BREAK LINES

DIMENSION LINES

PHANTOM LINES

Engineering Aid 3 - Beginning Structural engineering guide book
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Figure 3-31.-Breaking extension lines and leaders at points of intersection. radius of an arc is to be indicated, there is an arrow at only the end of the line that touches the arc. The other end, without an arrow, terminates at the point used as the center in drawing the arc. The arrowhead on a dimension or leader line is an important detail of a drawing. If these arrowheads are sloppily drawn and vary in size, the drawing will not look finished and professional. The size of the arrowhead used on a drawing may vary with the size of the drawing, but all arrowheads on a single drawing should be the same size, except occasionally when space is very restricted. The arrowheads used on Navy drawings are usually solid, or filled in, and are between one eighth and one fourth of an inch long, with the length about three times the spread. (See fig. 3-32.) With a little practice, you can learn to make good arrowheads freehand, Referring to figure 3-32, first define the length of the arrowhead with a short stroke as shown at A. Then draw the sides of the arrowhead as indicated at B and C. Finally, fill in the area enclosed by the lines, as shown at D. LEADERS Leaders are used to connect numbers, references, or notes to the appropriate surfaces Figure 3-32.-Method of drawing an arrowhead. or lines on the drawing. From any suitable portion of the reference, note, or number, a short line is drawn parallel to the lettering. From this line the remainder of the leader is drawn at an angle (dog leg) to an arrowhead or dot. In this way, the leader will not be confused with other lines of the drawing. If the reference is to a line, the leader is always terminated at this line with an arrowhead, as shown in figure 3-33. However, a reference to a surface terminates with a dot within the outline of that surface. BREAK LINES The size of the graphic representation of an object is often reduced (usually for the purpose of economizing on paper space) by the use of a device called a break. Suppose, for example, you want to make a drawing of a rectangle 1 ft wide by 100 ft long to the scale of 1/12, or 1 in. = 1 ft. If you drew in the full length of the rectangle, you would need a sheet of paper 100 in. long. By using a break, you can reduce the length of the figure to a feasible length, as shown in figure 3-34. Figure 3-33.-Use of a leader. Figure 3-34.-Use of proper line conventions for (A) short break, and (B) long break. 3-23

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