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Quality
Further decomposition of DP-111 is based on the necessary
steps to attain high quality manufacturing processes via statistical
process control methods, process design and optimization (robust
design), as shown in Figure 2. Thus, FR-111 and DP-111 together
state that manufacturing contributes to a high quality product
not just by simply being within the tolerance specifications
but by having a process centered on the designer’s target
value with minimal variation.
The first requirement of manufacturing processes with minimal
variation from the target is Process Stability (FR-Q1). A
process is said to be in a state of control when there are
no assignable causes of variation present and instead only
common causes. Assignable causes are non-random events, that
when eliminated or corrected result in the process returning
to a state of control (i.e. process is once again stable).
Examples include tool wear and failure, improperly adjusted
devices (torque guns, spindle speed), chips caught under fixtures,
plugged coolant lines, etc. Therefore, the design parameter
that achieves process stability is the elimination of assignable
causes of variation, DP-Q1. This DP corresponds to the first
major step in statistical process control techniques used
to quickly detect the occurrence of assignable causes at the
machine. Quick detection allows investigation of the process
and permits corrective action to be taken before many non-conforming
units are manufactured. However, a stable process alone is
not a sufficient condition for high quality manufacturing
processes. For example, a process may be unstable, yet produces
parts that are within the specification limit. The next two
functional requirements provide the necessary conditions for
high quality given a stable process.
A process that is stable and has a sufficiently small standard
deviation may still be producing an excessive number of out-of-tolerance
parts. Figure 1 shows a process distribution with a mean that
is too close to the upper specification limit and thus is
producing defective parts despite an acceptable standard deviation.
FR-Q2 gives the requirement to deal with ill-centered process
means. To correctly place the process mean at the required
design target involves adjusting process parameters DP-Q2.
Manufacturing system engineers have only freedom to control
m and s of the process (through operational process adjustment)
since the specification limits LSL and USL are dictated by
product design.
Figure 1 Centering process mean on the target, adapted
from Bothe, D. R. (1997)
The third high-level quality requirement is to reduce variation
in process output – FR-Q3. Variation that is seen in the output
of a stable process is the result of the existence of uncontrollable
noise factors in the process. Noise factors as defined by
Phadke (1989) are the parameters that cannot be controlled
by the designer and lead to the variation causing quality
loss. To reduce variation in process output requires the reduction
of process noise, DP-Q3. Further decomposition of DP-Q3 leads
to the requirements to reduce noise factors in process inputs
and sensitivity to noise in the output.
The dependencies as shown in Figure 2 state that a stable
process (FR-Q1) is first required, followed by centering the
mean (FR-Q2), followed by reducing variation in process output
(FR-Q3). The complete decomposition of the quality branch
is shown in Figure 2.
Figure 2: DP111
is based on the necessary steps to attain high quality manufacturing
processes via statistical process control methods, process
design and optimization (robust design).
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