Drill bit position sensor

Longitudinal position of the end of a drill bit (21) is determined in order to accurately control depth of a hole to be drilled. A drill bit (21) is rotated at a predetermined speed and caused to penetrate a light beam of known vertical position. The light beam, which is generated by a light emitting diode (50) and received by a photo transistor (54), is modulated by the flutes at the tip of the rotating bit to cause the photo transistor to provide an output signal that is modulated at a frequency that is a fixed multiple of the speed of the rotating bit. An operational amplifier (74) having a feedback circuit (80) that is resonant at the frequency of the modulated electrical signal from the photo transistor receives the photo transistor output signal as its input. The amplifier output experiences a sharp increase in magnitude upon receipt of the input signal at the resonant frequency of its feedback circuit. Thus a reference position of the bit is signalled and stored by the bit spindle control (18) that will move the bit longitudinally to a depth determined on the basis of the known bit reference position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to drilled hole depth control, and more 
particularly concerns simple, inexpensive measurement of a reference 
position of the end of a rotating drill bit. 
2. Description of Related Art 
In a typical automatic drilling or routing machine a drill bit or drill 
point is inserted into a chuck of a spindle that securely grasps and 
rotates the bit. The chuck is moved relative to a workpiece in X and Y 
positions, horizontally, to locate the horizontal position of the bit for 
proper location of the hole to be drilled. The spindle with its bit is 
then moved vertically in a Z axis direction to cause the rotating bit to 
penetrate a workpiece to a predetermined depth so as to drill a hole to a 
predetermined depth. Depth of the hole is controlled by the machine tool 
controller, which calculates and commands all of the tool positioning, 
including the Z axis positioning for hole depth drilling. Although the 
machine tool control can precisely command the magnitude of Z axis motion, 
precision of hole depth control requires precision information concerning 
the Z axis position of the bit end. As each bit is inserted into a spindle 
it may occupy different longitudinal positions relative to the spindle, 
depending upon how far into the spindle the bit is inserted. Even if the 
position of the bit relative to the spindle be precisely known, it is 
still necessary to calibrate the system to precisely determine the height 
of the bit end at some given Z axis position of the machine tool itself. 
In a typical drill bit locating system of the prior art a pressure plate 
system is employed in which the tip of the drill bit is caused to contact 
the pressure plate. When sufficient pressure is applied to the plate by 
the drill bit a signalling circuit is activated that enables the machine 
to know and store the Z axis position of the drill tip. Tips of drills, 
particularly with very small diameter drills, are subject to significant 
abuse in the practice of this method, because the pressure contact will 
dull the tip and shorten its life span. Moreover, accidentally applied 
excess pressure may break fragile small diameter drill bits. 
Other systems have employed a very fine diameter laser beam that is 
directed at the tip of the drill bit to identify drill bit Z axis position 
with good inherent accuracy. However, such a laser system is excessively 
expensive. 
Accordingly, it is an object of the present invention to provide a drill 
bit end position sensor that avoids or eliminates above mentioned 
problems. 
SUMMARY OF THE INVENTION 
In carrying out principles of the present invention in accordance with a 
preferred embodiment thereof a drill bit is rotated at a known speed and a 
light beam is projected along a path of known position. The beam is caused 
to be modulated at a predetermined frequency related to the bit rotation 
speed by moving the end of the drill bit into the path of the beam while 
the drill bit is rotating. The fluted end of the rotating drill bit 
modulates the beam, and occurrence of this modulation is detected to 
enable indication of the position of the drill bit at the time of 
modulation. 
According to a specific feature of the invention, modulation of the light 
beam is caused to provide a modulated electrical signal having a frequency 
that is a fixed multiple of speed of the drill bit rotation. This 
modulated electrical signal is applied to a circuit that is resonant at 
the sensing frequency, thereby utilizing the high change in impedance of 
the resonant circuit at resonance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As illustrated in FIG. 1, a work table 10 movably mounts a drill spindle 
carrier 12 for motion in X and Y directions, as indicated by arrows 14,16, 
under control of drive motors (not shown). A drill spindle 18 carried by 
the support structure 12 includes a motor (not shown) for vertical or Z 
axis control of a chuck 20 as indicated by arrow 15 in which is removably 
mounted a drill bit 21. Support structure 12 moves horizontally in X and Y 
over the surface of work table 10 to position the drill bit at a desired 
location with respect to a workpiece 22. The drill bit is then rotated, 
and the spindle lowers the rotating bit to drill a hole in the workpiece. 
In accordance with principles of the present invention, a drill bit end 
position sensor 30 is fixedly mounted on the work table in a known Z axis 
position. To utilize the bit end position sensor 30 the support structure 
is moved to cause the drill bit to be positioned over a drill bit 
receiving hole 32 formed in the sensor 30, and the spindle is operated to 
rotate the drill bit at a predetermined rotational speed and to 
simultaneously lower the bit slowly into the sensing hole 32. Structure 
within the sensing hole 32, to be described in detail below, together with 
associated electrical circuitry, then measures the Z axis position of the 
tip of the drill bit in a fixed reference system. This Z axis position of 
the tip is then stored by the machine tool control, which can thereafter 
accurately control Z axis position of the bit as it drills a hole in the 
workpiece 22, thereby controlling hole depth. 
The sensor 30 (FIGS. 2-5) includes a rigid depth sensing pad 36, having a 
circular aperture 38 in which is fixedly mounted a bushing 40. The upper 
end of the bushing is flush with an upper surface 42 of the pad 36, and 
the lower end of the bushing is provided with a pair of shallow 
diametrically opposed slots 44,46. A recess 48 in the pad 36 at one side 
of hole 38 mounts a conventional and inexpensive light emitting diode 
(LED) 50, and a recess 52 in the pad diametrically opposite the recess 48 
mounts a light sensitive or photo transistor 54. The light emitting diode 
and photo transistors are held in place by a lower cap 60 that is fixed to 
the pad 36 and includes lateral flanges 62,64 which contact and support 
lower ends of the light emitting diode and photo transistor structures. 
Slots 44 and 46 together provide slots of very small dimensions that allow 
only a very narrow beam of light to be received by the photo transistor 
54. The received beam of light in a typical embodiment has a height of 
approximately 0.003 inches, which is the vertical height of slots 44 and 
46. The sensor establishes a known fixed elevation of slot 44 and, 
therefore, of the beam of light received by the photo transistor. 
Principles of the invention take advantage of the fluted construction of 
the common drill bit or drill point and utilize these flutes on a rotating 
bit to modulate the vertically narrow light beam received by the 
transistor 54. The tip of a typical drill bit is illustrated in FIGS. 2 
and 3 and includes first and second tapered and angulated flute ends 67,69 
to provide the bit end with different shapes when viewed at different 
angles. The drill bit is rotated at a fixed known speed and slowly lowered 
into the calibration hole of the bushing 40. When the tip of the drill bit 
begins to penetrate the light beam, the amount of light received by the 
photo transistor is slightly modulated by the two flutes of the rotating 
bit. 
Reception of the modulated light beam by the photo transistor produces an 
electrical signal from the photo transistor that is modulated at a 
frequency that is a fixed multiple of the bit rotation speed. Typically 
the bit is rotated at 1,500 revolutions per second to provide an output 
electrical signal that fluctuates at 3,000 cycles per second. 
FIG. 6 illustrates circuitry of an arrangement for signalling time of the 
initial penetration of the drill bit into the light beam. Light emitting 
diode 50 may be an infrared diode that directs a light beam to a photo 
transistor 54. Current from a source of positive potential is fed through 
a current limiting resistor 70, through the light emitting diode 50 to 
ground. The positive potential is applied to the collector of light 
sensitive transistor 54, which has its emitter connected to an input 
terminal 72 of an operational amplifier 74, having its second input 
terminal 76 grounded. The amplifier has an output terminal 78 which is 
connected to its input terminal 72 by means of a negative feedback circuit 
80. Feedback circuit 80 comprises a coil 82 and a capacitor 84 that are 
configured to have a resonant frequency that is a fixed multiple of the 
rotational speed of the rotating bit. In a presently preferred embodiment 
the resonant frequency of the circuit 82,84 is 3,000 hertz. Quiescent 
current from the output of amplifier 74 is fed back through the relatively 
low resistance (in the order of 600 ohms) of coil 82 to hold the voltage 
at input terminal 72 of the amplifier to nearly zero volts. At frequencies 
other than the resonant frequency (3,000 hertz in the illustrated example) 
the impedance of the parallel resonant LC circuit 80 is approximately the 
same as the very low resistance of the coil 82. 
If the input signal at terminal 72 varies at a frequency other than the 
resonant frequency of circuit 80, the very low feedback resistance 
provides an output voltage at terminal 78 of substantially zero volts. 
When the spinning drill tip modulates the light at resonant frequency, the 
parallel resonant circuit 80 provides a very high impedance in the 
amplifier feedback circuit. Therefore, even the very small input current 
pulses produced by the spinning drill bit cause the output of amplifier 74 
to provide a high AC output voltage. This voltage is coupled via a 
capacitor 86 and via a pair of voltage limiting diodes 88,90, to the input 
of an operational amplifier 92 that is connected to a negative voltage via 
a resistor 94. The output of amplifier 92 is connected to the base of a 
transistor 96, having its collector connected to a positive source of 
voltage through resistors 98,100 and a light emitting diode 102 that 
provides a visible output signal. An output from the collector of 
transistor 96 on a line 106 provides a signal that occurs at the time of 
the initial penetration of the drill bit end into the light beam, and thus 
an indication of the drill bit vertical position. 
During quiescent condition resistor 94 reduces the voltage at the input to 
amplifier 92 to a value less than the grounded voltage at the second input 
to this amplifier. The voltage applied to amplifier 92 is limited by the 
sum of the forward voltage drops across diodes 88 and 90. Normally, during 
quiescent condition, the output of amplifier 92 on line 104 is high, which 
turns off transistor 96 and also turns off visible light emitting diode 
102, whereby the output of the system on line 106 is high. 
When a resonant signal occurs at the input of amplifier 74, its output goes 
high, and capacitor 86 and a second capacitor 108 connected between an 
input of amplifier 92 and ground, together with diodes 88 and 90, 
effectively form a voltage doubler that causes the input 91 of amplifier 
92 to go high. The amplifier output on line 104 then goes low to turn on 
transistor 96, turn on the visible light emitting diode 102, and provide a 
low output signal on line 106. Occurrence of this output signal signals 
the machine controlling computer that the end of the drill bit is at the 
known height (Z axis position) of the light beam from LED 50 and 
subsequent Z axis motion of this drill bit is then all related to the 
known height of the light beam. 
There has been shown and described a drill point position sensor having 
greatly increased sensitivity and accuracy but which employs simple and 
inexpensive components and does not subject the drill bit to possibly 
damaging contact forces.