- Make a sector of plywood or plastic (cut it with a router or jigsaw)
-
Wrap some nylon threaded rod around the sector (can be pre-bent to the
sector radius in boiling water)
- Apply JB weld in a fillet on both
sides of the nylon threaded rod, let dry
- Remove threaded rod (nylon won't
stick to the JB Weld very strongly, even
without a release agent)
-
Trim/clean up the rough edges
Presto...a large sector drive! How big? Mine is about 11 inch
radius. You
can make yours larger if you want. (This may not
work very well on smaller
radii unless you use finer/thinner threaded
rod.) I used 5/16 -18 threaded
rod. Use the same diameter/pitch
threaded rod (nylon, brass, steel?) for
the worm. Attach to a stepper
or synchronous motor.
Wanna make a continuous worm? Do it in two steps. First mold a
partial
section on the disk. Then, the second molding step is the rest
of the
disk...with the ends of the threaded rod firmly meshed into the ends
of the
first molded worm gear section. Can this really work? It
did for me...on
the first try...a seamless/continuous 21 inch diameter worm
gear with 1187
teeth.
I've been able to find nylon threaded rod in six foot lengths. That can
make a pretty big worm! ;-)
With a big enough worm you can use a "direct drive" system....which means
low/zero backlash, and only one source of periodic error (one worm meshing
with one worm gear, no reduction gear train). One motor, one worm, one
big
worm gear. Choose whatever worm gear size and threaded rod pitch
will meet
your design needs.
If done carefully, this is easily good enough for wide angle imaging. I
have not tested it for longer focal length photography possibilities, but
periodic error in my first version is smooth, not jerky. I bet an
autoguider could handle it better than many mass produced, inexpensive drive
units!
Currently I'm using these gears to drive my sixteen inch, f/6 dobsonian.
With surplus steppers rated at 6 volts, driven at 24 volts, using a small
flywheel to smooth rotor resonance problems (lower fundamental resonance
freq of the rotor), and some series resistors for current limiting at low
step rates...I can halfstep slew this large telescope at 2.75-3 degrees per
second. Duck!
Tom Krajci
PS. Is this an original idea? Nope. See Sky & Tel ATM
articles from 1974
and 1979. The difference is that nylon threaded rod
is now easily
available.