DML straightness material condition modifiers and relation to position/orientation at MMC 1

[Burunduk]

When the straightness tolerance at MMC is used in conjunction with an orientation or position tolerance at MMC, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value and does not contribute to the IB or OB of the position or orientation tolerance. The collective effect of the MMC size and form tolerance produces a VC, OB, or IB resulting from the form tolerance but does not affect the IB or OB created by any orientation or position tolerances on the feature.

Why is the above rule of not specifying larger tolerance for straightness than for position/orientation applies only to DML straightness @MMC & Position/orientation @MMC, and does not include DML straightness @RFS & Position/orientation @MMC?

I think that if DML straightness @RFS & Position/orientation @MMC is applied the size of the unrelated AME may be greater than MMC size, however we wouldn't want it to become greater than the VC for Position/orientation @MMC, since we want the surface of the cylinder to be limited by the VC gage, without any unrealizable form allowance - same logic as for not wanting the VC for DML straightness @MMC to become larger than the VC for Position/orientation @MMC that will limit the surface anyway. Is this not the guiding logic?

 

[pmarc]

but there is an exception to that: DML straightness or DMP flatness specified MMC (or LMC).
In these cases, each cross-section is evaluated for departure from MMC (or LMC) in the true sense of "material" (amount) condition, and gets a local bonus for form deviation according to the local size.

Not that it matters much, but are you sure about the DML straightness @MMC?

 

[Burunduk]

pmarc,
The caveat with the derived medial line @MMC is that the sizes used for the local tolerance and center point determination are circumscribed (for external cylinders) or inscribed (for holes) circles. So if the surface of the feature is deformed enough, the size of the local perfect circle that only touches the high points may not always indicate the "amount of material" systematically. A circular element might measure as MMC but as long as the roundness is not perfect (and it never is) it's true that it doesn't really mean "maximum possible" material - but it is the only case for any probability for that. As you hinted, this matters little. The terminology of "maximum material" should not be interpreted as a scientifically accurate description anyway, but it does represent something.