Straight-Line and Shape Cutting on Carbon Steel Plate

What Oxy-Fuel Cutting Does

Oxy-fuel cutting (OFC) uses a preheat flame to raise carbon steel to its ignition temperature—roughly 1,400–1,600 °F—then directs a high-pressure stream of pure oxygen at that spot. The steel burns (oxidizes) rapidly, and the gas stream blows the molten iron oxide out of the joint, leaving a kerf. Because the process relies on oxidation, it works well on plain carbon and low-alloy steels but poorly on stainless steel, cast iron, and aluminum, which form oxides that resist further burning.

Understanding the Kerf

The kerf is the width of material removed by the cut. On a hand-held torch cutting 1/4 in. plate with a standard cutting tip, the kerf is typically around 1/8 in. wide. You must account for this when scribing layout lines: always cut to the waste side of the line, not through the center of it. If you center the kerf on the line, you lose roughly 1/16 in. from your finished piece—enough to fail a dimensional check.

Tip Selection and Pressures

Cutting tips are sized by the thickness they are designed to cut. Consult the tip manufacturer's chart (posted in your lab) for the correct tip orifice size and oxygen and acetylene pressures for 1/4 in. and 3/8 in. plate. As a general rule, thicker plate needs a larger tip and higher cutting-oxygen pressure. Using an undersized tip causes incomplete penetration and heavy slag. Using an oversized tip wastes gas and produces a ragged, wide kerf.

Before striking any arc or lighting any flame, verify that both regulator gauges read within the tip manufacturer's recommended range and that hose connections are leak-free (use approved leak-detection solution—never an open flame).

Straight-Line Cutting Technique

1. Scribe the line with soapstone on a cleaned plate surface. Use a steel rule held firmly against the plate edge; draw one clean line rather than multiple overlapping strokes.

2. Preheat the starting edge. Hold the torch so the preheat flame cones are roughly 1/8 in. above the plate surface. Wait until the spot glows bright orange-red.

3. Open the cutting-oxygen lever smoothly and begin traveling along the line at a steady pace. The travel speed should allow the oxygen stream to pierce completely through the plate—you will see sparks exiting the bottom of the cut when penetration is full.

4. Maintain tip angle: keep the tip perpendicular to the plate surface (90°) for a square cut face, or tilted to the required bevel angle for bevel cuts.

5. Travel speed: moving too fast leaves an incomplete cut or heavy slag on the bottom; moving too slowly produces excessive melting and a wide, irregular kerf. Aim for a smooth, consistent speed throughout.

A magnetic straightedge guide clamped parallel to your layout line gives a physical fence for the torch body and is the most reliable way to hold dimensional tolerance on straight cuts.

Circle Cutting with a Magnetic Guide

A magnetic circle-cutting guide attaches to the torch body and pivots around a center pin pressed into the plate. Set the radius to half the desired diameter—for a 4 in. disk, set the radius to 2 in. Preheat the starting point at the circle's edge, open the cutting-oxygen lever, and rotate the torch smoothly around the pivot. Keep even pressure on the pivot pin so the radius does not drift. Complete the circle by returning to the starting point without stopping, then release the cutting-oxygen lever.

Check roundness by laying a circle template over the disk. Any deviation over 1/8 in. at the measurement points requires the cut to be repeated.

Bevel Cutting on 3/8 in. Plate

Bevel cuts are used in weld joint preparation. A 45-degree bevel is common for full-penetration groove welds. Use a dedicated 45-degree bevel guide clamped to the plate edge to hold the torch at the correct angle throughout the cut. After the cut, use an angle grinder with a grinding disc to remove slag and a flap disc to dress the face. Verify the angle with a protractor before recording results.

Slag Removal

Slag is the solidified iron oxide that adheres to the bottom edge of a cut. Never measure or submit a cut for inspection without removing all slag first. Use an angle grinder or chipping hammer and wire brush. Slag left in place can hide under-cuts and dimensional errors, and it interferes with downstream welding.

Safety Requirements for This Lab

PPE: Shade 5 filter goggles (ANSI Z87.1) are required for oxy-fuel cutting; standard clear safety glasses are not sufficient. Wear a leather jacket or leather sleeves, leather gloves, and steel-toed boots. Synthetic fabrics ignite easily—do not wear them.

Fire and explosion prevention: Keep the work area clear of flammables within a minimum 35-foot radius or use fire-resistant barriers. Have a fire extinguisher within immediate reach. Never cut directly on a concrete floor—trapped moisture can cause explosive spalling; use a steel cutting table with a slag catcher.

Cylinder safety: Secure oxygen and acetylene cylinders upright and chained. Never allow oil or grease to contact oxygen fittings. Acetylene pressure must never exceed 15 psi at the regulator—above that pressure, acetylene becomes unstable.

Backfire and flashback: If the flame pops back into the tip (backfire), release the cutting-oxygen lever and both needle valves immediately. If you suspect a flashback (flame traveling back into the hoses), close both cylinder valves at once. Allow equipment to cool before inspecting hoses and check valves.

Ventilation: Cutting carbon steel produces iron oxide fume and, if the plate has any coating, additional hazardous fumes. Use local exhaust ventilation at the table and confirm general shop ventilation is running.