Why do parts come out oversized if I do not account for bend deduction?

Because metal physically stretches in tension around the outer radius during bending. If you cut a flat blank by simply adding Outside Flange 1 + Outside Flange 2 (e.g. 2" + 3" = 5"), the stretched metal will cause the finished part to measure 5.10" or 5.15", ruining precision tolerances.

How do I determine the actual K-factor of my shop press brake?

Cut a scrap test coupon of known length (e.g. exactly 4.000"). Bend it at 90 degrees. Measure the two outside legs with calipers (e.g. 2.120" and 2.120"). The actual Bend Deduction is (2.120 + 2.120) - 4.000 = 0.240". Plug that BD back into the formula to solve for your machine exact K-factor.

What is the 8x rule for press brake V-die opening selection?

For air bending mild steel up to 1/4" thick, the industry gold standard is selecting a bottom V-die opening equal to 8 times the material thickness (V = 8 * T). Using an undersized die increases required tonnage and causes punch indentations.

Why does grain direction matter when bending sheet metal?

Sheet metal has microscopic rolling grains from the mill. Bending parallel to the grain creates high stress concentrations that crack the outer radius. Bending perpendicular (or at 45 degrees) to the grain allows the metal to stretch smoothly without fracture.

What is Outside Setback (OSSB)?

OSSB is the distance from the intersection of the two outside flange tangent lines (the apex corner point) to the beginning of the curved bend radius line on the outside surface: OSSB = tan(Angle / 2) * (Radius + Thickness).

Free Sheet Metal Bend Allowance & Deduction Calculator

📐 Sheet Specs & Bend Geometry

Material & Typical K-Factor

Material Thickness T (in)

Inside Bend Radius R (in) Punch tip radius (e.g. 1/16" = 0.0625")

Bend Angle (°) Included angle

Flange 1 (in) Outside leg 1

Flange 2 (in) Outside leg 2

⚡ Blank Layout & Deductions

Flat Blank Length -- Total cut length before bending

Bend Deduction (BD) -- Subtract from (L1 + L2)

Bend Allowance (BA) --

Outside Setback (OSSB) --

Recommended V-Die Opening --

Minimum Flange Height --

Estimated Press Brake Tonnage --

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The Mechanics of Sheet Metal Plastic Deformation

When flat sheet metal is bent over a press brake die, the material undergoes both tensile and compressive stresses. The outer fibers of the bend stretch in tension, while the inner surface compresses. Somewhere between the outer stretched skin and inner compressed skin lies the Neutral Axis—a theoretical plane that neither stretches nor compresses.

The K-Factor: Locating the Neutral Axis

The K-factor represents the position of the neutral axis as a fraction of total material thickness (T):

K-Factor = Distance from Inside Surface to Neutral Axis / Sheet Thickness T

In unstressed sheet, the neutral axis sits at the 50% midpoint ((K = 0.50)). However, during air bending, compression displaces the neutral axis inward, causing (K) to range from 0.33 to 0.45 depending on material hardness and ductility.

Bend Allowance (BA) vs. Bend Deduction (BD)

Two methods exist to calculate the flat pattern blank:

Bend Allowance (BA): The actual curved arc length along the neutral axis around the bend radius: BA = (pi / 180) × Angle × (Radius + K × T). Flat Length = Flat Flange 1 + Flat Flange 2 + BA.
Bend Deduction (BD): The amount by which total outside flange lengths must be reduced to account for elongation: BD = (2 × OSSB) - BA. Flat Length = Outside Flange 1 + Outside Flange 2 - BD.