Kanbans

Pull scheduling. This ensures that production is pulled by demand and not just pushed, for example, by forecasts which may result in levels of stock that are either too high or too low. Pull scheduling is usually implemented through a simple but effective signal card system (the kanban system). 

In the remainder of this chapter, we first provide a more detailed discussion of JIT/TPS in Section 2, including its philosophy and implementation issues. In Section 3, we examine the kanban system, widely used in JIT for control of production and inventory, and present a case study of JIT/kanban implementation. Section 4 follows with an examination of JIT s relation to complementary approaches such as total quality management (TQM) and total productive maintenance (TPM), together with a case study of their joint implementation. In Section 5, we examine lean production as an extension of JIT, explore the relationship of JIT to theory of constraints (TOC), and conclude with a brief consideration of applications to service industries of JIT, TOC, and other manufacturing-based approaches. 

The success of JIT, as weU as kanban, is contingent on meeting several prerequisites (1) smoothing of volume and variety (2) development of a flexible, multiskilled workforce and (3) implementation of continuous improvement and autonomation. These prerequisites are discussed in the sections below. In addition, achievement of high quality levels is also essential to implement JIT. For this purpose, JIT and TQM efforts should be closely linked with each other, as will be discussed later. 

KANBAN AS A DECENTRALIZED CONTROL SYSTEM FOR JUST-IN-TIME... 

The Japanese term kanban simply means card or ticket. They are typically enclosed in a protective vinyl cover and contain the following information part number and name, process name where the kanban is to be used, number of units in the standard conteiiner and type of packing, number of kanban cards issued, preceding process outbound stockpoint number, and subsequent process inbound stockpoint number. There is a one-to-one correspondence between the cmds themselves and the standard parts containers that they represent. Furthermore, the cmds eilways circulate together with the actual material flow. Through the kanban system, workers understand their operations procedures and standards and learn and share the information required for process control. In this way, kanban functions as an information system as well as a means of visueil control. 

I w I withdrawal authorizing kanban I p I production ordering kanban... 

Figure 3 Flow of Kanban Cards and Containers between two Processing Areas.

In considering kanban as a decentralized control system, the following control parameters are necessary number of kanbans in circulation number of units in the kanban standard container and kanban delivery cycle a-b-c (where b is number of deliveries per a days and c indicates the delivery delay factor as an indication of replenishment lead time). For example, 1-4-2 means that every 1 day the containers are delivered 4 times and that a new production order would be deUveted by the 2nd subsequent delivery (in this case, about a half-day later, given four deliveries per day). 

Minimizing work-in-process inventory is a goal of JIT. In keeping with this, the number of units per standard container should be kept as small as possible, with one being the ideal. Furthermore, the number of deliveries per day (p d) should be set as frequently as possible so as to synchronize with the takt time of the subsequent process, and the delivery delay factor c should be kept as short as possible. Ideally, the number of kanbans in circulation between two adjacent workstations also should be minimized. However, in consideration of practical constraints, a tentative number of kanbans may be calculated as follows ... 

This tentative number of kanbans is reconsidered monthly because the daily leveled production requirement may differ under the new monthly production schedule. In addition, the number of kanbans is sometimes further reduced by systematically removing them from circulation in the system. The resultant reduction of work-in-process inventory will stress the production system and reveal the weakest point for further improvement. Thus, the kanban system is part of the approach used in JIT to move toward the goal of stockless production and achieve continuous improvement of the production system. 

As previously noted, the kanbtui system is particularly appropriate for high-volume, repetitive manufacturing environments. However, in comparison to the situation when the kemban system was originally created, many industries now face tremendous increases in the variety of pr ucts and parts coupled with lower volumes for each individueil item. This is seen also in the automobile industry, where more than 50% of parts now have their respective kanbans circulate less than once per day (Kuroiwa 1999). Such a low frequency of circulation leads to undesirably high levels of work-in-process inventory, and the kanban system ends up functioning the same way as the classic double-bin inventory system. 

Figure 5 Analogy of Drum-Buffer-Rope in (a) Theory of Constraints and (b) Kanban System.

Kuroiwa, S. (1999), Just-in-Time and Kanban System, in Seisan Kanri no Jiten, T. Enkawa, M. Kuroda, and Y. Fiikuda, Eds., Asakura Shoten, Tokyo, pp. 636-646 (in Japanese). 

Sugimori, Y., Kusunoki, K., Cho, E, and Uchikawa, S. (1977), Toyota Production System and Kanban System Materialization of Just-in-Time and Respect-for-Human System, International Journal of Production Research, Vol. 15, No. 6, pp. 553-564. 

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