Caculating current needed for extruder stepper motor?

goldentuna · 19 Dec 2016, 16:46

I just wanted to confirm I was figuring the current correctly for my extruder stepper motors.
Based on the wiki article: https://duet3d.com/wiki/Choosing_stepper_motors

I figure that my current should be ~ 850ma for this stepper motor: (short body Nema 17) https://goo.gl/Pu8xLX
Rated Current/phase 1.0A

1.0 * 85% = 850ma ??

Am I figuring this correctly?

And since I have 2 extruders/motors my config.g would be like:
M906 X800 Y800 Z800 E850:850 ; Set motor currents (mA)

dc42 · 19 Dec 2016, 20:34

The optimum current for an extruder motor is high enough to push the filament through the hot nozzle, but low enough so that if the nozzle is temporarily obstructed, the motor skips steps instead of grinding a groove into the filament. 85% of rated current is a reasonable maximum value so as not to heat up the motors much.

deckingman · 19 Dec 2016, 20:46

This kind of got me thinking. If the nozzle gets partially blocked and the extruder starts grinding the filament, it's kind of obvious that something is amiss whereas missed steps may not be so obvious. Is there any way that the firmware could detect missed steps and flag something up on DWC that something is amiss?

dc42 · 19 Dec 2016, 21:04

On my Ormerod extruders (which is also the design I use on my Delta), it's very obvious when the extruder motor skips steps. I can hear it and see the gears spin backwards.

If the extruder steps/mm is high enough then the stall detection built into the drivers on the Duet WiFi could be used to detect skipped steps.

deckingman · 19 Dec 2016, 22:30

Are skipped steps always accompanied by the loud(ish) clicking noise? Sorry if that sounds like a daft question but I've only once had a significant hot end blockage so not much experience of what the manifestation of skipped steps is like.

T3P3Tony · 20 Dec 2016, 00:25

It depends on the extruder, the clicking on our old extruders is pronounced but it's not that pronounced on a Titan for example

DjDemonD · 20 Dec 2016, 08:51

I think you can usually hear it, sure its more obvious on a Wade's extruder but I can hear it on the titan on the rare occasion when something jams. It should never grind the filament, if it does the pinch wheel/idler is not tight enough.

T3P3Tony · 20 Dec 2016, 10:35

Its not just if its not tight enough - the idler can be set at the right tension and still grind filament if the extruder current is set high enough.

deckingman · 20 Dec 2016, 11:50

@T3P3Tony:

Its not just if its not tight enough - the idler can be set at the right tension and still grind filament if the extruder current is set high enough.

I can vouch for that!

Dougal1957 · 20 Dec 2016, 14:05

And so can I.

goldentuna · 21 Dec 2016, 07:07

It turns out - and hence my OP question - that one of my Titan extruders was in fact skipping. I thought at first the motor wasn't strong enough .. or that there was a clog in the nozzle - or that .. well.. 1/2 a dozen other reasons I was checking.

As it was .. I just didn't have the current set correctly. It was set at 400mA (it probably came from a copy/paste doing the config) .. and it really should have been 850mA

Once I made that adjustment - so far, it's been working great.

Thanks for everyone's input and help.