Last reviewed 16 Sept 2026 · 8 min read
Sections of solids
Sectioning shows interior details of objects that would otherwise appear as confusing hidden lines. An imaginary cutting (section) plane cuts the object; the portion between the observer and the plane is removed, and the remaining part is projected.
Section planes
| Section plane | Front view | Top view | True shape of section |
|---|---|---|---|
| Parallel to HP (⊥ VP) | Line parallel to XY | Section in true shape | Seen in top view |
| Parallel to VP (⊥ HP) | Section in true shape | Line parallel to XY | Seen in front view |
| Perpendicular to VP, inclined to HP (AIP) | Line inclined to XY (cutting plane trace) | Apparent section (foreshortened) | On an auxiliary plane parallel to the section plane |
| Perpendicular to HP, inclined to VP (AVP) | Apparent section | Line inclined to XY | On auxiliary plane parallel to section plane |
- Apparent section — the projection of the cut surface on HP or VP (not true shape).
- True shape of section — obtained by projecting on a plane parallel to the section plane (auxiliary view).
Hatching (section lines)
- Thin continuous lines, usually at 45° to the principal outline or axis, uniformly spaced (commonly about 1–2 mm on small areas, larger for big areas).
- Adjacent parts in an assembly are hatched in different directions or spacing.
- For thin sections (e.g. gaskets, sheet metal), the section may be fully blackened.
- Large areas may be hatched only near the boundary.
- Hatching lines should not be parallel or perpendicular to the main outline.
- Standard material symbols are used in building drawings (brick, concrete, earth, etc.).
Cutting plane representation
- Shown in the adjacent view by a thin chain line, thick at ends and changes of direction, with arrows indicating viewing direction and letters (e.g. section A–A).
Types of sectional views
| Type | Description / use |
|---|---|
| Full section | Cutting plane passes completely through the object — the whole view is sectioned |
| Half section | For symmetrical objects — one quarter removed; half the view is sectional and half external; centre line separates them; hidden lines usually omitted in both halves |
| Offset section | Cutting plane bends (offsets) to pass through features not in one plane; offsets not shown in the sectional view |
| Aligned section | Features (ribs, holes, spokes) at angles are rotated into the cutting plane for true representation |
| Revolved section | Cross-section rotated 90° in place on the view (e.g. arms, spokes, beams) |
| Removed section | Revolved section moved outside the view and labelled |
| Broken-out (partial) section | Small portion removed to show a local detail, bounded by a freehand break line |
| Auxiliary section | Section on an inclined plane projected in an auxiliary view |
Conventions — parts not sectioned longitudinally
When the cutting plane passes along the axis of the following, they are shown unsectioned: shafts, bolts, nuts, screws, rivets, pins, keys, washers, balls, ribs/webs, spokes of wheels, gear teeth. (Sectioning them conveys no useful information.)
Sections of solids — shapes obtained
| Solid | Section plane | Section shape |
|---|---|---|
| Prism | Parallel to base | Same as base |
| Prism | Parallel to axis | Rectangle |
| Pyramid | Parallel to base | Similar polygon (smaller) |
| Pyramid | Through apex | Triangle |
| Cylinder | Parallel to base | Circle |
| Cylinder | Parallel to axis | Rectangle |
| Cylinder | Inclined to axis (cutting all generators) | Ellipse |
| Cone | Parallel to base | Circle |
| Cone | Through apex | Isosceles triangle |
| Cone | Inclined, cutting all generators | Ellipse |
| Cone | Parallel to a generator | Parabola |
| Cone | Parallel to axis (or steeper than generators) | Hyperbola |
| Sphere | Any plane | Circle |
Methods for sections of cones/cylinders: generator (line) method — divide the base into equal parts and draw generators; circle method — horizontal cutting circles on a cone.
Development of surfaces
Development is the unfolding of the lateral (and base) surfaces of a solid onto a plane, showing the true size and shape of every surface. Developments are essential for sheet metal work — ducts, hoppers, chimneys, funnels, pipes, tanks, trays, transition pieces.
Methods of development
| Method | Used for |
|---|---|
| Parallel line development | Solids with parallel lateral edges/generators — prisms, cylinders |
| Radial line development | Solids with edges/generators meeting at an apex — pyramids, cones |
| Triangulation | Transition pieces and oblique solids — surfaces divided into triangles whose true lengths are found |
| Approximate method | Doubly curved surfaces (sphere) — not truly developable; approximated by gores or zones |
Development of common solids
- Right prism: rectangle of length = perimeter of base, height = axis height; lateral edges as parallel lines.
- Right cylinder: rectangle of length and height .
- Right pyramid: series of triangles with slant edges as radii, drawn around the apex (radial lines).
- Right cone: sector of a circle of radius (slant height) with sector angle
- Frustum of cone: annular sector between radii (full cone) and (removed top cone).
Truncated and cut solids: plot points where the cutting plane intersects edges/generators in the views; transfer their true distances (from the base or apex) to the corresponding lines in the development; join with straight lines (prisms/pyramids) or smooth curves (cylinders/cones).
- For pyramids and cones, true lengths along slant edges/generators are found by projecting horizontally onto a true-length edge (the extreme generator/edge parallel to VP).
- Seam/joint usually placed along the shortest edge to save welding/riveting.