- CLR/D describes bend tightness; D/t describes wall slenderness.
- 1.5D is a common starting point; 1.0D and below usually requires more capable tooling.
- Tangent length is a gripping requirement, not disposable allowance.
- 3D bends require defined tangent points, rotation angles, end orientation and section-distortion limits.
Values in this guide are practical starting points for RFQ, DFM and prototype validation. They do not replace load calculations, regulations, material certificates, qualified procedures or approved production drawings.
Screen with CLR/D and D/t
Lower CLR/D means a tighter bend; higher D/t means a thinner wall. For rotary-draw bending, CLR≈1.5D is a common design starting point. Around 1.0D or below usually needs capable mandrel/wiper tooling and tighter material control. Above D/t of roughly 25–30, wrinkling and flattening risk rises and early trials are advisable. Use both ratios together.
Define Acceptable Section Change
Round-tube ovality may be expressed as (max OD−min OD)/nominal OD×100%; wall thinning as (original t−minimum t)/original t×100%. For ordinary non-pressure parts, ovality ≤5–8% and thinning ≤10–15% can be discussion ranges. Pressure, fatigue or precision-fit parts require calculated and code-specific limits rather than generic values.
Reserve Tangent Length for Tooling
Straight tangents are not cosmetic allowance; they are grip zones for clamp and pressure dies. Required lengths depend on machine/tooling. Obtain minimum front grip, rear tangent and tangent-between-bends values before design freeze. A short final end may be achieved by bending long and trimming later. Closely spaced bends may require compound tooling or a different sequence.
Define Rotation and Tangent Geometry
Overall height and width are not enough. State CLR, bend angle, straight distance between tangent points, rotation between bend planes, end orientation and datum. Define left/right mirror relationships and identify which face of square/rectangular tube lies on the inside radius to prevent a 90° orientation error.
Keep Holes and Seams Out of Critical Zones
Holes in the bend zone distort; place functional holes on tangents and produce them after bending where practical. Weld-seam orientation affects consistency and is often controlled near the neutral axis or in a supplier-validated position. Sequence flattening, swaging, piercing and bending so earlier operations do not remove grip or mandrel access.
Compensate Springback with Data
Stainless, high-strength steel and aluminum generally spring back more than mild steel. The supplier compensates through overbend, tooling and program settings. Product drawings should state final free-state geometry rather than an arbitrary overbend angle. Verify end points, rotation and section change on first article using a gauge or 3D measurement.