variant shown here the recording device 42 comprises two inductive proximity switches 43 44 as can be seen in fig1 the inductive proximity switches 43 44 are fixed permanently to the headstock 12 and are located on elevations indicated by 45 and 46 chosen in a such a way that the signal ring 39 which rests on the collar 38 optimally dampens the field of the inductive proximity switches 43 44 although the spindle 16 rotates at coupling speed 27 the signal ring 39 hereby ensures a permanent damping of the proximity switches 43 44 it can also be seen in fig 1 that the bore hole 29 in which the signal rod 30 is borne ends in the annular collar 38 so that the upper end 47 of the signal rod 30 can be brought into contact with the annular flange 40 of the signal ring 39 however when the tool holder 18 has been moved out of the recess 17 the annular flange 40 rests on the collar 38 and is released by the signal rod 30 the annular flange 40 can however rest permanently on the signal rod 30 and thus help the spring 34 push the signal rod 30 and thus the key 33 downwards with the aid of the tension spring 41 in this way the signal transmitter 31 consisting of the signal rod 30 and signal ring 29 comprises a sensor device 48 together with the recording device 42 which in the state shown in fig1 indicates that the catch 32 is completely free the tool.