Copyright Goodheart-Willcox Co., Inc. 20 Section 1 Introduction to Drafting and Print Reading show center lines crossing in the center of a circle quite as well as drawings drawn by hand. In these cases, the drafter should adjust the scale of the center line so the short dashes make a plus. Notice that the center line extends beyond the boundary of the shape description, extending past the visible linework ap- proximately 1/8″ to 1/4″. Center lines are also used to indicate paths of motion, as shown in Figure 2‑2. In addition, on some drawings, only one side of a part is drawn, and a symbol is placed on each end of the center line to indicate the other side is symmet- rical (identical in dimension and shape). When these symbols are added to the center line, the line can be called a symmetry line. Section View Lines Some lines are used primarily in section view drawings, which are discussed in Unit 6. Section views are views that show the object as if it has been cut through. Additional conventions are needed to express these views on a drawing. For surfaces that are assumed to be cut, section lines are used. These are thin, continuous lines usually drawn at an angle, most commonly 45°. Section lines indicate the surface of an object in a section view that was “cut” by a “cutting plane.” Sometimes section lines have dashes to indicate a particular material. General purpose section lining is the same as the cast iron pattern shown in Figure 2‑5. This type of section lining is commonly used for other materials in section views, unless the drafter or designer wants to indicate the specific material. Some CAD pro- grams refer to the lines within a section-lined area as “hatching” or “cross-hatching.” For section view drawings, a cutting-plane line is often shown on the view adjacent to the section view to help the print reader know where the “cut” is made. The cutting-plane line should be a thick, dashed line. It usually terminates in a short line at 90° to the cutting plane, with arrowheads in the direction of sight for viewing the section. Letters may be used to indicate the section, but are not required. There are currently three different choices for the drafter when creating a cutting-plane line. Refer to Figure 2‑1. The most common cutting-plane line features a long dash and then two short dashes. Goodheart-Willcox Publisher Figure 2‑3. A visible line is a thick, continuous line representing all edges and surfaces on an object visible in the view, while hidden lines represent hidden features. Visible lines Hidden lines