4.8885 29 My'99 L 6822338 PREFACE. In this work the authors have attempted to provide:-(1.) A large number of dimensioned illustrations which may serve as good drawing examples for students, examples ranging in difficulty from the simplest machine detail to a set of triple-expansion marine engines. (2.) Illustrations and descriptions of a great variety of machine details, which may assist the designer in selecting the form of detail best suited to his purpose. (3.) Many rules and tables of proportions, based on scientific principles or on numerous examples from actual practice, which may be useful to the experienced designer for the sake of comparison with the results of his own practice, and which may, to some extent at least, take the place of the well-filled notebook and collection of designs usually possessed by the experienced designer, but which the young engineer or draughtsman can scarcely be expected to have. (4.) Numerous examples showing the application of the principles of mechanics to the calculation of the proportions of parts of machines. The illustrations given are very numerous, and they have all been specially prepared for this work from working drawings, and the authors have been at great trouble to obtain examples representing the best modern practice in machine design. The authors would here acknowledge their great indebtedness to the many engineers and engineering firms throughout the country who have generously given them drawings and much valuable information, which they feel sure will prove useful to students, draughtsmen, and engineers. They would also record their indebtedness to the leading engineering papers, and to the published Proceedings of the various engineering societies, English and American, for particulars of examples of modern practice, which they have either incorporated directly, or have made use of in drawing up the numerous rules and tables which occur throughout the work. In the introductory chapter, besides several brief articles on drawing appliances and the making of working drawings, there is a collection of problems in practical geometry which are very often required in machine drawing; but the student must not imagine that the amount of geometry there given is all that he will require; in fact, as machine drawing is simply the application of practical geometry to the representation of machines, it is evident that a thorough knowledge of the latter subject will be of immense advantage in the study and practice of the former. The brief statements of various principles and rules of mechanics given in Chapter II. are inserted more for the sake of reference for the student than for the purpose of teaching those principles, and the student who wishes to succeed with the subject of machine design will do well to make a special study of mechanics. The numerous dimensioned illustrations, which may be used as drawing examples, are not, as a rule, accompanied by any definite instructions as to what views should be drawn or what scales should be used. The student should, however, not be content with simply reproducing the views shown to the dimensions marked on them, but should project from them other views and sections. This is of the greatest importance, and is always insisted on by good teachers. The student must, of course, select his scale from a consideration of the dimensions of the object he is to represent, the number of views he is to show, and the size of his drawing-paper. The great importance of the power of making intelligible freehand sketches of machine details can scarcely be overrated, and the student should practise this art, not only from illustrations in books, but from the actual machine details themselves. Fully dimensioned freehand sketches, made by the student himself, from actual machines or machine details, form excellent examples for drawing practice. Such sketches should be made in a notebook kept for the purpose, and no opportunity should be lost of inserting a sketch of any design which may be new to the student, always putting on the dimensions if possible. The student should also record in his notebook any new rules for proportioning machines which he may meet with in the course of his reading, or intercourse with others, whether subordinates or superiors in his profession, taking care, however, in each case to state the source of such information for his future guidance and for reference. In conclusion, the authors would say that they have in this work, upon which they have expended a large amount of thought and labour, made use not only of the great experience of many engineers as recorded in the engineering press and the Proceedings of the engineering societies, but also of their own not inconsiderable experience in the workshops, the drawing-office, and the class-room, and they hope that the result of their efforts may be that many young engineers who wish to advance themselves in mechanical engineering-one of the most interesting and important of professions—may not only have their knowledge and power extended, but may be assisted in learning to combine science with practice. January 1893. D. A. L. A. W. B. |