Manufacturing as a system and a business
Every manufacturing business is a system:
- Inputs: materials, energy, people, machines, money, information.
- Process: the steps that change the materials.
- Outputs: products (and waste, which must be managed).
- Feedback: measurements and customer comments used to improve.
Roles
Design engineer, manufacturing (process) engineer, CNC machinist, tool and die maker, quality inspector, maintenance technician, production supervisor, purchasing and sales staff.
From idea to market
- Product development: research the need, sketch and model (CAD), build a prototype, test, refine.
- Marketing: study customers, set the price, package, advertise and distribute.
- Factors to consider: cost, safety, regulations, environment and recycling, quality, time to market, and ethics.
Process planning and managing production
A process plan lists each operation, the machine, the tooling, the order and the time. Key ideas:
- Cycle time: time a station takes per part.
- Bottleneck: the slowest station. It sets the output: parts per hour = 3600 ÷ bottleneck time (s).
- Line balance (%) = sum of station times ÷ (number of stations × bottleneck time) × 100. Higher means less waiting.
- Production types: one-off (custom), batch, mass (assembly line), continuous (chemicals).
- Lean manufacturing: cut waste (waiting, extra movement, defects, overproduction), use just-in-time stock and continuous improvement (kaizen).
Labour cost
Labour cost per part = wage per hour × hours per part. Faster cycle times, good training, safe workstations and sensible automation lower the cost per part.
Choosing materials
Engineers choose by properties, process and cost:
- Mechanical: strength, hardness, toughness, stiffness, wear resistance.
- Physical: density, melting point, thermal and electrical conductivity.
- Chemical: corrosion resistance.
- Manufacturability: can it be cut, cast, moulded, welded or 3D printed easily?
| Material | Density (g/cm³) | Good for |
|---|---|---|
| Steel | ≈ 7.85 | strong frames, shafts, tools |
| Aluminium | ≈ 2.7 | light parts, aircraft, cans |
| ABS plastic | ≈ 1.05 | cases, toys, 3D printing |
Other families: cast iron, brass and copper, composites (carbon fibre), ceramics, wood. Material choice also links to science: stress = force ÷ area, and mass = density × volume.
Tools, machines, CNC and control systems
Processes
- Material removal: turning (lathe), milling, drilling, grinding.
- Forming: casting, forging, bending, moulding, extrusion.
- Joining: welding, riveting, fasteners, adhesives.
- Additive: 3D printing layer by layer.
- Finishing: polishing, painting, plating.
CNC (computer numerical control)
CAD makes the drawing, CAM turns it into a tool path and G-code (e.g. G01 X40 Y20 = move in a straight line to X 40 mm, Y 20 mm). The machine follows the code exactly and can repeat it thousands of times.
Measuring
Steel rule, vernier and digital callipers (0.02–0.01 mm), micrometers (0.01 mm), height gauges, dial indicators; units in mm and sometimes inches (1 in = 25.4 mm).
Control systems
- Open loop: runs a fixed program with no checking.
- Closed loop: a sensor feeds back and the controller corrects.
- PLC: a tough industrial computer that reads sensors and switches outputs.
- Pneumatics (compressed air: fast, clean, light loads) and hydraulics (oil: very large forces).
- Robots and electronics: motors, relays, switches, servo drives.
Safety
Safety glasses, no loose clothing or gloves near rotating machines, guards in place, emergency stops, lock-out before maintenance.
Quality assurance and standards
Quality control (QC) checks products and finds defects. Quality assurance (QA) designs the whole process so defects do not happen.
- Tolerance: the allowed variation, e.g. 25.00 ± 0.05 mm means 24.95 to 25.05 mm passes.
- Inspection: gauges, go/no-go gauges, coordinate measuring machines (CMM), sensors on the line.
- Statistical process control (SPC): measure samples, plot them on a control chart, act before parts go out of tolerance.
- Defect rate (%) = rejected ÷ total × 100.
- Standards bodies: ISO (international, e.g. ISO 9001 for quality systems), national bodies such as BIS (India), CSA (Canada), ANSI (USA), DIN (Germany).
Try it: take 10 coins or pencils of the "same" type. Measure each with a ruler or calliper, write the values, and find the smallest, largest and average. Set a tolerance: how many would pass?
Key formulas and definitions
- Parts per hour = 3600 ÷ bottleneck cycle time (s)
- Line balance (%) = Σ station times ÷ (stations × bottleneck time) × 100
- Labour cost per part = wage per hour × hours per part
- Mass = density × volume; stress = force ÷ area
- Tolerance band: nominal ± tolerance (e.g. 25.00 ± 0.05 mm)
- Defect rate (%) = rejected ÷ total × 100
Worked examples
1. Stations take 40, 55, 45 and 30 s. Find the bottleneck and parts per hour.
Bottleneck = 55 s (drill). Parts per hour = 3600 ÷ 55 ≈ 65.5, so about 65 parts per hour.
2. For the same line, what is the line balance?
Sum = 40 + 55 + 45 + 30 = 170 s. Balance = 170 ÷ (4 × 55) × 100 = 170 ÷ 220 × 100 ≈ 77%.
3. A part's diameter must be 25.00 ± 0.05 mm. Which pass: 24.96, 25.06, 25.05, 24.94?
Range is 24.95 to 25.05. Pass: 24.96 and 25.05. Fail: 25.06 (too big) and 24.94 (too small).
4. A bracket has a volume of 40 cm³. Find its mass in steel and in aluminium.
Steel: 40 × 7.85 = 314 g. Aluminium: 40 × 2.7 = 108 g. Aluminium is about 3 times lighter.
5. A worker earns 300 per hour and makes one part every 4 minutes. Labour cost per part?
Hours per part = 4 ÷ 60 = 0.0667 h. Cost = 300 × 0.0667 = 20 per part.
6. Out of 800 parts, 12 are rejected. What is the defect rate?
12 ÷ 800 × 100 = 1.5%.
Common mistakes
- Speeding up a station that is not the bottleneck and expecting the line to go faster.
- Choosing the strongest material for every part, instead of the one that fits weight, cost and process.
- Mixing up quality control (checking products) with quality assurance (designing a process that prevents defects).
- Wearing gloves or loose sleeves near a lathe or drill, where they can be caught by rotating parts.