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Fillet

In one sentence: Round selected edges of a Part shape with a constant or variable radius to remove sharp corners.

Overview

Workbench: Part · Menu: Part → Fillet
Toolbar: Part Modeling · Shortcut: none

Fillet creates a Part::Fillet object that applies curved, round transitions along the selected edges of a shape. Unlike Part Design Fillet — which operates on the Body tip — Part Fillet operates on any standalone Part shape.

Intuition

Sharp edges cause stress concentrations, are hard to deburr in manufacturing, and are hazardous to handle. Fillets replace the sharp edge with a smooth circular arc cross-section. In addition to structural benefits, fillets make parts look more polished and injection-moulded.

When to use it

  • Rounding external corners of a machined block.
  • Applying draft-radius fillets before injection mould analysis.
  • Softening internal corners of a cut feature.

When NOT to use it

  • Inside a Part Design Body — use Part Design Fillet on the Body tip.
  • Bevelled edge (flat, not rounded) — use Chamfer.
  • Edges that are tangent or near-tangent — OCCT may fail to fillet them.

Step-by-step walkthrough

  1. Select the shape (the whole solid, not individual edges) in the model tree.
  2. Part → Fillet.
  3. In the Fillet dialog, the shape's edges are listed.
  4. Select the edges to fillet (Ctrl+click for multiple).
  5. Set the Radius.
  6. Click OK.

Parameters

Parameter Type Default Description
Shape Part object — The solid to fillet
Edges Edge list — Edges to fillet (selected in dialog)
Radius Distance 1 mm Fillet radius

Common mistakes and pitfalls

Fillet failure on co-planar edges

After a Boolean Fuse, the merged solid may have internal seam edges where two faces were originally co-planar. Fillets on these edges fail. Use Refine Shape before filleting.

Radius too large — fillet exceeds the face size

If the fillet radius is larger than the adjacent face allows, OCCT raises an error. Reduce the radius or shorten the adjacent face.

Select the shape, not individual edges, when launching the tool

Fillet operates on the parent shape and lets you select edges inside the dialog. If you pre-select edges before launching, the dialog may not show all available edges.

Python API

import FreeCAD as App
import Part

doc = App.newDocument()

box = doc.addObject("Part::Box", "Box")
box.Length = 30; box.Width = 20; box.Height = 15
doc.recompute()

# Parametric fillet via Part::Fillet object
fillet = doc.addObject("Part::Fillet", "RoundedBox")
fillet.Base = box
# Select all 12 edges with radius 2 mm
fillet.Edges = [(i, 2.0, 2.0) for i in range(1, 13)]  # (edge_index, start_r, end_r)
doc.recompute()

print(f"Filleted volume: {fillet.Shape.Volume:.2f} mm³")

Note: Edge indices in the Edges property are 1-based. The tuple format is (edge_index, start_radius, end_radius) — use the same value for both for a constant-radius fillet.

See also