In article ,
"Michael Koblic" wrote:

Now I am still confused: Why go to all this kerfuffle to smooth out a bit of
metal, particularly if it is flat. I see the grits available are anywhere
between 50 and 1200 so one can take pretty big chunks of material. Why can't
you achieve the same result (on a flat and accessible surface at least) with
steel wool/Scotchbrite pad? Or just plain old sand-paper for that matter (if
you are using the coarser grits). How is the lapping compound behave
differently from these other gizmos?

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.

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