In Mechanical Drawing, Parts I, II, and III, the common drafting instruments and materials are described, and hints given regarding their use; the fundamental geometrical problems are solved; the principles of orthographic projection are stated, and their application to intersections and developments illustrated. A careful study of these Parts, with the actual drawing work incident thereto, should have given the student considerable facility in producing good line work; he should now be able to draw neatly and accurately any simple piece which may be given him, correctly applying the principles as described.
In producing working drawings the principles already laid down are constantly used, and the more they are at the finger ends of the student the easier his work will become. The principles of projection must be thoroughly understood and fixed in the student's mind in order that he may devote himself with the greatest application to the actual detail of the drawing, and he must not be compelled at every step to turn back to find out how to make the simple projections.
Sectional Elevation Of New Crossley Oil Engine.
Courtesy of Crossley Brothers, Manchester, England.
A working drawing is a drawing which completely instructs the workman, so that he is able actually to make in the shop the object which the drawing represents; in other words, a working drawing conveys to the mechanic all the information necessary to make the object. The student should constantly keep before him the idea that the workman must take a drawing, and, without any further instructions verbal or written, produce the object as the draftsman intended it to be made. The instruction supplied by the drawing should not only cover the form and size of the object, but also the kind of material of which it is to be made, the number of pieces desired, and the finish of its surfaces. A drawing, therefore, is a sort of abbreviated language, or shorthand method of conveying an amount of exact, detail information, which it would take many pages of manuscript to convey.
A second point to be noted in connection with a working drawing is that the workman has no time to puzzle over a mass of lines and figures more complicated than necessary. This means that special attention must be paid to making the drawing as simple as possible; all lines and figures which are unnecessary, beyond the point of conveying complete information, are hindrances rather than helps to the workman; moreover, it takes the draftsman's time to make these extra lines and figures, and thus the drawing becomes unnecessarily expensive. A good drawing, therefore, not only implies accuracy and completeness but also simplicity and directness.
Fig. 1. Conventional lines Used in Machine Drawings.
The secret of a clear drawing, as far as the line work is concerned, lies not only in absolute uniformity in the making of the lines, but in choosing certain characteristic lines to convey different ideas. The most common kinds of lines used are shown in Fig. 1 and described below, and the purposes of their use are stated.
Full lines represent the portions of the object which are visible; they should be bold and clear, heavy on detail drawings, say 1/32" wide, and lighter on an assembled drawing.
Invisible lines represent the hidden parts of the object; they consist of short dashes regularly spaced, the spaces being about ¼ the length of the dash; the dashes should never have a greater width than that of the full line, and usually should be slightly less. A drawing is much easier to read if the full lines force themselves on the eye, while the dotted lines, by their lighter character, are left in the background.
Center or axis lines consist of alternate long and short dashes, finer than the main lines of the drawing.
Fig. 2. Flanged Coupling Giving Practical Application of Fig. 1.
Some draftsmen prefer not to use "dash and dot" center lines, but make them continuous fine lines. Either style is good.
Dimension and extension lines are made fine, like center lines, and may be either full or dotted, according to the preference of the draftsman; the full line is preferable on account of its bolder character and the shorter time it takes to make it.
Extension lines start a short distance away from the edges of the object, so as to break up the continuity of the lines of the object and the extension line.
Dimension lines are run between the extension lines, terminating at the extension lines in arrows. The extension lines should always run a short distance beyond the point at which the dimension line touches them.
Shade lines are used for the purpose of more clearly bringing out to the eye the projecting edges of the object on the shadow side, and should be the heaviest lines on the drawing; the proper effect is secured if these lines are made nearly twice as heavy as the principal lines of the drawing.
Fig. 2 shows a flanged coupling in which the lines given in Fig. 1 are applied to an actual problem. In the lower half of the elevation observe how the invisible parts are shown by dotted lines.
Imagine a rectangular block placed within a glass box, and the surfaces projected to the top, front and right-hand side, as in Fig. 3; now open the box in the manner indicated in Fig. 4 and we have three views of the object on a plane surface, i.e., the drawing paper of the draftsman. These views are called top plan, front, and side elevations respectively, and are denoted in the figure by the letters T, F, and 5. If more views are required, the arrangement is shown in Fig. 5. The bottom plan B is found below the front elevation, and the left side elevation S' is found on the left of the front elevation, the same principles of projection being used as in the former case.
The above procedure is equivalent to tracing on each side of the box the outline of the object as observed by the eye, when directly in front of each side of the object; after this is done the unfolding of the box results in the outlines shown in Fig. 4.
If we consider the front elevation of the object as our starting point, then the top plan is above, the bottom plan below, the view of the right-hand side is on the right of, and the view of the left-hand aide is on the left of the front elevation. This arrangement of views is easily remembered and is very logical; it is the most common method of projection in drafting work, and will be used throughout this book. For such a simple object as that considered above, two views only are necessary, a front elevation and top plan, but machine drawings frequently require three views, top, front, and side, and sometimes more.
Fig. 3. Rectangular Block Within a Glass Box, the Surface Projected to the Top, Front, and Right-Hand Side.
Fig. 4. First and Preferred Method of Showing Top, Front, and Right-Hand Side Views of a Rectangular Block on a Plane Surface.
Some draftsmen prefer the method of projection shown in Fig. 6, by which the lines of the object, instead of being observed through an imaginary glass partition and traced thereon, are projected away from the eye upon surfaces beyond the object; the surfaces are then unfolded as before, with the result, as shown in Fig. 7, that the front elevation, being the starting point, the top plan is below, the bottom plan is above, the left-hand view is at the right of, and the right-hand view is at the left of the front elevation. This system of projection has few advantages for machine drawing, and has been largely superseded by the former method.
The interior construction of machine parts, especially if at all complicated, can seldom be clearly or completely shown by dotted lines. A large number of dotted lines on a drawing is very confusing, and in many cases renders the drawing useless. Sectional views are used to overcome this difficulty, and as an unlimited number of sections can be taken, it is always possible to make clear the interior construction of any piece, however complicated.
Fig. 5. Method of Showing Five Views of a Rectangular Block on Plane Surface.
Fig. 6. Second Method of Showing an Object and Its Projection in Vertical and Horizontal.
(Not Advised for Machine Drawing).