Oxy-Fuel Cutting Fundamentals

Oxy-fuel cutting heats steel with a flame until it can burn, then blasts it with pure oxygen. The oxygen burns through the hot iron and blows the waste out of the kerf. No power supply is needed, the rig is portable, and it can cut steel many inches thick, so it remains a standard tool for field work, repair and demolition. It only works well if you set it up in the right order, every time.

What You Will Be Able to Do

• Identify the parts of an oxy-fuel rig and set it up.
• Leak-check, light and adjust a neutral flame.
• Make clean straight and bevel cuts.
• Tell a backfire from a flashback, and respond to each.
• Shut down and bleed the rig safely.

What Can and Cannot Be Cut

Oxy-fuel works on carbon and low-alloy steel, because iron oxidizes readily in oxygen and its oxide melts below the metal. Stainless steel and aluminum do not oxidize that way. Their oxides form a tough skin, so the cut stalls. Use plasma or another process for those.

The Rig

Keep these rules in mind on any rig:

• Fittings. Oxygen fittings use right-hand threads. Fuel fittings use left-hand threads, often with a notched nut, so that the two systems cannot be cross-connected.
• Hoses. Use hose rated for your fuel. Grade T hose suits all common fuel gases, while grade R is for acetylene only.
• Flashback arrestors. Many rigs carry them at both ends of each hose, at the regulator outlet and the torch inlet, on the oxygen side and the fuel side. Follow the equipment maker and your employer. They stop a flame from moving back through the hose to the regulator or cylinder.
• Cylinders. Keep them upright and chained, with valve caps on when not in use. Laying acetylene on its side lets the acetone inside migrate into the regulator.
• Storage. Separate stored oxygen and fuel gas by 20 feet, or by a noncombustible barrier at least 5 feet high with a half-hour fire rating.

Acetylene Limits

Warning: Never set acetylene above 15 psi. Above that pressure, acetylene can decompose explosively, even without oxygen.

Acetylene also has a limit on how fast you can draw it, commonly one-seventh of the cylinder's contents in an hour. Drawing faster can pull acetone into the hose and torch.

Oxygen Rules

Industrial oxygen makes things burn far faster than ordinary air does.

• Keep oil and grease off every part of the oxygen side. Oil and grease can ignite on their own in pure oxygen. Never lubricate oxygen fittings.
• Never use oxygen to blow off clothes or equipment. Oxygen soaks into clothing and hair, and a spark can then ignite them violently.
• Never use oxygen to ventilate a space, or in place of compressed air.

Setting Up

Follow the same order each time.

1. Secure the cylinders, and remove the caps.
2. Stand to the side, and crack each valve briefly to blow out dust.
3. Attach the regulators, then the arrestors and hoses, using the correct wrench.
4. Back out the regulator adjusting screws until they feel loose.
5. Stand to the side, and open the oxygen cylinder valve slowly, all the way.
6. Open the acetylene valve about 1/4 to 1/2 turn, so that you can shut it off quickly. OSHA limits this to 1-1/2 turns, and prefers less than 3/4 turn.
7. Set working pressures from the tip maker's chart. A rough start for a number 1 tip is about 25 to 30 psi oxygen, and 5 to 7 psi acetylene. Tip numbers vary by maker.
8. Leak-check every connection with leak-test solution. Bubbles mean a leak. Never check with a flame.
9. Purge each hose separately, by briefly opening and closing each torch valve in a clear area.

Lighting and Adjusting

1. Open the torch fuel valve a little.
2. Light it with a flint striker. Never use a lighter or matches.
3. Open the fuel until the smoke clears and the flame stands just off the tip.
4. Slowly bring in oxygen with the preheat valve, until the feather disappears.
5. Press the cutting lever. The preheat flames may change a bit, so adjust them to neutral while the lever is held.

Making the Cut

1. Protect your eyes and skin. OSHA's minimum shades for oxygen cutting are 3 under 1 inch, 4 from 1 to 6 inches, and 5 over 6 inches. Many shops use shade 5 for general cutting, or 6 for heavy work. Wear leather gloves and flame-resistant clothing.
2. Mark your cut, and plan where the piece will fall.
3. Preheat the edge. Hold the inner cones about 1/16 to 1/8 inch above the steel. Heat until it reaches about 1,600 to 1,800 F, a bright cherry red.
4. Press the lever. Sparks should stream out the bottom.
5. Set your angle. Start vertical, then tilt the tip 5 to 15 degrees forward, once the cut is established.
6. Move steadily. Too fast, and the cut will stall. Too slow, and slag welds to the edge. For 1 inch plate, roughly 8 to 12 inches per minute is typical, and your chart comes first.
7. Finish cleanly. Release the lever and move the flame away.

Keep the torch steady with your free hand, a straightedge or a track. A smooth cut has nearly vertical drag lines and square top and bottom edges.

If the cut stops, release the lever, reheat the spot and start again. Do not keep blowing oxygen on cold metal.

Bevel Cuts

To bevel a plate for welding, tilt the tip to the angle you want. Bevels need a slower speed, and sometimes a larger tip, because the cut path through the metal is longer. A track burner gives a much smoother result than a hand cut.

Cut Quality

Before welding, grind a torch-cut edge to clean metal. Oxide and scale left on the edge cause porosity and poor fusion.

Backfire and Flashback

For a sustained backfire or flashback:

1. Close the torch oxygen valve.
2. Close the torch fuel valve.
3. Close the cylinder valves if it is safe.
4. Let the torch cool, and inspect the torch, hoses and arrestors before you use them again.

Common causes include a dirty or overheated tip, touching the tip to the work, wrong pressures and leaks. Follow your equipment maker's instructions if they differ.

Coated Steel and Tight Spaces

Galvanized steel. Cutting it releases zinc fume, which causes metal fume fever and other lung harm. Work outdoors or use local exhaust, and use the respiratory protection your employer requires. Grind the coating back from the cut line first to reduce the fume.

Confined spaces. A leaking torch can raise oxygen levels and turn clothing into fuel, or fill the space with fuel gas. Test the air, and use forced ventilation, not oxygen. Take the torch and hoses out when you leave, even on a short break. Confined space rules apply.

Fire Watch

Sparks can travel 35 feet or more, and they can fall through gaps in floors. Keep combustibles clear, and follow your hot work permit. OSHA's minimum is a fire watch of a half hour after the work stops. Recent NFPA 51B editions call for at least 1 hour, so your local rule or permit decides how long.

Shutting Down

Makers differ on whether to close oxygen or fuel first, so follow your torch's instructions. Many tell you to close the oxygen first, and the steps below follow that order.

1. Close the torch oxygen valve, then the fuel valve.
2. Close both cylinder valves.
3. Open the torch oxygen valve and bleed the line until the gauges read zero, then close it.
4. Do the same for the fuel line.
5. Back out both regulator adjusting screws.
6. Coil the hoses, and put your tools away.

Common Mistakes

• Opening a valve while standing in front of it. Stand to the side.
• Skipping the leak test. Leaks turn into fires.
• Using a greasy glove on the oxygen side. That is an ignition source.
• Cutting over your own feet or a hose. Plan where the slag and offcut will go.
• Leaving hoses pressurized overnight. Bleed the lines each time.

Check Your Understanding

You need to open a new acetylene cylinder. How far do you turn the valve, and why?

About 1/4 to 1/2 turn. That gives you enough gas and lets you shut it off quickly in an emergency.

A coworker's gloves are covered in bearing grease, and he reaches for the oxygen regulator. Why stop him?

Oil and grease can ignite on their own in pure oxygen. He needs clean gloves.

Your torch starts to squeal, and the handle gets hot. What do you do?

Treat it as a flashback or sustained backfire. Close the oxygen, then the fuel, then the cylinders. Inspect it all before you use it again.

Your cut keeps stalling, and sparks spray up off the top of the plate. What is probably wrong?

You are moving too fast. Release the lever, reheat the spot and restart at a slower speed.

Key Takeaways

• Oxy-fuel burns carbon steel, and it will not cut stainless or aluminum.
• Follow the setup order, and leak-check every time.
• Never set acetylene above 15 psi, and keep oil away from oxygen.
• Use a neutral flame, preheat to bright cherry red, then tilt forward once the cut is going.
• For a flashback, shut oxygen first, then fuel, then inspect.
• Plan for fire, fume and falling metal before you light up.