Inside radius depends on ductility, thickness, rolling direction, tooling and bending mode. In air bending, the resulting radius is often governed by V opening, not simply punch-tip radius. Mild steel can often start near R≈1t, 304 stainless near 1–2t and 5052-H32 near 1t; less-ductile 6061-T6 generally needs a larger radius based on product-specific data.
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.
THREE CRITICAL VARIABLES
1. Rolling direction: a bend line perpendicular to grain is generally safer than parallel; retain orientation through nesting. 2. V opening: V≈6–10t is a common starting range, but press capacity, short flange and target R alter selection. Insufficient tonnage or a flange falling into the die makes the setup infeasible. 3. Springback: materials with a high yield/elastic ratio spring back more; control by overbend, tool compensation or bottoming, validated by trials.
VALIDATION METHOD
For a new grade/temper, cut 5–10 coupons parallel and transverse to grain and bend them 90° using candidate R and V values. Inspect outside cracking, angle, actual inside R and spread. Measure with radius gauges or profile measurement rather than assuming CAD geometry. State the required formed inside radius on the drawing; the fabricator selects tooling to achieve it.
PROJECT CHECKLIST
Grade/temper; thickness tolerance; grain; R/t; V/t; short flange; tonnage; springback; crack acceptance; lot variation.