Plate Cutting: Plasma, Oxy-Fuel, and Mechanical Methods

Plate Cutting: Plasma, Oxy-Fuel, and Mechanical Methods

Cutting hull plates accurately is one of the most critical steps in ship construction. A plate cut even a little outside tolerance forces costly rework during fit-up and can compromise weld quality. This lesson covers the two primary thermal cutting methods used in marine fabrication—plasma arc cutting and oxy-fuel cutting—along with the mechanical finishing steps that bring every edge to drawing specification.

How Plasma Arc Cutting Works

Plasma arc cutting (PAC) forces a high-velocity jet of ionized gas (plasma) through a constricted nozzle. The arc between the torch electrode and the workpiece reaches temperatures high enough to melt any electrically conductive metal instantly, while the gas stream blows the molten material away. Because it relies on conductivity rather than oxidation, plasma cuts both ferrous metals (steel) and non-ferrous metals (aluminum), making it the standard thermal cutting tool in modern shipyards.

Key setup variables:

• Consumables – electrode and nozzle/shield must match the amperage and material. Using worn consumables produces a ragged, beveled edge and excessive dross.
• Amperage – set per the machine's material-thickness chart. Too low leaves dross; too high can widen the kerf beyond tolerance.
• Standoff distance – the gap between the nozzle tip and the plate surface, typically 1/16" to 3/16" for hand cutting. Dragging the tip damages the nozzle and worsens cut quality.
• Travel speed – move too fast and you get incomplete penetration; too slow and you get excessive dross on the bottom edge and possible warping of thin plate.

For CNC plasma tables the machine controller manages travel speed and standoff automatically, but the operator must still load the correct cut program, verify consumable condition, and confirm material thickness before starting.

Plasma Cutting Aluminum

Aluminum requires clean, oxide-free plate and dry cutting gas. Aluminum oxide (the surface film) melts at a much higher temperature than the aluminum beneath it, so the arc must break through the oxide layer immediately. If the plate has heavy oxidation or contamination, pre-clean with a stainless-steel wire brush dedicated to aluminum only. Never use the same brush on steel and aluminum—steel particles embedded in aluminum cause corrosion and weld defects.

How Oxy-Fuel Cutting Works

Oxy-fuel cutting (OFC) is a two-stage process used on steel only. First, a preheat flame (typically oxy-acetylene) raises a small zone of the steel to its kindling temperature (roughly red-orange heat). Then a high-pressure stream of pure oxygen is introduced through the cutting oxygen valve; the steel oxidizes (burns) rapidly, and the iron oxide is blown away. Because this process depends on the metal oxidizing, it does not work on aluminum, stainless steel, or other metals whose oxides are too stable or whose melting point is below the oxidation temperature.

Tip selection matters. Cutting tips are sized for specific thickness ranges. Using the wrong tip produces a rough, incomplete cut or excessive slag. Always consult the tip manufacturer's chart.

Preheat properly. The preheat flame must bring the surface to full kindling temperature before the cutting oxygen lever is opened. Opening oxygen too early produces a sputtering, incomplete cut.

Comparing Cut Quality

Plasma cuts on steel typically produce a narrower kerf and less heat-affected zone (HAZ) than oxy-fuel. Oxy-fuel cuts on thicker steel (above about 1") can be more economical on operating cost, but edge quality and squareness depend heavily on operator skill and tip condition. For marine-grade weld preparation, both methods produce edges that may require light grinding to meet the bevel angle and surface quality called out on the drawing.

Edge Preparation After Cutting

All thermally cut edges must be inspected and dressed before fit-up:

1. Remove dross – dross (re-solidified oxide) on the bottom edge must be chipped or ground off. Dross left on a weld joint causes porosity and lack of fusion.

2. Grind to bevel angle – use an angle grinder with a grinding disc or flap disc. Check the bevel angle frequently with a bevel protractor or weld gauge.

3. Deburr mechanical cuts – sawn or sheared edges have burrs that must be filed or ground flush.

4. Clean the edge – wipe away grinding dust and any oil or moisture before fit-up.

Dimensional Tolerance

The cutting list specifies the finished dimensions of each plate. The acceptance criterion for this course—and a common shipyard standard—is within 1/8 inch (3 mm) on all edges. Measure each finished plate with a steel tape and record results on the cut-plate inspection form before presenting it to the instructor.

Safety First

PPE required for all thermal cutting: shade-appropriate face shield or welding helmet (plasma and oxy-fuel produce intense UV/IR radiation), safety glasses underneath, leather gloves, leather apron, and leather boots. Ear protection is required whenever angle grinders are in use.

Fire watch: Remove or wet down combustibles within the hot-work area. A fire extinguisher must be within arm's reach throughout cutting operations. Welding curtains protect adjacent workers from arc flash and spatter.

Fume control: Plasma cutting generates metal fume and ozone. Work with local exhaust ventilation or wear a supplied-air or PAPR respirator rated for metal fume. Galvanized or coated plate releases toxic fumes—never cut coated material without specific training and ventilation controls.

Cylinder safety (oxy-fuel): Secure oxygen and fuel cylinders upright in an approved cart or chain them to a wall. Never store oxygen and fuel cylinders together without the required separation. Always crack the cylinder valve briefly before attaching the regulator to blow out dust. Open the oxygen cylinder valve fully; open the fuel valve only one to one-and-a-half turns.

Lockout/tagout: Before changing plasma consumables or adjusting any machine guard, shut down the machine and follow the facility's lockout/tagout procedure. The plasma arc can restart unexpectedly if the trigger is bumped.