Weird print issue: X and Y get shifted at random heights

[mattanonymous]

Any idea what causes this? I've had it happen on several large parts. I'm slicing with Slic3r, but the rendering of the g-code looks fine when viewed in pronterface.

Also, toward the end of printing an identical part, the printer just stopped. The nozzle stayed and bed stayed heated and the homing g-code wasn't run (I assume because it didn't make it to the end).

[mattanonymous]

I can't seem to edit my post, but here's a link to a bigger pic: http://imgur.com/NYhP2Jd

[jimc]

Skipped steps is usually from:

too high accelleration settings.
xy movement speed too high
part curling or warping causing nozzle collision
overextrusion causing nozzle to collide or drag heavily on surface of the part.

[beerdart]

I was having the same random skipping I tracked it down to the board briefly powering off. I installed a bed relay and have not had any skipping since.

[adamfilip]

possible interfuption/miscommunication between computer via USb (if connected)

[Chadd]

You may also want to check to make sure your Y axis isn't hitting something. I had something similar to this happen and it didn't show up on smaller prints. What was happening in my case is that I added some clips to the corners of the bed to keep the nuts on the bottom of the leveling bolts from turning. And on occasion one of those would drag on the brace at the rear of the printer that the y motor mounts to and would cause it to offset a layer like that. I finally heard it while I was printing a larger print and figured out what it was.

Hard to tell for sure but to me it only looks like the Y axis is offset, that was the same that was happening to me.

[mattanonymous]

Forgot to mention I have the 10 inch model, so there's already a bed relay installed.

I was printing from the SD card.

I updated Marlin from the latest github, so I'll have to check those settings compared to the original Makerfarm ones.

I don't think there anything is interfering with movement, but I'll double check when I get home today.

Could it be loose belts? What's the rule of thumb for belt tightness/tension?

[sniffle]

At a minimum enough tension that there is no sag in the belts along either axis but if you can get it so that its a notch or two tighter thats better :-)

[gmay3]

I've had the same issues. Exact X or Y axis shifts in your print are almost always due to the belt slipping either by being too loose or the grub screws on the X and Y motor pulleys becoming loose allowing the pulley to turn on the stepper motor shaft.

For your specific picture, the likely culprit is loose belts. The belts heat up during the print and expand, which is why the shift starts a certain layer into your print. Most of the heat usually comes from the stepper motor itself and is transferred to the belt through the metal pulley. Try tightening the belts slightly more and see if that helps!

[mattanonymous]

Thanks for the tips guys! The belts do have some slack in them.

The y-belt hasn't been adjusted since the belt guide (http://www.thingiverse.com/thing:359773) I was using cracked. Do you guys use a belt guide or is it excessive?

Also, I noticed that the bed leveling screws toward the back are tightened to the point that they don't clear the slots on the back of the frame. I plan to fix that soon.