From: "Jerry" 
To: "ATM List" 
Reply-To: "Jerry" 


     Gears with larger prime numbers of teeth would cause longer intervals
before repeating relative positions. But the simplest answer is to have a
belt drive to at least one of the moving parts, turn table or over arm. The
belt slippage will vary with the drag of the tool or lap. The drag changes
at least a little all the while as you grind or polish. It is quite
unlikely to repeat perfectly the same pattern.
     I am not sure that if you had a somewhat large stroke to rotation ratio
that you would have much of a problem in the shape on the glass even if it
repeated exactly. I guess that if you had integer ratios like 1:1 or 1:3
etc. it would be a big problem. But with something like maybe 1:12 or more
I don't think you would have much problem. And you could just stop the
machine every now and then and rotate the bottom piece just a bit somewhat
as you would probably do if you were working the glass by hand on a barrel.
I have only used a belt driven machine so this is only opinion.
        I note you said "almost precisely the same path."
"Almost precisely"
is not the same as "precisely".  It may be if you allowed the
machine to run longer that you would see that it varies more than appears
with a short run time.


Jerry

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