Jacob":15r7v7zf said:
I'd guess not enough "stiction" with water on a wooden bench. I've always used a machine bed but I imagine glass, granite, even MFC, would do just as well as long as they are impermeable and very smooth.
I wasn't clear about that. The glass lap was on a wooden bench. I used water on the glass lap because I didn't want swarf to run over the sides and strip the wax and oil off of the wooden bench.
I had a table saw at the time, but it was very poor flatness front to back (about a hundredth hollow) and the glass would easily flex in that. On the bench, I couldn't get a .015 feeler under a starrett edge regardless of the pressure I placed on the glass. that kind of accuracy isn't really necessary, but it doesn't hurt to have.
For what it's worth, if grit particles or chip got under the glass on one end (from lifting the lap up for any reason and replacing it on the bench without sweeping any foreign particles off), I could put a feeler under the straight edge. I only checked the first couple of times I used the lap, but I was surprised by just how much the glass flexes.
I don't have a power jointer, so the TS at the time was my only truly flat other surface that I could've put a significant amount of pressure on.