Why splines are a problem
A spline is stored as control points, knots and weights, not as the path the machine follows. Every program that reads it has to approximate it, and two programs can approximate it differently. Lines and arcs are read the same way everywhere.
Shops ask for simple geometry, and their rules differ. SendCutSend recommends converting splines to polylines on export, because polylines are more widely accepted, and in Inventor replacing splines by a linear approximation of about 0.010 in. MEILLER accepts only lines and arcs, no polylines. TAUPITZ asks for polylines where possible. Lines and arcs are the common ground.
Many shops also ask for the old R12 DXF format: Autodesk's Inventor help says that many machine tools expect files in AutoCAD R12 format, and Xometry tells customers to save the file as an R12 DXF. Saving to R12 converts newer objects: Autodesk lists that lightweight polylines become Release 12 polylines and that ellipses, regions and other objects become lines, arcs and circles.
How to replace splines
Short straight lines: auto-fix in the CutMeter check replaces each spline with a polyline of short lines that follow the curve. The simplest geometry any cutting program reads.
Arcs: if you choose arcs, auto-fix fits arcs to the curve. Far fewer pieces, a smoother path and a closer length. By default CutMeter writes the repaired contour as one polyline; for a shop that accepts only separate lines and arcs, as MEILLER does, set Write as to Lines and arcs in auto-fix.
R12: when you download the fixed file as R12, CutMeter always writes curves as arcs inside an R12 polyline (POLYLINE with VERTEX entities), because R12 has no lightweight polyline.
Worked example
An arch: one cubic spline with control points 0,0; 30,40; 70,40; 100,0 (mm), closed by a straight base line of 100 mm.
CutMeter measures the spline at 120.97 mm, the whole contour at 220.97 mm, and reports a spline, fixable. Replaced by 96 short lines the contour is 0.0019 mm shorter; replaced by 15 arcs it is 0.0002 mm shorter.
The same arch, three ways
| Variant | Entities written | The spline becomes | Length change |
|---|---|---|---|
| Original drawing | SPLINE × 1, LINE × 1 | 1 spline | 0 mm |
| Auto-fix, short lines | LWPOLYLINE × 1 | 96 straight lines | -0.0019 mm |
| Auto-fix, arcs | LWPOLYLINE × 1 | 15 arcs | -0.0002 mm |
| Auto-fix, R12 file | POLYLINE × 1, VERTEX × 16, SEQEND × 1 | 15 arcs | -0.0002 mm |
| Auto-fix, Lines and arcs | ARC × 15, LINE × 1 | 15 arcs | -0.0002 mm |
Questions
What is a spline in a DXF?
A free-form curve (the SPLINE entity) stored as control points, knots and weights. CAD programs draw it smoothly; a cutting program has to turn it into lines or arcs first.
Will my shop cut splines?
Ask the shop, because rules differ: SendCutSend recommends converting splines to polylines, MEILLER accepts only lines and arcs. Auto-fix serves both: arcs in a polyline by default, or every piece as its own line or arc with Write as set to Lines and arcs.
Arcs or short lines: which is better?
Arcs, where the curve allows: fewer pieces, a smoother edge and a length closer to the curve. Short lines suit controllers that read nothing else.
Should I save my DXF as R12?
If your shop asks for it, yes. R12 is the oldest common DXF version, and newer objects are converted when you save to it. Look at the drawing after saving: check that curves and polylines came through as you expect.
Does the replacement change my quote?
Barely: in the example the cut length changes by thousandths of a millimetre. CutMeter shows the change of every fix in the report, so the price follows the file that is cut.
Sources
- SendCutSend: How To Export a DXF from Autodesk Fusion sendcutsend.com
- SendCutSend: How to Export from Autodesk Inventor for SendCutSend sendcutsend.com
- MEILLER: Laserteile richtig als DXF-Datei anlegen (German) meiller-lasertechnik.de
- TAUPITZ: Anforderungen an CAD-Dateien in der Blechbearbeitung (German) taupitz.com
- Autodesk: About Saving Drawings to Previous Drawing File Formats help.autodesk.com
- Autodesk Inventor: Flat Pattern Export Dialog Reference help.autodesk.com
- Xometry: How to Convert Image Files to DXF for Sheet Cutting xometry.com