Problem 1

Samples of n=4 items each are taken from a manufacturing process at regular intervals. A quality characteristic is measured, and x-bar and R values are calculated for each sample. After 25 samples, we have:

∑X-bar = 107.5

∑R = 12.5

Assume that the quality characteristic is normally distributed.

a) Compute control limits for the x-bar and R control charts

b) Estimate the process mean and standard deviation

c) Assuming that the process is in control, what are the natural tolerance limits of the process?

d) If the specifications limits are 4.4 ± 0.2. What is the process capability? Is the process capable of meeting the specifications?

e) Assuming that if any item exceeds the upper specification limit it can be reworked, and if it is below the lower specification limit, it must be scrapped, what percent scrap and rework is the process producing?

f) If the unit cost of scrap and rework are $2.4 and $0.75, respectively, find the total daily cost of scrap and rework.

g) If a process average shifts to 4.5 mm, what is the impact on the proportion of scrap and rework produced?

Problem 2

The metal body for a spark plug is made by a combination of cold extrusion and machining. The occurrence of surface cracking following the extrusion process has been shown by Pareto diagrams to be responsible for producing virtually all of the defective parts. To identify opportunities for improvement, you have been using a control chart to monitor the process.  The probability of a false alarm is 0.0056

a. On average, how many samples will you take before getting an out of control signal? 

b. What is the probability that there will be no false alarms in the next 15 samples taken?  

c. What is the probability that there will be at least one false alarm in the next 50 samples?

d. In a standard control chart the control limits are 3 standard deviations away from the mean, and the probability of a false alarm is 0.0027.  Given that the probability of a false alarm for this chart is .0056, how many standard deviations away from the center line are the control limits? 

e. Suppose the process mean shifts such that the probability of detecting the shift is 0.30.  What is b? 

f. What is the probability of failing to detect the shift by the 9th sample collected?