The Japanese concept of JIT

Just-in-Time, basically a Japanese concept is a programme designed to enable the right quantities to be purchased or manufactured at the right time without waste. It provides for the required flow of production to be maintained with zero inventory, (no buffer stock) at each stage of the supply or manufacturing chain.

The overriding feature of JIT is that materials or parts are generated in the exact quantity required and just at the time they are needed. A classic JIT system consists of a series of manufacturing units each delivering to one another in successive stages of production. The amount provided by each unit to the next unit is precisely what the latter need for the next production period (usually one day). There are no safety margins in the form of buffer stocks, live storage, or work-in –progress.

JIT aims to minimize inventory and its associated costs within the whole manufacturing or supply chain and meet the requirements of each internal or external customer immediately on demand (bearing in mind that within an organization. Successive links in the manufacturing/supply chain are customers of the previous link in the chain, whether or not this is inside or outside the organization.

JIT aims to achieve a continuous production flow to meet demand or supply requirements without delay or waste. It ensures total reliability in the delivery of required parts and materials on time.

The main feature of JIT is that it operates as a ‘pull system, producing on demand, i.e., ‘making to order.’

The use of Pull rather than Push system: - ‘Push’ systems produce to forecast the demand or make to stock. They have the advantage of increasing the predictability of scheduling and machine loading, leading to computerized material planning and production scheduling systems. But because forecasts are fallible, a ‘push’ system can result in excessive stocks.

‘Pull’ systems produce on demand, i.e., ‘make to order.’ They, therefore, eliminate the need to manufacture for stock and minimize stock levels in finished goods stores or distribution centres. ‘Pull’ systems are less dependent on computers. But they have to be able to respond to sudden and unexpected demands. They, therefore, need careful planning and good plant maintenance to work correctly. A ‘Pull’ system is an appropriate approach in the JIT process, but there will be many circumstances in which it is impossible to use it exclusively.

If there is low inventory in the JIT system, any fluctuation in the final stage of production or operation creates variations in the requirements at previous stages. These variations become progressively larger for processes further away from the final step. A minimum lot or batch size, ideally a lot size of one, is used to prevent this variation. Small lot sizes help reduce non-conformities because they ease the rapid detection of problems and, therefore, achieve solutions more quickly, before large quantities are generated.

JIT calls for small lots and frequent production runs. The JIT operation helps to control excess materials in the process, but it can create the problem of too much time being spent setting up machines. A system developed by Shigeo Shingo can be used for this purpose.

JIT does not accept processing time as a constant and tries to reduce it as much as any other time. The rules developed by Hernandez for analyzing and reducing process time are.

Establish a processing time goal at the beginning of the design change. Change the design to achieve the goal if necessary. Bear in mind that products may have to be disassembled to carry out line repairs. Therefore, set disassembly goals. Define the process clearly and ensure that its documentation match the engineering documentation. Ensure that the manufacturing engineers have clear procedures to cover the process step by step at each work centre.

Kanban is a Japanese word meaning visible record. A Kanban system ‘pulls’ parts and components through the production processes when and only when needed. Therefore, the movement of materials is controlled entirely by the usage of parts. It is an essential element in a ‘pull’ system. Each component or part has its special container designed to hold a precise, preferably small quantity.

Every container has two Kanban cards, a production or P-Kanban for the unit producing the part and a conveyance or C-Kanban for the work centre using it. The container travels between the two units and one Kanban is exchanged for another along the way.

No parts are made unless there is P-Kanban to authorize it. Without this authorization the production unit will halt and engage in other maintenance, quality improvement or cleaning activities. If this happens too frequently action is taken to change flows, batch sizes or production methods.

Kanban system can only operate as part of a JIT process. It is interesting to note that a complete JIT system incorporating Kanban systems as developed originally by Toyota does not rely on computerization, which is the case with systems such as flexible manufacturing systems (FMS), Computer integrated manufacturing(CIM) and Materials requirement planning and Manufacturing resources planning(MRP I and II).

The basic rules for operating a Kanban system are (a) move a Kanban only when the lot it represents is consumed. (b) No withdrawal of parts without a Kanban is allowed. (c) The number of parts issued to the subsequent process must be the exact number specified by the Kanban. (d) A Kanban traveling card should always produce its parts in the quantities withdrawn by the subsequent process. (e) Defective parts should never be conveyed to the subsequent process. (f) The Kanban should be processed on every work centre strictly in the order in which they arrive at the work centre.

A material handling system in a JIT must ensure the prompt delivery of parts to the work centres in the required quantities.

Paced Production system is at times called the ‘hard’ approach. It is often adopted in conveyor-time manufacture in a JIT system. In this technique operations are carried out in sequence directly on the track or conveyor, the operator faces the bench and the product is not removed to an adjacent bench. In the alternative ‘soft’ approach the operation is completed off the conveyor and then returned so that a further operation downstream can perform the subsequent operation.

Cellular manufacture is a non-conveyor type operation. A characteristic feature of cellular manufacturing in the JIT system is that batches are tiny. Thus the amount of idle time between cells is minimized, and as all the quantities are kept to a minimum, the real work – in-progress is very low.

Process capability is a statistical technique designed to ensure that the process continuously satisfies design requirements under operating conditions. It provides a basis for continuous process improvements and can be a valuable part of a JIT system.

A Total Quality Management (TQM) involves intensive and long-term effort to create and maintain the high standards of product and process quality required to operate a JIT system and to satisfy customers (internal and external) at each point in the manufacturing and distribution chain.

A complete JIT system, as developed by Japanese firms such as Toyota and Matsushita, may take years to introduce. It requires far-reaching changes to the production system and techniques and even more fundamental changes to the attitudes and activities of all concerned.

The writer is former Director General, EPB.

He can be contacted at [email protected]