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 saidthat,—
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-drawingor 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.
Mechanical drawings should be made with reference to all the processes that are required in the construction of the work, and the drawings should be responsible, not only for dimensions, but for adaptation to fitting, forging, pattern-making, moulding, and so on.
Every part laid down should have something to govern it that may be termed a “base”—some position which, if understood, will suggest size, shape and relation to other parts. Searching after a base for each and every part and detail, the draughtsman should proceed upon a regular system, continually maintaining a test of what is done.
A mechanical drawing consists chiefly of three views:
1. The plan or top view.2. The side elevation.3. The end elevation.
1. The plan or top view.2. The side elevation.3. The end elevation.
In addition to the above, drawings are used to show interior portions of the figure; these are termed “sections,” and they may be taken where any plane crosses another.
Note.—The wordelevation, as applied to mechanical drawings, means simply a view; hence a side elevation is a side view, or an end elevation is an end view.The wordplanis employed in place of the word top; hence a plan view is a top view or a view looking down upon the top of the piece.Ageneralview means a view showing the machine put together or assembled, while a detail drawing is one containing a detail, as a part of the machine or a single piece disconnected from the other parts of the whole machine.
Note.—The wordelevation, as applied to mechanical drawings, means simply a view; hence a side elevation is a side view, or an end elevation is an end view.
The wordplanis employed in place of the word top; hence a plan view is a top view or a view looking down upon the top of the piece.
Ageneralview means a view showing the machine put together or assembled, while a detail drawing is one containing a detail, as a part of the machine or a single piece disconnected from the other parts of the whole machine.