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Illustrative example
The following example illustrates how low-level design decisions
can be traced to high-level system objectives with the help
of the MSDD. Let us consider the equipment selection for tempering
steering gear racks with two different machine concepts. Assume
that each machine is equally capable of producing parts to
the desired specifications. One machine is a large draw furnace
capable of processing the aggregated demand from customer
operations in the departmental plant. This machine is a “process
island.” The throughput time is 1 ˝ hours. Its cycle time
is 5 seconds. Parts flow continuously through the machine
on conveyors. The second concept uses an induction tempering
process. The machine is narrow and is a single cycle automatic
machine. The machine processes a part every 54 seconds at
the same pace of its customer assembly cell. Eleven machines
would have to be purchased to have the same capacity as the
draw furnace. Figure 1 shows a sketch and additional information
of both machines.
Figure 1: Two different equipment concepts
The MSDD illustrates the impact of the different equipment
concepts relative to the manufacturing system design. The
draw furnace has a very short cycle time and is fed by multiple
upstream machines. Thus, it becomes difficult to identify
disruptions where they occur (FR-R112), which in turn may
lead to hiding disruptions (if one machine of the multiple
machines at the upstream process fails, the loss in production
capacity does not require an immediate response and may go
unnoted). As a consequence, throughput time variation reduction
(DP-112) is difficult to achieve (see left arrow in Figure
2).
The cycle time of the draw furnace is five seconds, which
makes it very hard to balance the system (FR-T221) causing
process delay (FR-T2). The size of the draw furnace also hinders
the ability to establish a material flow oriented layout to
reduce transportation delay (FR-T4). Both effects will eventually
increase throughput time (FR-113).
The cycle time of the draw furnace is five seconds, which
makes it very hard to balance the system (FR-T221) causing
process delay (FR-T2). The size of the draw furnace also hinders
the ability to establish a material flow oriented layout to
reduce transportation delay (FR-T4). Both effects will eventually
increase throughput time (FR-113), which is shown in the middle
arrow of Figure 2.
Figure 2: Impact of the draw
furnace in achieving high-level system objectives. The design
of the draw furnace (DP) makes it difficult to satisfy the
marked low-level FR’s. The arrows illustrate, which high-level
system objectives are at risk to be satisfied
The draw furnace also has ergonomic weaknesses (right arrow
in Figure 2). The cycle time of 5 seconds prevents man-machine
separation (FR-D1) and the size of the machine requires a
lot of walking (FR-D23). Figure
2 summarizes the discussed relationships. The induction-tempering
machine would avoid the stated problems. The machine could
be integrated into a manufacturing cell to achieve simplified
material flow paths (FR-R112), to balance the system (FR-T221),
to reduce unnecessary walking (FR-D23), and to allow the operator
to operate multiple machines (FR-D1).
The discussed example illustrates the importance of understanding
how low-level design decisions can affect the achievement
of high-level system objectives. It also showed how the MSDD
can provide a communication platform for the system design.
Engineers working on equipment selection can determine how
their decisions will affect the layout designers and vice
versa.
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