Line types used in technical drawing are used for different purposes to provide specific information to the people looking at the drawing. Drafting students or those reading the drawings have to learn what they mean, just as one learns a new language. It is a basic requirement and learned early in drafting instruction.
Object Line
Object lines are solid heavy lines, .7 mm to .9 mm. These lines define the shape of the object portrayed and are the outermost outline of the object. A round bar is shown as a circle in one view and a rectangle in the other. Both would be drawn with object lines.
Center Line
A center line is a .3 mm to .5 mm line that alternates between short and long dashes. It is used to identify a hole as shown from the side. If a hole were in a plate, the center line would locate the center in the view where the feature isn’t shown.
Hidden Line
A hidden line is a .3 mm to .5 mm dashed line. It shows features, such as holes, in a view they are not in. The feature will be shown in another view of the drawing.
Break Line
A break line is a .3 mm to .5 mm or .7 mm to .9 mm line with “Z” breaks, for a flat object, and “S” breaks, for a round object. These are used to show that a portion of the part is not shown. The area left out will not have any features that are unique yet is the same as what is shown. An example would be a rod that is threaded on both ends. Break lines would be used to eliminate the section between the threaded sections to shorten the object.
Section Line
A section line is a .7 mm to .9 mm line drawn at angles, normally 45, 30 or 60 degrees, to show a feature more clearly. The cutting plane line is a .5 mm dashed line with arrows on the end to show where it slices through the material.
