LightBurn says your shape is not closed? The contour has a gap.

You import a DXF, set the layer to fill, and LightBurn refuses or fills the wrong regions. On screen the outline looks closed. It almost certainly is not: somewhere on that contour, two endpoints miss each other by a hundredth of a millimetre.

Why software insists on closed shapes

Fill and engrave work by scanning across the shape and switching between inside and outside at every boundary crossing. Kerf offset shifts the whole contour inward or outward by half the beam width. Both operations only make sense when the boundary actually separates an inside from an outside, and that requires a path that returns to its exact starting point. A contour that misses closure by a hundredth of a millimetre is, topologically, a line with two loose ends. The software cannot know whether that gap is an accident or something you meant, so it treats the shape as open and refuses.

That is why the error feels absurd. You can stare at the drawing at normal zoom and see nothing wrong. The gap only becomes visible when you zoom into a single corner until the two endpoints separate. Note that open is not always wrong: a single engraving line is allowed to be open. The trouble starts when you ask for fill or offset.

Where the gaps come from

Most CAD programs store a drawing as a pile of independent entities: this LINE, that ARC. On export to DXF, each segment goes out separately, and the contour exists only implicitly, through endpoints that happen to land on the same coordinates. Three things break that. Drawing without endpoint snapping means the segments never met exactly in the first place. Floating point rounding during export nudges coordinates in the last decimal places. And explode operations turn a clean closed polyline into loose segments before the file is even written. None of these produce a visible defect. All of them produce gaps.

Find and close the gaps

LightBurn

Edit > Select open shapes highlights everything that is open, which answers the first question: where. For gaps up to roughly 0.05 mm, Edit > Auto-Join selected shapes (Alt+J) merges segments whose endpoints sit close together. Close Path connects the two loose ends of one selected shape, but only when they are within 0.5 mm of each other. For anything larger there is Edit > Close selected paths with tolerance, where you set the distance threshold yourself and choose whether the endpoints get pulled together or bridged with a small extra segment. The details are in LightBurn's own Open vs Closed Shapes documentation.

QCAD

The free QCAD route has two steps, and the order matters. First run the detection for zero-length entities and delete what it finds, because degenerate leftovers confuse the joining step. Then use "Polyline from segments" with a tolerance of 0.1 to collect the loose lines and arcs into proper closed polylines.

VCarve, Aspire and other Vectric tools

Select the vectors and use Join Vectors with a tolerance. Endpoints within the threshold get merged and the contour becomes a single closed vector that toolpath operations accept. If you cut on a CNC or plasma table rather than a laser, the open paths guide for SheetCAM and VCarve covers that side in more depth.

Choosing the tolerance

The tolerance is a trade. Too small and gaps survive the join. Too large and the software welds endpoints that were meant to stay apart, for example the mouth of a narrow slot or two parts drawn close together. Start small and raise it only as far as needed, then check the result: if two neighbouring shapes became one, you went too far. A sensible starting point is just above the largest gap you actually measured, not a guess.

Fix the file, not every import

Joining shapes in LightBurn fixes this one job. The DXF still carries the gaps, and the next program, or the next person you send the file to, starts the same hunt from zero. If the file is yours, re-export with endpoint snapping enabled and avoid exploding polylines before export. If it came from a customer, repair it once, save the repaired version, and feed that one file to every tool downstream.