CMM granite surface plate grade ?

[JohnR]

Anybody have an idea of what the minimal grade of the granite surface on a cmm. Looking at buying a
junk unit for the granite with a table. The column was modified by a towmotor. A lot cheaper than I could buy a used POS and I would suspect
with less abuse.
Thank you.

[John Hasler]

You'll also be getting a bunch of super-high resolution encoders, bearings and other very expensive precision components. Can I interest you in selling what's left after you remove the table?

[Inspector]

We had our CMM's calibrated every year but I never paid attention to if he verified the flatness too. I would guess they are an inspection grade as they are a measuring tool and the table is often used as a data plane. It will make a great surface table.

Pete

[EricM]

I don't believe granite CMM tables have the same flatness / repeatability requirements as a true surface plate. In my experience, the CMM tables are NOT used as a datum reference, since the CMM software creates the datums on the part being measured. The granite is used typically for stability. I have seen some CMM's that did not use granite at all for the tables, but a epoxy resin composite material. I have never seen a CMM granite table calibrated as though it were a surface plate.