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.
A Worked Example end to end
Formulas are easier to trust once you have seen them run on a real part. The example below is a 2.0 mm SPCC bracket with a single 90° bend, formed in a 16 mm die. Both flanges are dimensioned on the outside faces, which is how most drawings arrive.
| Step | Calculation | Result |
|---|---|---|
| Inside radius set by the die | R = 0.156 × V = 0.156 × 16 mm | 2.5 mm |
| K-factor for this radius-to-thickness ratio | R is about 1.25 × t, so K ≈ 0.40 | 0.40 |
| Bend allowance | BA = 90 × (π / 180) × (R + K × t) = 1.571 × (2.5 + 0.8) | 5.18 mm |
| Outer setback | OSSB = tan(45°) × (R + t) = 1 × 4.5 | 4.5 mm |
| Bend deduction | BD = 2 × OSSB − BA = 9.0 − 5.18 | 3.82 mm |
| Flat length for 40 + 30 mm outside flanges | L = 40 + 30 − BD | 66.18 mm |
The K-factor is the part of this that cannot be looked up reliably. Typical values sit near 0.33 for a radius below the material thickness, around 0.38 to 0.42 when the radius is one to two times thickness, and approach 0.5 for larger radii. We refine it empirically per material batch using test bends, because two parts cut from different coils can differ enough to move the flat pattern by a tenth of a millimetre — which matters when a hole sits near a bend.
Frequently Asked Questions
Should the drawing dimension flanges inside or outside?
Whichever way your inspection will check them, as long as it is stated. We convert either way; what causes problems is a drawing dimensioned outside while someone inspects inside.
Do you use bend allowance or bend deduction?
Both, because they answer different questions. Deduction is used when you start from outside dimensions, allowance when you are building the flat from the neutral axis. Either leads to the same flat length if the inputs agree.
Why do two suppliers quote different flat lengths for the same part?
Different assumed K-factors and different die openings. Neither is wrong until the die is fixed — which is why we confirm the tool set before releasing a program.
Questions about a specific part are usually faster to answer against the drawing — send it through the review route below.
Before you send the RFQ. Reference radius assumptions behind the allowance first if this part is still at drawing stage — most cost drivers on a bent part are set there. For anything already specified, flat pattern development is where to send it. Related: bending theory and K-factor and drawing review before quoting.