Technical Drawing · Lesson 8

Hatching, types of section, conic sections, true shapes of cut solids

WAECNECO

What the syllabus asks

WAEC: WAEC/NECO Technical Drawing, Solid Geometry item 16 (sectional views and true shapes of sections of prisms, pyramids, cylinders, cones and spheres; conic sections) and Mechanical Drawing item 4 (types of section and conventions). (Not a JAMB UTME subject: Technical Drawing is absent from JAMB’s official IBASS e-syllabus list.)

NECO: WAEC/NECO Technical Drawing, Solid Geometry item 16 (sectional views and true shapes of sections of prisms, pyramids, cylinders, cones and spheres; conic sections) and Mechanical Drawing item 4 (types of section and conventions). (Not a JAMB UTME subject: Technical Drawing is absent from JAMB’s official IBASS e-syllabus list.)

What you will learn

Explain the purpose of sectional views and show cutting planes correctly.
Apply hatching rules for single parts and assemblies.
Select and draw full, half, offset, aligned, part, revolved and removed sections.
Apply the conventions for parts not sectioned.
Draw sections and true shapes of prisms, pyramids, cylinders, cones and spheres.
Produce sectional views of machine parts and buildings.

Key points

Sections reveal internal detail without hidden lines.
Cutting plane: chain line, thick ends, arrows, letters.
Hatch thin at 45°, even, same direction per part.
Never hatch empty space.
Know full, half, offset, aligned, part, revolved, removed.
Do not hatch shafts, fasteners, keys, ribs cut lengthwise.
Cylinder oblique section: ellipse, major d/cos α.
Cone: circle, ellipse, parabola, hyperbola, triangle.
True shape by auxiliary projection.
Building sections use material symbols.

Start here: Why sections are used

Hidden lines are useful for simple parts, but inside a valve body, a pump casing or a gearbox they become a confusing tangle of dashes. A sectional view shows the object as if it had been cut by an imaginary cutting plane, with the part nearer the observer removed. The inside is then drawn with visible lines, and the solid material that the plane cuts through is shaded with thin parallel lines called hatching.

Benefits

Internal shapes become clear: bores, recesses, ribs, threads.
Dimensions can be applied to visible lines instead of hidden lines (a rule of good dimensioning).
Assemblies show how parts fit together, which part is which, and the materials used.

The cutting plane

The position of the cut is shown on an adjacent view by a cutting-plane line: thin chain, thick at the ends and at changes of direction (BS 8888/ISO 128 type, inherited from BS 308). Arrows at the ends point in the direction of viewing, and capital letters label the plane, for example A–A. The sectional view is titled SECTION A–A.

What is shown in a section

The cut surface (hatched).
All visible edges beyond the cutting plane, in the direction of viewing.
Hidden lines are normally omitted in sections, unless needed for clarity.

Think of a sliced loaf of agege bread: the cut face shows the inside texture, and you can also see the crust edges beyond the cut. A sectional view shows both.

Where the cutting plane goes

Choose the plane so that it passes through the most important internal features, normally along a main centre line of symmetry. If the plane lies on a centre line of symmetry and the section is placed in normal projection, the cutting-plane line and labels may be omitted (BS 8888 allows this when there is no doubt), but in examinations always show it: it costs little time and earns marks.

Syllabus links

WAEC solid geometry item 16 requires sectional views and true shapes of sections of prisms, pyramids, cylinders and cones (including the conic sections). Mechanical drawing item MD4 requires the types of section and the conventions. Both appear in this unit.

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Practice questions

1. Section (hatching) lines are normally

thin lines at 45°
thick lines at 45°
thin lines at 90°
dashed lines

Show answer

A: thin lines at 45°
Thin, evenly spaced, usually 45°.

2. Adjacent parts in an assembly section are hatched

identically
not at all
with thick lines
in different directions or spacing

Show answer

D: in different directions or spacing
So the parts can be told apart.

3. Which of these is normally NOT sectioned when cut along its axis?

A housing
A pulley rim
A shaft
A casting

Show answer

C: A shaft
Solid shafts, bolts, keys, ribs and webs are not hatched lengthwise.

4. A half section is used for

developments
symmetrical objects
irregular castings only
threads only

Show answer

B: symmetrical objects
One half shows outside, the other the section.

5. In a half section, hidden lines in the outside half are

usually omitted
always shown
hatched
drawn thick

Show answer

A: usually omitted
The section already shows the interior.

Key words

Section: View of an object cut by a plane.
Cutting plane: Imaginary plane making the cut.
Hatching: Thin parallel lines on cut material.
Full section: Section with the plane right through the object.
Half section: Half outside view, half section.
Offset section: Section on a stepped cutting plane.
Aligned section: Section with an inclined part revolved into line.
Part section: Local broken-out section.
Revolved section: Cross-section turned and drawn on the view.
Removed section: Cross-section drawn away from the view.
Rib: Thin supporting web of a casting.
Web: Thin plate joining parts of a casting.

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