Master the calculations behind sheet metal flat pattern development. Learn about bend allowance, bend deduction, K-factor, and how to calculate accurate blank sizes.
Introduction
Accurate flat pattern calculation is essential for producing bent parts that meet specifications. At Fulei Metal, our CNC software handles calculations automatically, but understanding principles helps verify results.
Key Concepts
Bend Allowance (BA)
The length of material consumed in the bend, measured along the neutral axis.
Bend Deduction (BD)
The amount subtracted from outside dimensions to calculate flat pattern length.
K-Factor
Ratio of distance from inside surface to neutral axis, divided by thickness. Ranges from 0 to 0.5. Typical: mild steel 0.33, stainless 0.44, aluminum 0.33.
Calculating Bend Allowance
BA = 0.017453 x Bend Angle x (Inside Radius + K-factor x Thickness)
Example: 90-degree bend, 2 mm steel, 2 mm radius, K=0.33. BA = 0.017453 x 90 x (2 + 0.66) = 4.18 mm.
Calculating Bend Deduction
BD = 2 x Outside Setback – Bend Allowance
Where Outside Setback = Tan(Angle/2) x (Radius + Thickness).
Example: OSSB = Tan(45) x 4 = 4.0 mm. BD = 8.0 – 4.18 = 3.82 mm.
Calculating Flat Pattern
Using BA: Flat = Inside Flange 1 + BA + Inside Flange 2. Using BD: Flat = Outside Flange 1 + Outside Flange 2 – BD. Both give the same result.
K-Factor Values
Mild steel air bending: 0.33. Stainless air bending: 0.44. Aluminum soft: 0.33. Aluminum hard: 0.40. Bottom bending: higher than air bending.
Determining K-Factor Experimentally
Cut a known-length blank, bend to 90 degrees, measure flanges, calculate actual BA, back-calculate K-factor. This gives the most accurate value.
Practical Considerations
CNC software calculates automatically. For multiple bends, sum all straight sections and bend allowances. Formulas work for any angle. Variable radii require separate calculations.
Common Errors
Wrong K-factor, incorrect inside radius, angle measurement errors, unit errors.
Verification
First article measurement after bending. If parts are consistently too long, decrease K-factor. If too short, increase K-factor.
Conclusion
Accurate calculations are fundamental to sheet metal fabrication. Modern software handles them automatically, but understanding principles helps verify results. At Fulei Metal, our CNC systems ensure every flat pattern is correct.