Introduction
Sheet Metal Forming involves forming and cutting operations performed on thin metal sheets, strips, and coils.
Characteristics
- Thickness typically 0.4 mm to 6 mm
- High surface area to volume ratio
- Operations performed at room temperature (usually)
- High production rates
- Low material waste
Classification of Sheet Metal Operations
1. Cutting Operations
- Shearing, Blanking, Punching, Piercing, Trimming, Notching
2. Bending Operations
- V-bending, Edge bending, Flanging, Hemming
3. Drawing Operations
- Deep drawing, Shallow drawing, Redrawing
Cutting Operations
Shearing
Shearing = Cutting operation where sheet metal is cut along a straight line
Process:
- Upper blade (punch) moves down
- Lower blade (die) is stationary
- Material experiences shear stress
- Fracture occurs along shear plane
Shear Force: F = S × t × L
Where:
- S = Shear strength of material (MPa)
- t = Sheet thickness (mm)
- L = Length of cut (mm)
Clearance (c):
- Gap between punch and die
- Typical clearance = 5-10% of sheet thickness
- Too small → excessive force, poor edge quality
- Too large → burrs, rough edge
Blanking and Punching
Blanking: Cutting operation where the cut piece (blank) is the desired part
- Blank = Product
- Scrap = Remaining sheet with hole
Punching: Cutting operation where the hole is desired
- Hole = Product
- Slug = Scrap piece removed
Punch Force: F = S × t × P
Where:
- P = Perimeter of cut (mm)
Die Clearance: c = 0.075 × t (for soft materials) c = 0.10 × t (for hard materials)
Other Cutting Operations
Piercing: Creating holes (similar to punching)
Notching: Cutting out a portion from edge
Trimming: Removing excess material from edges
Parting: Separating a part from the remaining strip
Slitting: Making lengthwise cuts
Bending Operations
Bending Fundamentals
Bending = Forming operation where sheet metal is deformed along a straight axis
Key Parameters:
- Bend angle (α): Angle through which material is bent
- Bend radius (R): Inside radius of bend
- Bend allowance (BA): Length of neutral axis in bend region
Bend Allowance
Bend Allowance Formula: BA = (π/180) × α × (R + k×t)
Where:
- α = Bend angle (degrees)
- R = Inside bend radius
- t = Sheet thickness
- k = Factor (typically 0.33 for R < 2t, 0.5 for R > 2t)
Simplified (for 90° bend): BA ≈ 1.57 × (R + 0.5t)
Minimum Bend Radius
Minimum Bend Radius: Smallest radius that can be bent without cracking
Factors affecting minimum bend radius:
- Material ductility (more ductile → smaller radius)
- Sheet thickness (thicker → larger radius)
- Bend direction relative to rolling direction
- Material condition (annealed vs work-hardened)
Typical values:
- Soft aluminum: R(min) = 0
- Half-hard aluminum: R(min) = t
- Soft steel: R(min) = 0.5t
- Hard steel: R(min) = 2t
Spring-back
Spring-back = Elastic recovery after bending force is removed
Spring-back Angle: Δα = α(initial) - α(final)
Factors increasing spring-back:
- Higher yield strength
- Lower elastic modulus
- Smaller bend radius
- Thinner sheet
Compensation Methods:
- Overbending
- Bottoming (coining)
- Stretch bending
Types of Bending
V-Bending:
- Most common
- Sheet bent between V-shaped punch and die
- Can be air bending or bottoming
Edge Bending:
- Bending along edge of sheet
- Wiping die used
Flanging:
- Bending edge perpendicular to sheet
Hemming:
- Folding edge over itself (180° bend)
- Used for stiffening and edge finishing
Bending Force
Approximate Bending Force: F = (k × S × w × t²) / L
Where:
- k = Constant (≈ 1.33 for V-bending)
- S = Tensile strength
- w = Width of part
- t = Thickness
- L = Die opening width
Deep Drawing
Deep Drawing Process
Deep Drawing = Forming a flat blank into a hollow vessel without excessive wrinkling, thinning, or fracturing
Process:
- Blank placed over die opening
- Punch pushes blank into die cavity
- Blank holder prevents wrinkling
- Material flows from flange into cup walls
Drawing Ratio
Drawing Ratio (DR) = D₀/Dp
Where:
- D₀ = Blank diameter
- Dp = Punch diameter
Limiting Drawing Ratio (LDR):
- Maximum DR achievable in single draw
- Typical LDR ≈ 2.0 to 2.2
- Higher LDR → more severe drawing
For deeper cups: Multiple drawing operations (redrawing) required
Blank Size Calculation
For cylindrical cup:
Blank Diameter: D₀ = √(Dp² + 4×Dp×h)
Where:
- Dp = Cup diameter
- h = Cup height
Drawing Force
Approximate Drawing Force: F = π × Dp × t × S × (D₀/Dp - 0.7)
Where:
- S = Tensile strength of material
Blank Holder Force
Purpose: Prevent wrinkling of flange
Typical value: 30-40% of drawing force
Drawing Defects
Wrinkling:
- Wrinkles in flange or wall
- Caused by: insufficient blank holder force, large blank, thin material
Tearing/Fracture:
- Cup bottom or wall tears
- Caused by: excessive drawing ratio, insufficient lubrication, sharp punch radius
Earing:
- Wavy edge at cup top
- Caused by: anisotropy in sheet material
Surface Scratches:
- Caused by: poor lubrication, rough die surface, hard particles
Other Sheet Metal Operations
Stretch Forming
- Sheet stretched over form block
- Used for large panels (aircraft, automotive)
Spinning
- Sheet rotated and formed over mandrel
- Manual or CNC
- Used for axially symmetric parts
Hydroforming
- Fluid pressure used to form sheet
- Good for complex shapes
- Minimal tooling
Incremental Forming
- Localized deformation by moving tool
- Flexible, no dedicated dies
- Low production volumes
Sheet Metal Materials
Common materials:
- Low carbon steel: Most common, good formability
- Stainless steel: Corrosion resistance, higher strength
- Aluminum alloys: Lightweight, good formability
- Brass/Copper: Excellent formability, decorative
- Titanium: High strength, aerospace applications
Advantages of Sheet Metal Forming
- High production rates
- Good dimensional accuracy
- Excellent surface finish
- Minimal material waste
- High strength-to-weight ratio
- Complex shapes possible
Limitations
- Limited to thin materials
- Spring-back issues
- Tooling costs high for low volumes
- Size limitations
- Anisotropy effects