Open contours in a DXF: why the laser won't cut the part, and how to close them

What “open” means to a controller

A closed shape encloses an area: it has an inside and an outside, so the CAM program or the controller knows where to put the kerf and which piece is the part. A single line has neither, and nor does a rectangle whose corners miss by 0.05 mm. CypCut's manual says the software expects all graphics to be closed and shows non-closed ones in red; CypCutPro's inner/outer detection does not count an unclosed graphic as a contour at all. SheetCAM's manual is the same: if two ends don't meet, the shape is open and cannot be cut out. SendCutSend's checklist for its customers simply says: no open contours.

Where open contours come from

• Lines drawn without end snaps: on screen they meet, zoomed in they miss by a few hundredths of a millimetre. CypCut's manual warns that a drawing from AutoCAD can look closed and not be.
• Outlines exploded into single lines and arcs and then edited: one piece trimmed short, another extended past the corner. CypCut's manual tells you to split off and delete such an excess end before you join the contour.
• Overlapping lines: two copies on top of each other, or one partly over another, so three or four ends meet at one point.
• Curves exported as thousands of short lines, or art traced to DXF, that stop a fraction of a millimetre short of their start.
• Gaps drawn on purpose: a slot drawn as three sides, a sketch line, a scribe line on the wrong layer.

Find the gaps

1. Drop the file on the DXF checker. It counts the open contours on cut layers and lists every gap it closed, with the largest one in mm. It runs in your browser; the file is not uploaded.
2. Show the path ends. SheetCAM's tip works in any program: a closed shape has no start and no end, so an end marker on a “closed” outline marks a gap. In CypCut, View highlights unclosed graphics and Select → Select unclosed curve picks them.
3. Zoom in at each marker. Most gaps sit at a corner or where an arc meets a line.
4. Join the outline into one piece (JOIN in AutoCAD), snapping to end points, then export again.

Fix it in CAD or let the importer join it?

Every importer has a join tolerance. SheetCAM moves ends that lie within its import link tolerance so they touch. CypCut merges unclosed lines on import and has Combine Near with a precision you type in; CypNest's Join Nearest merges breakpoints within its join precision. SheetCAM's manual adds a warning worth keeping: the tolerance should not be used to compensate for poor drawings. A tolerance cannot tell a rounding error from a real gap — joining a 0.01 mm miss is harmless, joining the 2 mm mouth of a slot changes the part.

The limits in this table are our own rule of thumb (estimate), set to match the 0.1 mm gap that Zorocut closes by itself.

What Zorocut's pre-cut check does

• End points closer than 0.005 mm are treated as one point.
• Two pieces whose ends are up to 0.1 mm apart are joined with a short straight line, and the check warns you, for example “Closed 3 small gaps (largest gap 0.042 mm)”.
• A long outline whose own two ends almost meet is closed up to 0.2 % of its length, at most 0.5 mm: up to 0.2 mm on a 100 mm outline, 0.5 mm from 250 mm up. This is for exported splines and traced art that stop just short of their start.
• Exact duplicate and zero-length segments are removed, and specks smaller than 0.2 mm both ways are dropped as drawing noise. Every repair is listed.
• Where more than two segments meet at one point, the check warns that the contour there is ambiguous — usually an overlapping line or an end sticking past a corner. A line that only partly overlaps another is not removed: delete it in CAD.
• Whatever is still open on a cut layer is an error, and that geometry is not exported. Zorocut Nest draws the open contours in red in the Check view; the online nester can leave the open lines out and nest the closed parts.
• Open paths are allowed on mark and engrave layers: a layer whose name contains MARK, ETCH, SCRIBE or ENGRAV.