There are a couple of ways to get there. You can mount a separate Z mechanism on the same carriage as your plasma head and let it have it's own motor and driver (independent Z2). At that point you have total control of both processes provided:
1. Your Z is a real axis and part of the toolpath
2. You can change the POST in the CAM to work with two Z's
3. Your hardware and control software can support the additional axis.
We helped program a machine recently that had to do mixed processes (drill, route and plasma cut. It even had to be able to mount two router motors and use a different tool in each, all on the same table. It ended up being a 4 axis (5 motor/5 driver) setup with there being X (dual drive), Y , Z1 and Z2. Either Z could be: plasma, spindle (high speed) or special process (marker, center punch, drill)
For the average hobby guy or small shop building a plasma table I don't advocate a mixed use table unless they don't have the physical space for two tables and they don't mind spending the time to switch over the physical setup. For a shop that builds a custom product or needs the flexibility of doing multiple processes on one machine that saves handling and labor, the multi-use machine starts to make sense.
Because the plasma control (with DTHC) through MACH3 uses a normal Z axis (even for THC control) switching to another process on that or another axis is a 10 second software profile change. By writing a custom POST in SheetCAM, the tool number automatically determines the active axis, retracts the non-active axis, AND applies an automatic offset so it does not have to be done manually in the CAM program. It will switch back and forth between the heads and position them properly based entirely on which tool number you select in SheetCAM. Since the feedrate and even the process (peck drilling, etc) are a function of the tool definition, all of that is automatic too. It does it by switching the Axis call (turns what would be Z moves in the g-code to "B" axis moves and apply a fixed XY (and B if needed) offset for just that process.
On a table designed to do 2 axis of motion and there is no true Z (only a torch lifter and independant ATHC) your options start to diminish. Most often it is not as simple as just adding another axis (slide and motor) because the software does not support the new axis. You can, of course, make it totally manual and just build something to do the one process you want and trigger it externally, but things like offset, feedrate, spindle RPM and peck cycles will be difficult to impossible to do.
Tom Caudle
www.CandCNC.com