Six Sigma Quality at Flyrock Tires
Executive Summary

The process of creating tires at Flyrock Tires involves 20 different steps to take the rubber from bales to final curing. Given this complexity and the high production volume (the factory produces about 10,000 tires per hour), it takes only a small margin of error in each of these steps to begin to compound and result in a high defective rate. For both public safety and their reputation, Flyrock strives to minimize the number of defects. The answers to the questions asked by this case form a good base for evaluating the production and extrusion process at Flyrock. The company begins by setting expectations for what defect rates should be under ideal conditions as well as setting …show more content…

Question 6
Assuming that we return to the case of the worn bearing in question 3 where extrusion produces a mean thickness of 403 thou even though the setting is 400 thou., we find that the proportion of defective sheets under a six-sigma process equals 0%. P(x<410) = P(z<410-403/1.667) = P(z<4.199) = 1
P(x<390) = P(z<390-403/1.667) = P(z<-7.8) = 0
P(-7.8<z<4.2) = 1 – 0 = 1
Proportion of defective sheets = 0%

Looking at the control limits in question 5 we can find the probability that a sample taken from the extruder with the worn bearings will be out of control:

We begin by finding the probability of the samples being “in control”:
P(In Control) = P(396.837<x<403.163)
P((398.419-403)/0.527<x<(401.581-403)/0.527), which equals
P(-8.69<z<-2.69) = 0.0036
We can then subtract by one to find the probability of being out of control:
P(Out of Control) = 1 – 0.0036 = 0.9964

This tells us that based on the control limits from question #5, the probability of a defect under a six-sigma process with