Sheet Metal Forming: 20 important points:
1️⃣ Yield Strength (σy)
The stress at which a material begins to deform plastically. Crucial to predict forming limits and springback.
2️⃣ Tensile Strength (UTS)
Max stress the material can withstand before failure. Used to define forming severity.
3️⃣ Elongation %
Indicates ductility. Higher elongation = better formability (e.g., ~35% for deep drawing steel).
4️⃣ Strain Hardening Exponent (n-value)
Defines material’s ability to uniformly deform. High n-value improves stretchability.
5️⃣ Anisotropy Ratio (r-value)
Ratio of width strain to thickness strain. High r-value = good drawability.
6️⃣ Forming Limit Diagram (FLD)
Graphical tool used to predict necking/failure zones during stamping.
7️⃣ Thickness Reduction (%)
Calculated during stretching:
Δt% = [(t₀ – t₁)/t₀] × 100
Critical to track material thinning beyond safe limits.
8️⃣ Drawing Ratio (DR)
DR = Blank Diameter / Punch Diameter
Safe range: 1.8–2.2 for deep draw. Exceeding = risk of tearing.
9️⃣ Bend Allowance (BA)
BA = (π/180) × Angle × (R + K × t)
Used to compute flattened length in bending operations.
🔟 Bend Deduction (BD)
Amount by which the total length is reduced after bending. BD = FL – (Leg A + Leg B)
1️⃣1️⃣ K-Factor
Ratio of distance from neutral axis to thickness. Used in flat pattern calculations. Typical K = 0.3–0.5 for mild steel.
1️⃣2️⃣ Springback Angle
Rebound angle after unloading the bend. Dependent on yield strength, tooling radius, and material type.
1️⃣3️⃣ Blank Holding Force (BHF)
BHF = 0.015 × σy × t × D × μ
Prevents wrinkling during deep draw. Must be optimized.
1️⃣4️⃣ Punch Force (F)
F = Perimeter × Thickness × Shear Strength
Used in blanking or piercing operations.
1️⃣5️⃣ Deep Drawing Force (F)
F = π × d × t × σy × ln(d/dp)
Where d = blank dia., dp = punch dia. Used to design press capacity.
1️⃣6️⃣ Stretch Forming Strain
ε = ln(Lf / Lo)
Used to calculate strain in hemispherical or free-form stretching.
1️⃣7️⃣ Wrinkling Analysis
Controlled via BHF, draw bead design, and corner radius optimization.
1️⃣8️⃣ Necking Criteria
Based on Considère Criterion: dσ/dε = σ
Important for predicting failure before fracture.
1️⃣9️⃣ Tool Radius Design
Punch radius ≥ 5t, Die radius ≥ 8t (t = sheet thickness)
Improves flow, avoids cracks.
2️⃣0️⃣ Material Utilization Efficiency
% Efficiency = (Final part area / Blank area) × 100Critical in cost analysis and nesting strategy.