Setting laser parameters: power, speed and line interval
are explained briefly when you hover over or tap them.
Whether an engraving comes out clean or a cut goes through is decided at its core by four values: power, speed, passes and, when engraving, the line interval. With fibre lasers, frequency is added. This article explains what each value does, how they relate and which starting values you can test with.
The four basic values
Power is usually given in the software as a percentage of maximum power. So 50 % on a 20 W diode laser is something different from 50 % on a 40 W machine. This is why percentage values can never be transferred one to one between machines.
Speed is the pace at which the laser head travels over the material. The slower it is, the longer the beam acts on one spot. Watch the unit: many GRBL machines work in mm/min, manufacturer software often in mm/s. 60 mm/s is 3,600 mm/min. A wrong unit is one of the most common reasons for burnt or barely visible results.
Passes repeat the same path. Two passes at medium speed often give cleaner cut edges than one very slow pass, because the material cools briefly in between and chars less.
Line interval applies only to area engravings and images. The laser scans the area line by line, and the line interval determines how far apart the lines are.
Line interval, LPI and DPI
Line interval, LPI (lines per inch) and DPI describe the same thing from different directions. The conversion according to the LightBurn documentation: DPI = 25.4 divided by the line interval in mm. In xTool software you often find lines per centimetre instead.
| Line interval | DPI or LPI | Lines per cm |
|---|---|---|
| 0.2 mm | 127 | 50 |
| 0.1 mm | 254 | 100 |
| 0.08 mm | 318 | 125 |
| 0.05 mm | 508 | 200 |
Smaller is not automatically better. If the interval is too small, the lines overlap, the engraving takes much longer and images become dark and muddy. If it is too large, visible stripes appear and you need more power for the same darkness. The ideal interval is where the lines just touch without overlapping. This depends on the laser spot of your machine. For wood with a diode laser, 0.08 to 0.1 mm is a common start. You find the right value with the interval test in LightBurn or a test grid, see Material test.
Frequency with fibre lasers
Fibre lasers work in pulses. The frequency in kilohertz states how many pulses are emitted per second. At the same power, roughly: low frequency means fewer pulses with more energy per pulse, which removes more material and looks rougher. High frequency means many weaker pulses, which puts more heat into the surface and is better suited to smooth markings and annealing colours on stainless steel.
With Q-switch sources the pulse duration is largely fixed, about 100 ns according to the source manufacturer JPT. MOPA sources additionally allow the pulse duration to be changed (2 to 500 ns at JPT) and cover a much larger frequency range. This makes colour marking on stainless steel possible. Which frequencies your machine allows is stated in the data sheet of the laser source. Diode lasers and most hobby CO₂ lasers have no frequency control in this sense.
How the values relate
What matters is the energy that goes into the material per area or per millimetre of travel. Roughly:
- Double the power at the same speed has a similar effect to half the speed at the same power. It is not entirely linear, because heat flows away and materials react differently.
- A smaller line interval puts more energy into the area. If you reduce it, you often have to reduce power or move faster.
- When cutting, a second pass does not always replace slower travel. With thick materials it often helps to set the focus lower for the second pass, see Laser focus.
- Diode lasers usually engrave at high power and control darkness via speed. According to the LightBurn documentation, CO₂ lasers typically engrave at low power and go slower for darker results.
When adjusting, always change only one value at a time. Otherwise you will not know what brought the improvement.
Starting values by power class
The following table is a rough guide. The values are compiled from manufacturer libraries, retailer tables and community collections, which in part contradict each other considerably. The same wood from a different batch, a dirty lens or a different focus shifts the result greatly. So laser a test grid before every new material. All values apply with air assist and focus on the surface, speed in mm/s.
| Material and task | 10 W diode | 20 W diode | 40 W diode | approx. 50 W CO₂ |
|---|---|---|---|---|
| Engrave plywood (area) | 50 to 80 %, 100 to 200 mm/s | 20 to 50 %, 100 to 300 mm/s | 20 to 50 %, 200 to 400 mm/s | 15 to 35 %, 300 to 500 mm/s |
| Cut 3 mm plywood | 100 %, 3 to 8 mm/s, 1 to 2 passes | 100 %, 5 to 12 mm/s, 1 pass | 100 %, 10 to 25 mm/s, 1 pass | 50 to 80 %, 15 to 30 mm/s, 1 pass |
| Cut 3 mm acrylic | black/opaque only: 100 %, 2 to 5 mm/s, 2 to 4 passes | black/opaque only: 100 %, 3 to 10 mm/s, 1 to 3 passes | black/opaque only: 100 %, 5 to 12 mm/s, 1 to 2 passes | clear too: 70 to 100 %, 15 to 40 mm/s, 1 pass |
| Engrave natural leather | 30 to 50 %, 100 to 200 mm/s | 15 to 35 %, 150 to 300 mm/s | 10 to 25 %, 200 to 400 mm/s | 10 to 25 %, 300 to 500 mm/s |
| Engrave slate | 40 to 70 %, 100 to 200 mm/s | 20 to 40 %, 150 to 300 mm/s | 10 to 30 %, 150 to 300 mm/s | 20 to 40 %, 200 to 400 mm/s |
A few notes on the table:
- Diode lasers with blue light (around 450 nm) do not cut clear or light translucent acrylic, because the light passes through. This is not a question of settings.
- Leather here means vegetable-tanned natural leather. Faux leather often contains PVC and does not belong under the laser. More on this under Prohibited materials.
- With CO₂ tubes, many manufacturers recommend not working at full power permanently, because it ages the tube. Check your machine’s manual.
How to proceed
- Start with the manufacturer values from your machine’s software, if available. They are matched to the machine.
- If a value is missing, take the middle of the range from the table above.
- Laser a test grid with power and speed as axes on an offcut of the same material.
- Choose the best square and note the values with machine, material and date.
Never leave the laser unattended while doing this, and keep a fire extinguisher within reach. Flames arise quickly, especially when testing at slow speeds.
Continue with: Material test, Laser focus and Air assist.