Z axis motors lack torque

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Z axis motors lack torque

Postby geoffs » Tue Aug 05, 2014 5:58 am

New (delayed) build and I've struck a problem with the Z axis.

I had movement in all axes although Z was very slow. I've tidied the wiring up and now, instead of the Z axis motors turning, they just vibrate and squeal.
FIrst thing was to check the wiring, no porblem found. Swapping motors between drivers confirms this
I've swapped individual motors to X and Y drivers and all motors work OK on all drivers - problem only occurs when Z axis motors are paralleled.
New stepper driver makes no difference, existing stepper driver swapped from Z to X axis and works fine.
Adjusting current on stepper driver only changes the frequency/voume of the squeal.
To ensure it wasn't mechanical binding, I removed the couplers from both Z axis motors.

I then setup in firmware to mirror the Z axis using the spare extruder driver and Z axis works perfectly.
This is an acceptable solution as I don't envisage using a second extruder but I'd like to find the coause of the problem if possible

Electronics is Ramps 1.4 with 4988 drivers, Repetier firmware, testing with Pronterface and LCD controller. 12V 30A power supply
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Re: Z axis motors lack torque

Postby cvoinescu » Tue Aug 05, 2014 10:45 am

The cause is likely insufficient current for your Z motors. If they're low-impedance motors, the drivers can have trouble supplying what they need with two motors in parallel. What's the current rating of your Z motors?

One thing to try would be to wire them in series, rather than parallel.
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Re: Z axis motors lack torque

Postby geoffs » Tue Aug 05, 2014 12:55 pm

The motors are from ATI as supplied with the Hadron kit, model is KL17H247-168-4A
Current is listed at 1.68A, impedence at 2.8mH.
The mirroring is working OK now I've properly adjusted the current but I'll give them a try in serial mode. The troubleshooting I did yesterday has messed up the neat wiring so I have to redo it anyway.
I now have a problem with the X axis motor. It vibrating no matter what current I set. I had previously tried to remove the pulley (stuck fast) and may have put some strain on the bearings. If I can get the pulley off, I'll use the spare extruder motor as a replacement.
I did a test print (without extruder) and motion was quite smooth if a little slow (apart from the aformentioned X axis motor) Steppers were cool enough to touch after 30 minutes or so.
I'll try the test print again after re=slicing with Cura (Octoprint on a Raspberry Pi)
Last edited by geoffs on Wed Aug 06, 2014 12:06 am, edited 1 time in total.
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Re: Z axis motors lack torque

Postby cvoinescu » Tue Aug 05, 2014 3:53 pm

The speed has nothing to do with current or anything like that. A stepper motor either goes the speed it's commanded to, or doesn't. When it doesn't, you'll know -- usually it vibrates and doesn't move, or makes loud noises when it misses steps -- and, in any case, it loses position (it's no longer where it was commanded to be). It never just goes slower or faster.

So if your speed is not what you expect, but the movement is otherwise accurate and the position correct, look for the the cause on the software side: maybe you set the max speeds too low in firmware, maybe your slicer was configured with too low a speed, and so on.

As for your Z motors, normally, they should work in parallel on an Allegro A4988 driver.
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Re: Z axis motors lack torque

Postby geoffs » Thu Aug 07, 2014 8:58 am

Thanks for the serial suggestion,. It worked very well, I could find very little mention of it as an alternative.
Motor action was smooth and reliable. The stepper driver did get quite hot but nothing a small fan couldn't take care of.
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Re: Z axis motors lack torque

Postby cvoinescu » Thu Aug 07, 2014 11:50 am

You're welcome. And you're right about serial connection being rare, although I'm not quite sure why.
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Re: Z axis motors lack torque

Postby geoffs » Fri Aug 08, 2014 2:11 pm

Motion sorted, now on to the joys of making the QU-BD extruder work. ;)
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Re: Z axis motors lack torque

Postby TLHarrell » Fri Aug 08, 2014 4:13 pm

geoffs wrote:Motion sorted, now on to the joys of making the QU-BD extruder work. ;)


That sounds like much more hassle then the z axis.
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Re: Z axis motors lack torque

Postby cvoinescu » Fri Aug 08, 2014 7:26 pm

TLHarrell wrote:
geoffs wrote:Motion sorted, now on to the joys of making the QU-BD extruder work. ;)


That sounds like much more hassle then the z axis.

Don't.

Actually, you won't believe us until you try, so go ahead.

But after you sunk a few dozen hours, ask a friend to print the parts for a Greg's Wade Reloaded for you (yeah, ugly and big, but extremely reliable!), buy the hobbed bolt and other hardware for it on eBay (or buy a complete kit with the printed parts too), and get a genuine J-Head MK V-BV hot end (from hotends.com, not eBay). It'll cost more than twice as much as the QU-BD, and it'll look much less neat, but it'll work from the first attempt, and keep working. And you can reuse the stepper from the QU-BD!
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Re: Z axis motors lack torque

Postby geoffs » Fri Aug 08, 2014 11:34 pm

I have the parts to Bart's extruder modification and a Mk 7 drive gear so I might put some time into getting that to work.
If (when) that all fails, I can fall back on the printer at the hacker space to make a real extruder. :D
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