I want to retrofit CNC to an old pattern torch. The basics of the mechanics will be simple. Build a frame with a stepper at each corner. Drive with four cables, each with a ring at the end that fits onto the drive magnet above the torch. To return to original pattern following use simply lift the rings off and place onto pegs on the frame for storage.

Like this IPAnema with half the cables and working only in a 2D plane, and doesn't need to be anywhere near that fast.

In other words what I want to do is like a flycam that doesn't move up and down, and is a heck of a lot smaller than the systems used at racetracks. ;-)

The hard bits. Software. Can Mach3 or LinuxCNC control such a system? Cable winding without having the drum diameter change from the cable layering or moving up and down. The method used on the IPAnema does the trick but looks quite expensive to have made and its bulky. How about a work envelope that isn't a quadrilateral? The space the arm can reach is pretty much a half circle, though in use it never gets used up close to the column or at full extension.

I know there's ready made cable pulleys with spiral grooves for drive systems that need the cable line to stay in plane, but I haven't been able to find any new or surplus. I've seen them in really old daisy wheel printers like a Diablo 630, but who has a few of those laying about these days? Parts of the carriage drive from one of those would do the job. The Diablo 630 had a massive carriage and it was capable of zipping it through 15" of travel in 0.04 second. Even with a heavy printer cabinet (those things were LOUD) mine would shake it around like it was having its own personal earthquake.

As for motor size and power, the torch won't need much. I can easily move it with just the tip of one little finger. 100 oz-in would likely be grossly overpowered. Torch on/off is manual on this old thing, no auto height control, doesn't need it. Never gets used on anything less than 0.25" Cutting speed is most often quite slow.

I've seen several kits with four NEMA17 motors, power supply and integrated control board for $200 or less which ought to do the job just fine, assuming the software will work it.

Give it more power, a floating head + Z axis with THC and these old two joint arm pattern torches could make inexpensive CNC plasma tables - but that's not needed on *this* one. (Going to do a boringly conventional, large gantry for plasma.)