"Just-in-Time" means making "only what is needed, when it is needed, and in the amount needed." For example, to efficiently produce a large number of automobiles, which can consist of around 30,000 parts, it is necessary to create a detailed production plan that includes parts procurement. Supplying "what is needed, when it is needed, and in the amount needed" according to this production plan can eliminate waste, inconsistencies, and unreasonable requirements, resulting in improved productivity.
In the TPS (Toyota Production System), a unique production control method called the "kanban system" plays an integral role. The kanban system has also been called the "Supermarket method" because the idea behind it was borrowed from supermarkets. Such mass merchandizing stores use product control cards upon which product-related information, such as a product's name, code and storage location, are entered. Because Toyota employed kanban signs for use in their production processes, the method came to be called the "kanban system." At Toyota, when a process refers to a preceding process to retrieve parts, it uses a kanban to communicate which parts have been used.
- Why use a supermarket concept?
A supermarket stocks the items needed by its customers when they are needed in the quantity needed, and has all of these items available for sale at any given time.
Taiichi Ohno (a former Toyota vice president), who promoted the idea of Just-in-Time, applied this concept, equating the supermarket and the customer with the preceding process and the next process, respectively. By having the next process (the customer) go to the preceding process (the supermarket) to retrieve the necessary parts when they are needed and in the amount needed, it was possible to improve upon the existing inefficient production system. No longer were the preceding processes making excess parts and delivering them to the next process.
Two kinds of kanban (the production instruction kanban and the parts retrieval kanban) are used for managing parts.