Re: [ATM] Stubborn Oblate Spheroid

[Mike Lockwood <melockwo@uiuc.edu>]

Jason,

As others have noted, the goal right now is to polish out your mirror. 
  However, understanding what's going on will help you later on.

Jason Hissong wrote:
> Yes, I have read your post before, but in the process of digging deeper 
> in eliminating it, I did not do what you suggested (applying weight on 
> the trailing edge of the stroke). Thanks for pointing me back towards 
> that way.  I will try this tonight.

What I mean by this is, as you push the mirror foward, apply the 
downward force to the back of the tool (most apply it in the center or 
think they are) back farther towards you - maybe 1/2" from the center, 
towards you.  When you reverse the stroke, you can either ease off on 
the pressure, or apply a little more to the side of the mirror that is 
away from you.

For me, I apply slightly more pressure on the forward stroke by using 
the heel of my hand to apply a little more pressure.  On the reverse 
stroke, my fingertips apply a little more pressure.

This may take some practice.  We're not talking about a lot of 
pressure here.  It's a somewhat subtle shift in hand pressure.

What is really going on is you are trying to reduce the tendency of 
the leading edge of the lap to dig in.  That causes the oblate sphere.

For me, the oblate sphere is not the problem - it's the 
turned-down-edge that appears when I get and oblate sphere.  The edge 
of the lap rides up the "rim" of the oblate sphere, and when the lap 
goes over the edge it hacks it off and you get TDE.

>  When I do my strokes, I use weights 
> to rest on the center of the tool and this helps with the polishing (as 
> far as I can tell).  Did not know if offsetting the weight back a little 
> would help (making sure it is offset towards me)?

The weight distribution needs to change with the forward and back ward 
stroke, and a fixed weight can't do this.  It's a real pain to move 
the weight for each stroke.  The pressure on the mirror/lap should 
come from your hands/arms.  The grinding stand must be at the proper 
height for this to be comfortable to do.

I recommend you get rid of the weight.

> The other concern 
> that I had was if I go MOT, I thought that would deepen the center 
> instead of working the edge.

Exactly.  An oblate sphere has a center that is too shallow, and an 
outer region that rises up too much.

> The temperature in my room is about 76 degrees and according to the 
> graphs I seen at 
> http://www.atmsite.org/contrib/Shrader/Acculap/pitchtest_graph_lg.gif 
> the standard Acculap pitch should work.  The medium may be good for my 
> figuring.

I don't know - I have never used Acculap.  At this temperature I'd 
probably be using Gugolz 64 with a little 73 mixed in.

	Mike Lockwood

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