Laying Out a Fillet Between Two Surfaces
A fillet rounds the sharp intersection between two surfaces with a constant-radius arc that stays tangent to both. The arc doesn't start exactly at the corner; it starts a calculated distance back along each surface, called the setback, which depends on both the fillet radius and the angle between the surfaces.
Why Included Angle Changes the Math
At a square 90° corner, the setback distance happens to equal the fillet radius exactly, which is why many machinists default to that shortcut without thinking about it. Once the angle opens up past 90° or tightens below it, that shortcut breaks down, and the actual setback can be meaningfully larger or smaller than the radius itself.
Arc Length and Chord
Arc length is how much material the fillet actually removes along its curved path, useful for estimating cycle time on a filleted edge. Chord length is the straight-line distance between the two tangent points, useful for checking a fillet against a print dimension that was measured straight across rather than along the curve.