Ecnerwal wrote:

If, by some miracle, it is "flat", then using scotchbrite or sandpaper
will render it not flat - on the microscopic precision scale one cares
about with lapping - areas where the stuff is pressed harder will wear
down more, etc.. If it's not flat to start with, scotchbrite or
sandpaper won't do any thing to improve it, as they conform to the
surface (removing material form both low and high spots) and also have
the above behavior of possibly putting new gouges (on a fine scale)
into the surface.

A lap only grinds at the high points of contact between the rigid
(soft) lap and the rigid (harder) item being lapped, where the (even
harder) abrasive trapped in the soft lap scrapes the high points of
the item being lapped. Thus, it makes the surface flatter, or
rounder, or more whatever shape the lap is.

This brings up all sorts of follow-up questions, like what should be the
nature of the flat surface that does the lapping. Does it need to be lapped
itself? Or would, say, fly-cutting a piece of soft metal such as brass or
aluminium be sufficient. If the lapper has to be softer than the lappee then
a plate of glass would presumably not be suitable.

Revisiting the shaft affair, what should one have done? The profile is
round, not flat. Would you apply the lapping compound to the bushings and
spin the shaft in them? Would the bushings still be useable afterwards?
Should one have a set of "lapper bushings"?

Anyway, this has been very helpful to me. Thanks.

--
Michael Koblic,
Campbell River, BC

P.S. I have jsut become a proud owner of the Machinery's Handbook. It has a
fairly extensive section on lapping yet even they assume the basic
understanding of the concept. So all I need now is some fine, soft,
close-grained cast iron, flour of emery and some sperm oil...:-)