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Im making a round rotary table, about 300 mm in diameter -- looking to get a tolerance within ±0.01 mm around the perimeter
With step angles of 1.8° Im looking at tolerance of ~ 5 mm.
Question: What gear reduction would I have to use, to get ±0.01 mm?
When you say tolerance of .01mm, do you mean you want to be able to position any point on the table perimeter within .01mm? In any position? Or just a few?
How much load? Must it turn continuously? If loading is light any reasonably consistent gear reduction can be used, such as a worm gear, with a preload mechanism. Preload mechanisms are easier if it doesn't need continuous rotation.
Gear cogging can be programmed out if the gears are cut well.
Load max = 2-3 lbs (0.9 - 1.3 kg)
It will pause for 10 seconds every 25.7° (where the tight tolerance is necessary)
So you want to index 14 positions? There are better ways to do this.
Just roughly position the table, than have a wedge shaped pneumatic clamp align it and lock it in place.
Great insight, thank you
If you're indexing the same index for every station, a shot pin solution is far and away the best.
If you need arbitrary positioning:
For that size, I've gotten good results machining a timing belt profile directly into the perimeter of my plate. HTD belts have a fully round tooth profile that's straightforward to machine. 20:1 with a 300mm plate means a 15mm drive sprocket. With a 3mm belt, you'd be looking at about a 16 tooth pulley on the motor.
20:1 is about the most you can hope to do with a single stage of reduction before things get silly.
For .01mm resolution at 300mm diameter, you'd need a servo with a 5000 PPR encoder. That's not hard to come by. If you need to position against a dynamic load, everything gets harder, of course.
If you're wedded to a stepper and can't use a shot pin, you're either looking at precision worm drive gearboxes or a harmonic gearbox, and about 200:1 gearing. Worm drive is okay if you don't need to reverse and you don't need to go particularly fast.
Sure!
You want a positional accuracy of .01mm at the rim of the disc.
With a diameter of 300mm, the circumference is 300mm*pi. So about 942mm. Divide that by your accuracy requirement, and you get 94,248 positions.
A standard stepper is 200 positions per rotation. Microstepping that will reliably get you within half to a quarter of a step. So a 200:1 gearbox will get you about 80,000 to 160,000 reliable positions.
It's not an even multiple, so you're stuck microstepping either way.
Alternatively, size your table so that the circumference is an even multiple of the step angle and gearbox ratio.
For example, with a 200:1 gearbox, half-stepping with a 200 step motor, you'd go backwards:
(200 steps (motor) *2 (half step) *200 (gearbox ratio) *.01 (accuracy))/pi (circumference to diameter) = 254.65mm diameter
Or with a 250:1 gearbox
(200 steps (motor) *2 (half step) *250 (gearbox ratio) *.01 (accuracy))/pi (circumference to diameter) = 318.31mm diameter
But, realize with a worm gear box you'll have to either always approach from the same direction, or add a preload mechanism, because worm boxes have backlash.
Hope that helps. But again, if you don't need arbitrary positions, just a fixed 14, then just drive the wheel directly off the stepper, add a tapered shot pin on a preloaded linear rail driven by a pneumatic cylinder and call it a day.
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