Re: [ATM] mirror grinding

[Mark Holm <mdholm@telerama.com>]

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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