Bend allowance and bend deduction do the same job from opposite directions: both get you from a 3D part to a correct flat blank. Bend allowance is the arc length of the bend measured along the neutral axis, and you add it to your flange lengths. Bend deduction is the material you subtract from the outside flange lengths to reach the same flat size. Designers and CAD tend to use allowance. The shop floor tends to use deduction, because operators measure outside dimensions. Here are both formulas, a worked example, and how to pick.
Bend allowance (BA) is the length of the bent region along the neutral axis. You add it to the straight (tangent) leg lengths to get the flat:
Flat length = Leg A + Leg B + BA
The formula is BA = (π / 180) × A × (R + K × T), where A is the bend angle in degrees, R is the inside radius, K is the K-factor and T is the thickness. For a common shortcut The Fabricator gives BA = [(0.017453 × inside radius) + (0.0078 × thickness)] × bend angle, which is the same relationship with the constants worked out.
Bend deduction (BD) is what you take off the sum of the outside flange lengths, the ones measured out to the sharp apex, to reach the flat. You subtract it:
Flat length = Leg A + Leg B − BD
The formula is BD = 2 × (R + T) × tan(A / 2) − BA. The first part, (R + T) × tan(A / 2), is the outside setback (OSSB), the distance from the tangent point out to the apex. So bend deduction is just twice the outside setback minus the bend allowance.
Say you bend 2 mm mild steel to 90 degrees with a 2 mm inside radius and a K-factor of 0.44.
Two legs drawn to the inside tangent at 30 mm each give a flat of 30 + 30 + 4.52 = 64.52 mm. The same part drawn to the outside apex at 34 mm each gives 34 + 34 − 3.48 = 64.52 mm. Same blank, two routes. (Recompute for your own K-factor and radius, these numbers are an illustration.)
Use whichever matches how the part is dimensioned. CAD models and neutral-axis work lean on bend allowance. Press brake setups and shop drawings lean on bend deduction, because the operator is measuring to the outside of the part, not to an imaginary tangent line. Most CAD systems will give you either, and a bend deduction table per material and thickness is what most shops keep by the machine.
Bend allowance is added to the inside (tangent) flange lengths and equals the neutral-axis arc length of the bend. Bend deduction is subtracted from the outside (apex) flange lengths and equals twice the outside setback minus the bend allowance. Both produce the same flat blank, they just start from different dimensions.
Use BD = 2 × (R + T) × tan(45°) − BA, and since tan(45°) = 1 that simplifies to BD = 2 × (R + T) − BA. Plug in your inside radius, thickness and bend allowance. For 2 mm steel at a 2 mm radius and K = 0.44 it works out near 3.5 mm.
Yes. The K-factor sets the bend allowance, and the bend deduction is built from the bend allowance, so the same K-factor drives both. Dial in your K-factor from a test bend first, then the rest follows.