Inventory Management & Lean Manufacturing
Managing What You Need, When You Need It
Every manufacturing business must manage its inventory — the raw materials, components and finished goods it holds in stock. Too much stock ties up capital, occupies warehouse space, risks obsolescence, and can mask inefficiencies in the production process. Too little stock means production lines stop because a critical component has run out. Inventory management is the discipline of finding and maintaining the right balance, using systems such as just-in-time (JIT) and material requirements planning (MRP) to coordinate supply with demand.
Just-in-Time (JIT) Manufacturing
Just-in-time (JIT) manufacturing is an approach in which materials arrive exactly when they are needed in the production sequence — not before and not after. Originated and perfected by Toyota in Japan, JIT aims to eliminate the waste of holding inventory. Advantages include: reduced warehousing costs, lower capital tied up in stock, reduced risk of materials becoming obsolete or damaged in storage, and the ability to respond quickly to changes in customer demand. Disadvantages include: very high dependency on supplier reliability — if a delivery is late, the production line stops; there is no buffer stock to absorb disruptions; and the approach requires excellent supplier relationships, accurate demand forecasting and a very disciplined production schedule. JIT works best in stable, predictable production environments.
Just-in-time (JIT) manufacturing orders and delivers materials only when needed, eliminating large stock holdings. Key advantage: reduced warehousing cost and capital tied up in stock. Key disadvantage: total dependency on supplier reliability — a late delivery stops production immediately with no buffer stock to absorb the delay.
Material Requirements Planning (MRP)
Material requirements planning (MRP) is a computer-based approach that takes the production schedule (what products are to be made, in what quantities and by when) and works backwards through the bill of materials for each product to calculate exactly what raw materials and components must be ordered and when. MRP provides detailed ordering schedules, reduces the risk of running out of stock, and highlights when resources such as machine capacity and labour are insufficient to meet the schedule. Advantages include improved coordination between production and procurement, reduced waste from over-ordering, and better visibility of future capacity requirements. Disadvantages include the significant cost and complexity of implementing and maintaining MRP software, and the fact that MRP is only as accurate as the data fed into it — errors in the bill of materials or inaccurate lead-time data will cascade into incorrect ordering decisions.
JIT is a production philosophy — it drives the entire approach to scheduling and supplier management with the goal of eliminating inventory. MRP is a planning tool — a software system that calculates ordering schedules from a production plan. In practice many companies use MRP to plan their ordering schedules while also applying JIT principles to minimise stock levels. They are complementary rather than mutually exclusive approaches. An exam question asking you to distinguish between them should focus on: JIT = production philosophy, pull-driven, minimal stock; MRP = computer-based calculation tool, push-driven from production schedule.
Lean Manufacturing and the Seven Wastes
Lean manufacturing is a philosophy developed from the Toyota Production System that seeks to eliminate waste (called "muda" in Japanese) from every aspect of production. The central insight is that any activity consuming time, materials or labour without creating value for the customer is waste that should be eliminated or minimised. Seven categories of waste are identified in lean manufacturing, and understanding each one is essential for the OCR exam.
The seven wastes are: Transportation — unnecessary movement of materials between locations adds time and risk of damage without adding value. Inventory — holding more stock than needed ties up capital and space. Movement — unnecessary movement of people (operators walking to fetch tools, bending to reach components) wastes time and risks injury. Waiting — time spent by operators or machines idle (waiting for materials, instructions or the previous operation) adds lead time without adding value. Over-processing — carrying out more operations or finishing to a higher standard than the customer requires (for example, polishing a surface that will be hidden inside an assembly). Over-production — making more than is needed, earlier than needed — widely considered the most damaging waste because it generates all other wastes. Defects — products that do not meet specification require rework or scrapping, wasting all materials and labour invested in them.
The seven categories of lean waste are: Transportation, Inventory, Movement, Waiting, Over-processing, Over-production, Defects. The mnemonic TIM WOOD helps recall them. Over-production is considered the most damaging because it generates all other wastes — for example, producing too many parts creates excess inventory, which requires transportation, which risks damage (defects).
Reducing transportation waste cuts lead time and damage risk. Reducing inventory frees up capital and floor space and improves responsiveness to design changes. Reducing movement waste by redesigning workstation layouts improves operator productivity and reduces musculoskeletal injury. Reducing waiting by synchronising production stages (balanced production flow) increases throughput. Reducing over-processing by specifying only the quality level customers require cuts cycle time. Reducing over-production by only making to order (pull production) eliminates the need to hold finished goods stock. Reducing defects cuts scrap, rework and warranty costs, and improves customer satisfaction. Each waste reduction directly improves efficiency, responsiveness and cost.