Re: Newbie seeking advice on mirror grinding

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

.

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