I had a go at retensioning my three belts using @LiamN’s method, and went for a target of 125Hz, which on my SO3 seems to be the sweet spot (it will be interesting to hear what value others end up using)
280mm span on the right Y belt, using two #201 endmill shafts:
280mm span on the X belt:
280mm span on the left Y belt:
What I found out today while doing this:
- Previously, to tighten my belts I used to choose a given length of belt loop locking onto itself when the tensioner is fully tight against the plate, and then add or remove one teeth or two from the loop and re-tighten to adjust tension and have them “guitar string tight”.
- I found out that one teeth can make a significant difference in tension using that method, and it did not allow me to reach exactly 125Hz (say, one measurement was at 110Hz, and just one teeth tighter got me 140Hz)
- So I went for finding the loop length that gave me a value above 125Hz when fully tightened against the plate, and then slightly turned the bolt counterclockwise while measuring with the phone app, until I was at exactly 125Hz (+/-1 Hz). It takes very little untightening of the bolt, so I doubt to the bolt will go loose, time will tell.
- after I tuned both my Y belts to 125Hz, I did the “home then manually jog to the front plate” test, and I now have my Y rails contacting the front plate at (almost) the same time, but more importantly with the exact same offset as the one I get when moving the gantry manually with the machine powered off (which means that the belt stretch is even on the left and right sides, and does not introduce additional shift). I still have a 0.5mm gap on the left side when the right Y plate is touching the front plate, but I chose to live with this tiny offset from perfectly square last time I shimmed my gantry (and 0.5mm over the Y travel is not much, I’m not even sure what the tolerances on the Y plates and their powder coating are…)
Thanks again @LiamN for opening the path to a much more reliable way to adjust belt tension, while keeping it very simple (it takes about a minute to grab two endmills and a phone to do the test).