GD & T Rule 1 3

[vellingiri]

Hi All,

I understand the GD & T rule 1 & it says

Perfect form at MMC, if it is so
then Why not perfect form at LMC also

for ex. for the shaft dia. of 20 +/- 0.1, the LMC is 19.9 mm
If all the element are at LMC of 19.9 mm, then it is also a perfect form ? Is it not ?

Please any one clarify abt. my understanding.

 

[Belanger]

There is no need for perfect form at LMC because on a shaft the concern is usually that we do not exceed the largest "envelope." Recall that form means shape, and for now let's just consider straightness or bending of the shaft...

Using your example, let's say that the shaft needs to fit into a hole that has a worst case of 20.1 mm. If the shaft has a cross-sectional diameter of 20.1, then it should be obvious that it must be perfectly straight in order to assemble.

But think about it -- if we manufacture a shaft with a cross-sectional diameter of 19.9, it now has a some looseness when it fits into that 20.1 mm hole. So that LMC shaft should not be required to be perfectly straight. Heck, we can bend it 0.2 mm before it would cause an assembly problem (because 19.9 + 0.2 = 20.1).

Thus, there's no need to require perfect form at LMC; in fact it would be detrimental to the function. Having said that however, it should be noted that the 2009 standard does now say that perfect form at LMC is required if the feature has a geometric tolerance with the "L" modifier on it. (Because the concern for that part is the smallest envelope, not the largest.)

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

 

[vellingiri]

Hi John-Paul Belanger,

Thanks.



Vellingiri
Coimbatore,India