Usually THC reads arc voltage of a plasma machine and adjusts the height of the Z Axis motor for cutting. I've seen a couple of oxy height sensing products in the net such as Hypertherm OHC ( Oxy Height Sensor ) and Rutex r9812 Oxy Complete Kit that suits my application, but the spirit of building your own kit by yourself is not there. Anyone here can provide some information or if even possible, circuit schematic designs regarding this height sensor? I'm using Mach3 by the way.