Inspection always involves evaluating the quality of some characteristic in relation to a standard
Evaluation may be described as: the "inspection act” and consists of the following actions that apply to each quality characteristic:
1. Interpretation of the inspection.
2. Measurement of the quality of the characteristic.
3. Comparing 1 with 2.
4. Judging conformance.
5. Disposing of conforming cases.
6. Disposing of non-conforming cases.
7. Recording the data obtained.
The word “specifications” is used in the generic sense as the standard for the characteristic. It may consist of a written transcription, drawing, a photograph, a physical sample, an oral instruction, a hazy memory.
When measurement is done, the words commonly used to describe it are:
1. With the aid of mechanical measuring instruments — gauging or calipering.
8. With the aid of electronic measuring instruments — testing.
9. With the aid of chemical or metallurgical measuring instruments — testing or assaying.
The purpose of inspection is to determine whether products conform to specifications = “acceptance inspection” or "product acceptance." However, there are other purposes as well. This section is concerned primarily with acceptance inspection, but it deals also with those purposes of inspection which are closely allied to acceptance inspection, e.g., rating of accuracy of inspectors.
The purposes of inspection:
1. To distinguish good lots from bad lots--usually called, “acceptance sampling or sampling inspection.”
Distinguishing features: prime purpose is to classify lots of product as to whether they are acceptable or non-acceptable. Results of the sampling are used to make this classification. Data from sampling is usually made available to producing department.
10. To distinguish good pieces from bad pieces--usually called, “detail inspection, 100% inspection, or sorting, or classification.”
Distinguishing features: 1.) Supplier (or incoming) inspection: if done by purchaser on material bought from another company. 2.) Process inspection: if done between departments of the same company, 3.) Final inspection: if done by producer prior to shipment of finished goods to the customer.
3. To determine if the process is changing--usually called, “Control sampling.”
Distinguishing features: Control sampling: prime purpose is to see if the process is changing. Usually done through Shewhart control charts which compare averages of samples to statistical limit lines. Detects the entrance of significant causes of variation. Any classification of product is incidental.
4. To determine if the process is approaching the specification limits--usually called, “PRE-Control.”
Distinguishing features: PRE-Control: prime purpose is to see if the trend of change within the process is such that there is danger of producing a defective product. Usually done through charts which compare measurements on individual units of product to narrowed specification limits.
5. To check rate the quality of product--usually called, “product auditing or quality rating.”
Distinguishing features: prime purpose is to qualify the quality of product. Usually, the seriousness of defects is recognized by assigning demerits or weights depending on the severity of the defects. Results are usually charted as demerits per unit of product.
6. To rate the accuracy of inspectors--usually called, “accuracy inspection, or over inspection, or accuracy reading, or check inspection.”
Distinguishing features: prime purpose is to measure the effectiveness of inspectors in finding the facts. Comparison is made between (1) defects found by the inspector and, (2) defects which should have been found by the inspector. The ratio of (1) to (2) is the accuracy of the inspector.
7. To measure the precision of the measuring instrument
Distinguishing features: prime purpose is to measure the ability of the instrument to reproduce its own readings under like conditions. Usually involves repeat checks by the same instrument on the same unit of product. May involve checks by more than one instrument on the same unit of product.
8. To secure product-design information--usually called, “qualification testing.”
Distinguishing features: prime purpose is to judge the service capability of the product. Sometimes involves tests of increased severity.
9. To measure process capability--usually called, “process-capability measurement.”
Distinguishing features: qualifies inherent variation of process.
*Inspection includes, in all instances (1) interpretation of the specifications, (2) measurement of the product, and (3) comparison of (1) with (2). Inspection also includes additional elements depending on the purpose (as noted above).
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