Sheet Metal DXF Unfolding, K-Factor & Press Brake Prepress Guide

1. The Physics of Sheet Metal Bending & Neutral Axis Shift

When a flat metal blank is bent around a press brake punch and V-die, the inner fibers experience intense plastic compression while the outer fibers undergo extreme tensile stretching. Somewhere between the compressed inner face and the elongated outer face lies the Neutral Axis—the theoretical plane that experiences zero length change during the bending cycle.

The position of this neutral axis is represented mathematically by the K-Factor ($K$):

2. Core Mathematical Formulas: BA, OSSB & BD

To produce an unfolded flat blank that folds precisely into the desired 3D enclosure dimensions, 3 interdependent values must be calculated:

Bend Allowance (BA)

The actual arc length of the material along the shifted neutral axis:

Outside Setback (OSSB)

The linear distance from the theoretical sharp apex of the outside flange intersection to the tangent point where the curvature starts:

Bend Deduction (BD)

The total amount subtracted from the sum of the outside flange dimensions to obtain the true unfolded flat blank length ($L_{\text{flat}}$):

3. Critical Corner Relief Geometry in DXF Layouts

Whenever two perpendicular flanges fold adjacent to each other, the compressed material at the corner intersection will buckle, bulge, or tear the metal unless an intentional Corner Relief Notch is programmed into the 2D DXF vector cut path.

• Circular Hole Relief: Place a laser-cut hole at the bend line intersection with diameter $D \ge T + R$.
• Rectangular Tear Relief: Cut a slot with width $W \ge T$ and depth extending at least $1.0\text{ mm}$ past the tangent bend line.
• Smooth Radius Fillets: Never leave a sharp 90° internal corner on a flat blank cut path. Always apply a minimum fillet radius $R_{\text{fillet}} \ge 0.5 \times T$ to eliminate stress concentration and laser micro-cracking.

4. Press Brake Air Bending Tonnage & Die Sizing

In air bending, the sheet rests on the shoulders of the V-die and the punch descends to form the angle without bottoming out. The required machine tonnage per linear meter is governed by the V-die opening width ($V$):

5. Standard DXF Layer Convention for Automated CAM

When preparing unfolded sheet metal DXF files for laser nesting (Trumpf TruTops, Bystronic BySoft, Amada VPSS), organize your vector geometry into clear CAD layers:

• 0_OUTER_CUT (Continuous White/Blue): 100% closed outer perimeter of the flat blank.
• INTERNAL_PIERCE (Green): Closed inner cutouts, mounting holes, and bolt circles.
• BEND_UP_90 (Dashed Yellow/Orange): Centerline of the bend arc with direction and angle attributes.
• ETCH_MARK (Red): Low-power laser surface etching for part numbering, grain direction, or bend alignment ticks.