Sheet Metal K-Factor Test Bend Calculator
Measure a formed test coupon and use its flat length, outside legs, inside radius, and bend angle to solve the K-factor that fits that bend. The result also shows the bend deduction and allowance behind the answer.
Solved K-factor0.4
- Measured bend deduction (mm)
- 2.7
- Implied bend allowance (mm)
- 3.3
- Outside setback (mm)
- 3
- Neutral-axis depth (mm)
- 0.6
- Radius-to-thickness ratio
- 1
- Calibration check
- Within the usual geometric range (greater than 0 and no more than 0.5).
All lengths must use the selected unit system; changing units does not change the K-factor.
The finished outside legs are measured to the theoretical outside mold-line apex.
How to use this calculator
- Choose millimeters or inches, then use that unit for every length measurement.
- Micrometer the actual coupon thickness and measure the finished inside radius after bending.
- Enter the angle through which the sheet turns, not the included angle between the finished faces.
- Measure both finished outside legs to their theoretical outside mold-line apex and enter the original flat blank length.
- Review the K-factor and calibration check before using it in a bend table or CAD rule.
How a test bend solves the K-factor
A K-factor locates the neutral axis within sheet thickness during a bend. Rather than assuming a value, this calculator works backward from a coupon that has already been formed. That makes the answer specific to the material, thickness, grain direction, tooling, radius, and forming method used for the test.
Let T be measured thickness, R the finished inside radius, α the bend angle in degrees, θ that angle in radians, L the original flat length, and F1 and F2 the finished outside legs measured to the outside mold-line apex. The calculator first finds the measured bend deduction:
BD = F1 + F2 − L
It then finds outside setback and the bend allowance implied by the coupon:
OSSB = (R + T) × tan(θ ÷ 2)
BA = 2 × OSSB − BD
The ordinary bend-allowance relationship is BA = θ × (R + K × T). Rearranging it gives the reported result:
K = (BA ÷ θ − R) ÷ T
Measurements that matter most
The outside-leg measurements and flat length drive bend deduction, so small errors there can move the solved K-factor noticeably. Measure each leg to the theoretical point where the extended outside faces meet, not to a tangent point. A square, a height gauge, or a simple inspection layout can help establish that apex consistently.
Use the formed inside radius, not only the punch-tip radius. Air bending can produce a radius that differs from the punch, while coining and bottoming can behave differently again. The angle is the angle through which the material turns: a right-angle bend is 90 degrees. Entering the included angle reverses the geometry and can produce an implausible result.
What the result does and does not represent
A value greater than 0 and no more than 0.5 is the usual geometric range for this convention. A result outside that range is still shown because it is useful diagnostic information, but it calls for checking the angle convention, apex measurements, radius, and blank length before changing a production bend table.
This is a single-bend geometric model. It does not separately model springback, coating thickness, tensile variation, rolling direction, tool wear, press brake deflection, or inconsistent thickness. Make several coupons under the same setup and average the results when the part tolerance warrants it.
Worked example
For a 1.50 mm coupon bent 90° around a finished 1.50 mm inside radius, start with a 100.00 mm blank and measure two 51.35 mm outside legs to the mold-line apex. The bend deduction is 51.35 + 51.35 − 100.00 = 2.70 mm. Outside setback is (1.50 + 1.50) × tan(45°) = 3.00 mm, so the implied bend allowance is 2 × 3.00 − 2.70 = 3.30 mm.
Using θ = 90° × π ÷ 180 = 1.5708 radians, the solved K-factor is (3.30 ÷ 1.5708 − 1.50) ÷ 1.50 = 0.4006. The neutral-axis depth is 0.4006 × 1.50 = 0.6008 mm, and the radius-to-thickness ratio is 1.00.
Common questions
How do I measure a test bend for K-factor?
Cut a coupon, record its edge-to-edge flat length, form one bend, and measure the actual thickness and finished inside radius. Measure each outside flange from its free edge to the theoretical apex formed by extending the two outside faces. Use the same unit for every length.
Is bend angle the included angle or the angle of bend?
Enter the angle through which the sheet turns. A standard right-angle bend is 90 degrees. The included angle between the finished faces is a different convention and should not be entered here.
Why is my calculated K-factor above 0.5?
A value above 0.5 usually indicates that one measurement or the angle convention needs checking. Confirm that the legs reach the outside mold-line apex, the radius is the formed radius, and the blank length was measured before bending. Springback and rounding can also make difficult measurements less reliable.
Should I use the punch radius or the finished inside radius?
Use the finished inside radius measured on the coupon. The punch radius can differ from the resulting sheet radius, especially in air bending. The equation describes the formed part, so the finished radius is the relevant value.
What is the difference between bend allowance and bend deduction?
Bend allowance is the length of material consumed through the bend along the neutral axis. Bend deduction is the amount subtracted from the two outside flange dimensions to obtain the flat length. This calculator derives both from the same test coupon.
Do I need a separate K-factor for each material and tooling setup?
Usually, yes. Material lot, thickness, grain direction, die opening, punch and formed radius, and bend method can all change the result. Keep the test conditions with the value when adding it to a bend table or CAD rule.