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Gage at LMC

Gage at LMC

Gage at LMC

(OP)
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

RE: Gage at LMC

You can't do that. To see why, you'd have to write out a table showing two columns: one for the hole size and one for the corresponding hole position. Per the original spec, a hole of .495 would be allowed .015 total position tolerance. A hole of .496 would get .016, and so on until you get to a hole size of .505, which would be allowed a position tolerance of .025.

If we go with the suggestion from your designer, then the position tolerance STARTS at .005. But that in no way correlates to what the original requirement was.

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

RE: Gage at LMC

J-P,

Will the above approach never accept a bad part, BUT will reject some of functional good parts?
What do you think?

RE: Gage at LMC

I am not sure where the "if the size tolerance is taken from the position tolerance" idea comes from, but it is true that you can't design a hard gage that would verify feature's LMC virtual condition boundary. It is simply because that boundary always lies inside of material of inspected feature.

J-P,
I believe you meant: "a hole of .495 would be allowed .025 total position tolerance. A hole of .496 would get .024, and so on until you get to a hole size of .505, which would be allowed a position tolerance of .015". Am I right?

RE: Gage at LMC

Well, first I must modify the numbers I wrote before -- that's the danger of firing off a reply too quickly :)

Per the original spec, a hole of .505 would be allowed .015 total position tolerance. A hole of .504 would get .016, and so on until you get to a hole size of .495, which would be allowed a position tolerance of .025.

But I still think that the suggested modification by the designer isn't going to work, because that would be gaging a .495 size hole to be positioned within .005 -- much tighter than the .025 allowed by the print's spec for a .495 hole. So yes, the suggested gage could reject a good part.

I think this stems from a misguided notion that the LMC modifier creates a "negative" bonus tolerance. Not true.

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

RE: Gage at LMC

Yes, pmarc .... I noticed it after I posted it, and made the correction :)

I must say that I've heard this logic before, and it's from a faulty understanding of the concepts. People think of LMC as the opposite of MMC and thus they try to make the concept of bonus tolerance also work in the opposite way. But bonus tolerance is always additive.

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

RE: Gage at LMC

Resultant condition at MMC = Virtual boundary at LMC
Isn't that enough?

RE: Gage at LMC

Resultant condition at MMC for a hole is a boundary that lies inside of material of this hole. You can't build a hard gage to properly verify non-violation of that boundary.

RE: Gage at LMC

gabimot,

I recall being told in school that a GO gauge can check all the available features, but a NOGO gauge checks one feature only. The NOGO checks your LMC.

--
JHG

RE: Gage at LMC

Pmarc,

"Resultant condition at MMC for a hole is a boundary that lies inside of material of this hole. You can't build a hard gage to properly verify non-violation of that boundary. "

I agree, BUT if you are checking the MMC VC and checking the size tolerance (which you have to check it anyway) THEN you can qualify the part, don't you think?
The MMC RC cannot be bigger than .520 which is equal with LMC VC which is the design intent, right? So, why this is NOT a valid inspection method? Just asking.

Am I missing something?

RE: Gage at LMC

To be sure that we are on the same page...

The hole in OP's question is toleranced in following way:
500±.005 (hole)
Pos Ø.015 LMC to A, B, C

How is the gage pin toleranced? And does it have to go or no-go?

RE: Gage at LMC

Has to go.
and is checking Pos Ø.005 MMC to A, B, C

RE: Gage at LMC

And what are size limits of the gage pin?

RE: Gage at LMC

Greenimi.... we all agree that a go gage (functional gage) can be made to check a position tolerance specified as .005 MMC. But in what possible way is a position tolerance of .005 MMC equivalent to the original print requirement of .015 LMC?

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

RE: Gage at LMC

Pmarc,

Go pin size: .490
(.500-.005-.005)

RE: Gage at LMC

greenimi,
I have to repeat after J-P: "What possible way is a position tolerance of .005 MMC equivalent to the original print requirement of .015 LMC?"

RE: Gage at LMC

I did not say it is equivalent!
I did say---see my replay from 2 days ago....

"Will the above approach never accept a bad part, BUT will reject some of functional good parts"

Again, this approach NEVER accept bad parts (bad parts per the original requirement position at LMC)

Do you agree?

RE: Gage at LMC

So you want to use a gage pin of dia. .490 to verify if dia. 500±.005 hole located by Pos Ø.015 LMC to A, B, C does not violate its resultant condition boundary?

RE: Gage at LMC

Re:"So you want to use a gage pin of dia. .490 to verify if dia. 500±.005 hole located by Pos Ø.015 LMC to A, B, C does not violate its resultant condition boundary? "

pmarc,
I want to use a gage pin of dia. .490 to verify if dia. 500±.005 hole located by Pos Ø.015 LMC to A, B, C does not violate its VIRTUAL condition boundary.
How? By checking VC at MMC (.490) and if the size is in tolerance (Ø.500±.005) then the RC MMC is .520.

RE: Gage at LMC

I see what greenimi is saying.

Here is a table showing the LMC and MMC values.
LMC_vs_MMC.png

What I'm not sure is "will reject some functional good parts"

RE: Gage at LMC

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/getfile.aspx?folder=8...

RE: Gage at LMC

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.

RE: Gage at LMC

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.

RE: Gage at LMC

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

RE: Gage at LMC

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.

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