Hidden Line Representation in Section Views.
In section view drawings, hidden line representation is omitted in that part of the view with the section lining. Threee different orthographic representations of the hub are shown: (A) A standard orthographic drawing of the hub showing the front and top views; (B) a full section in the front view but with hidden lines inappropriately included in the background; and (C) a correct conventional representation of the full section in the front view which omits these hidden lines. In all cases the top view of the drawing is a standard orthographic view complete with visible and hidden lines. Because the cutting plane is obvious, the cutting plane line is omitted in the top view.
Line Precedence in Section View Drawings.
Visible object lines take precedence over hidden and centerlines. Hidden lines take precedence over centerlines. Cutting plane lines take precedence over centerlines when locating the cutting plane for the section view. Centerlines are included in the section view, but hidden lines are omitted.
Parts Not Sectioned.
Certain features of engineering parts are generally not sectioned even though the cutting plane may pass through them. When the cutting plane passes through the length of supporting ribs, lugs, and other thin parts, the feature is represented without section lines to avoid a misimpression of solidity. Here the cutting plane passes through a thin rib and a lug. The section view represents the cylindrical body and base of the part with section lining, but the thin rib and lug are not sectioned in accordance with this convention.
Conventional Representation of Mechanical
Elements in Section Views.
In addition to thin structural features, parts not sectioned also include standard mechanical elements such as shafts, bolts, screws, nuts, rivets, keys, pins, bearings, and gear teeth. Here the shaft, bolts, and nuts of the assembly are not sectioned even though they are cut by the cutting plane.
Treatment of Webs and Spokes in Section Views.
When shown in section views (case A), the web is sectioned to indicate the continuity of the material throughout the circumference of the wheel. when shown in section views (case B), spokes are not sectioned to indicate gaps around the circumference.
The Use of Alternate Sectioning.
When both a web and ribs are present in section views of cylindrical parts, the alternate sectioning rule applies. (A) The base web is sectioned according to normal practice. (B) When ribs are present in addition to the web, the usual convention for ribs would result in a view identical to case A. To eliminate this misrepresentation, a technique called alternate sectioning is employed. In this technique only alternating section lines are shown in the cross-hatching style as depicted in case B.
Foreshortened Features in Section Views.
(A) True orthographic projection sometimes yields a misimpression of objects. Foreshortening of features such as holes, lugs, ribs, spokes, and arms should be avoided. (B) Recommended practice is to rotate the feature into the plane of projection to yield an aligned view. Here both the hole and the rib have been rotated.
Foreshortened Features in Section Views.
(A) True orthographic projection sometimes yields a misimpression of objects. For example, foreshortening of the spokes should be avoided. (B) Recommended practice is to rotate the spoke into the plane of the section view. The spoke itself is not sectioned in keeping with conventional practice. In addition, the rear spoke projection is omitted here for clarity.
The use of Partial Views in Sectioning.
In section views of symmetrical parts, partial views may be used. For example, only half of the top view of the rod base is shown. This is permissible because the rod base is circularly symmetric and the missing half of the top view can easily be inferred by the reader. When using half views in sectioning, the cutting plane is omitted because the cut through the object is obvious.
Cutting Plane Intersection Practices in Setion Views.
(A) When the true projection of the intersection results in only a minor distortion, the true line of intersection is disregarded and a simplification is used. In this case, the true projection is replaced with a simple straight line projection. (B) In the case where the cutting plane intersects large features such as this large through hole, the true orthographic projection is shown.