Sheet Metal Fabrication CAM
Sheet Metal Corner Tear Relief & Flat Blank Prepress Guide
When two sheet metal flanges meet at a 90° corner, bending them without a corner relief notch induces severe triaxial stress at the apex. This guide details how to calculate, position, and export clean DXF relief geometry to guarantee zero-crack press brake results.
1. The Physics of Corner Tearing
During press brake forming, sheet metal elongates on the outer bend radius while compressing on the inner radius. Where two orthogonal bends intersect, material from both flanges tries to occupy the exact same spatial coordinates. Without a relief notch, the localized stress concentration factor ($K_t$) spikes above 4.0, causing immediate outer fiber tearing or corner wrinkling.
The Golden Relief Formula
Minimum Relief Diameter (D_r) ≥ Inside Bend Radius (R_i) + Sheet Thickness (T)
For heavy gauge stock or low-ductility alloys (such as Aluminum 6061-T6), scale diameter to D_r ≥ 2.5T.
2. Relief Notch Geometry Types Compared
- Circular / Round Relief: The industry standard. A laser or punch cut hole centered at the intersection of the two bend tangent lines. Offers smooth stress distribution ($K_t pprox 1.15$).
- Teardrop Relief: An expanded 45° conical transition. Essential for heavy-duty structural brackets and press brake tooling with acute air dies ($K_t pprox 1.05$).
- Rectangular / Square Notch: The easiest to punch on turret presses with standard square dies, but leaves sharp interior $90^\circ$ corners that act as stress risers ($K_t pprox 2.20$).
- Obround / Slotted Relief: An elongated oval oriented along the $45^\circ$ corner bisector, providing extra clearance for weld seam backings.
3. Flat Blank DXF Layer Standards
When exporting flat patterns from CAD/CAM software to laser nesting programs (Trumpf TruTops, Bystronic BySoft, SigmaNEST):
LAYER 0: Outer Cut Perimeter (Closed polyline DXF with relief notches)
LAYER BEND_UP: Green dashed centerline (90° Up bend + Deduction data)
LAYER BEND_DOWN: Red dashed centerline (90° Down bend)
LAYER MARK: Text etching for part numbers and bend sequence
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