Page 79 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

– In an oblique drawing one of the principal faces of the object is kept parallel to the plane of projection, but in
isometric, none of the faces of the object is parallel to the plane of projection.

Even though one of face of the object is positioned parallel to the picture plane, we still get a pictorial
view and the projections are inclined to both HP and VP.
Because one of the principle faces of the object is parallel to the plane of projection, in the oblique projection, the
projection of this face and faces parallel to it will appear in true size and shape. In the oblique projection of a prism
is shown in Fig 19, the faces ABCD and EFGH are parallel to the plane of projection and they appear to be true
in size and shape. The other four faces which are perpendicular to the plane of projection do not appear in true
shape. (all these four faces are seen as parallelogram) However the vertical edges of these faces are parallel to
the plane of projection and hence the projection of these edges will measure to their true lengths.
Projection of edges such as AE, DH, BF and CG which are perpendicular to the plane of projection will measure
differently depending on the angle of inclination of the projectors. If the inclination of the projectors is 45° the
projections of these edges measure to their true lengths. If the angle is less than 45° the projection of such
perpendicular edges will measure less than the true length, if the angle of inclination of the projectors is greater
than 45°. Projection of such perpendicular edges will measure more than the true length.
Oblique projections nevertheless have an unique advantage what we want to make pictorial drawings of object
having curved features. For making isometric views of a curved feature we have first to draw their orthographic
views in order to find out the offsets of points lying on the curve. But this difficult procedure is not necessary in
the case of oblique views.
For example the component shown in Fig 20 has several curved features. While drawing oblique view of this
component the curved features are drawn to true shape using compass. This is relatively easier method in
comparison to the drawing of the same component in isometric view.
Fig 19 Fig 20

Inclination of projectors: While projectors can have any angle of inclination, usually oblique views are drawn to
either 45° or 30°. The inclined lines in the oblique drawings are called receding lines (Fig 21) and they are more
commonly drawn to 45°. The scale of lines along receding may be 1:1 (true lengths) or 1:2 (half of the true length).
Oblique drawing drawn to 1:1 (Fig 22) are called as cavalier projections and those drawn to 1:2 (Fig 23) are
called as cabinet projections. These two terms are of academic interest only and mostly we refer these views as
oblique projections/drawings only.
In comparisn a cabinet projection will look less distorted than a cavalier projection.

Fig 21 Fig 22

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CITS : Engineering Drawing (Mechanical) - Exercise 5