What is K-factor in sheet metal bending?
K-factor is the ratio of the neutral axis distance from the inner radius to the material thickness (K = t_neutral / thickness). During bending, the inside material compresses and the outside stretches. The neutral axis is the line where neither tension nor compression occurs. A K-factor of 0.38 means the neutral axis sits 38% of the way through the material, which directly affects bend allowance and flat blank length. Published values range from 0.33 (tight r/t < 1) to 0.50 (theoretical centerline, never achieved in practice).
How do I measure K-factor for my specific tooling?
Bend a test piece of the exact material and thickness. Measure: flat blank length before bending (L₀), and both leg lengths after bending (A and B). Then: K = (L₀ − A − B + 2×OSSB) / (π/180 × angle × thickness), where OSSB = tan(angle/2) × (radius + thickness). Most shops use published Machinery's Handbook values as a starting point and dial in their actual K from test bends. For production runs with tight tolerances, always run a test piece first.
Why does my flat pattern come out wrong?
Four common causes: (1) Wrong K-factor — using 0.5 (centerline) instead of your actual K causes every bend to be off. (2) Using BA when you need BD — bend allowance and bend deduction are different. You lay out a flat pattern using BD, not BA. (3) Not compensating for springback — the part springs back after the die lifts; you need to over-bend. (4) Tooling method changes K — air bending and bottom bending produce different K values on the same material. This calculator uses air-bend values from Machinery's Handbook by default.
What is the difference between bend allowance, bend deduction, and OSSB?
Bend Allowance (BA) is the arc length along the neutral axis — how much material the bend actually consumes. Bend Deduction (BD) is how much to shorten the flat blank so the formed part comes out to the correct outside dimensions. BD = 2 × OSSB − BA, where OSSB (Outside Set-Back) is the distance from the bend tangent line to the mold line intersection. In practice: you use BD for flat pattern layout. Each bend requires one BD to be deducted from the sum of leg lengths.
What K-factors should I use for different materials?
Published air-bend starting values for r/t 1–3 (most common shop scenario): Mild steel 0.38, Stainless steel 0.34 (work-hardens more, neutral axis shifts inward), Aluminum 0.41 (soft alloys allow more plastic flow outward), Galvanized steel 0.38 (same as mild steel). Tighter radii (r/t < 1) lower K; larger radii (r/t > 5) raise K toward 0.45. This calculator auto-selects the K from the material + r/t combination you enter.
How much springback should I expect, and how do I compensate?
Springback (elastic recovery after press brake lifts) varies by material and r/t ratio. Approximate values: Mild steel 2–6°, Stainless steel 5–10° (most problematic — needs the most over-bending), Aluminum 4–9°. Compensation: set the die angle to (target angle − springback). For 90° with 4° springback, set the die to 86°. Stainless at tight r/t can need 10°+ of over-bend. The estimate shown by this calculator is a starting point — verify with a test bend.
Can I use this for multi-bend parts like channels and boxes?
Yes. Add a row for each bend in the table. For a U-channel with 2 bends, enter 3 leg lengths and 2 bend rows. For a 4-sided box enclosure, enter 4 bends and 5 legs (or 4 legs for a closed top). The calculator sums all legs and deducts each bend's BD, giving you the total flat blank length in one shot. Each bend can have a different angle, radius, and K-factor override if your part geometry varies.
How does IronKit calculate flat blank length?
Flat blank = sum of all leg lengths − total bend deduction. For each bend: BA = (π/180) × angle × (radius + K × thickness); OSSB = tan(angle/2) × (radius + thickness); BD = 2 × OSSB − BA. K-factor is looked up from the Machinery's Handbook table based on material and r/t ratio (inside radius ÷ thickness). You can override K per bend if you've measured your tooling. The formulas are per Sheet Metal Handbook (Oberg, Jones, Horton) and Machinery's Handbook 31st edition.