Figs. I and 2 show an object made of sheet metal drawn according to first and third angle methods of mechanical drawing.
In both, the following principles are evi dent: the top view is directly over or under the front view; the side views are in the same horizontal line as the front view and the width of the side views is the same as the width of the top view.
An object is sometimes composed of a com bination of units whose dimensions are in dif give an even tone. It must be understood that the portion sectioned to show the interior in one view is not removed from the other views, but is indicated on the view to which the cut ting surface is perpendicular, by a broken line consisting of alternate and short dashes. When a figure is symmetrical along an axis, half only may be sectioned and the other half shown in an outside view. This shows in one view what would otherwise require two.
Working drawings are drawings which give all the information necessary for the complete construction of the object represented. They are divided into two main classes, assembly drawings and detail drawings, which are gen erally made in accordance with the principles of orthographic projection.
An assembly drawing shows the object with all its parts in their proper positions. A few of the principal dimensions are given and the parts may be indicated by distinguishing marks or numbers which serve as a guide to the erector. The over-all dimensions are service able in determining the space required. Detail drawings give minute particulars regarding the form and construction of each part of the ma chine and must be made clearly to be self-ex planatory.
All dimensions should be accurately shown (Fig. 3) and it must be borne in mind that the dimensions given are those necessary and most convenient for the workman who is to make the piece. Therefore, the draftsman must put himself in the place of the pattern maker, blacksmith or machinist and must men tally construct the object represented. He must foresee whether the object can he cast or forged practically and economically and must know which dimensions would give in the best way the required information.
All unnecessary labor in drafting should be avoided. The drawing should give the instruc tions explicitly, but should not be so elaborate as to require a waste of time in its execution. Accordingly, if there is a way of representing an object by a few lines, without sacrificing clearness, that method should be used. There are many details such as screw threads, springs, bolts, rivets, gear wheels, etc., which occur so frequently in working drawings, that easy methods of representation, or conventions, have been universally adopted.
Since a complete record of the work is nec essary, original drawings are never allowed to leave the drafting-room but the drawings are duplicated by tracing in ink on transparent paper or cloth and a photographic print is made on sensitized paper. Any number of prints may be made from one tracing. The tracing is the °master drawings and must never be taken from the office.
The steps to be taken in making a working drawing may be summarized as follows: (1) Pencilling on paper.
(a) Choice of views—Make only as many views as are necessary to describe the object Some thought will be involved as to which view will show the object to the best advan tage: whether a section will explain the con struction better than an exterior view; whether an auxiliary view will save one or more other views.
(b) Choice of scale and size of paper— Since the drawings are to be filed for reference it is necessary to establish some sheets of standard size, which would determine the scale.
• (c) Block in all views and take care that the location of views on sheet will allow for title and notes.
(d) Complete outline.
(e) Dimension.
• (f) Add section lines, notes, title, border, trimming line. (2) Tracing on cloth.
(a) Stretch cloth on pencilled drawing. Dust with chalk or prepared pounce and rub off with a cloth to remove any traces of grease which sometimes prevents flow of ink.