Have you tried a different brand of belt? Maybe one with carbon fibers for better stretch resistance?
I wasn’t aware that they were available with carbon fibers. I think I saw some with kevlar. I will do a search and see what I find. I don’t think that stretch is the issue as much as lack of accuracy when the belt is made. Stretch should be uniform along the length and therefore easily compensated for. I realize that making a rubber belt that has the tooth spacing held to 0.003" along it’s length is a tall task. Probably not going to get a lot better than what I have with any sort of belt.
It is likely I was remembering Kevlar, not carbon fiber. I’ll try to find my reference. I believe Gates makes them.
EDIT: Belts are here: McMaster-Carr
Suitable pulleys same source.
Several Shapeoko heavy users have switched their belts to steel belts. You can check this thread for info and sources.
Good information in the thread. More to think about. As a luthier, I cut thin wood at fairly slow speed. I don’t know if I would see any benefit unless the tooth spacing on a steel or kevlar belt is more accurate.
The idea of steel belts is that it reduces the possible belt stretch and is more sturdy preventing belt breakage.
I have never broken a belt and uniform stretch can be compensated for, so I am not sure the steel is an advantage for me. If someone has actually measured belt accuracy in terms of tooth spacing and found it to be superior to the alternatives, that would definitely be worth the investment.