How many of you have to dumb your drawings down for people who don't understand GD&T 19

[Eric Gushiken]

Just curious. How many of you have to dumb your drawings down for people, even engineering teams, who do not understand GD&T. I encounter this quite often and it is very FRUSTRATING! I know the ramifications and I want the product or system to have the best chance of success with the least problems during assembly and in service but when you just can't convince people even though you are able to communicate the issue crystal clearly it often makes me wonder if I should just be a people pleaser and try not to cause a ruckus and just make the money or should I just be an ass and fight with them, then move on to the next contract that I'm inevitably going to fight with again.

 

[3DDave]

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.

 

[geesaman.d]

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.

 

[jmec87]

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.

 

[ctopher]

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

 

[Eric Gushiken]

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.

 

[3DDave]

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?

 

[geesaman.d]

'"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).

 

[3DDave]

"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.

 

[Eric Gushiken]

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

 

[ctopher]

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

 

[Eric Gushiken]

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.

 

[3DDave]

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.

 

[geesaman.d]

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.

 

[Burunduk]

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.

 

[Brian Malone]

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.

 

[Burunduk]

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?

 

[Brian Malone]

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.

 

[Burunduk]

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.

 

[CWB1]

"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.