Re: [ATM] 12.5 inches BVC mirror polishing and figuring difficulties

["Bob May" <bobmay@nethere.com>]

I'm assuming a full sized pitch lap with this.
Just put the mirror on top of the lap and do some strokes from about 2"
offset to the center with the pressure on the edge over the lap.  Press
often as the lap will tend ot deform with this stroke.  You want to have one
hand in front of the other rather than side by side as this works on the
area just inside the edge.
The other stroke will be to do a steady back and forth with a total stroke
length of about 3" and a W of about 2" just to keep things level.  Again,
frequent pressing should be done.  The strokes should take about 1 second
out and another second back for best results.  Going faster will tend to
warm the glass and pitch and make things walk about.
The thing that you have to remembe iwth pads is that they will tend to make
hills and valleys for two reasons.  First is that they cut fast and that
means heat.  Second is that they aren't going to go perfectly flat like
pitch will to the surface of the glass.  This means that you have to
randomize the strokes a lot more than you do with pitch to get a decent
surface.  When going to pitch from a pad surface that is zoney, you have to
insure that you don't do repetitive strokes as this won't cure the problems
put in by the pads.  Change the length of the strokes and often press to
insure that the surface of the lap is congruent with the mirror.
I'll also note that when you have a great drag when you move the mirror on
the lap for best results.  If the lap is grabbing and slipping, you need to
press to get it working right.  A perfect lap feel is like there is grease
between the two surfaces that allows for a hard slow pull without easy
sliding.  I've had guys that would have to throw their weight behind a well
working lap interface to get the mirror to move across the lap and that at
only a slow pace of about two seconds from one end of the stroke to the
other.  Sliding with your  fingertips isn't doing any work.  You should be
able to feel when the lap is hitting a hill also as the grip goes up a bit.
I'll also note that this is usually the slowest part of a mirror.  Get
through this and do a 1/10th wavefront surface and you'll be very happy.
Kind of like childbirth for a woman.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

_______________________________________________
ATM mailing list http://www.atmlist.net/