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]

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

 

[sendithard]

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.

 

[Burunduk]

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.

 

[3DDave]

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.

 

[Burunduk]

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.

 

[3DDave]

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.

 

[Burunduk]

"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.

 

[3DDave]

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.

 

[Burunduk]

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

 

[3DDave]

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

 

[Burunduk]

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

 

[dik]

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

 

[3DDave]

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.

 

[Burunduk]

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.

 

[3DDave]

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?

 

[3DDave]

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.

 

[Burunduk]

 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.

 

[Eric Gushiken]

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!"

 

[ctopher]

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

 

[geesaman.d]

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.