Page:Sheet Metal Drafting.djvu/22

8 but as long as a definite logical system is followed it does not make much difference what system it is.

Center Lines.—Center lines are, in general, lines of symmetry; that is, they usually divide the views of an object into two equal though not exactly similar parts, since one is right-handed and the other left-handed. In some cases, however, the two parts are sometimes unequal, as well as dissimilar.

Center lines are used to aid in dimensioning; to line up two or more related views; and to fix definitely the centers of circles. All circles have two center lines at right angles to one another, usually a vertical and a horizontal center line.

Extension Lines.—Extension lines are used to extend object or projection lines in order to line up related views and to insert dimensions without placing them on the object itself, causing

a confusion of lines. They should fail to touch the object by about $$\scriptstyle{\frac{1}{16}}$$ in.

Dimension Lines.—Dimension lines are used with arrowheads at each end to show the limits of a given dimension. The lines are broken at some point, usually near the center, in order to insert the figures.

5. Dimensions.—Dimensions up to 24 inches are, in general, given in inches, as 16½". Above 24 inches, practice varies, but, in general, feet and inches are used as 3'-2½", for 3 feet 2½ inches.

Vertical figures about ⅛ in. high are usually used for dimensions. Fractions should be as large as whole numbers and care should be taken to see that the figures do not touch the dividing line. The dividing line of a fraction should be on a level with the dimension line as in Fig. 14.