Fasteners
Fastener questions test thread conventions and the approximate proportions used to draw bolts, nuts, rivets and joints.
Threads
Metric threads have a 60° included angle and are designated M followed by the nominal major diameter, e.g. M20. Whitworth (BSW) has 55°. Square, ACME (29°) and buttress threads are used for power transmission. Pitch is the distance between crests; lead is the distance moved in one turn. In conventional representation, an external thread shows the major diameter as a thick line and the minor diameter as a thin line; an internal thread reverses this. In section, hatching runs to the major diameter for an internal thread.
Fasteners
Hexagonal head bolts, nuts and washers are drawn with approximate proportions: head height about 0.7D, nut thickness about 0.8D, washer thickness about 0.15D. A stud screws into one part and takes a nut. Rivet heads include snap, pan, countersunk and flat. Riveted joints are lap or butt with single or double straps.
Worked example
An M20 coarse bolt has a 2.5 mm pitch. The tapping drill for the nut is 20 − 2.5 = 17.5 mm. Approximate nut thickness = 0.8 × 20 = 16 mm; head height = 0.7 × 20 = 14 mm. Reasoning: the minor diameter is roughly major diameter minus pitch, so the drill must be smaller than 20 mm.
Traps
Calling M20 the minor diameter; using 55° for a metric thread; drawing a thread as two plain circles only; forgetting the thin minor line; using the wrong drill size; omitting the washer.