y axis shifts mid print?

for some reason the y axis loses its position or something mid print, causing it to shift the rest of the layers. of course, being a long print, i walk away, and it happens when i am not there, so i cant tell if the y axis is hanging or what. i've included a pic of it. any suggestions? maybe just the file i am trying to print (attached)? this is my first printer, so still learning. prusa i3, marlin firmware, ramps 1.4.

i've been able to print the calibration squares and they did fine.

You're dropping steps most likely. This can be either from an electrical cause or a mechanical cause.

The main electrical cause is that the stepper driver is set too low. I don't know what kind of electronics you have, but most boards have stepper-driver sub-boards on them, one of which is for the Y axis. This will have a small trimmer potentiometer on it that you can use to adjust the current output. Too low and you will lose steps, too high and the motor will overheat. It's a bit of a balancing act.

Mechanically, if the axis is not free-moving, it can bind and fight with the motor. Make sure the axis is not "sticking" at any place along its travel.

Also, if the overall axis is heavy and you have rapid changes in direction or speed, you could lose steps. You'd need to look at your acceleration and jerk settings in this case.

thanks for the reply. i'll turn up the juice on the stepper driver and see if that helps. i dont really understand the acceleration and jerk settings, but they are set to the defaults:

#define DEFAULT_MAX_ACCELERATION {9000,9000,100,10000}
#define DEFAULT_ACCELERATION 3000
#define DEFAULT_RETRACT_ACCELERATION 3000

#define DEFAULT_XYJERK 20.0 // (mm/sec)
#define DEFAULT_ZJERK 0.4 // (mm/sec)
#define DEFAULT_EJERK 5.0 // (mm/sec)

should these be moved up or down? it is a little heavy, since i had to use an aluminum plate for the y carriage, cause the wood and abs that i had initially kept warping, was just about impossible to get the bed level. i printed an enstop holder, and it came out fine, but it's not as big as the other object.

Another note, one that I found with my extruder that I have since left behind, is check that your hotend isn't being moved in the holder. The extruder that I came with my i3 kit did not have any means of securing the hotend to the extruder, other than a snug fit. This caused me a lot of grief when I would print and would have up to 4 Y-Axis shifts. I believe that jbernardis is correct, but it might be worth checking it regardless.

An unlikely but possible possiblity is that something is broken and the belt is loose. I had a shifted print one time and found that my printer had actually broken in four places [which spurred me building my own printer on the basis that a reliable printer has no printed parts] due to either my grease or de-greaser. My belts had skipped, not my motors.

thanks for all of the suggestions. i turned up the pots, and it seems to have made a difference, although i ran into another problem while trying to test it-the extruder quit feeding. seems the hobbed bolt i got off ebay for cheap is nothing but crap, keeps eating into the filament and filling the groves, loosing traction. going by the hw store today and getting a new one, will try the dremel method.

Maybe you're trying to do things too fast. The plastic will melt at a certain rate. If you push faster than that, back pressure will build and eventually the bobbed bolt will slip, likely scraping off plastic as it does so. The extrusion rate has to keep up with the feed rate, so if is very possible to go too fast for your extruder. You could try slowing things down, and then slowly building the speed back up to see where you can get to.

is there any way to calculate the best rate for the material being used? not the steps, i've already done that and it's accurate.

@tiimbo Regarding acceleration, try reducing those values. I had the same problem as you on my Prusa i3, where at some point in the print the y-axis was shifting and reducing the acceleration has completely solved the problem (assuming the other factors such as stepper current, binding etc are all OK).

Here are the acceleration settings I'm using in Marlin. As you can see, they are MUCH less than the defaults!

#define DEFAULT_MAX_ACCELERATION      {180,180,100,200}    // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.

#define DEFAULT_ACCELERATION          60    // X, Y, Z and E max acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION  60   // X, Y, Z and E max acceleration in mm/s^2 for retracts

@amrbekhit thanks for that. i turned up the pots on the stepper drivers, and it seems to have fixed the problem. i think what you suggested and what i did accomplish the same thing, except my motors will generate more heat and move a little faster though. but, if i have it happen again, i will definately be looking into changing the accel. if i'm wrong, please correct me!

i was able to print out a complete part last night. seems that the extruder temperature was not being reported accurately. i borrowed the thermal imager from my FD that has a temp readout on it, and i needed to set my extruder temp to 248 in the host sw/slic3r to actually get it to 230. once i did that, seems like it printed fine. i dont really know what to change to get it accurate, as the hotend came assmebled, so i dont know what thermistor it is using, just a 100k one.