Technical Laser Cutting Parameters: How Each Setting Changes the Cut

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  Technical laser cutting parameters: beam quality and focus position
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  How power, speed and duty cycle stay in balance
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  Assist gas type and pressure change the edge chemistry
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  Kerf width, taper and heat-affected zone
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  Piercing technique decides whether the hole starts clean
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  Parameter starting points by material and thickness

Every cut starts with the focused spot. Beam quality, usually quoted as M², tells you how tightly the raw beam can be concentrated. A fiber source with M² near 1.1 gives a spot a few tenths of a millimeter wide. A CO2 source behaves differently at the same power because the 10.6 μm wavelength couples into steel and reflects off copper. Spot size is not a setting you dial in directly. It falls out of the source, the collimation length and the focal length of the lens you bolted into the head.

Focal length is the choice you actually make. A 5 in lens gives a small spot and a shallow depth of focus, which suits 1–3 mm sheet at high speed. A 7.5 in lens spreads the same power over a wider spot and reaches deeper into 8–12 mm plate. On thick material the kerf widens and the cut face roughens, but the beam keeps enough energy density at the bottom to avoid a hanging dross line.

Focus position is where the waist sits relative to the top surface. Zero focus puts the smallest spot right at the surface. Positive focus lifts the waist above the sheet, which widens the kerf at the top and helps oxygen cutting on mild steel. Negative focus pushes the waist into the material, which is the usual setup for nitrogen cutting stainless above 4 mm. A shift of 0.5 mm changes dross behavior noticeably, so treat focus as a fine control, not a rough one.

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  Small spot, thin sheet5 in lens, high speed, tight kerf
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  Wide spot, thick plate7.5 in lens, deeper penetration
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  Focus shift0.5 mm moves dross from bottom to top

Power and cutting speed are locked together. For a given material and thickness there is a band of speed where the cut is clean. Too slow and the beam dwells, the kerf widens, and the edges show a coarse striation pattern with heavy oxidation. Too fast and the beam does not fully penetrate, so you get a partial cut and a shower of sparks off the top surface. The window is often only 10–15 % wide.

Average power is not the whole story on a modulated source. Peak power and pulse frequency decide how much energy lands per unit length. When cutting 1 mm stainless with nitrogen, a higher peak power at lower frequency gives a cleaner edge than the same average power delivered continuously. The parameter list on the controller usually exposes frequency, duty cycle and peak power separately for this reason.

Duty cycle matters on reflective and heat-sensitive alloys. Cutting copper or brass with a fiber source means a large share of the beam bounces back, so the head runs hot and the nozzle erodes faster. Reducing duty cycle and raising assist pressure keeps the process stable. The same logic applies to thin titanium, where excess heat input warps the part and leaves a colored oxide that has to be removed later.

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  Cut too slowWide kerf, heavy oxidation, coarse striations
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  Cut too fastPartial penetration, sparks off the top
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  Reflective alloysLower duty cycle, higher assist pressure

Oxygen and nitrogen do different jobs. Oxygen reacts with the melt and adds chemical energy to the cut, which is why 6 mm mild steel cuts quickly with O2 at 0.5–1.5 bar. The trade-off is an oxidized edge and a heat-affected zone a few tenths of a millimeter deep. Nitrogen is inert. It blows the melt out without adding heat, so the edge stays bright and oxide-free, but the required pressure climbs to 12–20 bar on stainless and the cutting speed drops.

Nozzle diameter sets the gas flow pattern. A 1.5 mm nozzle concentrates pressure at the kerf and suits thin sheet. A 3 mm nozzle spreads the flow and helps thick plate, where the kerf is wider and the melt has farther to travel. Stand-off distance matters just as much: 0.5–1 mm keeps the jet coherent, while 3 mm or more lets the flow disperse and the cut quality falls off.

Air and compressed air are sometimes used on thin mild steel to cut cost. The edge is oxidized and not weldable without cleaning, so this is a choice for brackets and covers, not for parts that go straight into a weld fixture. If the drawing calls out a weldable edge, nitrogen or oxygen with a post-process pickling step is the safer route.

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  OxygenFast on mild steel, oxidized edge
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  NitrogenBright edge, 12–20 bar on stainless
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  AirCheap, but not weld-ready

Kerf is the material the beam removes. On 1 mm stainless with a 5 in lens it runs about 0.15 mm; on 10 mm mild steel it can reach 0.6 mm. Kerf width sets the minimum inside radius you can cut and the amount of material you lose on a nest. A tight kerf means more parts per sheet, but it also means less room for the melt to escape, so assist pressure has to rise to compensate.

Taper is the difference between kerf at the top and kerf at the bottom. It comes from focus position, gas flow and the divergence of the beam through the plate. On a good setup, taper stays under 0.05 mm over the full thickness. If the top is wider than the bottom, the focus is usually too high. If the bottom is wider, the focus is too deep or the gas pressure is too low.

The heat-affected zone is the band next to the cut where the microstructure changed. On mild steel it is typically 0.1–0.3 mm and usually harmless. On 17-4PH or 4130 it can harden the edge enough to matter for later machining. If the part needs a tight tolerance after cutting, leave 0.3–0.5 mm of stock and finish it on a mill rather than fighting the HAZ at the laser.

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  Kerf 1 mm stainlessAbout 0.15 mm
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  Kerf 10 mm mild steelUp to 0.6 mm
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  Taper targetUnder 0.05 mm over thickness
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  HAZ on mild steel0.1–0.3 mm, usually harmless

Most cuts do not start at the sheet edge. They start with a pierce, and the pierce is often what damages the part. A continuous blast pierce burns a pit through the material and throws spatter onto the top surface. On 3 mm mild steel that spatter lands within a few millimeters and can end up on a visible face. On thicker plate the pit itself is deep enough to leave a crater at the lead-in.

Pulsed piercing breaks the same hole into many small pulses. Each pulse removes a thin layer of melt, so the hole forms gradually with far less spatter. It takes longer, often 1–3 seconds per pierce on 6 mm stainless, but the surface stays clean and the nozzle lasts longer because the reflected energy is lower. For parts with a cosmetic face, pulsed piercing is the default.

Where the pierce sits also matters. Put it in scrap, not on the part outline, whenever the nest allows. On a small bracket with no scrap nearby, pierce on the part and then run a lead-in arc that carries the pierce crater off the finished edge. A lead-in radius of 1–2 mm is usually enough to keep the crater out of the tolerance zone.

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  Blast pierceFast, heavy spatter, crater risk
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  Pulsed pierceSlower, clean surface, longer nozzle life
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  Pierce placementIn scrap, or use a 1–2 mm lead-in arc