A set screw is a type of screw generally used to secure an object within another object.
A potentiometer knob with a set screw for locking it in place.They are used to prevent relative motion between two rotating parts, such as the movement of pulley on a shaft. For this, a set screw is screwed into the pulley hub so that its end-point bears firmly against the shaft. The fastening action is by friction between the screw and the shaft.
Typically, set screws are not efficient and so are used only for transmitting very light loads. To reduce the chance of slipping and to increase load capacity, a detent may be drilled in the shaft into which the set screw's point enters. The detent, however, must be precisely aligned with the threaded hole prior to fastening. For longer life, they are usually made of steel and case hardened.