Author Topic: Weird problem with one axis only  (Read 1702 times)

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Offline anzacinexile

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Weird problem with one axis only
« on: April 17, 2017, 11:23:11 AM »
I hope that someone can shed some light on a peculiar problem I have with a new build CNC Mill.

First the details

Running ESS, plugin version ESS_Mach3_160630_u4f1b
CNC4PC C25S BOB
Mach3 version 3.043.062
Longs motors 34HS5435C-O2B2
DM860A motor drivers set with 5000 steps / rev
4 core CPU Windows 10 with 16GB of memory

The problem

After carefully setting up the system X and Y axis are working fine and are very accurate however, Z is causing me real problems. The motor is completely isolated from the machine so there are no mechanical issues. I have even swapped the complete drive chain Z for Y and the problem remains with Z so I know there’s no issues with the ESS, BOB, power supplies or ESS configuration.

The problem is Z will run flat out in both directions no problem. It will run feeds up to 500mm/min and 1000mm/min up to max speed without issues HOWEVER, between 600 and 900mm/min the motor stalls with a horrible scream.

As I said, the motor is on the bench and with swapping axis, the problem remains with the Z motor. The only other items which might have a bearing is that with very slow feeds (1mm/min up to 100mm/min) the motor sounds very rough. I even tried an experiment where I set an extremely slow motor acceleration and on ramping up to maximum speed, the motor seemed to go through very rough bands until it got to 100mm/min. From 100 to 500mm/min it smoothed out until it got to about 600mm/min and the motor promptly stalled and screamed until a commanded speed of about 1000mm/min whereupon it took off without any further issues up to maximum speed.

I’m beginning to suspect the motor (and yes, I have checked the wiring)

Thoughts would be appreciated.

Thanks
« Last Edit: April 17, 2017, 11:33:04 AM by anzacinexile »

Offline Tweakie.CNC

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Re: Weird problem with one axis only
« Reply #1 on: April 18, 2017, 03:07:58 AM »
Quote
The problem is Z will run flat out in both directions no problem. It will run feeds up to 500mm/min and 1000mm/min up to max speed without issues HOWEVER, between 600 and 900mm/min the motor stalls with a horrible scream.

This problem is typical of 'mid-band resonance'.

Some stepper motor drivers (typically Gecko http://www.geckodrive.com/support/step-motor-basics/mid-band-instability.html ) incorporate a system that more or less overcomes the mid-band problem but there is the alternative of fitting a dynamic balancing device known as a damper or dampner. These devices are quite light-weight and rattle - they incorporate small internal weights which move when the stepper hesitates (just before the stall) and in so doing changes its mass and thus prevents the stall. I have made my own dampners which I use on a couple of steppers that I usually run at around 2000 rpm and they really do work well.
Just one tip, avoid the heavy ‘puck’ type solution – it changes the mass alright but can cause other problems (don’t ask me how I know that).

Hope this helps.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline anzacinexile

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Re: Weird problem with one axis only
« Reply #2 on: April 18, 2017, 05:21:13 AM »
This sounds interesting Tweakie, the link sounds exactly like the symptoms I'm seeing.
Can you get these dampers commercially or do I have to resort to more metal bashing?
Thanks for the reply, my head hurts from banging it against a wall!!!!

Offline Tweakie.CNC

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Re: Weird problem with one axis only
« Reply #3 on: April 18, 2017, 06:08:38 AM »
Stop banging the head - everything is simple once we know the answer.  ;D

This thread shows a pic of the ones I made and the link to the Zone should get you started. http://www.machsupport.com/forum/index.php/topic,17670.0.html

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline anzacinexile

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Re: Weird problem with one axis only
« Reply #4 on: April 18, 2017, 06:13:01 AM »
Cheers buddy, much appreciated