Setting up a Surface plate question

I acquired a used CMM surface plate which I will use in my shop as a general surface plate. The dimensions of the plate are 56” x 52” x 6” thick black granite. The plate was originally supported on a stand at 4 points with felt pieces. The only adjustment available are at the stand feet. Upon inspecting under the plate I noticed ther are 5 threaded inserts all 1/2-13. Three of these inserts are located at the approximate Bessel points that you might expect. The other 2 threaded inserts are a mirror of the 2 point side. I thought this was odd.

My question: why would the original owner have supported at 4 points instead of 3 and why only use a piece of 1/2” felt instead of an adjustable foot or something similar? I don’t think this plate is big enough to need more than 3 support points. I’m a little confused since the plate manufacture took the time to install 5 thread inserts. maybe this was for transportation purposes or some other reason? My plan was to support on the 3 Bessel points and have the plate checked/ resurfaced if needed. Any reason not to proceed with this approach?

Generally you level the plate on three points (Airy or Bessel, not too much difference) and adjust the other two supports to just touch the plate and avoid any tipping, which is rather likely in the case you place something heavy on the table near one of the corners adjacent to the single support.

Paolo

In general, the granite plates used for CMM work surfaces are not necessarily made to the same flatness standards as a dedicated stand-alone metrology surface plate. The CMM provides its own axis references for measurement purposes. While one might use the immediate surrounding area of the granite for purposes of establishing a plane alignment for a small part (if tolerances permit, and you're in a hurry), the granite is not as flat as a "real" surface plate, because it doesn't need to be. The planar alignment for the part is typically created from the part characteristics themselves.

That said, nothing wrong with your plan, just don't expect A-grade flatness from the plate.