Tert Book -OF- Mechanical Drawing, BEING An Explanation of the Principles of Geometry and Compiled with Original Illustrations BY JOHN S. ROOKE, Teacher of Metal Working and Mechanical Drawing in the Workshop NEW EDITION. PRINTED FOR THE AUTHOR. 607108 724 19 AUG 5 1993 SC •R67 Introduction. The purpose of this book is to aid pupils in the study of the principles underlying mechanical drawing. It is not intended to make the study easy, for the pupil cannot be benefited in that way. For that reason, though the drawings are made sufficiently accurate to serve as illustrations of the text, they are not so accurate or complete as to permit their use as flat studies to be merely copied without disclosing the copyist's ignorance of the principles intended to be taught. Nor are the explanations made so full and complete as to relieve the pupil of the necessity of thinking. In short, this book by its text and illustrations, will enable the earnest pupil to study the art and get a firm grasp upon the principles of projection, but it will be of no use to the careless pupil who seeks to skim over his studies and make nothing more than a show of understanding. The subject is presented in a way that is likely to be interesting, and the problems set are of great practical value. The study is not an easy one-if it were it would be of less value than it is-but those who give it serious attention will soon find that what at first seemed hard to understand has become quite simple, and that they have been prepared by a thorough mastery of principles to understand and make the most complex mechanical drawings. This is the end in view. Copying from flat drawings is useful only as an exercise with the pen and other instruments; the mastery of the principles of projection gives the pupil the key to original work. Philadelphia, September 1902. JOHN S. ROOKE. Orthographic Projection. Our first illustration is intended to show what is meant by orthographic projection; the pupil will hereafter have to imagine these planes represented by glass. In Fig. h we have a rectangular prism placed within a case of plates of glass upon which the projections of the prism are made. These plates of glass represent the planes of projection, and can be revolved about the axes a b and b c, until all are in one plane as in Fig. b, which is called the plane of the paper. h is called the horizontal plane, a' the front vertical, and c' the side vertical. Suppose c' to be revolved about b c until it is in the same plane as a', and that c' and a' are revolved about a b until they are in the same plane as h", then we have Fig. b. The projections from all points of an object perpendicular to these planes of projection are called orthographic projections. The projection on h" is called the top or plan, on a' the front elevation, and on c' the side elevation. It can readily be seen without any reference to the planes that these views are arranged as common sense would suggest, bringing the top to the top, the front to the front, and the side to the side. In the illustrations the small letters h and v are used with all other letters to indicate in one case the horizontal and in the other the vertical planes. |
