Splines, polylines and arcs in a DXF: why curves turn into thousands of short lines

Five ways to store a curve

Two details catch importers out. On a closed polyline the last vertex's bulge belongs to the closing segment back to the first vertex — miss it and a rounded corner comes in sharp. And a SPLINE can be written with fit points only, without knots or weights, so a reader has to handle both forms.

“It converts all arcs to lines”

That complaint comes from a SheetCAM user on the PlasmaSpider forum in 2023, about a free nesting tool: “it converts all arcs to lines (g1) motion”. His point was that once the arcs are gone, SheetCAM cannot see holes or arcs, so features that depend on them — hole detection, torch-height lock on arcs — stop working. Short segments cost time as well. SheetCAM's manual says drawings made of very many small segments slow processing, give very large G-code files and can slow machining down. CypCut has an option to smooth curves with NURBS interpolation to improve cutting speed, and a curve-precision setting: the lower it is, the more speed is limited on curves.

What controllers and CAM do with the rest

• CypCut smooths curves automatically on import and has a manual Smooth with a precision you set.
• CypNest's Auto Smooth reworks curves made of many line segments, with options to leave Bézier curves alone and to identify circles.
• SheetCAM turns runs of lines that follow an arc back into an arc within its arc fitting tolerance, and drops detail within its max detail reduction error — the manual suggests about 0.020 in for plasma.
• Each of these changes the geometry a little, by a tolerance you have to know. A file made of arcs gives them nothing to change.

Export settings by program

• Fusion: SendCutSend recommends the export option Convert Splines to Polylines, because polylines are more widely accepted and give more consistent results; the default tolerance is usually enough.
• SolidWorks: the same company's SolidWorks guide says the opposite for that program — exporting splines as splines is ideal.
• AutoCAD: SendCutSend's AutoCAD page says nothing about splines. Draw holes with CIRCLE and radii with ARC or a fillet, and they stay exact.
• Inkscape: its DXF exporter writes each Bézier segment as its own SPLINE; with Flatten Béziers on, curves become short lines; straight runs go out as LWPOLYLINE by default. Or skip the DXF: Zorocut reads the SVG directly — see SVG to DXF.

What Zorocut does with curves

• ARC and CIRCLE are read as they are, and polyline bulges become true arcs.
• SPLINE and ELLIPSE are sampled and fitted with the fewest arcs and lines that stay within 0.01 mm of the curve. The pre-cut check lists them, for example “Approximated with arcs and lines: 4 SPLINE”.
• A SPLINE with fit points only is joined with straight lines through those points, with a warning to check its shape.
• Arcs inside a block inserted with different X and Y scales are no longer circles: they are approximated and reported too.
• Curves already broken into LINEs stay lines: Zorocut does not fit arcs to them. On a closed outline of more than 200 segments it merges lines that lie on one curve within 0.005 mm, so the file gets lighter, but a hole drawn as 64 chords still exports as 64 chords. Redraw it as a circle in CAD.
• The exported DXF holds only LINE, ARC and CIRCLE, and the original arcs are moved and turned, never re-approximated — see Export DXF for CypCut and FSCUT.