Screws

The essentials

A machine screw or cap screw is like a bolt without a nut: it passes through a clearance hole in one part and screws into a tapped hole in the other. Screws are classified mainly by the shape of their heads, and each head suits a particular situation such as a flush surface, limited space or the need to lock a part on a shaft.

Key points

• Round head screw: a dome-shaped head with a slot for a screwdriver; used for light work where the head may project.
• Cheese head screw: a cylindrical head with a slightly rounded top and a slot; gives a deeper slot and can be sunk in a counterbored hole.
• 90° flat countersunk head screw: a conical head with an included angle of 90° and a flat top, which sits flush with the surface in a countersunk hole.
• Hexagonal socket head screw: a cylindrical head with a hexagonal recess, tightened with an Allen key; useful where there is no room for a spanner around the head and for high tightening force.
• Grub screw (set screw without a head): fully threaded with a slot or socket at one end; screwed in completely so nothing projects. Used to stop relative movement, for example holding a pulley or collar on a shaft.
• Screws are sketched with proportions based on D; threads are shown conventionally.

Exam tips

• Link each head to its typical need: flush surface → countersunk; no projection at all → grub; confined space → socket head.
• In sketches, show the slot or socket clearly in the top view.

Common mistakes

• Calling the countersunk head angle 60°; for these screws it is 90°.
• Confusing a grub screw with a stud; a grub screw is threaded fully and has no nut.