Wire and Bar Drawing
Wire Drawing Process
Wire Drawing = Pulling a wire or bar through a converging die to reduce cross-sectional area and increase length
Process:
- Wire pulled through tapered die
- Cross-section reduced
- Length increased
- Diameter reduced
Key Features:
- Cold working process
- Strain hardening occurs
- Multiple passes required for large reductions
- Intermediate annealing may be needed
Drawing Die Geometry
Die Components:
- Entry zone - Guides wire into die
- Approach angle - Converging section (typically 6-20°)
- Bearing/Land - Cylindrical section for sizing
- Back relief - Exit section
- Exit zone
Die Angle (α):
- Typical: 6° to 20°
- Smaller angle → lower stress, longer die life, more friction
- Larger angle → higher stress, shorter die life, less friction
- Optimum: ~12-15°
Area Reduction
Area Reduction (r): r = (A₀ - Af)/A₀ × 100%
Where:
- A₀ = Initial cross-sectional area
- Af = Final cross-sectional area
Typical Reductions:
- Single pass: 20-50%
- Total reduction: up to 99% (multiple passes)
Drawing Force
Drawing Stress: σd = Yf × ln(A₀/Af) × (1 + μ/tan α)
Where:
- Yf = Average flow stress
- μ = Coefficient of friction
- α = Die semi-angle
Drawing Force: F = σd × Af
Maximum Reduction per Pass:
- Limited by tensile strength of drawn wire
- Wire must not break at exit
- Typically 50-60% maximum
Drawing Speed
Drawing Speed:
- Depends on material and reduction
- Steel wire: 5-50 m/s
- Copper wire: up to 100 m/s
- Higher speeds → heat generation
Wire Drawing Machines
Types:
- Single-draft machine - One die, one pass
- Multi-draft machine - Multiple dies in series
- Continuous drawing - Wire passes through multiple dies without stopping
Bull block: Large rotating drum that pulls wire through die
Drawing Defects
Center burst (Chevron cracking):
- Internal cracks along centerline
- Caused by: excessive die angle, high reduction, insufficient lubrication
Surface defects:
- Scratches, scoring
- Caused by: die wear, poor lubrication, hard particles
Seams:
- Longitudinal surface cracks
- Caused by: defects in starting material
Uneven diameter:
- Caused by: die wear, speed variations
Tube Drawing
Tube Drawing = Similar to wire drawing but for hollow tubes
Methods:
- Tube sinking - No mandrel, tube drawn through die (reduces diameter and wall thickness)
- Tube drawing with mandrel - Mandrel controls inside diameter
- Tube drawing with plug - Floating plug controls wall thickness
Bar Drawing
Bar Drawing = Drawing of bars with larger cross-sections
Differences from wire drawing:
- Larger cross-sections (>10 mm diameter)
- Lower speeds
- May be done hot or cold
- Used for: round bars, hexagonal bars, square bars
Metal Spinning
Conventional Spinning
Metal Spinning = Forming axially symmetric parts by rotating a blank and pressing it against a mandrel
Process:
- Circular blank clamped to rotating mandrel
- Roller or tool presses blank against mandrel
- Blank gradually formed to mandrel shape
- Multiple passes may be needed
Key Features:
- No material thickness change (ideally)
- Blank diameter = final part diameter
- Used for hollow, axially symmetric parts
- Manual or CNC
Applications:
- Cookware, light reflectors, bells, musical instruments
- Aerospace components (nose cones, rocket casings)
- Decorative items
Advantages:
- Low tooling cost (only mandrel needed)
- Flexible (easy to change design)
- Good for low to medium production
- Minimal material waste
Limitations:
- Limited to axially symmetric parts
- Slower than deep drawing
- Requires skilled operators (manual)
- Size limitations
Shear Spinning (Flow Forming)
Shear Spinning = Spinning process where wall thickness is intentionally reduced
Process:
- Blank thickness reduced as it's formed
- Material flows in axial direction
- Wall becomes thinner, length increases
Sine Law: tf = t₀ × sin α
Where:
- tf = Final wall thickness
- t₀ = Initial blank thickness
- α = Mandrel half-angle
Applications:
- Rocket motor casings
- Pressure vessels
- Thin-walled cylinders
Advantages over conventional spinning:
- Higher strength (work hardening)
- Better dimensional accuracy
- Thinner walls possible
Tube Spinning
Tube Spinning = Spinning process starting with a tube instead of flat blank
Applications:
- Reducing tube diameter
- Forming tube ends
- Creating complex tube shapes
Comparison: Drawing vs Spinning
| Aspect | Drawing | Spinning |
|---|---|---|
| Starting form | Flat blank | Flat blank |
| Tooling cost | High (punch, die, blank holder) | Low (mandrel only) |
| Production rate | High | Low to medium |
| Part complexity | Limited | More flexible |
| Wall thickness | Controlled thinning | Constant (conventional) or controlled (shear) |
| Symmetry | Axially symmetric | Axially symmetric |
| Best for | Mass production | Low volume, prototypes |
Lubrication in Drawing and Spinning
Importance:
- Reduces friction
- Lowers drawing force
- Improves surface finish
- Extends die life
- Prevents galling and scoring
Lubricants:
- Wire drawing: Soap solutions, oils, polymer coatings
- Tube drawing: Oils, greases, phosphate coatings
- Spinning: Waxes, soaps, oils
Material Considerations
Suitable Materials:
- Ductile metals: steel, aluminum, copper, brass, stainless steel
- Must have sufficient ductility to avoid fracture
- Strain hardening beneficial for strength
Annealing:
- Intermediate annealing between passes
- Restores ductility
- Allows further reduction
- Typical for copper, aluminum in multi-pass drawing