I converted a PM-932 a couple of years ago using a Nema 42 Stepper Motor With 4200 Oz-In Holding Torque for the Z axis. Probably way oversized but it moves the head like it's not there and have never missed steps. Not saying this is the "correct" size. It's what I used.Could anyone who has made a cnc conversion on the pm-940m tell me what would be the correct size motor (either step or servo) to use on the z-axis. Thanks.
A lot of holding torque, but the useful RPM is limited on those motors. I have some experience with that particular motor, and found that below about 450 RPM it worked fine, but above that, the torque drops off rapidly and the motor can decouple running unloaded. If your RPM and acceleration can be limited it should work fine.I went the full limit and went with a nema 42 at 4200 oz. I'm also going to use a ball screw on the z axis. Any comments on this? I also ordered the centroid acorn system. This will be a long project but I think it will be fun and useful.
Good luck with that.I have no plans at present to change over to a high speed spindle so going slower will be just fine.