Splines vanish or turn jagged when you import a DXF?

The part looks perfect in Fusion 360. In LightBurn, SheetCAM or your CAM tool, the straight lines arrive fine and the curved sections are missing, zigzagged or broken into coarse facets. Those curves left your CAD as SPLINE entities, and SPLINE is the entity hobby importers handle worst.

Why your file contains splines you never drew

Fusion 360 exports sketch fillets and conic curves as SPLINE entities by default. A bracket with four rounded corners ships four splines, even though you only ever clicked "fillet". Autodesk documents the resulting compatibility problems in its own knowledge base article. Other parametric CADs behave similarly: anything that is not a line, an arc or a circle tends to leave the building as a spline.

The entity itself is the problem. A LINE stores two coordinates. A SPLINE stores control points that mostly do not lie on the curve, a knot vector, and optional weights, and the importer has to evaluate real NURBS math to reconstruct the shape. Get any part of that wrong and the curve deforms. The old R12 dialect, still a common export target because everything reads it, cannot represent a SPLINE at all, so exporters aiming at R12 must approximate the curve or silently drop it.

Three failure modes, three fingerprints

What you see after import tells you what the importer did with the spline:

The third case still cuts. It just cuts slowly and leaves visible flats on what should be a smooth curve. The first two cases produce parts you cannot use at all.

Convert splines before they leave your CAD

Fusion 360

Install the free DXF Spline to Polyline add-in from the Autodesk App Store. It converts splines while saving the sketch as DXF, so fillets and conics arrive as polylines every importer understands. Pair it with the right export route (right-click the sketch, then "Save As DXF"); our Fusion 360 DXF guide covers why that route matters for units too.

AutoCAD

Use PEDIT to convert a spline into a polyline: start the command, pick the spline, and accept the conversion. Do this on every spline before exporting the DXF. The result is chorded, so check the fit tolerance against what your machine needs.

No conversion possible? Try SVG instead

If your cutting software also reads SVG (LightBurn does), export SVG rather than DXF. SVG has no NURBS entity, so the exporter flattens curves to Bézier paths, which vector importers handle far more reliably. This is the standing advice on plasma forums for spline-heavy files.

If you cut with LightBurn: update it

LightBurn shipped DXF spline importer improvements in version 1.5. An update will not rescue every malformed file, but it fixed known spline bugs, so rule that out before you rebuild geometry.

What DXF Medic does and does not do with splines

DXF Medic's parser reads SPLINE entities. That makes it useful in exactly the situation above: a file that imports blank or mangled elsewhere can be opened and inspected, and you see what it actually contains, spline by spline, entity by entity. Anything the parser could not read is named in the report instead of silently vanishing, so you stop guessing which tool in the chain lost your curves.

What it does not do: convert splines to polylines or arcs. That conversion involves a tolerance decision, and it belongs in the CAD that still knows the true curve. Use the Fusion add-in or PEDIT for the conversion, and use the diagnosis to confirm what your export really contains before and after.