Having learned the general principles involved in making a correct mechanical drawing of any part of a machine, or the machine as a whole, it might be assumed that the student was in a position to work up complete shop drawings of any piece of apparatus when given the necessary data. However, this is hardly the case. The previous work took up the subject from the standpoint of proper portrayal, the proper way to represent a given object in the form of a drawing, without emphasizing the use to which it is to be put.
While it is absolutely necessary that the draftsman have a thorough knowledge of the theory and practice of line drawings, it is also essential that he go farther and attain as well a complete understanding of the uses to which the drawings are to be put. He must look at the whole drawing or set of drawings as a means to an end, the building of the machine or piece of apparatus.
We cannot emphasize too strongly the necessity for a thorough grasp of the work done heretofore. One cannot hope to attain proficiency in machine drawing unless one has previously learned to make line drawings accurately and correctly. It is not a question of having the necessary information available in the form of books or instructions, but of having the information in one's mind, and the ability to produce the drawing at one's finger tips. The principles set forth in the previous works and the elementary training secured in the thorough mastery and study of those principles will alone form the proper foundation for the following work and help to produce an efficient draftsman.
The making of drawings will now be taken up from the standpoint of their practical use in the shop for the production of a complete machine. We must look at the drawing no longer as a "picture" but as a practical form of instruction to the pattern maker, to the foundryman, to the machinist, or to the assembler. Our object is no longer to show the machine or the part, but to give to the shopman such information that he may build the machine.
Variations from the theoretical principles heretofore set forth will be found in plenty, but every variation will have its practical reason. Only a small part of the whole of a piece may be shown, when that small part tells the whole story to the shopman. The proper laws of projection may not be followed or the crosshatching may be omitted entirely from a cross section, but these liberties will be taken by the draftsman only that the drawing may be more clear.
It should not be assumed because of the above statements that a knowledge of the essential principles will not help in the making of practical drawings. The truth, is quite the contrary. Unless one knows the principles from the beginning to the end he dare not take liberties for fear these liberties will confuse instead of clarify the work.
As stated in one of the earlier books on this subject,* the two chief essentials of a shop drawing are:
(1) Absolutely complete and definite instructions from designer to workman.
(2) Least possible cost in dollars and cents of production of the drawing measured by the draftsman's time.
Of the above the first is the easier to determine, once the drawing is in the shop and in the hands of the workman. The least question as to form or dimension stamps the drawing as bad and the draftsman as a poor workman. This does not mean that a drawing must be a mass of lines and dimensions nor that everything must be shown on each drawing; in fact, the confusion which would result from such drawings would be as bad as the uncertainty caused by incomplete work. The exact shape and every necessary dimension must be shown, but no unnecessary line must be drawn to hinder and confuse the workman.
*Charlos L. Griffin, Machine Drawing, Part III A.
In this connection, it is well to state that many manufacturers prefer that much of the information be given in the form of notes or tables, if it will help to eliminate confusing lines or dimensions on the drawings. Such practices vary widely in different shops, and no definite rules can be laid down.
As to the second point - the cost of the drawing - it is harder to tell when "cheapness" is a real economy. The first point is so firmly fixed as a part of the second that for the cost you must always consider the two together. If a perfect shop drawing can be made cheaply, that is real economy. To make a poor shop drawing cheaply is the greatest extravagance. A draftsman may produce a fairly good shop drawing but may reach this end by unsystematic and haphazard work; the result is high cost of the drawing and at best only fair results. Another draftsman may, although apparently working at a slower rate, reach the same end by careful and systematic work in less time. The drawings of the second man will be cheaper, and the chances are that his care and systematic procedure will in time assure the production of better and better work.
System is essential to cheap drawings. No draftsman can hope to start his work in a careless and haphazard manner and complete it in a reasonable time or even be sure that it is complete when it seems to be. A definite start, a definite system of building up the drawing from that start, and a definite end in view will go far toward teaching the draftsman to produce good drawings at the minimum expense.
With the above points constantly in mind, we can proceed to a demonstration of how a set of shop drawings are produced. In order to get the most good from this demonstration, much of it must be worked out in detail by the student himself. It is to be hoped that this work will go far toward instilling in his mind the principles involved and the necessity for constant thought, close application, and hard work.
The theoretical considerations involved in the design of a direct-current generator are beyond the scope of this work, but the production of shop drawings of such a machine, once the designing engineer has supplied the data, will make an excellent study.