Multi-Bend Flat Pattern (Developed Length)
The flat blank length for a sheet-metal part with several bends: flat = mold-line - n_bends x BD, the sum of the outside (mold-line) flange dimensions minus the bend deduction per bend (from bend-allowance). A U-channel of 2 + 4 + 2 = 8 in mold-line with 2 bends at 0.1355 in BD develops to 7.73 in; a hat section of 12 in mold-line over 4 bends develops to 11.46 in -- more bends pull more material out of the blank. A layout aid; confirm the first part against a test bend, since the real BD shifts with tooling, material, and grain.
Formula and source
flat = mold_line - n_bends x BD; the per-bend bend deduction BD = 2 x OSSB - BA comes from the bend-allowance geometry.
Machinery's Handbook (sheet-metal bend geometry) by name; SME Sheet Metal Forming by name; press-brake developed-length practice.
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