Page:Sheet Metal Drafting.djvu/21

Rh '''4. Lines.'''—In Fig. 13 the names and uses of various kinds of lines which are used in making a drawing are shown. This figure also shows a table of the relative weights of these lines.

Border Lines.—The border line needs no detailed explanation.

Object or Projection Lines.—The visible object line is a line that represents any definite edge that may be seen from the position that the observer assumes in obtaining the given view. The invisible object or projection line represents a line or edge of an object that cannot be seen from the observer's point of view but which actually exists and may be seen from some other position or point of view. For instance, the drawing board cleats on the bottom

could not be seen from above, but could be seen from the sides or when the board was held above the eye. For the sake of determining the relation of such cleats to some other member that might be required on the top of the board, the cleats would be shown by invisible or broken lines on the top view.

Bending Lines.—The lines that are drawn on a layout or pattern of an object to indicate the location of an edge in the completed object are called bending lines, since they locate on the layout or pattern the line along which a bend must be made. These lines differ from the regular object or projection lines only by having at each end a small free-hand circle drawn upon them. Bending lines are drawn differently by different authors and in some shops,