Integrating Kanban And Other Schedulingmethods
Production Control coordinates multi-step processes
, often with multiple products. Kanban is just one of several ways to achieve this coordination. Physical Linking is another way to coordinate. Here, each part in the process moves in synchronization and each step starts simultaneously. Processes must have the same lot size and co-location.
In Broadcast, a final assembly operation builds directly to schedule. The schedule is simultaneously "Broadcast" to upstream subassembly and supply operations. They build the needed parts in "Line-Set Order" with a small time offset for delivery. This system does not require co-location. It does require identical lot sizes (usually one) for all processes.
Kanban scheduling systems are useful when lot sizes differ between process steps, processes are unbalanced or when distance introduces time lag or variability. These systems slightly de-couple the processes.
Materials Requirements Planning (MRP) works from Bills Of Material (BOM), routings, inventory records and forecasts. It plans each process step for each product, subassembly and item. The system accumulates demand for each work center and each time period. In theory, MRP allows effective scheduling under the most difficult conditions of variability and disconnected process.
This ability to connect disjointed production comes with a price. MRP permits some forms of sloppy engineering. The administrative costs are high. Throughput times are long and inventory turns low. Errors in inventory, BOM's or lead times disrupt the system. About 70%-90% of installed MRP systems do not meet their user's needs.
Re-Order Point (ROP) systems store each item and issue to downstream work centers on request. ROP signals a resupply when the inventory is just sufficient to cover the resupply time.
ROP systems are simple. They require steady and predictable withdrawal rates and predictable replenishment times. But, these conditions are rare. Typical systems have very high inventories and experience frequent stockouts.
A Hierarchy Of Methodology
The figure below shows how the methods form a hierarchy of simplicity and flexibility. The best system is the simplest. Where the process allows, Physical Link is the system of choice. Broadcast, Kanban, MRP and ROP follow in desirability.
A system designer would examine each process and each product group in turn. He/she attempts to apply physical linkage. If the necessary conditions do not exist and process change is impractical, Physical Linkage is rejected and Broadcast is considered. This process follows down to the least desirable systems, MRP & ROP.
In practice, Kanban scheduling systems are often a good choice. They can be a transition between MRP and ROP approaches and Physical Linkage. Scheduling a dedicated cell in which all jobs follow the same route through the call is much simpler than scheduling a multi - products facility, and the control of inter - stage stocks is much easier. Reporting and recording of progress between stages can be dispensed with, and a simple visual control system such as Kanban can be used. It is simplest from an area called a Kanban Square is marked out on the floor or bench between stages for each product. The square is just big enough to accommodate an agreed number of containers of the product. In the simplest case, between stages 1 and 2 three would be one square capable of holding one box of ten of the intermediate items produced by stage 1. When the box is taken by stage 2, the empty square is the signal for stage 1 to produce another box of ten. When the space is again occupied by a box of ten items, stage 1 stops producing that item.
Alternative signals to unoccupied space can be used. For example, as originally used in Japan a card can be the signal. Kanban is the Japanese for signal or card. In the above example there would be one or more cards for each items produced by stages 1 and 2. The card is a license to produce. When a full box is produced by stage 1, the card is placed in the box which is then passed to stage 2. When the box is taken for stage 2, the card is returned to stage 1 and authority for the production of another box of items. If a stage has no cards, it stops production
Kanban works best in a repetitive environment. It is very simple to introduce and provides effective control. Production stops, however, only when all the Kanban cards are accompanying products; that are when all the authorized inter - stage stocks are at the permitted maxima. If product is sold or used intermittently, Kanban fills the line with components for the products, which are not going to be needed for some time. Kamban is in fact nothing more than Order Point control using the two-bin system as with Order Point, it can be likened to trying to drive a car forwards whilst looking only backwards. This can be done if the road is straight. That is if the future can be safely assumed to be the same as the past, but is difficult if there are bends ahead.
Kanban can also be used outside a cellular environment if an item is used repetitively. For example, the supply of 5-liter containers to a packing line, which is filling them with a variety of products, could be put under Kanban control, with the package component warehouse supplying a further pallet whenever a Kanban signal is received. The 5-liter container would be taken off the kitting list for each separate filling order, simplifying both paper work and handling and reducing the need for line - side storage space.
by: jamesmiller
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