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Re: [ATM] mirror grinding
I would love to be at that stage lol. I am hogging out 2 - 12" f5 mirrors
on the same tool for a binoscope. I now have to make a new tools as I have
worn out this one. The mirrors were not prehogged out. I have worn the
outer edge of the tile down to 1/8 of an inch. and they are starting to peal
off. So tonight I will make a new tile tool and hog out the remaining
glass, I am just shy of 1/8 in deep in the center, I need to get to
9/64ths. each mirror so far is within 1/100,000ths of an inch of eachother
according to the machine shop up the street. pits,,,,yea I have pits lol
lol lol. they all grind and polish out though if you take each grit as a
seperate project. Clean everything, change cloths, wash everything down, I
have a small room, no windows, sealed off wth plastic, with a air filter in
there when polishing. I had dust get in there once and caused a scratch,
nothing that once around the lap didnt fix though. Dont stress over the
pits.
>From: Mark Holm <mdholm@telerama.com>
>To: atm@atmlist.net
>Subject: Re: [ATM] mirror grinding
>Date: Tue, 21 Jun 2005 23:21:41 -0400
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>Cristian wrote:
>
> > Have I used too a large magnification or will the mirror
> > always have some sort of scratch or pit under high magnification.
>
>Until you polish it out, you will always see pits and perhaps a few
>small scratches under magnification.
>
>The thing about pits is that they always have a size distribution. At
>any grit size, you will a lot of middle sized pits, a fair number of
>smaller ones, and a few larger ones. Look over a reasonable area of the
>mirror, getting a good idea of the size distribution. Make sure to
>check both the center and near the edge as grinding is usually more or
>less efficient at one of these. Then grind 2 or 3 more wets of that
>grit. If the distribution of pit sizes doesn't appear to change, you
>are probably far enough along with that grade. It is common for there
>to be some difference in appearance from the center of the mirror to the
>edge. If the appearance doesn't change after two or three wets, then
>you are probably done, even if there is some appearance difference from
>center to edge.
>
>Scratches are another matter. A few very small ones, that do not appear
>any deeper than the pits, may not be a cause for worry, but generally,
>they shouldn't be there. If you see more near the edge, you may be
>flaking off bits of glass at the edge. Is your bevel still there? Some
>report that they have less scratching trouble if they regrind the bevel
>with each grade of grit as they move through the grit sizes. Use a
>paste of grit on a piece of hard tile or a small piece of glass for
>this. Make the bevel grinding motions parallel to the mirror surface.
>That is, if the mirror is lying on its back, the bevel grinding motions
>should be horizontal and tangent to the mirror edge with the tile tipped
>at about a 45 degree angle.
>
>Perhaps your tool bevel needs work too. If the tool is glass, it's
>bevel should get the same fine grinding treatment as the mirror's bevel.
>
>Other causes for scratches include letting the grit mixture get too dry
>near the end of a wet, so that, when you slide the mirror and tool
>apart, they tend to stick. Grinding needs to be lubricated with water.
> Don't let it get too dry.
>
>--
>Mark Holm
>mdholm@telerama.com
>
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