How do you handle people who belittle GD&T and point to their success without it? 1

[Tunalover]

My boss today discredited my use of the fixed fastener formula of Y14.5. Since he had never heard of it (he's an EE) and probably because I'm a new guy at the company, he essentially discredited my use of the formula and said "we've made hundreds of thousands of parts here over the years I've been here and we've never experienced fit problems between our PCBs and their housings."

I tried to tell him that any process change, tooling change, or supplier change can cause holes to drift away from their true positions. If they do, and the parts still pass inspection, then the parts are still accepted because the formula guarantees that the parts will fit together as long as all the holes come in within their allowable size and positional tolerances. He summarily dismissed the entire subject before I had a chance to show him how the shape of the tolerance zone alone provides a 57% bigger area for the hole centers to land. He stated that GD&T should only be used for fit-critical situations because the symbols drive up the cost of the parts.

How can you talk sense into someone with this lack of knowledge and appreciation for GD&T?

Tunalover

 

[KENAT]

You can't.

I've posted about this before, especially back in my early days.

thread1103-193705 was the closest I found with a quick look.

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[3DDave]

About the best one might do, and it will take a lot of work, is to gather position variation data and do a statistical analysis to see if the current processes might result in a problem. If there isn't a problem and there isn't one predicted, then there isn't much need to avoid it.

 

[MikeHalloran]

the symbols drive up the cost of the parts

He's not wrong about that, >in the context of his mechanically unsophisticated world<, i.e. if he is equally dismissive of his mechanical part suppliers as he is of you.

DO NOT try to educate him any further; he already knows everything, because he is an EE, and you are not.
There is a current discussion around here on that very subject, but I'll just reiterate that arguing with that boss is a career decision. ... that I have made too many times, and it never turned out well for me.

Find something else to do, and try to steer the company away from jobs/products where tolerance problems can not be solved with a hammer.

... or get a better job while you still have one to leave voluntarily.



Mike Halloran
Pembroke Pines, FL, USA

 

[CheckerHater]

How do you handle people who belittle GD&T and point to their success without it?

Try to learn from them. Analyse. Follow 3Dave's advise and study the process. Maybe your numbers, that are fine by the "formula", are laughable from the shop's point of view. That will not add you any credibility.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[MadMango]

What? You don't just reference a Surface Profile in general notes?

"ALL SURFACES ARE APPLICABLE TO A PROFILE TOLERANCE OF .060" WITH RESPECT TO DATUM A (PRIMARY),DATUM B (SECONDARY), DATUM C (TERTIARY)."

Sarcasm

"Art without engineering is dreaming; Engineering without art is calculating."

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[drawoh]

tunalover,

Several thoughts here...

Are your parts being machined? Machining is an accurate process, which makes it forgiving of bad drafting. You may be able to do the math and show him that parts meeting the specified tolerances cannot be guaranteed to assemble. EEs are supposed to be impressed with math.

On my machined parts, I tend to apply all-around profile tolerances of 0.4 or 0.8mm. I don't know how the shops cost these. I figure that calling up a 0.8mm[&nbsp;]profile on a machined part shows that I don't give a s**t. Something else almost certainly is way more important.

I am writing up notes here on DFMA. I am convinced that drafting is a major cost issue. If the shop cannot make sense of your drawings, they will anticipate time on the phone asking you questions, and they will anticipate doing free re-work. They will charge more. There was a good discussion here a while ago on thread1103-322065s, on how to tell the shop which dimensions matter.

--
JHG

 

[JNieman]

You can emphasize that he isn't wrong, and that your suggestions are not replacements, but improvements. They aren't in competition with existing processes, but simply another, leaner, more efficient way of doing things. It, in no way, seeks to invalidate decades/centuries or performance; that'd be ludicrous.

MikeHalloran, on the other hand, is not wrong when he supports GD&T being a cost INCREASER in many industries or types of fabrication. It is. It simply is. There are simply many shops that work with nothing but calipers, tape measures, and even micrometers who will not give a crap about your true position.
The moderately educated ones will convert the T.P. to a +/- dimension so they can measure it in two directions with old fashioned tools and call it good, raising the price for the additional 'overhead work' of conversions.
The less moderately educated ones will ignore the symbol altogether, raise the price to cover any rejects due to their ignorance.
Then there are the ones who will ignore your print, make the part to "their tolerances" and say "this is all we promise to hold, regardless of what the print says" and you're now behind schedule, over budget, and lack good parts.

That's my experience with the "cheap shops". Both where I've worked, and whom I've used as subs

The shops who understand and grok the GD&T language will be shops that are likely already a bit pricier because they hire more-than-minimum wage laborers, have a reputation of quality, probably work to certain standards, and so forth.


I'm sorry- my post is absolutely one of the "sitting on the fence" positions. There's truth to the fact that the world indeed still spinned and parts were still successfully designed before the standardized use of tolerancing specified in Y14.5. You can indeed work to the 'old fashioned' way of doing it and get the parts you need. It's up to you on whether or not you this manager must change his ways, can change his ways, or if you can do it your way without him agreeing. There's no good answer. Pick your battles and decide if this is a hill you truly want to die on.

 

[TheTick]

I would never put GD&T on a PCB or other EE-heavy thing because most of the industry just doesn't work that way.

 

[KENAT]

Other thing to consider with PCB mounting is that the screws will 'chew' through the board a few thou pretty easily so slight mis-alignments aren't as obvious as they may be with some components.

Where it can be an issue is if it manages to chew thorough the coppper on a hole or something but in most cases I'm not sure this would cause a function problem.

Then there is the issue that simple application of calculations in B4 are 'worst case' not statistical so values may be a conservative.

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[drawoh]

KENAT,

Yet another thing to consider is that PCBs are CNC machined, especially the holes. The specified tolerances may be inadequate, but the process cannot exploit the sloppiness. Until such time as PCBs are sand cast or welded, there is no problem.

--
JHG

 

[btrueblood]

Dangit, drawoh. Now I'm gonna have to dream up a welded PCB just for the heck of it.

 

[KENAT]

Sure I've been told the holes on pcb's will hit +-.003 day in day out on size & position pretty much regardless.

I don't bother applying GD&T to pcbs - in fact the electrical guys do all the definition I don't touch the drawings.

I just make sure they put a hole big enough to accommodate the tolerance build up from their process and on the mating part. I then GD & T the holes on the mating part converting the +- to pos equivalent etc.

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[Belanger]

I love the last part of this thread's title: "their success without it."
Yeah, there have been a lot of people or companies that got by for a while without using updated techniques, but eventually it caught up with them.

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

 

[SwinnyGG]

Other thing to consider with PCB mounting is that the screws will 'chew' through the board a few thou pretty easily so slight mis-alignments aren't as obvious as they may be with some components.

Where it can be an issue is if it manages to chew thorough the coppper on a hole or something but in most cases I'm not sure this would cause a function problem.

This can create a problem, if the PCB is a multi-layer design that uses one or more screws to force ground plane contact with the housing.

I had a problem once (at the prototype level, so we caught it) where a hole was out of position, causing the screw to bind. The screws in the assembly were very small, so a small deviation in installation torque caused by the screw thread grinding through the edge of the hole was enough to prevent the screw from seating, so the PCB had intermittent ground plane contact at this location. Took the EE guys a period of tail-chasing over an intermittent noise problem to figure out it was mechanical and kick it back to us.

Turned out to be a problem with the prototype casting tool- the PCB was produced via CNC and was right on print.

 

[MikeHalloran]

The other problem with screws chewing copper off a PCB is that the copper ends up somewhere, eventually.


Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

Sure it can cause problems, but a lot of times it won't so Tunalovers manager thinks it isn't a problem.

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[Belanger]

But if he doesn't understand GD&T, how much longer will he be viable in the job market?
He's just been lucky so far. The next time he sends out his resume, others who have equal skills but also GD&T will get the job.

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

 

[KENAT]

Belanger, for an EE that's going to be a minor limitation in employability.

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[MikeHalloran]

I have worked for tunalover's manager, or equal, too many times.

Mike Halloran
Pembroke Pines, FL, USA