ATM Roller driven GEM

[Brad Davy <bdavy@fred.net>]

I'm currently experimenting with adapting a wood roller drive to a GEM.  So
far I'm optimistic about the results.  The drive uses 4 turned wooden
circles (5 inches, 6 inches, 7 inches, and 11 inches respectively), with
each circle mounted to a 3/8 inch steel shaft that functions as the roller
for the next.  Driven by a 24 volt 200 step motor.  The scope (8" f8
newtonian) is mounted on the 11 inch circle, with a 2" solid steel shaft
for the RA axis.  The declination axis is a 1-1/2" galvanized pipe with a
flexible coupling acting as a clutch.  I've balanced the entire assemble,
and its actually quite easy to turn by hand, given the total weight (this
is not a portable scope). 

Now for the hard part.  The 11 inch circle, while easy to turn by hand,
does not have sufficient friction with the 3/8 inch steel drive roller, and
is not turning.  I'm looking for a material to cover the surface of the
wood to provide better friction.  I've tried rubberized cork gasket (left
over from CB 245 construction), that was almost enough but the necessary
compression left dents in the cork that could lead to tracking errors.
I've also tried rubber strips cut from a bicycle inner tube (surface was
actually more slippery!), as well as a thin coating of clear silicone caulk
(did not adhere tightly enough to the wood).  Emery paper also proved to
slippery, since I think the steel shaft is too hard.  My next attempt will
be to turn a groove in the 11 inch disk and mount an old fan belt in the
groove, then spray the fan belt with "belt dressing".  

Does anybody have any other recommendations of what I might try?  A steel
cable wrapped around the disk will work, but then clutching the assembly
becomes more of a problem.  My current method for clutching is simply to
release the pressure between the drive roller and the disk.

Thanks for any suggestions.
Brad
bdavy@fred.net               http://www.fred.net/bdavy/home.html