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[Reply #15 on:]

I will obviously gear them to achieve somewhere under 1 degree per step

Typically, stepper motors step 1.8 degrees per step, for a total of 200 steps per revolution. Using 1/2 stepping, that is already under 1 degree per step. Gearing should let you get down to the 0.1 degree per step range, or lower. Gearing and micro-stepping could let you take pictures in 5000+ unique positions per revolution.

[Reply #16 on:]

Typically, stepper motors step 1.8 degrees per step, for a total of 200 steps per revolution. Using 1/2 stepping, that is already under 1 degree per step. Gearing should let you get down to the 0.1 degree per step range, or lower. Gearing and micro-stepping could let you take pictures in 5000+ unique positions per revolution.

Yes, thats the ideal situation my steppers however are at 7.5 degrees per step (48 steps per rev)

I can gear it down considerably with the 17 tooth gear. but gearing it to .1 degree would take up a lot of room!

[Reply #17 on:]

I can gear it down considerably with the 17 tooth gear. but gearing it to .1 degree would take up a lot of room!

Worm gears give good reduction per space requirement. A single start worm with a 30 tooth gear will get your 7.5 degree step down to 0.25 degree.

[Reply #18 on:]

I can gear it down considerably with the 17 tooth gear. but gearing it to .1 degree would take up a lot of room!

Worm gears give good reduction per space requirement. A single start worm with a 30 tooth gear will get your 7.5 degree step down to 0.25 degree.

at £15 a piece.. or am I just being a cheapskate?

on the note of working out gear ratios I'm getting into a bit of a muddle...

on my motor's output shaft I have a 17 toothed gear. I'm trying to work out the number of teeth per step so... (answers rounded to 2d.p)

48/17= 2.82 - so that is the amount of teeth per step

7.5 degrees/2.82=2.65 - the angle of rotation per tooth

So I want the out put gear to turn .5 of a degree per step.

2.65/0.5=5.3 - the ratio between the angle per tooth on our small gear and the angle on the big gear.

5.3*17=90.1 - this is the ration multipled by the small gears teeth which gives us 90.1  (we will call it 90)

so I need a 90 tooth gear to achieve a 0.5 degree rotation per step? am I right or am I crazy?

CHeers

[Reply #19 on:]

I've ordered all of the electonic bits I need, I think it'll be easier when I've got everything in front of me too much to think about in my head!

[Reply #20 on:]

at £15 a piece.. or am I just being a cheapskate?
RS is not renowned for cheap as it's more geared (no pun intended) toward business instead of consumer. You can get cheaper stuff but they give no details of worm pitch to calculate the reduction. Maybe  eBay would do stuff like this.

on the note of working out gear ratios I'm getting into a bit of a muddle...
so I need a 90 tooth gear to achieve a 0.5 degree rotation per step? am I right or am I crazy?
I work it out at 1.416 degree per step using a 17:90 ratio. To get a 0.5 degree per step you would need a 17:255 ratio.

[Reply #21 on:]

You can achieve massive gearing reductions in a small space using epicyclic gearboxes, and they also package well because they are self contained and have co-axial input and output shafts.

[Reply #22 on:]

This could be just what you're looking for.  It's easy to connect to Arduino.

http://www.amazon.com/Bescor-MP101-Video-Motorized-Head/dp/B0000AUR21

[Reply #23 on:]

This could be just what you're looking for.  It's easy to connect to Arduino.

Does it have positional feedback?

[Reply #24 on:]

This could be just what you're looking for.  It's easy to connect to Arduino.

http://www.amazon.com/Bescor-MP101-Video-Motorized-Head/dp/B0000AUR21

I've seen these, I wan't to make my own for the hell of it and it'll probably work out cheaper.

[Reply #25 on:]

at £15 a piece.. or am I just being a cheapskate?
RS is not renowned for cheap as it's more geared (no pun intended) toward business instead of consumer. You can get cheaper stuff but they give no details of worm pitch to calculate the reduction. Maybe  eBay would do stuff like this.

on the note of working out gear ratios I'm getting into a bit of a muddle...
so I need a 90 tooth gear to achieve a 0.5 degree rotation per step? am I right or am I crazy?
I work it out at 1.416 degree per step using a 17:90 ratio. To get a 0.5 degree per step you would need a 17:255 ratio.

Of Course you are correct! I understand now

stepper angle / output gear angle  and then multiply by the output gear teeth.

This is a problem. If I invested in a 1.8 degree stepper I would only need a 17:61 reduction.

I ideally want to keep the design as simplistic as possible, so it is robust and wont fail easily in the field.

Worm gears are a strong possibility.. Just need to find some a little bit cheaper with the sizes specified!

[Reply #26 on:]

Yeah the worm gear pricing has shocked me a couple of times and I too would like to find a reliable source of less expensive worm gears.

Keep in mind: the cheaper you go, the more likely it is to fail in the field...

[Reply #27 on:]

Bescor.  It does not know it's current position unless it's at the end stop.  I ripped out the internal mechanical stop, and made a dowel that can be removed to allow for continuous rotation.  Starting from the end, you can move for x seconds, at y speed, and you'll be at the same angle every time.  The speed is determined by the DC voltage to the motor.  So there is no way to predict how fast V volts will be.  It is not a stepper motor.  But it is mechanically sturdy to support a large DSLR and long lens.  Perfect for GigalPixel photography using the end stop for the edge of each each row.

[Reply #28 on:]

Yeah the worm gear pricing has shocked me a couple of times and I too would like to find a reliable source of less expensive worm gears.

Keep in mind: the cheaper you go, the more likely it is to fail in the field...

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I'm going to a shop later today that might be ale to order a set in its plastic though so I'll reject it if I think it's not going to be man enough.

I've seen a rig use the metal worm gears, I guess I may have to just pay the price as I don't really want problems with gears while it's in the field.

[Reply #29 on:]

Yeah the worm gear pricing has shocked me a couple of times and I too would like to find a reliable source of less expensive worm gears.
Keep in mind: the cheaper you go, the more likely it is to fail in the field...

In reply #20 I did post a cheaper alternative I found, £2.69 for a worm and a 40 tooth gear. The site has zero information like worm pitch so you cannot work out the ratios. eBay also has some stuff but they are all plastic so may not last as long as you hope.