More questions about Z axis drive

Just wanted to let anyone who commented in the thread titled "Questions about Z axis drive" know that that I appear to have all three axis on my printer working more or less OK now.. I made changes so that the Z axis has a stepper motor with a 16 tooth pulley turning a jackshaft with a 40 tooth pulley.. Then a 1 to 1 pulley arrangement drives the three acme screws that raise and lower the bed... I also set the optioning so that the stepper uses full steps instead of 1/16 steps.. This gives me about 250 steps per mm. the bed now moves up and down nicely (smoothly and quietly) even with a bit of weight on it. But.....

The next thing I wonder about this.. I have optical endstops and they appear to be working properly... I am currently set to "home" the Z axis in the negative direction (minimum distance to the nozzle). I am using the latest version of Marlin (downloaded it a couple days ago from github) and Pronterface for testing... If I tell the printer to "home" the Z axis, the bed moves up smoothly and quietly (and fairly quickly) until the bed triggers the end stop... Then the bed will then retract a couple mm and then move upwards again until the endstop is triggered again. This time the bed does not move as quickly... I assume this is due to of a routine that is in place to accurately stop the bed at the right place without overshooting... The problem is while the bed is moving slowly this second time, the stepper motor makes a god-awful racket.
The sound is similar to the sound made when the bed jams and the motor skips steps...but the bed does move.. When I command it to move only a tenth of a mm or two it does without problem. I was wondering what is going on...

I believe that all three axis are working good enough that I could do printing but this is bugging me..Once I get this figured out I can start getting the hotbed working and the extruder to properly squirt plastic.

Bobby

Why did you switch from 1/16 microsteps to full steps? You need microstepping to get smooth motion, especially at low speeds.

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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].

You can adjust the speed of that final move on homing. I forgot where I saw it in the config now.
Homing 'bump' is it? Something like that.
In most recent Marlin it has been slowed right down.
It is quite noisy on 1/16 stepping. Full steps must sound rough.

a

Edited 1 time(s). Last edit at 04/01/2015 03:25PM by alan richard.

RE: Why did you switch from 1/16 microsteps to full steps? You need microstepping to get smooth motion,

As I understand it, you get more torque at full steps.. And smoothness isn't as big a problem for the z axis because when printing it will only move a small amount between layers then stop until that layer is done.. With the setup I have, it would have taken 4000 1/16 microsteps to move the bed 1 mm... For full steps it still takes 250 to go 1 mm ... or 25 steps to move .1 mm... I would think that resolution would be fine ..
The X and Y axis are still set to 1/16 microstep and I think this is where the smoothness is needed..

Please let me know where and why my reasoning would be wrong

RE: It is quite noisy on 1/16 stepping. Full steps must sound rough.

It's not bad at all.... When it is moving a distance up or down you hear a soft whirr. It is had to tell just how noisey it is when moving only small amount because the noise doesn't last long but it doesn't seem to be loud then... That is why I was asking about the noise.... It only sounds awful when doing the "homing bump" .

Bobby

The "homing bump" is because the stepper motor is moving at low speed without microstepping, possibly also exciting a resonance. Enabling microstepping will almost certainly fix it. It's not unusual to have 4000 microsteps/mm on the Z axis, that's exactly what my Cartesian printer uses. Don't worry about the torque, a stepper motor geared down that much should be capable of producing sufficient torque even with microstepping.

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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].

Just wanted to let you all know I was working on my printer some again today.. I add a strap so the z axis is 1/4 stepping instead of full stepping and re-flashed to 1000 steps per mm.. This did quiet the homing bump down a lot.. Enough that while I might change it all the way back to 1/16th steps, I am not going to worry about it now.. I am going to move on trying to get my extruder and heat bed working properly..

Thanks!

Quote
Bobby-S
RE: Why did you switch from 1/16 microsteps to full steps? You need microstepping to get smooth motion,

As I understand it, you get more torque at full steps.. And smoothness isn't as big a problem for the z axis because when printing it will only move a small amount between layers then stop until that layer is done.. With the setup I have, it would have taken 4000 1/16 microsteps to move the bed 1 mm... For full steps it still takes 250 to go 1 mm ... or 25 steps to move .1 mm... I would think that resolution would be fine ..
The X and Y axis are still set to 1/16 microstep and I think this is where the smoothness is needed..

Please let me know where and why my reasoning would be wrong ...

You don't get more overall torque at all whatever the microstepping mode you choose. It's the incremental torque that decreases as you increase the number of microsteps: roughly, double the number of microsteps steps and you halve the incremental torque. The incremental torque just affects exactly on which microstep your motor will stop (i.e. microstep precision), it does not affect the overall torque at all.
It does not matter whether you are talking about the X, Y or Z axis: more microsteps means more accurate and smoother motion, less vibrations and resonances, and less "roughness", and there is no loss in overall torque (I am simplifying things a little bit, see [www.micromo.com] for details).

Summarizing: whenever possible, go for the maximum number of microsteps your driver supports.

Edited 1 time(s). Last edit at 04/06/2015 03:38PM by AndrewBCN.