Process tolerance limits

Tolerance limits are limits that include a specific proportion of the population at a given confidence level. In the context of process control, they are used to make sure that production will not be outside specifications. It is proposed to obtain long-term and short-term estimates of the process tolerances and to use them in capability analysis as an alternative method to the use of capability ratios. This includes monitoring a tolerance chart to assess continuously process tolerances, similar to a chart of process capability indices. Examples are provided for the continuous assessment of process tolerances and use of estimated tolerance limits as a diagnostic tool in process control.