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Prusa getting off

Posted by kos 
Prusa getting off
January 02, 2014 02:42AM

Just built Prusa I3 from this kit and now I'm trying to print 0.5mm thin wall. I'm getting a strange behavior: sometimes motors stop shortly and then continue going, but the model is now off a bit, like if part of the movement was skipped.

Here's what first few layers look like:

Video of the process (z height is a bit off here too): [youtu.be] ,check out how it drifts more and more at 1:00

I first noticed it when calibrating the extruder. Procedure was to extrude 100mm of the rod and measure actual length. Results were consistent unless stepper made one or more small stops in which case a few mm were skipped. It was looking like an overheating problem because I was getting more and more skips over time and they disappeared if I let it to cool down a bit. I however couldn't find any hot parts by touching them, and printer was fresh during test print above.

Motors are cold, stepper drivers are cold. Steppers Vref=0.6. Here are my changes to Marlin_v1

any help appreciated
Re: Prusa getting off
January 02, 2014 05:33AM
It looks to me like something is binding.
Re: Prusa getting off
January 02, 2014 07:17AM
is it possible the belts are a bit tight or the pulley's lose.
Re: Prusa getting off
January 02, 2014 09:04AM
Might be running the steppers too fast and they are locking up at times. I had the same issue until I realized the homing speed is mm/minute and the max speed is mm/sec. Big difference when you set them to the same number...
Re: Prusa getting off
January 03, 2014 01:26AM
Thanks everyone

I was actually wrong, this is overheating, Y axis driver gets particularly hot very fast.
Reducing acceleration and feed rate helped a bit. Installing heatsinks helped a lot, but it still overheats after a couple minutes. Trying to decrease Vref now, but it doesn't look right to me..
Re: Prusa getting off
January 04, 2014 03:04PM
The pauses might be due to I/O lag in the USB if you'e printing directly from the host instead of from an SD card. I used to get that when printing from a Mac before using an SD card.

Did you test if your bed moves freely before you installed the belt? The motor might be working too had if it's not.

As far as the extruder, you should be able to push the filament through your hotend with your hands without too much force. If it's too hard, your hot end might be binding. The geared extruder should have plenty of torque unless the temperature is too low or there's a problem with your hot end.
Re: Prusa getting off
January 10, 2014 06:09AM
Vref=0.6 seems a bit high to me. I asked folks at 3dprinterczar (where I got my Prusa i3) and they suggested 0.42V. Here's what I did: I started off at 0.42V, and gradually tuning down the voltage. Eventually, I stopped at a point where the motors sound very smooth (you could barely hear them). It turned out to be around 0.38V.

Alternatively, you could mount a fan on your ramps (you will need to print a case first) to cool it down.
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