Wiley Horne -- So. Calif.
>Thom said:
"As far as the technique for spotting the bow is concerned, if you dropped a plane on a surface plate and then grabbed one end and wiggled it, the bowed plane would rotate around the center point. If it were perfectly flat it would rotate around the 30% from the end point, and so on. With the right ink there should be an ink print as well. "
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Thanks. I think I follow you. Please check me. Let's say the machinist has done the grinding, and there is an interior high spot that is ~2 thousandths. He's put the work down on the thinly inked plate, and 'wiggled it' to locate that 2 thou bearing point. Intuitively, you know where the high spot has to be on the work. So you pick up the work, and much of the bottom will have picked up ink, but you know where the high spot has to be, so you look for that dab of ink (with an uninked immediate surround) where you know the high spot has to be.

So, if I follow, if the surface you're working is a tiny bit convex, the ink is an aid, but you're relying on finding the bearing point mechanically. You're done when you get an even coverage of ink, and the work (by feel) is dead stable on the plate.

Now I'm wondering if a machinist, during the 'grinding down' phase, doesn't make sure he grinds it just a le-e-etle bit concave, and then scrapes the periphery, which is where there will be ink pickup. I.e., if you want to get flat, it's easier to start out a little hollow.

Wiley ..... speculating