Drawings are, in general, of two kinds, pictorial and working. A pictorial drawing represents an object as it appears, while a working drawing represents the object as it really is, Fig. 4. The latter is of the utmost importance to the workman for it tells him concisely all about the object, - its size, shape, kind of material, etc.

Fig. 5 shows two views of a common wood spool. The front view is the view one would get by looking at the object from the front; the side view, the view one would get by looking at the side of the object; a top view, the view one would get by looking at the top of the object, the observer in each case being so far away from the object that the views show the real shape of the object and not its perspective. The side view will be found at the side of the front view and the top view will be found directly above the front view. An examination of Figs. 6 and 7 should enable one to fix the relationship of the views in mind. Fig. 6 represents an object within a "cage" where the views have been drawn upon transparent screens. Fig. 7 shows the cage as it opens out so as to bring all of the views in one plane, as they must be on drawing paper.

Fig. 6. Mechanical Drawing Cage

Fig. 6. Mechanical Drawing Cage.

Fig. 7. Cage Unfolded

Fig. 7. Cage Unfolded.

The various kinds of lines in a working drawing have different meanings. The very light lines of indefinite length are known as construction, extension, or projection lines. They are the first lines drawn. The heavy lines represent visible edges or outlines of the object. The broken or dotted lines represent hidden edges or outlines. Those lines having arrow barbs and numbers are known as dimension lines, and the barbs indicate the extreme limit of the measurement while the number indicates the amount. Lines through the middle of an object dividing it into two equal parts are known as center lines.

Section At AB

Section At AB.

Fig. 8. Drafting Conventions

Fig. 8. Drafting Conventions.

Fig. 8 shows how nails and screws may be represented. This illustration also shows two other conventions, the cross-section and the broken view. A cross-section represents an object as it would appear if cut, and is indicated by a shading, known as cross-hatching, as shown. A broken view is used when, for any reason it is not advisable or possible to represent the full view. Irregular lines indicate the missing part and the dimensions indicate the true size.

Small objects are drawn full size, that is, the object and drawing are of the same dimensions. A drawing issaidtobedrawntoscalewhenitspartsare similar in proportion to that of the object represents. There are various scales used, such as 1/4"=1' (1/4 inch=1 foot); 3"=1', known as a quarter scale; and on very small objects we may have such scales as 1/2"=1",etc Whatever the scale used, the figure on the drawing represents the size of the object's corresponding part.

Fig. 9. Drafting Tools

Fig. 9. Drafting Tools.

Working drawings are best made with instruments. Fig.9 shows a set. The T-square is used in making horizontal lines, the lines being drawn from left to right The triangles are used in the making of vertical and oblique lines, the lines being drawn from the T-square.