should be differentiated from hidden, extension,dimension, or center lines.CONSTRUCTION LINESUsually the first lines that you will use on adrawing are construction lines. These are the samelines that you used to lay out your drafting sheet.They will also be used to lay out the rest of yourdrawing. Line weight for construction lines is notimportant since they will not appear on yourfinished drawing. They should be heavy enoughto see, but light enough to erase easily. A 4H to6H pencil with a sharp, conical point should beused. With the exception of light letteringguidelines, all construction lines must be erasedor darkened before a drawing is reproduced.CENTER LINESCenter lines are used to indicate the centerof a circle, arc, or any symmetrical object.(See fig. 3-24.) Center lines are composed of longand short dashes, alternately and evenly spaced,with a long dash at each end. They should extendat least one-fourth in. outside the object. Atintersecting points, center lines should be drawnas short dashes.A very short center line may be drawn as asingle dash if there is no possibility of confusingit with other lines. Center lines may also be usedto indicate the travel of a moving center, as shownin figure 3-24.VISIBLE LINESThe visible edge lines of the view are drawnas solid, thick lines. These include not only theoutlines of the view, but lines defining edges thatare visible within the view. (See fig. 3-25.)HIDDEN LINESHidden edge lines are drawn with short dashesand are used to show hidden features of anobject. A hidden line should begin with a dashin contact with the line from which it starts,except when it is the continuation of an unbrokenline. (See fig. 3-26.)To prevent confusion in the interpretationof hidden edge lines, you must apply certain stan-dard techniques in drawing these lines. Ahidden edge line that is supposed to join a visibleor another hidden line must actually contact theline, as shown in the upper views of figure 3-27;Figure 3-25.-Use of visible edge lines.Figure 3-26.-Use of hidden edge lines.Figure 3-24.-Use of center lines.Figure 3-27.-Correct and incorrect procedures for draw-ing adjoining bidden lines.3-21
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