An oxy-acetylene torch produces a flame that can reach 3,480°C (about 6,300°F), making it one of the hottest fuel-gas combinations available for cutting, welding, brazing, and heating metal. Using one safely comes down to a consistent routine: proper setup, correct lighting sequence, the right flame adjustment for your task, and a careful shutdown. Here’s how each step works.
Equipment and Safety Gear
A standard oxy-acetylene rig includes two cylinders (one oxygen, one acetylene), a regulator for each cylinder, twin hoses (green for oxygen, red for acetylene), a torch body with oxygen and fuel valves, and interchangeable tips sized for different jobs. You’ll also need a friction striker to light the torch. Never use a cigarette lighter or match.
Two safety devices sit between the cylinders and the torch. Check valves prevent gas from flowing backward through the hoses. Flashback arrestors go a step further: they detect and extinguish a flame that’s traveling backward toward the cylinders, which is the most dangerous failure mode in an oxy-fuel system. Most modern setups install flashback arrestors near the torch inlets, with check valves at the regulator outlets. Confirm both are in place before every use.
For eye protection, OSHA requires shaded goggles or a face shield. The minimum shade depends on what you’re doing. For gas welding thin plate (under 1/8 inch), a shade 4 lens is the OSHA minimum, though ANSI recommends shade 5. For oxygen cutting plate under 1 inch thick, shade 3 is the minimum, with ANSI recommending shade 4. As material gets thicker, you step up to darker shades. A good rule of thumb: start with a shade that’s too dark and work lighter until you can see the work zone clearly. Wear flame-resistant gloves, long sleeves, and leather boots. Remove anything synthetic that could melt onto skin.
Setting Up the Cylinders
Acetylene cylinders must always be stored and used valve-end up, in a vertical position. Inside each acetylene cylinder is a porous mass soaked in a solvent (acetone) that keeps the acetylene stable. Tilting or laying the cylinder on its side can allow that solvent to leak out through the valve, creating a fire hazard and making the gas less stable. If a cylinder has been on its side, stand it upright and wait at least 15 to 20 minutes before opening the valve.
Secure both cylinders to a wall, cart, or post with a chain so they can’t fall. Before attaching regulators, slowly open the oxygen cylinder valve briefly to blow debris from the valve outlet. Do not crack the acetylene valve for the same purpose. Instead, wipe the acetylene valve outlet clean with an oil-free rag or blow it out with compressed air or nitrogen. This matters because acetylene is far more reactive, and even a small spark from debris could ignite it.
Attach the oxygen regulator to the oxygen cylinder and the acetylene regulator to the acetylene cylinder. The fittings are intentionally different (acetylene uses left-hand threads) so you can’t mix them up. Tighten connections firmly with a wrench. Before attaching, confirm each connection is clean and free of oil or grease, which can ignite on contact with high-pressure oxygen.
Pressurizing and Leak Testing
With regulators attached and the pressure adjusting screws backed all the way out (turned counterclockwise until loose), slowly open the oxygen cylinder valve fully. Open the acetylene cylinder valve no more than one and a half turns, so you can shut it off quickly in an emergency. Now turn each regulator’s adjusting screw clockwise to set your working pressure. The exact pressure depends on your tip size and task, but most welding tips run between 5 and 7 psi on both oxygen and acetylene.
One hard rule: never set acetylene delivery pressure above 15 psi. OSHA regulation 1910.253 prohibits using acetylene above 15 psig because the gas becomes unstable at higher pressures and can decompose explosively, even without an ignition source.
Once pressurized, test every connection for leaks using a soap-and-water solution (or a commercial leak detection fluid). Brush it onto each fitting, including the cylinder-to-regulator connection, hose connections, and torch fittings. Bubbles mean gas is escaping. If you find a leak, close the cylinder valve for that gas and try tightening the connection. If bubbles persist, take the equipment out of service. You can also do a static leak test by closing all torch valves and cylinder valves, then watching the regulator gauges. A dropping pressure reading means gas is leaking somewhere in the line.
Lighting the Torch
Before lighting, purge each line separately. Open the acetylene torch valve for a couple of seconds to flush air from the fuel line, then close it. Do the same with the oxygen torch valve. This prevents a mixed-gas pocket in the hoses that could pop when ignited.
To light, open the acetylene torch valve about half a turn. Hold the striker at the tip and squeeze to create a spark. The acetylene will ignite with a yellow, smoky flame. If the flame jumps away from the tip, reduce the acetylene flow slightly until the flame seats back against the tip. Now slowly open the oxygen torch valve. As you add oxygen, the smoky yellow flame will shrink and change color. Adjust until you get the flame type your work requires.
Reading and Adjusting the Flame
The flame you set determines the quality of your work. There are three types, and each looks distinctly different.
A neutral flame has a balanced oxygen-to-fuel ratio. It appears as a semi-transparent blue or purple cone with a clearly defined inner cone and no feathery edges. This is the default for most welding and brazing work because it neither adds carbon to nor oxidizes the metal surface.
A carburizing (reducing) flame has too little oxygen. You’ll see a feathery, lighter-colored “acetylene feather” extending beyond the inner cone, and the flame may appear more opaque or yellowish at the edges. This flame introduces carbon into the weld pool, which is useful for hardfacing or certain specialty applications but generally undesirable for standard welding. It also produces carbon monoxide, so ventilation is critical.
An oxidizing flame has excess oxygen. The inner cone gets shorter, more pointed, and a more transparent blue. It hisses or roars noticeably louder. This flame oxidizes the metal surface, which weakens most welds. Outside of a few specific tasks like brazing certain alloys or platinum soldering in jewelry work, you’ll want to avoid it.
For most work, start with slightly too much acetylene (you’ll see the feather), then slowly increase oxygen until the feather just disappears into the inner cone. That’s your neutral flame.
Working With the Torch
Hold the torch at roughly a 45-degree angle to the workpiece for welding, with the inner cone about 1/16 to 1/8 inch from the metal surface. The hottest point is just beyond the tip of the inner cone. For cutting, a separate cutting attachment or torch head directs a high-pressure oxygen stream through the center of the preheat flame. You heat the metal to a bright cherry red, then press the cutting lever to release the oxygen jet, which burns through the steel.
Move at a steady pace. If you go too slowly, you’ll melt too wide a puddle and distort the metal. Too fast, and you won’t get full penetration. When welding with filler rod, dip the rod into the leading edge of the puddle rather than holding it in the flame, which wastes material and creates spatter.
Shutting Down Properly
The shutdown sequence matters as much as the lighting sequence. Close the oxygen torch valve first, then the acetylene torch valve. This extinguishes the flame quickly and avoids a loud pop that can occur if you close acetylene first (which momentarily creates an oxygen-rich blast).
Next, close both cylinder valves: oxygen first, then acetylene. Now bleed the remaining pressure from the lines by opening each torch valve separately until the regulator gauges read zero, then closing the torch valves again. Finally, back out both regulator adjusting screws until they spin freely. This relieves spring tension on the regulator diaphragms, extending their lifespan. Confirm all four gauges (two delivery, two cylinder) read zero before walking away.
Maintaining the Torch Tips
A torch tip in good condition produces a clean, round flame pattern. Over time, tips collect spatter, carbon buildup, and dents, especially cutting tips that tend to get banged around on the job. A damaged tip causes uneven flames, increased popping, and a higher risk of backfire, where the flame travels backward inside the tip and damages internal seals and gaskets.
Clean tip orifices with a tip cleaner set, which is a small keyring of calibrated wire files. Insert the correct size into the hole and push straight through without twisting, which can enlarge the opening. For cutting tips with multiple preheat holes, clean each one individually. If the tip face is uneven or rounded, use a flat, fine-pitch file to square it up, then rebore the holes. A tool called a “Tip Nip” is specifically designed to resurface and recondition tip profiles.
For two-piece tips used with alternative fuel gases, clean the flutes on the inner piece with a small stainless steel brush and use a thin blade to straighten any flutes that have twisted from overtightening. Replace tips that are cracked, badly pitted, or won’t produce a stable flame after cleaning. The cost of a new tip is negligible compared to the weld quality and safety problems a worn one creates.