RE: ATM 1st mirror - machine use

[Randy Johnson <randyj@nwlink.com>]

From: "Tom Krajci" <krajcit@det5wg57.barksdale.af.mil>
Date: Wed, 25 Sep 1996 08:53:05 GMT+7
Subject: ATM 1st mirror - machine use

Hi Tom,

Here are my notes on the subject of machine work: 

>1.  Reduce the weight, and to what value?

I used about 8 1bs in my polishing work on a 6" project and it seemed to
proceed fairly rapidly, 4-5 hours on pitch as I recall.

>2.  Are stroke size/shapes different for machine work versus hand 
>work?  (I'm using TOT, 1/4 strokes in a "W" that provides about 3/4" 
>overhang at each side of the "W.")

No! See Bob Pfaff's remarks below.  I had exactly the problems he said I
would before implementing the strokes that he prescribed.

>3.  Go to hand work at a certain fine grit?

Maybe.  I think the machine could and probably would work fine though I
found in my last mirror by using a short (1/4 overhang, center over center,
fine grinding stroke, I seemed to set up a more spherically uniform shape
and thus a quicker polish at the edge than I had experienced on my previous
mirrors.)  This was done by hand.  

>4.  Could I even polish by machine?  Maybe for the first few hours?

Sure!  That's where it shines.  If you keep the lap irrigated you can let
the machine run for quite a while by itself.

I'm at the other end of the spectrum from you in that I have used my machine
only for polishing so far and used the Armstrong method for all of my mirror
grinding to this point.  

The rest of this is excerpted from a response that Bob Pfaff had to a
similar inquiry that I had to yours when I started using machines. The whole
article is in the ATM archive dated January 21, 1996.

Bob's Machine Use Primer...

If your machine is turning counter cloclwise (this the normal stroke)
the pin should move from the center to 1/4 diameter to the left.
You may let the pin move just a little bit to the right (1/4" to 1/2")
When I first used my machine I let it swing to left to right by equal
amounts and had donut rings in my mirror. My pro friend Gerry Logan
set me streight. 
As for speed a 6" can run from 75 to 100 rpm, a 12.5" 30 rpm and the 24" I ran 
it at 12 rpm. For figuring you can vary the stroke to do what has to be done.

One problem with machines doing larger mirrors is the spindle table
is not stiff enough and this can cause astigmatism in your mirror.
I had this problem doing a 13" mirror on a partical board table 1.5"
thick. I replaced it with a round steel plate 1.25" thick. I have
read that cement tables work ok also. Astigmatism is the thing
watch for using a machine.

More tips to avoid astigmatism: Keep the mirror running true and keep
the runout as low as possible. For you large thin mirrors cover
the table with a 1/4" to 3/8" layer of pitch. Groove it about ever 2.5".
Lay a piece of 4 mil plastic on top and then lay your mirror on top.
Place the mirror on the table in the same position each time.
This works much better then carpet because it does not give as the
tool overhangs the mirror but it does flote the mirror. The plastic
will keep the mirror from sticking to the table. 

Nothing like the smell of carbide in the morn
Bob Pfaff
Randy Johnson           Seattle, WA

randyj@nwlink.com
randyj@radarinc.com