Z-Axis Resolution

I'm pondering a belted Z-axis conversion for my Ender3 but I'm unsure of what max mm/step I should be targeting. I've seen designs that use worm gear setups ranging from 20:1 to 40:1. The mm/step for 20-tooth GT2 on 1.8deg stepper is 0.2mm if I'm not mistaken. Why does the resolution need to be so much higher for the Z than the X and Y axes?

Using a worm gear reducer in the Z axis isn't so much about increasing resolution (though it does that) as it is about preventing bed drop when Z motor power is cut. The gear reduction also increases lifting power of the motor at the expense of speed, but high speed isn't really needed in the Z axis, so it's a good trade off. You need to use high quality gears because any error in the worm gear surface will show up as a repeating error in the Z axis of your prints.

Really high quality prints may be made in layers that are 50 to 100 um thick, so those are good targets for the minimum resolution in Z, but if you don't print that fine (who has the time?) you can live with less resolution.

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Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Thanks for the response DD. I've read your blog on your less-than-successful attempt at a cheap replacement for the Rino and I learned a lot from it. I'm sourcing my gears from Misumi so I'm hoping for better results.

Does micro-stepping increase the resolution of an axis or is it best practice to calculate using the native stepper resolution?

Edited 1 time(s). Last edit at 09/19/2019 12:32PM by Rewas.

Microstepping and resolution is a complex topic- in general, with a motor that has a 5% step accuracy spec (typical), microstepping beyond 16:1 doesn't increase resolution (it depends on the load and the motor torque), but reduces noise and vibration.

It is considered good practice to ensure that the layer thickness you print is a whole number multiple of the full step distance in the Z axis. That means you want to select the pulley and gear ratio carefully. For example, if you use a 30:1 gear ratio on a 200 step per rev motor, it will take 6000 full steps to get one rev of the gearbox output shaft. If you put a 30 tooth, 2 mm pitch pulley on that shaft, the bed will move 60 mm. That means the whole step distance is 60/6000= 0.01 mm=10 um, so any print layer thickness that is a whole number multiple of 10 um should be good (which is probably any print layer thickness you're ever going to want to use). With that kind of full step resolution, the microstepping ratio you use doesn't really matter. Pick one that let's the axis move as fast as you want (10mm/sec is usually sufficient) and as quietly as you want (noise usually isn't a problem with the Z axis because of the way the Z axis is used in a printer). IRIC, UMMD's Z axis is 20 um/full step.

Let me know if the Misumi gears work out. I'd like to take another crack at making my own gearbox.

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Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I will let you know. I'm fairly confident that they will work as it is the same worm gear set that is used in the V-King CoreXY, which has been built successfully by quite a few people.

One thing that puzzles me about the "layer height = multiple of native step" (aka Magic Numbers) is the effect that baby-stepping to adjust the Z offset has on the layers landing on a full step. I have always adhered to that "rule" but it seems to me that there is no way to insure that the motor will stay on an even step. Thoughts?

It doesn't have to land on a full step position, just in a consistent offset from the full step position.

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Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]