What is the best way to verify a small surface plate 9" x 12" and smaller? I'm looking for a method to check repeatability and flatness. We currently use a Optodyne Laser system for larger plates with a Rahn repeatometer. I have yet to hear any good method for smaller plates.
It depends on the accuracy your looking for. If it isn't that accurate, you can get by with a Rahn repeat-o-meter. If the accuracy is more important, I would calibrate the flatness of it like a granite parallel bar using a stand to place the plate on so that you can get a good zero, and the use a hite-indicator to move across the plate to see the deviation. I think it is too small for a planekator.
For the best accuracy, a coordinate measuring machine is the best, but CMMs are prohibitively expensive and have to stay put under tightly controlled temperature conditions.
For the next best, we use a Mahr-Federal automatic profiling system, which consists of two differential electronic levels, an amplifier and software that does the profiling using the Moody Method. It does 9" x 12" surface plates and larger, up to as big as you can reach with the cables on the unit. The uncertainty is 6 µin. per foot (or ±0.1 arc second). Best part is it's portable, you can take it on-site. Go to the Mahr website www.mahr.com and search for electronic levels, the part number is EMD-832P-50-W1 (note: it won't show if you search for the part number, download the .pdf catalog for electronic levels instead.)
Thanks for the input. What about for smaller surface plates 9" x 12" and smaller?
I would imagine just so long as both electronic leveling heads can sit fully on the surface plate and have room to be moved around to different points from all corners to the middle, it'll work. The bottom of each head is about 2" x 4".