Bend allowance is the length of the neutral axis between the bend lines in sheet metal bending. It represents the arc length of the bend measured along the neutral axis, accounting for material stretching and compression during the bending process.

Understanding bend allowance is critical in sheet metal fabrication to calculate accurate flat pattern dimensions before bending. Without proper bend allowance calculations, parts will be undersized or oversized after bending, leading to scrap and rework.

The K-factor represents where the neutral axis lies within the material thickness during bending:

• K = 0.33: Typical for soft materials (aluminum, soft steel) - neutral axis at 1/3 thickness from inside.
• K = 0.5: Common for hard materials (stainless steel, spring steel) - neutral axis at mid-thickness.
• K = 0.4-0.45: Medium hardness materials (mild steel, brass).
• Material dependency: K-factor varies with material properties, grain direction, and bend radius.
• Testing required: Precise K-factors are determined through bending tests for specific materials and processes.

1. Select units: Choose millimeters (mm) or inches (in) for your measurements.
2. Enter material thickness: Input the sheet metal thickness.
3. Input bend angle: Enter the angle of the bend (0-180 degrees, typically 90).
4. Set inside radius: Enter the inside bend radius (often equals punch radius).
5. Adjust K-factor: Use 0.33 for soft materials, 0.5 for hard, or custom value.
6. Click Calculate: Get bend allowance, bend deduction, setback, and arc length.
7. Apply to flat pattern: Use bend allowance to calculate developed length for fabrication.

Bend allowance calculations are essential for:

• Flat Pattern Development: Calculating unfolded sheet metal dimensions before bending.
• Press Brake Operations: Programming CNC press brakes with accurate bend parameters.
• CAD/CAM Design: Creating accurate sheet metal models with proper bend compensation.
• Bracket Fabrication: Manufacturing brackets, enclosures, and structural components.
• HVAC Ductwork: Sizing duct sections for proper fit after bending.
• Automotive Panels: Forming body panels and chassis components.
• Electronics Enclosures: Creating boxes, chassis, and mounting plates.
• Quality Control: Verifying part dimensions match design specifications.

Two methods exist for calculating flat patterns:

• Bend Allowance (BA): Material added to account for the bend. Flat length = L1 + L2 + BA, where L1 and L2 are leg lengths.
• Bend Deduction (BD): Material subtracted from total length. Flat length = L1 + L2 - BD, where L1 and L2 are mold line dimensions.
• Which to use: BA is preferred when working from leg dimensions; BD when working from mold line dimensions.
• Relationship: BD = 2 × OSSB - BA, so both methods yield the same flat length.
• Industry preference: North American shops often use BD; European shops often use BA.

Scenario: 90-degree bend in 2mm thick aluminum with 1mm inside radius, K-factor 0.33.

1. BA = (3.14159/180) × (1 + 0.33 × 2) × 90 = (0.01745) × (1.66) × 90 = 2.607 mm
2. OSSB = tan(90/2) × (1 + 2) = tan(45) × 3 = 1 × 3 = 3 mm
3. BD = 2 × 3 - 2.607 = 6 - 2.607 = 3.393 mm
4. For 50mm + 50mm leg lengths: Flat = 50 + 50 + 2.607 = 102.607 mm
5. Or using mold lines (53mm + 53mm): Flat = 53 + 53 - 3.393 = 102.607 mm (same result)

Typical values for sheet metal bending:

• Minimum bend radius: Generally equals material thickness (1T) for soft materials, 2T-3T for hard.
• Standard angles: 90° most common; 45°, 60°, 120°, 135° also frequent.
• Thickness range: 0.5mm to 6mm for typical sheet metal; thicker requires specialized equipment.
• Tolerance: Bend angle typically ±0.5° to ±1°; leg length ±0.1mm to ±0.5mm depending on process control.
• Springback: Material returns slightly after bending; compensate by over-bending 2-5 degrees.