Workshop Reference Chart

K-Factor Chart by Material and Bend Radius

Typical K-factor values for mild steel, stainless steel and aluminium, with the neutral-axis theory and a repeatable test-bend procedure for finding your own value.

K-factor locates the neutral axis inside bent sheet metal. It is a small number with a large effect: every bend allowance, bend deduction and developed flat pattern depends on it. Published values are useful for quoting and first-off work, but K-factor is not a fixed material property like density. It represents the combined behaviour of material, thickness, inside radius, bend angle, grain direction, tooling and bending method.

The tables below are practical starting points for air bending common sheet. Select the material table, find the thickness row and then choose the column closest to the finished inside-radius-to-thickness ratio, written R/T. When a design falls between columns, interpolate rather than jumping to the next value. For close-tolerance work, use the empirical test procedure later on this page.

Typical K-factor by material and R/T ratio

Mild steel

Stainless steel

Aluminium

How to choose a value from the chart

Calculate R/T using the finished inside radius, not automatically the punch nose. For 2 mm mild steel with a 3 mm inside radius, R/T is 3 ÷ 2 = 1.5. The chart suggests K = 0.35. If the measured radius were 4 mm, R/T would be 2.0 and K = 0.38. The change looks small, but it acts on every bend.

Thickness rows are included because material behaviour and common processes change across sheet ranges. R/T remains the stronger selector: a 1 mm sheet around a 1 mm radius and a 3 mm plate around a 3 mm radius share R/T = 1, though they may not bend identically. Treat the thickness adjustment as guidance rather than artificial precision.

How K-factor comes from the neutral axis

During bending, fibres near the inside surface compress and fibres near the outside surface stretch. Between them is a layer whose developed length changes very little: the neutral axis. In flat sheet the neutral axis is near mid-thickness. As plastic bending begins, it shifts toward the inside surface because tensile and compressive strains are not symmetrical.

Here tₙ is the distance from the inside surface to the neutral axis and T is sheet thickness. If the neutral axis is 0.66 mm from the inside of 2 mm sheet, K = 0.66 / 2 = 0.33. A theoretical neutral axis at exact mid-thickness gives 0.50. Tighter bends generally pull it farther inward, creating lower values; larger R/T ratios tend toward 0.50.

The neutral-axis radius is R + K × T. Multiplying that radius by the bend angle in radians gives bend allowance:

This explains why K-factor affects flat length. It does not directly predict springback, required tonnage or minimum safe radius. Those are related forming questions, but they require material strength, temper, tooling and process information beyond neutral-axis location.

How tooling and process influence K-factor

In air bending, the sheet contacts the punch and die at three main points. Die width and penetration influence the achieved radius, while elastic recovery changes the final angle. In bottoming, the sheet is pressed more closely against the tooling. Coining uses much higher pressure and plastically sets the bend region. Each creates a different strain distribution.

Grain direction matters too. A coupon bent parallel to the rolling direction may not match one bent across it, especially in aluminium and harder material. Protective film, surface finish and coatings can slightly change thickness or friction. Batch chemistry and tensile properties introduce further variation. A useful shop database therefore identifies material specification, nominal and measured thickness, grain, machine, punch, die, angle and process alongside K.

Measure K-factor empirically from a test bend

The most reliable K-factor is calculated backwards from a simple coupon made with the actual production setup. Cut a straight blank long enough to form two measurable legs. Record its flat length before bending. Measure actual thickness with a micrometer, bend it to the target angle, then measure the finished inside radius and two outside mould-line leg lengths. Use consistent sharp-corner projections rather than measuring along the curved surface.

1. Prepare the coupon. Use production stock, maintain the intended grain direction, deburr it, and record flat length L.
2. Make the bend. Use the production press brake, punch, die opening and bend method. Allow springback to settle.
3. Measure geometry. Record final angle A, thickness T, inside radius R, and outside legs X and Y.
4. Find bend deduction. BD = X + Y − L.
5. Find outside setback. OSSB = tan(A/2) × (R + T).
6. Recover bend allowance. BA = 2 × OSSB − BD.
7. Solve K. K = [BA ÷ ((π/180) × A) − R] ÷ T.

Worked test-bend example

A 2 mm mild-steel coupon starts at 96.20 mm. After a 90° bend, the two outside legs measure 50.00 mm each. The finished inside radius is 2.00 mm. Bend deduction is 50 + 50 − 96.20 = 3.80 mm. At 90°, outside setback is tan(45°) × (2 + 2) = 4.00 mm.

Repeat the coupon at least three times and average the result. A large spread indicates inconsistent radius, angle or measurement. Test other R/T ratios rather than assuming one K-factor fits every tool. Record enough decimal places during calculation and round the production blank only at the end.

Common K-factor mistakes

• Using outside radius: formulas require the inside bend radius.
• Confusing bend angle and opening angle: verify that A is the amount of bend through which the material turns.
• Assuming K is a universal material constant: it belongs to a material-and-process combination.
• Measuring legs from inconsistent datums: outside mould lines, tangent points and bend lines produce different arithmetic.
• Rounding too soon: small per-bend errors accumulate on multi-bend parts.
• Ignoring actual thickness: coated sheet and mill tolerance can differ from nominal stock size.

Frequently asked questions