Bolts and nuts

The essentials

A bolt is a cylindrical rod with a head at one end and a thread at the other; a nut is a block with an internal thread that screws onto the bolt. Together they clamp two or more parts that have clearance holes. Hexagonal heads and nuts are the most common because a spanner can grip them from many positions; square heads and nuts are used for rough work and where the head must not turn, for example when it sits in a square recess.

Key points

• All sizes are given as proportions of the nominal diameter D. Proportions commonly used in school drawing:
• Width across flats (S) of a hexagonal or square nut and head: 1.5D + 3 mm. For M20, S = 33 mm.
• Thickness of nut: D. Height of bolt head: about 0.8D (for M20 about 16 mm).
• Chamfer: the corners of hexagonal nuts and heads are chamfered at 30°. This produces arcs on each visible face in the front view.
• A hexagon has a width across flats and a larger width across corners; the views showing "across corners" display three faces, while those showing "across flats" display two.
• Thread length on the bolt is kept long enough for the nut to be fully engaged, with a small part projecting beyond the nut.
• Square nut: four faces; in a view across corners two faces are seen.

Exam tips

• Start the views of a hexagonal nut with the circle and hexagon in the top or side view, then project to the front view.
• Draw chamfer arcs neatly; examiners check them.

Common mistakes

• Making the nut thickness the same as the bolt head height.
• Drawing the hexagon with S as the distance across corners.
• Forgetting the 30° chamfer on nuts and bolt heads.