Lol breakfast tacos. I do owe him a beer, or at least some help with a plasma table when the time comes.
I built my chassis to this design
https://www.instructables.com/id/CNC-Plasma-Table/
It runs pulleys. With a jet cutter, you don't really need the torque from a screw drive, but you want speed. Especially for thinner material that you have to cut faster. That's not to say that you can't do it with screw drives, and you'd probably have less backlash. This is a little cheaper I guess. It runs V-groove bearings on 2X2 box tube turned diagonally. One day I will use this one to build another one, and I will probably make a 4X8 or even a 5X10. I'll use linear bearings next time and something beefier for the drive. Maybe ball screws, maybe just bigger pulleys/belts. This works, but if you want to do really fine detail, the backlash could be better. I run two X-axis steppers and one Y, all big (3 Nm?) NEMA 23's. Overkill, I'm sure, but the price wasn't much different. The Z-axis is a smaller stepper, I forget off the top of my head what it is, and that is a screw drive with awl thread and a captured, floating nut.
It's actually controlled by an Arduino Uno with USB into my laptop. I liked being able to CAD in the old house, make the CAM files, and then go out into the garage, plug the laptop in and burn. There's no tower that has to stay in the shop and get files transferred onto it. I have a GRBL shield (another board with pins that fits directly onto the UNO), and originally had 8825 drivers. Drivers take step pulses and a direction (high/low) signal and a DC power input to run the motors with current that can't go through the control board.
The Uno is running GRBL firmware, which is mostly used for printers and some routers. The 8825 motor drivers connected right to the shield, but weren't skookum enough for my giant NEMA 23's (should have known just by looking at them... but the spec's checked out). I swapped them out to TB6600's and stopped loosing steps/overheating drivers.
The tricky part with using 3D printer firmware was the Z-axis. With a plasma, the Z-axis has to be both open-loop and closed-loop, depending on the situation. The X and Y axes are both open-loop full-time. Printers, routers, mills, are all open-loop, meaning you put in a command and the machine does it with no feedback to modify the toolpath. The Z-axis on a plasma is controlled with feedback from the arc voltage while cutting to maintain a set arc length. So my controller sends Z steps and Z direction signals through relays. The relays are controlled by the coolant pin on the Uno. Normally, the steps/dir are passed from the Uno through the relays to the drivers. That allows the CNC (Uno) to control the Z for clearance moves during rapids, and to probe the material before each pierce. The Z axis hardware floats on bearings. The CNC drives it down into the workpiece until it touches and keeps going until a limit switch opens. Then it sets the Z height to some negative number (whatever it takes to go from touching the work to opening that switch). Then it can go to pierce height, start the arc, lead-in and drive down to cut height. At that point, the "coolant" is turned on, switching control of the Z-axis driver to a Proma torch height controller (THC). The Proma gets a divided arc voltage from my Powermax 45XP power unit and adjusts the Z axis height to maintain that arc voltage to the target voltage, within a tolerance. With the THC, if the bed isn't 100% parallel to the axes, or the work warps from the heat, you still maintain the desired arc height for a clean cut.
It would probably be easier to use an old tower with a printer port running Mach CNC software, rather than the Uno/GRBL route. It's been done, for plasma, a million times so you don't have to figure this stuff out on your own. The printer/serial port also runs at a higher signal voltage than USB, which increases your signal to noise ratio and makes interference less of an issue. I had to shield/ground all of my cabling and drive a separate ground rod just for the table to ground the EMI.
I got a lot of my hardware on ebay, waiting 3 weeks for orders from China. Order spares, it's saved me a few times. I figure all-in, I probably have $1,500 in the table. The plasma was $2,200 with a machine torch. I spent a few hundred more to have a hand torch too. They can be very easily swapped on the Powermax. I had the laptop. For CAD, you can download Draftsight 2D for free. Inkscape is another free program that can create .dxf (vector files) that can be used to cut. It's more of a graphic design program, so its good for anything decorative like signs. For CAM, I spent $120 on SheetCAM, and it was money well spent. The CAM program takes your CAD file and turns it into a toolpath. You can get a free trial version that just limits how many lines of G-code it will make. There is a text file called a Post-Processor that takes that toolpath and turns it into G-code, which is what the CNC controller reads. SheetCAM comes with maybe 40-50 post-processors which are set up for different CNC controllers (such as Mach or GRBL). The post-processor can be customized, which is how I tell the controller when to turn on the "coolant" to trade control of the Z-axis, and how to probe the material.
I'm probably rambling now... if you get into building one, you'll have questions. I'll do my best to help you out if you need it!
