Use a Cutting Torch
An oxygen/acetylene torch is a dangerous cutting system, but with precautions and practice you can use it to cut steel to size and shape. To use a cutting torch safely, follow the instructions below.
Contents
Steps
Preparation
- Understand the cutting process. The initial combustion of the acetylene gas heats the steel to a molten state. By adding a pressurized stream of oxygen, the cutter ignites the steel to burn 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. Aluminum, stainless steel, and other metals and alloys cannot be cut with a cutting torch.
- 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.
- Marking and measuring tools. These will help you lay out your cut correctly, and can include a soapstone marker, a square, and a ruler.
- 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. This also means no loose pockets of cuffed pants or shirts that would allow slag to catch and burn through. Fire retardant clothing is recommended but, if not available, wear well fitting cotton clothes. Nylon and most other common synthetic clothes will rapidly burn if caught on fire!
- 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.
- 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 {{safesubst:#invoke:convert|convert}} or further. Do not allow the flame to contact concrete, especially fresh concrete, as it make cause it to expand and crack violently, sending small bits of concrete flying.
- 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 be toxic.
- 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.
Set Up the Cutting Torch
- Hook up the gauges to the appropriate tanks. Typically, oxygen tanks and hoses are green, acetylene hoses are red, and they are paired 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. Since the fittings are made of brass and can easily be damaged, tighten them with an appropriately sized wrench.
- 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 one turn of the wrist. This is for safety reasons. Never allow acetylene gas pressure to exceed 15 PSI, at higher pressures, acetylene becomes unstable, and may ignite spontaneously or explode. To make sure the gas is regulated to the correct pressure, do the following:
- 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.
- To vent the atmosphere from the acetylene hose, open the gas valve on the cutting torch handle until you hear gas escaping, then observe the low pressure gauge to see if the pressure remains steady during flow and ensure that you have this regulator set correctly.
- Close the acetylene valve on the torch.
- Turn the oxygen regulator pressure off or down and then adjust the oxygen pressure. To turn the regulator pressure down, back the regulator gauge up a few turns. When you’re done, proceed with the following steps to adjust the oxygen pressure:
- Open the main oxygen tank valve all the way. The valve is a double-seated valve and, if it’s not opened all the way, the oxygen will leak out around the valve-stem O-ring because of the high cylinder pressure of 2200 psi.
- 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.
Use the Cutting Torch
- 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.
- 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 hear 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.
- Adjust the acetylene valve until you have a yellow flame about {{safesubst:#invoke:convert|convert}} 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.
- Turn the forward oxygen valve on slowly. The flame color will turn from yellow to light blue as sufficient oxygen is supplied to completely combust the acetylene. Increase the oxygen until the inner blue flame begins to shrink back toward the tip.
- Open the oxygen valve more to increase the flame size until the length of the inner flame is just over 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 hear 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.
- 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; however, 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.
- 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 continue to allow the flame to heat the metal more.
- 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 too quickly.
- Continue cutting until you have parted the metal or finished the cut. Make sure the slag and any droplets 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 it to cool naturally 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 an instantaneous cloud of very hot steam.
- This is only for mild steel, as water cooling will affect quench- and temper-type steels.
- Chip the slag from the cut. You can also grind the cut smooth if a more precise fit is desired.
Tips
- Make sure all hose connections, gauge/regulator fittings, and other attachments are tight. Leaking gas at these fittings can produce an instant fire.
- Always transport gas cylinders in a vertical (upright) position.
- Keep animals and children away from areas where this type of hot work is being done.
- Keep the torch tip clean.
- Have flashbacks installed on both ends; it's much safer than only one fitted.
Warnings
- Use a cutting torch only in a well ventilated area, away from combustible materials.
- Having a fire watch is required on OSHA-regulated projects.
- Using a blow back or back flow preventer is highly recommended.
Things You'll Need
- Cutting torch set, with fuel and oxidizer tanks.
- Striker, marking tools, and safety equipment.
