Why Internal Corner Radius Matters
Every internal corner in a CNC-milled part must have a radius equal to or larger than the radius of the cutting tool used to machine it. Sharp 90-degree internal corners are physically impossible to produce with a rotating cylindrical end mill. Specifying a zero-radius internal corner forces the machinist to use EDM or multiple small-tool passes, increasing cost by 200 to 500 percent compared to a design with proper radii.
The standard recommendation is to specify an internal corner radius of at least 1/3 of the pocket depth. For a 12 mm deep pocket, the minimum corner radius should be 4 mm, which allows use of an 8 mm diameter end mill. Smaller radii require smaller tools that are weaker, slower, and more prone to breakage — a 2 mm end mill machines at roughly 1/4 the material removal rate of an 8 mm tool.
Optimal Radius Sizing Guidelines
For most applications, an internal corner radius of 3.0 mm (0.118 inch) provides an excellent balance between machinability and design function. This radius accommodates standard 6 mm end mills available in every machine shop. When mating parts require tighter corners, use a radius of 1.5 mm minimum — but expect a 20 to 30 percent cost increase due to the need for smaller tooling and multiple finishing passes.
Critical stress-bearing corners should have a radius-to-wall-thickness ratio of at least 0.25 to avoid stress concentration. A corner with R0.5 mm in a 4 mm thick wall creates a stress concentration factor (Kt) of approximately 2.5, while R1.0 mm reduces Kt to about 1.8. For fatigue-loaded parts in aerospace or automotive applications, increasing corner radii from 1.0 mm to 2.0 mm can improve fatigue life by 40 to 60 percent.
Design Techniques for Corner Relief
When mating parts require a sharp corner fit, use corner relief features instead of demanding small radii. A dog-bone relief adds a small circular cutout at each corner, allowing the mating part to seat fully without interference from the tool radius. T-bone relief extends the relief along one wall only. Both approaches maintain full functionality while using standard tooling.
Another effective technique is to add an undercut or chamfer on the mating part rather than demanding sharp corners on the pocket. This shifts the design complexity to the easier-to-machine component. For parts with many pocketed features, standardize on two or three corner radii across the entire design to minimize tool changes — each tool change adds 15 to 30 seconds of non-cutting time per operation.
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