DIY CNC Rotary 4th Axis


Here’s a great CNC hack that adds a ton of functionality, a DIY rotary 4th axis!

[Jim] had started this project over a year ago when he originally ordered the gearhead off eBay, but like many good intentions, sometimes projects just get pushed to the back burner until necessity forces action. That necessity was entering our Trinket Contest, and he decided to finish it off just so he could put a HaD logo on a piece of PVC for us!

Unfortunately it took him a bit too long, and he only finished it last week — but luckily he had a fallback plan, and submitted his CNC Etch a Sketch project instead, which won him a Trinket anyway!

The 4th axis uses a 276oz-in stepper motor which is directly coupled to a Harmonic Drive Systems 11:1 planetary gearhead. It’s extremely accurate, has minimal backlash, and by using a 10 microstepping Gecko stepper drive, [Jim] is getting about 61 steps per degree of rotation. Not bad for a home-made setup!

Check out his blog for a great write up on the project, and stick around after the break to see the 4th axis in action.


  1. medix says:

    Nice build, but definitely *not* a planetary gear head as described. This looks to be a harmonic drive:

    • medix says:

      No wait.. I lied. Gear ratio isn’t high enough. ;)

    • Jim says:

      This version of the gear head from Harmonic Drive Systems is planetary. I do have a true zero backlash harmonic drive gear set from HDS that I also found on eBay for cheap. One day I will finish building a better rotary axis with that one. The backlash from this planetary gear head is rather small, it’s listed as less than 1 arc-min in the datasheet, it’s really not a problem at all. If my trusty Hp15c calculator is correct, it works out to be only about 1 microstep. To small to really worry about for rotary carving.

      • medix says:

        Either way, still really awesome!

        I got excited when I thought it was a harmonic drive. I built a massive octagonal robot several years back using harmonic gearboxes in the drive train. It was a beast!

        • Jim says:

          About 3 years ago I bought a cheap rotary table that I converted by taking off the handle and replacing with stepper motor. I used thrust bearings and whatever I could think of to minimize the backlash but it was a big FAIL. The internal worm gear mechanism was so sloppy that backlash was measured in degrees. Its been sitting on the shelf collecting dust. Since then I found out about Harmonic Drives and what high end CNC/Robotic systems use for rotational axis. A new harmonic drive is big $$$$ so eBay to the rescue. The gear head I used is planetary but made by Harmonic Drive Systems so it is very precise. It’s simple and just the right size for my small cnc. Since the stepper motor, gecko drive and gearhead was bought off eBay, it wasn’t that expensive. Under $150. I had upgraded my mini lathe to a bigger 5″ 3 jaw chuck. The 3″ chuck wasn’t being used anymore so I put it on the gearhead.

  2. Tom says:

    this would be realy cool to make cylinder seals or ink rollers for making prints

  3. matthew venn says:

    how do people generate tool paths for a 4 axis? Do they just remap the y axis to rotation?

  4. sneakypoo says:

    Should feed rates vary that much?

    • Jim says:

      That’s pretty good that you noticed that. In the video the axis is programmed to move 10ipm but the rotary axis seems to move faster than that. I’m using MACH3 CNC control software. I’ve only used the rotary axis a few times so haven’t been able to track down the problem yet. There is some setting in the software that is mis-configured.

      • t-bone says:

        On a HAAS there’s a controller setting for “4th Axis Dia.” (or maybe it’s radius). The controller will turn the 4th axis so that the feed rate remains constant if you’re at that diameter.

      • Ken says:

        Mach3 has diameter/radius compensation that converts feedrates on rotary axes under the assumption that they are engraving at a fixed diameter. Probably the same as on the Haas.

        If you get fancy and use your rotary axis at varying radii (esp near r=0), you’ll want to look at G93 Inverse Time Feed Mode.

        • Jim says:

          Thanks that was it, once I put the diameter of the tube in the rotary “A” axis compensation, all is good. The rotary axis moves at the right speed. It’s in the Mach3 settings menu tab.

  5. David says:

    what kind of software could be used for controlling the 4 axis?

  6. fartface says:

    you certainly do not need 4th axis for this. a 3 axis can easily do this. just have a moving head on Y and rotate on X. and honestly, I would not even have a Z other than CUT or !CUT to make some very cool things.

  7. Josh Martin says:

    Meshcam has 4th axis support and is totally free for 2 weeks.
    I have yet to use it as I’ve been too poor for a $200 harmonic drive setup (servo+ drive, or plain drive + stepper) yet :(

  8. RandyKC says:

    Does anyone know of any open source software to generate 4 axis gcode and then preview generated milling or printing paths.

  9. Josh Martin says:

    OpenScam will simulate 3 axis stuff but he is working on 4th, past that I am not sure if there is any free or open source simulators that do 3rd or 4th axis simulations.
    I think CNC toolkit for 3dmax does 4-5th axis stuff and is open source or at least free.

  10. Scaramouche says:

    Now just turn that pvc into a pressure vessel and you’ll be automatically entered into a special kind of contest. Check the comments section afterwards to see if you’ve won.

  11. blade says:

    Looks like how the C-Axis works on the lathes I run at work, to be honest. Of course, I spend all day turning raw cast or ductile iron into machined castings and plastic garbage cans of cast iron, and we don’t generally do anything that intricate.

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