How to Use a Cutting Torch

Cutting 3/8 mild steel angle with a torch.

For some building and repair projects, an Oxygen/Acetylene Torch is the best tool to use for cutting steel to size and shape. This is a dangerous cutting system, but with precautions and practice, you can cut steel into various shapes using this method, where other tools simple are not up to the task. Here are some steps for using this tool.

Understand what the oxygen/acetylene torch actually does. The initial combustion of the acetylene gas heats the steel to a molten state, then, by adding a pressurized stream of oxygen, the steel is ignited and burned away through the kerf of your cut. This is why this process is called burning steel, and steel and carbon steel are the only materials it is used to cut. Cast iron, aluminum, stainless steel, and other metals and alloys can not be cut with a cutting torch.

A typical oxy-acetylene cutting rig on a cart.

Get the proper tools for the job. Besides the torch assembly, described in detail throughout the steps, you should have the following:

Fire extinguisher. For most uses, a compressed air and water extinguisher will work, but if oil, plastics, or other combustible materials are around, an "ABC" extinguisher is recommended.

Tools assembled that are needed for this work.

Marking and measuring tools. These will help you lay out your cut correctly, and can include a soapstone marker, a square, and a rule.

Safety equipment, including a set of cutting goggles and heavy leather gloves.
Proper clothing is a must. Do not wear loose fitting clothes, clothes made from flammable synthetic materials, or clothing with frayed or torn edges that may ignite more easily than tightly woven clean hemmed garments.
Sturdy leather soled work boots are recommended, since rubber soled shoes will allow hot slag to burn through pretty quickly. Laced boots are preferable, since slag can drop into the top of pull on types like engineer's boots and cowboy boots.
A striker, for correctly igniting the torch. Using matches or a cigarette lighter is very dangerous, a striker is made especially for igniting torches, and reduces the risk of serious injuries.

Using an I-Beam for a cutting table on a clean concrete shop floor.

Set up your work in a safe environment. Working on bare earth or a concrete slab is highly recommended, since sparks are going to fly many feet from the cutting location. Dry material like paper, sawdust, cardboard, and dried plant foliage or grass can be ignited from 15 feet or further.

Place the stock you are going to cut on supports at a comfortable working height. A steel table is ideal, since you can brace yourself to steady the torch while heating and burning it. Never use a surface that is flammable or has flammable materials spilled on it. Also, beware of material that has metallic oxide coatings, such as lead paint, chromate primers, and zinc plating, since inhaling the fumes from these can have a toxic effect on the torch operator.
Mark your cuts with a soapstone marker, allowing for grinding if a precise fit is required. You may use a permanent marker if a soapstone is not available, but the mark will tend to disappear just ahead of the torch flame. For very precise cuts, you may find a specialized jig will give better results, but that is not within the scope of this article.

Clearly marked oxygen (oxidizer) tank. These tanks are pressurized to 3000 PSI, do not let them fall over.

Hook up the gauges to the appropriate tanks. Typically oxygen tanks and hoses are green, and acetylene hoses are red, and they are pair together with the ends separated to attach to their respective tanks. The acetylene hose will have reversed threads, and the fitting will be a male type, to prevent accidental switching of these two hoses/gauges. Tighten them with the correct size wrench, since the fittings are made of brass, and can easily be damaged.

Note the bolded red line on the right upper side of the scale, indicating pressure is too high.

Make sure the acetylene regulator is turned off by backing the Tee handle out a few turns, then turn the gas valve on top of the tank on. Turning it about 4 turns is usually sufficient, although some gauge instructions recommend turning it open completely. Follow the following points to make sure the gas is regulated to the correct pressure. Never allow acetylene gas pressure to exceed 15 PSI, at higher pressures, acetylene becomes unstable, and may ignite spontaneously or explode.

After turning on the main acetylene tank valve, open the regulator valve by turning the tee handle clockwise. This should be done very slowly, while watching the low pressure gauge. Open only until the pressure indicated is between 5 and 8 PSI.
Open the gas valve on the cutting torch handle until you hear gas escaping to vent the atmosphere from the acetylene hose, then observe the low pressure gauge to see if the pressure remains steady during flow, to assure you have this regulator set correctly.
Close the acetylene valve on the torch, and continue to the next step, to attach and adjust the oxygen regulator.

The acetylene valve is on the left, oxygen on the right on this torch.

Turn the oxygen regulator pressure off or down by backing the regulator gauge up a few turns, then proceed with the following steps to adjust the oxygen pressure.

Open the main oxygen tank valve on about 2 to 3 turns. Always remember, the oxygen tank is under extremely high pressure, so the gauge should not be opened fully, only open it enough to allow a continuous flow at the proper pressure.
Open the regulator valve slowly, watching the low pressure gauge as you do so, until the pressure reads between 25 and 40 PSI.
Open the oxygen valve on the torch to allow the atmosphere to vent out of the hose. Note that there are two valves on the oxygen side of the cutting torch assembly. The valve near the hose connections will control the flow of oxygen to the mixing chamber for the combustion, or heating operation of the torch as well as the oxygen jet side, so no oxygen should vent from the torch tip until either the cutting lever is depressed or the valve further up the torch is opened. To begin with, open this first valve several turns, to insure sufficient oxygen is available for both functions. Next, open the forward valve slightly until the hose is purged, about 3 to 5 seconds for a 25 foot hose.
Close the forward valve.

Put your gloves and goggles on before lighting the torch. Check your work area one more time for combustible materials, and you should be ready to go.
Prepare to light the torch. Open the acetylene valve again, allowing the oxygen remaining in the mixing chamber to purge for a few seconds, then shut the valve until you can barely here gas escaping. Holding the striker in front of the torch tip, with the tip facing inside the striker (or toward the ignition source, for electronic strikers), squeeze the handle. A small yellow flame should appear at the tip when the sparks from the striker ignite the acetylene.

The yellow flame of pure acetylene burning before the oxygen valve is opened.

Adjust the acetylene valve until you have a yellow flame about 10 inches long. Make sure the flame begins at the torch tip, if too much acetylene is supplied the flame will jump, or be blown away from the tip.

The forward valve that controls the oxygen to the cutting tip. The chrome lever is the oxygen jet valve.

Turn the forward oxygen valve on slowly. The flame color will turn from yellow to light blue as sufficient oxygen is supplied to complete combust the acetylene. Increase the oxygen until the inner blue flame begins to shrink back toward the tip. This flame doesn't appear in the photos provided since the camera's discretion on this bright light source failed to distinguish it from the outer flame.

Inside the bright, white flame is a small blue flame, invisible in this photo.

Open the acetylene valve more to increase the flame size until the inner flame is about the length of the thickness of the steel you are intending to cut. For 3/8 inch cold-rolled plate, or mild, steel, a 1/2 inch inner flame is about right. If you here a blowing noise, or the blue flame seems erratic and feathery, there is probably too much oxygen at the flame, reduce it until the flame is stable, and the inner flame is a clean, cone shape.

The flame is held steady, very close to the metal to heat it prior to beginning the actual cut.

Bring the tip of the inner flame to the surface of the steel you are going to cut. You will need to heat the steel with this flame until a molten pool of metal forms and luminesces at this location. For 1/4 inch plate steel at room temperature, this should take about 45 seconds, it will take longer for heavier material, or material at low temperatures. Keep the tip of the flame steady, and about 3/8 of an inch from the metal's surface to concentrate the heat in one location.

A shower of sparks and burning, molten steel is blown backwards until a hole is burned through the steel.

Push the cutting valve handle down slowly to release the oxygen jet which ignites the molten steel. If a violent reaction instantly starts, the steel has ignited, and you can gradually increase pressure until the jet is cutting completely through the metal. If no reaction occurs, the metal is not hot enough to ignite, so you can let the oxygen handle release and allow the flame to heat the metal more.

Note that now that the torch is cutting through, the burning metal is washed out the bottom of the cut in the jet of oxygen.

Begin moving the torch tip slowly along the line of your cut when the jet is cutting through the steel. You should observe that almost all of the sparks and molten slag are being blown out the back or bottom of your cut. If this flow of superheated material slows or backs up, slow your forward speed or stop and let the metal heat more. It is better to cut too slowly than to try to cut to fast.

Tapping out the red hot plug after the cut is complete.

Continue cutting until you have parted the metal, or finished the cut. Make sure the slag and any drops of heated metal do not get underfoot. Even sturdy boot soles will burn through if you find yourself standing on a large piece.

Cool the workpiece with plenty of water, or allow to cool if you are in no hurry. Be aware that dipping a superheated piece of steel into a bucket or stream of cold water will create and instantaneous cloud of very hot steam.

Note the slag that dripped through the cut and solidified on the bottom when this angle was cut.

Chip the slag from the cut, or grind the cut smooth if a more precise fit is desired.

Cleaning the cutting tip with tip cleaners, small round files made for this purpose.

Keep the torch tip clean.

Make sure all hose connections, gauge/regulator fittings, and other attachments are tight. Leaking gas at these fittings can produce an instant fire.