Uneven laser engraving: causes and solutions

Highlighted terms are explained briefly when you hover over or tap them.

One side of the engraving is deep and dark, the other pale. Or the area is even, but the edges are noticeably darker than the middle. Uneven engravings usually have a mechanical or optical cause, and less often it is down to the settings themselves. Work through the points below in order.

Which pattern do you see?

Focus and uneven material

The most common reason for alternating light and dark is the focus. The laser spot is only small and sharp within a narrow range. If the material sits higher or lower at one point, the spot becomes larger there, the energy spreads out and the engraving becomes lighter.

Typical triggers:

• Warped wood: Plywood and solid wood often bow by 1 to 2 mm. With a diode laser that has a short focus range, that is enough for visible differences.
• Honeycomb bed with dents: A bent honeycomb bed lifts the material at individual points.
• Items that do not lie flat: cutting boards, coasters with a chamfer, curved lids.

What helps:

• Press the material flat with weights, magnets or hold-downs. Place hold-downs so that the laser head does not hit them.
• Check the focus in several places, not just in the middle.
• With heavily warped material, take a different piece. Constantly working against the material costs more time than it saves.

How to set the focus correctly is explained under Laser focus.

Bed or frame not level

If the material lies flat but the engraving is still lighter on one side, the bed or frame may be crooked. With open diode lasers the frame often stands directly on the table. If the table is uneven, the frame warps.

Here is how to check: set the focus in one corner and then move the head to the other corners without the laser on. Measure the distance each time with your focus gauge. If the corners differ from one another, the bed is not parallel to the travel plane. Then place the frame on a level board or shim the bed until all corners match.

Belts and wheels

Loose belts and wheels with play make the laser head swing on after a change of direction. This shows up as waves, wobble or offset lines.

• Belt tension: The belt should be taut, but not like a guitar string. For some machines the manufacturer specifies a method; stick to it.
• Wheels (eccentric nuts): If the laser head or a carriage wobbles by hand, adjust the eccentric nut slightly until there is no play left and the wheel still turns easily.
• Acceleration: According to the LightBurn documentation, line wobble often comes from acceleration that is too high. Their solution: lower the Y-axis acceleration in the machine settings.

Always check the mechanics before you move on to software corrections. The LightBurn documentation also advises ruling out loose belts and other mechanical problems first.

Dark edges: overscan

When engraving areas, the laser travels back and forth line by line. At the end of each line it has to brake and accelerate again. During that time it moves more slowly, and if it keeps lasing then, the edge receives more energy than the middle. The result: dark stripes on the left and right.

Overscanning extends each line beyond the engraving. The laser accelerates outside the area, only fires at full speed and brakes again outside.

• On GRBL machines you switch on overscanning in LightBurn per layer in the Cut Settings Editor and set the distance. It is calculated as a percentage of the speed.
• DSP controllers (for example Ruida on CO₂ lasers) do this automatically, according to LightBurn.
• Galvo lasers do not use overscanning; there, delay values even out the edges.

Overscan needs space. If the engraving is close to the edge of the work area, the head can hit the stop.

Offset edges: scanning offset

If the laser engraves bidirectionally, that is, in both directions, the forward and return lines do not line up exactly because of the delay when the laser switches on and belt stretch. The edges then look blurred, doubled or frayed.

LightBurn offers Scanning Offset Adjustment in the device settings for this. According to the documentation, you proceed like this:

1. Create a 50 x 10 mm rectangle as a fill, line interval 0.5 mm, with a power that marks without charring. On GRBL machines, overscanning at least 5 %.
2. Engrave the rectangle at several speeds, at least two, preferably more.
3. At each speed, measure the offset between the line ends.
4. Enter half of the measured value as the Line Shift for the respective speed and activate the correction.

For speeds in between, LightBurn calculates the values itself. As a quick alternative you can engrave unidirectionally. That costs time but removes the offset.

Dirty lens

Smoke and resin settle on the lens. Then less power arrives, and the engraving gets weaker over weeks without you having changed anything. A coating can also create spots in the beam. Clean the lens regularly according to the manufacturer’s instructions, on open diode lasers usually with a lint-free cloth and isopropyl alcohol. Air assist keeps the lens clean for longer, see Air assist.

Wood grain and resin

Wood is not homogeneous. The dark annual rings are denser and often richer in resin than the light early wood. They react differently to the laser. The result is a blotchy or striped engraving that follows the grain exactly. With plywood there are also glue pockets and filled spots.

This can only be adjusted to a limited extent. It becomes more even with wood that has a fine, uniform structure, for example lime, maple or birch. More on this under Laser engraving wood.

Power too close to the minimum

Lasers do not work equally stably at every power. CO₂ tubes fire unreliably below a certain threshold. In the LightBurn forum, users report values around 7 to 12 %, depending on the tube and power supply. Diode modules also start irregularly at very low values.

If your engraving becomes blotchy at very low power, increase the power and move faster to compensate. You can find the stable range with a material test.

Quick checklist

1. Material lies flat, focus checked in several places?
2. Bed or frame the same distance from the head in all corners?
3. Belts tensioned, wheels without play?
4. Overscanning active for dark edges?
5. Scanning offset set or unidirectional tested?
6. Lens and nozzle clean?
7. Wood with an even grain chosen?
8. Power well above the minimum?

Continue with: Laser focus, Material test and Laser parameters.