Machine Design

Digital Spindle Lets CNC Run Faster

A digital open-loop vector drive lets a three-axis vertical-machining center run at top speed without fear of a spindle fault. The Personal CNC or PCNC from Tormach LLC in Waunakee, Wis., contains logic that tells the drive to back off to the highest possible sustainable speed when the load is too great. Targeting inventors and machinisthobbyists, PCNCs also work well for companies requiring CNC for secondary operations that don’t need more than 0.001-in. accuracy.

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Earlier versions of the machine used an analog drive to power the spindle motor. However, analog devices are subject to temperature drift, meaning users had to manually tune the potentiometers.

Tormach tested several drives before focusing on open-loop vector drives. They provide better slip compensation and more torque at low speeds than volt/Hertz drives because they track and respond to a motor’s load qualities. More testing pinpointed one model with additional programming functions that let it sense when the motor couldn’t attain a commanded speed. These functions let Tormach program the “fold-back logic” into the drive that manages speed when loads are too high. It also solved the thermalrepeatability problem. In addition, the drive boosted power from 1.5 hp (5 A continuous and 7 A peak) to 2 hp. The bigger overload capacity allows stable carry-through during tough cutting conditions. Also, cutting speeds went from 21 to 40 ipm at 1,200 rpm. And increased torque at low speeds allows efficient use of large-diameter drills.

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