ATM bearings

["Bob Walker" <tac@i1.net>]

Automobile wheel bearings were suggested--seems to me bearings from the
other end would be better.  I've seen pictures of equatorial mounts made
from auto-truck rear axles (rear wheel drive!).  If weight isn't an issue
this would certainly be sturdy.  If weight is important, consider using
differential bearings instead of wheel bearings.  They are much larger in
diameter--around 3" ID.  This allows for a large diameter, tubular shaft.  A
3" OD tube would be much stiffer than a 1" OD solid shaft of equivalent
weight per length.  This would allow you to build an extremely stiff, but
fairly bulky, mount that would be no heavier than a smaller one with smaller
bearings and solid shafts.  As a bonus. I believe you could have a coaxial
polar alignment scope that would look through a hole in the declination axis
(with the dec axis at one specific orientation, of course) because a 1" or
so hole drilled across a 3"  tube probably wouldn't weaken it much.  You
could have your polar scope without sacrificing wide bearing spacing and
stiffness.  Of course, none of this meets the minimal-machining
specification, but you were just waiting for an excuse to buy a metal lathe,
weren't you?

 *  Best regards, Bob
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