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Drop Bow Pen
PROPORTIONAL  DIVIDERS

Engineering Aid 3 - Beginning Structural engineering guide book
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45.132E Figure 2-19.-Divider maintenance: (A) Evening the legs of dividers;  (B)  Sharpening  divider  needlepoints. pinhole. Since the same center is often used for both the compasses and dividers, it is best that needles on both be the same size. If the compass needle is noticeably larger, grind it until it is the correct  size. To  make  a  compass  needle  smaller,  wet  one side of the whetstone and place the needle with its  shoulder  against  this  edge.  Then  grind  it  against the whetstone, twirling it between your thumb and forefinger (fig. 2-20). Test it for size by inserting it in a hole made by another needle of the correct size. When it is pushed as far as the shoulder, it should  not  enlarge  the  hole. The  screw  threads  on  bow  instruments  are delicate.  Because  of  this,  you  should  take  care never  to  force  the  adjusting  nut.  Threads  must be  kept  free  from  rust  or  dirt. If   possible,    it  is  best  to  keep  drawing instruments in a case, since the case protects them from  damage  from  falls  or  unnecessary  pressures. Then, too, the lining of the case is usually treated with a chemical that helps prevent the instruments from  tarnishing  or  corroding. To protect instruments from rusting when they are not in use, clean them frequently with a soft cloth and apply a light film of oil to their surface 142.35 Figure 2-20.-Shaping a compass needle. with  a  rag.  Joints  on  compasses  and  dividers should  not  be  oiled.  When  the  surface  finish  of instruments   becomes   worn   or   scarred,   it   is subject  to  corrosion;  therefore,  a  knife  edge  or an   abrasive   should   never   be   used   to   clean drafting  instruments. Beam  Compass The BEAM COMPASS (fig. 2-21) is used for drawing circles with radii larger than can be set on a pivot joint or bow compass. Both the needle- point attachment and the pen or pencil attachment on a beam compass are slide-mounted on a metal 45.134X Figure  2-21.-Beam  compass. 2-15







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