One-stage pull production uses only one stock area or point of use for inbound materials and parts. See Figure 4-6. In a two-stage pull system, supermarkets are established between subprocesses to store kits for use by the downstream process. See Figure 4-7.
Supermarkets
The kanban system was inspired by the way supermarkets work to supply customers with products. In the kanban system, supermarkets are the areas where produced parts, components, subassemblies,
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On-order
Figure 4-7. Two-Stage Pull
Supplier
and finished goods are stored. These supermarkets or stores are located near the area where the parts are produced. When the downstream process (the customer) withdraws parts from an upstream process (the supermarket), the upstream process replen-ishes the withdrawn product by making another amount equal to what was withdrawn.
Withdrawal and production-ordering kanbans control this process, ensuring that there are always parts available when needed, but that no more than needed will be made. Supermarkets are most often used to store component kits between major steps in the process, reducing the number of places that downstream process workers have to go to withdraw the parts they need. The super-market is the primary feature that distinguishes one-stage and two-stage pull systems.
Water Beetles and Milk Runs
The term water beetle comes from the insect called a whirligig. It moves swiftly across the surface of water, spins, and makes rapid and unexpected changes in direction. In the Toyota Production System this name was given to the conveyor—an operator who delivers parts to the other operators in his cell or on the line so that they don’t have to get up to replenish their work stations to make the next set of products.
In the kanban system, frequent transportation of parts becomes necessary. Usually hourly deliveries are needed. For simplicity, we have described the withdrawal and production process in terms of a single withdrawal location. The reality is that most production items require numerous parts that come from several preceding processes. This can be a time consuming and confusing aspect.
Designating a carrier to withdraw parts from the preceding
processes and deliver them to workers greatly simplifies the kanban process. Also, this person quickly becomes an expert in the with-drawal and production kanban process, making it possible to identify and eliminate errors.
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K e y T e r m
K e y P o i n t
Benefits of the water beetle.When a water beetle supports the line:
1. Operators don’t have to leave the work area to find parts or tools.
2. Waiting time becomes visible.
3. The water beetle becomes the pace maker for the line; delays and progress become visible.
4. Water beetles can fill-in for absent operators or for less than “full work” operations.
Tools needed by the water beetle.To do the job well, the water beetle will need:
1. A pushcart with:
a. Small wheels for easy maneuvering
b. A checklist for pickups and deliveries to be made
c. A layout of cells or lines so that pick up and placement can occur easily
2. A picking list to:
a. Clearly identify what and where to pick up b. Clearly identify where and when to pick up c. Limit the number of items on the list
3. Visual displays to help identify supplies to be picked up or delivered 4. Pickup buckets for ease of transfer to and from the pushcart
5. Storage shelves where:
a. Items are stored so that they are easily transferred to the push-cart—nothing above the shoulder or below the knee
b. Gravity is used whenever possible to minimize lifting 6. A supply area at the line that:
a. Makes transfer from pushcart to the line easy b. Is large enough for only one kit
Milk runs.Milk runs are the name given to the path the water bee-tle takes in his or her delivery and pickup runs. Figure 4-8 shows a layout that allows the water beetle to serve a number of cells in one pass from supermarket and back again.
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K e y T e r m
C H A P T E R 4
72 Figure 4-8. Milk Runs
Re-supply Area
Water beetle on milk run.
In this example, three water beetles are supplying six cells.
T A K E F I V E
Take five minutes to think about these questions and to write down your answers:
1. Consider where a two-stage pull system might be beneficial in your factor y.
2. What is the purpose of water beetles?
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S U M M A R Y
Pull production works as a feedback system starting with the downstream customer. The kanban system has emerged to coor-dinate production with the movement of parts and components between processes. Kanban means “card” or “sign” in Japanese.
A kanban system uses cards and other visual signals to control the flow and production of materials. The kanban system functions as a communication system, as pick up and work order infor-mation, to eliminate overproduction waste, as a tool for visual control, and as a tool for promoting improvement. There are two major types of kanbans. Transport kanbans (including sup-plier and withdrawal kanbans) and and production kanbans (including production-ordering and signal kanbans).
The kanban system supports level production. It helps to main-tain stable and efficient operations. The question of how many kanbans to use is a basic issue in running a kanban system. The number of kanbans you need is dependent on the number of pal-lets or containers and their capacity. Lead times, safety margins or buffer inventory, and transportation time for kanban retrieval are also important factors.
You pull material from the upstream process only as you need it to replenish what you have used to produce parts, components, or final assemblies. This is in direct contrast to the standard oper-ating procedure of push systems: “If I have material at my station I will make more parts.” Pull production requires behavior according to a new rule: “I will not produce unless signaled!”
Upstream to you may be the preceding process or it may be a supplier of raw materials or subassemblies. You withdraw from the place where the materials are either produced or stored.
Withdrawal kanbans identify the location of these materials so that there is no confusion about where to find them when you need them.
You withdraw only what you need to complete your operations.
The material you pull from the upstream process can be
con-In Conclusion
tained in a number of ways. Whatever you pull must include a withdrawal kanban, which states the exact quantity that is con-tained in the pallet or lot that you withdraw. You only produce in order to replenish what has been withdrawn from your work process. If you work in the inventory warehouse for raw materi-als and parts purchased from outside suppliers or other plants, you will replenish raw materials as they are withdrawn for use in your assembly production processes. If you work in sub-assembly you will produce the parts and components as they are withdrawn for final assembly. If you are in final assembly you will produce finished goods as customers order the products.
One-stage pull production uses only one stock area or point of use for inbound materials and parts. In a two-stage pull system, supermarkets are established between subprocesses to store kits for use by the downstream process. The kanban system was inspired by the way supermarkets work to supply customers with products. In the kanban system, supermarkets are the areas where produced parts, components, subassemblies, and finished goods are stored. These supermarkets or stores are located near the area where products or parts are produced. When the downstream process (the customer) withdraws parts from an upstream process (the supermarket), the upstream process replenishes the withdrawn product by making another amount equal to what was withdrawn.
Withdrawal and production kanbans control this process, ensur-ing that there are always parts available when needed, but that no more than needed will be made. Supermarkets are most often used to store component kits between major steps in the process, reducing the number of places that downstream process workers have to go to withdraw the parts they need. The super-market is the primary feature that distinguishes one-stage and two-stage pull systems.
A water beetle is an operator who delivers parts to the other oper-ators in his cell or on the line so that they don’t have to get up to replenish their work stations to make the next set of products. In the kanban system, frequent transportation of parts becomes necessary. For simplicity, we have described the withdrawal and production process in terms of a single withdrawal location. The
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reality is that most production items require numerous parts that come from several preceding processes. Designating a carrier to withdraw parts from the preceding processes and deliver them to workers greatly simplifies the kanban process. Also, this person quickly becomes an expert in the withdrawal and production kanbans, making it possible to identify and eliminate errors in the process. Milk runs are the name given to the path the water beetle takes in his or her delivery and pick up runs. Milk runs allow the water beetle to serve a number of cells in one pass from supermarket and back again.
Now that you have completed this chapter, take five minutes to think about these questions and to write down your answers:
• What did you learn from reading this chapter that stands out as par ticularly useful or interesting?
• Do you have any questions about the topics presented in this chapter? If so, what are they?
• What additional information do you need to fully under-stand the ideas presented in this chapter?
R E F L E C T I O N S