Functional vs. Process Driven GD&T

[pmarc]

Hello,
This time a little bit less specific question:
I am looking for some good real life examples showing differences between function and process driven GD&T, and somehow proving that function-based dimensioning and tolerancing is indeed beneficial in comparison to the latter approach.

Did anyone ever experience a situation when for instance manufacturing dept. insisted on adjusting GD&T scheme to their needs (tolerance values, datum structures, etc.), but at the end of the day, after doing some stacks, it occured that tolerances were unnecessarily tighten from functional point of view?

Any input will be really valuable.
Thanks a lot.

 

[KENAT]

You trying to persuade me to get a copy CH.;-)

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[CheckerHater]

Looks like HUGE selling point. I should charge commission.

 

[SeasonLee]

pmarc

I have read Alex Krulikowski's "Advanced Concepts of GD&T" Chapter 13 The Datum System, and some of the shop floor mechanic fully agreed what Alex said while I talked with them regarding the datum setup, but I can’t open your attached file, here is my post.

SeasonLee

 

[fsincox]

CH,
Thanks, Does he explain how 2768 is supposed to work in any more detail than Henzold? Henzold just basically introduces the concept like the ISO standard does, IMHO.
Frank

 

[CheckerHater]

Frank,
No, he is trying to avoid duplicating standard.
He points out “grey” legality of “no rejection” clause, as in “who is to decide if part actually works?”

 

[KENAT]

I'm saying nothing, honest.;-)

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[drawoh]

drawoh,

Your last paragraph is a very common scenario. Your last sentence is spot on.

Thanks.

Note, however, that we subcontract all of our fabrication. We are the customer and we are always right. My impression is that the OP is not the customer. Depending on how his office politics work, he could be frequently, and possibly always wrong.

JHG

 

[fsincox]

You don't need to, point to Ken
Frank

 

[CheckerHater]

Another true story from the trenches.
The company starts outsourcing. The company doesn’t use GD&T. They actually buy/build gauges and ship them following the prints. Company gets back good parts.
Now, what are the alternatives?
In order to get back good parts without sending out some heavy iron (or living, breathing engineer) is for your GD&T to have enough data for gauges to be built locally.
And that will make your GD&T (just a little bit) process-oriented.

 

[powerhound]

drawoh,

All I was saying is that it happens more often than one would like to think. Ultimately the part has to conform to the print per the DRF. Nothing says a part has to be built that way. Whether you establish a datum then drill a hole WRT it or drill the hole and establish the datum later makes no difference. What does matter is how it's checked. If it checks out then how it was made is irrelevant. Simplistic, I know, but that's really my point.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fsincox]

powerhound,
I think most of us here, believe that.

You guys may have just talked me into getting Alex's book.
Frank

 

[CheckerHater]

Frank,

You are a bit late.

Alex's "Advanced" is long out of print. :-(

 

[powerhound]

You can find it on Amazon.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II