Conducting a Process Capability Study
The steps for conducting a process
capability study are:
Preparing for the study.
Comparing the output to the spec.
action to improve the process.
A process capability study measures
the capability of a specific piece of equipment or a process under
specific operating conditions.
Step 1: Preparing for the Study
To prepare for
Define the processing conditions.
Assure sufficient raw materials are
Make sure the measurement system is reliable.
Determining the Process Output
To determine the process output,
run the process and collect data as you would if you were setting up
a control chart.
Make sure the process is stable using the same
methods as for setting up a control chart.
Since common process
capability calculations are based on a stable, normally distributed
process, if the process is not stable, you should not conduct a
process capability study.
Calculate the process mean and process
variation for the measured output.
Step 3: Comparing Process Output
to the Spec
A specification normally consists of the nominal, or
ideal, measure for the product and the tolerance, which is the
amount of variation acceptable to the customer. It is often referred
to as “the spec.”
The distance between the upper spec limit (USL)
and the lower spec limit (LSL) is called the total tolerance, or T.T.
The Cpk for a process is determined by calculating the Cpu and the
Cpl. The Cpk is the lower of those two numbers.
Step 4: Taking
Action to Improve the Process