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Bend allowance vs bend deduction: when to add or subtract

Solve the same right-angle bend with tangent lengths and outside dimensions, and avoid applying the unfolding compensation twice.

By LinkReady · Updated

Check where each dimension ends

BA is the neutral-axis arc length through the bend. With straight lengths a and b ending at the arc tangent points, flat length is a + b + BA. With outside dimensions A and B extending to the virtual sharp, it is A + B − BD. These pairs describe different segments.

Two routes, one part

Assume one uniform-thickness bend: T = 2 mm, formed inside R = 3 mm, angle turned from flat θ = 90°, illustrative K = 0.4, and outside A = 60 mm, B = 40 mm.

BA = (π/2) × (3 + 0.4 × 2) ≈ 5.969 mm

SB = (R + T) × tan(45°) = 5 mm

BD = 2 × SB − BA ≈ 4.031 mm

Outside route: L = 60 + 40 − 4.031 ≈ 95.969 mm

Tangent route: a = 55, b = 35; L = 55 + 35 + 5.969 ≈ 95.969 mm

Avoid double compensation

After subtracting SB from each outside dimension, you have a and b: add BA only. Subtracting BD again deducts compensation twice. Similarly, do not manually subtract another deduction from a flat pattern already produced with a validated bend table.

With identical geometry and parameters, both routes must return the same L. A discrepancy is a reason to check dimensions and units before rounding.

Use a repeatable pre-cut check

  1. Label outside or tangent dimensions on the drawing.
  2. Confirm angle, formed inside radius, thickness, and the chosen K or bend table.
  3. Check the arithmetic using the worked example or web calculator.
  4. For additional mobile unfolding tasks, review SheetMetal Master’s supported workflows and validate its results with your process data.

Scope and references

This example excludes springback compensation, interacting bends, and seams. K = 0.4 is not a universal material value. See the dimension conventions in SOLIDWORKS bend allowance and deduction help and methods in Inventor custom unfold equations.