Setting base alignment off physical surface

[[Ma...]]

Howdy, Just received our new contura CMM and i'm playing around with some cad models and i have a part sitting on top of a grinding vice, the bottom side of the part is datum A and im measuring flatness to it. do i have to modal in a new surface and place points to probe or is there a way i can enlarge a plane from a surface???
Thanks!
-Max

[[Ma...]]

I'm having troubles with understanding of your text.

You can not extend anything for flatness in meaning like you can do for flatness ( changing nominal dimensions will affect results )
If you need to report flatness you should flip a part and scan whole plane as you should.

Otherwise i got your question wrong.

[[Ma...]]

My apologies, Im new to this whole metrology thing so please bear with me.
I understand flipping the part to measure flatness but i don't believe I'm quite there yet, I was curious if one could use a physical object, such as a parallel for milling machines, to reference a feature on a part and use that parallel to create dimensions from.
Does that help at all?

[[Ma...]]

You can scan that plane or use plane from fixture, but you can not make flatness on that if you don't measure it correctly

[[Ky...]]

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Hello Max,

To expand on what Martin was talking about, the definition of "flatness" in this case is, effectively "all measured points on the feature must lie between two parallel planes that are a certain distance apart", where the certain distance apart is the flatness tolerance.

If you "expand" the plane like you mentioned, you'd be measuring whatever the part is sitting on (the granite or whatever fixture you have) instead of the part, so you'd be doing the flatness of that plane, which is not related in anyway to the plane you want. As Martin mentioned, it works for position or using it as a datum, since those are defined in a different way, but not a great idea for form.