Making a cconstant velocity gearing

Well i can suggest that worm and worm wheel is goo enough if machined properly to have minor backlash.Buy material wise i suggest nylon for it.I tried with this material and it gives better results as any plastic material is considered for gear.

Car windscreen wiper motors and most electric window motors are worm and wheel drive. With worm drive you do not have any gear backlash but the worm has an end float which allows for growth etc. while the gearing is good the ratio is way to fast for 1 revolution per day. so I'd suggest using say a windscreen wiper motor driving a small pulley from a shredder or photocopier and making up a large wheel for belt drive (again no backlash in belt drive). Windscreen motors generally run at about 60 rpm so your ratio will need to be 86,400:1. this brings me to plan "B" use an old electric clock, belt driven at 2:1 from the hour hand drive.

Although I do have at least one harmonic drive/stepper motor in my stash, I haven't used one to drive a telescope. However, since a telescope essentially magnifies any error in the drive chain, I suggest that:

1. You use the worm to drive the telescope and the harmonic drive to drive the worm (rather than vice versa, as I believe you suggested. A well made worm gear should be absolutely smooth - The harmonic drive does have teeth, so there is going to be at least a bit of cogging. The worm will divide that cogging by its ratio.

2. This is too sensitive an application to use anything like all-thread. You need very precise and smooth threads, preferably ground; very good bearings supporting the shaft, and shaft bearing surfaces very accurately concentric with the worm.

3. Although this is a pretty large telescope by amateur standards, hopefully it is well balanced, so the worm gear does not need to be very strong. Good luck on making the teeth come out right at the end of the revolution without an indexing system... I'd start searching junkyards/surplus vendors for a gear that can be adapted.

Why not belts & pulleys? All the parts would be obtainium.

First: I should say that I have no experience in any of these areas.

However I think you need to define for at least yourself over what period of time jitter is to be considered a problem. In other words are you just looking through the scope or are you taking photos? If photos how long will the exposure be? For what period of time will you want to accurately track one point in the sky? If you're looking for differences over a period of several days, your servo will be very different from that required for a few second "exposure".

Can you fit an anti-backlash gear somewhere in the drive to keep the train preloaded?

I will suggest worm drive connected with VFD to motor so that you can reduce size of worm gear.

Hi

Just to add a word of caution, be wary of using automotive parts - see Guest's contribution mentioning windscreen wiper motors. Most automotive motors are built to last about 2500 hours of continuous running, which is about 4 months of running 24/7. Depending on the usage you want get out of your drive this may be too little. Also some motors have straight winding slots which lead to 'cogging', i.e. the motor does not exhibit true constant angular velocity when running, which in a telescope application may be unacceptable, even with the worm drive.

What it all boils down to is that if the life expectancy is OK for you, try it. Usually a motor from a wreckers yard is cheap, and if it works - good. If not, it was not expensive!

Regarding further reduction gears - I would bet on a number of worm reductions to remove any gear induced vibrations. Maybe someone else has some ideas on harmonic drives with helical teeth?

Good luck!

There is a very good book called "Gears and Gear Cutting" by Ivan Law from Brittain. It is part of the Workshop Practice Series produced produced by Special Interest Model Books. I don't have the ISBN with me but it is available from many sources and retails for about 9 quid in UK or $20 Aus. Another author in the series called Harold Hall shows some good ways to eliminate backlash when machining which may interest you. I think it was Wikipedia that had a good story about the type of gearing you've described (harmonic drive).