Quantcast USE OF THE DRAFTING SCALE

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one  leg  between  the  thumb  and  the  first  and second  fingers,  and  hold  the  other  leg  between  the third  and  fourth  fingers.  Place  the  second  and third fingers on the inside of the legs; the dividers are  opened  by  spreading  these  fingers  apart. Dividers are closed by squeezing the thumb and first   finger   toward   the   fourth   finger   while gradually  slipping  out  the  other  two  fingers. To transfer measurements on a drawing, set the dividers to the correct distance, then transfer the measurements to the drawing by pricking the drawing surface very lightly with the points of the dividers. To measure off a series of equal distances on the  line,  set  the  dividers  to  the  given  distance. Then  step  off  this  distance  as  many  times  as desired by swinging the dividers from one leg to the other along the line, first swinging clockwise 180 degrees, then counterclockwise 180 degrees, and  so  on. In  dividing  either  a  straight  line  (fig.  3-12, view   B)   or   a   curved   line   (fig.   3-12,   view   C) into a given number of equal parts (for example, four)  by  trial,  open  the  dividers  to  a  rough approximation of the first division (in this case, one  quarter  of  the  line  length)  and  step  off  the distance   lightly,   holding   the   dividers   by   the handle and pivoting the instrument on alternate sides  of  the  line  at  each  step.  If  the  dividers fall   short   of   the   end   of   the   line   after   the fourth  step,  hold  the  back  leg  in  place  and advance  the  forward  leg,  by  guess,  one  quarter of the remaining distance. Repeat the procedure until the last step falls at the end of the line. Be careful during this process not to punch holes in the  paper,  but  just  barely  mark  the  surface  for future  reference.  To  identify  prick  marks  made with  small  dividers  for  future  reference,  circle  the marks lightly with a pencil. USE OF THE DRAFTING SCALE Accuracy  in  drawing  depends  to  a  great extent  upon  correct  use  of  the  scale  in  marking off  distances.  You  should  place  the  edge  of  the scale parallel to the line being measured (fig. 3-13). To  eliminate  shadows  cast  by  your  body  or  hands, point  the  desired  scale  face  away  from  you  for horizontal  measurements  and  toward  your  left  for vertical   measurements.   With   a   sharp   pencil, mark  off  short  dashes  at  right  angles  to  the scale at the correct distances, aligning the mark carefully  with  the  scale  graduation.  Have  your eye  approximately  over  the  point  being  measured, Figure 3-13.-Use of the drafting scale. and  make  light  marks  to  denote  the  point  of measurement. When  setting  the  compass  to  a  given  radius or  when  setting  divider  points,  never  place  the sharp points of these instruments on the scale. Lay out the desired radius or distance on a straight pencil  line  by  using  the  scale  in  the  manner described  above.  Then  adjust  the  compass  or dividers   to   the   indicated   length   by   using the  measured  line.  A  scale  surface  marred  by pinpricks is difficult to read and is unsuitable for accurate  work. In  making  successive  measurements  along the  same  line,  make  as  many  measurements  as possible  without  moving  the  scale.  If  a  number of  distances  are  to  be  laid  out  end  to  end,  hold the scale in one position and add each successive measurement  to  the  preceding  one.  If  the  scale is  moved  to  a  new  position  each  time,  slight errors   in   measurement   may   accumulate.   For example, four successive measurements of 1 5/8 in. each should give an overall length of 6 1/2 in., not  6  9/16  in.  Therefore,  make  as  many measurements as you can without changing the reference point. This will avoid cumulative errors in the use of the scale. Note that your pencil touches the scale only for the purpose of marking a point on the paper. Never use a scale as a straightedge for drawing lines.  A  typical  office  ruler  has  a  metal  edge; it  is  a  scale  and  straightedge  combined.  But  a draftsman’s  measuring  scale  is  for  measuring only;    it    is    not    a    ruler.    A    scale    properly used will last for decades, but a scale used as a straightedge  will  soon  have  the  graduations  worn away. 3-10



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