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Banding, ghosting and double lines in laser engraving
Stripes across a fill, or text with a shadow beside it, look like an expensive machine problem. They are almost always a belt, a loose grub screw or an uncalibrated scanning offset - all free to fix.
Check your job time →1. First, the two-minute test that splits the problem in half
Turn off bidirectional (back-and-forth) fill and engrave the same test again. If the doubling disappears, the machine is mechanically fine and you have a scanning offset problem. If it is still doubled, it is mechanical. This one test saves you from taking the gantry apart for a software setting.
2. Scanning offset: the software fix
In bidirectional engraving the head fires on both the left-to-right and right-to-left passes. Because the head takes a moment to respond, the two directions land slightly apart - so vertical strokes double up. Most controllers have a scanning-offset or reverse-interval adjustment: you engrave a calibration pattern at several speeds and enter the measured offset for each. It is speed-dependent, so a value that fixes 200 mm/s will not fix 800 mm/s.
3. Belt tension: the most common mechanical cause
A slack belt lets the head lag and overshoot, which smears detail and produces uneven bands. Too tight is also wrong - it loads the bearings and can make the motor skip. Aim for a firm guitar-string feel with no visible sag. Check both axes, and check the belt is seated squarely in the pulley rather than riding a flange.
4. Loose pulley grub screws and carriage play
A grub screw backing off a motor shaft is a classic intermittent fault: the pattern shifts a fraction each pass and you get ghosting that comes and goes. Grip each pulley and try to twist it against the shaft - there should be no movement. Then rock the head by hand: any play in the wheels shows up as a shadow on every vertical edge. Adjust the eccentric nuts until the head rolls smoothly with no rock.
5. Horizontal banding across a large fill
Even stripes at a regular pitch usually mean the line interval is fighting the spot size - each pass is not quite overlapping the last. Tighten the interval slightly, or check focus, since an out-of-focus spot is wider than the interval assumes. Irregular banding that changes across the sheet points back at flatness and focus instead.
6. Speed and acceleration
Every machine has a speed above which its frame cannot keep up. If the artefact only appears at high speed, engrave slower and see. That is not a defeat - it is the machine telling you its real limit, and a slightly slower job that comes out clean is faster than three re-runs.
Rule of thumb: Work in this order: bidirectional off (diagnose), then scanning offset, then belts, then pulleys and wheels. Changing several at once means you will never know which one it was.
Open the calculator →FAQ
Why does my laser engrave double lines?
Usually bidirectional scanning offset: the head fires slightly late on each direction, so the two passes land apart. Turn off bidirectional fill to confirm - if the doubling goes away, calibrate the scanning offset for each speed you use.
What causes horizontal lines in laser engraving?
Either the line interval does not match the actual spot size, or focus is off so the spot is wider than expected. Irregular banding across a sheet usually means the material is not flat.
How tight should laser belts be?
Firm, like a guitar string, with no visible sag - but not so tight that it loads the motor bearings. Check both axes and make sure the belt sits squarely in the pulley.
Does ghosting mean I need a better laser?
Rarely. Ghosting is nearly always belt tension, a loose pulley grub screw, play in the carriage wheels, or an uncalibrated scanning offset. All of those are free to fix.