Scales of production
- One-off (job or bespoke) production: a single, often unique item made to a customer's needs. Skilled workers, general tools, high cost per item, high quality and choice. Examples: a bridge, a custom kitchen, a prototype.
- Batch production: a fixed number of identical items made together; machines are then reset for the next batch. Uses jigs, templates and moulds. Flexible, but there is down time while changing over. Examples: bread, school uniforms in sizes, furniture.
- Mass production (flow / line production): very large numbers of identical items on an assembly line. Each worker or robot does one small task. Low cost per item, but huge set-up cost and little variety. Examples: cars, phones, bicycles.
- Continuous production: the line runs 24 hours a day, because stopping is costly. Examples: oil, steel, chemicals, paper, soft drinks.
Job production is a business-studies name for one-off; flow production is the name for mass/line production.
Cost and choosing a method
Every method has a set-up (fixed) cost (machines, moulds, tooling) and a variable cost for each item (materials, labour, energy).
Cost per item = set-up cost ÷ quantity + variable cost per item.
One-off has almost no set-up but high cost per item. Mass production has a huge set-up cost, so it only pays when the quantity is huge: then the set-up is shared over millions of items. This is called an economy of scale. Choose a method by quantity, how much variety customers want, the skill and tools available, and the deadline.
Lean production, just in time and technology
Lean production means making products with as little waste as possible: wasted time, materials, stock, movement and faulty parts. Tools include kaizen (small, continuous improvements suggested by workers) and good layout.
Just in time (JIT): materials arrive exactly when needed, so little stock is stored. It saves space and money but needs reliable suppliers; one late lorry can stop the line. The opposite is just in case: keeping buffer stock.
Technology: CAD (computer-aided design) and CAM (computer-aided manufacture: CNC routers, laser cutters, 3D printers), robots and sensors make production faster and more accurate, but cost a lot to buy and need trained staff. Efficient use of materials: nesting shapes close together on a sheet, cutting to standard sizes, and reusing off-cuts reduce waste.
Quality, tolerances and processes
Quality control (QC) checks finished items or samples and rejects faulty ones. Quality assurance (QA) builds quality into every stage (trained staff, standards, checks at each step) so faults do not happen.
No part is perfect, so designers give a tolerance: the allowed amount of difference from the target. 50 ± 0.5 mm means any size from 49.5 mm to 50.5 mm passes. Go/no-go gauges, callipers and sensors check this quickly.
Main processes
- Wasting (removing material): cutting, drilling, milling, turning.
- Forming / redistribution (changing shape): bending, casting, moulding, forging, pressing.
- Addition / fabrication (joining): welding, gluing with adhesives, screws, bolts and rivets, 3D printing.
Try it yourself
Make 10 paper stars twice. First, cut each one alone (one-off style). Then make a card template and cut 10 together from folded paper (batch style). Time both. Then measure each star's width: if your target is 60 mm ± 2 mm, how many pass? In the 3D, step 6, slide the quantity from 1 to 1,000,000 and watch the best method change.
Key formulas and definitions
- Cost per item = set-up cost ÷ quantity + variable cost per item
- Tolerance: target ± allowed difference (50 ± 0.5 mm → 49.5 to 50.5 mm)
- Reject rate (%) = rejected ÷ total checked × 100
- One-off < batch < mass < continuous (quantity rises, variety falls)
Worked examples
1. A part is designed as 25 ± 0.2 mm. Which pass: 24.9, 25.3, 24.7, 25.2 mm?
Allowed range is 24.8 to 25.2 mm. 24.9 and 25.2 pass; 25.3 and 24.7 fail.
2. A mould costs 20,000 and each item costs 5 to make. Find the cost per item for 100 and for 10,000 items.
100 items: 20,000 ÷ 100 + 5 = 205 each. 10,000 items: 20,000 ÷ 10,000 + 5 = 7 each. Spreading the set-up over more items makes each one far cheaper.
3. A firm checks 400 items and rejects 12. What is the reject rate?
12 ÷ 400 × 100 = 3%.
4. Which method suits: (a) 1 wedding cake, (b) 300 school blazers in 5 sizes, (c) 2 million phone cases, (d) petrol?
(a) one-off, (b) batch, (c) mass (flow), (d) continuous.
Common mistakes
- Thinking batch means one at a time: a batch is a group of identical items made together.
- Saying mass production is always cheaper: it is cheaper per item only at very large quantities.
- Mixing up quality control (checking at the end) with quality assurance (building quality in at every stage).
- Reading ± wrongly: 50 ± 0.5 mm allows 49.5 to 50.5, not 49 to 51.