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Cavalier Projection
ANGLES IN OBLIQUE

Engineering Aid 3 - Beginning Structural engineering guide book
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may be drawn from O at any convenient angle. All three are equal in length, the length being the length of an edge of the original cube (which may be scaled down or up if the drawing is made other than  full  scale).  After  you  draw  the  axis, complete the projection by drawing the required parallel   lines.   All   the   edges   shown   in   the projection  are,  like  the  edges  on  the  original  cube, equal in length. Cabinet Projection The  first  thing  you  notice  about  the  cube shown in figure 5-48 is the fact that it doesn’t look like a cube because the depth dimension appears to be longer than the height and width dimensions. The  reason  for  this  is  the  fact  that  a  cavalier projection corrects a human optical illusion—the one  that  causes  an  object  to  appear  to  become smaller as its distance from the eye increases. This illusion. in turn. causes receding parallel lines to appear to the eye to be shorter-than they really are, and also to be converging toward a point in the  distance.  But  receding  parallel  lines  on  a cavalier projection appear in their true lengths, and they remain constantly parallel. Also, the far edges of the cube shown in figure 5-48 are equal in length to the near edges. The distortion in figure 5-48 is only apparent. It is sometimes desirable to reduce this appearance of  distortion.  This  can  be  done  by  reducing  the length  of  the  receding  axis  (OC  in  fig.  5-39). This axis can be reduced by any desired amount, but it is customary  to reduce  it by one half. Figure  5-49.-Cabinet  projection  of  the  cube  in  figure  5-48. (Note receding axis OC reduced by one half its length.) When  the  receding  axis  is  reduced  by  one  half, the  projection  is  called  a  CABINET  PROJEC- TION.  Figure  5-49  shows  a  cabinet  projection of a cube. The length of the receding axis OC has been  reduced  by  one  half.  As  you  can  see,  this representation  looks  more  like  a  cube. Cavalier  and  cabinet  projections  are  compared in figures 5-50 and 5-51. Oblique Drawing Techniques In   an   oblique   projection   drawing   of   a rectangular  object,  one  face  (usually  the  most prominent  or  most  important)  is  parallel  to  the plane  of  projection.  All  features  appearing  on  this plane, such as circles or oblique lines, are in their true dimension. However, in the side or top views, these   same   features   are   somewhat   distorted because  of  the  receding  axis  angle.  When  drawing these features, you can use various techniques to aid  you  in  their  construction. For  convenience,  the  angles  chosen  for  the receding  axis  are  either  30  degrees,  45  degrees,  or Figure  5-50.-Cavalier  projection.  Distances  along  front  axis and along receding axis are all true. 5-26







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