What Bend Allowance Means
When sheet metal bends, material near the inside radius compresses and material near the outside stretches. Somewhere through the thickness is the neutral axis, where length is assumed not to change. Bend allowance estimates the arc length of that neutral axis.
Accurate bend allowance matters for laser cut blanks, press brake work, formed enclosures, brackets, and any part where flange dimensions must land after forming. The right value depends on tooling, material, thickness, radius, grain direction, and process.
- Designers use it for flat-pattern CAD models.
- Fabricators use it to set up press brake jobs.
- Estimators use it to reduce scrap and rework.
How to Calculate Bend Allowance
The bend allowance formula is: BA = angle in radians x (inside radius + K-factor x material thickness). For a 90 degree bend, the angle in radians is pi / 2, or 1.5708.
For example, material thickness is 0.060 inch, inside radius is 0.090 inch, and K-factor is 0.33. BA = 1.5708 x (0.090 + 0.33 x 0.060) = 1.5708 x 0.1098 = 0.1725 inch.
Bend Deduction and Flat Pattern Length
Bend deduction is related but used differently. For a 90 degree bend, outside setback is often (inside radius + thickness) x tan(angle / 2). Bend deduction = 2 x outside setback - bend allowance for a two-flange bend.
Using the example above, outside setback for a 90 degree bend is (0.090 + 0.060) x tan(45 degrees) = 0.150 inch. Bend deduction is 2 x 0.150 - 0.1725 = 0.1275 inch. CAD systems may ask for K-factor, bend allowance, or bend deduction, so know which input is being used.
- Use test bends to calibrate K-factor for real tooling.
- Keep bend tables specific to material and thickness.
- Check whether dimensions are inside, outside, or flange dimensions.
Common Flat Pattern Mistakes
A common mistake is using one K-factor for every job. Air bending, bottoming, coining, large radii, hard materials, and different punch-die combinations can shift the neutral axis and change the developed length.
Another mistake is confusing bend angle with included angle. Some systems define the angle as the amount of bend, while others show the remaining included angle. A 90 degree formed corner must be entered according to the software or shop convention.
- Do not ignore minimum inside radius for the material.
- Do not mix metric and inch bend tables.
- Do not skip grain direction checks on crack-sensitive materials.
Frequently asked questions
What is K-factor in sheet metal?
K-factor is the neutral axis location expressed as a fraction of material thickness from the inside surface of the bend.
Is bend allowance the same as bend deduction?
No. Bend allowance is the neutral-axis arc length added into the flat pattern. Bend deduction is subtracted from outside flange dimensions to get flat length.
Can I use a standard K-factor of 0.33?
It can be a starting point for some air-bent parts, but production work should use bend tests or shop-proven bend tables.
Why do my formed parts come out short or long?
The bend data may not match the actual material, tooling, radius, or angle convention. Measure test parts and adjust the bend table.