Re: [ATM] Open Question Concerning Pitch Laps

[Mike Lockwood <melockwo@uiuc.edu>]

Bryan,

Bryan Stiles wrote:
> This question is a direct result of my prior posts about the inability
> to remedy a central hole in my mirror. I tried all the suggestions
> kindly provided but to no avail...I went back to fine grinding. In a
> nutshell the question is why does my lap quit responding?

Wish you hadn't gone back to fine grinding.  We're not done asking 
questions yet!  :)  A polished mirror is a nearly finished mirror, and 
something polished in can be polished out (with the exception of very 
badly turned edges).

> I am working on my second mirror and have come to a point where I seem
> to be unable to get the lap to make any difference in the figure of my
> glass. I have cold and warm pressed, faceted, trimmed, beveled,
> channelled, more pressure, less pressure, thinned CeO mix and
> thickened CeO mix. Unless I really try to do damage (i.e. don't rotate
> the mirror at all) I can't seem to get the thing to respond in the
> time frames mentioned in various replies and web sites.

This is evidence of a lap/contact problem, but there are MANY types of 
these.  In almost every case, when something doesn't respond, it is 
this type of problem, at least for me.

Now this doesn't mean you didn't press correctly - it might simply 
mean that your pitch is quite soft and contact was lost after a minute 
or two of work.

> Now I don't mean to paint too drastic a picture here. During the
> course of the 10 hours I raised a few spots and was able to smooth
> them out in fairly short order by concentrated effort, so I felt that
> things were working properly albeit slowly.

That's good to know, and very good experience for you.

> If this helps, the pitch is soft burgundy from Will-Bell. Room temp
> hovers between 68-70° F. I noticed after  channelling and beveling
> etc., that my central square was a bit too close to the actual center
> of the glass (what exactly does that cause?) so I scraped it down
> lower that the surround squares.

Okay, here are some questions (concerning your 8" F/6 mirror):
-------------------------------------------------------------
1)  What is the mirror substrate?  Pyrex?  How thick?
2)  What is the lap substrate?  How thick?
3)  How thick is the pitch layer?
4)  How large are the facets and channels?
5)  How long do the channels take to close under normal polishing?
6)  How much weight are you pressing with?
7)  Do you separate the lap and mirror after you finish pressing 
(before you polish)?
8)  How often did you press?  How much did you polish between presses?
9)  How large in diameter was the hole, and how deep?
10) What polishing compound are you using?
11) Do you have to make a new lap now that you went back to grinding? 
  (If so, it will act differently.)

	Mike Lockwood

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