A device for converting a rotating motion into a reciprocating motion comprises a rotatable shaft carrying a first grooved element, a second element constrained to move along an axis of said second element, and a pin carried on the second element. The pin may be engaged in the groove on the rotatable shaft so that as the rotatable shaft rotates, the pin moves in the groove thereby causing the second element to move along its axis. The shape of the groove defines the motion of the second element. There is also disclosed an electric brush assembly and an electric tool assembly comprising such a device.