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

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SUMMARY

Functional gages cannot be used to inspect features specified at Least Material Condition (LMC) according to "GD&T for Mechanical Design" by Cogorno. As features deviate from LMC toward Maximum Material Condition (MMC), a bonus tolerance is gained, which is the difference between the actual feature size and the LMC. Functional gages are designed to reject parts that are borderline MMC and are not intended for measurement. If there is any interference between the functional gage and the part, the part is rejected, highlighting the limitations of functional gages in LMC inspections.

PREREQUISITES
  • Understanding of Geometric Dimensioning and Tolerancing (GD&T)
  • Familiarity with Least Material Condition (LMC) and Maximum Material Condition (MMC)
  • Knowledge of functional gages and their applications
  • Basic concepts of tolerance and bonus tolerance in manufacturing
NEXT STEPS
  • Study the chapter on gages in "GD&T for Mechanical Design" by Cogorno
  • Learn about the application of bonus tolerance in GD&T
  • Research the differences between functional gages and measurement gages
  • Explore case studies on the use of functional gages in manufacturing processes
USEFUL FOR

Mechanical engineers, quality control inspectors, and manufacturing professionals involved in GD&T applications and inspection processes will benefit from this discussion.

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 ;)
 

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