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.

31 thoughts on “DIY CNC Rotary 4th Axis

    1. 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.

        1. 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.

    1. 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.

      1. 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.

      2. 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.

    1. For engraving on a cylinder, all you need to do is treat the rotary as the Y axis and engrave the logo as it if was on a flat plate. You’re not working with a 4 axes at once, just 2 plane axes and 1 rotational.

      For “proper” 4 axis stuff I’ve no idea what CAD/CAM software would work. I’m guessing it won’t be cheap though.

    2. For creating the gcode, it’s freeware

      True 4 or 5 axis CAM is several thousand and is not really a option for the home CNC hobbyist budget.

      His PVC engraved examples was what inspired me to finish making my 4th axis. I wanted to make LED lit PVC tube “candles” with engraved Halloween themes to be placed on our walkway leading up to the front door of the house. We get hundreds of kids coming to the house for candy each year. I didn’t finish in time but next year I will be ready.

      For controlling the actual CNC, I run MACH3 which is able to control up to 6 axis.

  1. 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.

  2. 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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