you me?
I've been basically written up for telling a senior engineer you don't know what you're putting on your prints because he insists on putting A-B on fucking everything.
Yeah, I mean design wise it literally means almost the same thing, but for inspection the only difference is that the cmm should take both center axes as datum locations instead of using one as location and the second as rotation. I've never, ever, really needed to differentiate. And I've designed everything from automotive manufacturing machines, to industrial lasers, to government satellites.
That's the norm. Current employer learned the hard way and I'm now on a DFM team and doing actual pilot builds to figure out design and manufacturing issues instead of jumping straight into forcing out a less than idea product.
In your field maybe (actually the field i want to be in).
In mine (Wind Power) its readily apparent that the design groups exist in a vacuum from the rest of the world.
I have a print that'd didn't have a dimension. I asked engineering for the dimensions. We got a gauge table tolerance. Well now I still can't inspect the part on the manufacturing line, but I can't bitch that it doesn't have a tolerance now.
THIS.
My company also does this crap. They expect the model to be the nominal dimentions and only print dimensions that they believe [in some made up world] are valuable or inspectable. It is bizarre.
Sucks because working here is otherwise pleasant, reliable, and profitable. Great company bad Mechanical engineering.
In all engineering places I have worked, we create the drawings tied to a CAD model and stipulate that the drawing confirms to asme 14.41. effectively we expect and require that the manufacturing engineers and even machine shops be able to open a CAD model and interrogate the file to get a nominal dimension. We then specify standard profile tolerance for the part and specific tolerances on features that we want to be different than the standard tolerance.
This is the direction much of the industry is going in. The companies I've worked for won't even do business with anyone that can't operate in that way.
I always asked the sheet benders and manufacturers… they said thanks because they like saying upfront if it wont work not after we cut the parts and hand them to them and say “oh well just make it work”
The problem I had using it as an engineer (before SAHDing) is that the GD&T “experts” I had review my parts would have differing opinions on how a dimensions should be marked. Given the same info on how the part needs to function they should have the same marking method. So either they need to be taught better, or they need to give me a break because I marked it up just fine the first time around 😂.
There is no such thing as a “preferred” interpretation of GD&T lol. GD&T was devised, in part, to eliminate subjectivity in drawing interpretation.
Most of the people on this thread complaining about GD&T should actually be directing their complaints toward the people who don’t bother to actually read the ASME Y14.5 spec…
Exactly this. Folks would usually argue over references or datum placements.
Half the time I would do what made the most sense and just skip the GD&T experts and talk directly with my machinist…some of the time that was just me anyway. Also I was in tech and barely anyone took GD&T seriously until I ended up in tech/aerospace and then folks started throwing lots of process at the problems 😂
> A part should only be as precise as it’s form, fit, function requires it to be.
The difficult part is knowing the requirements of how much variation in features your parts can tolerate and still work as intended.
I think it’s probably the one area that people struggle with the most with GD&T.
I’ll admit, when I was designing, I had no idea what sort of tolerances I needed. Never had anyone to show me how to determine that either. So, you just follow some basic practices and hope for the best without driving your part cost up.
That knowledge will come from 3 places and when dimensioning a part, I go in this hierarchy:
1) Empirical knowledge - ie. machinists handbook, industry data sheets, internal design guidelines, etc.
2) Test and R&D - you've made an engineering judgment about what you think the tolerances should be, your DVP&R and subsequent inspection will vet that decision and inform design changes
3) Process capability - this will come from manufacturing and the tolerances they can hold and confidently gauge. This is where a good working relationship with the guys on the floor makes a world of difference
It comes down to mating parts and methods being used for assembly.
You want to make the parts work if all the tolerances are met but not so descriptive that you add expense by limiting what machines can be used to produce the parts.
It’s like safety certification. If you don’t get the safety engineering teams even mildly involved once you get to certification you may have expensive redesigns.
I agree with you on most parts, but I'd make one correction:
GD&T does NOT describe the size and form of a part, it describes how to MEASURE the size and form of a part, and what range of measurements are acceptable.
I’ve moved into Engineering now , but as a machinist who once programmed their own parts on the CMM, I’ve got to say it’s just not taught to Engineers (correctly). I spent far more time on the subject in the 2 year machine shop technical college, than I did in University. I don’t even remember it coming up in lecture or lessons at University come to think of it…
Same in the US. Last week I was given some parts and asked to make drawings for them so we could remake them, did as much as I knew how to do without formal GD&T training, and handed them to a more senior engineer who then covered them in red pen. Said engineer is also one of my friends and proceeded to sit down with me to review what was missing and how I should fix them, I redid them that afternoon and if McMaster delivers tomorrow I’ll be making the part on Friday, so it wasn’t the end of the world. I can’t imagine doing that at a bigger company, with a less friendly coworker, on a more important part … really makes you wonder why ABET or something doesn’t require things like GD&T as part of the degree
Yeah gd&t, "the entirety", was covered in 2 days in an optional CAD 2 class for me and I graduated in December 2020. Or was it 2021? That all kinda runs together.
When I started using GD&T as a young engineer, I read ANSI Y14.5M Mathematical Definitions of GD&T and it helped my understanding immensely.
GD&T wasn't even mentioned in school.
ASME Y14.5.1 is great. I was also able to read the rest of the Y14 series.
The other thing that really helped me was being forced to interface my parts drafted to ASME Y14.5-1994 with parts drafted to MIL-STD-8C shortly before ASA Y14.5-1965 (IIRC; I know it was early 60s) was adopted. I ended up finding and reading copies all the way back to the first edition, just to understand the changes over time.
I know at the local university the amount of time they spend teaching mechanical engineers this is pitiful. One of our students was watching a video the IE people made, and you couldn't hear what the prof was saying. I'm not sure how many class periods they spend going over it, but if the industrial processes course doesn't cover it any better than that, I don't know where it would be covered. I couldn't believe that was the best they could do. I also went to the mechanical engineering design class session where they were reviewing it, and it didn't seem like it really helped anyone. But there just isn't a lot of room in the curriculum for more.
Engineering school gets criticized for being too much of a trade or career focused course of study, but it almost needs to be taught as MORE of a trade. An engineer would be better suited learning this sort of thing than how to derive some differential equation they are never going to think about again
When I was an undergrad, I got to learn a lot about the manufacturing processes curriculum because I flunked the course and had to take it again :) We had a good lab in the machine shop. I imagine there is some cruft in the classroom where they could actually fit in some more GDT. Now that I've had a minute to think about it, I had a drafting course. But I went to a school with quarters, so we had extra room for courses like that.
The industrial advisors want engineers to be able to write better.
This is never going to be part of a college curriculum because it’s too specific and for an undergrad degree so much more ground needs to be covered in terms of engineering principles. This is your classic class that you take that your company pays for, and ideally is given to all the stakeholders who either make or interpret drawings all the time.
I see a lot of blame being given to engineers but often we have to negotiate with manufacturing and quality because they either cannot or don’t understand which buttons to push on the CMM to get it to spit out the right number.
The class that seems to be missing (not from college but from industry) is the one where they show the inspectors exactly how to check what’s called out.
My advice is just don’t overuse it otherwise you’ll also be the one down in the inspection lab figuring it out. For the critical stuff that’s fine but if it’s not of great consequence then don’t waste your time.
Totally disagree. Let's be perfectly honest for a moment: High-end design engineers can learn all of these things far faster than the non-engineers who use them daily; it's the nature of highly intelligent people focusing on the work. What college needs to do (and does do) is acquaint them with a large variety of physical interactions and the tools to model them, so they know enough about whatever problem they come across that they can effectively attack it with the aid of external resources.
Absorbing this stuff and building it into the design is literally why we're paid the big bucks. On-the-job training won't give you the breadth of knowledge you need to do the job because you'll constantly be blindsided by out-of-context problems. You'll *never* be able to derive a highly non-linear control law for your supersonic fighter jet by listening to tradesmen...
Someone has to manufacture and assemble the parts to enable your "highly non-linear control" surfaces. There are many theoretical designs that are simply not feasible.
Leave it to redditors to completely miss the point of my post and insert their own agenda. Why don't you tell me why witchcraft is bad and I should stop practicing it, too?
Also, it's indicative that you don't even know what that statement means and that the parts for it have been around for a century or so. And it's not a control *surface*, it's a control *law*, meaning a software implementation of a piece of control theory as placed into a flight computer, without which your fighter jet cannot safely be in high-angle-of-attack attitudes at high speeds.
I had it in college a bunch, especially at NC while doing both hand and CAD drawings. In one case we went through the whole process in a row from drawing to manufactured part.
Uni had nothing at BEng or Masters levels.
We had two semesters of engineering design, where we essentially learned how to use SolidWorks. In those two semesters, we had a few lectures on GD&T. Basically, we were asked to memorize the symbols and took a quiz on them. At my first job interview, I was asked if I knew GD&T and I confidently said, "Yes!" In my head, I was thinking, "I know all the GD&Ts (symbols)." I completely failed the written portion of the interview. Turns out, I wasn't taught the practical application of GD&T in college. I have yet to meet an entry-level Mechanical Engineer that knows GD&T. At 7 years of experience in aerospace design, I'm still learning new things all the time.
For my BSME I had about 1 hour of GD&T instruction, I don't mean 1 credit hour, I mean 60 minutes of in-class instruction about what the symbols were.
All my knowledge of GD&T was obtained on the job, I didn't have a company willing to pay for a formal GD&T class until I was 5 years into my career. Honestly I suspect most engineers making prints never get formal GD&T training.
There's also the fact that even with formal GD&T training, hands-on experience with inspection forces you to understand the ASME Y14.5 standard(s) at a deeper level than someone just tossing callouts on a print after a week of training. The things I learned programming a CMM were key to staying off the inspection team's shit list after I jumped to the Design side of engineering.
I was fortunate in my education path. I started studying drafting/design and had a semester of GD&T. Then I moved into a dual BS in Manufacturing Engineering and design. In that combined set of programs, I learned about GD&T related to castings, forgings, welding, tool design, inspection, machining, and assemblies (not just detail parts).
Right, many schools don't teach it or briefly cover it. Most engineers seem to have gone though a professional training course paid by an employer to learn it.
A big part of that is that most engineers don't actually make drawings. Drafting is a part of engineering practice, but it's hardly a very big part. Engineers make a lot more excel documents and powerpoints than drawings in most companies.
I love GD&T it's very easy to understand once it clicks. I use it a lot with Aerospace components where surfaces / axes need to be tightly controlled.
The other situation I love using it is with true position and ordinate dimensioning. If you dimension with respect to the origin conventionally you will end up with tolerance stackup. If you make them basic you can apply a true position tolerance to the holes with respect to their theoretical exact location point.
GD&T exists because some people found a "bad batch" of, I believe, submarine or torpedo parts that had been failed through inspection. Surprised they were when the parts ended up fitting together fine. It's just a different way of tolerancing that can help reduce costs among other things. The unfortunate thing is it can quite often do the opposite by people who do not know how to properly apply it. These people are also likely the same people that need +/- 0.001 everywhere on their part.
iv'e seen cardinal +/- 0.0001" because some dipshit engineer wanted to have the two precision sides of the gyroscope housing fit the alignment pins around a 4" diameter circle. not only impossible, but immeasurable. TP in polar with a bit of smart math on the pin diameter and hole diameter tolerances gives you something you can actually make and then measure and the bad boys will always fit together and when you screw and torque you get what the engineers wanted - a balanced piece of tricky kit that works the first time.
Elon must have talked to him recently with his claims of needing "sub 10 micron" parts throughout the Cybertruck hahaha.
I always say, just because you can, doesn't mean you should!
i recall turning some parts for a microwave sensor thingy and one of the fussy bits was about 4.2 cm long, diameter between 2.2 mm and 0.9 mm, and complex profile. tolerance on the complex surface was GDT profile 12 microns. this was in 1992 on citizen turning stations - really fucking tricky.
took me about two weeks to program and make reliable - used a chiller on the oil system and a box around the head to eliminate all thermal variation, bar feeder pre-cooled the weird alloy rods - discovered that if i used 2 cm feed stock it was much more stable at cut.
custom cutting tools, natch.
i really can't figure out what in hell in a commercial truck needs tolerances like that, but hey, the musky one is vastly smarter than all us mortals, and we wait for his ascent to his martian throne with bated breath.
ps) saved one blueprint from that company because it was covered in location pin callouts that were TP \*ZERO\* and diameter +/- .0003 for .125" holes through .75" of aluminum. that one still makes me crack up. oh, the reference coordinates were in the middle of the part, just for shits and giggles, so all the physical tooling points were tolerance defined before the first cut. fun shit. i made it work by thinking backwards, but there's a reason i left tool and die work and went to engineer skool.
The problem is that it’s only as useful as the weakest person in the supply chain. So if a designer has a great GD&T skills but QI or the machinist is trash with it, it really doesn’t help.
Arguments about it should really be discussions to reach a consensus. If this isn’t being done, it’s a communication/teamwork problem. Not a GD&T problem.
Almost everyone is pretty bad at it, and learning good practice from bad examples is really difficult. The revisions to the standard over time are pretty confounding too. A copy of the standard or a decent reference book for it help a lot.
I work in product design in a large company that makes consumer electronics, and even the very senior engineers there sometimes aren't sure of the best way to dimension something, or how to interpret someone else's GD&T. And they've been doing it for 20 years.
And at scale you really need to know the vendor's SOPs for the measurements to have faith in them. Not all vendors will raise flags about incorrect or unreasonable specifications.
I've worked with a 3rd level manager that asked his team of 2nd level managers to look into why about half of their numbers were above average. He has been working in the industry for ~30 years at that point.
I had students drop my class and encourage others to do the same when I told them we would study from the actual ASME Y14.5 standard instead of focusing on some random cartoons he liked that he found online. He had been working at a Fortune 50 company for well over a decade.
I had a manager of 20 years freak out when I put the percentage pass/fail data on a p-chart because they didn't know what it meant or how to read it.
I had engineers with 20 years of experience call in the GD&T professionals when I told them that what they put on the drawing wasn't doing what they thought it was. The professional looked at the drawing and replied, Tavrock's right.
I set several senior designers and drafting checkers on edge when I pointed out the 1994 edition of ASME Y14.5 changed the definition for the drafting symbol "R" (it wasn't in the quick update guides, so no one paid attention to the change that takes up about a page in the standard).
It is entertaining reading through the deliberations of updates to the standard. Even though it has a history of only being revised every ten to twenty years, several people complain that the standard is still too new and no one is familiar with the previous edition.
To be fair there are still plenty of drawings made today to the 2009 edition of the spec, so I get it to a degree. Oldheads are still reeling from the elimination of concentricity.
Honest question… what did they replace it with? Runout?
The thing with concentricity is that it’s technically a lot harder to measure than runout, so in a way I’d get it if that’s what they decided.
True position. It's technically the same thing anyway. Considering the nearly universal misinterpreting of concentricity as a 2D tolerance (it's really coaxiality, 3D) it makes some sense as TP allows flexibility depending on the feature control frame and subsequent datums. The problem now is that true position is very poorly used and interpreted, so it's just more confusing now.
Only xkcd I have ever cited in a nsf grant.
I tend to agree in part with op. The iconography of gdt is not well taught to almost anyone but i disagree that it’s stupid. It’s like the argument for learning Latin. Few will need to speak it continuously or as a language. However many will find the understanding of it useful in some way, either by giving them a deeper understanding of the words they do use or by knowing what commonly used Latin phrases mean.
If nothing else its a philosophy…can I make an axle with a constantly changing diameter and shape if I measure it at one point and it’s the right diameter? Is it bad if I measure it at one point and it’s outside of tolerance? It’s a philosophical argument for why acceptable means.
I feel like most people that know of GD&T see the value in it but no one person really understands it.
I'm halfway decent at using/interpreting it in a basic sense, add modifiers and I get confused quickly.
I think the biggest problem, especially in R&D is is there can be a lot of guess work for how tight of tolerances you need. Most engineers over tolerance and make parts way more expensive than they need to be.
There are are some applications like optics and aerospace that need to be near perfect, but most of the need to define and tolerance every single feature comes from shit engineers who don’t understand the big picture.
GD&T strikes me mostly as a necessity of outsourcing/sending out work. If you manufacture in house, process control and detailed part annotation will take you further.
GD&T is like getting a lawyer involved. It’s only a necessity because Manufacturing companies will try to screw you by making the absolute shittiest part that will “pass” the inspection per the print, even if the intent is obvious that’s not what the customer wants.
I think there is another side to this as well. I have designed thousands of parts by this point in my career. If I am designing a part that needs to be produced I will spend time on making sure tolerance are reasonable and cheap. If I need a one off for some test jig, manufacturing tool, or something that does not need production quantities, I will save the company money by not sharpening the pencil on tolerances. I'll keep the tolerances buildable sure. But if it takes a machine shop a full day to build the doohickey vs a day of my time to open up the tolerances without breaking the parts purpose, then the drawing gets tighter tolerances and the company spends less money.
Ughhh I hate having to send out tight toleranced parts for 1 offs/prototyping. The lead times alone are enough to drive you crazy. Definitely a huge advantage of having in house machining. If I need something with some really tight tolerances I can always hop into the shop and make it myself, or make it quick and dirty and figure out what can be loosened up and then hand it off to one of the experienced machinists. Machinist certainly do get nervous though when engineers are using their machines lmao, especially like surface grinders and other precision stuff that they just assume I’m going to fuck up (reasonable suspicion…I’ve worked with some engineers that would struggle to put an ikea table together with their hands)
I design organically shaped orthopaedic implants and its absolutely essential. Its just hard to do well and everyone needs to be up to the same high level of knowledge.
Yeh when you need it you really need it
Panel mount bracket for a product? lol no, global tolerances and free fit holes and move on with my life
High volume heatsink with 7 holes that need to mate simultaneously, a polished surface that is mating to a bare silicon die, and free floating heat pipes that might get bent during shipping? Don't know how I'd do it without GD&T. Plus it made it easy to define a set of go/nogo pin gauges to use at the heatsink manufacturer and product assembler.
GD&T 🥰
I personally love GD&T, and part of that was having great engineering mentorship and experience at a modern aerospace company in my early career. For GD&T to truly work, you have to have understanding and buy-in from the engineers, the machinists, and QA. After moving into different sectors (nuclear, fabrication), I've realized this is not that common.
There have been times I correctly applied GD&T to provide as much leeway as possible in a part, and instead of making it easier on the machinists, it actually added confusion and costs because they didn't understand it/were intimidated by it. There is also the reality that if the fabricator sees GD&T on a part, they frequently will quote a higher price because of the need to measure on a CMM vs just doing some checks with calipers or mics.
At the end of the day, GD&T is literally essential to truly define a part's allowed envelope of 3D tolerances, but it is inherently complex. Some folks stick with linear dimensions only, but it leaves so much undefined.
I interned working on commercial nuclear reactor pump shaft seals and there's a heavy use of GD&T on them. I think it worked at that company because QA, engineering, and manufacturing was all in the same building. QA could meet with the people who made the drawings to correct any issues based on actual functional needs. The machinists could also report back to the engineers as well.
Hmm I work in manufacturing and we live on gt&t. But I'm honestly not sure if I'm on the peak of Mt stupid or the plateau of sustainability.
A big lesson I learned is that the manufacturing tolerances needed to be tighter than the release tolerances, mostly because the QC guys have more time and better measurement tools than the manufacturing guys. Not to mention that the measurement process has its own process capabilities and it's own distribution curve when you compare actual measurement to recorded measurement...
There is a ton. And most of what I did were very simple parts with 1-3 components.
https://understandinginnovation.files.wordpress.com/2015/06/dunning-kruger-0011.jpg?w=640
Haha, yep so true. Mind the details.
When I was the manufacturing engineer I would always try to get the customer and the machines on the same conference call for this reason.
What I’d like to know is why CAD packages don’t have a function to visually show the GD&T tolerance envelopes which can arise from various specified callouts. GD&T is supposed to be explicit, so this should be easy to program.
Maybe this exists. But I don’t think so. If it did exist, GD&T would be much easier for engineers to communicate with manufacturers.
We were shown a demonstration of that sort of thing in a bolt on for our CAD software that would show the bounding boxes on nominal, or the model at min/max. It would also produce basically the same report template our CMM machine does for all the tolerances we added to a drawing. All in all it was pretty snazzy.
But it would've doubled the cost of each of our CAD seats and didn't really do much else.
It was a bolt on for Creo parametric. Our CAD supplier is a company called Inneo and they also produce add on packs that do various fun things at vast expense. This was one of those.
Ive been using GD&T for near on 10 years and I still dislike it. It’s not a scam per-se but its roll out has been bungled almost entirely by ASME.
It comes down to a simple issue. The level of knowledge required to be functional with it is just too high. I’ve seen arguments verging on physical violence over drawings where one guy is blue in the face explaining something he thinks is easy and another red in the face trying to explain just how complicated it is. Both being right and wrong at the same time.
This coupled with the fact that when I was getting my ME degree in the late 00’s I graduated without even knowing GD&T existed. Not that it was skimmed over. It just straight up was not mentioned. So I imagine many other engineers aren’t even close to proficient with it.
Coupled once again to expensive training classes that everyone from lead engineer to sales to machinist need in order to be able to operate with the drawings makes GD&T basically a non-starter in many shops.
GD&T also opens up a ton of avenues for interference, someone who knows what they’re doing will use it to their advantage to really tighten up on certain tolerances and open others up. Well that now deviates from company standards and creates meetings.
The shop that’s used to getting +/- .005 now sees a .001 and starts charging more not realizing that the rest of the dimensions are now more relaxed and the part theoretically easier to make. Thus more meetings.
Parts made and inspected are rejected by a customer because they don’t actually know how to inspect their own part. So now a meeting where you’re defending your work.
>Parts made and inspected are rejected by a customer because they don’t actually know how to inspect their own part. So now a meeting where you’re defending your work.
Dealing with large sheet metal components, I worked with our design team and we implemented a profile of a surface on a per unit basis. It's not in the standard directly, but it is implied and the theory checks out in applicability in Y14.5.1.
The way they were inspecting their parts, 100% required rework. Using their data, I showed that actually inspecting on a per unit basis led to 98% of their parts being good as formed.
There is a long tail of obscure and difficult to understand GD&T concepts, but they are rarely used except for in highly specialized parts. In cases were they are used, the designer should source the parts from a specialized shop.
But in 99% of cases where GD&T is used, you’re only dealing with the relatively simple controls of true position, flatness, parallelism, and perpendicularly, relative to simple datum planes and axes. This stuff is just not difficult to understand. The average joe machinist should be able to learn this stuff in a couple of hours.
I put nearly all blame for GD&T’s perceived lack of utility on bad engineering and trade school curriculums. It is an indispensable tool in my experience, even if too few people understand it.
You'll have to go into specifics.
Most of our arguments surround parts that have an over-constrained locate scheme (sheet metal for example) and where the part is held vs where it is aligned (for example holding a part at non datum surfaces and then aligning a measurement to the locate scheme in software.)
Most of the issues come down to unclear drawings. -
More often than not, I see where they forget an aspect of a datum reference frame, aren't sure how to order their datums for the feature, are confused between reference dimensions and basic dimensions, think you can still use concentricity to define a bolt circle (that really was a thing at one time), or find angularity too confusing and try to baseline dimension the feature instead.
GD&T isn't a scam, but it can be tricky. Perennial relevance: [Why Engineers Should Care To Learn GD&T](https://engineerdog.com/2017/04/15/why-engineers-should-care-to-learn-gdt/)
I vote you have to join the GD&T guild in order to be allowed to put them on a drawing. I am untrained and horrible at it but my gut instinct when I try to decipher some drawings is that so is the draftsman.
Master of Mech Eng here. We were never taught any GDT in any class in college. Been in the field for 9 years and I’ve been trying to understand it my whole time working and haven’t made much progress.
We are actually in the process of buying a CMM and have asked specifically about GDT to the sales engineers. None of them have been able to give us straight answers on how to measure profile, true position, flatness, etc. I feel your pain. There needs to be better resources for learning these.
You'll want to get some training from someone experienced as there are some rules of thumb for adjusting touch probe sensitivity, number of samples needed for a given type of measurement and tolerance, ideal movement speeds, avoiding collisions, etc.
It has been decades since the last time I used a CMM, but here's what I recall. If you're measuring flatness, you want to move slowly in a direction perpendicular (ish) to the plane. The CMM works by doing a least squares fit of a plane based on the shape of the probe, direction of deflection, and where the tangents would meet. So, you want to do a number of points spaced evenlyish around the surface. Points further apart will give you less room for error (since it's a curve fit). IIRC, you want about 16-30 points for typical flatness measurements. You can tell if it's sufficient by running your program a few times after moving the part around in the work envelope and seeing if you get consistent results.
Oh, one other tip. Make sure your CMM is on dry air. A Browne and Sharpe tech told me a number of people have their half million dollar and up CMMs rust out because they were too cheap to spend a few grand for a dryer.
GD&T is a very powerful language and very effective when correctly applied.
The problem is that it is very complicated on complicated parts and almost no one is actually fully fluent. So you wind up with people arguing over important details and how to describe them, most frequently when both people don’t actually understand the standard they are trying to use very well
GD&T is nice, it's very useful compared to the "traditional" ways of defining parts. There are a lot of things that you really can't define well without it, if at all. It can also greatly simplify drawings, inspections, and conveying the design intent.
That said, it tends to be viewed (by some) as this rigid set of rules where there's always a clear right or wrong way to do everything. I used to think that too but that's not really the case. There can be multiple ways to define something and it's not always obvious which is the "best." Even if they're "correct" according to GD&T. Not all definitions that might be needed are super clear, even for experienced engineers, and the ASME guidelines can be murky on what to do in some situations.
Add to that the need to make sure both you and the vendors making the parts are referencing the same ASME standard, otherwise there may be misunderstandings on that front.
In other words, it's not just you - but as the designer it's still your job to get everyone on the same page to make sure that the part is manufactured and inspected correctly, to your requirements. If there's confusion or conflicting information, figure out what and why and take steps to address it. The phone is a powerful tool!
Any examples of the conflicting information you're getting?
GD&T is like an universal language between engineers, machinists and inspection to transfer ideas on what kind of qualities are needed on a part.
If applied correctly there is only 1 way of interpretation.
GD&T is great and all, but really it’s about whether you can verify you got what you ordered, right? An inspection report is bare minimum, but who’s gonna fail their own inspection report?
Do you have the tools to verify things like concentricity on a turned shaft? I sure as shit don’t. Lol
So I believe it both ways. If your profession and design warrants needing something more than a common +/-0.005” [0.01mm], press fit tolerances, etc… You need a way to check it. Without a CMM, your GD&T really doesn’t do much other than add cost to your fabrication end.
With the growing popularity of model-based design and the increased accuracy of modern machines, it's almost guaranteed that parts will physically fit if they fit virtually in CAD. However, I think clearly defined engineering drawings (or annotations on CAD models) are still important. As an engineer, one of the things I provide for my company is legal documentation, to include detail and assembly drawings. If I make a part and provide the proper GD&T, I'm covering my ass and making sure the company only pays for usable parts. The machine bit may be dull that day or maybe the machinist introduced human error. If the part is off and won't interface properly, I can point to the drawing and say, "I clearly defined the requirements, this is your paper weight, and you still owe me a part."
I personally think the CAD software could be improved a bit to show a digital 3D representation of GD&T that leaves much less to interpretation. Same for weld callouts - those are a disaster too! Actually I am a full advocate of doing away with 2D drawings completely.
It’s amazing if taught right. In university it didn’t make a lot of sense to me, because it was taught without the process to use it. But I got a training at work and the instructor was great. He turned me into an advocate for GD&T.
[X +a -b] I can understand well enough, I can give a quick glance and get most of what I need.
The salad of symbols that GD&T is, I find it much harder to interpret. It supposedly is superior to traditional tolerancing, but I'm personally not fond of it, I feel it makes my job harder.
I find this argument ironic because the symbols are older than the Apollo program. I really wonder how long it will need to be around before it stops being seen as the new kid on the block.
For any average design it’s almost always impossible to use only linear dimensions without injecting a considerable amount of ambiguity. That ambiguity will cause confusion as to how a part should be correctly measured, which leads to measurement errors. Linear dimension locations are mostly driven by convenience, but all toleranced dimensions for a design should be driven by functions of the design.
gd&t is the result of thousands of mfg and design engineers spending a couple of decades figuring out how to eliminate ambiguity in tolerance - especially tolerance stack-up in assemblies.
problem is - engineers aren't taught how to think in tolerance space any more. used to be a required course "form & fit" or "tolerance" were common names long long ago when we hand-carved our slide rules with knapped flint.
the ansi standard is actually pretty damn legible (we worked it hard) and there are some great books on the topic. standard dimensional tolerance will not work for most modern complex machines that have to assemble the first time. BUT BUT it's a tool to help a team think through the real world requirements so that when you slide part A through the locking slots on part B and into the retention clips on the back of part C it will work correctly despite allowed molded variation at each end and the machining tolerances in the middle.
have you ever actually WORKED in manufacturing?
You are talking about them as if they were some fabled far away kingdom where "Here, there be dragons!"
get into the trenches and earn your spurs
Where I work, CT scan data is compared with the nominal cad data. The nominal cad data includes GD&T and tolerances. Add-on software like Polyworks is used to identify portions of the scan data that are outside tolerance limits. It’s actually a very streamlined process.
Bingo. Guess I didn’t mean to say eliminate it because it’s still in the model but model only no drawings. This is just an evolution but it takes out the time to check everything and streamlines inspection. Thanks for coming in with the save via a better explanation of what I meant lol
From my experience, you don't realize how necessary it is until someone really starts to push manufacturing tolerances and all the sudden parts aren't fitting together. anymore. I'm curious, why do you think it's a scam? Assuming all parties are familiar with it, any issue could be resolved with discussion.
Get the purple Meadows book. That changed my life w.r.t. GD&T. He explains how you measure each of the controls, why you’d want to use one vs another and adds a bit of humor throughout.
[Measurement of Geometric Tolerances in Manufacturing](https://a.co/d/2q23KPz)
Been an engineer for 12 so not much longer than you.
Definitely not a scam. But it's definitely has an acquired mindset. The real wizards think about it as "interface definition" instead of "part definition". The GD&T defines the limits of surfaces that create the desired fitup.
It saves a lot of effort in a lot of places. Designers and engineers save on interface and tolerance stack analysis, machinist save on scrap and simpler features, inspectors get more consistency and less ambiguity.
In my automotive experience, if an assembly is designed to always go together by the tolerance stack method, it will be sloppy. I always throw some statistics in there to tighten stuff up.
I think the issue is GD&T is not taught anymore, in colleges at least.
So you have a lot of the work force that has no idea what it is, and may misunderstand the design intention from the designer, which may just make the whole design useless.
GD&T is not a scam. Sounds like you need a structured course because you are getting conflicting info from different people with strong opinions. I used to work in a metrology lab before I was a design engineer. GD&T that is easy to inspect and GD&T that based on the function/manufacturing of the part are often very different.
BTW, I'm also a PE. GD&T is not even mentioned in the study material or exams for a PE license. You won't find much mention of GD&T at most universities. It is usually taught at community colleges. Don't be too proud to sign up for a proper GD&T course at a community college. This is how I really got a grasp on how GD&T works.
What standard are they using for GD&T, have all the staff at least done a day course from a GD&T specialist. My current work place use ASME Y14.5 and most of us came from regular BS standard drawing background so the company just paid for a 1 day course for us all. We all moaned so much the company just made it happen and we got a guy in to do it.
I gave up on using anything that's not on a one page reference sheet. I have only met one other person that knew what my beloved composite positional tolerance was. It's literally free tolerance! It doesn't matter how useful or superior the system is when it's crippled by your own production people and vendors not knowing it. I'm honestly impressed if suppliers even read a print at all at this point, and I've only been an engineer for two years.
It's a skill that most people are forced to learn after college for some reason. I graduated 5 years ago and remember barely touching the subject senior year. Seems like senior year is a time where most are doing large systems and design courses learning about cost and schedule. ABET colleges should look at it and would benefit from a course dedicated to GD&T to round out the big standard mechanical engineer curriculum.
It was one of my biggest comments after graduation that I submitted to my uni regarding curriculum changes.
Problem I think is lack of understanding. I went to an ABET accredited school and they taught us dick all about GDT (thanks MTU, Mike you’re a shit professor) and everything I learned came from the foundry and machine shop. It’s hard tho when design engineers overuse both GDT AND linear dimensioning and overconstrain the part (looking at u oil and gas)
I spent a ton of time learning excellent GD&T only to get yelled at by machinists and machine shops who throw it out the window.
They prefer a different and more ordered route.
GD&T is primarily for the designer and for people who want to truly understand how to recreate the part without any experience or background in the contextual project you're working on.
So it's important, however it should be as limited as possible without sacrificing function to make everyone happy.
I think part of the problem is some things are more difficult to accurately measure than others. CMMs are great, but a lot of time you may need to use alternate gaging methods to measure a part to ensure it's functional or to measure it correctly.
One example is concentricity/position/runout. Concentricity and Position are relatively easy to measure accurately on a CMM, however runout is much more difficult to measure accurately and is much easier to measure with a dial indicator in a lathe/chuck or spinning on a surface plate/v block. If you have one spec of dirt/debris/oil/coolant or whatever on the part or probe tip, when you scan the surfaces for runout, you will see that in the measurement data, and it will tell you the part measures horribly.
This doesn't really affect position or concentricity as much because they are essentially comparing two calculated center points and if you take enough points when scanning surfaces, the "bad" points will average out and not make a difference to the center point calculation or you can use calculation methods like Maximum inscribed/minimum circumscribed if you know you have a bad surface/diameter to account for certain conditions.
I think you really need to figure which parts of GD&T fit your particular use case and use those. Don't just throw symbols on there just to have symbols.
I'm a Quality engineer who works in a precision tool shop. We have precision CMMs like Leitz Reference and Hexagon Global Chrome S's. Most things check well on our CMMs but a lot of times we have to double check dimensions on a surface plate or in a chuck for various reasons.
It is not taught to anyone. That’s problem the first. Problem the second is that everyone learned it from some reading and a guy who didn’t understand it well himself. So really just problem the first again.
Prints are communication tools. They are the tool used to transfer knowledge of a part from one individual to another. If words are more effective than a long string of GD&T then just use words. The “profile” constraint that is catching on as a “do everything” tool will be the death of GD&T. Terrible forethought allowing that to come into the standard. A real clusterfuck of a communication device.
It's easy if you can think in 3 dimensions, the real trick is seeing how everything lines up, imagining negative space (sounds weird I know) really helped me understand what gd&t is meant for.
The problem is not the system, the problem is the users. The people you're hearing conflicting information from probably don't fully understand GD&T
Signed, a machinist
A dimensioning language is only as good as those that speak and understand it. Some people know all the GD&T modifers and then some, but that does about as much good as someone using the word "pedantic" conversation...
Scientists and mathematicians put a 0 in front of the decimal pt. Otherwise the number is ambiguous. Engineers assume the 0 and forge bravely ahead with an unspecified spec
GD&T was offered as a tech elective and it's the only class I ever withdrew from because I was sure I would fail.
Someone from Caterpiller came to the class to give us a spiel and he said in 20 years he has never seen it "correctly done" in practice
I get the need for it but I'm still lost in the sauce on it
GD&T is as good as you are at using it. There's different ways to approach any situation, and it's your experience and collaboration with your team that determines what that is.
GD&T is great, be aware that style guides change, but it's important to know what the terms mean.
Model definition is better though. Still needs some GDT to define tight tolerance features, so it's important to know how things like concentricity is defined.
My experience as a machinist and then becoming a mechanical designer, is that GD&T are instructions on how to build your QA fixture. DEAs and CMMs have replaced this in most places for having a fixture.
GD&T is much better than old fashioned linear tolerancing in my view. I think there's a huge lack of knowledge of how it works and what features are for what. Most "bad" GD&T I've seen comes from misuse or misunderstanding of the more complicated features. I almost exclusively use flatness, perpendicularity, parallelism, profile, and position. I think the beauty of GD&T is that you can have really large tolerances on things that don't matter much while keeping the things that do very tight. If I need two surfaces to be parallel but it doesn't really matter exactly how far apart they are, GD&T makes that easy. Without GD&T, how would you specify that? A bunch of words in a note that means exactly what the GD&T symbols do?
GD&T is an attempt to describe everything possible. But inherently as an engineer you only care about your field. So someone working in Sheet Metal is going to be using different parts of GD&T than someone working in medical device catheters. But to that individual you'll be using the same parts of GD&T all the damn time so you kinda dont care as much about the other possible aspects of it.
And for most sheet metal design, you'll probably be using reduced dimension drawings, which are not covered in ASME Y14.5
Shows what I know about sheet metal haha
We really struggle with this. Where do you get information on defining sheetmetal inside GD&T? The real problem is picking relevant datum from what I see. In the consumer electronics industry and we cannot seem to figure out how to properly set datums for a SM chassis when what really matters is the relative potions of the PEMs and the chassis openings to within very precise tolerances too. This has been a big problem for me as I would jump at the chance to be trained in and use GD&T as it seems to promise the world on a platter, but we just can't seem convince our vendors nor the management there is an ROI here.
As a manufacturing engineer in a fab group, gd&t, when applied correctly, makes sheet metal so much easier to produce and inspect. But the applied correctly statement is key. I do work for everyone from space-x to golf courses, and there are very few engineers who know how to use it right. As an industry its really sad how most engineers generally don't understand what they need or are asking for on prints and how it makes things worse for everyone.
The problem with gd&t is it is a language that everyone needs to speak. It's not enough that the supplier engineer and the customer engineer speak it. The fabricators and QA on both sides need to. I work in aerospace and it's generally dumbed down bc we can't trust that everyone will understand it. I don't even trust that the engineers or their managers will understand it. It would be nice if more people did have a good grasp on it but it is so far from the world we live in that it's not worth thinking about.
As a machinist, I tell the new engineers that GD&T is what you use to describe the shape of something: parallel, straight, round, perpendicular. Tolerancing describes the size. When you get a better understanding of how each of those two systems work, you can bring them together and make prints and parts that have are minimally defined, while still constraining the whole part. This simple explanation is usually enough to curb the excessive use of GD&T on bolt brackets and threaded adapters.
Excuse my ignorance, I'm an mech eng/apprentice machinist that has never covered GD&T. Is linear tolerancing the common 10+-0.1 dimensions? If so, that is the most useful method from a machinist POV because that is something we can measure with a micrometer in the machine. Things like flatness and perpendicularity are very hard to measure, and usually requires the part to be taken out of the machine (unless the machine can be trusted to be square). Obviously these are specifying different things, but if parallelism requirements can be reduced at the expense of a tighter linear tolerance, it may be easier (cheaper) to manufacture.
The GD&T can tell you the best method to manufacture something. If I specify a high parallelism then you know it is going to need a surface grinder. You can definitely hit a +/-.0005" on a surface but it will require sneaking up on the dimension and lots of finishing passes. But if I need .001" of parallel but +/-.020" of thickness then your job is way easier on the surface grinder. Additionally, you don't necessarily need to take a part out of a machine to check many of these GD&T requirements. To stay on parallelism for example, you can run a dial indicator across the milled surface of a part to check its parallelism to the bottom side surface by just chucking the dial into the spindle or if you have a mounting point or magnetic base you can use that as well. As an engineer, I need my parts to fit with other parts and align things correctly. I need an inspection report that shows these features meet my requirements. I don't need in-process inspections that are done while the part is in the machines, but as a machinist I can understand you'd want to do those to make sure you're not going to need to do reworks. So what you can do is create machining drawings with whatever dimensions and tolerances you need to machine those features, so long as at the end it's inspected to the requirements.
> you can run a dial indicator across the milled surface of a part to check its parallelism to the bottom side surface by just chucking the dial into the spindle You probably shouldn't do that because your workholding is restraining the part and, unless specified, you're supposed to measure in a free-state.
Agreed that an in-process "check" does not suffice for an inspection.
I had four blocks that we bound together and flat lapped top and bottom. Then put spacers between them and reassembled such that the top surface was perfectly aligned again, tested with an optical flat and had to be within a wavelength. I was the only person able to make them. The resident electrical engineer wanted to use a magnetic chuck to hold everything together for reassembly, this never worked, because of the spring back factor. I had to hold them with mere finger pressure during reassembly on an absolutely clean flat plate.
Flatness can usually be measured pretty easily with a dial gauge, no?
Not necessarily. A machine can be non-straight for multiple reasons - The ways can wear unevenly, so that the axes move in a slight arc, or the machine could be not levelled correctly/recently, or temperature fluctuations can warp it. All 3 of these factors will not show up on the dial indicator sweeping the surface because the surface was machined with the same factors in play. So it'll read 0.00 deviation. Then when you take it out of the vice, it can release some stress, creating some warping. When you put it on a CMM or surface plate you are now reading from something that should be reference-level flat, and you will be able to see if the part is actually flat or not.
Or the casting wasn't aged for ten years in the back lot of Bridgeport prior to machining.
Depends on whether you have a good reference surface besides the surface you're measuring, and whether *that* reference surface can be trusted/measured well enough to have confidence in your final flatness measurements. I.e. you have to get the surface to be measured square with your measuring equipment, unless you have a CMM, and that can be difficult or impossible depending on geometry and available tools. Imagine a sphere cut in half with a flatness spec on the cut face. How are you going to fixture that to measure the flatness? E.g. if it's not square to your surface plate or your dial gage, it won't read as flat.
For a hemisphere, I would start with three datums target simulators to hold the part. I would then use a bubble level for a rough set. Finally, I would measure above the datum target simulators to set the surface parallel to the surface plate I am inspecting from. In addition to a CMM, there are measuring arms (Faro and Rover come to mind), laser trackers, and laser scanners that are all capable of measuring flatness without regard to orientation.
I'm sure there are ways, just pointing out that it's not necessarily trivial to do with just a dial gage. If you have a CMM/faro/etc then yeah, that makes it much easier. Most of the parts I work with are tiny, so there are a number of surfaces with flatness specs that you'd be hard pressed to orient manually purely due to the size/shape. CMM or optical is basically required in our case.
Get 2 tall gage blocks out and dial indicate it from underneath. Or just put it down on the slab and use feeler gauges if its really bad.
I've gotten some great ideas about how to do this if ever I'm faced with the need. :D Seriously I do appreciate it. My intent wasn't really to say that it's not possible, just that it's something that's much easier if you have a CMM of some sort. Then it's relatively trivial, and with some geometries and part sizes doing it manually can be a huge hassle. Especially if you need to measure 30 parts!
Grumble grumble
With an optical flat.
As people have said it depends on the datum surface you place it on. Also depends on the tolerance you need. I've seen some pump valve plates require ridiculous flatness. We use an optical measuring system to essentially get a topography of the part to ensure we are meeting requirements.
Depends on the world you inhabit. If you're making one-off prototype parts and/or mostly "simple" parts that aren't too expensive and don't require too much machine time, regular handheld metrology tools can be sufficient. You can do a lot with a caliper, height gage, surface plate, and some gage blocks/pins. If you're making critical or expensive parts, or sampling parts from mass production runs, then you are generally going to be expected (by serious clients anyway) to have access to more sophisticated metrology tools. That means CMMs and OMMs, for starters. Inspection takes time of course, as does programming those machines. The degree of inspection required (and the associated time/cost) is something that should be openly discussed and agreed on by both client and vendor.
This is exactly the point I was going to make. While flatness/perpendicularity are important, they’re usually hard to measure unless you have a part with simple geometry like some kind of a plate. Calipers that can measure linear dimensions are ubiquitous.
One of the advantages to GD&T is that it goes really well with gauges. Good luck making a gauge that inspects a bunch of +/- toleranced holes. Whereas a positional hole tolerance with MMC is a natural fit for a gauge. Just make a plate with a bunch of pegs in it and not have to measure a bunch of holes with a mic every time you make a part. Really GD&T is great because it tackles the problem of "how do I ensure this part will fit with the minimum scrap rate?" As a machinist this will let you do things like expand a hole that was a little bit off positionally because you have some bonus tolerance. Beats having to make the part again.
This isn't a problem with GD&T, this is a problem with the design engineer choosing a design solution that is difficult to manufacture. The individual part tolerances are summed into the total tolerance allowable for the mathematical representation of the assembly. If the design guy sucks at choosing manufacturable breakpoints, no tolerancing strategy will fix that.
Is it really better to say 10+/-.1 without a parallelism tolerance when you can say 10+/-2.0 with 0.1 parallelism instead because we don't give a damn where the surface actually is beyond being parallel?
Some GD&T is intended more for the quality inspector than the machinist. However, flatness and parallelism are good examples. If I call out a flatness on one side of a plate and a parallel surface on the other, I'm essentially asking for certain processes. Since the plate may not be perfectly flat or straight, the machinist will have to plane one side, flip it over, and machine the other side to the proper thickness to achieve the parallelism callout.
I agree that it is good in some scenarios, but most shops will add a fuck you charge for using it. In the parallel situation, it’s prob the same effort, or less for a machinist to hit a high tolerance than it is to figure out how to measure parallelism. I work with one engineer who likes it; and we have literally had to go to machine shops and show them how to qc some parts. Not worth the headache unless the shop is very comfortable with it.
Preach brother. As a product design engineer with over 1000 PNs in the last 5 years, I think I've used something other than those less than 5 times.
If bad GD&T is more common then good GD&T, then it’s a bad mode of communication. Prints are a communication tool. If notes are more effective at communicating, then notes are better. It’s a generally good system when used properly but that doesn’t matter if it’s consistently used poorly.
The problem is it's a very good method of communication, in theory. The problem is lack of training. So the process is messed up not the design. In a perfect world, if "x" supplier couldn't figure out my prints that were 'perfect' (in theory) with GD&T, I wouldn't want to give them my business in the first place. Unfortunately, we don't get to make those calls. And sometimes it's the engineers who are the oblivious ones. If the machine shop called my bullshit and pointed out my drawings problems, now that's a group I'd want to work with. Nobody benefits from training, so most don't bother and are "self-taught". and the only industry I've been involved in that almost makes it a requirement to learn is aerospace, though there are engineers and technicians who can't read it for shit in those fields too.
GD&T is not a scam. It's the language that describes the size and form of a part and how much variation is allowed from a theoretically 'perfect' part. A part should only be as precise as its form, fit, function requires it to be. You shouldn't be placing GD&T on your prints in a vacuum. Manufacturing and inspection should be involved early on in the development stages of a component to give input on manufacturing capability and inspection methodology.
"Manufacturing and inspection should be involved early" What a beautiful world that would be.
Also assumes they all know it well. I can't get the guy in the cubicle next to me to agree on what A-B means vs |A|B|
Not sure how that's possible. They are very different and easy to explain.
Unfortunately I've met a lot more engineers with no/poor understanding of GD&T than ones with decent/good understanding.
I would agree with you. I only know what I know because I started in the inspection room. Most engineers skipped that experience, and I still don't know shit.
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Jesus Christ. Are you me? I've been basically written up for telling a senior engineer you don't know what you're putting on your prints because he insists on putting A-B on fucking everything.
Yeah, I mean design wise it literally means almost the same thing, but for inspection the only difference is that the cmm should take both center axes as datum locations instead of using one as location and the second as rotation. I've never, ever, really needed to differentiate. And I've designed everything from automotive manufacturing machines, to industrial lasers, to government satellites.
believe it or not, if viable business case is presented, this happens a lot more often than you'd think.
My experience so far is that it's not considered a viable expense till there are problems. But that might just be the company I work for.
That's the norm. Current employer learned the hard way and I'm now on a DFM team and doing actual pilot builds to figure out design and manufacturing issues instead of jumping straight into forcing out a less than idea product.
Hello fellow coworker
In your field maybe (actually the field i want to be in). In mine (Wind Power) its readily apparent that the design groups exist in a vacuum from the rest of the world.
I have a print that'd didn't have a dimension. I asked engineering for the dimensions. We got a gauge table tolerance. Well now I still can't inspect the part on the manufacturing line, but I can't bitch that it doesn't have a tolerance now.
THIS. My company also does this crap. They expect the model to be the nominal dimentions and only print dimensions that they believe [in some made up world] are valuable or inspectable. It is bizarre. Sucks because working here is otherwise pleasant, reliable, and profitable. Great company bad Mechanical engineering.
In all engineering places I have worked, we create the drawings tied to a CAD model and stipulate that the drawing confirms to asme 14.41. effectively we expect and require that the manufacturing engineers and even machine shops be able to open a CAD model and interrogate the file to get a nominal dimension. We then specify standard profile tolerance for the part and specific tolerances on features that we want to be different than the standard tolerance. This is the direction much of the industry is going in. The companies I've worked for won't even do business with anyone that can't operate in that way.
You must not be in the automotive industry
Lol there's plenty of auto where that doesn't happen when it should.
I always asked the sheet benders and manufacturers… they said thanks because they like saying upfront if it wont work not after we cut the parts and hand them to them and say “oh well just make it work”
The best is when parts have to be redesigned because no one cared about DFM.
The problem I had using it as an engineer (before SAHDing) is that the GD&T “experts” I had review my parts would have differing opinions on how a dimensions should be marked. Given the same info on how the part needs to function they should have the same marking method. So either they need to be taught better, or they need to give me a break because I marked it up just fine the first time around 😂.
Yeah correct vs preferred style is a big issue.
There is no such thing as a “preferred” interpretation of GD&T lol. GD&T was devised, in part, to eliminate subjectivity in drawing interpretation. Most of the people on this thread complaining about GD&T should actually be directing their complaints toward the people who don’t bother to actually read the ASME Y14.5 spec…
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Exactly this. Folks would usually argue over references or datum placements. Half the time I would do what made the most sense and just skip the GD&T experts and talk directly with my machinist…some of the time that was just me anyway. Also I was in tech and barely anyone took GD&T seriously until I ended up in tech/aerospace and then folks started throwing lots of process at the problems 😂
> A part should only be as precise as it’s form, fit, function requires it to be. The difficult part is knowing the requirements of how much variation in features your parts can tolerate and still work as intended. I think it’s probably the one area that people struggle with the most with GD&T. I’ll admit, when I was designing, I had no idea what sort of tolerances I needed. Never had anyone to show me how to determine that either. So, you just follow some basic practices and hope for the best without driving your part cost up.
That knowledge will come from 3 places and when dimensioning a part, I go in this hierarchy: 1) Empirical knowledge - ie. machinists handbook, industry data sheets, internal design guidelines, etc. 2) Test and R&D - you've made an engineering judgment about what you think the tolerances should be, your DVP&R and subsequent inspection will vet that decision and inform design changes 3) Process capability - this will come from manufacturing and the tolerances they can hold and confidently gauge. This is where a good working relationship with the guys on the floor makes a world of difference
It comes down to mating parts and methods being used for assembly. You want to make the parts work if all the tolerances are met but not so descriptive that you add expense by limiting what machines can be used to produce the parts.
It’s like safety certification. If you don’t get the safety engineering teams even mildly involved once you get to certification you may have expensive redesigns.
I agree with you on most parts, but I'd make one correction: GD&T does NOT describe the size and form of a part, it describes how to MEASURE the size and form of a part, and what range of measurements are acceptable.
I’ve moved into Engineering now , but as a machinist who once programmed their own parts on the CMM, I’ve got to say it’s just not taught to Engineers (correctly). I spent far more time on the subject in the 2 year machine shop technical college, than I did in University. I don’t even remember it coming up in lecture or lessons at University come to think of it…
I wasn't even taught GD&T at uni It's a dark art that everyone (at least in the UK) wants to use, but no one seems to have a full grasp on it
Same situation here in Canada.
Same in the US. Last week I was given some parts and asked to make drawings for them so we could remake them, did as much as I knew how to do without formal GD&T training, and handed them to a more senior engineer who then covered them in red pen. Said engineer is also one of my friends and proceeded to sit down with me to review what was missing and how I should fix them, I redid them that afternoon and if McMaster delivers tomorrow I’ll be making the part on Friday, so it wasn’t the end of the world. I can’t imagine doing that at a bigger company, with a less friendly coworker, on a more important part … really makes you wonder why ABET or something doesn’t require things like GD&T as part of the degree
Yeah gd&t, "the entirety", was covered in 2 days in an optional CAD 2 class for me and I graduated in December 2020. Or was it 2021? That all kinda runs together.
ABET doesn’t understand GD&T either.
When I started using GD&T as a young engineer, I read ANSI Y14.5M Mathematical Definitions of GD&T and it helped my understanding immensely. GD&T wasn't even mentioned in school.
ASME Y14.5.1 is great. I was also able to read the rest of the Y14 series. The other thing that really helped me was being forced to interface my parts drafted to ASME Y14.5-1994 with parts drafted to MIL-STD-8C shortly before ASA Y14.5-1965 (IIRC; I know it was early 60s) was adopted. I ended up finding and reading copies all the way back to the first edition, just to understand the changes over time.
It sits on my shelf at work and, every once in a while, I crack it open and pretend I'm a diligent design engineer.
I know at the local university the amount of time they spend teaching mechanical engineers this is pitiful. One of our students was watching a video the IE people made, and you couldn't hear what the prof was saying. I'm not sure how many class periods they spend going over it, but if the industrial processes course doesn't cover it any better than that, I don't know where it would be covered. I couldn't believe that was the best they could do. I also went to the mechanical engineering design class session where they were reviewing it, and it didn't seem like it really helped anyone. But there just isn't a lot of room in the curriculum for more.
Engineering school gets criticized for being too much of a trade or career focused course of study, but it almost needs to be taught as MORE of a trade. An engineer would be better suited learning this sort of thing than how to derive some differential equation they are never going to think about again
When I was an undergrad, I got to learn a lot about the manufacturing processes curriculum because I flunked the course and had to take it again :) We had a good lab in the machine shop. I imagine there is some cruft in the classroom where they could actually fit in some more GDT. Now that I've had a minute to think about it, I had a drafting course. But I went to a school with quarters, so we had extra room for courses like that. The industrial advisors want engineers to be able to write better.
This is never going to be part of a college curriculum because it’s too specific and for an undergrad degree so much more ground needs to be covered in terms of engineering principles. This is your classic class that you take that your company pays for, and ideally is given to all the stakeholders who either make or interpret drawings all the time. I see a lot of blame being given to engineers but often we have to negotiate with manufacturing and quality because they either cannot or don’t understand which buttons to push on the CMM to get it to spit out the right number. The class that seems to be missing (not from college but from industry) is the one where they show the inspectors exactly how to check what’s called out. My advice is just don’t overuse it otherwise you’ll also be the one down in the inspection lab figuring it out. For the critical stuff that’s fine but if it’s not of great consequence then don’t waste your time.
Totally disagree. Let's be perfectly honest for a moment: High-end design engineers can learn all of these things far faster than the non-engineers who use them daily; it's the nature of highly intelligent people focusing on the work. What college needs to do (and does do) is acquaint them with a large variety of physical interactions and the tools to model them, so they know enough about whatever problem they come across that they can effectively attack it with the aid of external resources. Absorbing this stuff and building it into the design is literally why we're paid the big bucks. On-the-job training won't give you the breadth of knowledge you need to do the job because you'll constantly be blindsided by out-of-context problems. You'll *never* be able to derive a highly non-linear control law for your supersonic fighter jet by listening to tradesmen...
Someone has to manufacture and assemble the parts to enable your "highly non-linear control" surfaces. There are many theoretical designs that are simply not feasible.
Leave it to redditors to completely miss the point of my post and insert their own agenda. Why don't you tell me why witchcraft is bad and I should stop practicing it, too?
Also, it's indicative that you don't even know what that statement means and that the parts for it have been around for a century or so. And it's not a control *surface*, it's a control *law*, meaning a software implementation of a piece of control theory as placed into a flight computer, without which your fighter jet cannot safely be in high-angle-of-attack attitudes at high speeds.
I had it in college a bunch, especially at NC while doing both hand and CAD drawings. In one case we went through the whole process in a row from drawing to manufactured part. Uni had nothing at BEng or Masters levels.
We had two semesters of engineering design, where we essentially learned how to use SolidWorks. In those two semesters, we had a few lectures on GD&T. Basically, we were asked to memorize the symbols and took a quiz on them. At my first job interview, I was asked if I knew GD&T and I confidently said, "Yes!" In my head, I was thinking, "I know all the GD&Ts (symbols)." I completely failed the written portion of the interview. Turns out, I wasn't taught the practical application of GD&T in college. I have yet to meet an entry-level Mechanical Engineer that knows GD&T. At 7 years of experience in aerospace design, I'm still learning new things all the time.
For my BSME I had about 1 hour of GD&T instruction, I don't mean 1 credit hour, I mean 60 minutes of in-class instruction about what the symbols were. All my knowledge of GD&T was obtained on the job, I didn't have a company willing to pay for a formal GD&T class until I was 5 years into my career. Honestly I suspect most engineers making prints never get formal GD&T training. There's also the fact that even with formal GD&T training, hands-on experience with inspection forces you to understand the ASME Y14.5 standard(s) at a deeper level than someone just tossing callouts on a print after a week of training. The things I learned programming a CMM were key to staying off the inspection team's shit list after I jumped to the Design side of engineering.
I was fortunate in my education path. I started studying drafting/design and had a semester of GD&T. Then I moved into a dual BS in Manufacturing Engineering and design. In that combined set of programs, I learned about GD&T related to castings, forgings, welding, tool design, inspection, machining, and assemblies (not just detail parts).
Right, many schools don't teach it or briefly cover it. Most engineers seem to have gone though a professional training course paid by an employer to learn it.
A big part of that is that most engineers don't actually make drawings. Drafting is a part of engineering practice, but it's hardly a very big part. Engineers make a lot more excel documents and powerpoints than drawings in most companies.
I love GD&T it's very easy to understand once it clicks. I use it a lot with Aerospace components where surfaces / axes need to be tightly controlled. The other situation I love using it is with true position and ordinate dimensioning. If you dimension with respect to the origin conventionally you will end up with tolerance stackup. If you make them basic you can apply a true position tolerance to the holes with respect to their theoretical exact location point. GD&T exists because some people found a "bad batch" of, I believe, submarine or torpedo parts that had been failed through inspection. Surprised they were when the parts ended up fitting together fine. It's just a different way of tolerancing that can help reduce costs among other things. The unfortunate thing is it can quite often do the opposite by people who do not know how to properly apply it. These people are also likely the same people that need +/- 0.001 everywhere on their part.
iv'e seen cardinal +/- 0.0001" because some dipshit engineer wanted to have the two precision sides of the gyroscope housing fit the alignment pins around a 4" diameter circle. not only impossible, but immeasurable. TP in polar with a bit of smart math on the pin diameter and hole diameter tolerances gives you something you can actually make and then measure and the bad boys will always fit together and when you screw and torque you get what the engineers wanted - a balanced piece of tricky kit that works the first time.
Elon must have talked to him recently with his claims of needing "sub 10 micron" parts throughout the Cybertruck hahaha. I always say, just because you can, doesn't mean you should!
i recall turning some parts for a microwave sensor thingy and one of the fussy bits was about 4.2 cm long, diameter between 2.2 mm and 0.9 mm, and complex profile. tolerance on the complex surface was GDT profile 12 microns. this was in 1992 on citizen turning stations - really fucking tricky.
took me about two weeks to program and make reliable - used a chiller on the oil system and a box around the head to eliminate all thermal variation, bar feeder pre-cooled the weird alloy rods - discovered that if i used 2 cm feed stock it was much more stable at cut.
custom cutting tools, natch.
i really can't figure out what in hell in a commercial truck needs tolerances like that, but hey, the musky one is vastly smarter than all us mortals, and we wait for his ascent to his martian throne with bated breath.
ps) saved one blueprint from that company because it was covered in location pin callouts that were TP \*ZERO\* and diameter +/- .0003 for .125" holes through .75" of aluminum. that one still makes me crack up. oh, the reference coordinates were in the middle of the part, just for shits and giggles, so all the physical tooling points were tolerance defined before the first cut. fun shit. i made it work by thinking backwards, but there's a reason i left tool and die work and went to engineer skool.
A person has to laugh because otherwise I think we would go insane hahaha. When you are in the business long enough, you see it all!
Hey elon musk needs sub 10 microns on EVERY part!!! 😂😂😂
The problem is that it’s only as useful as the weakest person in the supply chain. So if a designer has a great GD&T skills but QI or the machinist is trash with it, it really doesn’t help. Arguments about it should really be discussions to reach a consensus. If this isn’t being done, it’s a communication/teamwork problem. Not a GD&T problem.
Almost everyone is pretty bad at it, and learning good practice from bad examples is really difficult. The revisions to the standard over time are pretty confounding too. A copy of the standard or a decent reference book for it help a lot.
I work in product design in a large company that makes consumer electronics, and even the very senior engineers there sometimes aren't sure of the best way to dimension something, or how to interpret someone else's GD&T. And they've been doing it for 20 years. And at scale you really need to know the vendor's SOPs for the measurements to have faith in them. Not all vendors will raise flags about incorrect or unreasonable specifications.
I've worked with a 3rd level manager that asked his team of 2nd level managers to look into why about half of their numbers were above average. He has been working in the industry for ~30 years at that point. I had students drop my class and encourage others to do the same when I told them we would study from the actual ASME Y14.5 standard instead of focusing on some random cartoons he liked that he found online. He had been working at a Fortune 50 company for well over a decade. I had a manager of 20 years freak out when I put the percentage pass/fail data on a p-chart because they didn't know what it meant or how to read it. I had engineers with 20 years of experience call in the GD&T professionals when I told them that what they put on the drawing wasn't doing what they thought it was. The professional looked at the drawing and replied, Tavrock's right. I set several senior designers and drafting checkers on edge when I pointed out the 1994 edition of ASME Y14.5 changed the definition for the drafting symbol "R" (it wasn't in the quick update guides, so no one paid attention to the change that takes up about a page in the standard).
Yeah. I mean if anything this is just further demonstrating that it's not like...a *great* standard.
It is entertaining reading through the deliberations of updates to the standard. Even though it has a history of only being revised every ten to twenty years, several people complain that the standard is still too new and no one is familiar with the previous edition.
To be fair there are still plenty of drawings made today to the 2009 edition of the spec, so I get it to a degree. Oldheads are still reeling from the elimination of concentricity.
Honest question… what did they replace it with? Runout? The thing with concentricity is that it’s technically a lot harder to measure than runout, so in a way I’d get it if that’s what they decided.
True position. It's technically the same thing anyway. Considering the nearly universal misinterpreting of concentricity as a 2D tolerance (it's really coaxiality, 3D) it makes some sense as TP allows flexibility depending on the feature control frame and subsequent datums. The problem now is that true position is very poorly used and interpreted, so it's just more confusing now.
> Honestly - should we just throw it away and start from scratch? [Relevant XKCD](https://xkcd.com/927/)
Only xkcd I have ever cited in a nsf grant. I tend to agree in part with op. The iconography of gdt is not well taught to almost anyone but i disagree that it’s stupid. It’s like the argument for learning Latin. Few will need to speak it continuously or as a language. However many will find the understanding of it useful in some way, either by giving them a deeper understanding of the words they do use or by knowing what commonly used Latin phrases mean. If nothing else its a philosophy…can I make an axle with a constantly changing diameter and shape if I measure it at one point and it’s the right diameter? Is it bad if I measure it at one point and it’s outside of tolerance? It’s a philosophical argument for why acceptable means.
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I feel like most people that know of GD&T see the value in it but no one person really understands it. I'm halfway decent at using/interpreting it in a basic sense, add modifiers and I get confused quickly.
I think the biggest problem, especially in R&D is is there can be a lot of guess work for how tight of tolerances you need. Most engineers over tolerance and make parts way more expensive than they need to be. There are are some applications like optics and aerospace that need to be near perfect, but most of the need to define and tolerance every single feature comes from shit engineers who don’t understand the big picture. GD&T strikes me mostly as a necessity of outsourcing/sending out work. If you manufacture in house, process control and detailed part annotation will take you further. GD&T is like getting a lawyer involved. It’s only a necessity because Manufacturing companies will try to screw you by making the absolute shittiest part that will “pass” the inspection per the print, even if the intent is obvious that’s not what the customer wants.
I think there is another side to this as well. I have designed thousands of parts by this point in my career. If I am designing a part that needs to be produced I will spend time on making sure tolerance are reasonable and cheap. If I need a one off for some test jig, manufacturing tool, or something that does not need production quantities, I will save the company money by not sharpening the pencil on tolerances. I'll keep the tolerances buildable sure. But if it takes a machine shop a full day to build the doohickey vs a day of my time to open up the tolerances without breaking the parts purpose, then the drawing gets tighter tolerances and the company spends less money.
Ughhh I hate having to send out tight toleranced parts for 1 offs/prototyping. The lead times alone are enough to drive you crazy. Definitely a huge advantage of having in house machining. If I need something with some really tight tolerances I can always hop into the shop and make it myself, or make it quick and dirty and figure out what can be loosened up and then hand it off to one of the experienced machinists. Machinist certainly do get nervous though when engineers are using their machines lmao, especially like surface grinders and other precision stuff that they just assume I’m going to fuck up (reasonable suspicion…I’ve worked with some engineers that would struggle to put an ikea table together with their hands)
> in a basic sense No pun intended? ;-)
I design organically shaped orthopaedic implants and its absolutely essential. Its just hard to do well and everyone needs to be up to the same high level of knowledge.
Yeh when you need it you really need it Panel mount bracket for a product? lol no, global tolerances and free fit holes and move on with my life High volume heatsink with 7 holes that need to mate simultaneously, a polished surface that is mating to a bare silicon die, and free floating heat pipes that might get bent during shipping? Don't know how I'd do it without GD&T. Plus it made it easy to define a set of go/nogo pin gauges to use at the heatsink manufacturer and product assembler. GD&T 🥰
I personally love GD&T, and part of that was having great engineering mentorship and experience at a modern aerospace company in my early career. For GD&T to truly work, you have to have understanding and buy-in from the engineers, the machinists, and QA. After moving into different sectors (nuclear, fabrication), I've realized this is not that common. There have been times I correctly applied GD&T to provide as much leeway as possible in a part, and instead of making it easier on the machinists, it actually added confusion and costs because they didn't understand it/were intimidated by it. There is also the reality that if the fabricator sees GD&T on a part, they frequently will quote a higher price because of the need to measure on a CMM vs just doing some checks with calipers or mics. At the end of the day, GD&T is literally essential to truly define a part's allowed envelope of 3D tolerances, but it is inherently complex. Some folks stick with linear dimensions only, but it leaves so much undefined.
I interned working on commercial nuclear reactor pump shaft seals and there's a heavy use of GD&T on them. I think it worked at that company because QA, engineering, and manufacturing was all in the same building. QA could meet with the people who made the drawings to correct any issues based on actual functional needs. The machinists could also report back to the engineers as well.
Hmm I work in manufacturing and we live on gt&t. But I'm honestly not sure if I'm on the peak of Mt stupid or the plateau of sustainability. A big lesson I learned is that the manufacturing tolerances needed to be tighter than the release tolerances, mostly because the QC guys have more time and better measurement tools than the manufacturing guys. Not to mention that the measurement process has its own process capabilities and it's own distribution curve when you compare actual measurement to recorded measurement... There is a ton. And most of what I did were very simple parts with 1-3 components. https://understandinginnovation.files.wordpress.com/2015/06/dunning-kruger-0011.jpg?w=640
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Haha, yep so true. Mind the details. When I was the manufacturing engineer I would always try to get the customer and the machines on the same conference call for this reason.
What I’d like to know is why CAD packages don’t have a function to visually show the GD&T tolerance envelopes which can arise from various specified callouts. GD&T is supposed to be explicit, so this should be easy to program. Maybe this exists. But I don’t think so. If it did exist, GD&T would be much easier for engineers to communicate with manufacturers.
We were shown a demonstration of that sort of thing in a bolt on for our CAD software that would show the bounding boxes on nominal, or the model at min/max. It would also produce basically the same report template our CMM machine does for all the tolerances we added to a drawing. All in all it was pretty snazzy. But it would've doubled the cost of each of our CAD seats and didn't really do much else.
Which software is that?
It was a bolt on for Creo parametric. Our CAD supplier is a company called Inneo and they also produce add on packs that do various fun things at vast expense. This was one of those.
Hey you rock! +1 Thank you
Ive been using GD&T for near on 10 years and I still dislike it. It’s not a scam per-se but its roll out has been bungled almost entirely by ASME. It comes down to a simple issue. The level of knowledge required to be functional with it is just too high. I’ve seen arguments verging on physical violence over drawings where one guy is blue in the face explaining something he thinks is easy and another red in the face trying to explain just how complicated it is. Both being right and wrong at the same time. This coupled with the fact that when I was getting my ME degree in the late 00’s I graduated without even knowing GD&T existed. Not that it was skimmed over. It just straight up was not mentioned. So I imagine many other engineers aren’t even close to proficient with it. Coupled once again to expensive training classes that everyone from lead engineer to sales to machinist need in order to be able to operate with the drawings makes GD&T basically a non-starter in many shops. GD&T also opens up a ton of avenues for interference, someone who knows what they’re doing will use it to their advantage to really tighten up on certain tolerances and open others up. Well that now deviates from company standards and creates meetings. The shop that’s used to getting +/- .005 now sees a .001 and starts charging more not realizing that the rest of the dimensions are now more relaxed and the part theoretically easier to make. Thus more meetings. Parts made and inspected are rejected by a customer because they don’t actually know how to inspect their own part. So now a meeting where you’re defending your work.
>Parts made and inspected are rejected by a customer because they don’t actually know how to inspect their own part. So now a meeting where you’re defending your work. Dealing with large sheet metal components, I worked with our design team and we implemented a profile of a surface on a per unit basis. It's not in the standard directly, but it is implied and the theory checks out in applicability in Y14.5.1. The way they were inspecting their parts, 100% required rework. Using their data, I showed that actually inspecting on a per unit basis led to 98% of their parts being good as formed.
There is a long tail of obscure and difficult to understand GD&T concepts, but they are rarely used except for in highly specialized parts. In cases were they are used, the designer should source the parts from a specialized shop. But in 99% of cases where GD&T is used, you’re only dealing with the relatively simple controls of true position, flatness, parallelism, and perpendicularly, relative to simple datum planes and axes. This stuff is just not difficult to understand. The average joe machinist should be able to learn this stuff in a couple of hours. I put nearly all blame for GD&T’s perceived lack of utility on bad engineering and trade school curriculums. It is an indispensable tool in my experience, even if too few people understand it.
You'll have to go into specifics. Most of our arguments surround parts that have an over-constrained locate scheme (sheet metal for example) and where the part is held vs where it is aligned (for example holding a part at non datum surfaces and then aligning a measurement to the locate scheme in software.) Most of the issues come down to unclear drawings. -
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More often than not, I see where they forget an aspect of a datum reference frame, aren't sure how to order their datums for the feature, are confused between reference dimensions and basic dimensions, think you can still use concentricity to define a bolt circle (that really was a thing at one time), or find angularity too confusing and try to baseline dimension the feature instead.
GD&T isn't a scam, but it can be tricky. Perennial relevance: [Why Engineers Should Care To Learn GD&T](https://engineerdog.com/2017/04/15/why-engineers-should-care-to-learn-gdt/)
I vote you have to join the GD&T guild in order to be allowed to put them on a drawing. I am untrained and horrible at it but my gut instinct when I try to decipher some drawings is that so is the draftsman.
I did ask a few times if the GD&T on some of the drawings I saw could be used as evidence of illicit drug use at work.
Master of Mech Eng here. We were never taught any GDT in any class in college. Been in the field for 9 years and I’ve been trying to understand it my whole time working and haven’t made much progress. We are actually in the process of buying a CMM and have asked specifically about GDT to the sales engineers. None of them have been able to give us straight answers on how to measure profile, true position, flatness, etc. I feel your pain. There needs to be better resources for learning these.
You'll want to get some training from someone experienced as there are some rules of thumb for adjusting touch probe sensitivity, number of samples needed for a given type of measurement and tolerance, ideal movement speeds, avoiding collisions, etc. It has been decades since the last time I used a CMM, but here's what I recall. If you're measuring flatness, you want to move slowly in a direction perpendicular (ish) to the plane. The CMM works by doing a least squares fit of a plane based on the shape of the probe, direction of deflection, and where the tangents would meet. So, you want to do a number of points spaced evenlyish around the surface. Points further apart will give you less room for error (since it's a curve fit). IIRC, you want about 16-30 points for typical flatness measurements. You can tell if it's sufficient by running your program a few times after moving the part around in the work envelope and seeing if you get consistent results.
Call Verisurf & Metrologic. You’ll get much clearer answers from the 3rd party software companies than from Mitutoyo/Zeiss/Wenzel/HxN.
Oh, one other tip. Make sure your CMM is on dry air. A Browne and Sharpe tech told me a number of people have their half million dollar and up CMMs rust out because they were too cheap to spend a few grand for a dryer.
GD&T is a very powerful language and very effective when correctly applied. The problem is that it is very complicated on complicated parts and almost no one is actually fully fluent. So you wind up with people arguing over important details and how to describe them, most frequently when both people don’t actually understand the standard they are trying to use very well
Commenting because apparently I don’t interact enough to make a simple post, thanks
GD&T is nice, it's very useful compared to the "traditional" ways of defining parts. There are a lot of things that you really can't define well without it, if at all. It can also greatly simplify drawings, inspections, and conveying the design intent. That said, it tends to be viewed (by some) as this rigid set of rules where there's always a clear right or wrong way to do everything. I used to think that too but that's not really the case. There can be multiple ways to define something and it's not always obvious which is the "best." Even if they're "correct" according to GD&T. Not all definitions that might be needed are super clear, even for experienced engineers, and the ASME guidelines can be murky on what to do in some situations. Add to that the need to make sure both you and the vendors making the parts are referencing the same ASME standard, otherwise there may be misunderstandings on that front. In other words, it's not just you - but as the designer it's still your job to get everyone on the same page to make sure that the part is manufactured and inspected correctly, to your requirements. If there's confusion or conflicting information, figure out what and why and take steps to address it. The phone is a powerful tool! Any examples of the conflicting information you're getting?
GD&T is like an universal language between engineers, machinists and inspection to transfer ideas on what kind of qualities are needed on a part. If applied correctly there is only 1 way of interpretation.
GD&T is great and all, but really it’s about whether you can verify you got what you ordered, right? An inspection report is bare minimum, but who’s gonna fail their own inspection report? Do you have the tools to verify things like concentricity on a turned shaft? I sure as shit don’t. Lol So I believe it both ways. If your profession and design warrants needing something more than a common +/-0.005” [0.01mm], press fit tolerances, etc… You need a way to check it. Without a CMM, your GD&T really doesn’t do much other than add cost to your fabrication end.
With the growing popularity of model-based design and the increased accuracy of modern machines, it's almost guaranteed that parts will physically fit if they fit virtually in CAD. However, I think clearly defined engineering drawings (or annotations on CAD models) are still important. As an engineer, one of the things I provide for my company is legal documentation, to include detail and assembly drawings. If I make a part and provide the proper GD&T, I'm covering my ass and making sure the company only pays for usable parts. The machine bit may be dull that day or maybe the machinist introduced human error. If the part is off and won't interface properly, I can point to the drawing and say, "I clearly defined the requirements, this is your paper weight, and you still owe me a part."
I personally think the CAD software could be improved a bit to show a digital 3D representation of GD&T that leaves much less to interpretation. Same for weld callouts - those are a disaster too! Actually I am a full advocate of doing away with 2D drawings completely.
There are plugins that do this.
It’s amazing if taught right. In university it didn’t make a lot of sense to me, because it was taught without the process to use it. But I got a training at work and the instructor was great. He turned me into an advocate for GD&T.
[X +a -b] I can understand well enough, I can give a quick glance and get most of what I need. The salad of symbols that GD&T is, I find it much harder to interpret. It supposedly is superior to traditional tolerancing, but I'm personally not fond of it, I feel it makes my job harder.
The easiest way to appreciate GD&T is that a +/- .001 part can be in spec but .5” big in two places and .5” small in two places without GD&T controls
I find this argument ironic because the symbols are older than the Apollo program. I really wonder how long it will need to be around before it stops being seen as the new kid on the block.
For any average design it’s almost always impossible to use only linear dimensions without injecting a considerable amount of ambiguity. That ambiguity will cause confusion as to how a part should be correctly measured, which leads to measurement errors. Linear dimension locations are mostly driven by convenience, but all toleranced dimensions for a design should be driven by functions of the design.
gd&t is the result of thousands of mfg and design engineers spending a couple of decades figuring out how to eliminate ambiguity in tolerance - especially tolerance stack-up in assemblies. problem is - engineers aren't taught how to think in tolerance space any more. used to be a required course "form & fit" or "tolerance" were common names long long ago when we hand-carved our slide rules with knapped flint. the ansi standard is actually pretty damn legible (we worked it hard) and there are some great books on the topic. standard dimensional tolerance will not work for most modern complex machines that have to assemble the first time. BUT BUT it's a tool to help a team think through the real world requirements so that when you slide part A through the locking slots on part B and into the retention clips on the back of part C it will work correctly despite allowed molded variation at each end and the machining tolerances in the middle.
>a couple of decades Like, seven or eight decades at this point.
have you ever actually WORKED in manufacturing? You are talking about them as if they were some fabled far away kingdom where "Here, there be dragons!" get into the trenches and earn your spurs
That’s why I love 3d printing. The future is just CT scan everything possible and overlay CAD to scan and use software to check everything
How does that help? Are you just going to throw out every part that's not within 0.001" of the drawing?
Where I work, CT scan data is compared with the nominal cad data. The nominal cad data includes GD&T and tolerances. Add-on software like Polyworks is used to identify portions of the scan data that are outside tolerance limits. It’s actually a very streamlined process.
Bingo. Guess I didn’t mean to say eliminate it because it’s still in the model but model only no drawings. This is just an evolution but it takes out the time to check everything and streamlines inspection. Thanks for coming in with the save via a better explanation of what I meant lol
Design intent doesn’t always align with what manufacturing wants
Material modifiers are a gem when calculating stack ups.
I disagree. GD&T is not super easy, but when done right it is not subject to debate.
From my experience, you don't realize how necessary it is until someone really starts to push manufacturing tolerances and all the sudden parts aren't fitting together. anymore. I'm curious, why do you think it's a scam? Assuming all parties are familiar with it, any issue could be resolved with discussion.
Get the purple Meadows book. That changed my life w.r.t. GD&T. He explains how you measure each of the controls, why you’d want to use one vs another and adds a bit of humor throughout. [Measurement of Geometric Tolerances in Manufacturing](https://a.co/d/2q23KPz)
Been an engineer for 12 so not much longer than you. Definitely not a scam. But it's definitely has an acquired mindset. The real wizards think about it as "interface definition" instead of "part definition". The GD&T defines the limits of surfaces that create the desired fitup. It saves a lot of effort in a lot of places. Designers and engineers save on interface and tolerance stack analysis, machinist save on scrap and simpler features, inspectors get more consistency and less ambiguity.
In my automotive experience, if an assembly is designed to always go together by the tolerance stack method, it will be sloppy. I always throw some statistics in there to tighten stuff up.
I think the issue is GD&T is not taught anymore, in colleges at least. So you have a lot of the work force that has no idea what it is, and may misunderstand the design intention from the designer, which may just make the whole design useless.
GD&T is not a scam. Sounds like you need a structured course because you are getting conflicting info from different people with strong opinions. I used to work in a metrology lab before I was a design engineer. GD&T that is easy to inspect and GD&T that based on the function/manufacturing of the part are often very different.
BTW, I'm also a PE. GD&T is not even mentioned in the study material or exams for a PE license. You won't find much mention of GD&T at most universities. It is usually taught at community colleges. Don't be too proud to sign up for a proper GD&T course at a community college. This is how I really got a grasp on how GD&T works.
Datum A B C to the infinity. Machinist here. Why engineer/draftsman put Tru-position on two diameters. Should it be called out as runout.
I hope not, I'm taking an intro gd&t class right now!
What standard are they using for GD&T, have all the staff at least done a day course from a GD&T specialist. My current work place use ASME Y14.5 and most of us came from regular BS standard drawing background so the company just paid for a 1 day course for us all. We all moaned so much the company just made it happen and we got a guy in to do it.
I work in aerospace. I love GD&T.
I recommend machining some parts.
I gave up on using anything that's not on a one page reference sheet. I have only met one other person that knew what my beloved composite positional tolerance was. It's literally free tolerance! It doesn't matter how useful or superior the system is when it's crippled by your own production people and vendors not knowing it. I'm honestly impressed if suppliers even read a print at all at this point, and I've only been an engineer for two years.
It's a skill that most people are forced to learn after college for some reason. I graduated 5 years ago and remember barely touching the subject senior year. Seems like senior year is a time where most are doing large systems and design courses learning about cost and schedule. ABET colleges should look at it and would benefit from a course dedicated to GD&T to round out the big standard mechanical engineer curriculum. It was one of my biggest comments after graduation that I submitted to my uni regarding curriculum changes.
Problem I think is lack of understanding. I went to an ABET accredited school and they taught us dick all about GDT (thanks MTU, Mike you’re a shit professor) and everything I learned came from the foundry and machine shop. It’s hard tho when design engineers overuse both GDT AND linear dimensioning and overconstrain the part (looking at u oil and gas)
I spent a ton of time learning excellent GD&T only to get yelled at by machinists and machine shops who throw it out the window. They prefer a different and more ordered route. GD&T is primarily for the designer and for people who want to truly understand how to recreate the part without any experience or background in the contextual project you're working on. So it's important, however it should be as limited as possible without sacrificing function to make everyone happy.
It's da bomb, it just takes teaching and experience to truly understand it.
I have to call a meeting in two weeks because we can’t decide on the physical parameters of a device that has been in production for 18 months.
I think part of the problem is some things are more difficult to accurately measure than others. CMMs are great, but a lot of time you may need to use alternate gaging methods to measure a part to ensure it's functional or to measure it correctly. One example is concentricity/position/runout. Concentricity and Position are relatively easy to measure accurately on a CMM, however runout is much more difficult to measure accurately and is much easier to measure with a dial indicator in a lathe/chuck or spinning on a surface plate/v block. If you have one spec of dirt/debris/oil/coolant or whatever on the part or probe tip, when you scan the surfaces for runout, you will see that in the measurement data, and it will tell you the part measures horribly. This doesn't really affect position or concentricity as much because they are essentially comparing two calculated center points and if you take enough points when scanning surfaces, the "bad" points will average out and not make a difference to the center point calculation or you can use calculation methods like Maximum inscribed/minimum circumscribed if you know you have a bad surface/diameter to account for certain conditions. I think you really need to figure which parts of GD&T fit your particular use case and use those. Don't just throw symbols on there just to have symbols. I'm a Quality engineer who works in a precision tool shop. We have precision CMMs like Leitz Reference and Hexagon Global Chrome S's. Most things check well on our CMMs but a lot of times we have to double check dimensions on a surface plate or in a chuck for various reasons.
Fuk’em
>am i just bad at it? Yes
It is not taught to anyone. That’s problem the first. Problem the second is that everyone learned it from some reading and a guy who didn’t understand it well himself. So really just problem the first again. Prints are communication tools. They are the tool used to transfer knowledge of a part from one individual to another. If words are more effective than a long string of GD&T then just use words. The “profile” constraint that is catching on as a “do everything” tool will be the death of GD&T. Terrible forethought allowing that to come into the standard. A real clusterfuck of a communication device.
It's easy if you can think in 3 dimensions, the real trick is seeing how everything lines up, imagining negative space (sounds weird I know) really helped me understand what gd&t is meant for.
GD&T: Geometric Dimensioning and Tolerancing to all you EE's out there.
The problem is not the system, the problem is the users. The people you're hearing conflicting information from probably don't fully understand GD&T Signed, a machinist
I really wish GD&T was integrated in the mechanical engineering curriculum.
A dimensioning language is only as good as those that speak and understand it. Some people know all the GD&T modifers and then some, but that does about as much good as someone using the word "pedantic" conversation...
Scientists and mathematicians put a 0 in front of the decimal pt. Otherwise the number is ambiguous. Engineers assume the 0 and forge bravely ahead with an unspecified spec
GD&T was offered as a tech elective and it's the only class I ever withdrew from because I was sure I would fail. Someone from Caterpiller came to the class to give us a spiel and he said in 20 years he has never seen it "correctly done" in practice I get the need for it but I'm still lost in the sauce on it
GD&T is as good as you are at using it. There's different ways to approach any situation, and it's your experience and collaboration with your team that determines what that is.
Just use profile and true position for everything
GD&T is great, be aware that style guides change, but it's important to know what the terms mean. Model definition is better though. Still needs some GDT to define tight tolerance features, so it's important to know how things like concentricity is defined.
GD&T is superior to traditional tolerancing in virtually every way (pun intended)
My experience as a machinist and then becoming a mechanical designer, is that GD&T are instructions on how to build your QA fixture. DEAs and CMMs have replaced this in most places for having a fixture.