ThomD
>I'm not going to say they don't grind it a little hollow. But I saw no evidence of it on the tape and don't see any need. When machinists grind this kind of thing down, I assume we are talking surface grinder. If that machine is any good the piece is in the tens without anyone doing anything by hand at all. If they did it so the center was hollow they would simply end up with a lot of material to scrape. Then let's say they have it a lot better but now it's a little proud in the center, because they worked the edge so well. Do they now have to hollow it out again? Sounds like a lot of work. If the erors are in the thous they can find them with rulers, or light, or gages, or indicatores, or by rotation about centers.

For me a takeaway is that here is a cheap to make device that takes away visible flakes or shavings, which is a lot faster material removal than we get with most of out wood workshop tools. We can choose to reduce the amount of grunt work by using a scraper to rough out. We can use it to beautify the surface of metal, a bit like a piece of burl, or we can finish the surface of the plane. And we don't have to do it to some incredible standard since those are for the guy who works upstream of even the guy who machines perfect tools.

Funny story, I was buying a surface plate a few months back, and the guy had bought a lot of plates out of some factory. The biggest one he had was a Starret pink granite inspection grade (above toolroom grade, above shop grade). It was 6'x4'x10", around here that is about 6k new. The only offer he had received on it was from a guy who wanted one as a pad for his woodstove. Isn't de-industrialization grand!