Reliability Engineering

14Process Control and Process Capability

One traditional approach to manufacturing and addressing quality is to depend on production to make the product and on quality control to inspect the final product, screening out the items that do not meet the requirements of the customer. This detection strategy using after-the-fact inspection is highly uneconomical, since the rejected products have already been produced. A better strategy is to avoid waste by not producing unacceptable output in the first place, focusing on prevention rather than screening. Statistical process control (SPC) is an effective prevention strategy to manufacture products that will meet the requirements of the customer (Duncan 1986; Montgomery 2005; Shewhart 1931).

This chapter covers process control systems, the different types of variation and how they affect the process output, and control charts and their use. It also covers how control charts and statistical methods identify whether a problem is due to special or common causes and the benefits that can be expected from using the control charts. It also covers what is meant by a process being in statistical control and process capability and its various indices and their applications.

14.1 Process Control System

A process control system (see Figure 14.1) is a kind of feedback system. Four elements of that system are important to the discussions that will follow:

1. The Process.  The process means the whole combination of people, equipment, input materials, methods, ...