Page 80 - DMGT206_PRODUCTION_AND_OPERATIONS_MANAGEMENT

Unit 4: Quality Control

In this case, it is assumed that the pattern of defects per unit follows the Poisson distribution. The Notes
mathematics of the Poisson distribution is used to establish the mean and control limits for the
c - chart.

Task TELCO manufactures valves for its engines on the Tata truck. 8 valves are used on
each truck. These valves are manufactured on a high-speed automated machine. The machine
is set up to produce a large run of valves of 10,000 numbers in each set up. The specified
dimension of the valve diameter is 2 + .001 - .002 inches.
To set up the machine and to create a control chart to be used throughout the run, 15
samples were taken with four valves in each sample. The complete list of samples and
their measured values are as follows:
Readings (in inches)
Sample Number Valve 1 Valve 2 Valve 3 Valve 4
1 2.0010 2.0006 2.0009 2.0011
2 1.9993 1.9966 2.0095 1.9994
3 1.9900 1.9880 1.9815 1.9808
4 1.9701 2.0010 2.1009 1.9980
5 1.9903 1.9809 1.9775 1.9930
6 1.9940 1.9701 1.9940 1.9805
7 1.9890 1.9920 1.9820 2.0020
8 1.9801 1.9860 1.9960 1.9960
9 2.1006 1.9890 2.1005 2.1007
10 1.9920 2.1007 2.1006 1.9790
11 1.9960 2.1006 1.9970 1.9890
12 2.0019 1.9960 1.9910 2.0011
13 1.9810 1.9910 1.9890 2.0003
14 1.9990 1.9930 1.9880 1.9840
15 2.0101 2.0002 2.0005 2.0009

Develop an X chart and an R chart and plot the values.
From the charts, what comments can you make about the process?

Self Assessment

Fill in the blanks:

6. The R chart indicates that the process is in ……………… .
7. The short run control chart plots interpretation of variables or attributes for several parts
on the ……………… chart.

8. SPC uses ……………… to determine if a process is within controlled parameters.
9. The ……………… is a measure of dispersion, i.e. it is the difference between the maximum
and minimum values in a sample.

10. The ‘p’ chart is used to control the ……………… defective in the output of an operation
while the operation is being performed.
11. The c-chart is useful where the opportunity for defects is large while the ……………… is
small.

LOVELY PROFESSIONAL UNIVERSITY 75