of the distance of movement in direction v is simple since it is directly proportional to the number of rotations of the drill bit 34 and or components rotatively rigidly connected thereto thus the first circuitry means preferably comprises a reed switch 59 and a permanent magnet 60 mounted in the worm gear 38 each rotation of the gear 38 resulting in the magnet 60 passing the reed switch 59 once and effecting operation thereof the first circuitry means further comprises a conventional digital counter 62 for counting the number of rotations of the gear 38 in a given sequence and digitally displaying that number fig4 shows one exemplary circuitry arrangement that is desirable for use with apparatus of fig3 the circuit of fig4 includes a digital counter 62 a monostable multi vibrator 63 which emits a pulse through transistor 64 to the digital counter 62 each time reed switch 59 is closed the multi vibrator 63 insuring that only one pulse is delivered to the counter 62 for each revolution a voltage comparator 65 for comparing the voltage delivered from photocell 24 to a reference voltage and a potentiometer 66 for adjusting the reference voltage which the voltage comparator 65 compares to the voltage from the photocell 24 four different voltage dividers are provided by resistors 67 68 69 70 71 72 and 92 100 line 73 is the main power line while line 74 is an.