[StrawberryBoi]

The tooth count is just a fundamental number, it will not give you the true ratio

I suppose if the manufacturer used a different diametral pitch on each pulley, that would be true... I'll confess that while I've never seen nor heard of such a thing, I've only designed splines and meshed gears, not toothed pulleys for timing belts, but given that the timing belt must have the same pitch as well to interface I'd be very much shocked. Backlash is an issue if it exists and unless he was only indexing to set locations without machining during rotation and could overshoot and return from the same direction to account for backlash, would not be something easily dealt with. Plus the manufacturing variation of the pitch from tooth to tooth would come into play as well, even with zero backlash. Just like the linear tolerance on ball screws.

I'll absolutely agree that the GT belts are a great system, it's all we use at work for actuation via belt systems.

I must be missing something here; by your calculation 360 degree rotation equals 12,000 steps (a whole number, micro steps included). That breaks down to 1 degree equals 33.3333333 micro steps (which has no practical value as it isn't a whole number and steppers are expected to move only in whole micro steps), again by your calculation. Your initial indexed moves were -90, 90, -90, 90 and then back to zero. Do the math and each of these moves yields something other than a whole number of steps, but by calculation tends to cancel errors out. Other angular series of moves may not yield expected accuracy. The question that arises is, how are rounding errors handled in the software and can they yield cumulative errors? That may be where all the angst is coming from.

Not meaning to throw a wrench into the works here, but; its my understanding there is also a tolerance issue here as well. Do servos or steppers always land on the exact designated step? Some of the info I've stumbled across suggests there is an acceptable range often set by the mfg. or end user. A bit like the tolerance set in any given tool path (loose for roughing and tight for finish cuts). I would be interested in hearing from others on this subject.

Sure, there will be some variation as to the exact angle that each step will span, however the variation will be very small, typically <+/- 0.005" at the airgap radius as an arc-length. If say, you have a 0.5" airgap that's <0.006° per micro-step of potential error. A good stator tooth has net profiles of <0.002" on both the rotor and stator, typically close to 0.001" average across large lots. That would be a fifth of that error per micro-step, physically in the motor. This ignores error in the control as well as the error introduced by friction torque, as the motor produces zero torque when exactly at step and there is always at least some torque, there will be a small alignment error due to that as well.

I'd be very curious to know if anyone has accurately measured actual step and micro-step error to provide this feedback on specific stepper products, but that's for another thread.

[mactec54]

I suppose if the manufacturer used a different diametral pitch on each pulley, that would be true... I'll confess that while I've never seen nor heard of such a thing, I've only designed splines and meshed gears, not toothed pulleys for timing belts, but given that the timing belt must have the same pitch as well to interface I'd be very much shocked. Backlash is an issue if it exists and unless he was only indexing to set locations without machining during rotation and could overshoot and return from the same direction to account for backlash, would not be something easily dealt with. Plus the manufacturing variation of the pitch from tooth to tooth would come into play as well, even with zero backlash. Just like the linear tolerance on ball screws.

I'll absolutely agree that the GT belts are a great system, it's all we use at work for actuation via belt systems.

The Timing Pulleys can be as much as +/- .030" some even more, so this can change the ratio, Teeth count is what will be the basic ratio, this type of Belt and Timing Pulley being use have backlash, so not great for indexing, if he loads it all in one direction you can over come some of these problems