Stepper "hiccup"

Twice now I've had a long print kaputted by the Y stepper doing a burst of skips.
Almost the worst sort of problem to diagnose - extreme intermittency...

Both times (separate nights) I had been printing smallish items for a couple of hours, then I hear the dreaded thump-thump-thump-thump of a jam/skip.
The head was in the middle of the print area - it lost 7 or 8 mm in the Y direction, then it had carried on as though nothing had happened - except it was extruding partially into empty air...
After clearing the rubble and starting again - worked flawlessly for another hour or so to finish the job...
I'm fairly certain I've got the Pololu current right, and again, it's right enough so that 99% of the time it works fine...

My setup: bog standard lm8uu Prusa Mendell, T5 belts/printed pulleys, Greg's extruder, 3mm PLA, cold glass bed + 2090 blue tape
HotEnd: PTFE thermal barrier, Brass bolt 3mm -> acorn nut .5mm, nichrome heater.
Slic3r, Repetier host, Repetier firmware. layer height 0.3mm, perims 20mm/s, fill 30 mm/s

What was I printing.....
well - conversation today getting an odd look: "Why are you looking tired Neil" - "Well boss, I was up late printing Alligators".

Edited 1 time(s). Last edit at 02/03/2012 12:58AM by nb99.

... could be dust (friction) on the rods and/or the stepper slightly underpowered ...

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Viktor
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Pulley loose on shaft? Belt rubbing/catching on something?

If the pulleys are in PLA, they could soften and get loose due to heat from the stepper shaft during long time printing.

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-Nudel
Blog with RepRap Comic

Make sure you don't have your end-stop wires running in parallel with your motor wires, keep them as separated as possible.

Bump up the motor current a tiny bit on Y and try again.

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[richrap.blogspot.com]

- stepper power : it seems to have plenty of torque - if I grab the bed, it takes quite a bit of force to make the stepper skip..

- belt does rub against the Y-idler fender-washer, but as it prints fine for extended periods I'm not sure that would be it

- I tightened the pully yesterday, again I'd wonder that that would give a 1-off slip during some hours of printing - but I take the point about heat buildup on a PLA pulley - I'll examine that further...

- End stop wires are twisted pair which I have kept well away from other wire, and as they have been an audible "several beat" skip which sounded like when a carriage runs into the wrong end, I wnder if a false end-stop trigger would do that?

Thanks folks, I will check out all those points, as it probably will end up being something I thought it couldn't be - must be some sort of corollary to murphy's law!

It could also be the driver overheating and cutting out momentarily or a bad connection at the motor plug.

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[www.hydraraptor.blogspot.com]

>>cutting out momentarily
OK - although when my Z-driver had too much current, and was doing that, there were brief *absences* of any stepper action as opposed to the loud thump-thump I have noticed when this one happens, and the Z one happened in little fits and starts (well, stops) rather than one isolated and elongated burst of audible "stepper bounces" (a bit over 1/2 a second of them)
Ditto with connector failure, presumably would cause a brief silence rather than forceful activity.

(but I shall go over all the plugs and check the temp on all the Pololus. I wonder if it would be worthwhile hanging one of the wee 40mm fans over the Pololus)

Unfortunately, whatever I try adjusting, there is no guarantee the effect will occur on any given night's printing - it's that intermittent..

Hopefully if I tune up all the mentioned points, it will just go away...

ta
nb

Could be the LM bearings seizing. I had one that would only move one way until I put a drop of oil on the rods.

It could also be the belt riding down the gap between the fender washer and the bearing and getting jammed. The belt wanders about so I imagine it could happen intermittently.

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[www.hydraraptor.blogspot.com]