CONTROL LIMITS AND TOLERANCE LIMITS

For a process that continually produces one type of a product, there are two concepts to monitor the process performance and accepted products:

1. CONTROL LIMITS

Control limits are the upper and lower acceptable performance values of the process.

The process performance is monitored by a graph (performance vs time) that called process control chart to judge how the process is stable.

The control limits are determined jointly by:

• standard statistical calculations based on previous similar process.
• principles to establish appropriate capability for a stable process.

The control limits are depicted by two lines drawn parallel to the time axis in the process control chart to represent the highest and lowest acceptable process performance.

The performance data (inputs to the graph) are the statistical information about the process performance that collected over time.

The process control charts are described as “the voice of the process”.

2. TOLERANCE LIMITS

Tolerance limits (AKA specification limits) are the upper and lower acceptable values of a specification of a product that developed by an ongoing process.

The product specifications is monitored by a graph (specification vs time) that called product control chart to judge how many verified products are acceptable.

The specification limits are determined by defining the highest and lowest acceptable degree of the specifications that can:

• fulfill the organization objectives.
• satisfy the customers.

The tolerance limits are depicted by two lines drawn parallel to the time axis in the product control chart to represent the highest and lowest acceptable a product specification.

The verification data of the products (inputs to the graph) are the statistical information that obtained from (either or):

• quality controlling results.
• customers satisfaction surveys.

that collected over time.

The product control charts are described as “the voice of the customer”.

Tolerance Limits are Early Alarm for Process Fixing:

The team defines a criterion for the results fall outside the tolerance limits and within the control limits to start fixing the process.

One of these criteria is the rule of seven which states “A run of seven or more consecutive points in a control chart, either above the mean, or below the mean, or continuously increasing or decreasing, may indicate the process may be out-of-control”.

These two concepts cannot be used interchangeably.

If the tolerance limits are used as control limits and a product found unacceptable, it will be late alarm to fix the process.

So, control limits should be wider than tolerance limits to give a time to fix the process before exceeding the control limits.

This guarantees continuous production of acceptable products.