In order to give tires of my model cars a final touch I was thinking of using a 4th axis for rotary engraving. For the vague idea, the intense investment and due to harsh software requirement it did not get a professional 4th axis set. It came in handy that I had a stepper motor (I changed from my y-axis) and a small chuck laying around. So I designed the pm4a.

This build was done on a desktop cnc-machine by Stepcraft from 6.5 mm birch plywood for the body and some 5 mm Pertinax for the gears using a 1.2 mm spiral toothed flute. Furthermore I used

• 2 Ballbearings Type 6000ZZ (10 x 26 x 8 mm)
• 1 Ballbearing Type 608ZZ (8 x 22 x 7 mm)
• small chuck (10 mm 3/8'' x 24 UNF) with SDS-Plus-adapter
• NEMA17 stepper
• a sharpened M8 machine screw and a selflocking nut (for the tailstock)
• some M3 machine screws

The two gears have an pretty uncommon cycloidal (round) teeth, which helped me to create smaller teeth, because they don't have any corners. Otherwise the teeth size would have been limited by the diameter of my milling bit.

The motor is driven by plugging it into the X-axis stepper driver. This way the machine won't move sideways, but instead the workpiece does. The distance of travel for one revolution is calculated depending of the diameter of the workpiece dived by the gear reduction of 6 : 1. For example my tires have 40 mm diameter. So the circumference is 40 mm x π (3.1415) = 125.66 mm / 6 = 20.94 mm. I entered this value in the controller settings.

When using the pm4a it'd be a good idea to use some kind of cover to prevent dust from getting into the gearing.

Visit https://www.zenziwerken.de/en/ for more interesting designs.