1 May 2013

Todays result: Modified Mendel90 y-idler-bracket

I tweaked the design for the bracket to add extra tension to the y belt as it is done up. The y bracket is responsible for the tension on the y belt. The y belt can be installed to within one tooth accurately. I used T2.5 belt so that is +/- 1.25mm as the belt doubles over. The last mm or so is achieved by the screw tension holding the y idler firm against the printer base. My printer is MDF. The screw hole for the bracket is stripped, and it quickly works its way free after tightening. Wood glue will hold it in place but only for a print or two. The belt being just a little loose has caused noticeable ripple patterns on prints as the bed wobbles back and forth.

The modification adds a slant to the back of the bracket. When tightening the screw the bracket acts as a leaver adding tension to the belt slightly. Trigonometry reveals that for T2.5 belt an angle of about 6 degrees will pick up the remaining slack.

I printed the bracket right way up, and then filed off the messy overhang. Flipping the model long way up would prevent this but also reduce the print area touching the bed. As the bracket screw hole on my printer has stripped I used a zip tie instead. The new bracket has removed the ripple effect on prints.

The modified scad file is available here. To use the Mendel90 repository is needed. Substitute the modified file for the original y-idler-bracket.scad in the directory Mendel90 \scad.

To install
  1. Setup the printer as follows 
    • Y-motor and bed in place
    • New y idler on the belt but unfixed to the base
    • Y-belt tighter than necessary
    • One belt clamp on the bed done up, the other tight enough to hold the position of the belt, but loose enough to give when pulled
  2. Place the y bracket on the base with the bearing side flat and screw hole elevated. Slide the bracket with the screw toward the screw hole until it is just able to reach. This should be the correct belt length to within one tooth.
  3. Tighten the belt clamps on the bed.
  4. Tighten the bracket screw until secure and the belt is tight

Angle to achieve desired tension

 Installed on my printer, this is before I did the math and has a 20 degree angle.