15.3 Oxy-Fuel & Plasma Arc Cutting

Task A-6 (Performs cutting and welding operations) carries 2 exam questions across six sub-tasks, and sub-task A-6.01 (Cuts material with oxy-fuel and plasma arc equipment) is the one a millwright uses almost daily — removing seized components, cutting access, freeing corroded fasteners and preparing repairs.

How Oxy-Fuel Cutting Works

Oxy-fuel cutting is a chemical oxidation process, not a melting process:

1. The preheat flame raises a spot on the steel to its kindling temperature, roughly 870 C (1,600 F) — a bright cherry red.
2. The cutting lever opens a stream of pure oxygen through the centre orifice of the tip.
3. The oxygen reacts with the hot iron, burning it to iron oxide and releasing large amounts of heat, which sustains the reaction.
4. The oxygen jet's kinetic energy blows the molten oxide (slag) out of the cut, leaving the kerf.

Consequence — the material limitation. Oxy-fuel only cuts metals whose oxides melt at a lower temperature than the metal itself. That means plain carbon and low-alloy steels only:

For anything other than carbon steel, use plasma.

Fuel Gases and Flame Types

The Three Flames

   CARBURIZING (REDUCING)     NEUTRAL                    OXIDIZING
   excess acetylene          balanced 1:1               excess oxygen

   ((( )))~~~~~~~            (()) ~~~~~~~~              () ~~~~~~~~
   inner cone + feather      sharp inner cone,          short, pointed,
   + outer envelope          no feather                 sharp hissing
   -> hardfacing, some       -> CUTTING and most        -> brass and bronze
      aluminum work             steel welding              only; oxidizes steel

Setting a neutral flame: open the fuel valve and light with a striker (never a lighter or match), increase fuel until the smoke clears, then add oxygen until the acetylene feather retracts into a sharply defined inner cone. On a cutting torch, set the neutral flame with the preheat valves, then press the cutting oxygen lever and re-adjust, because the cutting stream leans the flame.

Acetylene: The Non-Negotiable Rules

Acetylene is chemically unstable and requires specific handling that appears repeatedly on Red Seal exams:

1. Never exceed 103 kPa (15 psig) working pressure. Above that, acetylene can decompose explosively even without oxygen present.
2. Cylinders contain a porous mass saturated with acetone, in which the acetylene is dissolved. The gas is not stored as a free compressed gas.
3. Store and use cylinders upright. A cylinder that has been transported on its side must stand upright for at least 30 minutes (many employers require an hour) before use, or liquid acetone will be drawn into the regulator and torch.
4. Withdrawal rate is limited to about one-seventh of the cylinder contents per hour. Drawing faster pulls acetone out with the gas. Large jobs need a manifold of several cylinders.
5. Never use copper or copper alloys above 67% copper in acetylene service; acetylene forms explosive copper acetylide.
6. Open the acetylene cylinder valve only about three-quarters to one full turn, and leave the key on the valve so it can be shut instantly.

Oxygen: Equally Non-Negotiable

• Never allow oil, grease or any hydrocarbon near oxygen equipment. Oxygen under pressure causes hydrocarbons to ignite spontaneously. Do not handle oxygen fittings with oily gloves.
• Never use oxygen to blow off clothing, cool a work area, or pressurize anything. Oxygen-enriched clothing ignites explosively.
• Open an oxygen cylinder valve fully (it is a double-seated valve that seals at the back).
• Fitting threads are keyed: fuel gas connections are left-hand thread with a notched (grooved) nut; oxygen is right-hand thread. They cannot be cross-connected if the equipment is intact.
• Regulators must be backed off (adjusting screw released) before opening the cylinder valve, and the cylinder valve must be cracked open slowly while standing to the side of the regulator.

Backfire, Flashback and Arrestors

Flashback arrestors and reverse-flow check valves are required on both the torch inlets and the regulator outlets on modern equipment. A check valve alone stops reverse gas flow but does not stop a flame front; only a flashback arrestor quenches the flame.

Cutting Technique and Quality

A good cut shows near-vertical drag lines, a square top edge, a narrow uniform kerf, and dross that flakes off easily. Piercing a plate is done by holding the tip slightly high and angled, opening the oxygen gradually while raising the torch so blowback does not plug the tip, then rotating to the cut line.

Remember the heat-affected zone: a flame-cut edge is locally hardened and may crack. Cut edges on structural or load-bearing parts should be ground back to sound metal before welding.

Plasma Arc Cutting

Plasma cutting forces a gas (usually compressed air, sometimes nitrogen, argon-hydrogen or oxygen) through a constricting nozzle and ionizes it with an electric arc, producing a jet at roughly 20,000 C that melts the metal and blows it away.

The critical advantage: plasma cuts any electrically conductive metal — stainless steel, aluminum, copper, brass, cast iron, and carbon steel. It is faster than oxy-fuel on thin material, produces a much narrower heat-affected zone with far less distortion, and requires no preheating time.

Plasma requirements a millwright must manage: clean, dry compressed air (moisture or compressor oil destroys consumables and produces a poor cut), correct air pressure and flow, a good work clamp connection to clean metal, and correct standoff or a drag shield. Cutting with worn consumables is the most common cause of a poor plasma cut.

Plasma electrical hazard: the open-circuit voltage of a plasma cutter is high enough to be lethal. Never change consumables without disconnecting power, and never cut in wet conditions.

Carbon Arc Gouging (CAC-A)

Air carbon arc gouging strikes an arc with a carbon electrode and simultaneously blasts compressed air along the electrode to blow the molten metal away. Millwrights use it to remove old welds, back-gouge a joint, and cut cast iron and stainless. It is extremely loud and produces heavy fume and flying molten metal, so it demands full leathers, a face shield over the welding helmet, hearing protection and dedicated extraction. Leave carbon residue on the gouged surface and it will contaminate the repair weld — the groove must be ground clean before welding.

Hot Work Safety