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ISOMETRIC PROJECTION AND ISOMETRIC
DRAWING.
Figure 5-37 shows a cube
projected by ISOMETRIC PROJECTION, the
most frequently used type of axonometric
projection. The cube is inclined so that
all of its surfaces make the same
angle (3516) with the plane of
projection. As a result of this inclination,
the length of each of the edges shown in
the projection is somewhat shorter than the actual
length of the edge on the object itself. This reduction
is called FORESHORTENING. The degree
of reduction amounts to the ratio of 1 to the
cosine of 3516, or 1/0.8165. This means that if
an edge on the cube is 1 in, long, the projected edge
will be 0.8165 in. long. As all of the surfaces make
the same angle with the plane of projection, the
edges all foreshorten in the same ratio. Therefore,
one scale can be used for the entire layout;
hence the term isometric, which
literally means
"one-scale." Figure 5-38 shows an isometric projection as it would look to an observer whose line of sight was perpendicular to the plane of projection. Note
Figure 5-37.-Isometric projection of a cube. that the figure has a central axis, formed by the lines OA, OB, and OC. The existence of this axis is the origin of the term forms a 120-degree angle with the adjacent line, as shown. A quick way to draw the axis is to draw the perpendicular OC, then use a T square and 30/60 triangle to draw OA and OB at 30 degrees to the horizontal. Since the projections of parallel lines are parallel, the projections of the other edges of the cube will be, respectively, parallel to these axes.A rectangular object can be easily drawn in isometric by the procedure known as box construction. In the upperpart of figure 5-39, there is a two-view normal multi-view projection of a rectangular block. An isometric drawing of the block is shown below. You can see how you build the figure on the isometric axis and how you lay out the dimensions of the object on the
Figure 5-38.-Use of an isometric axis. isometric drawing. Because you lay out the identical dimensions, it is an isometric drawing rather than an isometric projection. |
 
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