ENGINEERING REFERENCE
Sheet-metal developed length and bend allowance: engineering guide
This server-rendered guide documents the physical basis, units, hand-check method and limits behind the interactive calculation.
Engineering Theory & Practical Use
A folded sheet changes length through the bend zone because the outside fibres stretch while the inside fibres compress. Between them is a neutral surface whose arc length is used to estimate the developed blank. The K-factor locates this neutral surface as a fraction of material thickness measured from the inside face. Bend allowance is the neutral-axis arc length; bend deduction is the amount subtracted from outside flange dimensions after geometric setbacks are considered. These terms are related but are not interchangeable.
Flat-pattern calculations support laser cutting, punching, shearing, nesting and press-brake planning. A sequential profile contains one more flange than bends, and each bend can have its own included angle, inside radius and direction. The production blank depends on material grade and temper, grain direction, thickness tolerance, tooling, forming method and the shop's measured bend database. The calculator preserves each bend and flange as traceable input data and provides a preliminary developed length and bend-line layout.
Governing Equations & Parameters
| Equation | Engineering meaning |
|---|---|
BA = (π / 180) θ (R + Kt) | bend allowance BA for bend angle θ, inside radius R, thickness t and K-factor K |
OSSB = (R + t) tan(θ / 2) | outside setback for a simple bend |
BD = 2 OSSB − BA | bend deduction for outside flange dimensions |
Lflat = Σ Lflange + Σ BA | developed length when entered straight segments are tangent flange lengths |
Step-by-Step Worked Example
- Consider two 90° bends in 2.0 mm mild-steel sheet with inside radius R = 2.0 mm and K = 0.40. Tangent flange lengths are 50, 80 and 40 mm.
- For each bend, BA = (π/180) × 90 × (2.0 + 0.40 × 2.0) = 4.398 mm.
- Total bend allowance is 2 × 4.398 = 8.796 mm. Developed length from tangent dimensions is 50 + 80 + 40 + 8.796 = 178.796 mm, reported as 178.80 mm before process compensation.
- Each bend line is placed after the preceding flange and allowance segments. A production trial coupon should then establish the controlled correction for the actual material lot, tooling and brake setup.
Design Limits, Safety & Standards
- Confirm whether source dimensions are tangent, inside, outside or apex dimensions.
- Use measured K-factor or bend-deduction data for released production blanks.
- Check minimum bend radius, cracking risk, springback, tooling clearance and grain direction.
- Applicable drawing, tolerance and material requirements may reference ISO 2768, ISO 8015, ASME Y14.5 or project-specific fabrication standards.
Reference basis: Classical neutral-axis bend allowance model; calibrate against controlled shop trials and the drawing's current fabrication requirements.
Engineering FAQ & Common Pitfalls
Is K-factor a material constant?
No. It changes with radius-to-thickness ratio, tooling, process, material condition and grain direction. Shop-qualified bend data is preferable.
Why is my finished part too large?
Common causes are mixing outside and tangent flange definitions, using an unverified K-factor, ignoring springback, or measuring to a different datum than the calculation.
Does bend direction change developed length?
Direction changes the folded profile and bend sequence. For otherwise identical simple bends it does not by itself change neutral-axis arc length, but it matters for tooling access and collision review.