Belt Size?

What size belts are used to move the x and y axis? I was watching a Shapeoko video and their 9mm belt slipped causing the cut to go bad.

The standard size is 6mm. If your machine is well calibrated you will have no issues with the belts. They will move the machine around faster then it can cut.

Any belt can slip if it has insufficient tension. As already mentioned, the standard width is 6mm, but there’s enough room to install a 9mm wide belt and pulleys. The wider belt improves stability and accuracy and will probably also decrease the chance of slipping. But, again, once a belt is tensioned properly, it takes a lot of force to make it slip.

I would guess that most “slipping” problems are actually the pulleys slipping around the axles of the stepper motors, and not the pulleys slipping on the belt. I had this problem on my Shapeoko prior to flattening one side of the axles to grip the pulley set screws.