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Manufacturing System Design Decomposition™
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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