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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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