Trust me - take the money. Unless you are hired to teach Y14.5 or ISO GPS you don't need to say a thing about it.

Teaching should be $50/hour per student. You are not being paid that much.

I do as much as possible, without making it less correct. I only use GD&T symbols and datum structures that I know I can confirm will make it through the supply chain correctly. I work at a relatively small company and I support incoming inspection so I get feedback on stuff I put on drawings. Our engineers don't have the luxury of making up new datum structures and having someone else be responsible to figure out a functional gauge - if our inspection equipment can't inspect it, it's a bad design choice.

GD&T is essentially the same as technical writing. The best content uses the simplest, most understandable language to convey the requirements. There is no award for using a more complicated symbology that is "tighter" than necessary (This isn't Perl or C, where obfuscation *can* win awards). Miscommunications and misunderstandings, no matter who is technically at fault, always cost everyone.

If you can't convince people then your communication is not as crystal clear as you think or your argument is not as persuasive as you think.

Be a customer-pleaser - if they don't want GD&T then don't use GD&T on their drawings. Don't fight your customers - give them what they want (as long as it is safe, even if not optimal from your perspective) and make the money.

I have been told on numerous occasions to leave off GD&T by either mgmt, or vendors.
Some vendors have quote higher cost due to GD&T on the dwgs.
Our new mngmt wants us to use GD&T, but they don't understand it themselves. They just think it should be there.
Our machinists and inspectors don't understand it, or care to.

Chris, CSWP
SolidWorks
ctophers home

ctopher, that is always the conundrum isn't it?

"GD&T will increase costs!" - False
Vendors quoting higher just for seeing some GD&T not realizing that the GD&T could be making things easier for them with looser tolerances and requirements.
And if something critical doesn't fit together whose fault is it? It's Engineering's because the tolerancing wasn't specific enough due to hole or pin positioning for instance.

Quote:

If you can't convince people then your communication is not as crystal clear as you think or your argument is not as persuasive as you think.

That is harsh. There are many who cannot be persuaded no matter how clear the communication is. As to whether someone is persuasive as they think when they are literally saying they aren't persuasive, that's just an odd comment to make.

I feel the GD&T industry isn't crystal clear or persuasive enough - why should that fall to the OP to fix?

'"GD&T will increase costs!" - False'

I've glanced at prints when I'm at a vendor and noticed some egregiously bad GD&T. I do try to ignore who it's from and respect nondisclosure, but I am drawn to look at the GD&T to see how it's being done elsewhere. And bad GD&T abounds.

Bad GD&T does increase cost. There are more ways to make bad GD&T practice than ways to make it well. It's our job as engineers to navigate all of that.

If your drawing uses GD&T effectively, there is nothing wrong with forcing the vendor to itemize the adders for GD&T. If they actually know what they're saying they can do it (I need a special CMM fixture, I need a CMM, I need to build a special functional gauge, etc).

"GD&T will increase costs!" - False

Really? To use it requires a substantial training cost and often an increase in the cost of inspection facilities. Once they are trained that workforce could work elsewhere for more money and may demand an increase in pay.

The question at that point is whether the increase in cost has a suitable benefit, but some won't look beyond the wage increases.

Good point about the training cost. I was just thinking about the manufacturing capability in relation to the tolerances.

I agree, it's training on their end.
One thing that's starting to help us, we are currently setting up to use MBD. I think it's easier to create GD&T in MBD, some vendors are slowly starting to get it.
I have also seen some poor GD&T on dwgs. Like anything else, everyone has their view/understanding how it works.
I have said before, no drafting skills, and no proper GD&T training, equals poor dwgs. Costs go up due to more revisions to fix errors, scrap due to parts made wrong, rework, etc.

Chris, CSWP
SolidWorks
ctophers home

To change the subject a little bit, I think it would be fun and entertaining if there was a YouTube video showing examples of horrible GD&T. I know this is unlikely because most drawings are proprietary and confidential but it would be fun to see nonetheless.

Most of the problematic applications I've seen have been fanciful selections of datum feature references without regard to the mutual orientation of the datum features or the sensitivity of the result to tiny variations in same.

Agreed. Weak and impractical datum structures are a sure sign of a broken system.

I've found some old drawings at my company where a shallow pilot feature is labeled -A- and on the opposite side of the part, far away, there is a runout measured against only A. And then I learned that they inspected it without even holding it on A. Too many of those!

My other pet peeve is seeing MMC on true position tolerance of threaded holes and bores that receive press-fitted components.

The other major ugly I see in our old drawings is a tolerance value of .001". Every parallelism, perpendicularity, runout, true position, all to .001", even on parts that are 80" across. While this is not impossible with the right materials and equipment, we weren't doing it. I think it came from "inspecting" the part on the machine before it's been unclamped. Our machine ways and stress relieving was not that good. This one doesn't wind me up because we didn't have a better way back then.

I've seen drawings with geometric tolerancing so bad they could be done better with just +/- and some notes. On the other hand, geometric tolerancing when done right is the only way to create function-focused unambiguous and uniform product definitions systematically.

GD&T was glossed over in my formal classwork in the 1980s and my first real working exposure and training was at a major aerospace company in the mid '80s. This company had resources to train the engineers and inspectors and their in-house machinists and they contracted to major companies that supplied the aerospace/aeronautics/military industries. Even within this environment GD&T was fairly well understood by most but not totally accepted - hence, the use of periodic training sessions. Upon leaving this employer and working 40+ years for companies that ranged from subsea equipment to medical instruments I have seen the application and acceptance of use of GD&T vary with the needs of the companies and their resources and the resources of their machine shops. Some machine shops did as previously mentioned: add cost on drawings using GD&T, others said 'no big deal. Some of the companies I have worked for did not have QA resources necessary to support the inspection for GD&T - be it the time required for hand calculation of tolerance conditions and ranges or buying software to do the calcs or having inspection equipment that could do the calcs and interpretations. The requirements of assembly assurance/interchangeability/performance were balanced against cost/percent of rejects to decide if GD&T vs. standard +/- tolerances and notes achieved what was needed. Generally, if no business benefit was found then GD&T was not a mandatory item.

Quote (Brian Malone)

Some of the companies I have worked for did not have QA resources necessary to support the inspection for GD&T - be it the time required for hand calculation of tolerance conditions and ranges or buying software to do the calcs or having inspection equipment that could do the calcs and interpretations.

Are you saying they were incapable of MMC bonus tolerance calculations or was it something more complicated than that?

Most were not incapable. Some chose not to worry about the potential bonus tolerance available if an MMC is considered. There is more expertise and interpretation required for checking beyond a straight Cartesian +/- tolerance zone.

Quote (Brian Malone)

Some chose not to worry about the potential bonus tolerance available if an MMC is considered.    

That does make sense. If the tolerance at MMC is not zero and measured smaller than specified, no need for calculations. Even if a calculation is needed, I'm not sure what QA resources and software could be required and missing for tolerance availability calculations though. To make it easy and mistake free, an excel sheet could do.

Quote:

"GD&T will increase costs!" - False
Really? To use it requires a substantial training cost and often an increase in the cost of inspection facilities.

40 years ago sure, but in recent decades GD&T has been taught in every trade-school and most engineering programs. 2/3 of my employers have offered ETI courses annually. The basic course is usually only interns and even many of them skip it for the advanced concepts and stacks courses. My other employer offered no GD&T training yet had no issues with employees using it.

I wouldnt forgo GD&T but do try to keep prints as simple as possible bc GD&T is like any other form of communication. Communicating with an abundance of words/callouts is easy but doing so often confuses the point. Effectively communicating with few words/callouts takes practice but is an art worth mastering bc in many circumstances a very direct yes/no or basic dim is all that's needed.

Do the advanced courses include trigonometry and developing the mean and standard deviation of tolerance stacks?

If I had the time and could legally divulge prints, I'd do as Eric suggested and showcase the poor GDT I see every day on youtube for fun. It would be crazy educational...much more so then a standard GDT video in my opinion. This is a great idea ;)

I'm gonna second the motion that 3dDave stated that GDT may or may not reduce costs...it's the typical in theory vs reality issue. I'm in the position where I am fixing legacy programs and the typical vision check on an optical was replaced by a CMM check. The prior CMM programmer wasn't using a Z datum so a cut depth feature(that was part of a profile) was floating in Z all it wanted to with no Z positional ramification. We just scrapped a lot of parts from bad GDT programming. Manually checking took an extra..let's say 3 minutes...would have caught the problem, but where I'm at it is go, go, go, make parts faster than you can check them...

GDT done right is good, done wrong hurts.

Quote (CWB1)

Effectively communicating with few words/callouts

For me, this is Limited(/Reduced) Dimension Drawing. Only critical dimensions and callouts are specified in orthographic views on the drawing sheet. The rest is general profile based on the functional DRF and the basic CAD model.

Ah, so anyone doing a tolerance analysis can ignore all the non-critical dimensions and tolerances.

The problems that can cause never get old.

Like, all the bolt holes line up, but the neighboring parts can't fit because this profile tolerance allows the parts to overlap.

That sounds like an incorrect application of a general profile tolerance, not a problem with the method or a general profile tolerance as a concept.

That's a problem of ever differentiating some dimensions as critical and the others, by default, not critical. Non-critical dimensions don't need to be analyzed. If they did, they would be critical.

But hey, truism is the best argument to make when there is no other at hand.

"Like, all the bolt holes line up, but the neighboring parts can't fit because this profile tolerance allows the parts to overlap." If this happened post-analysis, it indicates someone failed to see what he needs to analyze, regardless of whether the feature control frame is on the orthographic views or in the "UOS" note. It's not the fault of the method. If you caused it, consult a GD&T consultant.

I am a D&T (that's the properly abbreviated name of the Y14.5 standard) consultant - that's how I know that those who think they can separate out the "critical dimensions" are wrong. The analyst goes by what the critical dimensions are based on what the designer indicated was critical on the drawing. If they aren't critical they don't get analyzed.

It's the fault of the use of the UOS profile panacea to designers doing their job.

So whatever surface variations are controlled by the general profile tolerance, they are never analyzed? Is this what you advise your clients?

I advise that all dimensions are "critical dimensions" and lazy attempts to dodge that are to be avoided.

Yes, that is a great truism to keep in mind.

Could it be that your drawing is too complex to start with? I've not encountered this problem.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

As a truism the policy of using critical dimension drawings must be an anathema to the one who does so. I wonder why they would continue in the face of that.

Anyone smart would rather just go full MBD and be done with it rather than a crippled hybrid approach, thus directly inputting the model for tolerance analysis and inspection without resorting to duplicated information that might be falsified on a drawing by overriding dimensions and the like.

What are "critical dimension drawings"? If you mean limited or reduced dimension drawings, they do not have to repeat any information that is annotated on the model. And when the data set is done properly, they don't. Clearly you don't know enough about it, so for the good of your customers, I hope you avoid advising anything about or against it.

You should know - you mentioned them. "Only critical dimensions"

The actual measured values in the model had better duplicate the dimensions on the drawing, but since they do the drawing is redundant. In parametric modelers the model dimension used to define the feature in the model is used on the drawing so the part doesn't end up double dimensioned. Maybe your CAD isn't capable?

My introduction to critical dimension drawings was about 1983, so it doesn't matter to me if it upsets you to not use terms that came 29 years later.

LDDs and RDDs are for limited and reduced capability companies. Never worked for those, though in 1983 my boss's boss had the idea that engineering could just skip the work to do the analysis and hope it worked. It remains a dumb idea. Either full MBD or full drawings.

Seen some of my work with Shladot in the news.

Quote:

 The actual measured values in the model had better duplicate the dimensions on the drawing, but since they do the drawing is redundant

The drawing can only be considered redundant if there's a fully annotated (MBD) model (and everyone that needs the product definition at any given time has access to the model).

Normally all that is required to change a minimally dimensioned drawing into a fully dimensioned one is adding a multitude of basic dimensions that are otherwise represented by the model. The model supplied is not required to be annotated, and the drawing can be the only source for tolerancing information.

Apparently there are some gaps between the way you imagine or remember that method and how it should be done.

Here's a funny idea. Eng-Tips should sell a T-shirt for GD&T lovers showing a hole pattern with a composite positional tolerance and with MMC and MMB modifiers. Then have a large red circle with the diagonal line going through it like the Ghostbuster's symbol. Then below that image have the caption: "We Don't Need That Fancy Bullshit!"

Wouldn't that t-shirt be for GD&T haters?
They could be sized wrong too. A large would be a medium.

Chris, CSWP
SolidWorks
ctophers home

LOL, put the hole for the neck off-center to the shoulders and draw a tolerance zone around it. Then show datums around each shoulder.

Or....leave the MMB modifiers and the shirt could say "Shift happens."

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

"dumb your drawings down for people who don't understand GD&T"

Then for whatever reason the wrong people are interacting with the engineering drawings. Train them, hire different people or utilize knowledgeable job shops.

I've found that, generally, only the aerospace industry seems to have a good grasp of GD&T. However, one of the aerospace contracts I worked with had their own internal machine shop with state-of-the-art equipment but their machinists claimed they could only hold a tolerance of +/-.005". I know this is not true and asked if they meant +/-.0005"? No +/-.005". This was primarily for hole sizes and positions. This was causing issues with a tolerance stackup for an assembly I was doing for them and I tried to convince them that their machines are capable of doing better. They didn't like my attitude and let me go. +/-.005" is a very common title block tolerance for 3 place decimals and I think they had inexperienced machinists who just assumed that that is what their machines were capable of. I'm currently on another contract and the project is late and over-budget. Lots of back-and-forth with the team discussing and debating the GD&T, whether to use it or not, how to use it, etc. We need to use it to ensure the assembly will go together but there is always concerns that whatever shop they send the drawings out to either won't understand it and do a no-quote or jack the prices up way high, then when the parts come in this company's shop says they can't inspect anything with GD&T because they basically only have tape measures (these are large weldments with additional machining).