I worked on the drive for my mount some over the past week. I decided to go for a tangent arm style drive. I don’t want to do long term photography so I care about the errors associated with a tangent arm drive over time. If I could hold something pretty much steady for a little bit I’d be happy.
I have a 2.5″ tangent arm being pushed by a carriage on a 1/4-20 bolt driven by a 0.5 RPM motor, from Servocity, geared down to 0.3 RPM. Ideally, I need to run at 0.218 RPM according to the math. To vary the speed I’m using a simple potentiometer. The arm is made of wood and it is clamped down on the RA shaft via a bolt. To increase the friction between the wood and shaft I used a strip of sandpaper. Ideally, I’d move the object I want to track in place, turn on the motor and then tighten down the clamp bolt.
I’d say I’ve met with moderate success. It works intermittently. To test it I stared a Altair a lot through my 7mm eyepiece at 144x magnification. Most of the time I could decrease the rate at which it was moving and only rarely could I arrest the motion or push the star in the wrong direction. The few times I got it to track well I really had to crank down on the bolt holding the arm onto the RA shaft. Still there were a few times the star moved off to the left instead of down indicating it was working but that my RA shaft wasn’t pointed perfectly at the celestial north pole.
There are a couple issues that I think are giving me trouble. The first is that getting the arm to clamp tightly on the RA shaft so they turn as one is tough. Yeah I know, soft wood on steel is an issue. Making this piece of out metal would work better I think. To get it really tight though I think I’d have to have some sort of tool to use on the clamping bolt. The second issue I’m having trouble with is flex. As most of the drive is made out of wood there aren’t super tight tolerances. As the drive runs it removes these tolerances by jamming everything together but that takes time because the carriage only moves at 0.01″/min. I also have a spring on the threaded 1/4-20 bolt so I could unlock the gears to rewind the carriage. I’m pretty sure this is compressing some as it runs.
To fix things I need to increase the friction between the RA shaft and arm. I can’t permanently join the together and I’d like to avoid having to use something like an Allen wrench to tighten something like a set screw. To fix the tolerance issue I can try to make everything tighter or make the tangent arm longer. Making the arm longer requires the carriage to move faster which would remove the slack faster. Getting the carriage moving faster would require me to order some more gears if I want to rotate faster than 1 RPM. Of course making the whole thing out of Aluminum would probably work too…but thats beyond my current capabilities.
I’m still thinking up ways to get around these issues. But for now, here are some pics of the current setup.