How to measure your laser's kerf for press-fit joints
The short answer: cut a square of a known size, measure it with calipers and subtract. The difference is your kerf. Enter it once in the settings of any laser plan and the joints are compensated, so the parts press-fit.
What kerf is
A laser does not cut along an infinitely thin line: the beam burns away a strip of material, usually between 0.1 and 0.25 mm (0.004" to 0.010"). Cut a square and it comes out smaller by that width. A hole comes out larger by the same amount. For joints, that is the difference between a snug fit and a loose one.
How to measure it
- Draw a square of exactly 20 mm (3/4") in your laser software and cut it from the same material you will use for the project, at the power and speed you will use.
- Measure the square with calipers on two opposite sides and take the average. Say it measures 19.85 mm.
- Kerf = 20 − 19.85 = 0.15 mm. Enter that number in Material, laser and sheet in the plan.
If you do not have calipers, cut a 4 × 4 in (100 mm) square and measure it with a ruler: with ten times the length, the error you read is easier to see, and you can divide it by two sides.
Measure the real thickness too
Plywood labeled 1/8" often measures between 2.8 and 3.2 mm. The slots are as wide as the thickness you enter, so measure a sheet with calipers and type the real value. Then run a fit test and raise or lower the clearance by 0.05 mm at a time.
When to measure it again
- When you change the material or the sheet, because density and resin change how much the beam burns.
- When you change the power, the speed or the number of passes.
- After you clean or realign the lens or the mirrors, or when the tube gets old.
Frequently asked questions
What is a normal kerf?
Between 0.1 and 0.25 mm (0.004" to 0.010") on most CO2 lasers. Diode lasers are often a little wider. Measure yours, because it changes with the material and the focus.
Do I enter the kerf in the plan or in the laser software?
In the plan. The joints are compensated in the file, so leave any kerf or offset setting in your laser software at zero.
My parts are still loose or too tight. What now?
Run a fit test and change the clearance in steps of 0.05 mm. A small change is enough: do not retype the kerf unless the material or the laser changed.