The performance potential of a manufacturing process is given by the process capability. It is measured by the proportion of the output that complies with the specified tolerances. Such compliance can be understood within the framework of statistical distribution. Process capability is studied by computing standard deviation of the process or by computing the difference between the upper and lower limits of specification (USL-LSL).
Just when USL- LSL is within specified permissible limits and the process is in control following normal distribution, we can infer that the process is capable of meeting the specifications prescribed.
There can be three types of situations:
1. In situation one, the process is so crude that the specification lie within the control limit.
2. In situation two, the process is so sophisticated that the control limits lie within the specification limits.
3. This is an acceptable situation where the Control Limits (CL) and Specified Limits (SL) are close to each other. In fact, the CL lies within the SL.
Technological advancement has made it possible today to control the quality automatically. Quality control devices are being built into the machines, which control the quality of product during the process automatically. Diameters of automatic pistons coming off a line are examined automatically, and any significant deviation sends off an alarm signal.
Computers are also used. They may be â€˜on lineâ€™ or â€˜on line real timeâ€™. In case of â€˜on line real time computer operationâ€™ is connected at all times to a central computer or these may be satellite mini computers located at several key operations. The output of the machine or the process is constantly monitored and defects are reported â€œreal timeâ€? or quickly. The computer keeps a track on defects as well as good units turned out of the process of machine. If the proportion of defective goes up, the computer reports to the person responsible for locating the trouble or taking necessary actions. Sometimes the computer adjusts the process automatically.
In case of â€˜on lineâ€™ system, the operations are not connected directly to one computer. Quick reports are prepared by the inspectors deployed at various terminals in operating department such as welding, painting, assembling etc. Inspectors are prepared to watch defects and prepare reports but they are not supposed to notice trends in the defects.
This approach to quality control is used when a batch of product already exists and we wish to decide whether to accept it. This is when the products are made elsewhere and the sample is preferred total to total inspection. When the materials are purchased in a large quantity there may be some defective items in each lot of items. Purchased /Finished goods manufactured by complete inspection of the whole lot i.e. 100% inspection acceptance and use or production of defective can be avoided. But for the following reasons 100% inspection cannot be done:
1. It proves to be uneconomical as compared to the cost of inspection with the value and importance of the product.
2. In some cases inspection destroy the product itself and make the product useless. It means that 100% inspection of such products is equal to 100% destruction of the products before they are used. Fixation of a limit or the number of products to be tested is necessary.
3. 100% inspection is a time-consuming procedure and requires more labor.
4. Full inspection makes the inspectors careless. To satisfy the boss as well as themselves, they generally in most case, throw out a few items now and then from the materials and products received for inspection. The inspectors feel that inspection is throwing out of some item or the other. Inspectorsâ€™ carelessness may result in the rejection of a good lot or acceptance of a bad lot.
5. Handling the product may cause deterioration. Moreover, handling the large quantity is not an easy task.
Therefore, to assure better quality on one hand and to come out of the difficulties as mentioned above, acceptance sampling method i.e. method of sampling inspection is used. When the decision regarding acceptance or rejection of an entire lot of product or material or parts is taken on the basis of number of defective in a sample, it is called acceptance sampling.