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geared stepper motor for z-Axis? thoughts on step config

Posted by EljefedsecurIT 
geared stepper motor for z-Axis? thoughts on step config
June 29, 2015 12:48AM
Hi all!
I have an idea of using a NEMA 11 5:1 geared stepper motor for a z-axis belt and pulley concept for a large platform with a small motors that ride belts on the rails. the motor itself is a 11HS20, with 12Ncm torque, and 1.8 degrees/200steps/rev, and a 5.18:1 gear, that puts it at about .35 degress/step, or about 55.9Ncm (~4lb). that would give me in theory, 1028.571 steps/revolution, and at 1/6 microstepping, puts me right around 16457.14286 steps

now, if I did my math right, that would be some pretty high stepping, smiling smiley

I would like to ask if anyone could help me verify this? does this sound right? does Z-axis have a different set of math when using belts and pulleys vs. lead screws?

the design would depend on whether a pulley and looped belt would be more efficient, or a belt laid on the beam and having the motor ride the belt, which would align with my X & Y flat belts and motors riding along the rail.

would like some feedback on the concept, and if putting geared motors would affect how to put in the steps for z-axis, on say, repetier...
Re: geared stepper motor for z-Axis? thoughts on step config
June 29, 2015 07:47AM
I'm using a similar system but with kevlar string (on a 12 mm drum) rather than belts and I'm getting 430 steps per mm - a toothed belt pulley should be a similar diameter so I'd expect a similar value.

My build is based on this Sli3er

I'm quite pleased with the Z axis resolution - I'm still dialling in settings but it looks very even so far.
Re: geared stepper motor for z-Axis? thoughts on step config
June 29, 2015 08:10AM
The math is the same.

If I understand what you said, you're going to suspend the build plate using cords or belts and you're going to drive them using a geared motor. Lash in the gearbox might be a problem. You're relying on gravity to drop the bed as opposed to screw threads which physically engage nuts to move it up and down. That means if there is any misalignment that causes anything to bind you're going to have a problem.

When choosing a motor for any application, you calculate the required torque, then pick a motor to deliver that torque, then you select the electronics to power/drive the motor. You're starting with a motor (and probably electronics) and hoping it will work. Before you invest too much time and energy into this, calculate the torque required and see if the motor you've already selected will be able to do the job. Try the motor sizing tools page at the Oriental Motor web site.

The Z axis is normally pretty slow when driven by the typical screws. You may be making it slower with the gear drive- it will depend on the size of the pulleys you'll be using.
Re: geared stepper motor for z-Axis? thoughts on step config
June 29, 2015 03:37PM
fatfredie: is that using a geared motor?

the_digital_dentist, I agree in principle what you are saying.... I was looking at the belt & pulley system specifically because of the success I had building a delta printer, and how well it retains it's position even after powering off; I imagine that the tension on the belt and the tension on the carriages hugging the bars add to it's ability to hold it's position. that build use 60oz/in+ nema 17 steppers, so they were a bit of an overkill, but they never skip smiling smiley

the concept is to have the motor mounted to the bracket. the bracket is a custom design, that holds a 15x15 openbeam horizontally. this bracket rides vertically on another openbeam, using nylon oval roller bearing wheels. there would be a duplicate setup for the left/right side of the build plate. so, imagine something like a mendel or a coreXY, in terms of an 260mmx260mm plate, being held up by 2 beams that are cross-beamed, etc. totaling about 2lb total. add to it a build weight of another 2 lb of plastic, it should put me at about 5LB.

given that I am attempting to get into motors as compact as possible, I decided that NEMA11 motors would suffice for my X & Y torque aren't anywhere near stressed... but I really did not want to have to go with 2 NEMA 11's and a nema 17 for Z and the extruder sad smiley. OK on the extruder because, well that's been about as standardized as you can get and I'm not fighting that battle right now...

so, in looking for a small FF motors, the largest NEMA 11 could only pull 17oz/in. This is why I was looking at a geared nema 11, which would increase the torque, and the planetary gearbox lash would be at most .0018deg/step; not enough to warrant a major rethink in this design for z-height at such fine points... I would rather not have to settle and compromise and get a larger FF motor, but again I'm compromising on standardizing on NEMA 11 and NEMA 11 geared, which isn't consistent either, I know, but:

with such micro-micro- stepping, I'm not so much afraid of slippage; I'm more worried that the geared motor can actually handle being x16 micro-stepped, if even at all! The reason for that is, the firmware (repetier) and board (alligator board) I am using only have steps for 16 & 32 coded for it, respectively; I don't believe I can step it down any further without recoding the firmware... not that I couldn't but I'd rather not have to backport that into the repetier firmware. so I'm trying to figure out if I tune it to x16 microstepping, do the numbers sound about right?
Re: geared stepper motor for z-Axis? thoughts on step config
June 29, 2015 04:36PM
I suspect my Z axis will be a little heavier than yours - 1.25KG (2 3/4 lbs) for the plate itself plus some 20x20 extrusion for the support + 2 heaters and a plywood base. I'm using a geared nema17 stepper (http://e3d-online.com/Electrical/Stepper-Motors/Geared-stepper-motor) and it's close to the maximum weight the stepper will drive reliably - I'm thinking of counterbalancing it or swapping the 220x420x5mm aluminium sheet for something lighter.

There should be no problem microstepping the geared motor - it's just a standard stepper with a gearbox on it and you don't (usually) need high speeds from the Z axis.
Re: geared stepper motor for z-Axis? thoughts on step config
June 29, 2015 07:49PM
yea, my design doesn't account for a plywood base, just aluminum 260mmx260mmx3mm plate, a standard 220mm x220mm PCB mk2B heatbed, and about linear 700mm of 15x15 extrusion, so it is fairly lightweight. the big difference might be for me whether I put the motor on the frame and pull the belt, or mount the belt and pull the motor with the bed with the bracket. I wonder which would be easier on the motor design; whether I mount the motor to the bottom part of the frame like a delta pulley, or have it roll up and down like my X&Y will do horizontally, on the bracket along a stationary belt. I suspect either way, the gearing and the tension from the rollers should be able to hold it steady and stable on power down...or at least let it down gently, what happens when yours is not powered on? does it fall to the bottom, or is there some other way it holds it? are the gears good enough for holding it when they power down?

I've thought about counter-weights as well, but it doesn't scream "simple" to me anymore...as much as going to a bigger motor, or rethinking the stepper motor size or a the next step up on gear ratios. like you said,Z doesn't have to be fast, just accuratesmiling smiley

edit: although I *have* thought about a scissor lift with tension springs to complement the z-axis motor. it was a mental proof of concept, but I was mostly thinking about using that for stabilizing the outer edges using a hidden lead screw under the z-plate rather than to offset part of the weight during lift, and also possibly mitigate a crash scenario. but I couldn't figure out how bad the math would be for a scissor lift. maybe just using it with springs to put enough tension to offset some of the weight might be worth exploring if it still turns out too heavy....ideally something one could buy in home depot, right? winking smiley

Edited 1 time(s). Last edit at 06/29/2015 08:05PM by EljefedsecurIT.
Re: geared stepper motor for z-Axis? thoughts on step config
June 29, 2015 08:26PM
I've never seen a scissor lift that didn't wobble.
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