What an open contour is
A contour is closed when its last segment ends exactly where the first one starts. When the ends miss each other, by a few millimetres or by 0.001 mm, it is an open chain: the machine can still follow the line, but the software cannot fill the shape, so it does not know which side is the part.
Shops ask for closed contours in their file rules. SendCutSend writes that every cut path must form a closed shape: if your shape stops, so does the cutter. Xometry's DXF guide says the file should not contain open curves and asks to close them so that the sketch can be extruded. MEILLER requires a closed outer contour and warns that gaps lead to errors in production; TAUPITZ writes that lines that do not close lead to errors in the CAM software, and Tolery asks for contours that are closed and do not overshoot.
CutMeter joins segment ends that lie within the join tolerance (0.01 mm by default) and reports every chain that stays open. Two loose ends within ten times the tolerance of each other are reported as a gap at their position, and auto-fix can join them.
How to fix it
Auto-fix in the CutMeter check joins the two ends exactly; where both ends are on curves it adds a short straight line between them. The file you download cuts what was measured.
If your CAD writes ends that miss by a little, raise the join tolerance: the analysis then treats ends closer than the tolerance as joined. This changes the measurement only; the file stays as it is until you fix it.
In CAD, join the chain into one closed polyline and check that the command reports one closed object. Where a line is really missing, draw it: no tolerance should bridge a gap of millimetres.
Worked example
A 100 × 100 mm square drawn as four lines; the left edge stops 0.05 mm short of the corner.
At the default tolerance of 0.01 mm CutMeter finds 0 closed contours, 1 open chain, a net area of 0 mm² and these issues: Gap, Open contour. Both are marked fixable.
With a join tolerance of 0.06 mm the gap counts as joined: one closed contour of 10,000 mm².
After auto-fix the corner is joined in the file: one closed contour of 9,998.75 mm², cut length 399.975 mm.
The same square in three states
| State | Closed contours | Open chains | Net area | Pierces |
|---|---|---|---|---|
| Default tolerance | 0 | 1 | 0 mm² | 1 |
| Wider join tolerance | 1 | 0 | 10,000 mm² | 1 |
| After auto-fix | 1 | 0 | 9,998.75 mm² | 1 |
Questions
What is an open contour in a DXF?
An outline that does not close: its last segment does not end where the first one starts. CAM software cannot tell the part from the scrap, so it refuses the shape or cuts only a line.
Why will a shop not cut an open contour?
The cutting program needs a closed outline to decide on which side to cut and what falls out. With an open outline the part may stay attached to the sheet, or the job stops in programming. That is why shops ask for closed contours.
My gap is only 0.01 mm. Does it matter?
It can. Software that requires exact closure treats any gap as open. CutMeter joins ends within 0.01 mm by default and reports larger gaps, so you decide whether to join them.
Which join tolerance should I use?
Keep the default 0.01 mm for files from CAD. Raise it only for drawings converted from PDF or traced outlines whose ends miss by a little, and look at what gets joined.
Can CutMeter close every open contour?
No. Auto-fix joins ends that are close to each other. A contour whose ends are far apart is missing a line: draw it in your CAD.
Sources
- SendCutSend: Sheet Cutting Guidelines sendcutsend.com
- Xometry: Guide to Quoting with DXF Files xometry.com
- MEILLER: Laserteile richtig als DXF-Datei anlegen (German) meiller-lasertechnik.de
- TAUPITZ: Welche Dateiformate eignen sich für die Blechfertigung? (German) taupitz.com
- Tolery: Découpe laser en ligne (French) tolery.io