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The success of Japanese manufacturers has spurred the American manufacturing community to become quite familiar with previously unknown Japanese terms and methods, including kanban [2, 5], kaizen [8], poka-yoke [12], as well as the general philosophies of Just-in-Time (JIT) and total quality control (TQC) [3]. This Japanese education has been led primarily by the Toyota Motor Corporation, who was the first to devise and/or effectively implement many of these ideas [3, 5, 6, 7, 12]. However, while many terms have by now become widely known among those in production and inventory management, there is at least one which has received little attention in the academic and practitioner literature. Yet, in a recent discussion, Michael DaPrile [1], plant manager of Toyota Motor Manufacturing (TMM) in Georgetown, Kentucky, gave it credit as among the methods which they consider most basic to the Toyota production system. It is known at Toyota as "heijunka."
Given the relatively unknown character of DaPrile's response, we attempt to briefly explain the new term, the concept it represents, and its prerequisites and advantages. Using a brief review of the literature on Toyota manufacturing techniques, and input from our discussion with DaPrile, we find that heijunka represents a not-so-new idea whose implementation forms the gist of JIT production. Our interview with DaPrile also brings to light an important but subtle effect of heijunka not previously highlighted in the literature.
HEIJUNKA DEFINITION
Toyota officially defines heijunka as "distributing the production of different [body types] evenly over the course of a day, a week, and a month" in the assembly processes [13, p. 20]. It refers to the effort to match the daily production sequence to the actual mixed-model demand encountered by Toyota's retail dealers. In other words, heijunka is the term used to describe a mixed production system, where various and changeable sequences of mixed models are produced on the same assembly line. It is consistent with the well-recognized Toyota notion that "production must be viewed as something that naturally and faithfully conforms to firm orders" and reflects the Toyota mind-set against speculative production and the idea that market fluctuations must create inventories [7, p. 133].
However, heijunka goes a step beyond the basic idea of mixed-model production o match demand. It also incorporates...