CNC Pocket Design: Corner Radii That Cut Cost

CNC pocket design is where many parts win or lose on cost: a sharp internal corner forces a small cutter, light passes, and sometimes a second setup. Keep corner radii at least one-third of pocket depth, and 3 mm or larger where geometry allows, so a standard end mill can reach the floor without long-reach deflection.

Depth-to-width ratio matters as much as the radius. A pocket deeper than roughly four times the cutter diameter needs a long-reach tool, and chatter and wall taper appear quickly, especially in 304 stainless, which work-hardens under a rubbing cut. In 6061-T6 or POM/Delrin, a 6 mm carbide end mill handles about 20–25 mm of depth comfortably; beyond that, expect extra stepdowns, longer cycle time, and a higher quote.

Call radii explicitly on the drawing instead of noting sharp corners. A ±0.25 mm radius tolerance is normally achievable; tighter buys little. Pocket floors that seal against a gasket may justify Ra 0.8, but clearance or cosmetic floors rarely do—Ra 1.6 from a normal finishing pass is usually enough. Untoleranced dimensions fall under ISO 2768-m unless you state otherwise, so check that the default suits the fit.

If a square boss or insert must sit in the pocket, design a corner relief or clearance radius rather than demanding a true square corner. Square internal corners mean broaching or EDM, which adds lead time and cost well beyond the milling itself.

Send the 3D model with radii and depth limits in a drawing note; quoting gets faster and the first article is more likely to assemble.

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For more guidance on related work, see: Position Tolerance at MMC: CNC Hole Patterns That Fit.