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Approximate Cylinder

In one sentence: Fit a best-fit cylinder to a mesh segment and create a Part::Cylinder aligned to the estimated axis and radius.

Overview

Workbench: Reverse Engineering
Menu: Reverse Engineering → Approximation → Cylinder
Shortcut: none

Estimates the cylinder axis from the mesh face normals, then refines the axis and radius by iterative least-squares fitting. Creates a Part::Cylinder with the fit radius, height spanning the input extent, and placement aligned to the fit axis.

Intuition

If you have a scan of a bore or a shaft, the scan is a cloud of points on a cylinder. Approximate Cylinder finds the axis and radius of the cylinder that best fits those points, in the same way that Approximate Plane finds the best flat surface. The result is a clean Part::Cylinder you can use for measurement or as a reference feature.

When to use it

  • You have a segmented scan of a cylindrical bore, shaft, or pipe and need its axis and radius for engineering calculations or further modelling.
  • You want to measure the diameter of a scanned cylinder compared to its nominal dimension.
  • You need the fit cylinder as a reference axis for aligning other features.

When NOT to use it

  • Select only the cylindrical region — if the mesh includes non-cylindrical faces, the fit will be inaccurate. Use Mesh Segmentation to isolate the cylindrical region first.
  • Cone or tapered cylinder — the tool fits a right circular cylinder only. A tapered bore will produce a poor fit with high residual.

Step-by-step

  1. Use Mesh Segmentation or Manual Segmentation to isolate the cylindrical region as a separate Mesh::Feature.
  2. Select that Mesh::Feature.
  3. Choose Reverse Engineering → Approximation → Cylinder.
  4. FreeCAD creates a Part::Cylinder. Check the Report View for the RMS residual and fit parameters.

Parameters

No task panel — the fit runs automatically.

Result properties

Property Description
Radius Best-fit cylinder radius
Height Length of the input along the fit axis
Placement Positioned and oriented along the fit axis

Common mistakes and pitfalls

Fit is inaccurate — non-cylindrical faces included

Cause: The selected mesh includes flat faces or blending fillets adjacent to the cylinder.
Fix: Isolate just the cylindrical region using segmentation tools before fitting.

Fit fails silently (no Part::Cylinder created)

Cause: The algorithm could not converge — the input may not be sufficiently cylindrical.
Fix: Check that the selected region is approximately cylindrical. Try a different segment or use Approximate B-Spline Surface as a fallback.

Python API

import FreeCADGui as Gui
Gui.doCommand("Gui.runCommand('Reen_ApproxCylinder', 0)")

See also