4 Axis CNC Details

For my upcoming R2-D2 robot I needed a dome. More than that, a intricately detailed dome. I immediately knew my 3 axis CNC wouldn’t cut the mustard and neither would my 3D printer (footprint too small). I have always wanted to make a CNC with rotary axes… Let’s go.

My 4 axis CNC is more like a 5 axis CNC in the fact that it has two rotary axes, however it only has two Cartesian axes (X and Z). There’s no Y axis because I simply didn’t need one to cut a dome. Adding a Y axis would’ve added a great deal of complexity and cost (moving gantry, larger frame, more expensive and stronger parts needed etc).

Think I’ll just note here early that this machine is light duty and only built to machine wood/plastic as the spindle I’ll be using will be a dremel. Additionally the frame is fairly lacking in the strength department but I ain’t gonna be machining metals any time soon on there!

Starting with the X axis; it’s belt driven and powered by a NEMA 17 stepper motor. The stepper motor is mounted to a small sliding, adjustable mount that allows me to easily tension the belt. On the opposite side from the stepper motor are two skate board bearings (8x22x7mm) mounted on an M8 bolt which acts as idler pulley.

The Z axis is also powered with a NEMA 17 stepper motor, attached via cheap eBay coupling to the cheap eBay 8mm diameter leadscrew. The screw has a fair bit of wobble and whip to it so I may need to upgrade it.

The B axis too is powered by a NEMA 17 stepper motor, in fact (spoiler) the whole CNC uses NEMA 17’s. The motor has a 3.75 reduction (16T pulley on the stepper to a 60T pulley on the spindle mount). The belt is the classic GT2 as seen on every 3D printer ever. The stepper motor is attached via long M3 spacers and can be moved up and down to tension the belt.

The final axis: The C axis. This is the rotary table where the work piece is mounted. It’s the exact same mechanism as the B axis; a 3.75 reduction with 16T and 60T pulleys. The rotary table is mounted to a 200mm lazy susan bearing.

Here’s the electronics. I’ve temporarily thrown them in to an enclosure with a 80mm fan to keep the stepper motor drivers cool. There’s a 12V 10A power supply in there to power the RAMPS 1.4 board which typically is used for 3D printers. I had to modify the grbl (CNC controller software) source code to run on the RAMPS board as it doesn’t natively support it. I did quite a hack job and disabled a lot of safety features to get it all running! More work needed there for sure.

Just some final quick bits of info:

  • The machine is made from PVC (Grey plastic is solid PVC, white plastic is foam PVC – wouldn’t recommend this stuff, it’s too soft).
  • The black metal beams are OpenBeam 15x15mm aluminium extrusions.
  • Hardware design isn’t done, I’ll yet be adding some more parts to strength it up.

That’s it for now folks! Next post I’ll detail the custom software I’ve made to run this beast.


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