When measurements are taken at either the manufacturing or inspection stage trend monitoring and Statistical Process Control (SPC) can take place. Statistical Process Control (SPC) is the term used to cover the application of statistics to the control of industrial processes.

Measuring equipment, such as micrometers, callipers, inline gauging or even a CMM, can be connected to computers that can be networked together.

The option is then available to use software that can monitor the measurement results and obtain statistical data obtained

The way that the parameters of a component vary will influence many decisions during the production  process.

Variation is all around us ranging from the difference in the height of people, to the different types, colours and shapes of motor vehicles. So when dealing with variation in manufacturing it is important to monitor:

• Why do dimensions vary on components?
• What are the causes of variation within the methods of manufacture?

The use of statistical techniques to monitor the variation can help considerably to reduce the variation within the process and link the design, manufacture and measurement aspects of the process.

Tools and techniques used to monitor the process include:

• mean
• range
• standard deviation
• histogram
• control chart
• capability
• six sigma

Consider the case of inspecting a batch of 500 components by picking 50 components from a box of 500 and measuring them. The statistic derived from those measurements – such as the average size and the standard deviation of the sample will allow acceptance or rejection of the batch depending on the sampling plan that you have selected and the particular criteria used.

However, sampling in this way will not say a great deal about the manufacturing process.

Change the inspection process to one where every tenth component off the production line is measured and the average size, standard deviation and trends can be determined.

To determine trend in its simplest form this may involve measuring the size of every tenth item off the production line and measuring and recording the dimension on a graph that has the upper and lower tolerances marked on it. By taking note of the trends displayed on the graph it is then possible to predict when the process is going to produce components with dimensions that exceed the permitted tolerances, and take corrective actions such as adjusting the tool setting.

This trend monitoring is a powerful tool in the control of the process and allows the operator to make timely adjustments to the machine so that tolerances are not exceeded.

The data obtained from the measuring equipment and SPC can be fed back into the production process to allow for the relevant adjustments to be made to the machines, thus giving the opportunity to implement a prevention strategy.

A basic SPC control chart that plots the process variation with time. The basic aim of SPC is to minimise variation. By taking notice of the trends displayed it is then possible to predict where the process is going to produce components with dimensions that exceed the permitted tolerances, and take corrective actions such as adjust the tool setting.