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ADJUSTABLE  TRIANGLES
Dividers

Engineering Aid 3 - Beginning Structural engineering guide book
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45.127X Figure 2-14.-French curves. Figure 2-14 shows an assortment of french curves. In such an assortment you can find edge segments that can be fitted to any curved line that you need to  draw. French curves should be cared for and stowed in the same manner as triangles. DRAWING  INSTRUMENT  SETS So  far  we  have  discussed  only  those  instru- ments  and  materials  that  you  will  need  for drawing  straight  lines  (with  the  exception  of french  curves).  Many  drawings  that  you  will prepare will require circles and circular arcs. For this  purpose,  instruments  contained  in  a  drawing instrument set are used. Many types of drawing instrument   sets   are   available;   however,   it   is sometimes   difficult   to   judge   the   quality   of drafting instruments by appearance alone. Often their characteristics become evident only after they are used. The drawing instrument set shown in figure 2-15 is typical of those sets found in the standard draftsman  kit.  The  following  sections  describe these instruments. Some special-purpose instru- ments not found in the set will also be described. They  may  be  purchased  separately  or  found  in other instrument sets. Compasses Circles and circular curves of relatively short radius are drawn with COMPASSES. The large pivot joint compass (fig. 2-15C) is satisfactory for drawing circles of 1 in. to about 12 in. in diameter without  an  extension  bar.  The  pivot  joint  provides enough  friction  to  hold  the  legs  of  the  compass in a set position. One of the legs is equipped with a setscrew for mounting either a pen (fig. 2- 15B) or a pencil attachment on the compass. There is also an extension bar (fig. 2-15D), which can be inserted to increase the radius of the circle drawn. The  other  type  of  compass  found  in  the drawing  instrument  set  is  the  bow   compass (fig. 2-15K and 2- 15L). Many experienced drafts- men prefer the bow compass over the pivot joint compass. The bow compass is much sturdier and is capable of taking the heavy pressure necessary to produce opaque pencil lines without losing the radius setting. There  are  two  types  of  bow  compasses.  The location of the adjustment screw determines the type.  The  bow  pen  (fig.  2-15K)  and  bow  pencil (fig.  2-15L)  are  the  center   adjustment   type, whereas the bow instruments shown in figure 2-16 are the side adjustment type. Each type comes in two sizes: large and small. Large bow compasses are   usually   of   the   center   adjustment   type, although the side adjustment type is available. The large bow compasses are usually about 6 in. long; the  small,  approximately  4  in.  long.  Extension bars are available for large bow compasses. Bow compasses  are  available  as  separate  instruments, as  shown  in  figures  2-15  and  2-16,  or  as combination  instruments  with  pen  and  pencil attachments. Most  compasses  have  interchangeable  needle- points.  The  conical  or  plain  needlepoint  is  used when  the  compass  is  used  as  dividers.  The shoulder-end  needlepoint  is  used  with  pen  or pencil attachments. When many circles are drawn using the same center, the compass needle may tend to bore an oversized hole in the drawing. To prevent these holes, use a device called a horn center or center disk (fig. 2-151). This disk is placed over the center point.  The  point  of  the  compass  needle  is  then placed  into  the  hole  in  its  center. 2-12







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