After the exact and complete development of the steam-end layout, the student should be pretty thoroughly acquainted with the details of the cylinder. All the work thus far has been entirely for his own information, to get his ideas in visible shape, so that he himself can have a permanent record of them. This layout, however, is not in suitable form to finish up into a detail drawing. Its sketchy nature and the confusion of parts, especially if attempt were made to add dimensions, would render it somewhat difficult to be read by a workman taking it up as an unfamiliar subject. Hence it is now necessary to separately detail the parts, with the object in view of transferring, in the simplest and most direct manner, specific information to the workman which will enable him to construct the several parts. It is not enough now that the drawing be clear to the man who makes it; it must be absolutely clear to the shop mechanic, who has no means of knowing the designer's plans except through the information which the drawing gives on its face.
This requires that the draftsman should put himself in the workman's place, and forestall, by the explicit nature of his drawing, all possible questions which may arise in the shop. In this way only can he hope to avoid errors of construction and the continual annoyance of endless explanation of his orders.
Plate B is to be a finished drawing, and the first thing to do is to lay out the sheet. The standard sheet for details which has been adopted is 18X24 inches trimming size, with ½-inch margin all round, so that the working space is 17x23 inches. The rectangle for the title is to be laid off 2½ X4 inches in the lower right-hand corner, and must never be altered, either in size or position. This does not mean that other sizes are wrong, but once a standard system is adopted it must be strictly adhered to, both for artistic and commercial reasons. The scale to which the drawing is to be made is indicated in the title corner on every plate.
The scales permissible for shop drawings in the United States are those readily derived from the common foot rule, such as full size, 6 inches = 1 foot, 3 inches = 1 foot, 1½ inches = 1 foot. These are the most common, most easily read from an ordinary scale, and one of these can usually be adopted. The student should learn to read these from an ordinary scale without being confined to a special graduation. To do this it is not necessary to divide each dimension by 2,4, and 8 to get half size, quarter size, or eighth size, and then lay down the result. For half size, or 6 inches = 1 foot, ½ inch on an ordinary rule represents 1 inch. Hence, each half inch may be read as 1 inch, and its subdivisions accordingly, thus:
For 3 inches = 1 foot, or quarter size, ¼ inch represents 1 inch,and looks thus:
For 1½ inches = 1 foot, or eighth size, 1/8 inch represents 1 inch, and looks thus:
It is very easy to get accustomed to this, and it saves much time and trouble hunting up a special scale every time.
The other allowable scales, less common, but sometimes necessary on large work, are 1 inch = 1 foot, ¾ inch = 1 foot, ½ inch = 1 foot, 3/8 inch = 1 foot, ¼ inch = 1 foot, and 1/8 inch = 1 foot. To use these scales conveniently, special graduation is desirable.
The general arrangement of the sheet, number of views, and approximate space occupied, should be blocked out first. This can easily be done from the original layout. In general, several cross sections are preferable to a single view, which involves many dotted lines. Dotted lines are very convenient for showing invisible parts of an object, but they are often abused, and the drawing of a complicated piece made indefinite and confused thereby. As already stated, a working shop drawing is solely to convey information to the workman at the least possible cost. A careful consideration of this will settle the question of the number of views necessary, their character, and the amount of dotted line work desirable.
Never let the drawing become the master; always be master of the drawing. Do not draw an extra view if no use can be seen for it. Do not put in dotted lines if the detail is completely shown without them. Full lines, or lines which show' visible portions, must, of course, be shown completely.
The nature of the pencil work on Plate B should be the same as on the original layout; viz, sharp, definite lines and positive intersections. Above all things learn the habit of accurate workmanship, for it will save many errors and a vast amount of time. The draftsman must check himself at every line he draws. Slight errors in scaling will often throw parts out of proper relation to each other, and interferences, which the drawing does not show, will become apparent only when the parts get into the machinist's hands.
It is dangerous practice to project across from one view to the other. It only takes a slight irregularity or spring in the T-square to vary the location of lines very perceptibly from where they should be, and once out of scale from this reason it is almost impossible to work a view with any certainty. Rather than project across from view to view, the principal lines, at least, should be scaled off on each view, and it will be found that in the end time will be saved and greater accuracy secured.