How to Cut Metal with an Acetylene Torch

The blow torch has a cool name, but if you don't observe a few safety rules, the whole thing could blow.

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Look around you. Is there anything – anything – within a 25- to 30-foot radius that could catch on fire if, say, a shower of sparks were to land on it? Are you sure? What are you wearing? Does your shirt have open sleeves, an open collar? Is there a big tongue hanging out of your sneakers, or is there any other opening where a ball of fiery, molten metal could roll in?

These are the questions you have to ask yourself when you’re working with an acetylene cutting torch, known colloquially, in 1950s greaser garages everywhere, as a blow torch. That’s what Carlos Dos Santos, proprietor of Brooklyn Motor Works – where I’m in the process of reviving what was a particularly beat-up Toyota Land Cruiser– taught me in a brief tutorial. The message was clear: just about every mistake can cause burned skin, a fire, even – in the worst case – a giant explosion.

The blow torch is essentially a metal tube that mixes acetylene – an odorless colorless hydrocarbon gas – and oxygen to create a very hot pinpoint flame. That flame can be used for cutting and welding (although it’s not as efficient a way to weld metal as electric arc welding). Acetylene will burn by itself when it combines with atmospheric oxygen, but adding pressurized oxygen makes for a much cleaner, almost focused flame. The “blow” part of a blow torch is that extra jet of oxygen. The acetylene and oxygen mix comes out of a little ring of small holes in the tip of the torch, and the flame they create melts metal, turning it into a molten slurry. When you pull the torch’s trigger to activate the oxygen jet, the extra gas blows the molten metal out of the way, leaving a hole that, hopefully, was your intention to put there.

Carlos has been working with acetylene for years, and he started out under the tutelage of old-school tough guys who would arc weld and torch things wearing tank tops and other inappropriate clothing. It wasn’t the best example to follow, and he said he’d experienced his share of mishaps by following their lead.

“How do I know that molten metal can roll into your shoe if the tongue is hanging outside your pants?” he asked as I stood there with his baby-faced apprentice, “Junior, Jr.,” who was also soaking up knowledge. “Because it’s happened to me. There’s no easier way to lose control of your torch than when something under your clothes is scorching your skin.”

Other places those evil little globules can enter (and that Carlos knows from experience): Ear canals, shirt collars, upturned sleeves. The pain of being seared can lead to a momentary loss of control. Picture this: a glob of hot metal rolls down your shirt. You yell, jump back and involuntarily flail around as it burns its way slowly along your skin. In that moment, if you let go of the torch, it bounces, and lands aimes at the rubber oxygen and acetylene lines. Then? Explosion.

None of this is meant to suggest that acetylene torches are all disasters waiting to happen. Handled correctly, and with a great deal of respect for the harm they can do, they’re very handy tools. It’s just important to remember a few things when using them.

– Always check for flammable material around the area where you’ll be working before you light the torch.

– Make sure the regulators on the acetylene and oxygen tanks are set to the correct pressure (the wrong pressure can cause an explosion or a fire, surprise surprise).

– Never stand in front of a gas regulator (the thing on top of each tank with the gauges on it). Stand to the side. That way, if it blows up for some reason (and you never know), you won’t be in the direct path of flying parts.

– Always be aware of where your oxygen and acetylene lines are laying. They’re almost always paired, with the red one connecting to the acetylene tank and the green one connecting to the oxygen (because of different-sized connectors, it’s impossible to connect to the lines incorrectly). As mentioned before you don’t want any molten metal to roll across them and melt through

– Always wear a welding mask. Forget about those old movies with guys wearing goggles as they torch. You want to protect your face from lava splatter, right? Also, your eyes need protection from the brightness of the flame, so regular safety glasses won’t do.

– Dress appropriately. Wear non-flammable materials like cotton, wool and, best yet, leather. Synthetics will melt to your skin if they catch on fire. That’s why combat troops don’t wear synthetics in war zones; messy cleanup.

– Always have a fire watch when you work. Wearing a welding mask protects your face and eyes, but it also limits your field of vision to the thing you’re working on. You should have someone standing behind you to alert you in case something catches fire (very likely) or sparks get near the gas lines.

– Open the tank valves a couple of turns each by turning counter-clockwise (rightey-tightey, leftey-loosey). Never open the valves so much that you can’t quickly close them if something goes wrong.

– Set the acetylene outlet pressure to 7psi (but never, ever more than 15). The higher the acetylene pressure, the more unstable and dangerous it becomes.

– Set the oxygen pressure to about 15psi.

– On the torch head, open the acetylene valve a little bit (you don’t need too much here). Once you hear it hissing from the tip, strike a steel and flint striker (which makes sparks) near the head. The gas will catch fire and burn with a flame that’s bright orange, ragged and throws off a bit of smoke.

– Slowly add in oxygen with the other valve, turning the unkempt pure fuel flame into a chiseled pinpoint of blue fire. The center of the flame should be white, and the rest of it shouldn’t be too long.

Now, you’re ready to cut. Here’s how it’s done:

– hold the flame near (but not on) the piece of metal you’re trying to cut. Wait for it to turn shiny and lava-like. This means the metal is soft and ready to be reformed.

– Once the metal in front of the flame has returned to its volcanic state, carefully add in the oxygen jet.

– With the jet trigger squeezed, carefully move the torch across the surface to be cut. The metal should give way easily.

Take your time! Rushing can only cause mistakes, and mistakes can cause big problems.

That’s pretty much it. Using an acetylene torch ain’t rocket science, but the difference between doing it right and doing it wrong could be catastrophic. But don’t take my word for it, take a class. Learn the ropes yourself. Despite its potential to wreak havoc, the acetylene torch is a wonderful tool to have and to know how to use when you need to cut through 2-inch-thick steel in a jiffy.


Benjamin Preston


Benjamin Preston is an automotive journalist who holds the dubious distinction of having worked both as a writer and editor for the New York Times Automobiles section and as a mechanic at the Pep Boys in Fredericksburg, Va. As a journalist, his work has taken him to a few war zones, including Baghdad, Iraq, and the Detroit auto show.