is fitted into the wall of the cylindrical hollow body 60 and is identified with 90 in fig2 in order to monitor and control the screwing in depth and the screwing in torque of the screw caps 115 the drive shaft 53 for the wrench 140 is provided with a horizontal measuring disk 70 to be more precise above the wrench 140 the distance from the measuring disk 70 to the lower end of the wrench 140 corresponds approximately to the length of the wrench 140 the outer diameter of the measuring disk 70 which assumes the position shown in fig2 corresponds roughly to the internal diameter of the cylindrical hollow body 60 of the centering member 130 so that when the wrench 140 is lowered or raised the measuring disk 70 participates in this travelling movement in the direction of the arrow x6 on its outer circumference the measuring disk 70 is provided with a number of recesses such as notches grooves or otherwise configured indentations 71 which are arranged spaced apart at equal distances in the wall of the cylindrical hollow body 60 of the centering member 130 in the path of travel of the measuring disk 70 a proximity switch 75 is disposed which is constructed in a manner known per se and comprises a repeater circuit 76 with an oscillatory circuit coil 77 and a screening 78 the pattern of the field produced is indicated at 79 this proximity switch 75 is an electronic switching device.