27-08-2012, 10:05 AM
PROJECTIONS OF SOLIDS
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Projections of Solids in Simple Positions
A solid in simple position may have its axis perpendicular to
one reference plane or parallel to both. When the axis is
perpendicular to one reference plane, it is parallel to the other.
Also, when the axis of a solid is perpendicular to a plane, its
base will be parallel to that plane. We have already seen that
when a plane is parallel to a reference plane, its projection on
that plane shows its true shape and size.
Therefore, the projection of a solid on the plane to which its
axis is perpendicular will show the true shape and size of its
base.
Hence, when the axis is perpendicular to the ground, i.e. to the
H.P., the top view should be drawn first and the front view
projected from it.
Projections of solids with axes inclined to one of
the reference planes and parallel to the other
When a solid has its axis inclined to one plane and parallel to
the other, it projections are drawn in two stages.
a. In the initial stage, the solid is assumed to be in simple
position, i.e. its axis perpendicular to one of the planes.
If the axis is to be inclined to the ground, i.e. the H.P., it is
assumed to be perpendicular to the H.P. in the initial stage.
Similarly, if the axis is to be inclined to the V.P., it is kept
perpendicular to the V.P. in the initial stage.
Moreover
i. if the solid has an edge of its base parallel to the H.P.
or in the H.P. or on the ground, that edge should be
kept perpendicular to the V.P.; if the edge of the base is
parallel to the V.P. or in the V.P., it should be kept
perpendicular to the H.P.
ii. if the solid has a corner of its base in the H.P. or on
the ground, the sides of the base containing that
corner should be kept equally inclined to the V.P.; if the
corner is in the V.P., they should be kept equally
inclined to the H.P.
b. Having drawn the projections of the solid in its simple
position, the final projections may be obtained by one of the
following two methods