Layer Shifts

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A layer shift happens when, at a certain layer, everything shifts along to X or Y axis. This can happen for lots of reasons. The first thing to do is to try and determine which axis is having the issue. If your prints are shifting forward or backward, then you will need to adjust the Y axis. The reprap wiki has a comprehensive list of possible causes.


Mechanics or Friction

If your rods are not correctly aligned, you will need to adjust them. You might notice that the closer to the bed the more resistence there is. The gantry assembly may need to be pushed together tighter. I also read that some people loosened some of the spots and let the printer run then retightened everything. Just be careful that you don't loosen things so much that they come apart. Check the drive belt is snug, the pulley set screws are tight and that the belt is not rubbing on the idler. Shifts in x/y can indirectly be caused by z-axis binding. The print head fails to move up and the nozzle collides with the print, momentarily overwhelming the stepper. See http://rigidtalk.com/wiki/index.php?title=Z_Axis_troubleshooting


Lubricant

(thanks Carlton Dodd) If you have not already done so you may need to lubricate the rods. I used wheel bearing grease, from an auto parts store, to grease each of the rods. After I did this I ran the printer back and forth on each rod.


Acceleration speed

(thanks Jwo Fox-Lee and Wylie Hilliard) It also may help to lower the acceleration speed. This is set in the firmware. With a piece of heavy glass on my print bed I found I had to lower acceleration from 600 to 500 to stop the shifting.

In the firmware, marlin, change in Configuration.h "#define DEFAULT_MAX_ACCELERATION {800,600,100,10000} " to "#define DEFAULT_MAX_ACCELERATION {800,500,100,10000} " then recompile and flash using Firmware instructions.

This can also be changed and stored in eeprom using the LCD Controller, or from Repetier/Pronterface manual control using M201 and M500 commands. ref https://github.com/ErikZalm/Marlin


Obstructions

(thanks Josh Azevedo and Michael Gueterman) Make sure there is nothing in the way that is causing travel problems. You may also want to make sure the filament spool is not jerking back when it rolls over.

Trim pot adjustment

(thanks Katherine Campbell and John Logajan) If your stepper motors seem to freeze or skip a step it may be because you are allowing either too much or not enough current to the motors. There is a page Trim_Pots that will help you see what each symptom looks like. The trim pots can me adjusted with a very small screw driver in the 5 holes on the main board cover. Look for the small hole near the connector for the axis you are having troubles with. Also see Stepper Driver Adjustment http://rigidtalk.com/wiki/index.php?title=Stepper_Driver_Adjustment