Per unit measurement

[[Da...]]

Been having a discussion on ‘per unit’ measurements. Flatness and straightness seem fairly well defined in ASME but what about parallelism? Has anyone come across it, have any experiences of it?

It’s not sitting right with me as the application feels unbounded and almost impossible to control. The specific application is plane to plane parallelism.

[[Da...]]

I think I just had a class that covered that. As I recall, the first one tells you that you can be parallel within .02 over the area of the feature, but the second spec tells you that you can only be .01 in parallelism in any area of 25 long.

[[Da...]]

Yeah that’s right if it were applied like flatness and straightness

[[Jo...]]

That's kinda crazy.. So what is the flatness requirement for both ?

[[Ja...]]

Here's a good discussion about it: https://www.eng-tips.com/viewthread.cfm?qid=327653

As I understand it (and I'm probably wrong) you could try to argue extension of principal from flatness to parallelism. But that kind of falls apart because nothing in ASME supports creating a tangent plane of a surface on a per unit basis.

And overall I don't see what the localized orientation check would be good for. If the idea is something lands on that surface or rides along it and you want to reduce the variation from the rest of the surface then why not use flatness?

With the parallelism callout, if there was a form error similar to an 'o' ring groove on the surface, and if the bottom of the groove was parallel to the datum, would it pass?