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Cut by Polygon

In one sentence: Cut the mesh by drawing a closed polygon on the screen, removing mesh elements on one side of the projected polygon boundary.

Overview

Workbench: Mesh
Menu: Mesh → Meshes → Cutting → Cut by Polygon
Command: Mesh_PolyCut
Shortcut: none

Cuts the mesh by drawing a closed polygon in 2-D screen space. The polygon is projected onto the mesh and used as a cutting boundary. Mesh elements on the chosen side of the polygon are removed, leaving an open boundary.

Intuition

Think of using a cookie cutter: you stamp a shape from the top and remove everything outside (or inside) the stamp outline. Cut by Polygon does this from the current view direction — you draw the outline on screen and FreeCAD removes the mesh on one side.

The cut results in an open mesh boundary at the cut line. To close it, use Fill Up Holes or Fill Hole Interactively.

When to use it

  • Removing a region of a scan mesh that is outside your area of interest.
  • Cropping a mesh to a specific outline.
  • Removing a visible artifact or unwanted protrusion from a scan.

Step-by-step

  1. Orient the 3-D view so you're looking straight at the region to cut (use Top/Front/Side view for best results).
  2. Switch to Orthographic projection (View → Standard views → Orthographic).
  3. Choose Mesh → Meshes → Cutting → Cut by Polygon.
  4. Left-click to add polygon vertices in the 3-D view.
  5. Right-click to close the polygon and apply the cut.
  6. FreeCAD prompts: Inner (keep inside) or Outer (keep outside). Click the side to keep.

Common mistakes and pitfalls

The cut is performed in 2-D screen space

The polygon is a vertical projection through the view direction. In perspective mode or with an off-axis viewpoint, the cut boundary will be oblique. Use Orthographic projection and align the view to a standard plane for predictable results.

Result has an open boundary

Cut by Polygon leaves an open mesh edge at the cut. Use Fill Up Holes to close it if needed.

See also