From FreeCAD Documentation
- Make sure you have at least two objects in your document; the "source" object that you wish to project, and the "target" object where the projection will be made.
- Click on Projection on surface to launch a task panel with various options.
- Click on Select projection surface, and then click on the "target" surface where the projection will be created.
- Then click on the specific button to choose the type of subelement that you want to add to your projection object.
- Add face: select a source face.
- Add wire: select a source edge. The tool will extract the entire wire to which the edge belongs. For example, by choosing a single edge of a polygon, it will project the entire polygon.
- Add edge: select a source edge. The tool will only project the selected edge.
- Once a button is depressed, pick a subelement in the 3D view. If you wish to deselect it, pick the same element again.
- When you are satisfied with your selection, press the same Add... button to leave the selection mode.
- Then click on the specific radiobutton to choose the type of projection shape to create.
- Show all: it will show all types of closed wires and edges on the target surface. If a "face" subelement was selected in the previous step, a preview of a solid object extruded from the projection will be shown, depending on the value of Extrude height.
- Show faces: it will show a preview of a filled face on the target surface. This will only work if you selected a "face" subelement in the previous step; if you selected a closed "wire", only the edges (no face) will be created as projection; if you selected "edges", only those edges will be created as projection.
- Show edges: it will show a preview of the edges on the target surface. This will work whether you added a "face", "wire", or "edge" subelement in the previous step; even if you added a filled "face", only the edges will be created as projection.
- Press the OK to complete the operation and produce the new projection object.