How to Cut Metal with a Torch

How to Cut Metal with a Torch


Today we’re talking about oxy acetylene cutting as a lot of my friends are in the construction business and you may be used to working with wood and you’re looking at getting into metal. One of the most versatile tools that you can use is oxy fuel for both cutting and welding. I’m talking about oxy acetylene cutting and specifically how to do it. First of all, start with the tanks, you have acetylene, this is your fuel gas, and you have oxygen, this is your oxidizing agent and they mix together, and they create the flame that will eventually cut our steel. Oxygen is a high pressure cylinder, very high pressure, and you have a lot of potential energy there if you drop or knock off the valves so handle with care. Acetylene is even more tricky because acetylene doesn’t like being compressed. In fact, it will explode if you compress it over, I think it’s I forget the pressure number, but it’s a pretty low number. If you mix it with acetone it is stable at high pressure so there’s acetone and then there’s a foam core inside the bottle that keeps the acetone and the acetylene mixed together. Do not ever lay one of these down, because if you lay it down, acetone can separate from your acetylene and you can get an explosion. So read up on all the safety tips on acetylene as it can be very hazardous. It is a good idea to do a Hey Google, Hey Alexa, or Hey Siri search for acetylene cylinder safety procedures and the internal details of acetylene tanks before you start using a torch.  Next, you have the regulators. The best ones are Smith, Victor, or Forney. Oxygen is your high pressure, about 2000 PSI and about 100 PSI in the acetylene and it knocks it down to usable pressures for our torch. In this case, we’re using 3/8 inch material. Now it matters what size of material you have as it will change what type of tip you use, which will change what kind of pressures you send to that tip. In this case, we’re cutting 3/8 inch material, so we have a single art cutting tip and we’re running about 35 to 40 PSI for our oxygen and we’re running between 3 and 6 PSI on our acetylene. If you are cutting 16” x 20” heavy structural I beams or large diameter high pressure steel pipe you, obviously, would use a different tip and settings. All of that information on the different thicknesses and different tip sizes will be on a chart. That makes it really nice when you’re cutting a thicker piece material. What should my tip size and then what should my pressures be?


Let’s get to the torch. I’m using a two hose torch set up so I’ve got my oxygen coming in. This oxygen supplies both my preheat flame oxygen as well as my cutting blast oxygen. My preheat is set with a knob and my cutting is controlled by the lever valve on the back. Acetylene comes in and it only supplies fuel for the preheat flame. It’s what a lot of people don’t understand about this process. It’s not cutting the material with the heat of the flame. All the flame is doing is pre heating the material. The flames that you see coming out are actually called preheat flames. So I preheat the material and then I add the oxygen with the lever. Oxygen goes into the hot material, reacts with the iron and creates iron oxide. When iron oxide is formed from oxygen and iron, there is an excellent thermal reaction and it gives off so much heat that, in fact, that it melts the material and it preheats the material next to it. You keep putting in more oxygen from the top and it keeps reacting with the iron. More thermal reactions happen, and you keep melting. In the old days, especially when they make these huge steel slabs, you know, 8, 10 inches thick. They would use an oxy fuel torch, huge oxy fuel torches and were able to cut all the way through these 10 inch slabs because it’s not the heat of the flame at the very top that is making the cut, it’s the oxygen going through the material, reacting with the iron and creating enough energy to melt the material. Then your gasses come through. They create the pre heating flame and your oxygen comes through to make the cut. Talking about that preheat flame quickly, there’s three different flames that you can create with any oxy fuel torch. You have a reducing or carbon rising flame. That means you’re putting more acetylene into the flame than is being able to be combusted with the oxygen, you have a neutral flame, meaning you’re burning up as much acetylene as you’re burning oxygen and then you have an oxidizing flame, which means that you are putting more oxygen than can be consumed in the combustion process.


To light your torch, the safest and best way to light your oxy acetylene torch is to use a flint striker. Basically, this is a device that has a flint member that is fixed into a threaded socket that screws into a spring loaded member that moves back and forth against a hardened steel surface like a file. This assembly is held inside a protective steel cap about 1” in diameter x ½” deep. When the striker is activated by hand pressure, the flint moves across the steel file and creates sparks. These sparks, of course, will ignite the acetylene, and the steel cap will keep the flame from unexpectedly projecting too far. These strikers are made by many companies such as: Forney, Hobart, Ally Tools, Vas Tools, Hot Max, US Forge, Lincoln Electric, Worthington, Levado, and Tech Team Tech Team’s model 763 Flint Striker is the one we like the best because it has high quality construction with a durable zinc plating, and it contains 3 flints that can easily be rotated one to the next to the next as it wears down and becomes ineffective.

It probably also occurs to you that eventually these flints will wear out and oddly enough there are several companies that make replacement flints such as: Forney, US Forge, Shurlite, Zippo, and Tech Team We happen to like Tech Team’s item 761 contains 3 sets, each set having 3 replacement flints, which easily fits into their 763 3 Flint Striker. These can be bought from Lowe’s, Home Depot, Allies Welding Supply, or online from Amazon.


We want to set this up with a neutral flame to begin with. So we want to make that acetylene feather a long blue flame. We want to bring that back until you just have your nice, clean, neutral preheat flames out the front. Then you want to make sure that when you add your oxygen you keep at a neutral preheat flame, but you’re still putting that stream of oxygen through the middle, which creates an oxidizing flame that will cut through the material. Now to the practical part of it, a lot of practice, a lot of muscle memory. I like using a straight edge to help me get guided along. You can do it freehand, but if you’re like me, it’s going to wander everywhere. Frequently when you are using a torch to cut metal, you need to mark where you are going to cut. There are lots of ways to make a mark or a line that range from using a magic marker or sharpie, or just scratching the metal with an awl. The best way to make the marks you need is to use soapstone, which is the solid form of the same talc that is used in talcum powder. The handiest way to do this is to use pre-cut rectangular (127 x 12.7 x 4.8mm) prismatic pieces of talc, which are held in a special pen shaped device with a pocket clip that allows the talc to be firmly held and advanced as the talc wears down. These soap stone holders are available from many companies such as: Forney, Ally Tools, Hobart, Firepower, and Tech Team® We happen to like Tech Team’s model 759 the best because it has a durable zinc finish and a positive locking and advancing mechanism for the soap stone.

Of course, you also need soap stone, aka talc, to feed the holder as you wear out the soap stone. Those refills are available from several companies such as: Homee, Anchor, VasTools, Uniweld, Hobart, and also Tech Team® We like Tech Team’s 757 36 pc. of soap stone the best because it is high quality, bright white soap stone that leaves clear and easy to erase marks on almost any metal surface. I use a straight edge and then I want to travel at about maybe 28 inches per minute. That’s hard to know how to do, because I can’t even count to a minute, much less know whether I’m doing 28 inches per minute so a lot of it’s by feel. You want about 60 thousandths kerf width so a 60 thousands wide cut. You want your material dropping out the bottom cleanly. You don’t want it to be fusing back behind if it’s fusing back behind, you’re moving a little bit too slow. You want to increase your speed a little bit and then just practice. If you’re trying to do 24 inches minute will take you 15 seconds. Just practice 15 inch or 15 seconds to go across that line and you’ll get a feel for how many inches per minute you’re traveling. So that’s it for the quick and dirty tips of oxy fuel cutting. Go out, get yourself a rig and start cutting. One thing I forgot to mention earlier, don’t get cheap Chinese tanks online because you won’t be able to get them filled. Go to your local welding supply store, they’ll be able to rent you cylinders, or they’ll sell you cylinders. Work through them because they have to be approved in order for them to refill them.