Fig. 209.
Mechanical Drawing.
In distinction to “free-hand,” mechanical drawing is executed almost entirely by the use of the instruments previously described; hence its other term, instrumental drawing. To define it particularly it may be said that,—
Mechanical drawing is the correct reproduction of any figure or part of a machine, whether of full size or reduced in the proportion of one part to another; it also comprises the art of delineating the interior parts which are hidden from view in solid bodies.
A mechanical drawing is the vehicle for conveying the ideas of the designer to those who are to embody them in wood and metal, and the considerations which should govern its production are those which affect its clearness and legibility or those which facilitate reference to it.
Drawings consist of plans, elevations and sections; plans being views on the top of the object in a horizontal plane; elevations, views on the sides of the object in vertical planes; and sections, views taken on bisecting planes, at any angle through an object.
Drawings in true elevation or in section are based upon flat planes, and given dimensions parallel to the planes in which the views are taken.
Two elevations taken at right angles to each other fix all points, and give all dimensions of parts that have their axis parallel to the planes on which the views are taken; but when a machine is complex, or when several parts lie in the same plane, three and sometimes four views are required to display all the parts in a comprehensive manner.
A man must have either a natural talent for hand-drawing or years of experience, before he can produce a sketch and “dimension” it, fit to work from; hence the elementary character of the examples given for practice. A “pretty” drawing is not expected from a beginner; it should always be borne in mind, that correctness of dimensions and general clearness, rather than elaborate finish, are what will save the battle in the days of competition.