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Chapter 4 Drafting: Geometric Construction
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Engineering Aid 3 - Beginning Structural engineering guide book
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Figure 4-3.-Dropping a perpendicular from a given point to a line. Figure  4-3  shows  a  method  of  dropping  a perpendicular from a given point to a line, using a  compass.  To  drop  a  perpendicular  from  point P  to  AB,  set  the  needlepoint  of  the  compass  at P and strike an arc intersecting AB at C and D. With C and D as centers and any radius larger than  one-half  of  CD,  strike  arcs  intersecting  at E.  A  line  from  P  through  E  is  perpendicular  to AB. Figure  4-4  shows  a  method  of  erecting  a perpendicular  from  a  given  point  on  a  line.  To erect a perpendicular from point P on AB, set a compass  to  any  convenient  radius,  and,  with  P as a center, strike arcs intersecting AB at C and D. With C and D as centers and any radius larger than  one-half  of  CD,  strike  arcs  intersecting  at E.  A  line  from  P  through  E  is  perpendicular  to AB. BISECTION OF A LINE A line can be bisected by trial and error with dividers;  that  is,  by  setting  the  dividers  to  various Figure 4-5.-Bisecting a line. spreads  until  you  find  one  that  correctly  measures one-half  the  length  of  the  line. Geometric construction for bisecting a line is shown  in  figure  4-5.  To  bisect  the  line  AB,  use the  ends  of  the  line,  A  and  B,  as  centers;  set  a compass  to  a  radius  greater  than  one-half  the length of AB; and strike arcs intersecting at C and D.  A  line  drawn  from  C  through  D  bisects  AB. DIVISION  INTO  ANY  NUMBER OF  EQUAL  PARTS A  line  may  be  divided  into  more  than  two equal parts by trial and error with the dividers. Geometric  construction  for  dividing  a  line  into  any number of equal parts is shown in figure 4-6. To divide AB into 10 equal parts, draw a ray line CB from B at a convenient acute angle to AB. Set a compass  to  spread  less  than  one-tenth  of  the length of CB, and lay off this interval 10 times from B on CB. Draw a line from the 10th interval Figure 4-4.-Erecting a perpendicular from a given point on a line. Figure 4-6.-Dividing a line into any number of equal parts. 4-2







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