Orthographic views when carefully selected, may reveal the external features of even the most complicated objects. However, there are objects with complicated interior details and when represented by hidden lines, may not effectively reveal the true interior details. This may be overcome by representing one or more of the views ‘in section’.
A sectional view is obtained by imagining the object, as if cut by a cutting plane and the portion between the observer and the section plane being removed. Figure a shows an object, with the cutting plane passing through it and Fig. b, the two halves drawn apart, exposing the interior details.
A sectional view obtained by assuming that the object is completely cut by a plane is called a full section or sectional view. Figure a shows the view from the right of the object shown in Fig. a, in full section. The sectioned view provides all the inner details, better than the unsectioned view with dotted lines for inner details (Fig. b). The cutting plane is represented by its trace (V.T) in the view from the front (Fig. c) and the direction of sight to obtain the sectional view is represented by the arrows.
It may be noted that, in order to obtain a sectional view, only one half of the object is imagined to be removed, but is not actually shown removed anywhere except in the sectional view. Further, in a sectional view, the portions of the object that have been cut by the plane are represented by section lining or hatching. The view should also contain the visible parts behind the cutting plane.
Figure below represents the correct and incorrect ways of representing a sectional view. Sections are used primarily to replace hidden line representation, hence, as a rule, hidden lines are omitted in the sectional views.
A half sectional view is preferred for symmetrical objects. For a half section, the cutting plane removes only one quarter of an object. For a symmetrical object, a half sectional view is used to indicate both interior and exterior details in the same view. Even in half sectional views, it is a good practice to omit the hidden lines. Figure a below shows an object with the cutting plane in position for obtaining a half sectional view from the front, the top half being in section. Figure b below shows two parts drawn apart, exposing the inner details in the sectioned portion. Figure c below shows the half sectional view from the front. It may be noted that a centre line is used to separate the halves of the half section. Students are also advised to note the representation of the cutting plane in the view from above, for obtaining the half sectional view from the front.
Auxiliary sections may be used to supplement the principal views used in orthographic projections. A sectional view projected on an auxiliary plane, inclined to the principal planes of projection, shows the cross-sectional shapes of features such as arms, ribs and so on. In below given figure, auxiliary cutting plane X-X is used to obtain the auxiliary section X-X.
When specific features of an object that need highlighting are not located on the straight line of the cutting plane, an irregular-shaped cutting plane is imagined cutting the object, revealing the desired components. This is called an “offset view,” and is effective on complex objects. The bends in the imaginary cutting pane are always 90 degrees.
A “revolving view” is effective for elongated objects or the elongated section of an object. In this view, the cross-sectional shape of ribs, spokes, and other projections of the object are featured. The cutting plane cuts the object at an angle, but the drawing is rotated for a better view by the observer.
When only a small part of the object needs viewing, the cutting plane is not used. An irregular cut line removes a section of the object at the desired depth, leaving a “broken view.” A broken view is helpful when only specific interior details in a certain part of the object need featuring.