4th axis rotary

Just wondering if there has been anyone able to install a 4th axis?

Greg, I needed to do a bunch of repetitive cuts on 4" diameter PVC piping about a month ago, and threw together a rotary 4th axis working to do it. I used a spare 100 step/rev NEMA 17 stepper, turned a connector out of aluminum, and connected it to a Little Machine Shop 4" rotary table. For electronics I used a Ramps 1.4 stepper board, designed for 3D printers, with A4988 stepper drivers. I flashed the 6-axis GRBL onto the Ramps board and it behaves just like the 3-axis version.

The electronics were a little hard because the stepper I had was, naturally, a non-standard take-out from an old plotter and it had weird wiring. I swapped the X and Z axis LongMill steppers onto the Ramps board and used the new stepper as a rotating Y. It was a whole lot of trial and error to get the settings in the 6-axis GRBL working for both the old stepper and the LongMill steppers.

Software is the real rub. I tried GRBLGru, but he and I apparently don’t think alike and I couldn’t get it to do what I wanted. Some CAM software clams to do 4 axis but they don’t work as you would expect. I wasn’t willing to pay for 4-axis software when I only expected to do a few runs, and the only free CAD/CAM software I could find was Fusion 360. Once you figure out how to design for 4 axis milling it works quite well. It has cool built-in schemes for designing flat and wrapping around a cylinder, similar to the way you do extruded lettering on sheet metal, and dedicated 4-axis 2D and 3D toolpathing like in Manufacturing.

The only real issues I had after getting it running was slop in the drive. When you add the slop in the stepper with the slop in the cheap rotary table, trying to get accurate cuts was tough. On runs where the cutting could all be done in the same direction, especially along the Y axis, the cuts were pretty decent. If the direction changed back and forth things got a little ugly. Fortunately for what I was doing pretty wasn’t a requirement.

I’m sure the only changes that would be needed to get very good results are a good quality 200 step/rev stepper motor that is fast enough to allow good gearing and still maintain a n acceptable speed, and a decent rotary mount. I used the rotary table because it already had a chuck big enough to fit the PVC. The hot lick would be a harmonic drive - a strain wave gear box - which has almost zero backlash. There are 3D printable version on Thingiverse. I know you can do all this without spending a ton of money.

Anyway, give it a shot. I’m surprised more people haven’t tried it. And it’s really cool to watch.

Thank you for the info I’ve been looking at buying one of the 4th axis rotary off of Amazon. Im trying to see if the control box would be able to handle one.

Greg, you won’t use the control box. The Ramps board will do the controlling, and you need a separate power supply for the motors. The stepper drivers will take up to I think 30VDC, at least 24VDC. Just swap the motor wires from the LongBoard over to the Ramps.

Would you happen to have any pictures or videos of how you did all this?

Greg, I really didn’t document this very well, as with most projects I only do to see if I can figure them out, and then move on.

There’s really not much to it:
Electronics - Flash the 6-axis GRBL onto a Ramps board, which takes the place of the LongBoard. Install the plugin heat sinks, plug your new stepper into the Y-axis connector on the Ramps board, swap the X- and Z-axis plugs from the LongBoard over to the Ramps board, and connect your new 30V power supply to the Ramps board. The only small rub is making the wiring and GRBL settings work with both the new motor and the LongMill motors. With a newer, similar 4-wire 200-step/rev motor than I used it should be fairly simple.


Hardware - Connect the motor and the gearbox/rotary table/strain wave drive. Try to get motors and drives with as little backlash as possible, otherwise every time you change direction you get less movement than you want. Don’t try to skip the gearbox and go direct from the stepper unless you have a very strong stepper that can microstep. Unfortunately, precision costs. If you have a 3D printer there are some pretty cool strain wave designs on Thingiverse.


Design - If you’re into Fusion 360, it’s pretty good, and getting better quickly, with 4+ -axis milling. There are enough tools built into the education and personal accounts to do most 4-axis work. I had satisfactory results with GRBLGru and GRBL Plotter, but F360 was way easier and predictable.

There are videos on YouTube for adding a 4th axis. GRBLGru has some good ones, especially about using the Ramps board.

Have fun.

What if there was an official 4th axis option for the Longmill…

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Could you set up a similar rotary axis unit similar to those that are used on machines such as the K40 laser for engraving round items. I was thinking unplug the two Y axis LM and then plug in the rotary unit when required for those style of jobs?

Paul, leave the Y axis as is and use the x-axis as the rotary - fewer wires to screw with, and you get two motors moving along the rotary axis. And you don’t have to get the Ramps board configured for two motors in one axis. It was designed for 3D printer control and already has a double plug but it’s for Z.

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