Laser Power & Speed Test Generator
A power-by-speed grid is a third repeats, because the result depends on P/v — so cut a diagonal sweep instead, and get a real answer.
Your grid, with the frontier marked
The diagonal sweep — cut this instead
A power × speed grid is about a third repeats
What the beam does depends first on the line energy P/v, not on power and speed separately. So a 10 × 10 grid contains 63 distinct line energies, not 100 — 40% power at 20 mm/s is the same 1.2 J/mm as 80% at 40. The count is the Farey one, 2·Σφ(q) − 1, and it settles near 63% of the cells for any grid worth cutting. The cut/no-cut frontier therefore runs diagonally, and both triangles either side of it are foregone conclusions: lots of power slowly cuts through, a little power quickly doesn't.
But the leftover is exactly what you should be testing, which is why the grid isn't simply wasted. At equal line energy, faster is genuinely better. Heat diffuses during the dwell, so the char half-width goes as √(α·d/v) — one over the square root of the speed. Ten times faster at ten times the power gives a heat-affected zone 3.16× narrower. That's the familiar "cut fast and hot rather than slow and cool" as arithmetic rather than lore.
Which makes the right test a diagonal sweep at constant line energy, not a square. Find the ratio that just cuts, then push the speed along it until the tube runs out of power — and where it runs out is the answer. On a 60 W tube at 1.2 J/mm capped at 80% for tube life, that's 40 mm/s. No single grid cell states it, and a square grid spends most of its area confirming things either side of the frontier.
And steps finer than about 20% are measuring the plywood. Nominal 3 mm birch runs 2.6–3.1 mm, 17% of nominal, and the cutting threshold moves with thickness by roughly the same fraction. Two cells closer together than that will swap places between one board and the next, so whichever looked better is as likely to be the material as the setting. MDF varies about 7% and supports a finer step, which is a reason to dial settings in on it.
- Test on the board you'll cut. Thickness, density, glue and moisture all move the threshold, and plywood varies within a single sheet as well as between sheets.
- Watch the power cap as much as the result. Running a CO2 tube near full output shortens its life sharply, so the fastest setting found at 100% isn't the fastest worth using. Diode sources care less about this and more about duty cycle.
- Faster cutting narrows the kerf slightly, which shifts any joint compensation you calculated. Re-measure the kerf after changing speed, not just after changing material.
- Measure the threshold once, in J/mm. It transfers between machines of different wattage in a way that a power percentage never does — a 40% setting means nothing without the tube it was on.
How to use
- Measure the cutting threshold once, in joules per millimetre.
- Cut the diagonal sweep rather than a square grid.
- Test on the actual board you will use, not a similar scrap.
- Cap the power below full output to protect the tube.
Frequently asked questions
Why is a laser power and speed grid mostly redundant?
Because what the beam does depends first on the line energy, which is power divided by speed, rather than on power and speed separately. Forty per cent power at 20 mm a second delivers exactly the same 1.2 joules per millimetre as eighty per cent at 40, and to first order the material cannot tell them apart. A ten by ten grid contains 63 distinct line energies rather than 100, so about a third of the cells are the same test cut twice.
How many distinct settings does a 10x10 test grid really have?
Sixty-three. The number of distinct ratios of two integers up to n is the Farey count, twice the sum of Euler totient over one to n, minus one. It comes to 19 for a five by five, 63 for a ten by ten and 255 for a twenty by twenty — settling near 63 per cent of the cells for any grid worth cutting. The redundancy does not go away as the grid grows.
What is line energy in laser cutting?
Power divided by speed, measured in joules per millimetre — the energy deposited along each millimetre of the cut. It is the single most useful number to know about a material because it transfers between machines in a way a power percentage never does. A forty per cent setting means nothing without knowing the tube it was on, while 1.2 J/mm means the same thing everywhere.
Is it better to cut fast at high power or slow at low power?
Fast at high power, and the reason is calculable rather than folklore. Heat diffuses during the time the beam dwells on a point, so the char half-width goes as the square root of the diffusivity times the dwell — which scales as one over the square root of the speed. Ten times faster at ten times the power gives a heat-affected zone 3.16 times narrower for the same cutting energy.
What should I test instead of a power and speed grid?
A diagonal sweep at constant line energy. Find the ratio that just cuts your material, add a margin, then push the speed along that ratio until the source runs out of power. Where it runs out is the answer — it is the fastest you can cut this material cleanly, and no single grid cell states it directly. The sweep costs a strip of material rather than a square.
Why does the cut and no-cut boundary run diagonally?
Because cutting through depends on line energy exceeding a threshold, and lines of constant power over speed are diagonals on a power against speed plot. Everything above the diagonal cuts and everything below it does not, so both triangles are foregone conclusions: a lot of power slowly cuts through, a little power quickly does not. Only the band near the threshold carries information.
How fine should the steps in a laser test be?
No finer than the material varies, which for nominal 3 mm birch plywood is around 17 per cent — it runs 2.6 to 3.1 mm and the cutting threshold moves with thickness by roughly the same fraction. Two cells closer together than that will swap places between one board and the next, so whichever looked better is as likely to be the plywood as the setting. MDF varies about 7 per cent and supports finer steps.
Should I run my laser tube at full power?
Not routinely. Running a CO2 tube near full output shortens its life sharply, so the fastest setting a test finds at 100 per cent is not the fastest one worth using day to day. Capping around 80 per cent and accepting the slightly lower speed usually costs a few seconds a job and buys a great deal of tube. Diode sources care less about peak power and more about duty cycle and heat.
Does cutting speed change the kerf width?
Slightly, and in the same direction as the char. Faster cutting means less time for heat to spread, so both the heat-affected zone and the kerf narrow a little. That matters if you have calculated joint compensation for a press fit, because the compensation assumed a kerf measured at some particular speed. Re-measure the kerf after changing speed, not only after changing material.
Why do my settings stop working on a new sheet?
Because thickness, density, glue content and moisture all move the cutting threshold, and plywood varies within a single sheet as well as between sheets. A setting dialled in on last month material is a starting point rather than an answer. This is also why a test with steps finer than the material spread produces a different winner each time it is run.
How do I measure the cutting threshold for a material?
Cut a short line at a known power and speed and see whether it goes through, then adjust and repeat — but work in line energy rather than in the two numbers. Once you know that a material needs about 0.9 J/mm, that figure carries over to any machine and any speed, and you can compute the power needed at whatever speed you want to run rather than testing again.
What is a heat affected zone on a laser cut?
The band either side of the kerf where the material was heated enough to change without being removed — charring on wood, a slight haze on acrylic, a scorched halo on card. Its width follows the diffusion length during the beam dwell, so it narrows as the square root of the speed rises. Most complaints about scorched edges are really complaints about cutting too slowly at too little power.
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