Process Capability Indexes
A ball bearing design for wheel assemblies used in giant mining dump trucks requires the inner diameter to have a lower specification unit (LSL) of 60.24 cm and an upper specification unit (USL) of 60.38 cm. A simple random sample of 210 finished ball bearings, which were produced by an approximately normally distributed process, shows the sample average of 60.29 cm and sample standard deviation of 0.02 cm. Use 3 decimals in the following calculations. (a) Give an equation for the process capability index Cp, calculate and interpret it. (b) Give an equation for the upper one-sided index Cpu, calculate and interpret it. (c) Give an equation for the lower one-sided index Cpl, calculate and interpret it. (d) Give an equation for the process capability index Cpk, calculate and interpret it. (e) Among the above 4 indexes, which one is the most closely watched? Why?