Sections, auxiliary views and developments
This domain tests how internal form is revealed and how true shapes are recovered: section views, auxiliary views and surface development. Questions typically ask you to complete a section, identify what should be hatched, or develop a truncated cone.
Sections
- The cutting plane is a thick long-dash-dot line with arrows showing the viewing direction and letters matching the label on the section view.
- Cut material is hatched with thin continuous lines at 45 degrees, spaced 2 to 4 mm. Adjacent parts are hatched in opposite directions or at different spacings.
- Ribs, webs, shafts, bolts, nuts, keys and pins are not hatched when cut lengthwise, because hatching would suggest false mass.
- Half sections suit symmetrical parts; offset sections step the cutting plane through features not in one plane; aligned sections rotate features into the plane; revolved sections sit on the view.
Auxiliary views
Project perpendicular to an inclined face, using a reference line parallel to the inclined edge, to obtain the true shape. Transfer distances from the fold line of the adjacent view.
Developments
Parallel-line method for prisms and cylinders; radial-line method for pyramids and cones; triangulation for oblique shapes.
Worked example
Develop a cone of base radius 30 mm and slant height 90 mm. The development is a sector of radius 90 mm. Sector angle = 360 x 30 / 90 = 120 degrees. Draw an arc of radius 90 mm subtending 120 degrees; the arc length equals the base circumference of 188.5 mm.
Traps
Hatching a rib or shaft cut lengthwise; omitting the cutting-plane arrows; projecting an auxiliary view from the wrong reference line so the shape is not true; forgetting bend allowance in sheet-metal developments.