Experiences with steppers for dividing heads.

Firstly, thanks to all of you who contributed helpful comments to this project.
1. Some years ago I was given some scrap electronic equipment
from a local hospital (wouldn't happen now, what with Health and Safety paranoia and the Waste Electronics Directive!) and one of the things I retained was a Baekelite control knob with a metal skirt engraved 0 to 100 with the old style 1/4" shaft. This only came to light yesterday when shuffling through the junk box.
I have now pressed that into service as the indicator instead of the paper protractor previously discussed. Thanks anyway to Alan Bain for what was an interesting diversion.
2. Driving the stepper round by spinning the rotary encoder produced some stalls from the motor. I suppose that I'm not adept at generating a smooth acceleration manually! This experience has reinforced the need for protractors (or Baekelite knobs, as above) to record the position.
3. When the power to the system is off, the stepper shaft can be turned manually by the Baekelite knob, but of course it cogs into the 200 positions, with the stepper now acting as a magnetic brake. Suddenly I do not need any control electronics or rotary encoder, because I can drive the mechanism via the Baekelite knob and thus have nothing to gain by driving it from the rotary encoder!
It remains to be seen whether the magnetic braking effect of the stepper (amplified by the 80 turn reduction ratio) will be sufficient to resist cutting forces.
However, I shall retain the electronic controller in anticipation of the mechanism being automated in my forthcoming CNC experiments.
4. When cutting the worm, George Thomas's book suggested that I needed a pitch of 3.94 mm to interact with a 20 DP wheel. Not feeling adventurous, I reasoned that a 4mm pitch would engage but ride up the side of the gear slightly at both sides, thus removing any backlash problem. This does seem to be OK.
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On Fri, 7 Nov 2008 21:47:17 -0000, "Amateur Machinist"

I suspect that you will find it works just fine. If you need more resistance, have a switch that shorts the coils to act as a brake.
If you find you need more of a "cogging" effect when turning the knob, switch a suitable value of R across the coils.
Regards, Tony
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