Re: [ATM] Problems With Central Hole Unchanged

[Jonathan Bishop <jonathan_bishop@sbcglobal.net>]

Bryan,

Same thing happened to me ( and probably many others). It is useful to 
know how to get back to a sphere in these cases. I usually found that 
nice short W strokes were good for this.

Certainly no need to go back to fine grinding!

Good luck!

Jon Bishop

Bryan Stiles wrote:

>Good Evening Everyone, I am hoping that someone out there can give me
>a hand. I am working on a 8" f/6 mirror. When I attempted to figure
>this mirror, I worked about 15-20 with a W stroke. Apparently my
>stroke was too long and this produced a nice hole in the middle of the
>mirror about 4.25" in diameter. I have worked to try to get this out
>for about 4-5 hours now and don't seem to be getting anywhere.
>
>Here's what I have tried:
>1. MOT 1/3 COC. This didn't really do anything that I could see.
>Quickly abandoned this method after about 30 minutes.
>2. TOT 1/3 COC. This seemed to smooth the hole somewhat but not get rid of it.
>3. Cut a paper disc the size of the hole and cold pressed for ~30 min.
>More TOT 1/3 COC.
>
>The dang hole is still there. I have warm pressed, cold pressed, with
>mesh (microfacet), without mesh, more pressure, less pressure, thicker
>CeO and thinner CeO. I feel that I'm getting pretty good contact but
>I'm getting nowhere fast.
>
>I am looking at my mirror on my Foucault Tester (Homebuilt Stellafane
>Type) with the razor edge and Ronchi grating. The blade is coming in
>from the left and I am assuming the light to come from the right (thus
>a hole not a hill). When using the Ronchi grating (100 lpi) the lines
>will hardly deflect when crossing the hole.
>
>I am using soft burgundy and have rechanneled my lap 4 times now and
>am closing in on the fifth.
>
>I am seriously considering going back to 500 grit and trying it that
>way. I have really enjoyed making this mirror and don't mind the extra
>work but I am losing my patience with this problem. Any helpful advice
>would be very much appreciated.
>
>Thanks, Bryan
>_______________________________________________
>ATM mailing list http://www.atmlist.net/
>
>  
>

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