LEGO CNC mill

At the end of my first semester of Mechatronics at the University of Waterloo, we were assigned a LEGO Mindstorms NXT design project. We met the requirements in a day, so a partner and I decided to build a LEGO CNC mill, inspired by one on YouTube.

Three motors drive the X, Y and Z axes, each with a touch-sensor limit switch for homing. A separate high-speed LEGO motor spins a 1/16-inch drill bit through floral foam, which is soft enough to cut like butter.

LEGO motors have built-in encoders, and with our gearing one tick was about a thousandth of an inch. After testing several approaches, a feedforward plus PD controller held the average error under one tick. Cutting forces in foam are tiny, so a simple controller worked well.

G-code is well documented and straightforward to interpret, but files for complex parts were far too large for the NXT’s memory, even with custom compression. So we ran a Python server on a laptop that fed the NXT file chunks one at a time, watching for each chunk to be consumed and keeping one buffered.

We shimmed the linear rails to take up slack, used chain in the gear reductions for lower backlash, and built the frame rigidly to avoid low-frequency vibration. The finished machine carved an Iron Man head in about three and a half hours.