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

 

[Belanger]

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

 

[greenimi]

J-P,

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

 

[pmarc]

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?

 

[Belanger]

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

 

[Belanger]

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

 

[greenimi]

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

 

[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.

 

[drawoh]

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

 

[greenimi]

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?

 

[pmarc]

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?

 

[greenimi]

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

 

[pmarc]

And what are size limits of the gage pin?

 

[Belanger]

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

 

[greenimi]

Pmarc,

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

 

[pmarc]

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?"

 

[greenimi]

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?

 

[pmarc]

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?

 

[greenimi]

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.

 

[apekas]

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"