Re: [ATM] Fixing Tom's oblate sphere

[Mike Lockwood <melockwo@uiuc.edu>]

Tom,

Polk, Tom wrote:
> *****40-60 strokes per minute. Toward the end of a wet the strokes take
> longer. Friction is greater. 

If we define a stroke as one movement, either forward or backward, 
then that speed is OK.  If you are calling one stroke a forward AND 
back movement, then we want to be nearer 30.

> *****If I understand where you may be headed, I have two CEO ratios
> mixed, one for polishing at three rounded tablespoons per cup of H2O
> plus about 0.5 cc suds and one for cold pressing at three rounded
> tablespoons per 0.25 cup H2O and 1.5cc suds. But in use the leaner mix
> is rarely used. From a spray bottle a several shots are applied to the
> glass and lap after a cold press and I'm underway. The sponge paint
> brush that I apply the slurry with is used to apply more CEO.

For spherizing, you want the leaner mix.  In my experience that helps 
lap contact and encourages the surface to go more toward a sphere, 
provided you apply pressure properly.  Thick slurry encourages the 
tool to "float" on top of it, and can also help form an oblate sphere.

> *****I poured this lap on 3/27, re-channeled once and it's needing a
> minor one now. About 5-6 hours total wear.

That's just a little bit hard, but fine for this mirror, and probably 
very good for spherizing.

> Does the pitch lap feel like it's glued to the surface, or like it's 
> skidding on top of the polishing slurry?
> *****After a cold press, the action is fairly easy. After I water down
> the slurry, the friction increases. If my stroke is too fast, it does
> skid. It does not feel like it is glued to the surface, maybe 20 lbs of
> lateral force is needed to move the mirror or tool sideways.

You make a good point - if you move the mirror too fast, contact is 
lost and it skids.  This is a great principle for finding the right 
speed, and after a while it becomes second nature.

> Does the friction change during polishing sessions?
> ******Only as described above. Friction is smooth and even but does
> increase with time during a wet.

Interesting.  Do you let the slurry dry out?  I noticed above that you 
mentioned a "wet".  I don't think so much in terms of that when it 
comes to polishing - it is really one long process with a little 
slurry and water added along the way.  I want the feel of the work to 
be consistently smooth, but not easy to move.  Friction should be 
fairly uniform throughout the work.

> Does it feel different right after a cold press?
> *****Yes

OK, this is a clue that something is going on.  Try a leaner polishing 
mixture (you don't need the really thick stuff to press).  After 
pressing, get the tool/mirror moving, and don't separate them.  Then 
start polishing slowly, with the pressure I described.  Drag should 
start out heavy and stay that way.

Another question - does the friction vary during a stroke?

	Mike L.

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