Stages of Inspection

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I.  Purpose of Inspection

1.     To distinguish good lots from bad lots
2.     To distinguish good pieces from bad pieces.
3.     To determine if the process is changing.
4.     To determine if the process is approaching the specification limits.
5.     To rate quality of product.
6.     To rate accuracy of inspectors.
7.     To measure the precision of the measuring instrument.
8.     To secure products – design information.
9.     To measure process capability.

II. Stages of inspections

            1. Inspection of incoming material

            2. Inspection of production process

            3. Inspection of finished goods.

1. Inspection of incoming materials.

               It is also called receiving inspection. It consists of inspecting and checking of all the purchased raw materials and parts that are supplied before they are taken on to stock or used in actual manufacturing. Inspection may take place either at supplier’s end or at manufacturer’s gate. If the incoming materials are large in quantity and involve huge transportation cost it is economical to inspect them at the place of vendor or supplier.

               While doing receiving inspection following Inspection Criteria should to follow.

                    1. Sampling Method

                    2. Acceptable Quality Level (AQL)

                    3. Classification of defects

2. Inspection of production process.

              The work of inspection is done while the production process is simultaneously going on. Inspection is done at various work center of men and machines and at the critical production points. This had the advantage of preventing wastage of time and money on defective units and preventing delays in assembly.

3. Inspection of finished goods.

              This is the last stage when finished goods are inspected and carried out before marketing to see that poor quality product may be either rejected or sold at reduced price.

III Methods of Inspection

              There are two methods of inspection. They are 100% inspection and Sampling inspection.

              100% Inspection

   100% inspection involves inspection in detail of quality at each stage of manufacture. Where the test involved is non-destructive and every piece is separately inspected. It requires more number of inspectors and hence it is a costly method.

   There is no sampling error. This is subjected to inspection error arising out of fatigue, negligence, difficulty of supervision etc. Hence it is complete accuracy.

    It is suitable only when a small number of pieces are there or a very high degree of quality is required. Example: Jet engines, Aircraft, Medical and Scientific equipment.

   Sampling Inspection

   In this method randomly selected samples are inspected. This samples taken should be follow there own or standard sampling plan. Samples taken from different batches of products are representatives. If the sample prove defective. The entire concerned is to be rejected or recovered. Sampling inspection is cheaper and quicker. It requires less number of Inspectors. It’s subjected to sampling errors but the magnitude of sampling error can be estimated. In the case of destructive test, random or sampling inspection is desirable. This type of inspection governs wide currency due to the introduction of automatic machines or equipment which are less susceptible to chance variable and hence require less inspection, suitable for inspection of products which have less precision importance and are less costly. Example: Mass production Press shop products, CNC marching products, radio bulbs, etc.

   Destructive tests conducted for the products whose endurance or ultimate strength properties are required. Example: strength test, tear down test, compressibility etc.

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