What Is a Hairline Stroke in a Laser File?
A hairline stroke is the thinnest line a vector program can draw, in practice about 0.01 to 0.1 mm wide, used to mark a path that the laser should follow. The laser cuts along the centre of the path, so line width does not change the cut, but a hairline keeps the preview honest and avoids accidental fills.
You open a file and the outline looks fat and fuzzy, like it was drawn with a marker. Will the laser burn a fat line or a thin one? The answer is almost always "a thin one", and the reason is a small but important idea called the hairline stroke.
A hairline stroke is the thinnest line a vector program draws: a path with a stroke width so small it shows as a single thin line at any zoom. In laser files it marks the path the beam should follow, so it is the standard way to say "cut here".
Why lasers use hairlines
The laser does not burn the width you see on screen. Software such as LightBurn and xTool Creative Space follows the centreline of a stroked path and sends the beam along it. The real width of the cut comes from the beam and is called kerf, typically 0.1 to 0.25 mm in 3 mm plywood. A 2 mm stroke in your drawing does not make a 2 mm cut.
So why a hairline at all? Because it removes ambiguity:
- You can see exactly where the cut will go.
- A thick stroke can be mistaken for a filled shape and engraved.
- Thick overlapping lines hide duplicates.
- Some programs, Glowforge for one, treat strokes and fills differently, so a clean stroke-only path means a clean cut.
Typical hairline values
| Software | Common hairline | Note |
|---|---|---|
| Inkscape | 0.01 to 0.1 mm (about 0.04 to 0.4 px) | Turn off Scale stroke width when resizing |
| Illustrator | 0.001 to 0.25 pt | Use a tiny explicit value |
| CorelDRAW | Hairline preset | Thinnest line available |
| LightBurn | Not required | Follows the path and ignores stroke width |
| Our files | Red hairline for CUT | Black fills for ENGRAVE |
Avoid a stroke width of exactly 0 in SVG. Some programs treat it as "no stroke" and drop the line, so your outline vanishes. A value such as 0.01 mm is safe.
Stroke or fill: what the colour says
The hairline works together with colour, because laser programs sort shapes into layers by colour.
| Look | Meaning to the software | Result |
|---|---|---|
| Red hairline stroke, no fill | Line layer | Cut or score along the path |
| Black solid fill, no stroke | Fill layer | Engrave the area |
| Stroke and fill together | Two layers at once | Both happen, often unintentionally |
| Thick stroke, no fill | Sometimes a line, sometimes a shape | Unreliable, convert to a hairline |
Worked example
Take our reindeer ornament. Its outline is a red hairline around a 90 × 124 mm shape. The cut length is 0.47 m, so a 40 W CO2 spends about 44 seconds on it. Now imagine the outline was changed to a 3 mm stroke with a black fill. The software may see a filled band and engrave a thick border, and the cut is skipped. Set the stroke back to a hairline, remove the fill, and the cut works as before.
Pro tip: zoom in until 1 mm fills a hundred screen pixels. If the line still looks several pixels thick, it is not a hairline.
How to set a hairline stroke
- Select the cut paths.
- Set the fill to none.
- Open Fill and Stroke (Shift+Ctrl+F in Inkscape) and set the stroke width to 0.01 to 0.1 mm.
- Pick the stroke colour your laser layer expects, such as red.
- Convert text and shapes with Object to Path first.
Common mistake: assuming a thicker stroke gives a wider cut. It does not. If you need a wider slot, draw it as a shape at the real width you want, or offset the path.
Pitfalls
- Scaling: if "scale stroke width" is on, a hairline can balloon when you enlarge the design. See how to scale laser files.
- Export: some programs round tiny widths to 0 when exporting. Open the exported file and check.
- Mixed colours: two slightly different reds become two layers in many programs. Use one exact colour value for all cut lines, as described in cut, engrave and score layers.
For the whole file checklist, see how to prepare an SVG for laser cutting.
Questions and answers
What width is a hairline stroke?
In practice about 0.01 to 0.1 mm. Avoid exactly 0, which some programs read as no stroke at all.
Does a thicker stroke make a wider laser cut?
No. The laser follows the centre of the path. The cut width is the kerf, typically 0.1 to 0.25 mm in 3 mm plywood.
Does LightBurn care about stroke width?
No. LightBurn follows the path and ignores stroke width, but a hairline keeps the artwork unambiguous and works in every program.
Why did my outline get engraved instead of cut?
Probably the shape has a fill, or the stroke is so thick that the software reads it as a shape. Set the fill to none and the stroke to a hairline.
What colour should a hairline be?
Any colour that you map to a cut layer. Red is the common choice and is what our CUT paths use.