Z axis still missing steps, or another problem?

Despite reverting to the original gear which revolves smoothly (see thread about stuttering), I still have a problem with the Z axis not moving far enough between layers. I can see that the Z axis is moving between layers, but the work is coming out under-height.

I believe the M5 rod has a thread pitch of 0.5mm, and it looks like the z driving and driven gears have equal numbers of teeth. I am using the default layer height of 0.24mm, and the web interface is indeed increasing the reported Z position in steps of 0.24mm. So I should see very nearly one revolution of the stepper motor for every 2 layers. But I am seeing just under 1 revolution every 3 layers.

Has anyone else seen this?

I am tempted to increase the Z motor drive current and/or reduce the Z axis max feed rate and max acceleration.

EDIT: (1) I just abandoned that print. Web interface was saying Z=27.36, actual height of piece reached was 17mm.

(2) I just found that the flat in the z-driving gear isn't engaging securely with the flat on the motor. I can rotate the driving gear a whole tooth without the stepper shaft moving. This will screw up the bed compensation. So I think I need to make a new driving gear as well .

Edited 1 time(s). Last edit at 12/30/2013 06:04PM by dc42.

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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].

Hi dc42

I had someone else report that the z-gear could move, but I hadn't had a chance to test. I always felt it was pretty tight, and not under much load to move, but I could be wrong. I would be really surprised if it could rotate all the way around. To test, hold the motor shaft with a pair of pliers or adjustable wrench, and try and turn the z-gear. I can just force mine to move, but only a mm not all the way around, so I don't think it's significant. It's possible that the flat on the z-gear wasn't lined up - it's not the full length of the motor shaft. I'll bring it up at our New Year meeting, to see if the flat needs to be a bit bigger.

Have a look at your motor wiring, check it's well-seated. You can check the resistance of the coils to test; with the power off, you should be able to test them in situ on the board. Black/green and blue/red are the pairs, the resistance should be around 2.2ohms. We've also had one or two motors (out of 2500) that have damage to their pins from factory testing, and occasional dodgy soldering of the connectors inside the motor - they are very easy to take apart, just remember the order of the washers and bearings.

Otherwise, I guess we're looking at a stepper driver/board fault.

Also, M5 has a pitch of 0.8mm, which is why the axis has 4000 steps per mm; (full steps per rotation x microstepping)/pitch = (200 x 16) / 0.8 = 4000

Ian
RepRapPro tech support

Hi Ian,

My observations on stepper rotations make more sense if it is 0.8mm pitch, since 3 layers come to 0.75mm. So it looks like my stepper is moving correctly, i.e. just under 1 rev per 3 layers. I've increased the current to 1000mA and decreased the z-acceleration and max speed as a precaution.

But something is going wrong at least some of the time, because my current print is coming out under-height, and the actual height is less than the height reported by the web interface.

btw, I think I found a bug in the firmware. If I use M201 and M203 commands to specify just the Z values, then the X axis stops working. I changed it to specify all 4 values, and that problem went away. I took a quick glance at the firmware source code and I didn't see any reason for that.

Edited 1 time(s). Last edit at 12/31/2013 03:48AM by dc42.

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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].

Hi All

What about drilling and tapping the cog for a metal grub screw. This should provide a more positive fixing after all any backlash is undesirable.

Rory

Quote
Rory166
Hi All

What about drilling and tapping the cog for a metal grub screw. This should provide a more positive fixing after all any backlash is undesirable.

Rory

Yes, that would be more satisfactory. I think the backlash was the reason for the incorrect layer heights. With z-stepper current increased and accelerations and max movement rates reduced, the problem has almost gone away (web interface says 132mm at the end of a print job, ruler says about 127mm). But I noticed that when the z-axis moved between layers, sometimes I heard it as a smooth movement, other times there was a clunking sound. I think this difference depended on the bed position at which the z-axis movement took place, and hence the amount of bed compensation that the firmware had applied at that point. If there was no backlash to take up, the z-axis moved smoothly. If there is backlash to take up, then some of the stepper acceleration is done while the backlash is being taken up, so the stepper is rotating at quite a speed when the driven gear starts rotating. Hence the clinking noise, and probably missed steps.

Right now I'm having problems printing replacement gears, because every part I print is too tight. I think my print must be swelling slightly. I'm going to try reducing the filament feed rate a little - unless anyone has a better idea?

Edited 1 time(s). Last edit at 12/31/2013 03:31AM by dc42.

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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].

How are you generating your Gcode ?

Slice3r should be ok but it seems that if the gcode has X+Y+Z moves in it the firmware gets it wrong

try:

G28
G1 X100 Y100 Z100

Get pos says the Head is at X100,Y100,Z100 , but if you look at where it is it definatly is not
at 100Z !!

Quote
Rory166
Hi All

What about drilling and tapping the cog for a metal grub screw. This should provide a more positive fixing after all any backlash is undesirable.

Rory

I find it hard to believe that there would be any backlash there, it was a very tight fit, I was very impressed with the build quality, I would say 2-3kg, I did the press-on in a vise and supported the ND side, not on the inner ring of the bearing, on the end of the axel directly

When the fit is that tight its paramount to support the axel end, the inner mechanics are not meant to take that kind of stress - which brings me to my point, could the "missing steps" simply be a defect in the motor from mounting the gear?, could the rotor have moved on the axel? - it would surely reduce the momentum

If yours was a 2-3 kg press fit like mine, how did you all mount the gear? - did you press it on in a vise and did you support the axe? - or did you use a hammer?

Should be quite easy to test if the motor still have the 4000g.cm momentum

Erik

What happened is that my z-driven gear was off-centre, so I printed a new part. The new part was a tighter fit with the z-gear than the old, but I thought it was OK and would loosen up. In fact it became tighter, to the extent that at one point in its rotation, it was quite stiff to rotate. I think that is what caused the wear on the flat on the z-gear, leading to the backlash.

I reinstated the old wobbly z-driven gear and installed a newly-printed z-gear, and the problem went away.

Thanks for all your suggestions!

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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].

Re the z drive gear being off centre, how about making a part with three screws at 120 degree angles pointing towards the nuts. Tighten the screws till the rod is centred then fill with epoxy?

(not tested!)

regards
Andy

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Ormerod #318
www.zoomworks.org - Free and Open Source Stuff

Quote Ormerod168
"When the fit is that tight its paramount to support the axel end, the inner mechanics are not meant to take that kind of stress - which brings me to my point, could the "missing steps" simply be a defect in the motor from mounting the gear?, could the rotor have moved on the axel? - it would surely reduce the momentum

If yours was a 2-3 kg press fit like mine, how did you all mount the gear? - did you press it on in a vise and did you support the axe? - or did you use a hammer?"

This advice from ormerod168 sounds important to me I will put it in the suggestions thread for instructions.

Rory

Edited 1 time(s). Last edit at 01/04/2014 08:35AM by Rory166.

If you take the stepper motor apart (as I have a few times), all that is inside is a spring washer, on the end of the motor shaft. The shaft sits in bearings, so the spring washer is just there to reduce end float. It seems to cope with being flattened, and springs back. While it is sensible to support the back of the motor shaft (I think an M4 nyloc or M5 nut is perfect size), the forces we are dealing with, even the stiff printed pulleys, shouldn't cause it many problems. In fact, it's quite useful - if you push it on all the way, the spring washer compresses, and you can't push the pulley right up to the face of the motor, so there is always a gap to get it back off. Just don't hit it with a hammer, or maintain a large pressure on it for a long period of time.

Ian
RepRapPro tech support