Manufacturing Processes

Manufacturing turns raw materials into finished products. Knowing how parts are made is essential for designing parts that can actually be made economically.

Design for Manufacturing (DFM)

Design parts that are easy and economical to manufacture.

DFM Guidelines

1. Minimize part count: fewer parts means lower assembly cost.
2. Use standard materials: readily available, lower cost.
3. Standardize features: same hole sizes, radii, etc.
4. Avoid tight tolerances: only specify them when function demands.
5. Design for process: match design to manufacturing method.
6. Minimize machining: near-net-shape processes save time.
7. Consider assembly: easy access, self-locating features.

Material Selection

Common Engineering Materials

Selection Factors

1. Mechanical properties (strength, stiffness, toughness)
2. Physical properties (density, thermal, electrical)
3. Manufacturing (formability, machinability, weldability)
4. Cost (material plus processing)
5. Environment (temperature, corrosion, wear)
6. Availability (standard stock vs custom)

Example: Material Selection for a Bike Frame

Requirements: lightweight, strong, corrosion resistant, weldable.

Options:

• Steel: strong, heavy, weldable (pass)
• Aluminum: light, strong, weldable, corrosion resistant (best fit)
• Titanium: excellent properties but expensive (fail on cost)
• Carbon fiber: very light, strong, but expensive and hard to repair (mixed)

Choice: aluminum for performance bikes, steel for budget or touring bikes.

Primary Manufacturing Processes

Casting

Pour molten metal into a mold cavity, let it solidify.

Types

Advantages

• Complex shapes
• Large parts possible
• Good for mass production (die casting)

Disadvantages

• Poor surface finish (sand)
• Porosity issues
• High tooling cost (die casting)

Design Tips

• Uniform wall thickness
• Generous fillets and radii
• Draft angles for mold removal
• Consider parting line location

Forging

Shape metal by compressive forces (hammering, pressing).

Types

• Open-die forging: simple shapes, low tooling cost
• Closed-die forging: complex shapes, high tooling cost
• Hot forging: high temperatures, easier to form
• Cold forging: room temperature, better finish and properties

Advantages

• Excellent mechanical properties (grain flow)
• High strength
• No porosity

Disadvantages

• Limited complexity
• High tooling cost
• Material waste (flash)

Applications: crankshafts, connecting rods, gears, hand tools.

Extrusion

Force material through a die opening.

Types

• Hot extrusion: metals (aluminum, steel)
• Cold extrusion: soft metals, plastics

Advantages

• Consistent cross-section
• Good surface finish
• Efficient for long parts

Disadvantages

• Constant cross-section only
• High die cost

Applications: aluminum profiles, window frames, pipes, railings.

Rolling

Reduce thickness by passing between rollers.

Products

• Sheet metal
• Plates
• Structural shapes (I-beams, channels)
• Rails

Advantages

• High production rate
• Good surface finish
• Continuous process

Machining Processes

Remove material to create features and achieve precision.

Turning (Lathe)

Rotate workpiece, feed cutting tool to remove material.

Operations

• Facing (flat end surface)
• Turning (reduce diameter)
• Boring (enlarge hole)
• Threading (cut threads)
• Grooving

Tolerances: ±0.01 to 0.05 mm. Surface finish: Ra 0.4 to 6.3 µm.

Design Tips

• Minimize tool changes
• Avoid internal corners (tool radius needed)
• Use standard thread sizes

Milling

Rotate cutting tool, feed workpiece.

Types

• Face milling: flat surfaces
• End milling: pockets, slots, profiles
• Slot milling: keyways, grooves

Tolerances: ±0.02 to 0.1 mm. Surface finish: Ra 0.8 to 6.3 µm.

Design Tips

• Standard tool sizes (avoid custom cutters)
• Adequate corner radii (match tool radius)
• Minimize tool depth (avoid long, thin tools)

Drilling

Create holes using a rotating drill bit.

Standard drill sizes

• Metric: 1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20 mm
• Imperial: fractional (1/16", 1/8", etc.), number, letter

Depth limitation: typically L/D < 10 (length/diameter ratio).

Design Tips

• Use standard drill sizes
• Flat bottom requires an end mill (drills leave a conical point)
• Through holes are easier than blind holes
• Chamfer or radius hole entrance

Grinding

Precision finishing process using an abrasive wheel.

Tolerances: ±0.001 to 0.01 mm. Surface finish: Ra 0.1 to 1.6 µm.

Applications

• Precision shafts
• Bearing surfaces
• Tool and die finishing

Joining Processes

Welding

Fuse materials using heat.

Common Types

Advantages

• Permanent joint
• High strength
• No fasteners needed

Disadvantages

• Thermal distortion
• Requires skilled operator (TIG, stick)
• Difficult to inspect

Design Tips

• Accessible joint locations
• Appropriate joint type (butt, lap, T, corner)
• Account for weld shrinkage and distortion
• Material must be weldable

Brazing and Soldering

Join using filler metal with a lower melting point.

• Brazing: T > 450°C (silver, brass filler)
• Soldering: T < 450°C (lead-tin, lead-free)

Advantages

• No melting of base metal
• Join dissimilar metals
• Less distortion

Disadvantages

• Lower joint strength than welding
• Requires cleanliness

Applications

• Plumbing (solder)
• HVAC (brazing)
• Electronics (solder)

Mechanical Fastening

Bolts, rivets, screws, pins, etc.

Advantages

• Disassembly possible
• No heat required
• Simple equipment

Disadvantages

• More parts
• Potential for loosening
• Stress concentration

Adhesive Bonding

Join using structural adhesives.

Advantages

• Uniform stress distribution
• Seals joint
• Joins dissimilar materials

Disadvantages

• Surface preparation is critical
• Curing time required
• Difficult to disassemble

Sheet Metal Processes

Bending

Deform sheet metal to a desired angle.

Bend allowance

BA = θ(π/180) × (R + K × t)

Where:
- θ = bend angle (degrees)
- R = inside bend radius
- t = material thickness
- K = K-factor (≈ 0.33 for 90° bend)

Design Tips

• Minimum bend radius: R ≥ t (usually R = 2 to 3t safer)
• Bend perpendicular to rolling direction for strength
• Account for springback (material elastic recovery)

Punching and Blanking

Shear material using a punch and die.

Tolerances: ±0.1 to 0.25 mm.

Design Tips

• Minimum hole diameter: d ≥ t
• Edge distance: ≥ 2t from edge to hole
• Hole spacing: ≥ 3t between holes

Deep Drawing

Form 3D shapes from sheet metal (cups, cans).

Limitations

• Drawing ratio: diameter/height typically < 2
• Multiple draws for deep parts
• Requires annealing for ductility

Plastic Manufacturing

Injection Molding

Inject molten plastic into a mold cavity.

Advantages

• High production rate
• Complex shapes
• Excellent surface finish
• Minimal post-processing

Disadvantages

• High tooling cost
• Not economical for low volumes

Design Tips

• Uniform wall thickness (1.5 to 3 mm typical)
• Draft angles (0.5 to 2° for mold release)
• Avoid undercuts (or use slides/lifters)
• Generous radii (avoid stress concentrations)

Extrusion

Continuous profiles (pipes, channels, etc.).

Applications: pipes, window frames, profiles.

Blow Molding

Hollow parts (bottles, containers).

Process: heat plastic tube, inject air, expand against mold.

3D Printing (Additive Manufacturing)

Build a part layer by layer from a digital model.

Technologies

• FDM: extrude plastic filament
• SLA: UV cure liquid resin
• SLS: laser sinter powder
• Metal: laser melt metal powder

Advantages

• No tooling (direct from CAD)
• Complex geometry
• Rapid prototyping
• Low-volume production

Disadvantages

• Slow compared to traditional
• Surface finish varies
• Material properties may be anisotropic
• Size limitations

Tolerances and Fits

Geometric Dimensioning and Tolerancing (GD&T)

A precise language for specifying tolerances on drawings.

Common symbols

• Perpendicularity
• Parallelism
• Flatness
• Circularity
• Diameter
• Position

Tolerance Grades

ISO 2768 standard for general tolerances:

Fits

• Clearance fit: hole always larger than shaft (loose).
• Interference fit: shaft always larger than hole (tight, press fit).
• Transition fit: may be clearance or interference.

ISO fit system

• H7/g6: clearance fit (sliding)
• H7/h6: transition fit (locating)
• H7/p6: interference fit (press)

Cost Estimation

Factors Affecting Cost

1. Material (volume × material cost)
2. Processing (setup + cycle time × rate)
3. Tooling (amortized over production volume)
4. Finishing (painting, plating, heat treat)
5. Quality control (inspection, testing)
6. Assembly (labor + fixtures)

Production Volume Impact

Practice Problems

Problem 1: Bend Allowance

Calculate bend allowance for 90° bend, R = 3 mm, t = 2 mm, K = 0.33.

<details> <summary>Solution</summary>

BA = 90(π/180) × (3 + 0.33 × 2)
BA = 1.571 × 3.66 = 5.75 mm

</details>

Problem 2: Process Selection

Part: 1000 aluminum brackets, complex shape, tight tolerances. Which process?

<details> <summary>Solution</summary>

Die casting or machining from stock. Die casting is preferred if the volume justifies tooling cost and the shape suits the process. Otherwise, CNC machining from extruded or rolled stock. </details>

Key Takeaways

• Design for Manufacturing: make parts easy and economical to produce.
• Material selection: balance properties, cost, and manufacturability.
• Process selection: match the process to part requirements and volume.
• Tolerances: only specify as tight as the function requires.
• Cost drivers: material, tooling, setup, cycle time, volume.
• Standard features: standard sizes reduce cost.

Next Steps

Continue to 09-reference.md for the quick-reference formulas, conversions, and material data.