Median line at LMC

[Kedu]

Could a derived median line straightness be legally applied or modified at LMC per Y14.5-2009?
(looks like its "counterpart" median plane flatness it could)
If not, why not?
Is there any support for such of callout?

5.4.1.2 Violation of MMC Boundary.

In this instance, a diameter symbol precedes
the tolerance value, and the tolerance is applied
on either an RFS or MMC basis.


5.4.2.1 Application of Flatness RFS, MMC, or LMC to
Noncylindrical Features.

As an extension of the principles of para. 5.4.1.2, flatness may be applied on an RFS, MMC, or LMC basis to noncylindrical features of size.

 

[axym]

Kedu,

I would say that applying DML straightness at LMC is a valid extension of principle. If LMC can be applied to DMP flatness, then I don't see any reason not to apply it to DML straightness.

Having said that, I would still proceed with caution. Y14.5 allows LMC for DMP flatness, but does not provide details on how it is applied. There may be differences in what envelopes apply, as there are when LMC is applied to position tolerances.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,
May I ask, when would you use derived median line straightness modified at LMC? In other words, what would be a design scenario/case where this tool could be a good idea to be used.
Thank you for your input

 

[axym]

greenimi,

The only application I can think of would be on a cast bar that is going to be finish machined. The straightness at LMC would allow for significant bending/warpage and still guarantee that a cylinder of a certain size could be machined with full cleanup.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca