CGCADGuide.tools
Free 3D Printing Utility

Chordal Deviation STL Mesh Quality Calculator

Predict faceted stepping artifacts on circular features before exporting STEP/IGES to STL. Optimize polygon segment counts to achieve watertight, print-ready meshes.

Arc vs. Polygon Mesh Overlay

Interactive chordal sagitta geometry visualization

R = 5 mm36 polygon segmentsθ = 10°Ideal CircleSTL Polygon Mesh
Excellent — Imperceptible faceting

Your chordal deviation is well below the printer resolution threshold. Exported mesh geometry will be visually indistinguishable from original BREP curves.

Export Parameters

Configure STEP/IGES to STL export settings

1° (360 sides, ultra-fine)10° (36 sides, default)45° (8 sides, coarse)
Computed Mesh Metrics
Segments (N)
36
Sagitta (S)
0.0190mm
Chord Length
0.8716mm
Arc Error
0.127%
Recommended Export Settings
For your FDM (Standard) printer, use at least 10 segments (angular step ≤ 36.0°) for a 5 mm radius feature.

Understanding Chordal Deviation in CAD-to-Print Workflows

1. What is chordal deviation?

Definition: Chordal deviation (also called sagitta or chord height) is the maximum perpendicular distance between a chord segment and the arc it approximates. When CAD software exports BREP (boundary representation) geometry to STL mesh format, every curve is approximated by a series of straight line segments.

The Sagitta Formula: For a circle of radius R divided into N polygon segments, each segment subtends an angle θ = 360° / N. The sagitta is computed as:

S = R × (1 − cos(θ/2))

If S exceeds your printer's achievable resolution, the printed circular hole will exhibit visible flat edges (faceted stepping) rather than a smooth curve.

2. CAD software STL export settings guide

Most CAD platforms expose the chordal deviation parameter under different UI labels during STL export:

  • SolidWorks: File → Save As → STL → Options → Deviation (Fine/Custom tolerance).
  • Fusion 360: Export → Mesh → Refinement (Low/Medium/High/Custom).
  • Inventor: File → Save Copy As → STL → Surface Deviation.
  • FreeCAD: Part → Export → STL → Surface deviation (mm).
  • Rhino 3D: File → Save As → STL → Detailed Controls → Max angle.
  • CATIA: STL Rapid Prototyping → Tessellation Sag.

Rule of Thumb: Set the chordal deviation to a value that is at most half of your printer's stated XY resolution for best results. A value of 0.01 mm is typical for SLA/resin printers, and 0.05 - 0.1 mm for FDM machines.

Watertight Mesh Tip: After exporting STL, always run a mesh integrity check. Non-manifold edges, flipped normals, or gaps between polygon faces will cause slicing failures. Most slicers (Cura, PrusaSlicer) include built-in repair tools, or use open-source Meshmixer for advanced healing.

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