Bend Allowance vs Bend Deduction — Which to Use

The Decision Fork in One Minute

Bend Allowance — Working Outward From the Flat

Workflow: Establish flange lengths to the mold line (often inside), compute BA for each bend from angle, inside radius, thickness, and K, then sum flanges + allowances into one blank. Additive thinking: “how much stock does this bend consume?”

Where it shines: CAD unfolders, nesting from flats, and prints that already speak in developed length. Use the Bend Allowance Calculator when that is your shop’s language.

Watch-outs: If QC measures outside overall length, convert carefully — don’t paste BA into a BD-style sum of outside legs.

Bend Deduction — Working Inward From Finished Flanges

Workflow: Take the two outside flange lengths from the formed part, compute outside setback and BA, then BD = 2 × OSSB − BA. Subtract BD from the sum of outside legs to get the blank. Subtractive thinking: “how much do I pull out of the outside lengths?”

Where it shines: Hand layout from outside-dimension prints, many press-brake chart books, and shops that already stock BD tables by material and V-die. Use the Bend Deduction Calculator for that path.

Watch-outs: Wrong setback (inside vs outside) is the classic short-flange failure — see Sheet Metal Bending Troubleshooting.

What Shops and Software Usually Default To

• CAD sheet-metal packages — typically unfold with K / BA under the hood; the UI may still show BD for drafting convenience.
• Traditional brake shops — often talk BD and setback because outside flanges are what the print and the square call out.
• OEM tooling charts — frequently list BD (and recommended V-opening) for a thickness, not a lecture on neutral axis.

Neither convention is “more correct.” Match the method to the dimensioning system on the print and the tool your estimator already trusts. Gauge thickness for either path: Sheet Metal Gauge Chart.

Common Mistakes Mixing BA and BD on One Drawing

• Summing outside legs then adding BA — double-counts the bend zone; blank runs long.
• Subtracting BD from inside mold-line flanges — under-cuts the blank; flanges come short.
• Copying a BD number from a chart into a CAD BA field — same digits, different meaning.
• Changing K mid-job without regenerating BD — BA and BD both move; updating only one leaves the blank wrong.

Quick Selection Matrix