FreeCAD: External Geometry and Arc (freecadhub)
Continuing the freecadhub course — using External Geometry to project edges from existing features into a new sketch, and Arc to draw curved sketch geometry.
Continuing the @freecadhub course. Two new tools — one for referencing existing geometry in a new sketch, one for drawing curves.
New Tools
External Geometry (External Projection) — projects edges or vertices from existing solid features into the active sketch as reference geometry. The projected lines appear in purple/orange and are non-editable — they’re references, not sketch elements. Used to snap new sketch geometry precisely to the boundaries of an existing feature without manually re-entering coordinates.
The key use case: you’re sketching on a face of an existing Pad and need the new sketch to align exactly with the edges of that body. External Geometry pulls those edges in so you can constrain against them.
Arc — draws a circular arc in the Sketcher. Three variants:
- Arc by center — pick center, then start and end points; the most common
- Arc by three points — pick start, end, and a point on the arc
- Arc by edge — tangent to an existing edge
Arcs are constrained the same way as circles and lines — radius dimension, tangent, coincident, etc.
The Exercise
The part is a bracket-like block: a rectangular base Padded, with a cylindrical protrusion on top (built using an arc to form the rounded profile in the sketch), a rectangular Pocket on the face, and Fillet on the top edges.
Feature tree: Sketch → Pad → Sketch001 → Pocket → Fillet
The cylindrical top was not a separate Revolution — the arc was drawn directly into the base sketch profile, making the rounded top part of the initial Pad shape. External Geometry was used when sketching Sketch001 (the pocket face) to project the boundary edges of the Pad so the pocket could be constrained relative to the existing body geometry.
What Clicked
External Geometry is the bridge between sketches. Without it, every new sketch starts from scratch and has to be constrained from the origin. With it, you can lock new geometry to the existing body — the second sketch knows where the first one ended up.
Arcs in the base profile avoid a separate Revolution. A rounded top doesn’t always need to be a separate Revolution feature. If the shape has a uniform depth, drawing the arc into the initial sketch profile and Padding it once is simpler and produces a cleaner feature tree.