Abstract: The invention relates to a device for effecting the guiding and oscillation movements of a machine part, especially a welding head of the type mounted for oscillating motion either by a movement of translation or by a pendular movement in a certain direction under the action of an impulse-control system, the movements of the said machine part being limited by direct mechanical contact of said part at pre-determined points, the duration of each period of travel of the part being equal to a selectively-determined fraction of the interval between two successive impulses from the said control system, the remaining fraction constituting a stopping time between two periods of travel of said machine part.The machine part is actuated by a driving fluid and distributor system and further comprises means for regulating the speed of the said part in its outward and return travel periods by acting on the flow-rate of the driving fluid.The principal application is concerned with the welding together of metal plates.