Projection of Solids and Sections of Solids
What the exam tests
Projection of solids and sections tests prisms, pyramids, cylinders and cones in simple and inclined positions, and the effect of section planes. You must draw the sectional view, true shape of section, and hatching correctly.
Core rules
- Start with the solid in its simple position: axis perpendicular to HP, base on HP. Draw the plan first when the axis is vertical, then project the elevation.
- For an inclined axis, use the change-of-position method: draw the simple position, tilt the plan or elevation, then project the final view.
- A section plane is shown by a chain thick line with arrows at the ends; its trace is marked on the view where it appears as an edge.
- After sectioning, remove the cut part in the sectional view, draw the cut face with uniformly spaced 45° hatching, and retain visible edges behind the plane.
- The true shape of a section is found by projecting section points onto an auxiliary plane parallel to the cutting plane.
Worked example
A cylinder of diameter 50 mm and axis 70 mm stands on HP. A section plane perpendicular to VP and inclined at 30° to HP cuts the cylinder. Draw the elevation and mark the inclined section trace. Where the trace crosses the cylinder's extreme generators gives the section's major axis; the plan width gives the minor axis. Project points from the cut circle to an auxiliary plane parallel to the section plane to obtain the true ellipse. Hatch the cut face in the sectional view only.
Traps
Do not hatch the whole solid, only the cut surface. Hidden edges behind the section must not be drawn as visible outlines. A common error is to call the apparent section shape the true shape without auxiliary projection. Check that section-plane arrows point toward the removed part.