Jim Wilkins wrote:

Lapping implies a rigid guide holding the abrasive.

OK, now I get it. This was the critical element I was missing. So to lap,
say, a lathe way, you would spread the paste on a flat piece of brass or
aluminium and rub it on the ways?

I won't even ask how they lap the automotive valves...

--
Michael Koblic,
Campbell River, BC