Thermal cutting: plasma, oxy-fuel and laser

Most of the metal Kai had cut in his first two years went through the squaring shear, the notcher or a pair of snips. Thermal cutting does what those cannot: it cuts shapes of any outline in flat sheet, cuts openings in finished duct and casings, cuts steel angle, channel and plate for curbs and stands, and cuts stainless and aluminum that are tough on blades. Keelson used three thermal processes: the CNC plasma table in the shop, a hand plasma torch for the shop and field, and an oxy-fuel cutting torch for heavier carbon steel. Laser cutting was done for Keelson by a supplier with a fiber laser, for parts that needed very fine detail. This lesson covers how each works, what it can cut, and how to cut safely and cleanly. SMW-420 covers programming and running the CNC plasma table; here the focus is on the process itself and on hand cutting.

How plasma cutting works

Plasma is gas heated so hot that it becomes electrically conductive. A plasma torch forces a gas, usually compressed air for sheet metal work, through a small nozzle around a tungsten or hafnium electrode. A pilot arc starts inside the torch, and when the torch is brought to the work, the main arc transfers to the metal through the gas jet. The constricted arc reaches extremely high temperatures, melts a narrow line of metal, and the fast gas jet blows the molten metal out of the cut, called the kerf. Because the arc carries current to the work, plasma cuts any electrically conductive metal: carbon steel, galvanized steel, stainless steel, aluminum, copper and brass.

The main parts of a hand plasma system are the power supply, the torch and lead, the work clamp, the compressed air supply with a filter and dryer, and the consumables at the torch tip: the electrode, the swirl ring, the nozzle and the shield cap. Consumables wear with use. A worn nozzle with an enlarged, oval hole gives a wide, beveled, dross-covered cut; a pitted electrode starts poorly. Keelson's rule was to check consumables at the start of every shift and replace the electrode and nozzle as a pair.

Current, thickness and speed

A plasma system is rated by its output current, and the current decides how thick a metal it can cut and how fast. Manufacturers publish three thicknesses for each machine: a recommended thickness it cuts at a good speed with a clean edge, a larger thickness it can cut more slowly, and a severance thickness it can just get through with a rough edge. For sheet metal work the relevant range is thin: the duct, casing and hood gauges are well inside the recommended range of even a small hand unit, and the work is mostly about cutting cleanly and fast enough not to overheat the sheet.

Cutting speed matters on thin metal. Too slow, and the kerf widens, the edge melts and rounds over, dross builds underneath, and the heat warps the panel. Too fast, and the arc lags behind the torch, the cut does not go through, and molten metal sprays back up. The right speed shows as a clean cut with the spark stream leaving the bottom of the sheet angled slightly back from the direction of travel. For thin sheet, a lower current setting and a fine-cut nozzle give a narrower kerf and less heat than full power.

Keelson kept a cut chart taped to each machine, drawn up from the manufacturer's data and its own trials. The script below works from an illustrative version of the chart for the shop's 45 A hand unit, and estimates the time to cut the openings Kai had for the week.

# Hand plasma cut chart (illustrative values for a 45 A air plasma unit)
chart = [(0.040, 25, 200), (0.064, 35, 160), (0.125, 45, 100), (0.25, 45, 40), (0.375, 45, 16)]     # (thickness in in, amps, cut speed in ipm)
def speed(t):
    for th, amps, sp in chart:
        if t <= th:
            return amps, sp
    raise ValueError('beyond the recommended range of this machine')
jobs = [('grease duct cleanout door, 18 ga', 0.048, 55), ('lab exhaust branch opening, 20 ga', 0.036, 43), ('curb base plate holes, 1/4 in', 0.25, 36), ('casing access opening, 18 ga', 0.052, 87)]       # (opening, thickness in in, cut length in in)
total = 0.0
for name, t, length in jobs:
    amps, sp = speed(t)
    minutes = length / sp
    total += minutes
    print(f"{name:<34} {t:>6} in at {amps} A, {sp:>5} ipm: {minutes:5.2f} min of arc time")
print(f"total arc time {total:.1f} min; allow layout, setup and cleanup on top")

grease duct cleanout door, 18 ga    0.048 in at 35 A,   160 ipm:  0.34 min of arc time
lab exhaust branch opening, 20 ga   0.036 in at 25 A,   200 ipm:  0.21 min of arc time
curb base plate holes, 1/4 in        0.25 in at 45 A,    40 ipm:  0.90 min of arc time
casing access opening, 18 ga        0.052 in at 35 A,   160 ipm:  0.54 min of arc time
total arc time 2.0 min; allow layout, setup and cleanup on top

Arc time is a small part of the job. Laying out, clamping a straightedge or template, piercing, cleaning dross and checking the cut take longer. The script's real use is spotting work that is outside a machine's recommended range, where a bigger unit or another process is better.

Plasma technique on sheet

Aggie taught Kai to cut thin sheet with a drag tip or a standoff guide resting on the metal, which keeps the torch-to-work distance constant, and to guide the torch along a clamped straightedge or a template for straight lines and circles. Freehand cuts wander. A few habits make clean cuts.

• Start at an edge where possible. Piercing in the middle of a sheet blows molten metal back up at the nozzle and shortens its life; on thin sheet, tilting the torch slightly at the start and straightening it as the arc pierces helps.
• Keep the torch square to the work unless a bevel is wanted, and move at a steady speed.
• Support the work so the cut-off piece does not drop and pinch the torch at the end of the cut, and so the sparks go down into a safe area, not onto the floor, a hose or someone's boots.
• Clamp the work lead to the piece being cut, close to the cut.
• Mind the dross. Some dross on the underside is normal; heavy dross means the speed, current or consumables are wrong. Low-speed dross is a heavy bead that comes off in a lump; high-speed dross is a thin, tough line welded to the edge.

Cutting galvanized steel with plasma makes zinc fume, and cutting stainless makes hexavalent chromium fume, so the fume controls from lesson 2 apply. The plasma table's down-draft or water bed handles shop cutting; hand cutting needs a fume arm or an extraction table. Plasma is also loud and bright: hearing protection and at least a light filter shade are needed even at low current.

Oxy-fuel cutting

Oxy-fuel cutting works on a different principle from plasma. A preheat flame of fuel gas and oxygen heats carbon steel to its ignition temperature, a bright red. Then a jet of pure oxygen from the center of the tip is turned on, and the hot steel burns in the oxygen, forming iron oxide that the jet blows out of the kerf. The steel is burned, not melted. That only works on metals whose oxide melts at a lower temperature than the metal itself and which burn readily in oxygen: carbon and low-alloy steels. Stainless steel, aluminum, copper and brass form oxides that stop the reaction, so oxy-fuel cannot cut them cleanly.

Oxy-fuel is excellent for carbon steel from a few millimeters or an eighth of an inch up to very thick sections, needs no electricity, and is portable. On thin sheet, though, it is a poor choice: the preheat flame heats a wide area, the sheet warps, the edges melt and stick back together, and the cut is ragged. In a sheet metal shop it is used mainly for cutting steel angle, channel, pipe and plate for curbs, stands and supports, for removing old steel on site, and with a heating tip for bending and straightening.

• Set up the torch for the fuel used. Acetylene gives the hottest flame and fastest preheat; propane and other fuels are cheaper and safer to store but preheat more slowly and need different tips.
• Use flashback arrestors at the regulators and check valves at the torch, and purge each hose before lighting.
• Light the fuel first, then add oxygen to a neutral preheat flame; shut off oxygen first when finishing, then fuel, unless the torch maker's procedure says otherwise.
• Never use oxygen to blow dust off clothing or to ventilate a space. Clothing in oxygen-enriched air burns fiercely.

Laser cutting

A laser cutter focuses a beam of light, from a fiber laser or a carbon dioxide laser, into a tiny spot that melts or vaporizes the metal, while an assist gas blows the kerf clear. Nitrogen as the assist gas gives clean, oxide-free edges on stainless steel and aluminum; oxygen speeds up cutting of carbon steel. Lasers cut thin sheet very fast with a narrow kerf, a very small heat-affected zone and almost no dross, and hold tight tolerances for small holes and fine detail. They are expensive machines, and many sheet metal contractors buy laser-cut parts from a supplier, as Keelson did for the louver blades and perforated panels on the Eastgate project.

Laser beams used for cutting are invisible or nearly so and can blind instantly, which is why production lasers are fully enclosed with interlocked doors. Apprentices do not open or bypass a laser enclosure.

The heat-affected zone and cut edges

Every thermal cut heats the metal next to the kerf. That band, the heat-affected zone, may be harder, more brittle or discolored than the rest of the sheet. On carbon steel cut with plasma or oxy-fuel, the cut edge can be hard enough to crack when bent sharply, so parts that will be folded along a cut edge are better sheared or have the edge ground. On stainless steel, the heat tint along a plasma cut is an oxide layer that reduces corrosion resistance, and on food service or laboratory parts it must be ground or pickled away. On galvanized steel, the zinc is burned off a narrow strip along the cut, which is then protected with a zinc-rich paint where the part will be exposed. Lesson 4 covers edge preparation for welding.

Keeping cut parts accurate

A thermal cut removes a strip of metal as wide as the kerf, so a template or a straightedge must be offset by half the kerf from the line, on the waste side, or every part comes out undersize. The CNC plasma table applies this kerf offset automatically from its settings; on a hand cut, Kai clamped his straightedge so the torch nozzle ran along it with the arc on the waste side of the scribed line. He checked the first piece of any batch against the drawing before cutting the rest, measuring both length and squareness, because a straightedge that slipped by a thirty-second of an inch on one part slips on all of them. Parts are labeled as they come off the table, before the offcuts are cleared, so nothing gets mixed up.