Modela Pro MDX-650 3D Milling Machine
The spindle gets stuck while cutting when increasing the RPMs during the cutting. It is not due to the cutting depth
There could be multiple reasons for this
Clean around the ball screw and LM guide.
Replace the spindle and retest.
MDX-650A is not designed for milling hard metal. Therefore, if milling Copper, Roland recommends using the MDX-650A with the suitable conditions. If you use the MDX-650A in extreme conditions the spindle will be worn out before the referential time for the replacement.
If milling copper, the copper tends to have conducting properties. Therefore, milling copper dust might cause the short problem.