From FreeCAD Documentation
- During the command the 3D view cannot be changed. It is advisable to properly line up the 3d view first.
- Select one or more mesh objects.
- There are several ways to invoke the command:
- Press the Trim mesh button.
- Select the Meshes → Cutting → Trim mesh option from the menu.
- Define a polygon by picking points in the 3D view.
- Select an option from the 3D view context menu:
- Inner: removes the faces and parts of faces that are inside the polygon.
- Outer: removes the faces and parts of faces that are outside the polygon.
- Split: removes the faces and parts of faces that are outside the polygon, and creates a new mesh object containing them.
- Cancel: cancels the command.