Technical Drawing · Lesson 12

Limits and fits, threads, fasteners, keys, bearings, welding symbols and assemblies

WAECNECO

What the syllabus asks

WAEC: WAEC/NECO Technical Drawing, Mechanical Drawing items MD1–MD7 (tools; limits, tolerances and fits; screw threads and fasteners; sectioning; keys, cotters, pins, rivets, couplings, bearings; assembly drawings; symbols including surface finish and welding). Standards cited by WAEC: BS 308A (replaced by BS 8888). (Not a JAMB UTME subject: Technical Drawing is absent from JAMB’s official IBASS…

NECO: WAEC/NECO Technical Drawing, Mechanical Drawing items MD1–MD7 (tools; limits, tolerances and fits; screw threads and fasteners; sectioning; keys, cotters, pins, rivets, couplings, bearings; assembly drawings; symbols including surface finish and welding). Standards cited by WAEC: BS 308A (replaced by BS 8888). (Not a JAMB UTME subject: Technical Drawing is absent from JAMB’s official IBASS…

What you will learn

Describe mechanical drawing types, tools and the standards behind them.
Calculate limits, tolerances and fits and interpret ISO tolerance classes.
Name thread terms and forms and draw threads conventionally.
Draw nuts, bolts, studs, screws and locking devices to standard or proportional sizes.
Represent keys, cotters, pins, rivets, couplings, bearings, gears and springs.
Interpret surface finish and welding symbols and produce assembly drawings with parts lists.

Key points

Detail drawings for making; assemblies for fitting, with parts lists.
Tolerance = upper − lower limit.
Fits: clearance, transition, interference.
⌀25 H7/g6: 7–41 µm clearance; H7/p6: 1–35 µm interference.
ISO metric 60°; M12 × 1.75 coarse.
Lead = pitch × starts.
External thread: thick crests, thin roots; internal reversed.
M12 nut: 18 A/F (ISO 4032), 19 (DIN 934).
Do not hatch shafts, bolts, keys, pins, ribs cut lengthwise.
Weld symbols: arrow, reference line, symbol, size, all-round circle.

Start here: Mechanical drawing: purpose and tools

Mechanical (engineering) drawing describes machine parts and assemblies precisely enough for them to be made and fitted: a gearbox shaft for a tricycle, a pump coupling, a bracket for a generator. WAEC Paper 3 Section B offers a mechanical option, and Paper 1 Section B has ten mechanical questions for candidates who choose it. The syllabus items (MD1–MD7) cover tools, limits and fits, screw threads and fasteners, sectioning (td8), assembly drawings, and symbols including welding.

Tools and equipment (MD1)

The drawing instruments of td1, plus the workshop measuring tools whose readings appear on drawings: steel rule, outside and inside calipers, vernier caliper (reading to 0.02 or 0.05 mm), micrometer (0.01 mm), feeler gauges, thread pitch gauges, and surface plates. Drawings increasingly come from CAD (computer-aided design) packages, but the conventions are the same.

Types of mechanical drawing

Detail drawing: one part, fully dimensioned, with material, tolerances and finish; enough to make it.
Assembly drawing: all parts in their working positions, with a parts list; shows how they fit (12.10).
Exploded view: parts separated along their axes (td6).
Sub-assembly: a group of parts forming a unit within a larger assembly.

Standards

WAEC cites BS 308A. BS 308 was withdrawn in 2000 and replaced by BS 8888 (current edition 2025, verified on the BSI site; see notes), which applies ISO standards such as ISO 128 (lines), ISO 129 (dimensioning), ISO 286 (limits and fits), ISO 6410 (threads) and ISO 2553 (welding symbols). The conventions taught here are the ones examiners expect and are consistent with BS 8888.

Title block and parts list

Every sheet has a title block: drawing title, number, scale, projection symbol, units, material, drawn by, date, and tolerance note (for example GENERAL TOLERANCE ±0.5 UNLESS STATED). Assembly drawings add a parts list above the title block.

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Practice questions

1. In the drawing proportions for a hexagon nut, the width across flats is about

1.5d
0.8d
2d
d

Show answer

A: 1.5d
A/F ≈ 1.5d; for M16 this gives 24 mm.

2. In drawing proportions, the thickness of a hexagon nut is about

0.5d
1.5d
2d
0.8d

Show answer

D: 0.8d
Bolt head about 0.7d.

3. An M12 hexagon nut to ISO 4032 measures across flats

12 mm
24 mm
18 mm
19 mm

Show answer

C: 18 mm
ISO 4032 s = 18 mm (older DIN 934: 19 mm).

4. The pitch of an M12 coarse thread is

2 mm
1.75 mm
1.5 mm
1.25 mm

Show answer

B: 1.75 mm
ISO metric coarse M12 × 1.75.

5. A ⌀25 H7 hole is +0.021/0 and a g6 shaft is −0.007/−0.020. The fit is

a clearance fit
an interference fit
a transition fit
a force fit

Show answer

A: a clearance fit
Smallest hole 25.000 > largest shaft 24.993.

Key words

Detail drawing: Drawing of one part with all information to make it.
Assembly drawing: Drawing of parts in working positions.
Nominal size: Basic size from which limits are set.
Limits: Largest and smallest permitted sizes.
Tolerance: Difference between the limits.
Deviation: Difference between a limit and the nominal size.
Clearance fit: Shaft always smaller than the hole.
Transition fit: Fit that may give clearance or interference.
Interference fit: Shaft always larger than the hole.
Hole basis: System with H holes and varied shafts.
Pitch: Distance between adjacent threads.
Lead: Axial advance in one turn.

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