What a THC does, why almost every CNC plasma table needs one, how arc voltage sensing and ohmic sensing work, and which units are worth buying. This guide covers everything from basic function to tuning parameters so you can make an informed decision and get clean cuts from the start.
A torch height controller (THC) is the component on a CNC plasma table that automatically adjusts the torch height during cutting to maintain the optimal distance between the torch tip and the material surface. That distance — called the standoff or cut height — typically needs to stay within a range of about 0.06 to 0.15 inches depending on the amperage and material. A THC monitors this distance in real time and drives the Z-axis motor up or down to hold it steady as the torch moves across the workpiece.
Without a THC, the torch is locked at a fixed height above the table slats. The assumption is that the material surface is at a consistent, known position relative to those slats. In practice, that assumption is almost never true. Sheet metal arrives from the supplier with bow, twist, and edge wave. Table slats wear unevenly over weeks and months of cutting, creating high and low spots across the cutting bed. And the material itself warps during cutting as heat from the plasma arc causes thermal expansion — thin gauges especially will buckle and lift away from the slats mid-cut.
When the torch-to-material distance changes by even a few hundredths of an inch, the effects are immediate and visible. If the torch is too close, the arc column compresses, the kerf narrows, dross builds up on the bottom edge, and the consumables wear faster — or worse, the torch crashes into the material and destroys the nozzle. If the torch is too far, the arc column stretches, the kerf widens, the cut edges bevel, pierce reliability drops, and you get heavy dross that has to be ground off by hand.
The THC is often called the most underrated component on a CNC plasma table — and for good reason. Buyers tend to focus on the frame, the plasma source, and the cutting area, but the THC has more impact on day-to-day cut quality than almost anything else. A $30,000 table with a poorly tuned or sluggish THC will produce worse cuts than a $15,000 table with a responsive, well-calibrated THC. Our Buyer's Guide covers how THC quality fits into the overall purchasing decision.
The case for a THC comes down to a simple reality: the conditions that make fixed-height cutting work — perfectly flat material on a perfectly level table surface — almost never exist outside of a textbook. Here are the real-world factors that make a THC essential for most CNC plasma setups.
Even brand-new sheet stock straight from the steel supplier has measurable surface deviation. A standard 4x8 sheet of 14-gauge mild steel can have 1/4 inch or more of bow across its width. Thicker plate is more consistent but still has mill scale variations, edge wave, and oil-can distortion. Thinner gauges — 16, 18, 20 gauge — are worse because they have less inherent stiffness. If you are cutting drops and remnants (material left over from previous jobs), the flatness gets even more unpredictable. A THC tracks the material surface continuously and compensates for all of this automatically.
The slats (sometimes called fins or grate bars) that support the material on the cutting bed are consumable items themselves. The plasma arc erodes the slat tops over time, and some areas wear faster than others depending on where you typically load material and which zones get the most cutting activity. After a few months of production cutting, you can easily have 1/8 inch or more of height variation across the slat bed. Without a THC, this variation transfers directly to the torch-to-material distance. With a THC, the controller compensates transparently. For more on slat maintenance timing, see our Maintenance Guide.
This is the factor that catches most new CNC plasma operators off guard. When the plasma arc cuts through thin material (16 gauge and thinner), the intense heat causes thermal expansion in the area around the cut. The material buckles upward or downward — sometimes dramatically. On a 4x8 sheet of 18-gauge steel, you can see the material lift an inch or more off the slats adjacent to the cut path as the job progresses. A fixed-height torch cannot respond to this. It either crashes into the rising material or loses the arc as the material drops away. A THC with adequate response speed tracks these changes and keeps cutting cleanly.
The torch-to-material distance is not just a nice-to-have parameter — it directly controls four critical outcomes:
Edge Quality
At the correct standoff height, the arc column is tight and focused. The cut edges are square, smooth, and nearly dross-free. Too high and the edges bevel outward. Too low and you get a narrow, rough cut with heavy bottom dross.
Dross Formation
Dross (the re-solidified metal that sticks to the bottom edge of the cut) is directly related to arc column length. The correct standoff minimizes dross on both the top and bottom edges, reducing post-cut grinding time.
Consumable Life
Nozzles and electrodes wear faster when the standoff is wrong. Too close causes double arcing (the arc jumps to the nozzle bore), destroying the nozzle in seconds. Too far overheats the consumables because the arc column is stretched and less efficient.
Pierce Success
The initial pierce — where the arc punches through the material — is the hardest moment on consumables. Starting at the correct height with proper pierce delay timing maximizes pierce reliability and consumable longevity.
The only scenario where a THC is genuinely not needed is a very small table (2x2 or smaller) cutting only small, flat pieces that are clamped firmly to the bed. If every piece you cut is under 12x12 inches, perfectly flat, and held down so it cannot warp — and you maintain your slats meticulously — you can get acceptable results without a THC. But even in this narrow use case, a THC still improves consistency. The moment your work grows beyond that envelope, a THC stops being optional and becomes essential.
Modern THC systems use two complementary sensing methods, and understanding the distinction between them is important when choosing and configuring a THC. Most quality units use both methods together — one for initial positioning and one for active height control during the cut.
Arc voltage sensing is the primary method used by virtually all modern THCs to control torch height during cutting. It works on a straightforward physical principle: the voltage across a plasma arc is directly proportional to the arc length. A shorter arc (torch closer to the material) produces lower voltage. A longer arc (torch farther from the material) produces higher voltage. The relationship is nearly linear and predictable.
The THC reads the arc voltage through a voltage divider circuit (either built into the plasma source or provided as a separate module) and compares it to a target voltage set by the operator. If the measured voltage is higher than the target, the THC drives the Z-axis motor downward to bring the torch closer. If the measured voltage is lower, it drives upward. This feedback loop runs continuously — hundreds of times per second on a quality THC — so the torch tracks the material surface in real time.
Advantages
Limitations
Ohmic sensing uses electrical contact between the torch tip and the material surface to find the initial height before the arc fires. The THC applies a low-voltage signal through the torch nozzle and monitors for electrical continuity with the workpiece. When the nozzle touches the material, the circuit completes and the THC knows exactly where the surface is. The torch then retracts to the programmed pierce height (typically 0.15 to 0.20 inches above the surface) before the arc fires.
Ohmic sensing is significantly more accurate than the alternative initial height method — float switch sensing, where the torch body physically rests on the material and a spring-loaded switch detects contact. Float switches have inherent mechanical play and can be fooled by rust, scale, or debris on the material surface. Ohmic sensing detects actual electrical contact regardless of surface condition.
Advantages
Limitations
On a well-equipped CNC plasma table, the two sensing methods work as a team. Before each pierce, the ohmic sensor moves the torch down until it touches the material surface. The THC records that position, retracts the torch to pierce height, and fires the arc. After the arc transfers and the pierce is complete, the THC switches to arc voltage sensing and maintains the target cut height for the rest of that contour. At the next pierce point, the process repeats. This combined approach gives you the most accurate initial height and the most responsive in-cut tracking available.
Side-by-side comparison of the most common torch height controllers for CNC plasma tables, from budget signal converters to integrated production systems.
| THC Unit | Type | Approx Cost | Compatible Controllers | Best For |
|---|---|---|---|---|
| Proma Compact THC 150 | Arc Voltage | $300 – $400 | Mach3 / Mach4 | Budget / hobby tables |
| CandCNC MP-1000 | Arc Voltage + Ohmic | $400 – $500 | Mach3 / Mach4 | Small business |
| Mesa THCAD | Arc Voltage (signal converter) | ~$80 | LinuxCNC | DIY builders |
| Hypertherm Sensor THC | Arc Voltage + Ohmic | $800 – $1,200 | Mach3 / Mach4 / FlashCut | Production tables |
| FlashCut SmartTHC | Integrated | Included w/ system | FlashCut only | FlashCut tables |
| CandCNC DTHCII | Arc Voltage + Ohmic | $500 – $600 | Mach3 / Mach4 | Mid-range production |
Budget / Hobby
Proma Compact THC 150
Arc Voltage • $300–$400 • Mach3/Mach4
Small Business
CandCNC MP-1000
Arc Voltage + Ohmic • $400–$500 • Mach3/Mach4
DIY Builders
Mesa THCAD
Arc Voltage (signal converter) • ~$80 • LinuxCNC
Production Tables
Hypertherm Sensor THC
Arc Voltage + Ohmic • $800–$1,200 • Mach3/Mach4/FlashCut
FlashCut Tables
FlashCut SmartTHC
Integrated • Included w/ system • FlashCut only
Mid-Range Production
CandCNC DTHCII
Arc Voltage + Ohmic • $500–$600 • Mach3/Mach4
The Proma is the most popular standalone THC for hobby and budget CNC plasma builds. It uses arc voltage sensing only (no ohmic), connects to Mach3 or Mach4 via parallel port or breakout board, and has a straightforward setup process. The Proma has a reasonable response speed for hobby cutting speeds and handles mild steel and aluminum well. Its main limitation is that it lacks ohmic initial height sensing, so you rely on a float switch or manual touch-off for the starting height. For a hobby CNC plasma table running a few hours a week, the Proma is a strong value at its price point.
The MP-1000 from CandCNC adds ohmic sensing for initial height on top of arc voltage control during cutting. This combination gives you accurate, repeatable pierces and responsive in-cut tracking. CandCNC has a strong reputation in the CNC plasma community for customer support and documentation. The MP-1000 integrates with Mach3 and Mach4 and is a common upgrade choice for small-business tables that shipped with a basic or no THC. It sits in a sweet spot between the budget Proma and the premium Hypertherm units.
The Mesa THCAD is not a THC in the traditional sense — it is a voltage-to-frequency signal converter that feeds arc voltage data to LinuxCNC, which then handles the height control logic in software. At roughly $80, it is the most affordable entry point into torch height control, but it requires a LinuxCNC setup and the technical ability to configure the THC component in the LinuxCNC HAL (Hardware Abstraction Layer). The Mesa THCAD is the standard choice for DIY builders running LinuxCNC on a Mesa FPGA motion control card. It is not practical for Mach3/Mach4 users.
The Hypertherm Sensor THC is a premium unit designed specifically for Hypertherm plasma systems. It combines arc voltage sensing with ohmic initial height and integrates tightly with Hypertherm's CNC interface — reading divided arc voltage directly from the plasma source and using Hypertherm's published cut charts for optimal voltage targets. The Sensor THC has fast response times, robust corner lockout logic, and the fit-and-finish you expect from Hypertherm. The trade-off is price ($800-$1,200) and the fact that it is optimized for Hypertherm plasma sources — it will work with other brands but does not have the same level of integration. For production tables running Hypertherm Powermax systems, this is the reference-quality standalone THC.
The FlashCut SmartTHC is not sold separately — it comes integrated with FlashCut CNC control systems. FlashCut tables (and tables using FlashCut controllers) benefit from the tightest possible integration between the THC and the motion controller. The SmartTHC shares data bidirectionally with the controller: it knows the programmed cut speed, the current acceleration state, and the commanded tool path. This allows it to implement intelligent corner handling, predictive height adjustments, and automatic voltage target selection based on the cut parameters. If you are buying a table with a FlashCut controller, the integrated SmartTHC is a significant advantage over adding a third-party THC.
The DTHCII is CandCNC's higher-end THC, adding faster response times and more advanced filtering compared to the MP-1000. It includes both arc voltage sensing and ohmic initial height, with configurable response curves and corner lockout parameters. The DTHCII is a popular choice for mid-range production tables in the $20,000-$50,000 range that use Mach3 or Mach4 controllers. It bridges the gap between the consumer-grade MP-1000 and the premium Hypertherm Sensor THC, offering production-capable performance at a mid-range price point.
Getting a THC installed is only half the job. Proper tuning is what separates a THC that improves your cuts from one that causes new problems. Here are the key parameters you will need to set and adjust.
The target voltage tells the THC what arc voltage corresponds to the correct standoff height. Typical target voltages range from 100V to 150V depending on the material type, thickness, and cutting amperage. Higher amperages generally produce higher arc voltages at the same standoff distance. Most plasma source manufacturers publish cut charts that include recommended arc voltages for each material/thickness/amperage combination. Hypertherm's cut charts are the most comprehensive and widely referenced.
Pro tip: Start with the manufacturer's recommended voltage, then fine-tune by examining your cut edges. If the bottom edge has heavy dross and the top edge is sharp, the torch is too high — lower the target voltage. If the kerf is narrow and the nozzle wears fast, the torch is too low — raise the target voltage.
When the plasma arc first pierces through the material, the arc voltage spikes dramatically — often to 200V or more — before settling down to the normal cutting voltage. If the THC starts sampling immediately, it reads this spike as a massive height error and drives the torch downward aggressively. This is called "dive on pierce" and it can crash the torch into the material.
The sample delay (sometimes called "arc OK delay" or "THC delay") tells the THC to wait a set period after arc transfer before it starts controlling height. Typical values are 0.5 to 1.0 seconds. Thicker material needs longer delays because the pierce takes longer to complete. If you see the torch diving immediately after piercing, increase the sample delay.
Response speed controls how aggressively the THC reacts to voltage deviations. Too fast and the THC oscillates — the torch bounces up and down rapidly, leaving a wavy cut edge and accelerating wear on the Z-axis motor and drive components. Too slow and the THC cannot keep up with material surface changes at full cutting speed, which defeats the purpose of having a THC in the first place.
Start with the THC manufacturer's default response speed and adjust from there. If you see oscillation marks on the cut edge (a regular wave pattern), slow down the response. If you see height-related quality variations across the sheet — particularly when crossing warped areas — speed it up. Most THCs let you adjust response speed as a percentage or a gain value.
Corner lock (also called velocity lock or anti-dive) is arguably the most important THC setting after the target voltage. Here is the problem it solves: when the CNC torch approaches a sharp corner, the motion controller decelerates the X and Y axes to change direction. The plasma arc lingers longer in one spot, which changes the arc voltage — the voltage drops because the arc is heating a smaller area more intensely. The THC reads this voltage drop as the torch being too close and drives downward, digging the torch toward the material right at the corner.
Corner lock disables THC height adjustments whenever the cutting speed drops below a threshold — typically 80-85% of the programmed feed rate. Once the torch accelerates back to full speed after the corner, the THC re-engages. Without corner lock enabled, every inside corner and small radius on your parts will have a gouge or a divot where the torch dove. If your THC does not have corner lock capability, it is not suitable for production cutting.
Use our Amperage Calculator to determine the right power settings for your material, which will help you dial in the corresponding THC voltage targets.
Use this quick decision tree to determine whether a THC is essential, recommended, or optional for your setup. Start at the top and follow the path that matches your situation.
Yes — You need a THC. Thicker material means higher amperages, longer cut times, and greater consequences from incorrect standoff height. The torch-to-material distance matters more as thickness increases because the arc column must penetrate through more material. A THC is essential.
Yes — You need a THC. Larger sheets have more surface variation and are more likely to warp during cutting. Even new sheet stock has measurable bow and twist at dimensions above 12 inches. A THC compensates automatically.
Yes — You need a THC. On a 4x4, 4x8, or 5x10 table, the cutting area is large enough that slat wear, material bow, and thermal warping all become significant factors. A THC is not optional at these sizes.
Optional but still recommended. If every piece you cut is small, flat, and clamped to the bed so it cannot move or warp, you can get by without a THC. But even in this scenario, a THC improves consistency and extends consumable life. If your budget allows it, add one.
Add THC before cutting production work. If you are building a DIY table and have not installed a Z-axis yet, plan for one from the start. You can do test cuts without a THC, but do not take on paid work or cut material you cannot afford to waste until your THC is installed and tuned. The Z-axis motor, linear slide, and THC electronics should be in your build budget from day one.
If you answered yes to any of the first three questions — and most CNC plasma table buyers will — a THC is essential, not optional. The cost of a THC ($80-$1,200 depending on the unit) is a fraction of the total table investment and pays for itself quickly through reduced consumable waste, fewer re-cuts, and less post-cut grinding. When you request quotes from dealers, verify that the THC is included in the quoted price and ask which unit they supply.
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A torch height controller (THC) automatically adjusts the Z-axis height of the plasma torch during cutting to maintain the optimal distance between the torch tip and the workpiece surface. It compensates for material that is not perfectly flat, worn table slats, and thermal warping during cutting. The result is more consistent cut quality, longer consumable life, and fewer failed cuts.
For any table larger than 2x2 or any material larger than about 12x12 inches, yes. Sheet metal is never perfectly flat, and even small deviations in torch-to-material distance cause measurable changes in cut quality, edge bevel, and dross formation. The only scenario where a THC is truly optional is a very small table cutting only small, flat, clamped pieces — and even then, a THC improves consistency.
Arc voltage sensing measures the voltage of the plasma arc during cutting to determine torch height — higher voltage means the torch is farther from the material, lower voltage means it is closer. It works continuously during cutting. Ohmic sensing uses electrical contact between the torch tip and the material surface to find the initial touch-off height before piercing. Most modern THCs use both: ohmic sensing for initial height positioning and arc voltage sensing for height control during the cut.
Standalone THC units range from about $80 for a signal converter like the Mesa THCAD to $300-$600 for popular units like the Proma Compact THC 150 and CandCNC systems. Premium units like the Hypertherm Sensor THC cost $800-$1,200. Some table manufacturers include the THC in the table price as an integrated system, which is generally the best option for reliability and tuning.
Corner lock (also called velocity lock) is a THC feature that temporarily disables height adjustments when the cutting torch decelerates — such as going around a sharp corner. Without corner lock, the THC misinterprets the voltage change caused by the slower travel speed as a height change, causing the torch to dive toward the material. This can gouge corners, damage consumables, or crash the torch into the workpiece. Corner lock is essential for cutting any parts with sharp angles or tight radii.
Everything you need to know before buying — frame, THC, plasma source, software, and budget tiers.
Daily, weekly, and monthly maintenance checklists to keep your table cutting clean.
Diagnose and fix common cut quality problems — dross, bevel, double arcing, arc loss, and more.