Re: ATM RE: Trial Mirror:Grinding completed

["Richard Schwartz" <richas@earthlink.net>]

Before you start polishing, you should do the "red-out" test.

Hold the mirror up to a light, viewing the reflection of the light at a
grazing angle.  You should be surprized to see the light clearly, as if the
mirror is already polished.    Generally, rough surfaces can appear to be
polished if you view them at an angle nearly parallel to the surface.   The
finer the grinding of the rough surface, the less parallel you can go and
still have the appearance of a polished surface.

Gradually move the mirror so that the the reflection of the light is visible
while the light direction becomes less parallel to the surface.   You will
reach a critical angle where the light turns red, and beyond that angle the
reflection vanishes.

The angle where the light turns red depends on the roughness of the surface.
If you are ready to polish, that angle should be about 45 degrees.   If the
red-out angle is less than 45 degrees from the surface, you need to do some
finer grinding.

Also, the red-out angle should be the same all over the mirror.   A common
problem with beginners is good grinding at the center of the mirror, but
incomplete fine grinding at the edge.   This is easily revealed by the
red-out test, and is usually the result of using a solid glass grinding
tool.   You cure bad edge grinding by cutting channels in your grinding
tool.

I hope this helps,

. . . Richard



----- Original Message -----
From: "Ulhas Deshpande" <ulhas@nagpur.dot.net.in>
To: "Claude Serquet" <claude.serquet@actant.com>; "ATM" <atm@shore.net>
Sent: Monday, August 20, 2001 10:50 PM
Subject: ATM RE: Trial Mirror:Grinding completed


>
> Claude
> I would haveloved to carry on with finer grsdes also but I just dont have
> them. I continued with last wets of 303 for long duration hoping that the
> sludge would give a better finish. Well I hope that it will take
> longer,which would not matter as the machine is doing it,but eventually
the
> glass will polish. It has been two hours now but no sign of any polish.
> let's see and hope
> Ulhas
>
>
> >I'm just wondering that you start polishing right after #303 carbo, my
> >understanding was that you should grind until say #500 and then maybe 9,
> >or 5 um microgrit or AlOxyde powder. Or is a machine so that you really
> >can ignore these steps ?
>
>
>
>