Oxy-fuel cutting heats mild steel to 1,600F with a preheat flame, then blasts a stream of pure oxygen through the hot metal. The oxygen reacts with the iron in an exothermic (heat-generating) oxidation reaction that sustains itself, melting and blowing iron oxide out through the bottom of the cut. The result is a clean kerf through steel from 1/8" sheet to 6" plate and beyond.

The process only works on carbon steel and low-alloy steel. Stainless steel, aluminum, cast iron, copper, and brass don’t support the oxidation reaction. For those materials, use plasma cutting. But for mild steel, oxy-fuel cutting is simple, portable, inexpensive, and requires no electricity.

How the Cutting Process Works

A cutting torch tip has two types of orifices: multiple small outer holes for the preheat flame, and one larger center hole for the cutting oxygen.

Step 1: Preheat. Light the torch and adjust to a neutral flame. The outer preheat flames heat a small spot on the steel edge to bright cherry red, about 1,600F (870C). This takes 5-30 seconds depending on material thickness and tip size.

Step 2: Initiate the cut. Once the spot is cherry red, press the cutting oxygen lever. A jet of pure oxygen hits the hot steel. The iron reacts with the oxygen: Fe + O2 = FeO + heat. This reaction generates its own heat (roughly 1,800F more), sustaining the process.

Step 3: Travel. Move the torch steadily along the cut line. The preheat flames keep the steel ahead of the cut at ignition temperature. The cutting oxygen jet continues the reaction. The molten iron oxide (slag) blows out through the bottom of the kerf.

Step 4: Maintain. The process is self-sustaining as long as you maintain proper travel speed, standoff, and oxygen pressure. The preheat flames do most of their work on the top surface; the exothermic reaction provides the heat to cut through the thickness.

Required Equipment

Torch Setup

A standard oxy-fuel cutting outfit includes:

  • Oxygen cylinder (full at 2,200 PSI)
  • Acetylene cylinder (full at 250 PSI, never exceed 15 PSI working pressure)
  • Oxygen regulator (two-stage preferred for consistent pressure)
  • Acetylene regulator (two-stage preferred)
  • Twin hose (green for oxygen, red for acetylene, typically 25-50 ft)
  • Cutting torch (Victor, Harris, or Smith type)
  • Cutting tips (sized for material thickness)
  • Flashback arrestors (mandatory on both regulators)
  • Striker (spark lighter for igniting the torch)

For detailed setup procedures, see oxy-fuel regulator setup. For safety protocols, see oxy-fuel safety procedures.

Tip Selection

Cutting tips are sized by number, with each number corresponding to a material thickness range. The tip size determines how much oxygen and acetylene flow through the torch.

For tip sizing charts by thickness, see cutting torch tip size chart.

General guidelines:

  • #000 to #0: Sheet metal to 3/8"
  • #0 to #1: 3/8" to 5/8"
  • #1 to #2: 5/8" to 1"
  • #2 to #3: 1" to 2"
  • #3 to #5: 2" to 4"
  • #5+: 4" and thicker

Flame Adjustment

Proper flame adjustment is critical for clean cuts. An oxy-fuel cutting flame starts as acetylene (feathery, orange/yellow), then oxygen is added until the flame reaches a neutral balance.

Neutral Flame

A neutral flame has equal acetylene and oxygen in the mixture. The inner cone is sharp, well-defined, and bright blue-white. No feather extends beyond the inner cone. This is the correct flame for most cutting work.

Carburizing (Excess Acetylene)

If you see a feathery, whitish extension (acetylene feather) beyond the inner cone, the flame has excess acetylene. This adds carbon to the steel surface, which can prevent the cutting reaction from starting cleanly. Increase oxygen until the feather disappears.

Oxidizing (Excess Oxygen)

An oxidizing flame has a shorter, more pointed inner cone with a hissing sound. Too much oxygen in the preheat flame wastes oxygen and can cause the cut to start prematurely (before the steel is evenly heated). Reduce oxygen slightly until you see a distinct inner cone without the feather.

For cutting, some operators prefer a very slight oxidizing flame (the inner cone just slightly shorter than neutral). The extra oxygen in the preheat helps the cutting action start quickly. The difference from neutral is subtle.

Straight Cutting Technique

Starting the Cut

  1. Position the torch at the edge of the plate with the preheat cones about 1/16" to 1/8" above the steel surface.
  2. Aim the preheat at the very edge of the plate. Starting at an edge is easier than piercing because you only need to heat a thin section.
  3. Wait for the edge to reach cherry red (bright orange-red glow).
  4. Slowly press the cutting oxygen lever. You should see sparks and slag blow out the bottom immediately.
  5. Once the cut is established, begin moving the torch along the cut line.

Maintaining the Cut

Travel speed: Move at a pace that keeps the cutting reaction ahead of where you want the kerf to be. Too fast and the reaction can’t keep up, producing an incomplete cut. Too slow and you waste gas and overheat the surrounding metal.

Standoff height: Keep the preheat cones 1/16" to 1/8" above the surface. Too high and the preheat is inefficient. Too low and the tip may contact molten slag, causing backfires.

Torch angle: For straight cuts, hold the torch perpendicular to the plate surface. A slight drag angle (5-10 degrees tilted in the direction of travel) is acceptable and helps some operators maintain a smoother cut.

Watch the sparks. Sparks and slag exiting the bottom tell you about your speed:

  • Sparks stream straight down or slightly behind: correct speed
  • Sparks angle forward (ahead of the torch): too fast
  • Sparks lag far behind: too slow (wasting gas, widening the kerf)

Edge Starting vs Piercing

Edge starting (beginning the cut at the plate edge) is the standard technique. It’s faster, cleaner, and easier on the tip than piercing.

Piercing (starting in the middle of the plate) requires:

  1. Heat a spot to cherry red
  2. Raise the torch slightly (1/4" above the surface) to protect the tip from blowback
  3. Slowly press the cutting oxygen lever
  4. As the pierce breaks through, lower the torch back to normal standoff
  5. Begin traveling along the cut line

Piercing is harder on tips because the blowback from the initial breakthrough can clog the preheat orifices with slag. Clean the tip with a tip cleaner after heavy piercing.

Bevel Cuts

Bevel cuts angle the torch to produce a chamfered edge for welding prep. Common bevel angles are 22.5 degrees, 30 degrees, and 37.5 degrees (producing V-grooves of 45, 60, and 75 degrees when two beveled pieces are joined).

Technique for Bevel Cutting

  1. Tilt the torch to the desired bevel angle
  2. Position so the cutting oxygen jet hits the material at the bevel angle
  3. Travel speed is slightly slower than perpendicular cutting at the same thickness because the effective thickness through the bevel is longer
  4. Maintain consistent torch angle throughout the cut
  5. A guide or track burner helps maintain angle and speed for long bevel cuts

Combination (Land) Bevel

Many weld joints require a bevel with a flat land (root face) at the bottom. To cut this:

  1. Make the bevel cut first, stopping before the root
  2. Then make a straight vertical cut to establish the land
  3. Or use a specialty J-groove tip that cuts both profiles simultaneously

What Oxy-Fuel Can’t Cut

The oxidation reaction only works on metals where:

  1. The metal’s oxide melts at a lower temperature than the base metal
  2. The oxidation reaction generates enough heat to be self-sustaining
  3. The oxide is fluid enough to blow out of the kerf

This limits oxy-fuel to carbon steel and low-alloy steel. Materials that fail the test:

  • Stainless steel: Chromium oxide (Cr2O3) melts at 3,700F, far above the steel’s melting point. The refractory oxide blocks the cutting reaction.
  • Aluminum: Aluminum oxide (Al2O3) melts at 3,700F, three times the base metal’s melting point. It blocks the reaction.
  • Cast iron: High carbon content produces a pasty, viscous slag that doesn’t blow out cleanly. Cuts are ragged and unreliable.
  • Copper and brass: These don’t oxidize in a way that supports cutting.

For these materials, use plasma cutting.

Common Cutting Problems

Cut doesn’t go all the way through: Cutting oxygen pressure too low, travel speed too fast, tip too small for the thickness, or the steel isn’t preheated enough. Check tip size against the material thickness. Increase oxygen pressure or slow down.

Rough, jagged cut edges: Damaged tip (clogged or nicked orifices), travel speed too fast, or oxygen pressure too high for the tip. Clean or replace the tip. Reduce oxygen pressure if edges are gouged.

Heavy slag on the bottom edge: Travel speed too slow, tip too large, or oxygen pressure too low. The slag should blow clear of the bottom cleanly. If it hangs, adjust speed and pressure.

Torch keeps backfiring (popping): Tip touching molten metal, overheated tip, clogged orifices, or low gas pressure. Clean the tip, maintain proper standoff, check pressures. A severely overheated tip needs to be cooled by shutting off and letting it rest.

Cut starts but stops mid-plate: Lost the preheat. The steel ahead of the cut cooled below ignition temperature because travel speed was too fast, the tip is too small, or there’s a change in material (hitting a thick section, a different alloy, or heavy rust/paint that absorbs heat).

For cylinder sizes and cutting time estimates, see oxy-acetylene tank sizes.