I am quite unsure if this is the right place to ask this question, so if not please bear with me...
I am currently thinking about various possible solutions for keeping a camera (APS-C sensor, focal length 1000mm) tracked to a celestial position (AKA: "star tracker").
Various working DIY solutions based on the "barn door" principle do exist. Albeit a possible solution, I was feeling more like "hey, it's just a single axis... so, what about just taking a stepper motor with sufficient torque to hold the equipment and a gear with a sufficiently high ratio?!"
If I am right, this would however imply to use a 1:8400 ratio for achieving approximately 0,000215° per step (assuming 200 steps per revolution for the motor; microstepping doesn't seem to be precise enough?).
I simply can't find "stackable" gears for stepper motors, it seems. I can however find planetary gears (would not be required as backlash is not a problem with such a setup) for some (larger) stepper motors at ridicioulsy high price levels with ratios of up to 1:1000. I also can find configurable "toy"-gears (all plastics) on the dirt-cheap end even exceeding 1:10000. What I have not found so far is something inbetween...
So, may I dare to ask, of how you would solve the task?
Seems like this isn't easy as well, as I only can find worm gears up to a 1:100 ratio. This wouldn't be sufficient even if considering microstepping precise enough for the job.
Except for a few popular reduction ratios, gearboxes are custom designs and practical only for manufacturers of mass produced items.
It is extremely difficult for a hobbyist to come up with a nonstandard ratio, precision gearbox, especially if the output shaft has to support significant weight, without mechanical slop.
It is much easier to go with the barn door idea, using a combination of a geared stepper and a screw drive. The inherent nonlinearity of the rotation angle versus the drive extension can be corrected by simply changing the step timing.
Locutus74:
Seems like this isn't easy as well, as I only can find worm gears up to a 1:100 ratio. This wouldn't be sufficient even if considering microstepping precise enough for the job.
The usual way to increase the gear ratio is to use a series of gears in sequence. For example two 1:100 worm drives in sequence would give you a 1:100,000 ratio.
Robin2:
The usual way to increase the gear ratio is to use a series of gears in sequence. For example two 1:100 worm drives in sequence would give you a 1:100,000 ratio.
knew about that... but the usual "NEMA-XYZ" gearboxes seemingly cannot (edit: "easily") be stacked? At least they have completely different axis-diameters for the input and the output side... And up to now I have not seen online-examples of how to (or anyone in fact) stacking them.
Otherwise just "stacking" some of these (say one 1:40 and one 1:100 for a 1:4000 total ratio) would be just fine for me:
jremington:
It is much easier to go with the barn door idea, using a combination of a geared stepper and a screw drive. The inherent nonlinearity of the rotation angle versus the drive extension can be corrected by simply changing the step timing.
The barn door also is some sort of custom-ratio gearbox, isn't it?
And yes, of course the so called "tangential"-error (for the most popular solution it's a secant-error, btw...) can be compensated for quite easily if one uses a µC for controlling the speed of the motor (as well as the absolute position ("nr of steps turned so far from reference")).
So, yes, most probably it will be the next barn door tracker which will be built. But I still want to look for alternative solutions (if feasable for a halfway "decent" price tag ... that is if not more expensive than maybe 2/3-rds of the price of the commercially available solutions. And seemingly can be done using my lathe and mill (both non-CNC))..
Depending on how stout such an apparatus would be, and how much you wanna spend...
Why not start with the motor with the 100:1 planetary, that puts you down to 1:840...seems some of the plastic gears out there are relatively cheap and you could fab a gear train....
If you had big $ and needed ultra stout, could use a series of timing pulley reductions...
Worm gears do provide bigger reduction as zweiblum mentioned...
The other thought is to 3d print the appropriate gears for your train ...there are people printing gears to be used in iron gear trains for cutting metric threads under at least 1/3 hp(dunno how long they last) Fusion 360 (still free for tinkerers) has a great gear maker and can export an .stl file.
There's also the sparkfun motor controller (TC78H670FTG) that touts down to 1/128 of a step control, the video on that page also demos a hot dog cooker that tracks the sun with a parabolic mirror...
