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Fabrication

How to Vectorize for Laser Cutting and CNC

7 min read

A laser follows paths, and it follows every path it is given. A doubled line is cut twice — which on acrylic means a wider kerf and a scorched edge, and on plywood means the piece falls through mid-job. A path with a gap is a piece that never separates.

So laser preparation is almost entirely about path hygiene, and vectorization is the step where the hygiene is either established or lost.

Decide what is a cut and what is an engrave before converting

These are different operations with different requirements. A cut needs a single closed contour and nothing else. An engrave can be a filled region or a set of strokes, and a bit of extra node density there costs nothing but time.

Separate them in the source image if you can — one file for the cut outline, one for the engraved detail. Converting a combined image and then trying to pick the cut contours out of a hundred engrave paths afterwards is much slower than converting twice.

Trace filled shapes, not drawn outlines

This is the single most common mistake and the cause of nearly all doubled-line problems. If your source is a black outline drawing on white, the tracer sees a black ring and traces its boundary — which is two contours, one on each side of the stroke. The laser then cuts both, leaving a thin scrap of material where your line was.

Fill the shape solid in the source, so there is one region and therefore one boundary. If the artwork is genuinely a line drawing that has to stay a line drawing, convert it and then delete every inner contour by hand, or use the outline-to-centreline tools in your CAM software.

Straighten and de-skew before you trace, not after

A drawing scanned a degree and a half off vertical traces a degree and a half off vertical, and the error is now baked into hundreds of independent path coordinates. Rotating the raster before converting is one operation; rotating the result is one operation plus every straight line in the file being subtly non-orthogonal for ever.

The same applies to perspective. A photograph of a drawing taken at an angle has trapezoidal distortion the tracer will reproduce exactly. Scan it flat, or correct the perspective in any image editor first.

SVG into your CAM software, then set units deliberately

LightBurn, LaserGRBL, Lightburn's Ruida drivers and most CNC toolchains import SVG. Fusion 360 and other CAD packages generally want DXF, which Inkscape exports from an SVG in one step — that conversion is lossless for paths and is a well-trodden path.

Whatever route you take, set the physical dimensions explicitly once the file lands. An SVG's own size is expressed in a unit system that different tools interpret differently, and a part that arrives at 96/72 of its intended size is a classic and expensive surprise. Measure one known feature after import, every time.

Step by step

  1. 01

    Separate cut lines from engraved detail

    Prepare them as two images if you can. They are different operations and mixing them makes the file much harder to sort out later.

  2. 02

    Fill the shapes you intend to cut

    Trace a solid region, not a drawn outline. An outline traces as two contours and the laser will cut both.

  3. 03

    Straighten the scan

    Correct rotation and perspective in the raster before converting. Afterwards the error is spread across every coordinate in the file.

  4. 04

    Convert to SVG

    Upload and download. For CAD toolchains that want DXF, open the SVG in Inkscape and export one.

  5. 05

    Set the real dimensions after import

    Measure a known feature in your CAM software and scale to match. SVG units are interpreted differently by different tools.

Common problems

Every line was cut twice
You traced an outline drawing rather than a filled shape, so each stroke produced an outer and an inner contour. Fill the shapes in the source, or delete the inner contours before cutting.
A piece did not separate from the sheet
The contour has a gap. Zoom to the join and look — sub-millimetre gaps are invisible at normal zoom. Close the path in Inkscape, or use its 'union' operation on the shape.
The part came out the wrong size
A unit mismatch on import, almost always. Measure one known feature after importing and scale the whole file to match; do not trust the SVG's declared size.
Straight lines are slightly curved
The scan was skewed, or the source was a photograph with lens distortion. Correct the raster and convert again — this cannot be fixed properly afterwards.

Frequently asked questions

What format does a laser cutter need?
Most laser software takes SVG directly — LightBurn, LaserGRBL and the Ruida toolchain all do. CAD-based CNC workflows usually prefer DXF, which Inkscape exports from an SVG in a single step without losing path data.
Why does my laser cut every line twice?
Because the file contains two contours where you see one line. That happens when you trace a drawn outline: the tracer follows the boundary of the black stroke, which has an outside and an inside. Trace a filled shape instead.
Can I laser-engrave a photograph from a vector file?
No, and you should not try. Photo engraving is a raster operation — the laser modulates power per pixel — and your software takes the bitmap directly. Vectorizing a photograph first throws away exactly the tonal information the engraver needs.
How small can a cut feature be?
That is set by your kerf, not by the file. A typical CO2 laser has a kerf around 0.2 mm, so interior features below about half a millimetre close up. Check the traced file at its real physical size before cutting.
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