The accelerator assembly
The tolerances on the thickness of the washer are fairly large since the fit can be loose, but if it does happen to get too large, it can cause the accelerator to bind and create a potential problem for the driver. (Note: This part of the case has been fabricated for teaching purposes, and none of these data were obtained from Toyota.)
Let's assume that, as a first step to improving the process, a sample of 40 washers coming from the machine that produces the washers was taken and the thickness measured in millimeters. The following table has the measurements from the samples:
|
Sample
|
Observation
|
|
1
|
1.9
|
2.0
|
1.9
|
1.8
|
2.2
|
1.7
|
2.0
|
1.9
|
1.7
|
1.8
|
|
2
|
1.8
|
2.2
|
2.1
|
2.2
|
1.9
|
1.8
|
2.1
|
1.6
|
1.8
|
1.6
|
|
3
|
2.1
|
2.4
|
2.2
|
2.1
|
2.1
|
2.0
|
1.8
|
1.7
|
1.9
|
1.9
|
|
4
|
2.1
|
2.0
|
2.4
|
1.7
|
2.2
|
2.0
|
1.6
|
2.0
|
2.1
|
2.2
|
a. If the specification is such that no washer should be greater than 2.4 millimeters, assuming that the thicknesses are distributed normally, what fraction of the output is expected to be greater than this thickness?
b. If there is an upper and lower specification, where the upper thickness limit is 2.4 and the lower thickness limit is 1.4, what fraction of the output is expected to be out of tolerance?
c. What is the Cpk for the process?
d. What would be the Car for process if it were centered between the specification limits (assume the process standard deviation is the same?)
e. What percentage of output would be expected to be out of tolerance if the process were centered?
f. Set up X‾ and range control charts for the current process. Assume the operators will take samples of 10 washers at a time.
g. Plot the data on your control charts. Does the current process appear to be in control?