Abstract: In a pneumatically powered rotating grinding machine there is provided a safety unit (13) comprising a speed governor and an overspeed release mechanism the purpose of which is to ensure a predetermined maximum speed. For safety reasons, the motor speed must never exceed a certain critical speed level which is determined by the mechanical strength of the working tool attached to the machine. When choosing a working tool to be attached to the machine the maximum speed mark of the machine is decisive. By having a portion of the safety unit exposed through an aperture (57) in the tool housing (10) and having the maximum speed mark (52) of the safety unit (13) located at the exposed portion, the maximum speed mark of the machine will always be the same as that of the safety unit (13). The suggested overspeed safety device avoids the risk involved in the event that a machine carrying a certain maximum speed mark might be erroneously fitted with a safety unit (13) intended for a higher maximum speed.