What is the K factor in sheet metal bending?
The K factor describes where the neutral axis sits inside the material thickness during bending. It is expressed as a fraction of the thickness, measured from the inside surface. A K factor of 0.44 means the neutral axis is 44 percent of the thickness in from the inside face.
How do I calculate the K factor from a trial bend?
Measure the bend allowance that actually produced the correct flat length, then solve K = (BA / θrad − R) / t. The bend angle must be converted to radians first. Comparing the result with the value you assumed shows how far the estimate was out.
| Step | Calculation | Result |
|---|---|---|
| Angle in radians | 90 × π / 180 | 1.5708 rad |
| Assume neutral radius | R + 0.44t = 2.6 + 0.88 | 3.48 mm |
| Bend allowance | 1.5708 × 3.48 | 5.47 mm |
| Measured on the part | — | 5.00 mm |
| Solve for K | (5.00 / 1.5708 − 2.6) / 2 | 0.292 |
Why is the K factor not a fixed number?
The neutral axis shifts outward as the bend radius grows relative to the thickness, and it also depends on the material, the temper and how much the section is stretched. That is why the same material gives different K factors for a tight radius and a generous one.
Does the die opening change the K factor result?
It changes the inside radius, and the radius changes the K factor. Because the die opening largely sets the radius in air bending, changing the die will move the K factor even if the material is unchanged.
Related technical pages
Bend allowance calculator
Bend allowance, bend deduction and flat length from thickness, radius and angle.
Bend allowance guide
Setback, neutral axis and when each method applies.
V-die opening selection chart
The opening that sets the inside radius in air bending.
Minimum bend radius
The tightest radius the material can take without cracking.
Next step
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