I posted a few weeks ago on pretty much the same problem, and was told to tighten the hex screws in the tiny holes on each motor. This helped immensely, I have successfully milled copper.
The Z axis has started giving under pressure again and my tool is stripped (I don’t understand why these have to be so tiny) I’m ordering another tool which I’m sure will fix this issue, but this giving is a problem because it ruins my piece, wasted my time, and copper is not cheap.
Is there any upgrade I can do to keep my machines motors from giving? Looking for anything from small fixes like the tightening, to even replacing the motors, or replacing the belts with better ones, I just don’t know what exactly I should do.
Not sure where the belts are sourced or if better ones are available — early on, it was possible to replace the 6mm belts (on the X and Y) w/ 9mm and I bought mine from SDP/SI which seemed to be better quality — I believe they have an endless 6mm GT2 520mm belt or something close to it — not sure if it’s an improvement or no. I had to use a small pry bar to get my Z-axis taut enough: http://www.shapeoko.com/wiki/index.php/Shapeoko_3#Belts but the wooden shim technique suggested there is likely better.
The previous version of the Shapeoko used an M8 threaded rod for a Z-axis, and many people replaced it with an Acme rod. I actually sketched up plans for a replacement (I didn’t have much luck with the belt-driven Z-axis early on either) and did make notes on a B.O.M.: