Sharp internal corners create two problems: a rotating cutter cannot make a zero radius, and a small radius amplifies local stress. Inglis’s classic 1913 elliptical-hole analysis shows that a sharper notch raises stress concentration; real risk also depends on load direction, surface quality and fatigue cycles.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
DESIGN BY PROCESS
- CNC milling: inside radius cannot be smaller than cutter radius. For deep pockets, make the design radius about 1.2–1.5 times cutter radius so the tool does not reach full engagement in the corner; increasing depth/diameter raises chatter and breakage risk. - Laser/waterjet: a sharp contour is possible, but corner deceleration can increase heat effect, burn or dross; bend notches can also initiate cracks. - Sheet assemblies: use dog-bone or T-bone relief if a square insert must seat, while checking net-section strength and appearance. - Casting/forging: radii improve filling/flow and reduce local die loading.
EXECUTABLE CALLOUT
Do not state “all corners R0.5” without considering tools and thickness. Separate functional corners from non-functional ones: give a maximum permitted R where fit matters, and a larger typical radius such as R2 elsewhere. For fatigue parts, use FEA or testing to find hot spots and control machining marks, burrs and surface defects; a larger radius alone does not guarantee life.
PROJECT CHECKLIST
Process; cutter diameter and overhang; pocket depth; thickness; mating need; load direction; fatigue cycles; deburring requirement.