[ATM] Clarifying the Sharpie Test

[kb7h at onemain.com (Ken Hunter)]

Anytime you are grinding mirror on top, the center of the mirror will  be
worked more and the reverse is also true. To keep the same Radius of
Curvature i.e. "work the whole mirror" you will need to also grind with the
Tool on Top (works the edges more).

As the originator of the "Sharpie test", I regret to inform you that you are
doing it wrong. To perform the Sharpie test properly you clean both the tool
and mirror, make a grid, star etc of Sharpie lines and then carefully place
the tool and mirror together. NO GRIT, NO WATER etc.

Make a few VERY SHORT strokes in several directions (carefully with little
or no pressure) then check to see where the two disks are in contact. If
there is a definite zone getting attention and not the whole mirror, you are
not spherical!

This test is started between the rough and fine grinding stages of  grinding
(#220 or so) and continues during following grinding stages until the risk
of scratches becomes greater than the chance of not being spherical.
By the time you get to the finer grits, you will have the sphere you want
and the Sharpie test can be terminated. (15u)

The Sharpie test is often done with the current abrasive size and sometimes
the ATM will just grind away until all the lines are gone. This does not
test the sphere or the tool/mirror contact.

Following the Sharpie test, one must determine if the surface is ground
completely at the current grit size.

If you can meet these two requirements...
1. Pass the Sharpie test for SPHERE and...
2. Complete elimination of *OLD* current size pits.

It is safe to procede on to the next abrasive.

Ken Hunter



>
> Is this normal and should I go on to the finer grits?
>
> Thanks in advance,
>
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