Oxyfuel Cutting
Gases, flame, and safe setup
Oxyfuel cutting uses a fuel gas plus oxygen for preheat, then a jet of pure oxygen to oxidize and blow the steel away. Acetylene is the common fuel: never set acetylene delivery pressure above 15 psig, open the cylinder valve no more than about 1½ turns, and keep cylinders upright. Open oxygen cylinder valves fully. Hoses are color-coded (oxygen green, fuel red in the U.S.); install reverse-flow check valves at the torch and flashback arrestors at the regulators. Keep oil and grease away from oxygen fittings - that is a fire, not a nuisance.
Flame types. A neutral flame - a short, rounded, clearly defined inner cone - is correct for cutting steel. An oxidizing flame (short, pointed, hissing cone) leaves a poor, oxidized cut edge, and a carburizing flame (feathered acetylene plume) adds carbon and slows cutting. Set pressures from the tip chart, not by ear.
Technique. Preheat the start point to a bright cherry red, then open the cutting oxygen. Hold the tip about 1/8–1/4 in off the plate; too much standoff loses preheat and widens the kerf. A slight lag (drag) of about 1/8–1/4 in is normal. To pierce thick plate, tilt the torch roughly 15–20° so slag blows away from the kerf, then roll upright once through.
Backfire vs. flashback. A backfire is a pop or whistle with the flame briefly leaving the tip; the torch usually can be relit after checking. A flashback is a flame burning back into the hose or regulator - shut the torch oxygen valve first, then the fuel valve, then the cylinder valves, and inspect the whole train before relighting.
Worked example / rule of thumb. If sparks are spraying upward off the top of the plate, you are moving too slowly; if the cut keeps losing the kerf and the drag angle grows past the plate thickness, you are too fast.