Learn/GcodePilot/Automation & safety
Arc Voltage THC (AVTHC)
Updated v2.4.9At a glance
- FluidNC plasma fork holds cut height by arc voltage — sampled, compared to target, Z nudged in firmware
- Live arc volts, arc-OK, and correcting state on the DRO; setpoint from the post via M103 Q
- Operator override: bump target volts from the pendant or run g-code controlled — remembered per machine
- Armed/disarmed mid-program with M101/M102 — off during the probe, on after the pierce
- Arc-OK override: click (or hotkey) to confirm the arc and keep cutting when the sensor misses it
- Auto-Tune calibrates the voltage target to your machine's real electronics — see AVTHC Auto-Tune
On the FluidNC plasma fork, torch height control runs in the firmware: it samples arc voltage, compares
it to a target, and nudges Z to hold standoff — no host round-trip in the loop. GcodePilot configures it
from the machine profile (the Torch Height Control node, plasma machines only), the post sets each
cut's target with M103 Q<volts>, and M101/M102 arm and disarm the loop mid-program — off through
the probe, on once the pierce is done and the torch is at cut height. The fork ships with JetCad3 and
is required for AVTHC — flash it from the built-in tool; see
Machine Connection.
Reading the DRO
With AVTHC enabled the DRO gains an ARC row: the label is gray when the loop is off and orange when armed; the voltage readout is green while holding within tolerance, red while the loop is actively chasing the surface, and orange when armed but no arc has established yet. The arc-OK dot lights green when the arc has transferred — and it's clickable: if the arc is visibly cutting but a flaky or unwired sensor never asserts, a momentary click satisfies the pierce's arc-OK wait so the cut proceeds.
Operator override
The AVTHC pill on the bottom bar works in target volts and is safe to touch mid-cut — every action is
a realtime byte that never disturbs the running program. The pill has three modes. Gcode Controlled
(the default) leaves the program's M103 setpoints in charge, shown grayed. Touching −/+ (bindable
hotkeys, default − and =) or clicking the value enters Override, seeding your target from the
current effective setpoint — and from that moment the override value is what the firmware holds
automatically; the program's M103 lines are ignored until you click Gcode Controlled to hand the
setpoint back. Clicking the value swaps in an inline input for an exact target, and Disable forces
THC off for the rest of the run. Mode and override volts persist per machine and re-apply on reconnect.
Auto-Tune
The chart voltage assumes a perfect measurement chain; real dividers, ADCs and scale settings make the measured voltage land somewhere else — and the torch holds the wrong height while the readout looks right. Auto-Tune samples what the arc actually measures at true cut height right after each touch-off, walks the target onto it a little per pierce, and offers to write the calibrated voltage back into the cut chart. Full write-up: AVTHC Auto-Tune.
Manual Z comp — and snagging the arc reading
Flip on Manual Comp and the Z jog keys (PgUp/PgDn) hand-comp torch height mid-cut: the first
press switches AVTHC to Disabled in a seamless handoff and drives Z directly, the readout showing
Up/Dn while you hold. When the cut looks right, a quick + or − re-enables Override at the
live ARC readout — the loop snags the voltage the arc is reading right now as its new target, so
auto THC resumes holding exactly the height you just dialed in by hand, and further +/− taps nudge
1 V from there. Deliberately, this does not flip back to Gcode Controlled — your comped height wins
until you explicitly hand control back. While Manual Comp is driving, automatic height control is off:
the torch will follow your keys into or off the plate, so stay on it.
Tuning without re-posting
Right-click a cut in the toolpath view → Change AVTHC… to retarget or disable THC for one segment, one cut, or the whole program — useful for skipping a lead-in ramp or dialing volts on the next pass. See Run From Here & Cut Tuning.