Sorry I didn't get back to you. Totally missed the notification from your first reply.
Was trying to understand if you were building a 5 ton monster or a table top sized machine. Changes my advice
Looks like a solid design with Nema 23 425oz-in stepper motors and single axis microstepping drives. These shouldn't really chatter other than the hum of the drive's voltage chopping at the motor. If you hear chatter, groaning, or anything like that when it is stopped, then I'd be concerned you have noise problems.
Design for noise immunity is super important on a plasma CNC or the EMI emitted by the arc can cause real problems with your controls. You should definitely be using shielded cabled for any low voltage signals (limits, ESTOP, etc) and probably for the motor cabling as well. Make sure you have good grounding in your enclosure (including the enclosure itself.
I'm a mill and lathe guy, so a bit less experience with plasma, however I think you would be connecting to whatever port allows remote operation of the plasma unit (maybe the footswitch port?). You'd probably want a general purpose control relay.
Here is one option. I'd get probably a 5 or 10A contact rating. Make sure to get the mating socket if you go with one of these. Industry uses 24V for control pretty exclusively, so you are going to have a hard time finding a 5V relay. If you need one, Amazon and the arduino community might be your answer.
Buy JBtek 8 Channel DC 5V Relay Module for Arduino Raspberry Pi DSP AVR PIC ARM: Computer Components - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
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