[jimcolt]

It all sounds pretty simple...make sure you cover the following issues as well....like most of the available plasma THC systems do:

-Find the plate surface then retract to manufacturers suggested pierce height. (different for different power levels and thicknesses...must be operator setable)
-After pierce starts...activate a timer that freezes x, y and z motion until the pierce is complete. (different for different power levels and thicknesses...must be operator setable)

- Once pierce delay timer completes, index rapidly to a physical cut height as recommended by torch manufacturer. (different for different power levels and thicknesses...must be operator setable)

- Once x and y have achieved at least 80% of cut speed....activate arc voltage feedback circuitry to control height. (some THC systems sample the arc voltage at the beginning of every cut, after motion is near programmed speed and lock on that voltage, others use an operator set voltage that will maintain the physical cut height)
-when machine decelerates for corners and fine features....freeze the z axis to eliminate diving.
-when crossing the start kerf at the end of the cut....use kerf crossing voltage anomaly recognition to minimize torch diving.
-be sure to filter divided arc voltage to minimize hash and noise that is the result of the plasma arc interacting with the molten metal movement....voltage output varies quite a bit.
- As the torch electrode wears...the voltage reading will increase....causing the torch to be closer to the plate. Allow for automatic compensation if possible.

These are a few features that make plasma arc voltage height controls work well. I'll be happy to offer advice as I have been working with these devices for over 35 years. I would love to see a functional, stand alone THC system that would work on any cnc / plasma for a low price. Industrial THC's cost between $5000 and $12,000.....including torch lifter/drive.


Jim Colt

[Robin Hewitt]

Been thinking. Rarely a good idea at my age...

A stepper motor is the wrong tool for the job, it wants a little DC gear motor driving a screw.

A power op-amp to drive it so it can receive plus or minus volts to go up and down with a two wire connection.

It needs to be able to rise to the top and drop down to touch. I can sense those with two microswitches. If I split the motor power line with two diodes I can route the plus through the "top stop" switch and the minus through the "bottom hit" switch. That way it will stop automatically at top or bottom.

To do torch height I send the error to the motor. If it's way out it goes quick, if it's barely out it hardly moves at all.

That only leaves rise to the pierce height, still thinking on that, want to keep it simple.