Oxy-Fuel Cutting Fundamentals

Oxy-fuel cutting (often called "torch cutting" or just "burning") is the process of severing steel by preheating the metal to kindling temperature with a fuel-gas flame and then blasting a jet of pure oxygen through the heated zone. The oxygen rapidly oxidizes the iron, and the cutting kerf opens as the oxide is blown through. Oxy-fuel has been around since the early 1900s and remains the most portable, most affordable, and most field-proven cutting process for mild steel up to about 12 inches thick.

This guide walks through setup, flame adjustment, cutting technique, and the specific safety hazards that make oxy-fuel cutting different from arc work.

The Equipment Stack

An oxy-fuel cutting rig has six major components:

1. Oxygen cylinder - Green in North America, typically charged to 2,200 psi. Oxygen is not flammable on its own, but it dramatically accelerates combustion of anything that is.
2. Fuel gas cylinder - Usually acetylene (maroon, charged to about 250 psi), though propane, propylene, and MAPP-like substitutes are also used.
3. Regulators - One on each cylinder, reducing cylinder pressure to working pressure. Oxygen regulators read 20 to 60 psi working pressure; acetylene regulators typically 5 to 15 psi (never exceed 15 psi on acetylene - above that, acetylene can spontaneously decompose).
4. Hoses - Twin-line hoses, green for oxygen, red for fuel. Left-hand threads on the fuel connector, right-hand threads on the oxygen connector. The fuel fitting has a notch around the nut.
5. Flashback arrestors - Check-valve-and-flame-suppressor devices installed at the regulators and at the torch inlets. Required. If a backfire travels up the hose, these stop the flame from reaching the cylinder.
6. Torch and cutting tip - Cutting torches have three valves: one for fuel (often at the back of the handle), one for preheat oxygen (the knob next to the fuel), and the cutting oxygen lever (the "trigger" on top). Tips are sized by thickness. Common tip sizes: #00 for 1/8 inch, #0 for 1/4 inch, #1 for 3/8 to 1/2 inch, #2 for 1/2 to 1 inch, #3 for 1 to 2 inches.

Setup and Leak-Check - Every Time

1. Secure the cylinders upright with a chain or strap. A toppled acetylene cylinder can rupture.
2. Crack the cylinder valves - Open each valve for a fraction of a second with the regulator backed out, to blow dust out of the outlet. Stand to the side of the valve, not in front.
3. Attach regulators - Use the correct wrench. Do not overtighten.
4. Connect hoses and flashback arrestors.
5. Back out regulator adjusters (turn counterclockwise until loose) before opening cylinder valves.
6. Open the oxygen cylinder valve slowly and fully - Oxygen cylinders are always opened all the way.
7. Open the acetylene cylinder valve only 1/4 to 1/2 turn. This lets you shut it off quickly in an emergency.
8. Set working pressures per the tip manufacturer's chart. A rough starting point for a #1 tip: oxygen 30 psi, acetylene 7 psi.
9. Leak-check every connection with soapy water. Any bubbles = leak. Tighten or replace. Never check for leaks with a flame.
10. Purge each hose separately by opening and quickly closing each torch valve before lighting.

Any step skipped here is how welders lose eyebrows - or worse.

Lighting and Adjusting the Flame

Oxy-acetylene produces three flame types. Learn to recognize each.

• Neutral flame - Balanced oxygen and acetylene. Sharp, defined inner cone; no "feather" around it; quiet hiss. This is the flame you want for cutting and most welding.
• Carburizing (reducing) flame - Excess acetylene. A long feathery cloud surrounds the inner cone. Cools the cut, causes irregular oxide formation. Mostly used for brazing and some surface hardening processes.
• Oxidizing flame - Excess oxygen. Short, sharp inner cone; loud, shrill hiss. Burns the metal more aggressively but degrades cut quality and overheats the torch tip.

Light procedure:

1. Open the torch fuel valve about 1/8 turn.
2. Light immediately with a flint striker (never a lighter or match - an open flame near a rich acetylene cloud is a flashback waiting to happen).
3. Open the fuel valve further until the flame stops smoking and stands off the tip about 1/16 inch.
4. Open the preheat oxygen knob slowly. The flame will go from a long yellow feather to a short blue inner cone with an outer feather.
5. Continue opening oxygen until the feather disappears and the inner cone is sharp and defined - that is neutral.
6. Briefly press the cutting oxygen lever to confirm the flame stays neutral under cutting flow. Adjust as needed.

Making the Cut

The mechanics of an oxy-fuel cut are straightforward once the setup is right:

1. Preheat the starting edge - Hold the inner cones about 1/16 to 1/8 inch above the steel surface. Watch for a bright cherry-red color (around 1,600 to 1,800 degrees F) at the starting point. This takes a few seconds on thin plate, 20 to 30 seconds on 1 inch plate.
2. Press the cutting oxygen lever - The red-hot steel immediately begins to oxidize and sparks blow through the underside.
3. Set the travel angle - Start the cut with the tip vertical (90 degrees to the plate). Once the cut is established, tilt the tip 5 to 15 degrees forward ("drag angle") - this pushes the sparks downward and slightly ahead, improving cut speed and kerf quality.
4. Travel speed - Fast enough that the sparks fly downward and out the bottom in a clean stream. If sparks blow back toward you or stall on top, you are too fast and have lost the cut. Slow down, re-preheat, and restart. A good cut speed for 1 inch mild steel is about 8 to 12 inches per minute with a standard #1 tip.
5. Sparks direction is your feedback - A clean cut shows sparks blowing straight down and out. Sparks blowing up and forward mean the cut is not penetrating.

Bevel Cutting

To cut a bevel for a welded joint, tilt the torch tip to the desired bevel angle (commonly 30 to 45 degrees from vertical) and cut along a straight edge or track burner. Bevel cuts require slower travel and sometimes a larger tip than a straight-through cut at the same material thickness, because the effective cut length through the metal is longer.

Slag Removal

The cut edge will have slag (dross) clinging to the underside - a thin line of solidified oxide. On a clean cut with correct pressures and speed, the slag chips off with a light tap of a chipping hammer. Heavy, welded-on slag is a sign of:

• Cutting oxygen pressure too low
• Travel speed too slow (heat buildup melts instead of oxidizes)
• Tip too small for the thickness
• Dirty or damaged tip orifices

For weld prep, the cut edge must be ground clean to remove oxide scale and any heat-affected oxidation - welding over unground torch-cut edges causes porosity and lack of fusion.

PPE for Oxy-Fuel

• Shaded goggles or shaded cutting lens - For oxy-fuel cutting, shade 5 goggles are standard. For heavy oxy-fuel work on thick material, go to shade 6. Do not use arc-welding shades 10+ - you will not see the flame well enough to adjust it. A flip-up shade 5 lens on a standard face shield is a common setup.
• Leather gloves - Full gauntlet
• Leather jacket or sleeves - Sparks fly farther with oxy-fuel than with arc
• Leather boots with spats or leggings for overhead cutting - molten slag falls straight down
• No synthetics - polyester ignites in a spark shower
• Hearing protection for long cutting sessions - the torch hiss causes fatigue

Oxygen is Not Air - Critical Handling Rules

Industrial oxygen is about 99.5 percent pure O2, and it behaves nothing like the 21 percent oxygen in ordinary air. Enriched oxygen accelerates combustion so violently that materials which merely scorch in air will ignite and burn explosively in pure oxygen. Every year, welders are killed or badly burned by oxygen-related fires that had nothing to do with the torch flame itself. Memorize these rules before you ever crack an oxygen cylinder:

• Never use oxygen to clean clothing, blow off dust, ventilate a confined space, or "cool off" on a hot day. Oxygen-saturated clothing ignites from a single spark, a static discharge, or a nearby cigarette and burns through fabric in seconds. Use compressed air, never oxygen, for blow-off and cleanup.
• Never allow oil, grease, paint, or any hydrocarbon near oxygen regulators, hoses, fittings, or cylinder valves. Pure oxygen under pressure meeting a hydrocarbon can auto-ignite without any spark at all. Keep oily rags, greasy gloves, chapstick, hand lotion, and shop lubricants away from the oxy side of the rig. Wipe the cylinder valve outlet clean before attaching a regulator.
• Never lubricate oxygen regulator threads, fittings, or cylinder valves. If a fitting will not seat dry, replace it - do not "help it along" with anti-seize, pipe dope, or teflon tape. Oxy-rated thread sealants exist; standard shop lubricants are fire starters.
• Keep oily hands and tools off the oxygen side. Do not handle oxygen fittings with gloves you just used on chain, bearings, or anything lubricated. Oxygen-free tool handling is a habit, not a preference.
• Store cylinders separated. Oxygen cylinders stored next to fuel cylinders must be separated by 20 feet of clear space or a 5-foot non-combustible barrier at least 5 feet tall.

Treat oxygen with the same respect you give a live electrical bus bar. You do not see the hazard until it bites, and then it is already too late.

Backfire, Flashback, and Sustained Backfire

• Backfire - A momentary pop and flame extinguish as the flame snaps back inside the tip briefly. Usually caused by the tip touching the work or a heat-soaked tip. The flame often re-lights itself. Relight if needed and continue.
• Flashback - The flame travels back past the tip and into the torch or hose. Sounds like a continuous whistle or screech. Shut off the torch immediately. Most modern manufacturer guidance (Victor/ESAB, Miller, Harris, Smith) for a flashback event is to close the torch oxygen valve first to starve the in-torch combustion of oxidizer, then close the fuel valve, because the combustion is happening inside the torch or hose and killing the oxygen supply stops it fastest. Note that this is the opposite of the normal end-of-day shutdown sequence (fuel first, then oxygen) described below. Emergency procedures vary slightly by equipment maker - follow your specific torch manufacturer's emergency instructions, posted in the shop or printed in the torch manual. After shutdown, close both cylinder valves, let everything cool, and do not use the torch or hoses again until they have been inspected (and usually replaced). Flashback can destroy hoses and rupture cylinders. Flashback arrestors are your last line of defense, not a license to skip prevention.
• Sustained backfire - Flame burns continuously inside the torch tube. Treat like a flashback - oxygen off, fuel off, cylinders off, inspect before reuse.

Causes: dirty or damaged tips, overheated tips, incorrect pressures, dirty/leaky connections, reverse gas flow from a failed check valve. Prevention: regular tip cleaning, correct pressure setup, flashback arrestors installed at both the regulators and the torch inlets, and routine check-valve inspection.

Day 1 Checklist

• Cylinders secured upright, caps removed, valves cracked clear
• Regulators attached, adjusters backed out, hoses and flashback arrestors connected
• Pressures set per tip chart
• All connections leak-checked with soapy water
• Flint striker in pocket (no lighters or matches)
• Shade 5 goggles or shield, gauntlet gloves, leather jacket, closed boots
• Tip inspected and cleaned; cleaning wires on hand
• Area clear of combustibles 35 feet; fire extinguisher within 10 feet
• Cutting path chalked or marked; clear fall zone below the cut

Shutdown

1. Release the cutting oxygen lever.
2. Close the torch fuel valve (flame goes out).
3. Close the torch oxygen valve.
4. Close both cylinder valves.
5. Open the torch fuel valve to drain hose pressure. Wait for regulator gauges to read zero. Close it.
6. Back out the fuel regulator adjuster.
7. Repeat the drain-and-back-out procedure on the oxygen side.
8. Coil hoses. Store striker, goggles, and tools. Log any tip cleaning or consumable replacement needs.

Torch cutting is one of those skills that looks mystical until you do it 20 or 30 times. After that, it becomes almost automatic. Respect the hazards, set up carefully every time, and you will keep cutting steel long after the fancy plasma machines are down for service.