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I have recently acquired a 'CraftBot XL' 3D printer (sold by CraftUnique), which is a CoreXY 3D printer with linear rails. It is a second-hand purchase and the seller were a company that had several. This particular unit has an issue where sometimes during X-axis movement, the Y-axis would move as well. They replaced the stepper motors, but the problem persisted so they opted to sell the machine.

I have found a possible cause of this issue. When I tried to move the carriage around manually, I noticed a particular heavy resistance / "drag" on approx. 1/3rd to 1/2 of the X-axis motion, toward the left extent of the build space. The remainder of the X axis and the Y and Z-axes are completely free to move as they ought to.

The force required to move the carriage in the problematic part of the X-axis is (subjectively) quite large. Much much larger than any other movement. I believe that during some combined XY-manouevers (diagonal movement), this force is too large, causing the carriage to only move in the Y-axis, resulting in errant behavior (at best: layer shift, at worst: carriage getting stuck and the printer needing to be reset). The behavior arises during homing often, as the home position is in the problematic area. An additional reason that I think the X-axis force is too large is that, when this problem occurs and I ever so slightly help the carriage along the X-axis with my hand, the movement can complete without issue.

Steps I have undertaken to no avail yet:

  • Re-tension belts, make sure they are of roughly equal tension all around
  • Lubricate all axes with PTFE dry lube spray
  • Disconnect belts to see if they are causing the issue (they aren't, with belts completely disconnected the problem is unchanged)

This is a diagram of the setup:

CraftBot XL layout

What else could be causing this? I don't hear any out of place noises like metal grinding.

The next thing I want to check is the squareness of the gantry. I think I know how to measure it, but I'm not sure how to adjust it if it's off. Any help there?

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  • $\begingroup$ Please remove the tension from the belts or remove them completely; then see if the motion system still has a problematic issue with resistance. You can then exclude the belts from the equation. $\endgroup$
    – 0scar
    May 1, 2023 at 6:23
  • $\begingroup$ I have, and it persisted. Managed to accidentally pull one belt loose from the carriage so I will have to disassemble the whole extruder assembly now. As a bonus that will allow me to judge the linear rail and its carriage separately. $\endgroup$
    – Floris
    May 1, 2023 at 10:16

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I'm going to answer this myself, as I believe I have found the issue. It was purely a mechanical issue.

On this particular model, the extruder motor rides just on top of / above the crossbar that the linear rail is mounted under. The extruder motor was mounted very close to this crossbar, there was barely any clearance between the two. I figure that due to a slight deviation in height or non-squaredness in the ZY-axis, the motor started to heavily rub on the bar toward the left extent of the X-axis. I tested with a piece of paper, which I slid between the motor and the bar. This immediately caused the same "resistance" I was feeling before, but now everywhere along the X-axis.

After re-mounting the extruder motor about 1 mm higher, which the frame already allowed for, there is no longer any improper resistance noticable along any axis:

Adequate clearance between motor and bar

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