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

472 Introduction to Basic Manufacturing Processes and Workshop Technology
26.7.2 Waviness (Secondary texture)
It consists of those surface irregularities on the part, which are of considerable wavelength
of a periodic character. It may be caused by vibrations, machine or work deflection, chatter,
wrapping, etc. It appears in machining operations such as turning, milling, planning shaping,
broaching and grinding.
26.7.3 Profile
It is defined as the contour of any section through a surface on the part.
26.7.4 Lay
It is defined as the direction of the predominant surface pattern on the part. It is
ordinarily determined by the method used in the production of the article.
26.7.5 Flaws
These are surface imperfections such as minute cavities, holes, porosities, cracks, etc.
which appear at frequent intervals and at random locations. The flaws should not be available
on the surface.
Some of the well recognized methods for evaluating the roughness of a surface on the
parts are centre line average (CLA) method, envelope method and root mean square (RMS)
method.
26.8 QUALITY CONTROL
The term quality of product in production processes implies the best for the money
invested and does not necessarily mean the best. Therefore quality of the product is the
relative term and generally explained with reference to the end use of a product. A product
can be referred to be of good quality if it works very well and perform good function in a
particular situation for which it is meant or designed while in other situation it may not work
well and it is said to be of bad quality. The word control means regulation, which further
implies observations and manipulation. It suggests when to inspect, how often to inspect and
how much to inspect. The basic philosophy of quality control is both preventive and remedial.
Quality control strategy sometime identifies the important check measures and implements
them to check so that the defective items are not produced at all. When rejection of products
is increasing, some immediate corrective action should be evolved and taken suitably to
prevent further recurrence of product rejection of parts. Therefore well-planned inspection
will act to be an important tool of controlling quality of products. It checks the products at
specified stages while quality control attempts to bring the variable factors of manufacturing
under control for achieving the total product specification for meeting customary demands to
the maximum possible extent.
26.8.1 Stastical Quality Control
Full inspection of the finished product by usual inspection devices is time consuming and
costly. Also in a continuous production process 100% inspection may not be found practicable.
Certain statistical techniques have been formulated to evaluate materials, processes and
products by observing capabilities and trends in variations so that continual analysis predictions
may be made to control the desired quality level of the part. These statistical techniques are
called statistical quality control techniques. These techniques are