LaserCanvas
How laser cutting actually works
A lens concentrates the beam into a spot a fraction of a millimetre across. At that spot the material is vaporised or burned away, and moving the spot along a line turns that hole into a cut. Everything that goes wrong is about the spot being in the wrong place (focus), moving at the wrong rate (speed), or carrying the wrong amount of energy (power).
The beam does nothing until it is focused
Out of the source, the beam is several millimetres wide and will not cut anything. A lens brings it to a waist — the narrowest point, typically 0.08 to 0.2 mm across — and only there is the energy concentrated enough to do work. Above and below the waist the spot widens and the same power spreads over more area, so it heats rather than cuts.
This is why focus height is not a preference. A sheet 2 mm out of focus is not "slightly worse"; it may not cut through at all, while the wider spot dumps the same energy into a broader band and scorches it.
Cutting is a lot of small burns in a row
The head does not slice. It parks energy at one point until the material there is gone, moves on a fraction of a millimetre, and does it again. A cut is that repeated thousands of times along a path. Two numbers control it:
- Power — how much energy arrives per unit time. Usually set as a percentage, which maps to an S value in the G-code.
- Speed — how fast the spot travels, in mm/min. Slower means more time at each point, so more energy into it.
What actually matters is the ratio between them: energy per millimetre. Half the speed at the same power is roughly double the energy into the material. That is why 60% at 600 mm/min and 30% at 300 mm/min behave similarly, and why there is never one correct setting — only combinations that deliver the right energy.
Kerf: the cut has width
The beam removes a strip of material, not a mathematical line. That strip is the kerf, typically 0.1 to 0.3 mm depending on machine, lens and material. It matters the moment two parts have to fit together.
| If you cut | You get | Because |
|---|---|---|
| A 20 mm disc, on the line | About 19.8 mm | Half the kerf comes out of the disc on each side |
| A 20 mm hole, on the line | About 20.2 mm | Half the kerf comes out of the opening’s edge, so it measures wider |
| A slot for 3 mm ply | A slightly loose fit | The slot widens by one kerf unless you compensate |
Kerf compensation offsets the toolpath by half the kerf — outward for parts, inward for holes — so the finished piece measures what you drew. It only matters for fitted work; for a decorative shape, 0.2 mm is invisible.
Why the same settings behave differently tomorrow
- Material varies. Two sheets of "3 mm plywood" differ in glue, species and moisture. Glue lines resist the beam; a knot may not cut at all.
- Lenses get dirty. Smoke deposits on the lens absorb energy instead of passing it. A cut that worked last month failing today is usually a cleaning job, not a settings one.
- Diode modules fade over their life, and CO2 tubes lose output as they age.
- Ambient temperature changes CO2 tube output noticeably.
What the machine is actually told
Your design becomes G-code: a list of coordinates with the laser on or off. G0 moves with the beam off, G1 cuts, M4 S500 sets power, M5 turns it off. There is no notion of "a circle" by the time the controller sees it — only many short line segments.
That is worth knowing because it explains a class of problem: if a shape is in the wrong place on the machine, the G-code already had it in the wrong place. The controller does exactly what the file says.
Common questions
Why will my laser not cut through, even at 100% power?
Most often focus. At 100% power an out-of-focus beam still delivers the energy, but spread over a wider spot, so it chars a groove instead of cutting a slot. Check focus first, then whether the lens is dirty, then whether the material is one your wavelength is absorbed by at all — a diode will never cut clear acrylic regardless of power.
Is it better to do one slow pass or several fast ones?
Usually several fast passes for cutting. Each pass clears debris so the next reaches deeper, and less heat accumulates, which means less charring on the edge. One very slow pass tends to burn a wide, dark kerf and can set light to resin-rich material.
How do I measure my kerf?
Cut a square of known size, measure the finished piece with calipers, and subtract. If a nominal 20 mm square measures 19.8 mm, your kerf is about 0.2 mm. Measure on the material you will actually use, because kerf varies with thickness and material.
Does the beam come out perfectly straight?
It narrows to the waist and widens again, so the cut is slightly hourglass-shaped and, in thick material, noticeably tapered. Focusing a third of the way into thick stock rather than at the surface spreads that taper more evenly through the thickness.