What Is a CNC Gas Cutting Machine?

• 1
  How a CNC Gas Cutting Machine Actually Cuts Steel
• 2
  Thickness, Speed and the Heat-Affected Zone
• 3
  Which Metals Suit Oxy-Fuel Cutting
• 4
  Where Gas Cutting Ends and CNC Machining Begins
• 5
  Setup Details That Decide Cut Quality
• 6
  What Tolerance to Expect From a Gas Cut Part
• 7
  Gas Cutting vs Plasma, Laser and CNC Machining

The cutting medium is not the flame. The flame only heats the steel to its ignition temperature, roughly 870–1,100 °C for mild steel. Once the metal reaches that point, a separate stream of high-purity oxygen hits the spot and the iron oxidizes. That exothermic reaction supplies most of the energy that keeps the cut moving.

The tip of an oxy-fuel torch runs three separate gas paths: a preheat flame around the outside, a cutting oxygen jet in the center, and a shield. Acetylene gives the hottest preheat flame and the cleanest edge. Propane and natural gas cost less per hour and behave better on thick plate, but they need a longer preheat.

A CNC gas cutting machine only automates the motion and the gas sequencing. The controller moves the torch along the programmed path, opens the cutting oxygen at the right moment, and holds the standoff height. Kerf width, travel speed and preheat time still come from the tip size and the plate thickness.

So the machine is not smart about the process. It is repeatable. That distinction matters when you compare gas cutting to a machining center, where the tool geometry and the spindle define the result.

Gas cutting earns its place on plain carbon and low-alloy steel from about 6 mm up to 300 mm. Below 6 mm the heat input is hard to control: the plate warps, the kerf washes out, and plasma or laser usually wins on both edge quality and speed.

Cutting speed drops steeply with thickness. A 25 mm plate might run at 500–600 mm/min, while a 100 mm plate runs closer to 150–250 mm/min. Preheat time climbs too, from a few seconds to well over a minute at the top of the range.

Every cut leaves a heat-affected zone. On 25 mm mild steel the HAZ typically reaches 1–3 mm below the cut face, and it is harder and more brittle than the parent metal. If the part is a structural bracket that is fine. If it is a mating face for a bearing, it is not.

The kerf is also wide. Expect 1.5–4 mm depending on tip size and thickness. That kerf has to be accounted for in the nesting file, and it is one reason gas cutting holds loose tolerance compared with milling.

The reaction needs iron to oxidize. That makes low-carbon and low-alloy steel the natural fit: A36, 1018, 1045, 4130, 4140 and similar grades all cut cleanly. The lower the carbon and alloy content, the more predictable the edge.

Stainless steel is a poor candidate. Chromium oxide forms a refractory layer that blocks the oxygen jet, so the cut stalls or needs iron powder injection to keep going. Laser, plasma or waterjet handle 304 and 316 far better.

Aluminium, copper and brass also resist the process. Aluminium oxide melts at a far higher temperature than the metal itself, and copper conducts heat away from the cut zone too fast. Non-ferrous work belongs on a mill or a waterjet.

Cast iron cuts, but it is messy. Graphite flakes and sand inclusions interrupt the oxygen stream, and the edge often needs grinding afterward. For a one-off repair it works. For production parts we would not plan on it.

Gas cutting is a plate process. It produces a profile, a hole or a bevel. It does not produce a bore, a thread, a pocket or a flat face with a measured finish. Those features come from a machining operation after the plate is cut.

That is the normal sequence in our shop. A 40 mm A36 base plate is gas cut to rough profile with 3–5 mm of stock left on every critical face, then it goes onto a 3-axis or 4-axis mill for the bores, the counterbores and the mounting faces.

The rough cut saves material and time. Removing 40 mm of steel with an end mill is slow and expensive. Removing 3 mm is fast. The gas cut edge does not need to be pretty, only stable and free of deep dross.

If your part is under 6 mm thick, or it needs a finished dimension tighter than ±0.1 mm, skip gas cutting entirely. Go straight to laser or to machining. Mixing the two only adds handling and inspection steps.

Tip selection drives everything else. A tip rated for 25 mm will not cut 6 mm plate well, and using an oversized tip on thin plate overheats the edge and rounds the top corner. Match the tip to the thickness range stamped on it.

Standoff height matters more than most operators admit. The cutting oxygen jet loses coherence fast. Keep the tip 3–6 mm above the plate for thin material and slightly more for thick, and let the height control do the work if the machine has one.

Oxygen purity should be 99.5% or better. A drop of one percentage point costs speed and leaves dross on the bottom edge. If the cut suddenly needs more speed to stay clean, check the oxygen supply before touching the program.

Piercing thick plate is its own problem. A 100 mm pierce takes 30–60 seconds and throws molten metal upward. Most shops pierce off the part outline and lead in, which keeps the pierce crater out of the finished profile.

A well-run machine holds ±0.5 mm on plate up to 25 mm, widening to ±1.5 mm or more as thickness climbs. Thermal expansion during a long cut moves the plate, and no controller can fully compensate for that.

The cut face has a slight drag line and a small bevel, usually 1–3° per side. If the part needs a square edge, it gets machined or ground afterward. Design drawings should not call out a square edge on a gas cut face.

Holes under about 1.5 times the plate thickness are unreliable. A 20 mm hole in 40 mm plate tends to come out tapered and out of round. Drill it instead.

So the rule is simple: use gas cutting for profile and stock removal, then bring the part to a mill for anything measured. That split keeps cost down and keeps the tolerances honest.