Laser cutting, kerf, and the material you must never use

Two operations, one machine

A laser cutter does two different things and your file must say which is which.

• Vector cut: the head follows a path at speed, the beam cutting through. Driven by paths.
• Raster engrave: the head sweeps back and forth like an inkjet, pulsing the beam to burn a surface image. Driven by a bitmap, generated from your fill.

Most machines distinguish them by colour convention: a hairline stroke of one colour means cut, another means score, and black fill means engrave. The convention varies by machine and driver, so ask the operator rather than trusting any rule you read, including this one. What is universal is that the distinction is made by colour and stroke, so keep them on separate, clearly named layers.

Kerf, and why the parts do not fit

The beam removes material as it travels. That removed width is the kerf, typically 0.1 to 0.3 mm depending on material, thickness and focus.

The consequence:

• A cut-out hole comes out larger than drawn, by one kerf.
• A cut-out part comes out smaller than drawn, by one kerf.
• So two parts drawn to slot together exactly will be loose by two kerfs: about 0.4 mm, which is a wobbly joint.

Measure it rather than guessing. The standard test: cut a 20 mm square from a scrap of the actual material, measure the square and the hole with callipers, and the difference divided by two is your kerf.

hole measured  20.22 mm
part measured  19.78 mm
difference      0.44 mm  →  kerf 0.22 mm

Then offset your paths by half a kerf in the appropriate direction for a press fit, or design the clearance in deliberately.

Closed paths, always

A cut path must be closed. An open path leaves the piece attached by whatever gap remains, and it has to be snapped or cut free by hand, leaving a mark. This is the same discipline as the plotter: weld overlaps, close every loop, remove duplicates.

Duplicate paths matter more here than anywhere. Two identical paths stacked means the laser cuts the same line twice, which on thin material burns a wide, charred channel.

Material safety, which is not optional

Never cut PVC, vinyl, or anything containing chlorine. Lasering PVC releases hydrogen chloride gas, which corrodes the machine’s optics and rails and is seriously harmful to breathe. Sign vinyl is PVC. This is the single most important fact in this lesson, and reputable shops refuse the material outright.

Also refuse or check carefully:

• Polycarbonate: burns rather than cutting, yellows the edge, smokes badly.
• ABS: melts and produces cyanide compounds.
• Fibreglass and carbon fibre: resin fumes and abrasive dust.
• Anything you cannot identify. If the client cannot say what a sheet is made of, do not put it in a laser.

Safe and common: plywood and MDF (both give a charred edge, which is a look), acrylic / PMMA (cuts beautifully with a polished edge; cast acrylic engraves frosted white, extruded does not), paper and card, leather (vegetable tanned, not chrome), fabric, cork, anodised aluminium for marking.

Living hinges and the things a laser makes possible

A living hinge is a pattern of interlaced slots cut into rigid sheet that makes it bend. The spacing, slot length and gap determine the bend radius, and it is worth cutting a test strip in the actual material rather than copying numbers.

Similarly, inlay work, stacked layers, engrave-then-cut sequences: all are just ordering operations correctly in the file and on the machine.

The software

• LightBurn is what most small shops run. Paid, inexpensive, one-off licence, and good: layer-based, per-layer power and speed, kerf offset built in, and it drives most common controllers.
• Inkscape plus a CAM extension (Gcodetools, or the several laser plugins) is the free route and works, with more friction.
• The machine’s own driver, often a printer driver where you “print” from any application, using colours for operations.

Send SVG, DXF or PDF at 1:1 in millimetres, with layers named CUT, SCORE and ENGRAVE, and put the material and thickness in the filename.