Re: Newbie seeking advice on mirror grinding

chris.b@xxxxxxx wrote:

> I'm not sure how relevant my glass-pushing experiences are any more.
> But you can try the "pencil test" after hogging out with 80 to ensure
> you have a reasonably spherical surface before moving on to 120. Get
> the mirror and tool *dry and clean* with paper towels then draw lines
> across the mirror and tool with a soft pencil.
>
> Lower one piece of glass gently onto the other then rub the tool gently
> on the mirror (or vice versa) in short strokes *without pressing at
> all* and it will give you a rough idea of sphericity depending on how
> the pencil lines are worn away.
>
> Do *not* do this with wet glass or they may stick permamently due to
> suction. 16lbs per square inch of atmospheric pressure on the area of a
> mirrors is a lot of suction. Moisture will turn the glass into a giant
> sucker if the mirror has a hollow in the centre. So always get the
> glass clean and dry before bringing them gently together for this test.
> Do not expect perfection with the lines evenly worn away until later
> grades.
>
> My own references (now many decades old) always insisted on getting
> the mirror and tool spherical before smooothing began. 120 is the first
> smoothing abrasive. Not for removing lots of glass. 60 or 80 will
> always work much faster than 120. So make sure you use lots of 1/3
> C-over-C strokes after using the chordal strokes.
>
> It will *always* save masses of time if you are thorough with each
> grade of abrasive. Your glass will always come back to haunt you if you
> think you can cheat or make a large change with a later grade.
>
> Think of it like smoothing a roughly sawn piece of hard oak with the
> different grades of abrasive paper. You will soon realise how useless
> it is to try and smooth with finer abrasives when your oak is still
> rough. You'll want the roughest paper you can lay your hands on, or die
> trying.
>
> Chris.B
Thanks!

I was reluctant to go back to 80, but then I realized that my progress was
going 2 steps forward and 1 back and the correction is still large.

I am about to go back to 80 grit and do only a few wets and then test until
the sagitta is correct. Then I'll do the pencil test until its more
spherical and then back to the 120, not forgetting to clean my bucket,
throw out sponge and towel.
--
....Dave Dawson

"If you wrestle in the mud with a pig,
you both get dirty, and the pig likes it."
.

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