To illustrate we use a serial
pull process with inventories and queues. The triangle
for operation 1 represents the raw material inventory.
The triangle for operation 7 represents the finished
goods inventory. The d-shaped symbols for operations
2 and 4 are delays for the queues waiting for machining
operations 3 and 5.
We add a second pull location at
the output of operation 6. The value of 0.2 indicates
that for each unit leaving operation 7, 0.2 will leave
at operation 6. The two pull amounts are not independent,
the flow withdrawn from 6 will always be 20% of the amount
withdrawn from 7. If independent demands at the two locations
were required, separate processes would be defined for
each pull location.
Parameters for the operations are
on the Excel worksheet below. The worksheet is shown
in two parts for clarity. All operations use lot sizes
of 10. Only operations 3, 5 and 6 have nonzero operation
times. Nonzero scrap rates are at the three processing
operations, so their ratios are less than 1, and the
unit flows increase as we move backwards from operation
7. Defects are not included in the model, so there is
no need for inspection. Operation 6 models a batch process
where 10 items are processed at the same time.
The process uses raw materials as
listed in column J. The names of the raw materials are
important, because the use of like-named raw materials
by separate operations and separate processes are combined
for the economic analysis. The four system raw materials
for the example are: kit, part 1, part
2 and coat.
The entries in column J are called System
Raw Materials. Column K shows the number of units
of each raw material used by the operation. The measurement
dimension may differ between raw materials. In the
example, the kit, part 1, and part
2 raw materials are measured in units. Coat refers
to a material that covers the product in operation
6. We assume that coat has a liquid measure,
liters. In the example, we indicate that one kit is
required for each item passing through operation 1,
the raw material inventory. One part 1 is
required for each item passing through machine 1. Machine
2 requires only 0.5 units of part 2 for each
item passing through. This might represent some customer
selected part that is not required for every item.
Machine 3 implements a coating process that uses 0.4
liters per item. Some entries in column J are blank,
indicating that the associated operations use no raw
material. In this case the number in column K, the
default value, is irrelevant.
We title column L, the general raw
material column, Transact to stand for computer
transactions. Each item passing through an operation
requires a data transaction with the central computer
to identify the item and track its progress. The analyst
wants to compute the number of transactions required.
Although this might not fit the traditional definition
of raw material, it is an example of a quantity where
the count is important, not the time of passage. We see
a coefficient of 1 for the RM inventory and machine operations,
indicating that a computer transaction is required for
each item passing through these operations.
Note that raw materials do not affect
the flow of product through the system. The flow is caused
by the pull or push values associated with the operations.
With raw materials we count quantities that are related
to the flow.