BIZOMAT MAKEOS · Fabrication
Sheet Bend Allowance and Deduction
What developed length follows from this bend and K-factor?
Calculate one bend’s neutral-axis allowance and outside-dimension deduction using a supplied K-factor.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Sheet thickness | mm | Measured or drawing thickness. |
| Inside bend radius | mm | Achieved inside radius for the actual tooling and material. |
| Bend angle from flat | ° | Change in direction from flat: a right-angle bend is 90°, not its supplementary angle. |
| K-factor | Neutral-axis distance from the inside surface divided by thickness; calibrate with trials. | |
| Outside flange A | mm | Outside dimension to the theoretical sharp intersection of the outside faces. |
| Outside flange B | mm | Other outside flange dimension to that same virtual sharp. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Sheet thickness
- What it is
- The thickness of the sheet being bent, measured or from the drawing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.001 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Thicker sheet increases the outside setback and, unless K is 0, the bend allowance.
- Get it right
- Measure the actual sheet; nominal gauge thickness can differ.
What this means: Inside bend radius
- What it is
- The radius on the inside of the bend that the tooling and material actually produce.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger radius increases both the bend allowance and the setback.
- Get it right
- Use the achieved radius from trials, which can differ from the punch radius in air bending.
What this means: Bend angle from flat
- What it is
- How far the sheet turns from flat at this bend: 90° for a right-angle bend.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0.001° to 179°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The bend allowance is directly proportional to the bend angle.
- Get it right
- Enter the change in direction, not the included angle between flanges; a 135° included angle is a 45° bend.
What this means: K-factor
- What it is
- Where the neutral axis (the layer that neither stretches nor compresses) sits in the bend, as its distance from the inside surface divided by the sheet thickness.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher K-factor increases the bend allowance and so the developed length.
- Get it right
- It depends on material, tooling and process; calibrate it from trial bends rather than a generic value.
What this means: Outside flange A
- What it is
- The outside length of the first flange, measured to the virtual sharp: the point where the two outside faces would meet if the bend were a sharp corner.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.001 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It adds directly to the developed length.
- Get it right
- Do not measure to the tangent of the bend radius; use the theoretical outside intersection.
What this means: Outside flange B
- What it is
- The outside length of the second flange, measured to the same virtual sharp where the outside faces would intersect.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.001 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It adds directly to the developed length.
- Get it right
- Both flanges must reach the same virtual sharp; each must be at least the outside setback.
Method and formulas
- Bend allowance BA = angle (radians) × (inside radius + K × thickness).
- Outside setback SB = (inside radius + thickness) × tan(angle / 2).
- Bend deduction BD = 2 × SB − BA. Developed length = A + B − BD.
- Example: t = 2, R = 3, K = 0.4 and a 90° bend gives BA = 5.969026 mm, BD = 4.030974 mm; 30/40 mm outside flanges develop to 65.969026 mm.
Limitations
- K is process-specific, not a material constant. Outside-dimension conventions must match this method.
- No bend-force, minimum-radius, springback or manufacturability prediction.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- bend deduction
- flat pattern
- K factor
Who it is for
- Sheet-metal designers
- Fabricators
What you get
- Bend allowance
- Outside setback
- Bend deduction
- Developed length
Interpreting the result
- Cut and measure a trial strip before releasing a production flat pattern.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1346–1352. Topic reference; no source tables or recommended settings are reproduced.
- SOLIDWORKS Design Help: K-Factor — https://help.solidworks.com/2026/english/solidworks/sldworks/c_K_Factor.htm
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Shrinking and stretching curved flanges
In Hand Forming Sheet Metal by the Numbers
What it covers: Work out how much a flange on a curve must shrink or stretch, plan it as shrinker bites or tucks, compare a two-piece ring, and catch a flange folded over by bites taken too large.
Method explained here · BIZOMAT original book
Neutral axis, K-factor and bend allowance
In Sheet Metal Parts by the Numbers
What it covers: Where the neutral axis sits in a bend, what the K-factor means, how to calculate bend allowance for any angle, how sensitive the flat is to K, and how to calibrate K from trial strips.
Learning pathway
Develop a flat blank for a bent part
Learn how the neutral axis and K-factor set bend allowance, then size the blank for your own bend.
1 Learn
Read Neutral axis, K-factor and bend allowance in Sheet Metal Parts by the Numbers.
Where the neutral axis sits in a bend, what the K-factor means, how to calculate bend allowance for any angle, how sensitive the flat is to K, and how to calibrate K from trial strips.
2 Apply
Open Sheet Bend Allowance and Deduction and enter your own figures.
What developed length follows from this bend and K-factor?
3 Verify
Read the tool’s feasibility and verification checks under the result, and test your answer against its stated limitations:
- K is process-specific, not a material constant. Outside-dimension conventions must match this method.
- No bend-force, minimum-radius, springback or manufacturability prediction.
- Example values are illustrations. Replace them with values for your actual task.
4 Improve
Download the decision report or copy the link with your inputs to keep a record, then change one assumption at a time and compare.
Next reading: Springback and overbending in Sheet Metal Parts by the Numbers.
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