Sheet Metal K-Factor — Bend Allowance & Flat Pattern
Compute K-factor, bend allowance, and bend deduction for accurate flat pattern development. K-factor locates the neutral axis during bending. Essential for CNC brake programming, laser nesting, and first-pass fabrication accuracy.
Why This Construction Metric Matters
Why: Wrong K-factor produces incorrect flat patterns—parts don't fit, bends are off, scrap increases. Correct K ensures accurate bend allowance, proper blank size, and parts that assemble. Critical for CNC and laser nesting.
How: K = Neutral Axis / Thickness. Typical K: mild steel 0.33, stainless 0.35, aluminum 6061 0.45. BA = π × (R + K×T) × A/180. BD = 2×OSSB − BA. Flat length = Leg1 + Leg2 − BD. R/T ratio affects K.
- ●Build a K-factor chart for your brake, tooling, and materials.
- ●Springback varies by material; compensate in bend angle, not K.
- ●Use consistent units (mm or in.) for radius and thickness.
- ●Bend perpendicular to grain when possible.
Sample Projects - Click to Load
Mild Steel 90° Bend
Standard 90° bend in 0.125" mild steel
Aluminum Bracket
6061-T6 bracket with 45° bend
Stainless Enclosure
304 stainless box with 90° corners
Thick Plate Bend
0.5" thick steel with large radius
Air Bend Soft Die
Air bending with wide V-die
Bottom Bend Precision
Coining for tight tolerance part
Mild Steel (A36)
Most common structural steel, excellent formability
📏 Geometry Inputs
📐 Optional: Leg Dimensions (for Flat Pattern)
⚠️Planning estimates only. Verify with a licensed engineer or contractor before construction.
📐 Construction Industry Facts
Mild steel K ≈ 0.33; aluminum 6061 ≈ 0.45.
— ASTM
R/T < 0.5 uses lower K; R/T > 5 uses higher K.
— FMA
Bend deduction = 2×OSSB − Bend allowance.
— ASME
Verify K with test bends; document for your tooling.
— Industry
What is K-Factor in Sheet Metal Bending?
The K-factor is a ratio that represents the location of the neutral axis (the line that doesn't stretch or compress) during sheet metal bending. It's expressed as the distance from the inside surface to the neutral axis divided by the material thickness. The K-factor is critical for calculating accurate flat pattern dimensions, bend allowance, and bend deduction in sheet metal fabrication.
Neutral Axis
Location where material doesn't stretch or compress
Bend Allowance
Arc length along neutral axis
Bend Deduction
Material to subtract from flat pattern
Material Dependent
Varies by material type and R/T ratio
How to Calculate K-Factor and Bend Dimensions
K-Factor Formula
Typical K-factor values range from 0.25 (tight bends) to 0.50 (large radius bends)
Bend Allowance Formula
Where R = inner radius, T = thickness, A = bend angle
Standard K-Factor Values
When to Use Different K-Factor Methods
Standard K-Factor
Most common method using K = t/T ratio
Air Bend (Soft Die)
Material not bottomed in die, springback considered
Bottom Bend (Coining)
Material fully bottomed, minimal springback
Wipe Bend
Material wiped over edge, different neutral axis
Custom K-Factor
Enter specific K-factor value
Material K-Factor Reference
| Material | K-Factor | Range | Tensile (MPa) | Applications |
|---|---|---|---|---|
| Mild Steel (A36) | 0.330 | 0.30 - 0.40 | 400 | Brackets, Enclosures |
| Stainless Steel 304 | 0.350 | 0.32 - 0.40 | 515 | Food equipment, Architectural |
| Stainless Steel 316 | 0.360 | 0.33 - 0.42 | 515 | Chemical processing, Marine |
| Aluminum 6061-T6 | 0.450 | 0.40 - 0.50 | 310 | Aerospace, Automotive |
| Aluminum 5052-H32 | 0.430 | 0.38 - 0.48 | 228 | Marine, Sheet metal work |
| Copper (C11000) | 0.380 | 0.35 - 0.42 | 220 | Electrical, Plumbing |
| Brass (C26000) | 0.400 | 0.37 - 0.45 | 365 | Decorative, Hardware |
| Galvanized Steel | 0.340 | 0.31 - 0.39 | 400 | Outdoor structures, HVAC |
Why Accurate K-Factor Matters
Wrong K-factor produces incorrect flat patterns—parts don't fit, bends are off, and scrap piles up. Correct K-factor ensures accurate bend allowance, proper flat blank size, and parts that assemble correctly. It's essential for CNC programming, laser-cut nesting, and cost estimation.
R/T Ratio Guide
The R/T ratio (inner radius divided by material thickness) significantly affects K-factor. Smaller ratios require lower K-factors.
FAQs
Tight bends (low R/T) compress the inner material more, shifting the neutral axis inward. Large-radius bends (high R/T) behave more like pure bending with K ≈ 0.5.
K-factor is more flexible—one value works for any angle. Deduction tables are angle-specific but often provided by brake manufacturers. Use whichever your shop standardizes on.
Bend a test piece, measure the resulting dimensions, and back-calculate. Adjust K until flat pattern matches. Document for your tooling and material.
Expert Tips
- • Build a K-factor chart for your specific brake, tooling, and materials
- • Springback varies by material—account for it in bend angle, not K
- • Use consistent units: radius and thickness in same units (mm or in)
- • Grain direction affects bend—bend perpendicular to grain when possible
Common Mistakes to Avoid
- ✗ Using 0.5 for all materials—only accurate for large R/T
- ✗ Ignoring R/T ratio—same material needs different K for different bends
- ✗ Mixing inner radius with outer radius in formulas
- ✗ Assuming K is constant across different brake setups