[michiganiron]

Weve got an EZ-Router 8'x4' table, with a Hypertherm Powermax 1650, running on Mach3, and having some trouble with the THC. When I turn it on, it just seems to want to dive into the workpiece right away. The program will run fine otherwise. Once the tip dives into the piece, it seems to want to keep diving, the LED for "down" on the screen will stay on the whole time. The other thing i noticed is that even when the THC button is toggled off and you run a program, the "down" LED will stay lit for the entire run, although the torch head will not dive without the THC toggled on. Anyone have any similar experiences or any ideas? Thanks.

[Torchhead]

THC's "dive" for several reasons. It's all about gap volts. The gap of a plasma cut (distance from the end of the nozzle to the workpiece ) determines the gap voltage. The wider the gap, the higher the voltage. The THC measures the gap volts (like an altimeter) and compares that to the "Preset" (target) voltage you want. That voltage is determined by a chart from the plasma manufacturer. It varies with tip size, material thickness and type of material. The comparison results in a correction being issued to MACH. MACH determines how to apply the correction via the Z motor.

So, if the feedback voltage is too high (above the target voltage) then it drives the tip down until it reaches the correct voltage. If it's too low it lifts the head up. In short it is a loosely coupled servo system that is supposed to continually correct and hold the tip volts (and cut gap) within a narrow tolerance. Since with most cutting you are working with a gap of .063 while moving at 100+ IPM the margin for error is limited. If you are trying to cut with Fine cut or low currents and small tips on thin material you are typically at .040 down to .010 tip gap. Just remember the torch moves under THC control in the opposite direction as the gap volts until to reaches the "gap tolerance".

If your tip dives to the metal then either your target voltage is incorrect (too low) for the tip and material you are cutting, or the tip voltage coming back from the plasma is incorrect (too high) or the Z axis is setup in reverse. (Don't laugh, it happens more than you think!).

First thing is determine the actual tip volts while cutting with your machine. Setup a flat piece of material and cut a long straight line witht he THC button turned off in MACH. Watch the Tip volts DRO and mentally average it. The goal is to have the tip gap close to what it should be (based on your plasma torch) on a manual cut. That should yield a tip volt reading that is close to the numbers in the manual. Also watch the UP and DOWN LED's. Now program in the value you observed in the manual cut into the preset volts and run the cut again. It should start to servo up and adjust the torch up and down as it cuts. Confirm when the Down or UP LED is on the THC then the corresponding UP and DOWN screen LED's in MACH light up.

The piercing process causes a voltage spike, that if acted on by the THC, will drive the head down. There is a THC Delay setting in MACH that can be set to compensate for that (ignores the THC signals for the duration of the setting).

Some conditions that cause the tip slamming onto the metal as you cut are predictable. If you cut over a void, cross over, or too close to, an existing cut, or approach the beginning of a cut (lead-in) the head will dive. If the feedrate drops on tight turns the torch tends to cut more "air" and the voltage goes up (which drives the torch DOWN). Those conditions should not happen on straight cuts. More advanced controls will allow you to set the actual Target (Preset) voltage, hold it to + - 1 V or better, and will have a "Head Lock" (aka head freeze) feature that will not let the torch drop below a preset point.

TOM CAUDLE
www.CandCNC.com
Totally Modular CNC Electronics.