Parallelism Simulated Surface

[Mark F]

So I have a supplier that is using the surface plate as a simulated a surface (not a datum). Then they measure the top surface which is the actual datum A. So the callout is parallelism of the bottom of the part ( which rest on the granite) to datum A (top of part). This is a large part which isn’t easy to move around and the datum is on the top inner surface. Therefore we can’t rest datum A on the granite.

Is it legal to use a simulated surface face for parallelism? Instead of checking the parallelism of the bottom to datum A is it okay to check parallelism of datum A to the simulated surface (ie. granite)?

 

[Belanger]

No, because calling a surface a datum feature never controls the form (flatness). But calling out parallelism on a surface does control form. (Unless there is a "T" modifier.)

So by checking parallelism on the datum surface when that's not what the print calls for will perhaps reject a part that is bumpy/warped when the print's requirements might have accepted that part.

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

 

[pylfrm]

Mark F,

A parallelism tolerance requires the actual surface of the toleranced feature to fall between two planes of the given separation which are parallel to the datum plane. The method you describe would only work if the two surfaces are perfectly flat, and their outer perimeters are identical and exactly opposed. The measurement error on an actual part could go in either direction.

Can datum feature A rest on some sort of riser that is resting on the surface plate?


pylfrm