Surface profile actual value double worst vs absolute deviation

[sendithard]

ASME Y14.5 2018

The below speaks for itself, but why when you use ABC you double your worst, then when only AB you record only the largest deviation?

Thanks

 

[Burunduk]

Hi sendithard,
It's not about doubling the worst Vs. recording a surface deviation from basic.
The profile measurement always doubles the worst deviation.

It's about the different ways the surfaces are controlled by the two profile tolerances.
I think I assume correctly that the measured surfaces are the ones colored in purple.
In the AB Datum Reference Frame, the tolerance zone is floating in the longitudinal direction, because the degree of freedom in that direction is unconstrained. Therefore the measurement doesn't consider where the smaller surface is relative to datum C (datum C is not even recognized by this measurement).
In the measurement relative to ABC all 6 degrees of freedom are constrained, therefore the tolerance zone is fixed relative to all 3 datums, and any variation of the smaller surface in the longitudinal direction is recorded. Therefore the larger measured value.

 

[sendithard]

Ahhh...okay, I get it. For some reason, I get hung up on the callout creating two parallel planes as a tol zone where the nominal basic location would be in C. This is not correct as it is detached from C's gravity so you get a best fit so to speak on how far out the perpendicularity is vs B. The perp tol zone can travel to and from C as it is not constrained by C. Thanks for the input.

 

[Burunduk]

sendithard,
No problem.
How is it going with your Gom inspect investigations, such as the one on rotational "best-fit" about a datum axis? Any new videos you plan to put out?
I also have a question on the ability to recognize ASME Y14.5 Simultaneous Requirements in that context.
When you can, see your thread1103-497270, thanks.