vdoc

122 Part II: Metrology
of MSA. For a detailed discussion of analysis techniques for gage stability, gage
bias, and gage linearity, readers are referred to the AIAG reference manual for
MSA (3rd edition).
Part II.G
PROCESS VARIABILITY
All processes, no matter how well they are designed and executed, do possess
the component of variability. It is only the degree of variability that varies from
process to process. In fact, the point that variability exists in virtually any and all
processes cannot be overemphasized. The total variability of a process consists
of two components: parts variability and gage variability. Further, note that variability
in a process can be quantified by a probability distribution and it can be
expressed as
2 2 2
s = s + s
(13.1)
Total Parts Gage
Variability in the Measurement Process
Extending our conversation from the previous section, it is critically important
that we understand that the methods, procedures, tools, and equipment we use
to make measurements constitute an independent process that creates, and is susceptible
to, its own variation. This means there are two sources or components of
variation present in each measurement we take, as illustrated by equation 13.1 and
Figure 13.1.
Thus, it is important to ensure that the component of variability associated
with the measurement system does not consume an excessive amount of the variability
allowed in the process specification. So quantifying measurement system
variability is the purpose of MSA.
EVALUATING MEASUREMENT SYSTEM PERFORMANCE
In any process we usually focus our full attention on bringing the process under
statistical control. In other words, we focus our efforts on reducing the overall variation
in the process. In this chapter, we focus our entire attention on the measurement
system variation, which constitutes an important part of the total process
= +
Total variation Part variation Gage R&R variation
(Measurement variation)
Figure 13.1 Components of total variation.