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Chapter 13: G. Measurement System Analysis 135
Gage R&R Study Using Crossed Design
Gage name:
Date of study:
55
Reported by:
Tolerance:
Misc.:
Length in mm by Bolt
50
45
40
35
30
25
20
1 2 3 4 5 6 7 8 9 10
Bolt
Part II.G
Figure 13.12 Scatter plot for measurements versus parts (bolts).
In Figure 13.12 the clear circles represent the measurements of each part while
the black circles represent the mean for each part. In this case the spread of measurements
for each is not almost the same but is nevertheless very small. This
means that each part is being measured with the same precision and accuracy.
Greater variability among the black circles indicates that the measurement system
is quite capable of distinguishing the parts belonging to different categories. Thus,
combining the outcomes of the two diagrams 13.11 and 13.12, we can say that overall
the gage R&R variability is not very significant.
MEASUREMENT CAPABILITY INDICES (MCI)
As the process capability index quantifies the ability of a process to produce products
of desired quality, likewise the measurement capability index (MCI) quantifies
the ability of a measurement system to provide accurate measurements. In other
words, MCI evaluates the adequacy of the measurement system. There are various
measurement capability indices in use. One such measurement capability index
was defined earlier in equation (13.10). We discuss here the other two most commonly
used measurement capability indices, percent of process variation and process
specification. The measurement capability index (MCI) should always be used in
conjunction with the process capability index (PCI) (Barrentine 2003).