Kerf & Materials

The kerf is the width of material the beam actually removes as it cuts - the burned channel it leaves behind. On a typical CO2 laser it is small, roughly 0.1 to 0.3 mm, and it varies with the material, its thickness, the focus and the settings. Small as it is, it changes the size of everything, because the cut line in your file sits in the middle of the kerf. Half the kerf is taken from one side of the line and half from the other.

That split is exactly what loosens a joint. When you cut a slot, the beam eats into both walls, so the slot comes out wider than drawn - by about one kerf. When you cut a tab, the beam eats in from both edges, so the tab comes out narrower - again by about one kerf. Put them together and a press-fit that should be snug is loose by roughly two half-kerfs, one from each part. Worked example: a 30 mm tab into a 30 mm slot with a 0.2 mm kerf yields a tab near 29.8 mm and a slot near 30.2 mm - about 0.4 mm of slop where you wanted zero.

There are three ways to handle it. Kerf compensation: offset the cutting path by half the kerf - outward on parts you want to keep full size, inward on holes - so the finished dimension lands on your nominal. Most laser software and CAM can do this automatically once you tell it the kerf. Design allowances: draw the fit with the kerf already in mind (a slightly oversize tab, a slightly undersize slot). Test and measure: cut a small fit gauge in the real material and check with calipers before committing. Serious work uses all three.

A CO2 laser is happiest with organic and non-metallic sheet, and a handful of materials are the everyday staples. Plywood is the model-maker's default - birch aircraft ply cuts crisp and strong; but watch for internal glue lines and voids, which flare and leave uneven edges (cheap ply is worse). MDF cuts very cleanly and is cheap and dimensionally stable, but it produces a lot of smoke and fine dust and a scorched brown edge, so extraction matters. Acrylic is a favourite: cast acrylic vaporises to a beautifully flame-polished, glossy edge, while extruded acrylic cuts too but leaves a frostier edge and can be gummier - prefer cast for edge-quality work.

Beyond those, card, paper and mount-board cut fast and precise for models and packaging; leather cuts well but only vegetable-tanned leather is safe - chrome-tanned leather can release chromium compounds, so avoid it. Cork, felt, many fabrics, thin natural rubber, wood veneer and some foams all cut on a CO2 laser too. The common thread is that these are carbon-based materials that vaporise or burn cleanly at 10.6 microns without producing dangerous gases.

A practical note on thickness: a CO2 laser cuts from the top down, so there is a limit past which the beam cannot reach the bottom cleanly. A mid-power machine (say 60-100 W) comfortably cuts up to roughly 6 mm ply or acrylic, and higher-power machines push to 10-12 mm or more, but as thickness grows the edge tapers and chars more and cuts slow down or need multiple passes. Thick material is where a laser hands over to the CNC router of the next module.

This is the safety heart of the lesson, and it is absolute. Never laser-cut PVC or vinyl. When lasered, PVC releases chlorine and hydrochloric acid gas - toxic to your lungs and eyes, and corrosive to the machine itself, silently rusting the rails, optics and electronics from the inside. Vinyl stickers, faux-leather (often PVC-backed), some 'foam boards', pipe offcuts and cheap plastic sheet are common hidden sources. This single rule has kept more fablabs safe than any other.

The no-go list runs wider. Polycarbonate (Lexan) looks like acrylic but burns, yellows and will not cut cleanly - and it is often confused with acrylic, so identify before you cut. ABS melts, gums and can emit hazardous fumes rather than cutting. Fibreglass and carbon-fibre composites release fine glass dust and burning resin. Any material with a metallic or reflective coating can reflect the beam unpredictably. And any plastic you cannot positively identify goes in the same bin as PVC - if you do not know what it is, do not cut it.

Why so strict? Because the failure mode is not a ruined part - it is your health and an expensive machine. The gases from the wrong plastic are invisible and you may not smell danger until harm is done. When in doubt, test a tiny scrap for identification by other means (a burn test done safely outside the machine, a supplier data sheet), and default to a material you know is safe. The green list in the previous section is more than enough for almost any design job.

The cut edge tells you how well the job went. Because the beam converges to a focal point and then diverges, a laser cut is never perfectly square - it has a slight taper (kerf angle), narrower where the beam was focused and wider elsewhere. On thin material this is invisible; on thick material it becomes noticeable, so parts meant to stack or seat squarely need attention, and you sometimes flip focus to the mid-thickness to balance it.

On wood and MDF you get some char - a brown scorched edge - which is normal but controllable. Correct focus, adequate air assist, and a fast-enough cut (dwelling in one spot burns more) all reduce it; applying low-tack masking tape to the surface keeps soot off the face and peels away clean. On cast acrylic the same heat instead flame-polishes the edge to glossy clarity, which is a feature, not a flaw. Paper and card can scorch at the edge if the power is too high for the speed.

Thickness sets the outer limit. As material gets thicker, more of it sits far from the focal point, so cuts slow down, taper and char increase, and eventually the beam simply cannot clear the bottom - the part stays tacked in place. Multiple passes help a little but add char and heat. Batch and grain effects matter too: plywood voids, acrylic thickness tolerance, and moisture in wood all shift the result, which is why the same test-cut habit from the last lesson applies to every new material and every new batch.

Nothing in this lesson replaces the test cut, and it is worth stating as its own rule because it is where beginners lose the most sheet. Material varies - batch to batch, supplier to supplier, even across a humid week. A ply that cut in one pass last month may need two today; an acrylic sheet sold as 3 mm may measure 2.7 mm and cut differently. Trusting a number instead of the material in front of you is how a whole expensive sheet ends up half-cut and scorched.

So before any real job on a new material, cut a small test grid on an offcut of that exact stock: a few squares at stepped power and speed to confirm a clean through-cut, a scored line to check score depth, and - if the job has joints - a fit gauge, a little tab-and-slot pair, measured with calipers so you can dial in the kerf compensation. Keep the winning settings in a growing settings library keyed to material and thickness. That five-minute discipline routinely saves an hour and a sheet, and it is the difference between a maker who trusts their parts and one who hopes.