What Bend Deduction means
Bend deduction is the mirror image of bend allowance. Where bend allowance is measured along the neutral axis and added to the flat legs, bend deduction is measured against the outside dimensions of the finished part and subtracted from them.
Why it matters on the shop floor
The two describe the same physical correction from different reference points, and they must stay consistent with each other: a drawing dimensioned to outside dimensions, a flat pattern derived from a contradictory corner, is one of the most common causes of a first-article dimensional miss on a bent part.
In practice
Bend deduction is the more convenient number when a drawing is dimensioned to outside mould lines, which is how most enclosure and bracket drawings arrive. It grows with thickness and with the ratio of radius to thickness.
Formula
BD = 2 × OSSB − BA (OSSB = outside setback = (R + T) × tan(θ/2))R = inside bend radius · T = thickness · θ = bend angle · BA = bend allowance
What to remember
- Use one convention for the whole part: dimension to outside mould lines and subtract bend deduction, or dimension to the neutral axis and add bend allowance — never mix the two on one drawing.
- Bend deduction is always positive for acute and 90° bends; for very shallow bends the setback can approach zero.