CNC plasma cutting enables the rapid production of components from electrically conductive materials, such as structural steel, stainless steel and aluminium. However, the final result is determined not only by the plasma source, the cutting speed and the quality of the gases. The correct distance between the torch and the sheet metal is equally important.
CNC plasma cutting enables the rapid production of components from electrically conductive materials, such as structural steel, stainless steel and aluminium. However, the final result is determined not only by the plasma source, the cutting speed and the quality of the gases. The correct distance between the torch and the sheet metal is equally important.
THC, or Torch Height Control – an automatic torch height control system – is responsible for maintaining this distance during cutting. A well-configured system helps to minimise the impact of sheet metal undulation, stabilise cutting conditions and reduce the risk of premature wear of consumable parts.
What is THC in a CNC plasma cutter?
THC is a Z-axis control system that automatically adjusts the torch height above the material during cutting. In many systems, the controller utilises the relationship between arc voltage and the length of the plasma arc. When the actual distance between the torch and the sheet metal changes, the arc voltage also changes, and the system can raise or lower the torch accordingly.
This does not mean that every THC device works in exactly the same way. The measurement method, correction algorithms, response to corners, piercing handling and safety features depend on the CNC controller, plasma source, torch and machine configuration. Parameters should always be checked in the cutting tables and documentation for the specific system.
The most important task of THC is to maintain the cutting height within the limits required by the process. This helps to ensure the correct arc geometry, process stability and part repeatability across the entire sheet.
THC and IHS – two different stages of height control
In practice, the concepts of THC and material surface detection are often conflated, but they are distinct functions. THC is primarily responsible for height correction during cutting. Determining the position of the sheet surface before piercing begins is usually carried out by IHS, or Initial Height Sensing.
IHS may operate, for example, via a contact sensor, a mechanical sensor or another measurement method specified by the manufacturer. Once the surface has been detected, the controller sets a reference point, raises the torch to the piercing height, and, once piercing is complete, moves to the correct cutting height. Only then can the THC control take over the ongoing adjustment of the distance.
It is important to distinguish between these stages. The piercing height, cutting height and travel height are not the same parameter. Each should be selected in accordance with the technical specifications for the material, its thickness, current intensity, type of gas and consumables.
Why does the torch height affect the quality of the cut?
The distance between the torch and the material affects the arc concentration, the width of the cut gap, the angle of the edges, the amount of slag or burr, and the quality of the holes. The torch height also affects the service life of the nozzle, electrode and shield, particularly when piercing and when working on uneven sheets.
With correctly selected parameters, it is possible to achieve:
- a more consistent cut gap width;
- a cleaner edge and less slag;
- better control of edge bevel;
- more accurate workpiece dimensions after accounting for the kerf compensation;
- a lower risk of the torch coming into contact with the material;
- more predictable wear on consumables.
THC alone cannot replace the correct cutting speed, the appropriate current, good consumables, a properly functioning gas system or a rigid table mechanism. It is one element of the entire technological process.
What happens when the torch is set too high?
When the distance between the torch and the material is too great, the arc becomes less focused. This can impair energy transfer to the cutting zone and alter the geometry of the kerf.
Typical consequences of a burner set too high include:
- greater positive edge bevel;
- a wider cutting gap;
- poorer dimensional accuracy of workpieces;
- reduced quality of small holes;
- a greater risk of undercutting at an incorrectly selected speed;
- more finishing work required after cutting.
It is worth bearing in mind that similar symptoms may also result from a worn nozzle, unsuitable gas, too low a speed or an incorrectly selected flow rate. Diagnostics should always take the full set of parameters into account.
What happens if the torch is positioned too low?
Too small a distance increases the risk of the torch coming into contact with the material, spatter or deformed sheet metal. The risk is particularly high during piercing, as molten metal and intense spatter occur in the cutting zone.
The consequences may include:
- accelerated wear of the nozzle, electrode and shield;
- damage to the torch due to collision;
- arc instability;
- burn marks and increased burr formation;
- downtime associated with the replacement of consumables;
- damage to the workpiece or interruption of the process.
The height should be set in accordance with the manufacturer’s recommendations for the plasma source and consumables kit. A change in material, thickness, speed or current may require the settings to be updated.
How does the THC system work, step by step?
- Surface detection: the IHS system determines the position of the sheet metal and the reference point for the Z-axis.
- Piercing height setting: the torch rises to a safe height for arc initiation and to minimise the impact of spatter.
- Piercing the material: once the set piercing time has elapsed, the control system moves on to the next stage of the programme.
- Switchover to cutting height: the torch is positioned at the height required for stable arc guidance.
- Adjustment during cutting: THC analyses the signal – usually the arc voltage – and makes Z-axis corrections in accordance with the system settings.
- Travel and subsequent piercing: the torch rises to the travel height to minimise the risk of snagging on the workpiece or cut-out components.
The exact sequence may vary between systems. Therefore, parameters such as piercing height, piercing delay, cutting height, arc voltage, THC delay and travel height must be taken from the relevant process data.
Holes, corners and piercing – when does THC require special attention?
THC is very helpful with sheet metal undulation, but in certain sections of the path its response can worsen the result. This applies in particular to small holes, sharp corners, short arcs and areas where the machine slows down.
When the travel speed decreases, the conditions of the curve change, and the controller may misinterpret a change in voltage as a change in height. Depending on the system, functions such as THC lock, corner lock, anti-dive or temporary Z-axis freeze are then used. Their use should be in line with the controller’s capabilities and the manufacturer’s recommendations.
Punching is a separate stage of the process. It is usually carried out at a greater height than the actual cut to minimise the impact of molten metal on consumables. After piercing, the torch moves to the cutting height and only then begins the contour-specific adjustment.
The impact of THC on cutting quality and costs
A well-configured height control system can reduce the number of workpieces requiring rework and improve production repeatability. The benefits are particularly evident with larger sheets, thin sheets prone to warping, batch production and workpieces with complex contours.
THC can help to:
- reduced wear on consumable parts by minimising incorrect distances and collisions;
- less additional grinding or deburring;
- more consistent quality when the sheet position varies;
- better utilisation of the operator’s time;
- fewer unplanned stoppages.
The actual economic benefit depends on the type of production, the quality of the settings, the mechanical condition of the machine, the cutting parameters and the maintenance regime. THC does not automatically guarantee a perfect edge, but it helps to maintain the conditions necessary to achieve one.
How should a THC system be selected and set up?
The selection of the system should take into account the plasma source, the machine torch, the CNC controller, the Z-axis drive, the type of table and the planned range of materials. The best results are achieved with a complete solution in which all components are compatible and configured based on the manufacturer’s technical data.
Before commissioning, check:
- whether the system has the correct IHS method for the sheets being used;
- whether the plasma source provides the correct arc voltage signal to the CNC;
- whether the Z-axis drive operates smoothly and without excessive play;
- whether the height, speed and current settings are taken from the current cutting tables;
- whether the consumables are suitable for the process in question and are not worn out;
- whether the CAM programme takes into account piercing, overcuts, corners and small holes;
- do operators have a quality control procedure for the first workpiece?
Cutting without height control and with THC – a comparison
| Area | Without active height compensation | With correctly set THC |
|---|---|---|
| Sheet metal waviness | The torch can maintain a constant Z-axis position despite changes in the material level | The system can adjust the height during cutting in accordance with the arc signal |
| Edge repeatability | More dependent on the flatness of the material and manual adjustments | May be more stable with the correct process parameters |
| Parts wear | Higher risk of operating outside the correct distance | May be more predictable if collisions and incorrect heights are minimised |
| Small holes and corners | Require the correct programme and motion settings | Require the correct programme and, depending on the system, possibly a THC lock |
The most common signs of an incorrectly set torch height
If the cut quality varies between workpieces or deteriorates in different areas of the sheet, it is worth checking not only the torch height but also the entire process.
- a large amount of slag or burr;
- uneven or variable edge bevel;
- a cut gap that is too wide or irregular;
- problems with piercing;
- rapid wear of nozzles and electrodes;
- the torch striking the sheet metal or cut-out components;
- Z-axis waviness or excessive torch movement in corners.
Before changing the THC parameters, check the condition of consumables, the ground and cables, the gas pressure and quality, the cutting speed, the CAM settings, the condition of the table mechanism and the flatness of the material. Servicing work on electrical systems and the plasma source should be carried out by authorised personnel in accordance with the manufacturer’s instructions.
FAQ – frequently asked questions
Is THC necessary in every CNC plasma cutter?
Not every simple application requires advanced height control, but in batch production, with larger sheets, thin sheet metal and the need for consistent quality, THC is a very valuable component of the workstation.
Does the THC detect the sheet metal surface automatically?
In most cases, surface detection is carried out as a separate IHS stage. The THC is primarily responsible for regulating the height during cutting. The specific architecture depends on the system used.
Does THC improve the quality of small holes?
It can help maintain the correct height on straight sections, but with small holes and short curves, the THC’s response may need to be limited or temporarily disabled. Speed, cutting height, part quality and the CAM strategy are also key factors.
Why does the torch rise or fall at corners?
In a corner, the machine usually slows down, which can alter the arc voltage. If the THC interprets this change as a difference in height, it may unnecessarily correct the Z-axis. Corner lock or anti-dive functions may be helpful, provided they are available on the particular controller.
Summary
THC is one of the key systems in a CNC plasma cutter, as it helps maintain the correct torch height above the material during cutting. When combined with the correct IHS, up-to-date cutting tables, serviceable consumables and a well-prepared CAM programme, it supports process stability and edge quality.
The best results are achieved by treating height control as part of the overall process: from material flatness and piercing settings, through cutting speed, to the strategy for corners and holes. A CNC plasma system configured in this way can minimise rework, wear and tear on parts, and the risk of unplanned downtime.
