Introduction to Basic Manufacturing Processes and ... - always yours

466 Introduction to Basic Manufacturing Processes and Workshop Technology
26
CHAPTER
INSPECTION AND QUALITY CONTROL
26.1 INTRODUCTION
Inspection or checking of components or products with required specifications is very
minutely related with quality control. It is generally an accepted fact that no two things can
ever be exactly same. It also holds true with manufactured parts. Therefore certain variations
or deviations in dimensions and other product specifications are accepted. However, only few
produced articles or parts may be rejected if the deviations go beyond the specified quality
standards. Therefore it becomes essential to detect errors so that the manufacturing of faulty
product does not go uncorrected. The philosophy of inspection is only preventive and not
remedial. In other words the inspection of products is measuring or checking its quality in
terms geometrical tolerances of other specified feature of needed design. Generally, there are
three basic areas of inspection namely receiving inspection, in-process inspection and final
inspection. In the receiving inspection, inspections are performed on all incoming materials
and purchased parts. In the in-process inspection the products are inspected as they are in
processed in stages from starting station to finished station. In the final inspection, all
finished products or parts are inspected finally prior to delivering them to the customer.
The main motive of manufacturing is to process engineering materials and produce
desired and useful components or products to the specified shape, size and finish. The
specifications for the shapes, sizes and finishes on the products are furnished by the
manufacturing operations through specified process plan using part drawings or manufacturing
drawings. These specifications basically termed as called quality characteristics. The quality
of manufactured product is depend always upon the process capability of controlling
manufacturing functions which may lead to a certain amount of variation as a result of chance
and some cause. Also some chance or cause is inherent in any particular scheme of production
and inspection. The reasons for variation outside this stable system should be discovered and
corrected to minimize wastage and finally to improve quality.
26.2 TOLERANCES ON PARTS
It is difficult to manufacture any product or component to its exact size. Tolerance on
the parts is therefore the amount of variation in size tolerated to cover reasonable imperfections
in workmanship and it varies with different grades of work. Tolerance on a dimension can
also be specified as the difference between the maximum limit of size and the minimum limit
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