Kfactor Calculator

When sheet metal is bent, the outer surface stretches and the inner surface compresses. Somewhere between them lies a neutral axis whose length changes very little, and the K-factor simply states where it sits: the distance from the inside face of the bend to that axis, divided by material thickness. It is bounded by 0 and 0.5, since the neutral axis cannot migrate past the geometric centre of the sheet. Its value drives the bend allowance through BA = (bend angle in radians) times (inside radius plus K times thickness), which is the arc length the flat blank must supply to form the bend. The dominant variable is the ratio of inside radius to thickness. In a tight bend with R/t below 1, compression on the inside is severe and the neutral axis migrates inward, giving a K-factor near 0.33. As R/t rises past about 4 the strain distribution becomes nearly symmetric and K approaches 0.50. Material plays a secondary role: annealed aluminium and copper tend to run low, mild steel sits around 0.40 to 0.45, and stainless runs higher because of its springback behaviour. The honest caveat is that the K-factor is an empirical parameter absorbing springback, tooling geometry, die opening width, and grain direction all at once. Published tables get a shop close, but production accuracy comes from bending a test coupon, measuring the finished legs, and back-calculating the K-factor for that specific material, thickness, and press brake tooling combination.

Flat pattern development is as old as sheet metal work itself, but for centuries it was a layout craft rather than a calculation. Tinsmiths and boilermakers developed blanks by geometric construction on a layout table and allowed for the bend by rule of thumb, adjusting on the next piece if the first came out long or short. The press brake, which appeared in the late nineteenth century and spread widely in the early twentieth, made bends repeatable enough that the allowance itself became worth computing rather than guessing. Systematic bend allowance tables came from the aircraft industry, where thousands of formed aluminium parts had to be blanked accurately before assembly and scrapped material was expensive. Airframe manufacturers and military services published bend allowance charts indexed by thickness, radius, and angle through the 1940s, and equivalent tables entered general references such as Machinery's Handbook. The neutral axis position implicit in those charts was later expressed as the dimensionless K-factor so that it could be carried across unit systems and materials. Computer aided design made the parameter central: when solid modellers gained dedicated sheet metal environments in the 1980s and 1990s, unfolding a part required an explicit rule, and K-factor, bend allowance, and bend deduction became standard user inputs. The related Y-factor, defined as K multiplied by pi divided by 2, appears in some European and aerospace software for exactly the same purpose.