Defining
Production Control
Production
Control coordinates multi-step processes, often with
multiple products. Kanban is just one of several ways to
achieve this coordination.
Methods of
Production Control
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.
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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.
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.
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.
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