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What torch height control does

CNC torch height control (THC) adjusts the torch-to-work distance while a cut is underway. It reads arc voltage as a rough measure of that distance: with the same consumables, cutting current and material, voltage generally rises as the torch moves farther from the plate and falls as it gets closer. The THC raises or lowers the torch to keep voltage near a set point.

THC is most useful on plate that is not perfectly flat, or when long cuts and warped sheet make a fixed torch height unreliable. It does not replace the initial height-sensing step, and it cannot correct every problem. Incorrect amperage, worn consumables, poor air, or a bad cut chart will still produce poor cuts.

Check compatibility before buying

Start with the plasma cutter manual and the CNC machine’s wiring diagram. The cutter needs a way to provide an isolated arc-voltage signal, and the CNC controller needs a compatible THC input. Some cutters provide a divided-voltage output specifically for CNC; others require an approved interface or voltage divider. Open-circuit and raw arc voltage can be dangerous. Do not connect a controller directly to the torch lead or improvise a divider unless the cutter manufacturer and controller documentation explicitly support the arrangement.

Also check whether the cutter provides an arc-ok signal, which tells the CNC when a stable cutting arc is established. A THC that starts correcting before pierce delay ends can drive the torch into the plate or lift it during the pierce. Confirm that your machine can disable height correction during piercing and near the end of a cut.

If you are choosing equipment, compare a plasma cutter with a CNC divided-voltage port against adding an interface to a cutter you already own. The port is usually the simpler, lower-risk route. An interface can make sense when the existing cutter is otherwise suitable, but verify isolation, voltage ratio, and compatibility before ordering.

Choose the right THC setup

A basic THC is often enough if your table is rigid, your stock is reasonably flat, and you cut common thicknesses. Paying more for faster control or multiple sensing methods is worthwhile when warped sheet, production repeatability, or frequent material changes make height consistency important. For a retrofit, an isolated CNC plasma THC should match your controller’s input type and the cutter’s divided-voltage output.

Mount the torch and connect the signals

Install the torch in a rigid mount that can travel freely on the Z axis without twisting. Check that the lead does not pull the carriage or snag at either end of travel. Set the floating head or ohmic sensing hardware according to its instructions, then test that the Z axis can touch off and retract without binding.

With power disconnected, wire the divided-voltage signal, arc-ok signal, and any up/down control connections according to the equipment manuals. Keep signal wiring away from torch leads, motor cables, and other sources of electrical noise. Use the specified shielded cable and grounding arrangement; ground-loop problems can cause erratic height corrections. Do not assume that a connector’s pinout matches another brand’s just because the plug fits.

Before cutting, test the Z axis direction. A command to raise the torch must move it up, not down. Then test surface sensing and arc-ok without enabling automatic correction. If the torch dives, moves the wrong way, or reports an arc when none is present, stop and correct the wiring or configuration before striking an arc.

Set cut height, pierce height, and voltage

Use the plasma cutter’s cut chart for the material, thickness, amperage, and consumables. As a starting point, many air-plasma systems cut mild steel with a torch-to-work distance around 1.5 to 2.5 mm, but the correct figure varies substantially by torch and process. Set pierce height higher than cut height when the manufacturer’s chart calls for it; piercing too close exposes the nozzle and shield to molten metal.

Enter the chart’s initial voltage target only after the cut height and amperage are correct. Run a straight test cut at a conservative speed, then inspect the top edge, bottom dross, bevel, and kerf. Adjust one setting at a time. A voltage target is not universal: the same displayed voltage can correspond to different heights with different consumables, currents, air pressure, or worn parts.

Tune THC and diagnose common failures

Begin with THC disabled. Confirm that touch-off, pierce delay, and cut motion work reliably. Enable height control for a straight cut in flat scrap, then make small voltage or response adjustments. If the torch oscillates up and down, the control may be too aggressive, the voltage signal noisy, or the cut speed unstable. Reduce correction gain or response speed and check cable routing before changing the target voltage.

If the torch rides too high, inspect the voltage ratio setting, cut-chart target, and consumable condition. If it dives on corners, the machine may be slowing down while arc voltage falls; enable corner lockout or anti-dive if available. A torch that lifts at the end of a cut may be reacting to a voltage change as the arc exits the plate. Configure end-of-cut lockout and verify the CNC’s arc-ok timing.

Keep consumables clean and replace damaged electrodes or nozzles. A worn nozzle changes the arc and makes the voltage-to-height relationship less dependable. Likewise, water in the air line, poor work-clamp contact, loose connections, and warped stock can create symptoms that look like a THC tuning problem. Change one variable at a time and record settings that produce a clean test cut. For frequent setup changes, a CNC plasma floating-head torch mount can make mechanical touch-off more repeatable, though it is not a substitute for correct wiring or torch height settings.