Re: ATM first figuring attempt-hole in center, question

["Bill T." <twentiethwave@hotmail.com>]

Herb,

IMHO, you're too early in the game to be fiddling around with a 25% diameter
lap.  It'll probably just make that hole uglier, and you'll accumulate
roughness.  Save it for sparing use on a persistent, minor zone on a nearly
finished mirror.

Under the guise of going back to a sphere I'd try some pretty long center
over center stokes with the full sized lap on top.  For me, this has often
blended central holes back into the overall figure while keeping some degree
of overall correction.  On the way back to a sphere this way you often pass
pretty close a good and smooth parabola.  The trick is to check the mirror
frequently.  It's always worth a try.  If the magic doesn't start to happen
within say 20 minutes of long strokes, or if the edge is turning, just go to
normal W polishing strokes all the way to a sphere.

FWIW here's an 8" lap that does wonders on 20" mirrors.  The base is a piece
of 3/4 particle board pitch-glued to a 25 lb weight.  The closest I've been
able to come to "recipe figuring" moves this lap in an 11 to 13 vector W
stroke, biased towards the center of the mirror, with about 2 inches of
overhang.  Sometimes very brief spells of full size lap on top are required
to keep the edge under control: long strokes flatten out the edge, short
stokes steepen it -- but only briefly right after pressing!  For me, laps
with a diameter of about 3/4 mirror radius are the most useful for general
figuring.

http://www.kupercontrols.com/misc/figurelap1.jpg

Bill Tondreau