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

[Jerry Dove <jldove1@cableone.net>]

On Sat, 9 Jul 2005 21:17:15 -0700, you wrote:

	I just love this list!! I started my first mirror about five years ago and have been lurking for longer than
that. I've had to move twice and had five back surgeries in the last six months. But listening to you guys to help this
gentelman work through his growing pains and learning curve has inspired me to give it a go again and try to get mine
finished. I start tomorrow by pouring a new lap to finish polishing it with, then be bugging you guys for all the help I
can get. Mainly with the testing process. My is a 12.25" x 3/4" plateglass tabletop.


	I just met a young man a couple weeks ago that really wants to make his own scope too, so mybe we can work
through this together.

	Thanks for still being here after this long time with no progress.

		Jerry

		Kuna, ID.

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

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