I recently had a project that would have benefited from a 4th axis. I was able to work around it, but it shows me that I need to invest in one for future projects. I see Tormach offers the MicroARC. I also saw in the forum about not using anything bigger due to the clearances. Are there cheaper 3rd party options that work with the 440 and PathPilot, or is the MicroARC the only one that will work? Something with a live center on the other side would be preferable for longer, thin aluminum parts that I expect to cut on it.
In theory, any off the shelf rotary that is driven by a bipolar stepper motor will work. Way back in the day I built my own 4th axis with an amazon dividing head and a random nema17 stepper I had laying around. Worked pretty well, all things considered. There is someone in one of the FB tormach groups that built his own microarc clone and has plans available for reasonably cheap, but I don’t recall his name or website offhand.
There are some caveats to using anything not from Tormach:
You still need a stepper driver in the cabinet. You can (in theory) get one from where ever you’d like, but you won’t find one with a ribbon connector like the Tormach version. You can get a an adapter to go from the ribbon wire to pigtails, or you can roll your own method to connect everything. It will take some digging in the wiring diagrams to work out how to connect the driver to the machine controller but it’s not too difficult to work out.
You’ll also need to modify PP a bit to account for the steps per rotation of whatever you buy/build. I don’t recall the specific reduction in the MicroArc but I want to say it’s 90:1. The other 4th axis options from Tormach are different. When you select which one you have in PP, it will apply an axis scale factor based on the gear reduction for whichever rotary you selected but if you buy or build, you will have to edit that scale factor unless you happen to find something with the same gear reduction of one of the Tormach options.
As for a live center, that has little to do with which rotary you buy. Just pick one and either mount it to a subplate, or mill it shorter so it’s inline with the center of your chosen rotary.