Hong Life CalculatorsUnderstanding the bend allowance calculator — concept and principle
When sheet metal is bent, the material on the outside stretches and the inside compresses. To cut a flat blank that ends up with the right flange lengths, you need the bend allowance and bend deduction. This calculator finds them for a single bend and gives the flat blank length.
How it is calculated
- Bend allowance BA = θ × (R + K × T), with θ in radians, R the inside bend radius, T the thickness and K the K-factor.
- Outside setback OSSB = tan(θ ÷ 2) × (R + T).
- Bend deduction BD = 2 × OSSB − BA.
- Flat length = A + B − BD, where A and B are flange lengths to the outside mould line.
- Inside arc = θ × R; outside arc = θ × (R + T).
The principle
Somewhere inside the bent sheet is a neutral axis that neither stretches nor compresses; its length is the true amount of material in the bend. The K-factor gives its position as a fraction of the thickness from the inside surface (commonly around 0.3–0.5). Drawings usually dimension flanges to the outside corner (the mould line), which overcounts the material at the bend, so the bend deduction is subtracted to get the blank length.
Worked example
Default inputs: T = 2 mm, R = 2 mm, 90° bend, K = 0.42, flanges A = B = 50 mm.
- Neutral-axis radius = 2 + 0.42 × 2 = 2.84 mm.
- BA = (π ÷ 2) × 2.84 ≈ 4.461 mm.
- OSSB = tan 45° × (2 + 2) = 4 mm.
- BD = 2 × 4 − 4.461 = 3.539 mm.
- Flat blank = 50 + 50 − 3.539 = 96.461 mm. Inside arc ≈ 3.142 mm; outside arc ≈ 6.283 mm.
| Outside flange length A (mm) | Flat blank length (mm) |
|---|---|
| 12.5 | 58.961 |
| 25 | 71.461 |
| 37.5 | 83.961 |
| 50 | 96.461 |
| 62.5 | 108.961 |
| 75 | 121.461 |
| 100 | 146.461 |
How to read the result
The K-factor depends on material, thickness, radius and the bending method; shops often determine it from test bends. For several bends, subtract one bend deduction per bend. If parts come out long or short, adjust the K-factor based on measured test pieces.
Common mistakes
- Using the bend angle as the included angle (a 90° bend leaves a 90° corner; a 120° bend leaves 60°).
- Using an outside radius in the formula that expects the inside radius.
- Applying one K-factor to all materials and thicknesses.
- Forgetting to subtract a deduction for every bend.
Frequently asked questions
What is the difference between bend allowance and bend deduction?
Bend allowance is the length of the neutral axis in the bend; bend deduction is what you subtract from the sum of outside flange lengths.
What is a typical K-factor?
Often between about 0.3 and 0.5 depending on material and radius; test bends give the most accurate value.
How do I handle several bends?
Subtract one bend deduction for each bend from the total of outside dimensions.
Sources: Machinery's Handbook, Industrial Press (sheet metal bending) · SME, Tool and Manufacturing Engineers Handbook, Volume 2: Forming