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VERTICAL  LINES
Use of the Compass - CONTINUED

Engineering Aid 3 - Beginning Structural engineering guide book
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be  measured.  Extend  line  AO  on  to  C;  extend  line BO  on  to  D.  When  you  set  the  center  of  the protractor at O, make sure that both points c and a  are  on  line  AC.  Take  your  reading  at  point  d as well as at point b when you measure the angle. If you are laying off the angle BOA, protract and mark  point  d  as  well  as  point  b;  this  gives  you three points (d, O, and b) for establishing line DB. If  you  are  using  a  semicircular  protractor,  you can’t, of course, locate point d; but your accuracy will be improved by lining up c, O, and a before you  measure  or  lay  off  the  single  angle  BOA. PARALLEL   AND PERPENDICULAR LINES To draw a line parallel to a given line (fig. 3-6, view  A),  adjust  the  hypotenuse  of  a  triangle  in combination  with  a  straightedge  (T  square  or triangle)  to  the  given  line;  then,  holding  the straightedge firmly in position, slip the triangle to the desired position and draw the parallel line along  the  hypotenuse. To construct a line perpendicular to an existing line, use  the  triangle  and  straightedge  in combination, with the hypotenuse of the triangle resting  against  the  upper  edge  of  the  straightedge (fig.  3-6,  view  B).  Adjust  one  leg  of  the  triangle to a given line. Then slide the triangle along the supporting straightedge to the desired position and draw the line along the leg, perpendicular to the Figure 3-6.-Drawing parallel and perpendicular lines. leg that was adjusted to the given line. In the same manner, angles with multiples of 15 degrees may be drawn, using the triangle combinations shown in  figure  3-4. CURVED LINES Many drawings that you will prepare require the  construct  ion  of  various  curved  lines.  Basically there  are  two  types  of  curved  lines:  circles  and segments of circles, called arcs, which are drawn with  a  compass;  and  noncircular  curves,  which are  usually  drawn  with  french  curves.  In  this chapter we will discuss only techniques for using the compass and the french curve. Application of compass  techniques  in  geometric  construction  will be  covered  in  chapter  4. Use of the Compass When you are drawing circles and arcs, it is important  that  the  lines  produced  with  the compass  are  the  same  weight  as  corresponding pencil  lines.  Since  you  cannot  exert  as  much pressure  on  the  compass  as  you  can  with pencils,  you  should  use  a  compass  lead  that  is Figure 3-7.-Sharpening the compass lead and adjusting the point. 3-6







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