The following example shows how to do a process capability assessment.
You need to make some decisions regarding the procurement of bolts from three different suppliers. You have requested that the shank diameter (above the thread) have specifications of
1.500 ± .009 inches. The SPC studies done by the suppliers have indicated that their processes are behaving consistently in statistical control with the following process parameters.
Supplier 1: X (double bar) = 1.500 inches
σ x = 0.003 inch
Supplier 2: X (double bar) = 1.500 inches
σ x = 0.0022 inch
Supplier 3: X (double bar) = 1.4950 inches
σ x= 0.0015 inch
Which supplier would you purchase from? Why?
First of all, look at the graph below to understand the current situation.
Process Capability Assessment: Supplier 1
Process Capability Assessment: Supplier 2
Process Capability Assessment: Supplier 3
All three suppliers appear to be capable of meeting minimal requirements with respect to the conformance of their products to the customer specifications.
Supplier 3 has the least amount of variation. It is generally easier to center the process than to reduce the variation. It may be wise to go with Supplier 3 if the supplier agrees to center the process. Then the Cp and Cpk values will be: