Re: [ATM] Open Question Concerning Pitch Laps

[Kevin Whitefire <chaosopher23@yahoo.com>]

I had that central hole a few times, but I got rid of
it by staying on a fine grind stage for way longer
than I needed to.  Using my 9u alum-ox powder, I did a
central stroke that matched the radius of curvature
closely to a sphere, so fine grind makes the polishing
time fun or horrible.

While I didn't get a sphere (hyperboloid at -1.74) I
did get good lap contact.

Kevin of Eastern Iowa (it sucks here)
Seeker of the Darkness

--- Bryan Stiles <bryan.stiles@gmail.com> wrote:
> I have experienced a phenomenon on two separate
> occasions that I would
> appreciate some input on.
> 
> 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?
> 
> 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.
> 
> Case in point: As mentioned above I used too long a
> figuring stroke
> (for 10-12 minutes) and polished a decent hole in
> the center of my
> mirror (8" f/6). I tried for approximately three
> weeks to get that
> hole out. TOT 1/3-1/4 COC, smallish W stroke, etc.
> All told 10 hours
> of time pushing the thing around trying to fix the
> hole.  The upside
> was the glass was really, really clear.
> 
> 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.
> 
> I understand that it takes a while to clean up some
> mistakes (central
> hole, turned edge, etc.) but the times I've read
> about were on the
> order of an hour or two not 10.
> 
> 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.
> 
> Is this an issue with my patience or methods /
> materials? My resolve,
> I fear, is also being tested. :)
> 
> Trying Hard to Learn, Bryan
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


	
		
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