Determining the Cause of, and Fixing Axis Shift

Axis shifting is an undesirable symptom of a few possible underlying issues. Depending on the issue, the shifting may manifest itself slightly differently, or may be accompanied by other symptoms.

Misaligned belts

All printers are sent out fully tested with belts and pulleys properly aligned, but sometimes belts may stretch or loosen slightly during shipping. If your belts are misaligned, you may put a twisting moment on the left carriage which causes the whole carriage to bind up. Follow the directions in the true circle realignment video.

  • Binds happen in both manual control and during print
  • Shifting in both directions along Y axis. i.e. Forwards and Backwards
  • Shifting accompanied by loud buzzing or grinding sound
  • Carriage binds up often and at random

Loose set screws in pulleys

Every once in a while, a set screw loosens up and the pulley slips on the shaft. Solving your shifting may be as simple as tightening a set screw. If you are still getting shifting, try following the directions in the true circle realignment video.

  • Shifting in both directions along one axis. e.g. Forward and Backwards OR Left and Right.
  • Shifting happens seemingly at random
  • Not accompanied by any sounds
  • Can easily move carriage by hand or stop carriage while printing without causing a grinding sound.

Improperly calibrated trimpot

If your trim pot is set improperly, shifting may occur, but may have different results accompanying symptoms.
Current set too high:

  • Shifting in both directions
  • Very hot stepper motors
  • Shifting a result of overshooting
  • When motors are engaged but not moving, there may be a high pitched pulsing sound.

Current set too low:

  • Shifting in both directions
  • Shifting accompanied by buzzing or grinding sound

Both of these can be fixed by the following method:

  1. Center the print head over the platform.
  2. While connected to the printer, turn all the trimpot of the offending axis counter clockwise to reduce the power going to the motor. Take care not to short any pins elsewhere along the motherboard.
  3. In the software, home the offending axis. There should be no response.
  4. Slowly turn the trimpot clockwise, increasing power, until the axis starts to move. You will hear an increasingly loud buzzing sound until the motor has enough power to move the carriage smoothly.
  5. In the software, move the carriage back and forth a few times.
  6. If the carriage still binds, increase power slightly, otherwise run a test print.

Bad stepper chip

This was more of an issue with the Sanguinololu motherboard. The Pololu stepper driver chips may overheat and eventually fry the chip losing the ability to switch directions reliably. If you have a Sanguinololu motherboard and have the following symptoms, you may need a new stepper chip. You can either get a replacement from moc.eldoodilos|troppus#troppus, or order the more robust replacement.

  • Shifting in one direction only
  • Shifting not accompanied by any sound.

Bad rod

If you have a bent rod, the carriage will almost always bind up at the same location, depending on how bad the bend is. If the rods aren't smooth, the binding may be more sporadic, but can be determined by moving the carriage by hand when the printer is off. Contact moc.eldoodilos|troppus#troppus, then follow the directions for replacing your rail.

Bad stepper motor

This is the least likely case. The motors are very robust and do not tend to burn out. The ways to test for a bad motor or motor wiring are:

  • Disconnect your motor and try turning the motor shaft by hand. If you cannot turn the motor easily by hand, contact moc.eldoodilos|troppus#troppus and explain your process.
  • Disconnect your motor and measure the resistance between the wire pairs. You should measure few ohms between red and blue, and a few ohms between green and black. If you measure open circuit between red/blue or green/black, the wire or winding may be broken in the cable or motor; examine the wire crimping in the connector and check for pinched or broken wire leading up the motor.
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