Gage at LMC 1

[gabimo]

As per Y14.5 and Y14.43 theory a physical hard gage cannot be designed at LMC.

Our designer said, if the size tolerance is taken from the position tolerance then a gage CAN be made at LMC and be a valid inspection method. Is that really true?

Example: 500±.005 (hole)
Pos Ø.015 LMC to A, B, C

Gage to be pos Ø.005 MMC to A, B, C (size tol. ±.005= .010, .015 - .010 = .005)

What are your thoughts?

Thank you
Gabi

 

[pmarc]

greenimi,
OK, now I see what you mean, and my answer to your question is YES - this approach will never accept bad parts, BUT I in the same time would say that usefulness of this kind of gage is very low, because the amount of functionally good parts that could not be mounted in that GO gage is not acceptable from gage design standpoint (see attached table).

http://files.engineering.com/getfil...421d-4b64-9b8c-c1976067230e&file=RC_table.PNG

 

[greenimi]

Pmarc,
Re:” and my answer to your question is YES - this approach will never accept bad parts”
Amen.

I agree 100% with you. The usefulness is somewhat low. No doubt about.

But probably, this is the approach OP designer is talking about without explaining in details what’s what…..how…….”side effects”, all that good stuff, etc.

A case where this “sort of gage” is useful could be when you do not have a CMM readily available (or a vision system) or you do not know to use cartoon gages to inspect a part at LMC, but the parts must be somehow inspected at the machine (if possible). I know we can debate even my above statements for accuracy and usefulness …..

So, it is a risk and dimensional management………scrap functional good parts and you have something to check your parts with. Or use other inspection methods (the ones that can verify inside the material for the minimum material envelope) and qualify your parts per the drawing requirements.

 

[Michael854]

13 yes ago, the dimensional engineers of a placement firm convinced the company I was working at to go to LMC gaging. It was the interior systems integration program for a GM van.
LMC gaging was a colossal waste of time...at a time when the tier 1 supplier was trying to survive.
Eventually, all the DEs from that company were fired...I had to fix all the drawings.

 

[greenimi]

Just curious:
- Why you had to fix the drawings?

 

[Michael854]

I had to fix the drawings because tier 1 suppliers like to get reimbursed for their tooling outlays. If parts receive production approval, then the supplier gets its $ investment reimbursed. If the parts never receive PPAP approval on enough programs, then the supplier eventually file bankruptcy.
One component of PPAP approval was a 6 piece Cmm layout of parts produced during the "Run at Rate." In order to have any hope of part conformance, the drawings had to be changed.
The datum features were external fos's. The gage features were LMC holes and slots. Changing the tolerancing to mmc eliminated a great deal of float at the locators.