It’s well documented what kerfs are achievable with commercial 1KW+ laser cutters cutting steel, aluminium and plastics. I wanted to know what kerfs were achievable on our 40W Chinese built laser cutter so that I could work out what to add to allow for kerf when designing finger joints to be cut in 3mm ply or when cutting holes to be tapped. The kerf is the width of material removed during the cutting process.
Kerf is determined by material properties and thickness, the focal length of the lens and the gas used while cutting. Our laser cutter uses a lens with a 50mm focal length and uses compressed air to push out the vapourised/molten swarf.
By cutting a rectangle of material and then cutting 9 rectangles within it you get 10 cuts. When these 9 rectangles are pushed together at one end of the “frame”, the resulting gap at the other end is the sum of the kerfs. Dividing this gap by ten gives the average kerf for that material and material thickness.
The image below is a screenshot of the Alibre drawing. The vertical lines extend past the inside border of the frame so that the cut has time to penetrate the material before it crosses into the inside border of the frame. Some laser cutter software will compensate for penetration time, ours was cheap and so doesn’t.
Once cut I was careful to keep the parts together and not get them turned over. This is because the kerf will be slightly wider at the bottom than the top since the laser beam is not parallel, it gets wider the further it travels from the lens. For this reason a longer focal length lens gives a thinner kerf. Getting the parts turned over would result in the kerfs nesting together, to avoid this I drew a line on the top side before I removed it from the laser cutter.
Gaps were measured using a metric feeler gauge which allows you to add together slips of metal in different thicknesses to build up the thickness until it no longer fits in the gap. A PCB that was laying around was used to keep the small rectangles flat as the feeler gauge was used. Anything flat(ish) will do.
These values give me a good idea of what to add when designing parts for the laser cutter. I was curious to know how much the kerf would change with material but there’s so little change in kerf for these different materials that I’d just add 0.2mm to the dimensions if I wanted an interference fit.
I had had concerns when laser cutting holes in plastic which would later be tapped that the kerf would enlarge the hole too much to get a good thread in it. In future I’ll know to reduce the hole size by 0.1mm to compensate.