Fillets and Chamfers Without a Fillet Tool

Rig Cad has no fillet tool that you click on an edge. It has five ways to round or bevel geometry, each with a different cost and a different failure mode: a primitive's own corner radius, an extrusion's edge profile, a smooth SDF blend, a Minkowski sum, and a Surface Offset step. Knowing which one to reach for is most of the skill, and the companion project puts all of them side by side.

Table: Rounding routes compared. Costs are relative; the demo's measurements are in the sections below.

The general rule: round at the lowest level that owns the edge. A rounded corner that belongs to one primitive should be on that primitive; a fillet where a boss meets a plate belongs on the union that joins them; a soft-touch overall look belongs on the outermost node.

Box and cylinder both carry a cornerRadius. Box adds roundingMode: all rounds every edge, planar rounds only the edges perpendicular to the base plane and leaves the top and bottom faces crisp, which is what a plate that must sit flat on the bed usually wants. The first sample in the demo is a box with both options exposed.

The 2D primitives have the same option, and it is exact in the contour: the arc centre and radius are what you typed. Only the emitted polygon is faceted, by segments, which defaults to 8. Measured on a 20 by 20 rectangle at radius 5, the area came out 0.13 percent under the exact value at 8 segments and 0.002 percent under at 64. Raise segments when the corner is large or the dimension is toleranced; leave it low on decorative corners.

This is the route to prefer whenever it applies, because it costs no extra nodes and no sampling. Compose rounded primitives in 2D and extrude, as in Sketches to Solids, before you consider any of the others.

Linear Extrude can chamfer or round its top and bottom edges with edgeProfile and edgeRadius. The second sample exposes both. Chamfer is one extra ring of faces: the demo plate measured 220 triangles with a chamfer. Switching the same plate to rounded built edgeSegments offset layers around a rounded-rectangle profile and measured over 200,000 triangles, a thousand times heavier for a visually similar edge break.

So: chamfer for edge breaks on plates and lids; rounded only when the round is the point and the profile is simple; and when a rounded profile is heavy, lower edgeSegments and the profile's segments before anything else. Export for the Slicer has the numbers on what a slicer tolerates.

Turn on SDF Mode on a Union and its smooth value blends the children where they meet. This is the only route that fillets a junction, such as the boss on the third sample plate, and it comes from the field mathematics in Signed Distance Fields.

Three facts keep it under control.

smooth is a blend width, and its reach depends on blendMode. Two overlapping 10 mm circles unioned at smooth 7 added 16.6 mm² of fillet area in quadratic mode and 21.5 mm² in circular. The demo exposes the mode: circular is the closest to a machined fillet, chamfer bevels, stairs and columns step and scallop with blendSteps, and cubic, quartic, sigmoid, exponential, sq_root and polynomial shape the falloff. Measure the result rather than assuming millimetres.

A smooth union grows the whole outline, not just the concave corner. Two 20 mm boxes overlapping by 2 mm came out 24.1 mm across at smooth 7. clipToBounds, exposed on the demo as clipBlend, trims the result back to its original bounding box, which returned that measurement to exactly 20. It only reaches a bulge that leaves the box; a boss blended mid-face bulges inside the bounds and needs a following cutter.

Blend radius is bounded by feature spacing: fillet less than gap over two. Two 4 mm ribs 16 mm apart stayed separate at smooth 10. The same ribs 2 mm apart fused at smooth 8 into one component with a 63 percent larger volume. Check the connected-component count against what you expect, because fusion silently discards the reason you split the features.

The Minkowski node in sum mode sweeps its second child around the surface of its first. A box plus a sphere is a box with every convex edge and corner rounded to the sphere's radius, and that is the fourth sample. Because the sum grows the part by the radius in every direction, the demo shrinks the core by 2 * minkowskiRadius on each axis so the finished block keeps plateSize.

Minkowski rounds convex edges only. A concave edge, the inside corner of an L, is where two offset surfaces intersect, and it stays sharp. That is the same behaviour as a Surface Offset post-processing step, which is the Minkowski of a solid and a sphere expressed as an offset. Use whichever fits the tree: Minkowski when the rounding tool should be a node with children you can change, Surface Offset when it should be a step on an existing node's post-processing stack.

One trap: options.surfaceOffset on an ordinary node is not the offset. The SDF evaluator does not read it; the Surface Offset step is what moves a surface.

• A plate, boss, or slot that needs rounded corners: cornerRadius on the primitive, planar if it must sit flat.
• An edge break on a lid or plate: extrude chamfer.
• A fillet where two solids meet: SDF union with circular blend, clipToBounds on, cutters kept above it.
• A whole small part that should feel soft: Minkowski with a sphere, core shrunk by the radius.
• A finished node that needs a uniform radius after the fact: Surface Offset step.

Users arriving from feature-based CAD will find the mapping from fillet and chamfer commands in Coming from Onshape or SolidWorks and the Fusion 360 guide.