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What is a THC and why your DIY plasma table needs one

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If you’ve built or bought a DIY CNC plasma table, you’ve probably had this moment: the first straight cut on a fresh, flat sheet looks great. Then the sheet warps a little, the cut quality falls apart, the dross gets worse, and somewhere around the third part the torch clips the sheet and drags. Welcome to plasma cutting without a THC.

What THC actually means

THC stands for Torch Height Controller. It’s a small controller that continuously adjusts the distance between the plasma torch and the workpiece while the machine is cutting.

Why does that distance matter so much? In plasma cutting, the arc voltage depends directly on the gap between the torch and the metal. Double the gap and the voltage roughly doubles with it (the exact ratio depends on your plasma cutter, but the relationship is monotonic and very usable). That gives us a beautiful property: measure the arc voltage, and you know the torch height.

A THC does exactly that, in a loop:

  1. Read the arc voltage through a voltage divider.
  2. Compare it to a setpoint — the voltage that corresponds to your ideal cut height.
  3. If the voltage is too high (torch too far), move the Z axis down. Too low, move it up.
  4. Repeat — in SmartTHC’s case, a thousand times per second.

Why you can’t just “set the height and forget it”

On paper, you could pierce at the right height, lower to cut height, and let the gantry do its job. In practice, three things ruin that plan:

The sheet is never flat. Even a brand-new 2 mm sheet has waves in it. Lay it on slats and it sags between them. Across a meter of travel, the surface can easily move several millimeters — more than your entire optimal cut gap.

Heat warps the sheet as you cut. Plasma dumps enormous localized heat into thin metal. Parts lift, edges curl, long cuts turn into potato chips. The shape of your workpiece is changing while you cut it.

Consumables wear. As your electrode and nozzle wear, the arc characteristics drift, and with them the voltage at a given height. A fixed height that was perfect at 9 AM is wrong by the afternoon.

The result of ignoring all this: inconsistent kerf, heavy dross on the low side, beveled edges on the high side, and — the expensive one — the torch physically hitting the sheet or an already-cut part that curled up. One collision can destroy a nozzle, bend a torch, or scrap the whole plate.

What a THC buys you, concretely

Running my own table daily in a professional workshop, here’s what changed when height control came online:

  • Cut quality stays constant from the first part to the last, because the arc gap stays constant.
  • Consumable life goes up. Correct standoff is the single biggest factor in nozzle and electrode life.
  • Thinner sheets become practical. On 1–2 mm material, warp is so severe that cutting without height control is basically gambling.
  • You stop babysitting the machine. Start the job, walk away, come back to finished parts instead of a crashed torch.

The options on the market

There are roughly three ways to get torch height control:

  1. Commercial THC boxes (150–800 €). They work, but most are sealed black boxes. When the behavior is wrong — and it will be, on some jobs — you get a potentiometer and a prayer.
  2. THCAD-based setups with LinuxCNC. Excellent performance, but you need a PC running LinuxCNC, a THCAD card, interface hardware, and a serious time investment to configure it all.
  3. DIY controllers. Various Arduino projects exist, ranging from proof-of-concept to genuinely usable.

I built SmartTHC in the third category because I wanted the first category’s simplicity with the second category’s transparency, at a DIY price: an Arduino Uno R4, an LCD, a stepper driver and a handful of parts — around 40–60 € total, with fully open-source firmware you can read, audit and tune.

The part nobody talks about: anti-dive

Here’s the catch that most THC marketing glosses over. A THC’s job is to follow the sheet, and it does that by chasing arc voltage. But what happens when the torch crosses a hole — a slot it just cut, a pierce point, the edge of the sheet?

The arc suddenly has no metal under it. Voltage spikes. The controller reads “torch way too high” and slams the Z axis down — straight into the workpiece. This is called diving, and it’s responsible for a huge share of crashed torches in DIY plasma cutting.

Every serious THC needs an answer to this, and the quality of that answer is mostly what you’re paying for. It’s important enough that I wrote a whole separate article about it: Anti-dive explained: why torches crash on holes and how to prevent it. If you take one thing from this post, take this: when you evaluate a torch height controller, ask about its anti-dive logic first.

Do you need one?

If you cut thin sheet, large parts, or anything where the torch path crosses existing cuts: yes, unambiguously. If you only cut thick, flat plate with simple outer contours, you can survive without — but you’ll still get better cut quality and longer consumable life with one.

The good news is that in 2026, “adding a THC” no longer means a 500 € black box. An Arduino, a weekend of wiring, and open-source firmware will get you a controller that you actually understand — and when you understand your tools, you cut better parts.