CNC Torch Cutting Beginner's Guide

• 1
  What CNC torch cutting actually does
• 2
  Plasma cutting vs oxy-fuel cutting
• 3
  Kerf compensation and the CAM workflow
• 4
  Heat-affected zone, dross, and edge squareness
• 5
  What to design into a torch-cut part
• 6
  When torch cutting is the wrong process
• 7
  How a torch-cut plate order moves through the shop
• 8
  Choosing a torch process by thickness and material

A torch is a heat source. It raises a narrow band of metal above its melting or ignition point, and a gas stream pushes that molten or oxidized material out of the slot. The CNC part is the motion system: a gantry or a cantilever arm carries the torch along a toolpath generated from a CAD file, so the cut shape comes from the drawing rather than from a template or a chalk line.

That distinction matters more than it sounds. Manual torch work depends on operator steadiness, so hole diameters drift and nest layouts waste plate. Under CNC control the same part program repeats within the machine's positioning accuracy, which for a well-maintained cutting table is typically ±0.2 to ±0.5 mm over a 3,000 mm plate. That is not machining tolerance. It is plate tolerance, and it is the number you should design around.

CNC torch cutting is a 2D process. The torch cuts through the thickness in a straight vertical kerf, or with a slight bevel when a tilting head is used. You cannot produce pockets, counterbores, threads, or a face that has to seal against an O-ring. Those features belong to milling, turning, or a secondary operation after the plate is cut.

• 1
  Cutting, not shapingThe kerf removes material in a line; it does not generate a controlled surface.
• 2
  2.5D at mostBevel heads add chamfers and weld preps, not true 3D contours.
• 3
  Drawing drives the pathCAM output plus kerf offset defines the finished outline.

Plasma cutting uses a constricted electric arc. Gas passing through the nozzle is ionized into a plasma jet at roughly 20,000 °C or higher, and the jet melts the metal while the same gas blows the molten pool through the bottom of the plate. Because it is an arc process, it works on any conductive metal: carbon steel, stainless, aluminum, and copper alloys. A 100 A plasma supply will cut 12 mm carbon steel cleanly and sever 25 mm with a rougher edge.

Oxy-fuel cutting is a chemical process. A fuel gas, usually acetylene, propane, or natural gas, heats the steel to its ignition temperature, around 870 °C for plain carbon steel. A stream of pure oxygen then burns the iron itself, and iron oxide slag is blown out of the kerf. The steel supplies the fuel, so the process only works on carbon and low-alloy steel. It does not cut stainless, aluminum, or copper, because those oxides do not burn away the same way.

The practical split follows thickness. Plasma is faster and cleaner from roughly 3 mm to 25 mm, and it holds tighter kerf. Oxy-fuel takes over above 50 mm, where the arc becomes unstable and the kerf widens, and it can cut 150 mm plate with a properly sized tip. Between 25 mm and 50 mm, the choice depends on your edge requirement and on what gas you already have piped to the table.

• 1
  PlasmaAny conductive metal, 3–25 mm sweet spot, kerf 1.5–3.0 mm.
• 2
  Oxy-fuelCarbon and low-alloy steel only, 50–150 mm, kerf 2.5–4.0 mm.
• 3
  Both need clean dry gasMoisture causes porosity and dross, not a torch problem.

The torch does not cut a line of zero width. It removes a slot, and the width of that slot is the kerf. If you program the torch centerline straight down the part outline, every part comes out undersized by half the kerf on each side. Kerf compensation solves this by shifting the toolpath sideways, outward for a hole and inward for an outside profile, by exactly half the measured kerf.

Kerf is not a constant. It grows with plate thickness, with arc current, and with cutting speed. A 100 A plasma cut in 6 mm steel might leave a 1.5 mm kerf; the same torch at 200 A in 20 mm steel leaves closer to 3.0 mm. That is why the CAM programmer enters a kerf value matched to the process sheet, not a default. On a 200 mm bolt circle with a 2.5 mm kerf, getting this wrong shifts every hole by 1.25 mm.

The usual flow is: import the DXF or STEP file, nest the parts to use the plate, assign lead-in and lead-out points, apply kerf offset, then post-process to machine code. Lead-ins matter more than beginners expect. Starting the pierce directly on the part edge leaves a divot that no amount of grinding fully hides, so the pierce point is placed in scrap and the torch ramps onto the profile.

• 1
  Measure kerf, do not assumeCut a test coupon in the actual plate and measure it.
• 2
  Pierce in scrapLead-in length should exceed the kerf width by 2–3×.
• 3
  Nest with grain in mindLong thin parts bow along the rolling direction.

Every torch cut leaves a heat-affected zone. The metal beside the kerf has been heated above its transformation temperature and cooled quickly, so it is harder and more brittle than the parent plate. In 10 mm carbon steel the HAZ is usually 0.5 to 1.5 mm deep. For most structural brackets this is irrelevant. For a part that will be cyclically loaded in fatigue, or one that gets machined after cutting, it is the first thing to plan around.

Dross is the resolidified metal that sticks to the bottom edge. Low-speed dross is a heavy, bubbly deposit and means the torch moved too slowly, overheating the plate. High-speed dross is a fine, hard bead and means the opposite. Both are corrected with speed and standoff, not with more power. A clean cut has a light, easily brushed edge with a slight drag angle on the bottom.

Edge squareness is the other visible tell. Plasma cuts develop a bevel because the arc flares as it exits the nozzle. On thin plate the bevel is under 2°, but at 25 mm it can reach 5° to 8°, which on a 20 mm thick part means roughly 1.5 to 2.8 mm of taper. If your drawing calls for a square edge on thick plate, budget a milling pass on the profile. Oxy-fuel gives a squarer edge on thick steel but a wider kerf.

• 1
  HAZ depthRoughly 0.5–1.5 mm in carbon steel; remove before welding critical joints.
• 2
  Low-speed drossBubbly and heavy — increase travel speed.
• 3
  High-speed drossFine and hard — reduce speed or increase standoff.

Minimum hole diameter is the rule beginners break most often. A plasma torch cannot pierce a hole much smaller than about 1.5× the plate thickness, and the practical floor is usually 1.2× thickness for a hole that still has a usable edge. A 6 mm hole in 12 mm plate is a bad design for torch cutting. Drill it after cutting, or move it to a milled part.

Keep the smallest internal corner radius at roughly one-third of the plate thickness, or at least the kerf radius, whichever is larger. Sharp internal corners force the machine to slow down, which dumps extra heat into the corner and burns it. A 3 mm radius on a 10 mm plate costs nothing and improves the edge noticeably.

Leave margin between the part outline and the plate edge. A common rule is a minimum of 10 mm plus half the kerf, and more on thick plate. Cutting too close to the edge lets the plate spring as the cut releases residual stress, and the last 100 mm of the profile drifts. On long, narrow parts, cutting from both ends toward the middle or leaving tabs helps hold the geometry.

• 1
  Hole diameterKeep above 1.5× plate thickness; drill smaller holes later.
• 2
  Corner radiusAt least one-third of thickness, and never a sharp internal corner.
• 3
  Edge margin10 mm plus half the kerf, more as thickness increases.
• 4
  TabsLeave holding tabs on long parts to control spring-back.

Torch cutting wins on flat plate, moderate tolerance, and thickness. It loses whenever the part needs a controlled surface finish, a tight tolerance, or a feature that is not a through-profile. If your drawing shows ±0.05 mm on a bore, a pocket, a thread, or a sealing face, torch cutting is only the first operation. The plate gets cut oversize and the critical geometry comes off a machining center.

Laser cutting beats plasma below about 6 mm when edge quality and kerf matter, because the kerf is 0.2 to 0.5 mm and the HAZ is far thinner. Waterjet beats both when the material is heat-sensitive, such as titanium or a pre-hardened tool steel, or when you cannot tolerate any thermal effect at all. Neither laser nor waterjet scales economically to 100 mm plate, which is where oxy-fuel remains the practical answer.

The honest engineering position is that torch cutting is a roughing and blanking process. It produces the plate outline, the holes that will be drilled or reamed later, and the weld prep on a structural member. Treating it as a finished-part process is how tolerance arguments start between a shop and a customer.

• 1
  Use torch cutting forPlate outlines, brackets, gussets, weld preps, blank stock.
• 2
  Use laser or waterjet forThin plate, tight kerf, heat-sensitive alloys.
• 3
  Use milling forBores, pockets, threads, sealing faces, flatness under 0.05 mm.