Question

Problem 2

Krombach Auto Corporation, a German manufacturer of cars, investigates their steering wheel attachment process. A steering wheel needs some “Spiel”, a small range where moving the steering wheel does not turn the tires. This increases driving safety, as not every small movement by the often nervous driver is picked up by the car. However, if the Spiel is too large, drivers will feel uncomfortable and have the impression that the car does not react to their action. Krombach estimates that a Spiel of less than 1 degree is too little and causes too much nervousness. They also estimate that a Spiel of more than 4 degrees is too much and causes concern with the drivers. The steering wheel attachment process results in a Spiel with a mean of 1.7 degrees and a standard deviation of 0.4 degrees. Kromback aims for 3σ performance for this process.

1. Is the process capable of 3σ performance (the traditional C_p definition) of process capability? Explain your answer.
2. Does your answer in part (a) change when you also calculate C_pk? Explain your answer.

Answer 1

(a) Cp = [ USL-LSL/6x sigma]

= [ 4-1/6x0.4]

= [ 3/2.4]

= 1.25

With this capability value, the process seems to be capable of producing good quality items.

Cpk = Min [ USL-Mean / 3 x sigma, Mean -LSL / 3 x sigma]

=Min [ (4-1.7 / 3x0.4), ( 1.7-1/3x0.4)]

= Min [ 1.916, 0.5833]

= 0.5833

Now, with Cpk value of lee than one, it is revealed that the process is incapable of producing good quality items. The reason for this deviation is that the mean is not centered but shifted towards the lower specification limit.