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In January of 1999 I completed the conversion of a standard knee mill to CNC operation. The original mill was a used Acer Ultima 3VS with 3-axis digital readout and a Servo power feed. The picture shows the mill as delivered to my shop.

As a long-time Macintosh user, I wanted to use a Mac to control the mill. This meant writing my own software. Actually, I had done most of this work several years ago for my first CNC tool -- a router table. It took just a few days to update the old software to run the new mill.

I was able to purchase some surplus hardware, and get good advice, from Bill Griffin in the Los Angeles area. Bill is a consultant in CNC conversions and retrofits machines for hire.

I bought three large stepper motors. These were 56 frame motors with 1400 oz/in of torque. For drivers I acquired three Compumotor OEM 650 microstepping drivers. I built a small microcontroller to interface between the Macintosh serial port and the Compumotor drivers. It's barely more than a serial to parallel converter, but it also understands the pulse width requirements of the Compumotor drivers and will manage limit switches when they're added.

The power feed was to become superfluous, so I removed it and mounted one of the stepper motors in its place. The picture shows the X axis of the mill with the motor mounted to the right end of the mill table. The mounts are basically plates of 1/2" aluminum with a sliding fit to tension a 3/4" timing belt. I placed the top toothed pulley on the mill's crank shaft in place of the indicator dial. This allows me to crank the machine by hand when the motors are unpowered. The top plate is bolted to the existing bearing housing, so no new holes needed to be drilled in the mill itself.