complete control are not fully illustrated in the drawings they are illustrated only schematically since this constitutes prior art technology and does not affect the invention the location of the position e is selected such that the desired length of travel is achieved the second piston 6 is to move along a certain distance from its impact position before a point is passed at which the travel of the slide is reversed when this occurs the slide 4 starts to move and the pressure on the driving surface 5 of the second piston changes from low pressure to high pressure i e the motion of the second piston 6 changes from a return motion to become an impact motion also other solutions for the drainage of the drive chamber 14 b of the recoil piston are possible within the scope of the invention thus the position of the hammer piston can be determined using electronic sensors that identify a position that corresponds to the position e and a magnetic valve is subsequently operated in order to drain the drive chamber 14 b the sensors can be for example of inductive type or of capacitive type also electromagnetic radiation such as light for example may be used for detection it is in this case suitable that the sensor corresponds to the second control means and it can be mounted against the piston housing in order to measure either in the radial direction or in the axial direction the first control means can be constituted by.