I agree and I think it was sometime during the 1960's that the specs were dictated for, when or how on granite surface plate manufacturing. While machining is a weak link in metal working, my exposure has been full. My father-in-law would say; come on I need to go over to ---insert name-- shop and see if my project is finished. I got the tour of many machine shops, from high tech automotive to high tech aerospace and back to NHRA shops. At the time the cast iron surface plates seemed outdated compared to some of the granite plates. I only remember about three shops with cast iron plates but they were cordoned off as if they were sacred, and they were all much larger than any of the granite plates. The granite plates were black for the most part but a few shops had pink and black so my **why** kicked in. At the time I got the answer to the space under the bottom of the granite plates but not the technical detail.
The point in my comment is that many folks don't understand that more than three support points have the potential to stress the plate, plus the same three points are used when the plates are manufactured, along with any of the three points being high or low (out of level) don't increase of decrease stress to the plate. I was always being schooled, probably because I had the two year old **whys** always engaged because it was very interesting. My father-in-law was born in 1912 and his family moved from Pennsylvania to Los Angeles in the early 1920's. His father and grandfather were machinists, plus the changes he saw over his life time and career fill volumes. The generation who saw the first flight and then on to witnessing the first man on the moon is mind boggling.
Now I need to go look at how the cast iron surface plates are supposed to be supported. I know many metal objects were affected by WWII and the war effort, a few positive and negatives of both types of surface plates, but I never gave much thought to how they are supposed to be supported.
