Which should I choose Between NEMA 34 and NEMA 42?

The decision between NEMA 34 and NEMA 42 motors is primarily a decision about productivity -- NEMA 34 motors can remove material at a higher rate using higher feed rates and deeper cut depths.
The style of motor you choose is not the determining factor in what materials can be cut -- this is far more a function of the mechanical stiffness of the machine doing the cutting, as well as proper speeds, feeds and tooling. With our machine kits, either of our motor packages can cut hardwood, plastic, and non-ferrous materials such as aluminum.

Motors serve to accelerate and decelerate components of the machine (such as the gantry or z axis), as well as to push the cutting bit through material. nema 42 motors can take deeper passes through material and improve cutting speeds, especially on larger machines.

Generally, if you are using a machine for regular production work, higher power NEMA 34 motors will provide a fast return on investment. However, if you are primarily doing small production runs or prototyping work, nema 34 motors will likely be sufficient.