[Spanrz (Brett)]

Steve,

Haven't written off the plastic just yet, though my choices are very very limited.
A few weeks ago, I did buy a very cheap plastic worm gear from Jaycar, just on the off chance that I would have a go, seeming I was running into dead ends. It was the best of the plastics.
I have also disassembled (er... destroyed..hehe) a wiper motor, to get to the worm gear in that, as another alternative.

Also had a chat the other day to a guy who was fixing the large automatic gates at work. Mentioned where I can get any gears or anything close to the system they were using (the gate reduction box looked like a commercial worm gearbox), he didn't know, was his reply. They were heavily into mechatronics.
Hmm, can't seem to take a trick.

The hobbystore gear set, yes have thought about the accuracy a bit too.
But at the moment, it's the best candidate.

One thing that was going through my head as a real wild thing, was to try a larger stepper motor to drive it directly.
Yes a heathenous thought it is.
The torque would be well enough to drive the OTA, but the resonance of the motor at low speeds worries me. As Bojan has remarked the dropped steps is an issue, but it's playing with the finite settings that can reduce that over a analog driver.
The test on the smaller motors worked really well, just unsure how the larger motor would do, even though the driver units could accept the larger current draw.
If I was using the old analog drivers, this heathenous thought, would be driven out of my brain by the jitterness of it.
But the digital driver performs way above my expectations, to put this thought out of my mind.
I never did see a large motor drive at very slow speeds with this driver, so it's an educated guess.
No doubt a direct driven OTA with astro gear on it, will show up signs of the jitterness. It's a matter of how much.
So I am trying to think logically, but outside the square as well.