Choosing Pulleys

Something I just thought about.

Say Ive got:
Nema 17 Stepper, 1.8º, 200 Steps Per Revolution
GT2 Belts, 2mm Pitch

So, say I chose a 20 Tooth Pulley on the Motor
The math says that my resolution would be 12.5 Micron or 0.0125 mm Per Step

Am I correct in thinking that I'd only end up at accurate positioning if I only manually move or design parts with dimensions a multiple of 0.0125
I mean I realize that we're dealing with FDM machines here, and that even if it were that accurate in movement It'd be negated by multiple other factors, the shrinking of the plastic, belt elasticity, etc.

But, would it be better to use a 16 Tooth Pulley on the Motor?
The resolution would be 10 Micron or 0.01 mm

It reads like it would, theoretically.

I mean I don't ever design parts with dimensions like 10.0375. Nor do I 10.03 or 10.04. It wouldn't really ever matter other than just being 2 Microns more accurate and removing the fraction from the equation. Plus removing some microscopic if not invisible errors in the print.

Edited 1 time(s). Last edit at 01/04/2017 01:05PM by FA-MAS.

Actually it's 12.5 microns per 1/16 microstep. But if you use more than 1/16 microstepping, the motors very likely won't move on individual microsteps anyway.

On a delta printer where this is the Z axis resolution, I think it certainly makes sense to use 16 tooth pulleys instead of 20 tooth. Better still (or as well), use 0.9deg/step motors. On a Cartesian or CoreXY printer, I doubt that it makes any difference, because the XY resolution is less critical than the Z resolution, and the Z axis normally uses threaded rod or leadscrew drive.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Wouldn't there also be a torque vs speed compromise?

16t pulley have high(ish) torque vs low(ish) speed
20t pulley have high(ish) speed vs low(ish) torque?

Quote
friarfish
Wouldn't there also be a torque vs speed compromise?

16t pulley have high(ish) torque vs low(ish) speed
20t pulley have high(ish) speed vs low(ish) torque?

Yes; but the moving parts of delta printers are light weight, so torque is rarely in short supply.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

The idea of adjusting your CAD habits to match the printer isn't a bad one, though for 10s of microns in X and Y it isn't really very important because we're squirting liquid plastic out a nozzle. Where it can be important is the Z level positioning of features in your models. If you want an object to be 3.4 mm tall, that isn't going to work if you print in 250 um layers. Also, if you're in the habit of using an extra thick first layer, say 300 um and print the remaining layers at 200 um, 3.4 - 0.3 = 3.1, also not a multiple of 200 um.

For the last few years when I design parts I have been planning the print at 200 or 250 um layers (with no extra thick first layer) and trying to be careful about locating features at full layer boundaries. It isn't always possible, of course, but it helps, especially when multiple printed parts have to fit together or when prints have to fit with other objects. I don't know how the different slicers deal with the left overs- they may truncate the model or may add an extra layer. After you've been doing it for a while it becomes second nature and you don't really think about it much.

What would really be nice is if the CAD software people would set up a 3D printing mode where the XY grid can be set differently from the Z grid. That way you could set the Z grid to match the print layer thickness you plan to use and set XY grid any way you want, or maybe even to match the ustep size in the printer. Then just use the snap-to-grid and Bob's yer uncle.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]