[ATM] Concept review - dob roller drive

[Jay Kirkland <jaykirk2@compusmart.ab.ca>]

I've just rebuilt my 20" as a not-too-varied variation of Mel Bartel's 
brilliant tri-dob. For the altitude bearings I used 1/8" aluminum 
rolling on ball bearings. For the azimuth bearing I covered the top 
surface of the circular ground ring with a ringle ring of aluminum about 
1/16" thick. The ring has something like a 41" OD and 31" ID. The 
aluminum was cheap and the ring was really easy to cut using a router 
and a 1/4" straight upcutting spiral router bit. Do not use a carbide 
tipped bit when cutting aluminum, and do not use aircraft grade 
aluminum. A regular steel bit and regular aluminum and it'll cut like a 
knife through hot butter. I also used ball bearings for that axis. And 
teflon against the aluminum for the friction control. No matter what 
rubber for either part you use, you'll always end up with some 
spongieness, maybe not enough to feel, but enough to see at telescope 
magnifications. The rubbers may also have different hardnesses at 
different temperatures (from summer to winter) so you may have a hard 
time adjusting for different frictions. And rubber will also deform 
under the pressure of the bearings over time. I still haven't figured 
out where the motor drives are going on my scope yet. I don't see a way 
on my scope of combining my drives with my encoders, but that doesn't 
mean on other scopes it can't be done. Just check out Mel's site and his 
links for a lifetime of ideas. Plus all the parts you need to do it.

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