From: "Bill T." 
To: atm{at}shore.net
Reply-To: "Bill T." 


John,

The grinding action at any point on the mirror is proportional to the total
velocity contributed by both the arm speed and the rotation of the mirror.
If the table RPM's are high in relation to the arm speed, the edge gets
worked faster than the center.

I assume you have the curve already hogged out.  For grinding against an
approximate sphere, I've had great luck just parking the tool over the ~50%
zone of the mirror with a small amount of random motion.  For me, this
gives fast, easy, spherical results.

For an 8" mirror, that new 160 mm grinding tool would be exactly right
for this technique.  You may need to experiment with the exact position of
the tool -- monitor the ROC with a spherometer to find a tool parking
position around 50% of the mirror's radius where the curve stays constantly
the same, rather than getting gradually shorter or longer.  If the ROC is
getting short, move the tool a bit further out, and vice-versa.  Of course,
don't move the tool so far out that the center doesn't get enough grinding
action.

I admit it's cool to see that overarm flailing around.  I've tried all
sorts of grinding techniques involving lots of tool movement. I have to say
that the "parked around 50%" technique is the best.  This is also
true for gross polishing.  Save the overarm for figuring.

You need a lot of grit for the coarse grits.  And keep it wet.

Bill T.


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