Retrofit for esab shadow 2, 2axis burn table.

The motors and drives look pretty good. CMC have changed hands, but Advanced Motion Controls still exist. They have manuals and app notes for their stuff available.

Moving to stepper drive will slow the machine down considerably- if you mainly cut thicker stuff, that might not be an issue other than losing some rapids. Don't generally consider steppers to be usable over about 400rpm- look for that point in their torque curves, etc. If that works for your rack and pinions at the machine/cable/hose weight required and gets you to your cutting speed, a stepper swap is definitely going to be the easiest way to go and opens up basically all your grbl hardware options. You don't need to worry too much about extra overhead power since you're not using physical cutting tools.

However, if you have to go NEMA34- which wouldn't surprise me, that's a beefy-looking machine- then your easiest option there is to go closed loop (gets you a touch more power at any given rpm) because the kits include the matched drivers and are widely available fairly inexpensively. You can still use grbl, but you'd need hardware that easily allows for breaking out the STP/DIR pins to the stepper drivers. Doable with BlackBox, but not convenient and definitely not cost-effective if you don't already have one. Though you could still use OpenBuilds CONTROL with any other grbl hardware, even a plain Arduino Uno.

If those are analog servo drives running in velocity mode- which I'm guessing, with all the "signal"s and "ref"s and "tachometer"s on there- and you determine it's best to keep those for high-speed torque requirements or whatever, you'll need a control that can deliver 0-10V analog output for all three axes. Which to my knowledge off-hand, is only LinuxCNC with a Mesa 6i25 and probably a 7i77, though I'm sure there are actually other options out there. That's a... Pretty complex setup (I have a 6i25/7i76 setup on a mill), I wouldn't recommend it for a beginner on a time budget, though once it's actually done it's very powerful and pretty bulletproof for a working environment.

In an ideal world, you can get away with a NEMA23 stepper swap, if that'll give you the speed/torque requirements you need.

 

They all say they are. They're not. Figure 400rpm max under load. Acceleration settings matter here too- they're fairly easy to stall, especially open-loop steppers. They're not like servos where they know the motor is struggling and can dump more current to overpower the load- the step commutations are electronic. Think of them kinda like 200-pole brushless motors. So on an open-loop stepper, where the driver has no idea where the motor is, it just keeps commutating the current and the motor has stalled way back because the magnetic field has moved further on than it can recover. Makes a terrible grinding noise too, but it's pretty harmless.

Yeah, not sure about that. 1/4" shafts are pretty puny but they do take a fair bit of force, plus you have the mechanical advantage of the pinion gearing. That said, NEMA34s come with either 1/2" or 14mm shafts, usually, for the 3-9 ftlb range. If you can find a NEMA24, they often come with 8mm shafts which is a little better. Not as common a size though, usually run around 2ftlb.

A sensible mindset to take when going in semi-blind. It's a very large topic and takes years to really fully start to grasp, many decades to become an expert, I'm sure. It's not too hard to get up to speed with the basics though. Others have definitely done similar work to this in the past that you can probably use to help.

Any of the solutions I offered would be comfortably under $1000- even the LinuxCNC one. Of course you can add time and additional materials (cutting motor flange mount adapter plates, say) but it's not an enormously expensive- or even complex- project, just takes some specialist knowledge. Which of course isn't your friend when you're on a deadline.