Laser Cutting File Optimization: Paths, Order and Kerf

Machine time in laser cutting is dominated by travel, pierce points, and acceleration, not cutting. A poorly organized file can double job time: the laser head jumps back and forth across the sheet, pierces the material hundreds of times unnecessarily, and cuts outer shapes before inner details, causing parts to shift mid-job. Optimizing path order, reducing node count, and applying kerf offset last can cut the same design in half the time with no quality loss.

Path Order: Cut Inner Details Before Outer Perimeters

The single most important optimization is cut order. Always cut inner details (holes, slots, engraves, inner shapes) before cutting outer perimeters. If you cut the outer edge first, the part is freed from the sheet and can shift during subsequent cuts, ruining alignment. Inner cuts remain anchored by the outer material, so they stay in place.

For nested parts (e.g., a box with a cutout window inside a panel), cut from innermost to outermost: first the window, then the panel interior details, then the panel perimeter. Most laser control software (LightBurn, RDWorks, LaserCut) lets you assign layer order or manually reorder paths in a cut list. Use this to enforce inside-first cutting.

Minimize Travel with Nesting and Layout

Travel time — the laser head moving between cuts with the beam off — is wasted time. Tight nesting (packing parts close together on the sheet) reduces travel. For sparse sheets (parts spread out with large gaps), nesting can save 10-30% of total job time. For dense sheets, the savings are smaller but still measurable.

Manual nesting in design software (Inkscape, Illustrator) is tedious. Automated nesting software (Deepnest, SVGnest, RDWorks' built-in nester) arranges parts to minimize waste and travel. These tools export a nested layout you can import into your laser software. Nesting is especially valuable for production runs where the same parts are cut repeatedly.

Lead-In and Lead-Out Considerations

Lead-ins (short approach paths before the cut starts) and lead-outs (short exit paths after the cut ends) prevent burn marks at the start/end point of a closed path. They also add travel. If every part has a 2mm lead-in and 2mm lead-out, and you have 100 parts, that is 400mm of extra travel. Minimize lead-in/out length when quality allows, or disable them for hidden edges (e.g., internal corners that will be out of sight in the final assembly).

Reduce Node Count to Speed Path Computation

Vector paths with thousands of nodes take longer to compute and send to the laser controller. A path with 500 nodes cut at 20mm/s takes the same wall-clock time to cut as a 50-node path, but the controller spends more time processing the 500-node version, and the motion planning is less smooth. Auto-traced images and CAD exports often have excessive nodes. Simplify them before export.

In Inkscape, select all paths (Ctrl+A), then choose Path → Simplify (Ctrl+L) 2-5 times. In Illustrator, select all and choose Object → Path → Simplify, setting Curve Precision to 95-98%. For decorative elements, node reduction of 50-70% is common with no visible quality loss. For precision mechanical parts, use conservative settings (98-99% precision) to preserve fit.

When Not to Simplify

Do not simplify paths for precision parts with tight tolerances (e.g., gears, snap-fit joints). Simplification introduces small deviations (typically 0.01-0.1mm depending on settings) that can accumulate and affect fit. Simplify only decorative elements, text, and traced graphics where sub-0.1mm precision is not critical.

Layer Strategy: Batch Cuts, Scores, and Engraves

Laser control software assigns different speeds, powers, and modes (cut, score, engrave) to different layers. If your file has cuts and scores mixed on one layer, the laser may switch modes mid-job, slowing down due to mode-change overhead. Organize your file so all cuts are on one layer, all scores on another, and all engraves on a third. The laser will batch each operation type, reducing mode changes.

In LightBurn, assign colors to layers in the import dialog, then configure each layer with the appropriate cut settings. In RDWorks and LaserCut, layers are numbered (Layer00, Layer01, etc.) and you assign operations in the layer list. Plan your design so operation types map cleanly to layers: red = cut, blue = score, black = engrave, for example.

Apply Kerf Offset Last, in Laser Software

Kerf offset (compensating for material removed by the laser beam) should be applied in the laser control software, not in your design software. If you offset paths in Inkscape or Illustrator before export, and then apply kerf offset again in LightBurn, you will double-offset and ruin fit. Apply kerf once, at the last step, and document where you applied it.

LightBurn has a kerf offset feature in cut layer settings: set it to inward for outer cuts (to compensate for kerf widening the part) and outward for inner cuts (to compensate for kerf widening voids). If your laser software does not have kerf offset, apply it manually in design software using Path → Outset / Inset (Inkscape) or Object → Path → Offset Path (Illustrator). For detailed kerf guidance, see the laser kerf and press-fit guide.

Common-Line Cutting for Adjacent Parts

If two parts share an edge (e.g., tiles in a grid, box sides in a flat-pack), you can cut the shared edge once instead of twice by designing them as a single path with the shared edge omitted. This is called common-line cutting. It halves the cut time for that edge and reduces kerf loss (material removed by the beam).

Common-line cutting requires planning at the design stage. Draw adjacent parts so shared edges overlap exactly, then delete one edge. When the laser cuts, it will cut around the perimeter of the group, leaving the shared edge intact. After cutting, snap the parts apart along the shared edge (which was not cut). This technique is common in production laser cutting and CNC routing.

Limitations and Risks

Common-line cutting only works if the shared edge is straight or if both parts have matching curves along the shared edge. It also assumes the parts will be separated by hand after cutting, which may not be practical for small or fragile parts. Use it for large, flat parts (panels, tiles, gears) where the shared edge is easy to snap or cut by hand post-job.

Final Checks Before Cutting

Before sending the file to the laser, run through this checklist: (1) Inner shapes are cut before outer perimeters. (2) All paths are closed (no open paths that will cause backtracking). (3) Node count is minimized (simplified where appropriate). (4) Kerf offset is applied once, either in laser software or design software, not both. (5) Layers are organized by operation type (cut, score, engrave). (6) Lead-ins and lead-outs are set appropriately for the material.

Preview the toolpath in your laser software (LightBurn's Preview mode, RDWorks' Simulation) to confirm cut order and spot any unexpected travel moves. If the preview shows the laser jumping back and forth across the sheet repeatedly, reorder paths or re-nest parts.

For related workflows, see the LightBurn SVG import guide, the Glowforge SVG requirements, and the PNG to laser-ready vector guide. For file format considerations, check SVG vs DXF and the file formats reference.