I have an old plasma table, Aviator XLT that came with a Burny 1250 controller, Thermal Dynamics PakMaster 150XL and PCM-120 torch for the plasma. The Burny broke again and it was not feasible to repair so I updated the machine with new controls and servos. The CNC controller is a Masso G3 Touch with Automation Direct SureServo2 motors and drives. The unit uses a Masso DTHC for torch height control. The Masso is mounted on the outside of a metal enclosure that rides atop the gantry with the remainder of parts inside.

I do not know how the Burny was detecting the work surface while probing (maybe monitoring the Z axis servo amperage?) so I converted the torch to a floating head style with a limit switch to signal the Masso that the surface was reached. Because of the gantry design, this probing was not reliably accurate. However, I did make many test cuts with this setup that worked OK. The plasma worked, the machine worked, the DTHC worked; the only issue was unreliable probing. I always intended to use an ohmic sensing setup anyway so I moved on to adding a CNC4PC PTS-1 Plasma Touch Sensor. The machine style shield cup sits above the nozzle height so I bought a drag shield cup and ground down the prongs so that they were slightly lower than the nozzle. I tapped the shield cup and used a ring terminal to attach a wire to the cup that would feed back to the ohmic sensor's input. The shield cup is electrically isolated from the torch. The PTS-1 Ohmic sensing unit has a dry contact output that is opto isolated.

When I first tested the machine with the Ohmic sensing, the Z axis servo drive fried immediately when the plasma arced. I reviewed my setup and realized that I had placed the PTS-1 unit inside the control panel, even though the output from the unit was opto isolated, the signal wire from the torch shield was still going inside the cabinet. I thought this was the likely culprit so I moved the PTS-1 to the outside of the cabinet and tried again with a new servo drive. Upon the torch firing, the X axis servo driver was immediately fried.

Looking at the setup, I do not understand why this is happening. I did also realize that I believe the original setup on the table used the external shielding of the servo cables to bond the different axes together. The new setup uses internally shielded cables so there is no extra bonding of the axes. Could this be the problem? Is it likely that bonding the axes to the star ground would fix this issue? I do not want to experiment and toast another servo drive if possible.

I am clearly missing something, either the problem of no bonding was already an issue and it was a coincidence that the drives are frying after adding the Ohmic sensing, or something about the Ohmic sensing setup is to blame.

Thank you