laser-cutting-101-kerf-beam-focus

Laser cutting 101: kerf, beam, focus explained

Updated May 2026
TL;DR: A laser cuts by melting or vaporising a narrow strip of material. The beam is a tightly focused cone of light, not a thin line, so the slot it leaves (the kerf) has a real width and the edges have a slight bevel. Tiny changes in focus, tube age, lens cleanliness, or air pressure shift that cut by enough to ruin a tight joint. This article explains what is happening physically so you can read your machine's behaviour like a textbook.

What a laser actually is

A laser is a tightly controlled beam of light at one specific wavelength. CO2 lasers run at 10.6 microns (long-wave infrared, invisible to the eye). Diode lasers are around 450 nm (visible blue light). Fiber lasers sit near 1.06 microns.

Wavelength matters because different materials absorb different wavelengths differently. CO2 light is absorbed beautifully by acrylic and wood, badly by clear glass, and not at all by polished aluminum. Diode light is absorbed by anything dark, and bounces off anything reflective. This is why your machine cuts some materials and refuses to touch others.

The beam is a cone, not a line

The mental model that hurts most beginners: thinking of the laser as an infinitely thin sword that slices in a straight line. It is not.

The beam comes out of the laser tube as a parallel column, hits a focusing lens, narrows to its tightest point (the beam waist), and then diverges again on the other side. If you imagine looking at the beam edge-on, it looks like two cones tip to tip with the narrowest point at the focal plane. The beam waist is where the beam is at its most tightly focused. Above and below the waist the beam is wider.

This is the single most important thing to internalise. When the beam is moving through the thickness of your material, it has a width at the top, narrows toward the waist, then widens again. The width of the slot it leaves (the kerf) is not constant top to bottom.

Think of the laser like a needle that has a fat middle and pointed ends. Where the point lands matters.

Diagram of a focused laser beam passing through a material slab. The beam is shown as a cone narrowing to the beam waist at the focal plane, with the resulting hourglass kerf highlighted inside the material.

Kerf: the width of the slot

Kerf is the width of the slot the laser removes. For a CO2 laser cutting 3 mm acrylic, kerf is usually 0.1 to 0.3 mm depending on lens, power, speed, and air assist. For a 40 W diode cutting 3 mm TruFlat, kerf is closer to 0.15 to 0.4 mm.

Why this matters:

  • Tight joints (puzzle pieces, finger joints, snap-fits) need kerf compensation or they fall apart.
  • A 100 mm part cut on the inside of the line is 100 mm minus one kerf. Cut on the outside it is 100 mm plus one kerf. Cut down the middle it is 100 mm exactly. Most slicing software lets you choose.
  • Two cuts that should meet flush will have a kerf-wide gap unless you nudge one toward the other.

The right way to find your kerf is to cut a kerf test pattern. We have a kerf test generator on mylasertools.com that outputs an SVG you can run on any blank. Measure the slot with a digital caliper and write it down. Kerf is different per machine, per material, and per power and speed combo.

Why edges aren't perfectly straight

Because the beam is a cone, not a needle, the slot it cuts is slightly wider at one end than the other. On thin materials this is invisible. On 6 mm or thicker you can see and feel it.

Three reasons edges go off perpendicular:

  1. Beam profile. If the focal plane is at the top of the material, the kerf at the top is narrow and the kerf at the bottom is wider. If the focal plane is at the middle, kerf is widest at top and bottom and narrowest in the middle (a slight hourglass). If the focal plane is below the material, the kerf is widest at the top.
  2. Beam absorption. The first millimetre of material absorbs more energy than the last. By the time the beam reaches the bottom it has less power to remove material cleanly, so the cut can taper or leave a heavier char on the lower edge.
  3. Air assist direction. Most air-assist nozzles blow straight down. Below the cut, fumes and molten material are pushed sideways, which can flare the cut at the bottom edge.

Together these give every cut a small bevel and a small flare. The trick to a perfect-looking edge is choosing your focus position depending on what matters most to your finished piece.

What focus actually means

Focus is the distance from the lens to the surface of the material. The point where the beam is narrowest is the focal point. On most CO2 machines the focal length is fixed by the lens (a 2 inch lens has a 50.8 mm focal point below the nozzle). You set focus by moving the bed up and down so the surface of the material is the right distance from the nozzle.

There are three common focus strategies. The right one depends on what you are doing.

  1. Focus at the top of the material. Best when the top face is the visible face (signs, jewellery). Top kerf is narrowest, top edge is cleanest. Bottom edge gets a slight flare.
  2. Focus at the middle of the material. Best for cuts where you need both faces to look the same (boxes, snap-fits). The hourglass kerf is symmetric.
  3. Focus at the bottom of the material. Rarely used. Useful if the bottom face is the visible face or for very specific thick-acrylic effects.

If you change material thickness, your focus changes by half the thickness change. Going from 3 mm to 6 mm with mid-focus means moving the bed down by 1.5 mm. This is why automatic focus probes are worth the money on a busy laser.

Three side-by-side diagrams comparing focus positions. Focus at the top of the material produces a kerf that is narrow at the top and widens toward the bottom. Focus at the middle produces a symmetric hourglass kerf with both faces matching. Focus at the bottom produces a kerf that is wide at the top and narrows toward the bottom.

Cut, engrave, score, and mark

These are different operations because they use the laser differently.

  • Cut. Slow speed, high power, the beam removes material all the way through.
  • Engrave. Fast speed, moderate power, raster pattern. The beam removes a shallow surface layer, line by line, like a printer.
  • Score. Vector path like a cut but with low power, just enough to mark the surface without going through. Useful for fold lines, layout marks, and shallow decorative grooves.
  • Mark. Even lower power than score. The beam changes the colour of the surface without removing material. Common on metals (anodised aluminum), some plastics, and coated woods.

Most beginners mix these up. The rule of thumb: if you want it to fall out, cut. If you want a picture, engrave. If you want a line that does not go through, score. If you want a label that changes colour without leaving a groove, mark.

Why "the same settings" don't reproduce

This is the hardest part of laser cutting for beginners. You copy a settings recipe from a forum, run it on identical material, and the cut is worse. Why?

Eight things change between two seemingly identical cuts:

  1. Tube age. CO2 tubes lose power as they age. A new 60 W tube might output 65 W. A six-month-old tube might output 50 W. Diode modules degrade slower but still drop a few percent over time.
  2. Lens cleanliness. A fingerprint or smoke film on the focusing lens absorbs 5 to 30 percent of the beam.
  3. Mirror cleanliness on CO2 machines. Three mirrors, each adding tiny losses if dirty.
  4. Air assist pressure. More air clears fumes faster, gives a cleaner edge, but also cools the cut and reduces effective power. Less air leaves more heat and char.
  5. Focus drift. The bed sags, the material is not flat, the gantry rails are slightly out of square. Even 0.5 mm of focus drift is visible.
  6. Material batch. "3 mm cast acrylic" from one supplier is 2.8 mm. From another it is 3.1 mm. The plasticiser content differs. The masking tape on top changes thickness.
  7. Humidity. Wood and paper soak up moisture. Wet wood needs more passes or more power.
  8. Ambient temperature. Cold tubes need a warm-up pass. Hot tubes lose power.

The settings recipe is a starting point. Run a test cut on scrap before you commit to a real piece. Always.

Keep a notebook (or a spreadsheet) of your settings. Date them. Note the tube age, the lens, the air assist, and the material batch. Patterns will emerge.

Air assist, very briefly

Air assist blows compressed air through the nozzle at the cutting head. It does three things:

  1. Clears smoke and debris out of the cut so the beam keeps reaching the bottom.
  2. Cools the edge so the cut is sharper and less burnt.
  3. Reduces the risk of flame-ups, especially on wood and paper.

More air is generally better, but it has a ceiling. Too much pressure on thin material lifts the part or causes ridges. For most cuts, 15 to 30 psi at the nozzle is right. Engraving prefers less air than cutting because too much air dilutes the heat needed to mark the surface.

Bringing it together

When you watch a beginner's first cut go sideways, it is almost always one of these:

  • The focus is wrong (kerf widens, edges char).
  • The lens or a mirror is dirty (less power reaching the bed).
  • The material is not what they think it is (cast vs extruded, real walnut vs walnut-faced ply, etc.).
  • The settings are from a different machine or different tube age.

When everything is dialed in, a clean cut on cast acrylic looks like the edge was hand-polished. A clean cut on TruFlat looks like a knife went through it. That is your benchmark. If your edge is not at that benchmark, walk through the list above and the answer is in there.

Where to go next

Frequently asked questions

Q: How do I find my exact kerf? A: Cut a known shape and measure the slot. The simplest test is a 50 mm square. Cut it, then measure the cutout. The difference between the design (50 mm) and the measured slot is your kerf. Use a digital caliper. Do this once per material per machine per lens.

Q: Why does my cut leave a darker line on the bottom? A: Almost always it is air assist direction (fumes flaring out the bottom) plus focus being too high. Try lowering the focal point by half the material thickness or increasing air assist pressure.

Q: My cut works fine on day one, then degrades over the week. Why? A: Lens cleanliness. Smoke residue builds up on the underside of the focusing lens. Clean it with isopropyl alcohol and a lens-safe wipe every 10 to 20 hours of cutting.

Q: Can I make a perfect 90 degree edge? A: Yes, but only on thin material (under 3 mm) and only with focus at mid-thickness, sharp lens, and good air assist. On 6 mm or thicker the bevel from beam divergence is visible. If you need a perfectly square edge on thick acrylic, polish or sand the cut edge after.

Q: What is the simplest beginner mistake? A: Skipping a test cut. A 30 second test cut on a scrap of the actual material catches almost every problem before you ruin a real piece.

Q: Diode, CO2 or fiber: which is best for crafting? A: For most acrylic, wood and leatherette work, CO2 wins on speed and edge quality. Diode is cheaper and small-footprint, gets close to CO2 on plywood and leatherette but burns acrylic edges visibly unless the acrylic is coated or painted. Fiber is for metals and dark plastics, not crafting materials.

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