Oxy-Fuel Cutting and Related Processes
This domain tests oxy-fuel cutting and related processes: flame adjustment, cutting technique, kerf and drag, and the safe use of oxygen and fuel gases. Expect questions on neutral, oxidising and carburising flames and on flashback protection.
Core rules
- A neutral flame has approximately equal oxygen and acetylene; an oxidising flame has excess oxygen; a carburising flame has excess acetylene.
- Cutting is not melting alone: the preheat brings steel to ignition temperature, then the cutting oxygen jet oxidises the steel and blows the kerf clear.
- Keep the tip at the correct standoff, move at a steady speed, and watch drag lines; too fast leaves lag and incomplete cuts.
- Acetylene must be used within its safe delivery limit (commonly 15 psig maximum at the regulator) and cylinders must stand upright.
- Fit flashback arrestors at the regulator and torch.
Worked example
A 12 mm mild steel plate is to be cut by hand. Reasoning: select a tip for the thickness, set a neutral flame, preheat the edge to a bright orange, then open the cutting oxygen and move steadily. If the cut has heavy drag and dross, travel may be too fast or the tip may be too high. A carburising flame would add soot and delay the cut.
Traps
Do not use a cutting oxygen jet as a preheat flame. Do not exceed the acetylene delivery limit. Do not confuse a backfire (a pop at the torch) with a flashback (flame travelling back into the hose). Do not cut dirty or painted steel without considering fume.