Question

C-Spec, Inc., is attempting to determine whether an existing machine is capable of milling an engine part that has a key specification of 4 ± .003 inches. After a trial run on this machine, C-Spec has determined that the machine has a sample mean of 4.001 inches with a standard deviation of .002 inch. Calculate the Cpk for this machine.

Answer 1

0.3333

Explanation:

Given the following :

Sample mean(m) = 4.001 inch

Standard deviation(sd) = 0.002 inch

Key specification : = 4 ± .003 inches

Upper specification LIMIT ( USL) : (4 + 0.003) = 4.003 inches

Lower specification limit (LSL) : (4 - 0.003) = 3.997 inches

Cpk is found using the relation:

min[(USL - mean) / (3 * sd), (mean-LSL) / (3*sd)]

min[(4.003 - 4.001)/(3*0.002), (4.001 - 3.997)/(3*0.002)]

min[(0.002 / 0.006), (0.004 / 0.006)]

min[(0.33333, 0.66667)

Therefore Cpk = 0.3333

Because 0.33333<0.66667