Interesting idea for a custom build of a Primo

Many months ago, I had mentioned I had a 4th axis setup for my Primo’s.

Rotary carving/turning with a dedicated machine is an obsession of mine, and that rabbit hole is very deep. Commercial setups are rather insanely priced, i.e. Bobscnc Revolution, starting at around $1200, add $700 for software, so about $2000. Never mind the CNC Lathes/Turning Centers which are about the same price as a house.

After looking at the Revolution, it has came to mind that using the Primo as a basis for a rotary axis carving/turning lathe attachment might be possible. It uses the rotary A as a Y axis, i.e. rotary Y2A modified grbl gcode post processor.

My idea here is this. With a narrow Y, just wide enough for the Z Core, less than 10 inches, pinning the Y trucks centered, or custom designing a Y mount. X dual stepper series wired, Z normal, and Y as a rotary axis connected to the lathe spindle, behind a 3 jaw chuck via a closed loop stepper belt with a 3:1 ratio.
I have a 6 axis GRBL sheild and arduino mega to drive it. Exploring options now for gcode creation.

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That is very interesting. I have seen several builds that clamp the Y in place and put the rotary on the Y axis driver. But I haven’t seen anyone build a specifically rotary, 3 axis machine.

The core could definitely be simplified to only do one axis. You might even be able to attach it to the truck and only have one X motor. Two tubes along X, one mega foot at each end, a simplified Z core in the middle… And then a completely from scratch or off the shelf rotary axis.

I am sure we could rig something up. But maybe @Ryan should think about making one. That would be cool.

3 axis, where one is rotary, is easier to do in CAM. Well, easier in that you can trick 3D software to do it for you. Harder to think about when doing it. I don’t know how fusion works, but maybe the free version can simulate 2+1D CAM? Having accessible software (not $700) would be a key piece to make it a success.


LR3 feels more natural to convert to X/Z only, or to convert between rotary and cartesian.

For Primo you still need the Y tube through the core and then lock down Y motion, whereas the LR 3 you can bolt down the YZ plates and skip a ton of Y axis stuff altogether.


Your right, I think the LR3 is nearly set up for it. Slight modification to the YZ plates and it is almost purpose built. Are there any good inexpensive rotary axis or would that need to be fabricated? I know there were a couple 3D printed versions that seemed pretty solid.

There were some on aliexpress and the like. I haven’t ever tried one. But something with a few bearings, driven by a belt should be easy to make by aliexpress.

My only concern with the LR is the depth of cut. Because the entire gantry has to clear whatever rotary axis you choose. If you have a minimal chuck capable of holding a 6" work piece, and you try to work on a 1" dowel, you nee the Z to reach at least 2" past the top of the chuck. I don’t know what range for diameter is reasonable, but having the whole gantry move slightly above the top of the bit could be a tough constraint.

It seems like a really interesting design problem.

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What would a maximum length for this sort of work be?

Table leg, half a pool cue?

It’s true the X gantry is a constraint that is not present on the Primo.

Also remember that the router is overhung a bit in the Y direction, so that will add a little bit of clearance where the X gantry can be slightly below the top of the chuck and still miss. I would think 6 inches (or pretty close) difference between maximum and minimum diameter should be doable, even with a short bit.

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There are a lot of benchtop lathes sized in the 12"-18" size.

That seems like a reasonable “standard size” to me, but you know people are going to build larger and risk the flex. It doesn’t have to solve everything in the first version either. If the rotary axis can hold a longer piece than it can work on, you could engrave a pool cue or add detail to the top of a post without needing to build a 36" machine.


Replying to all here,
That is pretty much my thought process as well. The foot, the way I am thinking, would essentially be a linked and beefed up version of the Primo feet. The Y tube through the core, rather than a problem would actually beef up the rigidity of the machine given the narrow footprint and overhung router
The LR3 is a cool machine, but its Z depth of travel is an issue. That is why I am thinking Primo as a basis.
From my experience, the rotary wrapping for a 4th axis can be…hit or miss without the radial projection needed for a true and accurate rotary carving. It can come out stretched or compressed.
For hobbyists/tinkerers the 10x18 bench-top lathe would be a good option, being mostly for pen work. As for lengths, for my wants/needs I am in the 12x34 range as I have 2 identical floor model lathes of that capacity with chucks. I overachieve, use one to turn the other for sanding/finishing. The rigidity of the base lathe is not a question with the larger machine. That larger size allows such thing as drums, bowls and cups/chalices, chess sets, Newel posts and stair spindles. Longer lathes also have the option to do longer than base length through the use of a rolling steady rests on the tailstock side.
For basic CAD/CAM. using basic tools for the flat cam needed then running it through a rotary wrap conversion is probably the way. Scorchworks has the Gcode Ripper that can do rotary wrap and is free. I haven’t tested it very much. Leo did a modified version for his Marlin based laser work/3 jaw chuck 4th axis, quite out of my comfort zone on that kind of coding.
My research and searching over the last week or so shows a interesting disconnect as well. Closed loop Gt2 belts of a lot of sizes are available so that is a non-issue. On the other hand larger GT2, 10mm wide pulleys are fairly thin or priced like precious metals. It seems the larger size/teeth count in industry goes to the HTD spec. On the cited example of the Revolution, from what I can see they laser cut the pulley for the head side with a 180 or 360 tooth perimeter, and use a 20 tooth motor pulley. The back of the head is also marked with a degree wheel to assist in indexing the carving for A zero.

Well that is it until I have some more coffee…2:45 am here


Cup of coffee to the rescue.
For the Y axis to make it rigid I suppose I could use the Primo tower tops turned 90 degrees rather then pinning or custom stops

I would do something different.

I’d use two parallel tubes for the x axis. I’d mount the router between the two tubes.

That would make your forces in a triangle between the two tubes and the end of the end mill.

Z axis could be similar to mpcnc design.

Bonus would be if the router was mounted in a clamshell design so that it was supported by the rails on both sides. This would keep it from tilting left/right and forward/backwards. I would put the guide rails on the left and right with the stepper on the back since I think most of the forces would be in the side to side direction.

This would be very similar to the original lowrider except the x axis rails would be fixed and the router mount would move up and down.


I was JUST thinking about how to build a dedicated rotary cnc yesterday. Must be something in the air. Hopefully it wont take me the year that I didn’t looking for time to build tbe repeat. Some really good ideas here.

The non fixed rails of the LR1-2 were the weak point. They are pretty flexible. The shorter they are the more rigid, by far.

12" Z? So a 24" workpiece?

Software is a major concern for sure.

6 inches, the capacity is 12, max depth of cut would be at most 6

Ha, ha. No kidding. I’ve been planning on adding a rotary axis since before building my Primo. It’s one of the reasons I built a drop table. But after seeing @Legolor ’s recent post I started working on a concept.

My current concept is for a dedicated machine, mainly for rigidity (hoping to eventually cutting Aluminum and brass). I was thinking to have ~3.5” in Z travel for a max workpiece size of ~6”. I can’t really think of what I would make larger than that except maybe bowles but then I’ll have a cutter issue so I need to think about that a bit more.

I’m planning on using an Arduino/grbl shield for cost/simplicity but I believe that will make things more challenging if I want to run a laser. I’m also thinking about using a higher torque Nema 23 for the Y/rotary axis but that will require a non-standard driver so I may start with a 17.

Lots to ponder. :grin:

OK, been a few days, so an update on where I am on this.

Finally found a working GT2 pulley generator for FreeCAD. It generates the needed large size/tooth count pulley for the lathe spindle. Bonus is it also can size the center hole to fit the lathe spindle and add set screw. I have generated 2:1, 3:1, and 4:1 ratios. First prints of the 2:1 with a 0.8 nozzle look good, and the belt I have fits very well.
I have generated a linked foot STL for the ends, and this morning I am working on the tower top/Y axis mod to make rigid.
Have located 4 different rotary axis Post-processors that need slight modification, both GRBL and Marlin, for the standard CAD/CAM programs we use. Also located several for VCarve and ARTCAM/Carveco.
CAMotics currently doesn’t support 4th axis turning simulation, but I am told by the developer that it is in the works.
What a fun, and time consuming rabbit hole. I am sure if I was a Fusion360 guru this would go a lot faster. Oh well, carry on with what you know.


Which generator worked for you. Ones I tried were not so useful, or perhaps user error :confused:


Thanks,time to play with it🙂

Got delayed on the build/design, my 3d printer had some serious issues, back at it this week

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