Extruder motor skipping steps

I have a RepRapPro Fisher from around November 2015. It works fine but I have a very strange anomaly I can't really wrap my head around, and it's the skipping of the extruder drive. The printer is completely stock, so the drive is a geared one, driven by a Nema17 and the Duet electronics.

The nature of the skipping is weird. It usually starts mid-print, after 15-20 minutes of printing. There are times when the skipping starts already on the first layer but with empirical methods I found out that if I simply do not let the filament cool down inside the head, the early skipping (on the first layer) can be avoided.

Specs:

• Speed: I am printing rather slow, at 30mm/s on perimeters and 60mm/s for infills
• Current limit: Default Duet current limit of the Duet (which is running factory firmware), so it is at 1A.
• Filament: I tried all sorts of (non-Chinese) PLA filaments, also the one I got with the Fisher. It starts skipping after a while for each of them
• Temperature: 210C. I use these very same filaments on a Huxley of mine and I can print at 190C there.

What could it be that makes the Fisher start skipping steps mid-print after 15-20 minutes or so? If the skipping starts, no filament is getting through at all. The backskips of the motor are rather large, like 1/4 revolutions, but I think it's just because of the bowden. If the extruder is skipping and I apply some force on the filament by hand (push it in gently), I can "help" the extruder drive pretty easily. The whole drive feels sort of weak. When it is in the state of skipping repeatedly, it does not take a lot of force to rotate it even backwards if I want to.

What I am really interested in is what needs to be replaced, as this is not normal. Is it the motor or the electronics? How can I figure it out?

I helped someone else with a similar problem this afternoon. These are the steps we took:

1. The original filament (supplied with the machine I am told) didn't extrude cleanly, but formed a glob attached to the nozzle. We switched to a roll of eSun PLA I had.

2. Dismantled the hot end, unscrewed the nozzle from the heatsink, then screwed it back in but with thermal transfer compound on the nozzle threads this time.

3. Reassembled the hot end without the Bowden tube. Checked that we could push filament through the hot end when it was hot.

4. With the Bowden tube connected, the extruder drive could mostly extrude into free air at 5mm/sec, but occasionally skipped steps. We increased the motor current to 1200mA and the temperature to 210C, then it skipped steps only very occasionally.

5. With power off, swapped the E0 and X motor cables over on the Duet, then tested the printer after sending M569 commands to account for the swap. This was to check that there wasn't a fault on the E0 driver chip.

6. After experiencing a period of time when the extruder motor just vibrated instead of moving, checked all the connections in the extruder motor cable and re-made some.

[EDIT] 6a. Did several cycles of heat to 200C, extrude, cool to 100C, retract filament completely and cut off the end, to remove any foreign matter from the nozzle.

7. Did a 35 minute test print, initially at 1200mA current and 210C. It went well (no skipped steps noticed), so part way through we dropped the temperature to 205C. It continued to print well.

My conclusion is that the Fisher 1.0 extruder is slightly under-powered. It would be better with a higher torque motor or increased gearing. It's OK at 50mm/sec print speed, but can't consistently manage the ~5mm/sec rate needed for 100mm/sec print speed.

Edited 3 time(s). Last edit at 03/19/2016 07:12PM 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].

Dave

Would just like to Thank you for taking the time to help the Guy out he has been having issues for quite a while with his fisher and it is good to know that he is now happy that it is working.

Once again you have shown great support to the community even to giving up a Saturday afternoon to help someone in person for that I commend you?

Doug

Quote
dc42
I helped someone else with a similar problem this afternoon. These are the steps we took:

1. The original filament (supplied with the machine I am told) didn't extrude cleanly, but formed a glob attached to the nozzle. We switched to a roll of eSun PLA I had.

2. Dismantled the hot end, unscrewed the nozzle from the heatsink, then screwed it back in but with thermal transfer compound on the nozzle threads this time.

3. Reassembled the hot end without the Bowden tube. Checked that we could push filament through the hot end when it was hot.

4. With the Bowden tube connected, the extruder drive could mostly extrude into free air at 5mm/sec, but occasionally skipped steps. We increased the motor current to 1200mA and the temperature to 210C, then it skipped steps only very occasionally.

5. With power off, swapped the E0 and X motor cables over on the Duet, then tested the printer after sending M569 commands to account for the swap. This was to check that there wasn't a fault on the E0 driver chip.

6. After experiencing a period of time when the extruder motor just vibrated instead of moving, checked all the connections in the extruder motor cable and re-made some.

[EDIT] 6a. Did several cycles of heat to 200C, extrude, cool to 100C, retract filament completely and cut off the end, to remove any foreign matter from the nozzle.

7. Did a 35 minute test print, initially at 1200mA current and 210C. It went well (no skipped steps noticed), so part way through we dropped the temperature to 205C. It continued to print well.

My conclusion is that the Fisher 1.0 extruder is slightly under-powered. It would be better with a higher torque motor or increased gearing. It's OK at 50mm/sec print speed, but can't consistently manage the ~5mm/sec rate needed for 100mm/sec print speed.

Thank you again Dave for all your help yesterday! You have ended a three month struggle of frustration in five valuable hours of your weekend. As Doug said, incredibly kind of you to give up your afternoon to help me out, a complete stranger no less! Thank you!

Thanks for the suggestions! I'll start by increasing the current to 1200mA and if it doesn't help, I'll take apart the hot end. When you took the hot end apart, did you find the reason why it might be more difficult for the motor to push the PLA through if the material has been left in the hot end overnight?

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tml
Thanks for the suggestions! I'll start by increasing the current to 1200mA and if it doesn't help, I'll take apart the hot end. When you took the hot end apart, did you find the reason why it might be more difficult for the motor to push the PLA through if the material has been left in the hot end overnight?

If the filament wasn't retracted and/or the printer was powered down before the hot end had cooled to below 40, the filament could have expanded up into the heatsync part of the nozzle and therefore isn't melting properly now it's heating up

nice one dc ive been following that fisher with doug on another forum great to see its sorted

Hi DC,

How's tricks?
I was wondering if you could remember the supplier and model of the motors we were looking at as a potential upgrade?

The one I was probably suggesting is hard to get at present. This one [www.ebay.co.uk] may be a good choice if it fits in the available space. It is about 14mm longer than the one you have at present.

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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].

Great, thanks again!