Abstract:
A system for preventing the performance of an out-of-sequence reaming operation during the machining of holes in a lamination of fiber-reinforced plastic material. The system automatically stops or inhibits a reaming operation if a sensing tip of the reamer encounters an incompletely drilled pre-hole or blank material (i.e., no pre-hole) instead of a fully drilled pre-hole. The system does not require any human intervention to inhibit the reaming operation. In one embodiment, the human or operator cannot re-start the reaming process until the machine tool or reaming equipment is reset. In some cases the reset may require manual intervention.

Description:
BACKGROUND 
     The present disclosure relates generally to the machining of composite material. In particular, this disclosure relates to machining of holes in composite material. 
     Composite components are being utilized in a wide variety of articles of manufacture due to their high strength and light weight. This is particularly true in the field of aircraft manufacturing. Typical materials used in the manufacture of composite components include glass or graphite fibers that are embedded in resins, such as phenolic, epoxy, and bismaleimide resins. A composite lamination can be built up by laying successive plies of fiber tows (e.g., carbon fiber tows preimpregnated with a thermoset epoxy resin) around a mandrel and then curing. As more advanced materials and a wider variety of material forms have become available, aerospace usage of composites has increased. 
     Certain machining applications require drilling and/or reaming a hybrid stack-up of materials. A hybrid stack-up of materials may be defined as layers of discontinuous materials such as a carbon fiber-reinforced plastic (CFRP) composite material and titanium, aluminum or steel. For example, certain aircraft require a wing made from a composite material, such as CFRP, be joined to a titanium section of an aircraft body with fasteners that pass through holes made through the mating sections. When using fasteners to attach composite skins to metal substrates, coaxial holes must be drilled in both the skin and an underlying metal substrate. High-quality holes must be produced in such materials with dimensions within narrow tolerances. The wing-to-body join task typically requires a three-step conventional drilling process comprising a pilot drill, followed by a step drill, followed by a finish diameter reamer. 
     Reamers are cutting tools that are typically used to perform the final cutting operation on holes, particularly holes with small tolerances. Specifically, reamers perform secondary cutting operations after a hole has been drilled close to a desired final size. Reamers therefore typically have an external diameter that is slightly larger than that of the drilled hole, and are designed to finish the hole to within a small tolerance of the desired size and to provide a relatively smooth inner wall. 
     Standard reamers typically include a shank, a body portion at one end of the shank, and a plurality of teeth, such as 4-8 teeth, that are spaced around the body portion and extend outwardly therefrom to form the cutting surfaces of the reamer. Each tooth includes a rake face and a cutting edge that actually engages the workpiece in the course of reaming a hole. In some cases, the teeth are uniformly spaced around the body portion. In other cases, the teeth have non-uniform or irregular spacing. 
     During the machining of holes in composite material, severe delamination can occur if a reaming operation is performed prior to a hole being drilled or after a hole has been only partially drilled. The repairs for this kind of damage are very expensive and production flow must be halted until the problem is resolved. 
     Mechanics use visual inspection and tooling pins to assure that the pre-hole is complete before they move on to the reaming operation. Visual inspection of these holes is difficult in some areas due to limited access and time consuming. 
     Thus there is a need for a system to inhibit or terminate (without human intervention) the performance of a reaming operation if the pre-hole is missing or other dimensional characteristics do not meet the pre-hole requirements such as depth, diameter, orientation, and location. 
     SUMMARY 
     A system is disclosed for preventing the performance of an out-of-sequence reaming operation during the machining of holes in a lamination of fiber-reinforced plastic material. The system automatically stops or inhibits a reaming operation if a sensing tip of the reamer encounters an incompletely drilled pre-hole or blank material (i.e., no pre-hole) instead of a fully drilled pre-hole. The system does not require any human intervention to inhibit the reaming operation. In one embodiment, the human or operator cannot re-start the reaming process until the machine tool or reaming equipment is reset. In some cases the reset may require manual intervention. 
     A rotary cutting tool is also disclosed that comprises the following elements: (a) a body comprising an internal cavity that extends from one end of the body along an axis, the internal cavity comprising a first portion having a first cross-sectional area and a second portion having a second cross-sectional area less than the first cross-sectional area; (b) a plurality of teeth extending outwardly to form cutting surfaces near another end of the body; (c) a shaft disposed in the internal cavity along the axis, the shaft being movable axially relative to the body; and (d) a sensing tip coupled to or integrally formed with one end of the shaft and disposed outside the internal cavity. A spring, fluid piston or other suitable biasing member may be arranged to resist movement of the shaft from a first position toward a second position. The shaft and the internal cavity are configured so that another end of the shaft is disposed in the first portion of the internal cavity when the shaft is in the first position and is disposed in the second portion of the internal cavity when the shaft reaches the second position. 
     In addition, a method for using a rotary cutting tool having a leading tip axially movable relative to a plurality of cutting edges is disclosed. The method comprises the following steps: (a) mechanically resisting relative movement of the cutting edges and the leading tip toward each other; (b) actuating a drill motor that rotates the cutting edges; (c) advancing the rotating cutting edges forward along an axis of a pre-hole; (d) monitoring a parameter to determine when the value of that parameter reaches a specified threshold corresponding to the cutting edges advancing a specified distance relative to the leading tip after the latter has stopped advancing; and (e) stopping the rotation and advancement of the cutting edges when the value of the parameter reaches the specified threshold. The leading tip is carried by a shaft that is axially movable inside a body that carries the cutting edges. In a particular embodiment, the monitored parameter may be associated with a fluid that is ported or supplied to the body at an opening proximate to one end thereof. The fluid parameter monitored may correspond to a rate of flow of fluid through the body or a pressure in the line supplying fluid to the body. Step (e) is performed when the shaft position causes the fluid parameter to reach a threshold value. 
     Another aspect of the disclosed subject matter is a system comprising: a source of fluid; a rotary cutting tool comprising a plurality of cutting edges, a forward tip axially movable relative to the cutting edges, an inlet, an internal cavity in fluid communication with the inlet, and means for biasing the tip and cutting edges toward moving apart, wherein the forward tip is movable between a first position where the forward tip does not reduce the flow of fluid through the internal cavity and a second position where the forward tip reduces the flow of fluid through the internal cavity; a motor coupled to the rotary cutting tool for driving rotation of the cutting edges; a shutoff device for shutting off the motor; a subsystem for guiding the flow of fluid from the fluid source to the inlet of the rotary cutting tool, the subsystem comprising a sensor capable of producing a signal when a value of a parameter of the fluid reaches a specified threshold; and a controller coupled to the sensor for receipt of the signal from the sensor, the controller being programmed to output a shutoff signal to the shutoff device in response to receipt of the signal from the sensor. For example, the sensor can be a flow sensor or a fluid pressure sensor. 
     A further aspect is a system comprising: a reamer comprising an axial internal cavity having first and second openings at first and second ends of the reamer and a plurality of teeth extending outwardly to form cutting surfaces near the first end of the reamer, the internal cavity comprising a first portion having a first cross-sectional area and a second portion having a second cross-sectional area less than the first cross-sectional area; a sensor capable of producing a signal when a value of a parameter reaches a specified threshold; a flow path connecting the sensor to the second opening of the reamer; a shaft disposed in the internal cavity, the shaft being movable axially relative to the reamer from a first position toward a second position; and a tip coupled to or integrally formed with another end of the shaft and disposed outside the internal cavity. The shaft and the internal cavity are configured so that another end of the shaft is disposed in the first portion of the internal cavity when the shaft is in the first position and is disposed in the second portion of the internal cavity when the shaft reaches the second position, a flow through the internal cavity being reduced when the shaft moves from the first position to the second position. 
     Other aspects of the invention are disclosed and claimed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an isometric view of a sensing tip reamer in accordance with one embodiment. 
         FIG. 2  is a diagram showing a side view (magnified in scale) of the distal end of the sensing tip reamer depicted in  FIG. 1 . 
         FIG. 3  is a diagram showing an isometric view of the sensing tip partly depicted in  FIG. 1 . 
         FIG. 4  is a diagram showing an end view of the sensing tip. 
         FIG. 5  is a diagram showing a sectional view of a sensing tip reamer (of the type depicted in  FIG. 1 ) during reaming a completed hole in a metal/composite stack-up. 
         FIG. 6  is a diagram showing a sectional view of a sensing tip reamer (of the type depicted in  FIG. 1 ) which has been shut down in response to the sensing tip impinging on the bottom of a partially drilled pre-hole in composite material of a metal/composite stack-up. 
         FIG. 7  is a diagram showing a sectional view of a sensing tip reamer (of the type depicted in  FIG. 1 ) which has been shut down in response to the sensing tip impinging on composite material in and at the end of a hole that is slightly short (e.g., 0.050 inch) of being completed. 
         FIG. 8  is a hardware block diagram showing components of a system that incorporates a sensing tip reamer of any type disclosed herein. 
         FIG. 9  is a logic diagram showing steps of a process for automated shutdown of a reamer if a condition indicating an incomplete hole is detected. 
         FIG. 10  is a diagram showing an isometric view of a sensing tip in accordance with another embodiment. 
     
    
    
     Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
     DETAILED DESCRIPTION 
     The system disclosed herein is designed to prevent the running of a reaming operation prior to drilling the necessary pre-hole in CFRP material. A reaming technology is provided that will not harm the CFRP structure if a reamer were used out of sequence. A reamer having a sensing tip is installed in a powered drill motor that may be powered electrically, by pressurized air or other known means. The system shuts off the drill motor if the tip of the reamer encounters blank material. The system employs a control that shuts off the drill motor when a fluid parameter associated with a coolant or lubricant flowing through the reamer is restricted due to depression (relative to the reamer cutting edges) of the sensing tip, which is axially movable relative to the cutting edges of the reamer. The coolant or lubricant may comprise a liquid, gas, or mixture of liquid and gas such as a mist coolant or lubricant. Suitable gases may include air, inert gas or refrigerant. Depression of the sensing tip occurs when the tip meets blank material or an obstacle inside an incompletely drilled pre-hole while the cutting portion of the reamer continues to advance. If the tip is depressed by an amount sufficient to cause the magnitude of the monitored fluid parameter to reach a specified threshold, the control causes a shutoff valve to close in the case of an air-powered drive or opens a switch in the case of an electric drive, thereby stopping rotation and advancement, or advancement, of the reamer. The control may also shut down the metering pumps that provide air/coolant to the reamer. The system inhibits a reaming operation when blank or partially drilled material is encountered. 
       FIG. 1  shows an isometric view of a rotary cutting tool in accordance with one embodiment. The tool comprises reamer  10  and an integrally formed sensing tip/shaft comprising a sensing tip  16  and a shutoff shaft. The shutoff shaft is not visible in  FIG. 1  because it is inside the reamer  10 . The reamer  10  comprises a hollow body  12  having an internal cavity (not shown in  FIG. 1 ) that extends from a forward end to a rearward end along a central axis. The reamer  10  further comprises a plurality of teeth  14  extending outwardly to form cutting surfaces near the forward end of hollow body  12 . The cutting edges define an outer diameter of the reamer  10 . 
       FIG. 2  shows (on a magnified scale) the sensing tip  16  in its most forward position relative to reamer  10 . The sensing tip  16  comprises three contact arms  18  disposed radially outward at angles of 120 degrees. If any portion of the sensing tip  16  encounters an obstacle during reaming of a pre-hole, the sensing tip as seen in  FIG. 2  will move to the right relative to the cutting teeth  14 . In one specific implementation, the diameter of a hypothetical circle around the contact arms  18  may be in the range of 0.005 to 0.100 inch smaller than the reamer outer diameter. The tip  16  is relieved to minimize any impact to chip flow. In that same implementation, the tip is made of stainless steel. 
       FIG. 2  also shows that each flute between cutting teeth  14  is provided with one or more vent holes  4 . As will be described in more detail below with reference to  FIG. 5 , a jet spray of droplets of coolant suspended in air will flow through an internal cavity of the reamer and exit the reamer via the vent holes  4 , thereby cooling the cutting teeth  14  during the reaming operation. 
     The integrally formed sensing tip/shaft is shown removed from the reamer in  FIG. 3 . The contact arms  18  of sensing tip  16  are machined with a tri-lobed contact surface  20 , each lobe disposed at a 135-degree angle. The integrally formed sensing tip/shaft further comprises a shutoff shaft  22 . The shutoff shaft  22  comprises a guide body  24 , a plurality of circumferentially distributed, radially projecting guide features  26  and a retention slot  28 .  FIG. 4  shows an end view as seen from a vantage forward of the sensing tip  16 . The guide body  24  has an annular groove that receives a seal such as an O-ring  30 . The guide body  24  has an outer diameter greater than the outer diameter of the main portion of the shutoff shaft  22 . 
       FIG. 5  is a sectional view showing a sensing tip reamer (of the type depicted in  FIG. 1 ) during reaming a completed hole  40  in a stack-up consisting of metal  42  and composite material  44 . In the example depicted in  FIG. 5 , during advancement of the reamer  10 , the sensing tip  16  did not make contact with any obstacle inside completed pre-hole  40 , so the reaming operation did not need to be aborted. In other words, the entire pre-hole  40  was reamed completely. 
       FIG. 5  shows the internal structure of the reamer  10  and how shutoff shaft  22  is disposed inside the reamer internal cavity. The internal cavity comprises four circular cylindrical bores  32   a  through  32   d  which increase in diameter from right to left (as seen in  FIG. 5 ). A jet spray of air and suspended droplets of liquid coolant enters section  32   a  of the internal cavity via an opening  50  at the rear end of the hollow body  12  of reamer  10 . The air/coolant jet flows through sections  32   a ,  32   b  and  32   c , exiting the hollow body  12  through the aforementioned vent holes (not shown) between the cutting teeth  14 . As seen in  FIG. 5 , the guide features  26  hold the shutoff shaft  22  in a central position inside section  32   c  of the internal cavity of reamer  10 . The outer diameter of the guide features  26  is greater than the outer diameter of shutoff shaft  22 , creating an annular space between the outer surface of shutoff shaft  22  and the inner surface of section  32   c . The air/coolant flows through this annular space on its way to the vent holes (see items 4 in  FIG. 2 ) located between the cutting teeth  14 . 
     To assemble the sensing tip reamer, the shutoff shaft  22  is inserted into the internal cavity of hollow body  12  and then a set screw  36  is passed through the retention slot  28  in shutoff shaft  22 , the ends of the set screw being threadably coupled in diametrally opposed threaded holes (one such hole  6  can be seen in  FIG. 1 ) formed in the wall of the hollow body  12 . The set screw  36  retains the shutoff shaft  22  inside the hollow body  12  but, in cooperation with elongated axial slot  28  formed in shutoff shaft  22 , allows the shutoff shaft  22  to move axially between first and second limit positions relative to the hollow body.  FIG. 5  depicts shutoff shaft  22  in the first limit position, further axial movement rearward being stopped by impingement of the rearward end of slot  28  against set screw  36 . The sensing tip/shaft  16  is urged into the relative position seen in  FIG. 5  by a compression spring  38  which is placed between respective annular surfaces on the guide body  24  and on the hollow body  12 . In the absence of a force depressing the sensing tip, the sensing tip/shaft will remain in the relative position shown in  FIG. 5 . In this relative axial position, the rear end  22   a  of shutoff shaft  22  does not enter section  32   b  of the reamer internal cavity as seen in area  8 . 
     In accordance with alternative embodiments, the compression spring  38  can be omitted and the shutoff shaft  22  can be pressure biased in the forward direction by the coolant flowing into the internal cavity of the reamer. The forces on the shutoff shaft  22  are unbalanced and bias the shaft to the extended position when coolant pressure is present. 
     In contrast,  FIG. 6  shows a sectional view of the same sensing tip reamer depicted in  FIG. 5 , except that the sensing tip reamer has been inserted in a partially drilled pre-hole  46  (closed at the bottom) in the metal/composite stack-up  42 / 44 . When tip  16  impinges against the blank bottom of incomplete pre-hole  46 , the sensing tip/shaft ceases to advance while the reamer  10  continues its advance. The result is that the sensing tip/shaft moves rearward relative to reamer  10 . In particular, the end  22   a  of shutoff shaft  22  moves axially past position  8  and into section  32   b  of the reamer internal cavity. The end portion near end  22   a  which enters section  32   b  has an annular groove in which an O-ring  34  is seated. Because the diameter of section  32   b  is less than the diameter of section  32   c , where shutoff end  22   a  previously resided, the cross-sectional area of the annular space between the shaft end  22   a  and the internal cavity is reduced. In addition, the O-ring  34  further reduces the amount of air/coolant that can flow axially from section  32   b  into section  32   c . The result of the rearward movement of the shaft into section  32   b  of the reamer internal cavity is a reduction in the rate of air/coolant flow through the reamer (accompanied by a pressure increase in the line feeding air/coolant to the reamer). In other embodiments the O-ring  34  and the annular groove that it sits in can be omitted and a clearance fit between the rear end  22   a  of shutoff shaft  22  and section  32   b  can be provide which is capable of causing a suitable reduction in the rate of air/coolant flow through the reamer when rear end  22   a  is engaged with section  32   b . As will be explained in more detail later, this reduction in the fluid flow rate (or increase in pressure) is detected by the system. In response to the fluid flow rate falling below a specified threshold (or the pressure increasing above a specified threshold), the supply of pressurized air to the drill motor is shut off and the reaming operation is automatically aborted.  FIG. 6  shows the shutoff shaft  22  in a third position relative to the hollow body  12 , which third position is between the first and second limit positions. 
       FIG. 7  shows a sectional view of the same sensing tip reamer depicted in  FIG. 5 , except that the sensing tip reamer has been inserted in an incompletely drilled pre-hole  46  that is only a small distance (e.g., 0.050 inch) short of a completed hole. When tip  16  impinges against the undrilled material at the exit side of the incomplete pre-hole  46 , the sensing tip/shaft again ceases to advance while the reamer  10  continues its advance. The result is that the sensing tip/shaft will again move rearward relative to reamer  10 , eventually aborting the reaming operation. 
       FIG. 8  is a block diagram showing components of a system providing the functionality described above. The reamer  10  is coupled to a positive feed drill  52  (e.g., a Quackenbush positive feed drill) having a pneumatic motor. The pneumatic motor of drill  52  is powered by pressurized air from a plant air source  54  via a solenoid-actuated main air shutoff valve  56  and an air valve  58   [DMFI] . The operational state of shutoff valve  56  is controlled by a microcontroller  64  which can activate/deactivate a solenoid to respectively open/close the shutoff valve  56 . The microcontroller  64  can be programmed using an external personal computer  66 . When the system is in a locked state (i.e., key switch  68  is open), the system can be activated only by using a key to close key switch  68 . 
     When the system is activated, the microcontroller first opens the shutoff valve  56 . Pressurized air flows through shutoff valve  56  to air valve  58 . Some of the air flow (indicated by a line labeled “P ILOT  A IR ” in  FIG. 8 ) is diverted to an air distribution system that distributes pilot air to solenoid-actuated valves  70 ,  72 ,  74  and an air pressure sensor  62 . The air pressure sensor  62  outputs a digital signal representing the pilot air pressure to the microcontroller  64 . 
     When the pilot air pressure reaches a first specified threshold, the microcontroller  64  is programmed to open valves  70  and  72 , thereby supplying pilot air respective air pulse generators (not shown) which are used to send pulses of air to activate pistons of respective metering pumps (also not shown) that respectively meter motor oil and coolant from respective reservoirs  78  and  80 . If the pilot air pressure reaches a second specified threshold (higher than the first specified threshold and corresponding to a pressure buildup if no air is being supplied to the drill motor), the microcontroller is programmed to open a dump valve  74  to equalize the system. 
       FIG. 8  depicts the delivery of motor oil via a capillary tube (labeled “A IR  M OTOR  L UBE ” in  FIG. 8 ) to an air line  60  connected to air valve  58 . The motor oil is metered by a metering pump (not shown in  FIG. 8 ) coupled to an outlet of an oil reservoir  78 . In one implementation, the metering pump is an adjustable-stroke piston pump. As previously described, pilot air from air valve  58  is provided via open valve  70  to an air pulse generator (not shown) that sends pulses of air to activate the motor oil metering pump. 
     The pilot air is also received by a sensor  76 . In accordance with one embodiment, sensor  76  is a flow sensor that outputs an analog signal (0 V to 5 V) representing the rate of air flow through the sensor to the microcontroller  64 . (When there is no flow, the output of the sensor is 0 V.) In accordance with an alternative embodiment, sensor  76  is a fluid pressure sensor. The air (labeled “D RILL  B IT  A IR ” in  FIG. 8 ) flowing out of sensor  76  is supplied to the drill bit, i.e., reamer  10 , via an air line  82 .  FIG. 8  also depicts the delivery of lubricant (e.g., Micro-cut  26  coolant or Boelube oil) through a capillary tube (labeled “D RILL  B IT  L UBE ” in  FIG. 8 ) to line  82 , which feeds the air and lubricant to the reamer  10 . In one implementation, the lubricant (coolant or oil) is supplied to the reamer  10  via a ⅜-inch line for air with a ⅛-inch capillary for lubricant inside. In the implementation depicted in  FIG. 8 , the lubricant is Micro-cut  26  coolant. The coolant is delivered by a metering pump (not shown in  FIG. 8 ) coupled to an outlet of a coolant reservoir  80 . Again the metering pump is an adjustable-stroke piston pump. The valve  72  (under the control of microcontroller  64 ) opens pilot air from air valve  58  to an air pulse generator (not shown) that sends pulses of air to activate the coolant metering pump. 
     As previously described, following the actuation of the drill motor, a pre-hole in a composite/metal stackup can be reamed by the rotating cutting teeth of the advancing reamer  10 . In accordance with a flow sensor embodiment, the flow sensor output to the microcontroller is monitored to determine if the flow rate of air through the reamer has been sufficiently reduced due to obstruction of the sensing tip. The microcontroller is programmed to actuate closure of shutoff valve  56  as well as motor oil and coolant valves  70  and  72  in response to the air flow rate falling below a specified threshold. In accordance with a pressure sensor embodiment, the pressure sensor output to the microcontroller is monitored to determine if the air pressure in the supply line has increased sufficiently due to obstruction of the sensing tip. In this case, the microcontroller is programmed to actuate closure of shutoff valve  56  as well as motor oil and coolant valves  70  and  72  in response to the air pressure rising above a specified threshold. 
     One implementation of a method of aborting a reaming operation is depicted in the logic diagram of  FIG. 9 . First, the motor air supply is turned on (step  84 ). Then the coolant valve  72  is opened. Pressurized air (90 psi) and coolant are then supplied to the reamer (step  86 ). The air flow rate is measured by the flow sensor, which outputs an analog signal representing the air flow rate to the microcontroller. The microcontroller continuously monitors whether the coolant/air flow rate has fallen below 4 cfm (step  88 ). If not, then the main air shutoff valve is not closed (step  90 ). The 3-second loop  92  is the time the system takes to detect that a drop in flow has occurred. This is an adjustable setting in the system software. The setting should be selected to avoid any momentary events that may trigger a shutoff and still stop the system in time to mitigate any damage to the workpiece to be reamed. If the coolant/air flow rate has fallen below 4 cfm, then the main air shutoff valve is switched from open to closed (step  94 ). The microcontroller then sends a message to a user screen displayed on a personal computer (step  96 ). 
     Referring back to  FIG. 8 , the key switch  68  is a reset feature used after the system encounters an undersize or blank hole condition. It is preferred to keep this functionality away from the reamer operator so that he/she cannot not simply reset the system without removing the drill motor from the drill jig. The reset key resides with the drill motor set-up technician. This forces the operator to take a step back from the process to determine why the system tripped. 
     In the embodiment shown in  FIGS. 3 and 4 , the sensing tip and shutoff shaft were integrally formed. In accordance with an alternative embodiment depicted in  FIG. 10 , a sensing tip  100  is removable and rotatably coupled to one end of a shutoff shaft  98 . The rotating sensing tip  100  has two contact arms  18  and comprises a plug  102  having an annular groove. The plug  102  of sensing tip  100  is inserted into a socket  104  formed in a guide body  106  at one end of shutoff shaft  98 . The plug  102  is coupled to the socket  104  by inserting a slotted spring pin  108  into a radial opening in the socket wall. The radially inward end of slotted spring pin  108  sits in the annular groove  102  of the sensing tip  100 . This arrangement allows the sensing tip  100  to rotate relative to the shutoff shaft  98  while being coupled for axial movement therewith. 
     As in the earlier-described embodiment, the shutoff shaft  98  has a plurality of circumferentially distributed, radially projecting guide features  26  and a retention slot  28 . The guide body  106  has an annular recess that receives an O-ring  30 . The guide body  106  has an outer diameter greater than the outer diameter of the shutoff shaft  98 , providing an annular bearing surface for the spring  38  which urges the shutoff shaft and the reamer in opposite directions. 
     While a sensing tip reamer has been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings herein. In addition, many modifications may be made to adapt a particular situation to the teachings herein without departing from the essential scope thereof. Therefore it is intended that the claims set forth hereinafter not be limited to the disclosed embodiments. 
     In particular, a sensing tip arranged to block the flow of an air-coolant mixture to a reamer or drill when confronted by an obstruction is not limited to use with pneumatic drilling systems, but rather could also be incorporated in electrical drilling systems. In such embodiments, instead of closing a shutoff valve, thereby shutting down the drill motor, in response to rearward relative displacement of the sensing tip, the electrical drilling system would simply be shut down by changing the state of an electrical switch. 
     The method claims set forth hereinafter should not be construed to require that the steps recited therein be performed in alphabetical order or in the order in which they are recited. Nor should they be construed to exclude any steps being performed concurrently.