Page 104 - Six Sigma for electronics design and manufacturing

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Six Sigma and Manufacturing Control Systems
daily samples are then compared with a historical record to see if the
manufacturing process for the part is in control. In X , R charts, the
sample measurements taken today are expected to fall within three
standard deviations 3 s of the distribution of sample averages taken
in the past. In moving range (MR) charts, the sample is compared
with the 3 of the population standard deviation derived from an R
estimator of . When the sample taken falls outside of the 3 s limits,
the process is declared not in control, and a corrective action process
is initiated.
Another type of charting for quality in production is the precontrol
chart. These charts directly compare the daily measurements to the
part specifications. They require operators to make periodic measure-
ments, before the start of each shift, and then at selected time inter-
vals afterward. They require the operator to adjust the production
machines if the measurements fall outside a green zone halfway be-
tween the nominal and specification limits.
Precontrol charts ignore the natural distribution of process or ma-
chine variability. Instead, they require a higher level of operator
training and intervention in manufacturing to ensure that production
distribution is within halfway of the specification limits, on a daily ba-
sis. This is in direct opposition to six sigma concepts of analyzing and
matching the process distribution to he specification limits only in the
design phase, and thus removing the need to do so every time parts
are produced.
Moving range charts (MR) are used in low-volume applications.
They take advantage of statistical methodology to reduce the sample
size. They will be discussed further in the Chapter 5. In high-volume
manufacturing, where several measurements can be taken each day
for production samples, X and R control charts are used to monitor
the average and the standard deviation of production. It is important
to note that X control charts are derived from the sample average dis-
tribution, which is always normal, regardless of the parent distribu-
tion of the population , which is used for six sigma calculations of the
defect rate, and is not always normal, as discussed in the previous
chapter.
The X chart shows whether the manufacturing process is centered
around or shifted from the historical average. If there is a trend in the
plotted data, then the process value, as indicated by the sample aver-
age X , is moving up or down. The causes of X chart movements in-
clude faulty machine or process settings, improper operator training,
and defective materials.
The R chart shows the uniformity or consistency of the manufactur-
ing process. If the R chart is narrow, then the product is uniform. If
the R chart is wide or out of control, then there is a nonuniform effect