Rework of the original Y idler by gregsaun of the MK3 Bear full upgrade to include a belt tensioning mechanism.
Print with the same parameters as for the original (0.2mm, 3 perimeters, 30% infill).
- 2x M3 thin square nuts (taken from MK3 parts)
- 2x M3x10mm screws (MK3 parts)
Insert the thin nuts from the central side slot. The hole is intentionally tight, so do it carefully. Center the nut perfectly by pushing on the side slot, do not use an hex key or screw on the thread (doing so will damage the thin nut due to the tightness of the fit).
Insert the idler/belt, then use the original screw with nylock nut. Do not use force: the idler shouldn't bend inwards. Insert the M3 screws from the front but not tighten them. A drop of low/medium strength thread-locker is recommended. Pre-adjust the belt under the Y carriage until the belt has some tension (this idler allows for 1.5 teeth of adjustment).
Mount the idler on the frame and center it by pulling the carriage forward. I recommend to center visually first, ensuring that the entire belt is straight (you can move both the idler and the motor holder so that the belt is in line with the carriage). Then proceed to tighten the belt by turning both screws in small steps and at the same rate until the belt has the desired tension.
Finally, you can fine-tune the belt tracking by adjusting one screw only while moving the carriage to ensure the belt stays centered. Adjusting the belt tracking is usually better than just shifting a regular (fixed) idler left or right since it allows for all components to stay in a perfectly straight line.
The final depth of the screws is not perfectly identical due to the shape of the nut and axis adjustment, so do not rely on the insertion depth: rely exclusively on the belt to verify the alignment.
Again, with great thanks to gregsaun for his work.