Functional gages cannot be used to inspect features specified at LMC.

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dav2008
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"Functional gages cannot be used to inspect features specified at LMC."

What does that statement mean?

I am going through "GD&T for Mechanical Design" by Cogorno (McGraw-Hill) and it makes that statement on page 24.

As the size of the feature departs from LMC toward MMC, a bonus tolerance
is gained in the exact amount of such departure. Bonus tolerance
is the difference between the actual feature size and the LMC of the feature.
The bonus tolerance is added to the geometric tolerance specified in
the feature control frame. LMC is used to maintain a minimum distance
between features. The LMC is seldom used. Functional gages cannot be
used to inspect features specified at LMC.
 

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Functional gages aren't meant to take measurements. They're only intended to reject parts that are borderline MMC. If there is any interference between the functional gage and the part, the part is rejected.

An example would be using a gage pin to see if a shaft will fit through a hole. The size of the gage pin corresponds to the maximum diameter allowed for the shaft. If the hole is anywhere below MMC, and therefore larger than its smallest allowable size, the gage pin would not interfere with the hole. If the hole is larger than your allowable tolerance, the gage would not reject it.

I would be surprised if there isn't a chapter on gages in that book somewhere that could probably explain it a little better.
 


Skrambles said:
Functional gages aren't meant to take measurements. They're only intended to reject parts that are borderline MMC. If there is any interference between the functional gage and the part, the part is rejected.

An example would be using a gage pin to see if a shaft will fit through a hole. The size of the gage pin corresponds to the maximum diameter allowed for the shaft. If the hole is anywhere below MMC, and therefore larger than its smallest allowable size, the gage pin would not interfere with the hole. If the hole is larger than your allowable tolerance, the gage would not reject it.

I would be surprised if there isn't a chapter on gages in that book somewhere that could probably explain it a little better.

Ah ok thanks. I glossed over the word "functional", and the book does explain a bit more about them later on. I guess I should have figured out what a functional gage is before asking about it ;)