Tool School: Break Out the Torch
The basics behind cutting steel with flame
MIG welders and plasma cutters might carry more cache, but serious metal cred comes from the mastery of a simple flame. The oxy-acetylene torch setup is one of the most affordable versatile tools in any shop. With it, a competent operator can heat, weld, braze, silver-solder, and cut. We’ve used it for all of the above, but it’s most often used to heat steel for bending and to cut. Cutting with the torch is so incredibly simple and cost-effective compared to plasma cutting— especially for thicker materials—that it cannot be ignored. Fortunately, it’s also not that hard to learn. Here’s a quick primer to the tool every metal shop should have.
First, since this is a tool that can burn as well as explode, we need to address safety.
Key Safety Points:
1. Setup: This post assumes that the oxy-acetylene rig is already set up. If it is not, consult someone who can help you to do this correctly, or one of the many resources such as this manual.
2. Ensure the equipment is in good working order: Is the setup correct for the intended application? Is the correct regulator being used for the oxygen and for the acetylene? Are the regulators in good working order? Is the hose in good shape? Verify that there are no leaks anywhere in the system. If in doubt, consult someone who knows— is not one of those times when it’s okay to learn lessons the hard way.
3. Working with high-pressure gasses: Make sure that both gas cylinders are securely fastened and cannot fall over. Should the valve be damaged, the high pressure gas inside can turn the cylinder into a heavy rocket. Only open the cylinder valves about 1 1/2 turns so that they can be turned off quickly in an emergency.
4. Working with oxygen: High-pressure oxygen can react violently with oil or grease, and if your shop is anything like mine, there is oil and grease around. Make sure that it stays away from your oxy-acetylene welding rig and off of your welding gloves and clothing. Do not ever oil any of your gas regulators. Keep in mind that high-pressure, pure oxygen can make things that are a little flammable into things that are alot flammable.
5. Working with acetylene: Acetylene is as dangerous, if not moreso, than oxygen. It is unstable above around 15 psi, and as such, you should never operate your acetylene regulator near or above that output pressure. In practice, I use 5 psi for most jobs. Acetylene is only stable at higher pressures inside the tank because it is dissolved in Acetone. Because of this more complex storage arrangement, acetylene tanks must be stored upright. If they have been on their side, they must be left standing upright for as long as the cylinder was on its side.
6. Working with a flame: The safety considerations here are obvious: Keep the flame away from things you don’t want to burn (including your flesh), watch out for flammable materials nearby, and remember that whatever you are heating / cutting / welding will become seriously hot (around 1,600°F).
7. Safety equipment: In addition to the intense heat, torch cutting will throw off sparks. Gloves and shaded eye protection are a must at all times. A number-4 shade in the glasses or goggles is appropriate for most light to medium cutting. Clothes should be made of cotton or some other natural product. Do not wear cuffed pants or any other garment or footwear that can trap sparks. Protective leather welding gear is recommended for any serious cutting. Provide proper ventilation and wear a respirator if necessary.
8. Remember that no quick post online can convey all of the safety points needed to operate complex and dangerous tools. It is your responsibility to do your own research and to educate yourself as to the safety best practices. What is provided above is only a primer. Always make use of the best tool that you have available: common sense.
Understanding the Equipment:
A cutting torch consists of two hose connections: one for oxygen and one for acetylene, and either one or two shutoff valves. Some torches omit the oxygen shutoff valve at the hose connection, as the oxygen flow is also controlled by a mixing valve in a cutting torch. The torch provides passages for the oxygen and acetylene to flow into a mixing chamber and then out to the cutting tip. Acetylene flows directly through this circuit from the cutoff valve. Oxygen flows through two circuits in the torch. One oxygen circuit flows through the aforementioned oxygen mixing valve and then into the torch’s mixing chamber, from which it will leave through the torch tip mixed with the acetylene gas. The other oxygen circuit flows through a valve controlled by the lever on the outside of the torch body and, when the lever is depressed and the valve is opened, directly to the cutting tip without being mixed with acetylene.
On the torch, the cutting tips are swappable, and change based on the thickness of material being cut. Using the right one greatly helps the finished quality of the cut. Tip size selection depends on the specific manufacturer and model of the torch.
Adjusting the Flame:
The oxygen and acetylene valves on the torch should both be closed at this point. Open the valves on both cylinders, slowly, about 1 1/2 turns. Adjust the regulators as recommended by the same product documentation that you used to select the proper cutting tip size. If that information is not available, 5 psi for Acetylene and 20 psi for Oxygen are good starting points for light to medium cutting.
Begin by opening the acetylene valve on the torch very slightly and igniting the torch with a striker. The flame should be fairly small and sedate with a smoke of heavy black soot If the flame is larger, close the acetylene valve slightly before continuing with this process. Now, open the acetylene valve slowly, increasing the size of the flame. A little bit of valve movement goes a long way here. Continue increasing the amount of the acetylene in the flame until the black soot becomes less pronounced. Unless you are cutting very thick materials, the flame should not be particularly large. If the acetylene flame at this stage is larger than about a foot long for relatively thin materials, say 1/8″, then you are probably using too large a tip, setting the acetylene pressure too high on the regulator, or simply opening the acetylene valve too far in this step. One of the most common problems people have in learning to torch cut is using too large of a flame. Remember that you are oxidizing the metal, not melting through it. Use the smallest flame as you can get away with while still being able to preheat and cut.
Oxy-Acetylene Cutting Flame
Once the acetylene valve is adjusted, begin adding oxygen through the mixing valve on the torch. Start with the mixing valve fully closed and the cutoff valve at the hose connection, if one is present, fully open. Oxygen should be added to the mix by opening the mixing valve (again, slowly) until the flame is “neutral”; neither carburizing nor oxidizing. You can tell by observing the bright blue cones in the flame as you adjust the oxygen mixing valve. Initially, you should see two sets of cones; one that is very bright and very close to the cutting torch tip and another that is less well defined and a good deal farther away from the tip of the torch. This is what is known as a “carburizing” flame. As oxygen is added to the mix, the second cone collapses down onto the first set of cones. At the moment when the second cone has just disappeared into the first set of cones, the flame is neutral. Adding any more oxygen will result in an “oxidizing” flame.
The torch is now adjusted with a neutral flame and ready to cut.
Now it is time to cut some steel. Continuing to use your common sense, position the work to be cut such that the heat will not damage or ignite anything else and such that the slag falling from the cut will land safely.
To begin cutting, the lever on the torch should be in its “off” position, as it was when the flame was adjusted. The flame the torch produces in this state is for pre-heating the material to be cut. Though the common perception is that the torch melts through the metal, it actually burns through the metal by pouring high pressure oxygen onto a pre-heated section of steel: rapid oxidization, aka burning.
It is easiest to begin cutting from the edge of a part and more difficult to pierce. Position the torch just in from the edge of a piece of metal to be cut. As the metal is heated by the pre-heat circuit of the torch, the area just under the torch go through shades of red and into orange. At some point – and the feel for that exact color will come with experience – the metal is ready to cut. Squeeze the lever of the torch to start the main oxygen flow. If the torch immediately punctures the metal and begins to cut, all is well. If this does not happen, the metal is not yet hot enough to cut and you must continue to pre-heat the area.
Once the oxygen flow is cutting through the metal, begin to move the torch. It is difficult to see most markings when viewed through shaded safety goggles and in the glare of a 1,600°+ flame. Soapstone is a good solution for marking that overcomes this issue. Once the torch is in motion, hand-eye coordination becomes key. The torch needs to move at a fairly consistent and slow speed. It also needs to remain in the proper orientation to the piece being cut and at the correct distance. For starters, position the torch tip 1/8″ to 3/8″ away from the metal and orient it straight up and down. More practice and a study of specific situations and techniques will refine these suggestions.
Proper torch movement is harder to achieve than it may sound; this is where practice comes in. The most common issue in learning this skill will be that a rapid or jerky movement causes the flame to stop cutting. Simply stop, re-position the torch, and begin the pre-heat process again. When the metal under the torch tip is hot enough, begin cutting once again.
Practice, practice, practice.
When you are finished, first close the oxygen mixing valve and the acetylene cutoff valve on the torch to extinguish the flame. Next, close the valves on both gas cylinders. Gas left at pressure inside of the torch hoses can seep through the walls of the hose, potentially leading to mixing of the gasses. This is not a good situation. To prevent this, bleed off the gas in both the oxygen and the acetylene circuits by opening the valves on the torch, one at a time in turn. Close the valves again when you are finished so that the torch will not be left in an unexpected state for the next person who uses it. Finally, to preserve your regulators, back off the pressure adjustment thumbscrews a few turns.
Wait until the metal has cooled before handling it. When you do pick it up, you’ll notice that there is some unattractive stuff all around the cut, primarily on the back side. This is known as slag and it needs to be removed before you do much else with the part. Resist the temptation to break out the grinder here; most slag will chip off with a cold chisel and a hammer. Using this method, you’ll get a much better looking result faster, without filling your shop with airborne abrasive material. Grinding also runs the risk of cutting into your base material as you try to remove the slag.
The key points to success with oxy-acetylene cutting are summarized as follows:
1. Pay attention to safety and use common sense.
2. Remember that you are burning through the metal, not melting. Adjust the torch to have a neutral flame that is not too large for the work piece.
3. If the application of the oxygen flow doesn’t quickly punch through the metal, something is wrong. The most likely issue is that the metal has not been pre-heated sufficiently.
4. Practice smooth and consistent hand movements. Proper torch position and support are essential.
Don’t attempt what is described here unless everything above makes perfect sense to you. It would be wise to get some hands on advice from someone who is already familiar with these skills before you start.
If you would like to learn more about gas cutting, many community colleges and technical schools offer welding classes. There is also a wealth of information available online and in text books. As a reference, I use the textbook “Welding Principles and Applications” by Jeffus and Johnson.
Remember to use your common sense.