Title:
MEANS FOR DETECTING WEAR ON WELL DRILL BITS
United States Patent 3853184


Abstract:
The specification discloses sensor means in a rotary drilling bit for detecting the extent of wear on critical wear points in the bit during the drilling operation and means for signaling to the driller at a remote location from the bit the extent of such wear.



Inventors:
MCCULLOUGH D
Application Number:
05/266502
Publication Date:
12/10/1974
Filing Date:
06/26/1972
Assignee:
MCCULLOUGH D,US
Primary Class:
Other Classes:
175/40
International Classes:
E21B12/02; (IPC1-7): E21B13/00
Field of Search:
175/39 73
View Patent Images:
US Patent References:



Primary Examiner:
Murtagh, John E.
Assistant Examiner:
Favreau, Richard E.
Attorney, Agent or Firm:
Pravel & Wilson
Parent Case Data:


CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Pat. application, Ser. No. 69,549, filed Sept. 4, 1970, now abandoned.
Claims:
What is claimed is

1. Wear detecting means for detecting wear in drill bits used with drilling fluid in rotary well drilling operations comprising:

2. The invention of claim 1, wherein said wear detecting means includes a plug adjacent to the outer circumference of said drilling bit to detect a reduction in the gauge of such bit due to wear in drilling operations.

3. The invention of claim 1 wherein said drilling bit includes bearings for supporting roller cones thereon and said wear means are positioned adjacent said bearings to detect wear thereof.

4. The invention of claim 1 wherein said pressure responsive means includes a hydraulic cylinder having a piston therein that is adapted to be moved longitudinally with respect to such cylinder in response to pressure conducted through said fluid conduit means.

5. The invention of claim 1 wherein said means with said pressure responsive means for restricting flow of the drilling fluid used with said bit includes butterfly valve means operably connected to said pressure responsive means wherein said butterfly means includes an orifice therein for restricting flow of such drilling fluid.

6. The invention of claim 1 wherein said wear means includes a plurality of wear means at critical wear points in said bit and said means with said pressure responsive means includes butterfly valves each having an orifice therein of a different size to restrict the flow of drilling fluid to a different amount to thereby identify the particular critical wear point which is worn on the bit.

7. The invention of claim 1 wherein said drilling bit includes cutting teeth and said wear means are located adjacent to tips of said teeth of the said drilling bit.

8. The invention of claim 7 wherein said wear means adjacent to tips of such drill bit teeth include pin means to open said fluid conduit means when the wear means adjacent the bit tip has been worn away.

9. The invention of claim 1 including:

10. The invention of claim 9 wherein said rotatable member is formed with a plurality of ratchet teeth on the outer surface thereof and said means for rotating said rotatable member includes a pin having one end adapted to engage said ratchet means and the other end adjacent said wear means with means for urging said pin into engagement with said ratchet means and a restraining means on the end of the pin adjacent said wear means which is adapted to be worn away when said wear means is removed to release said pin for moving into engagement with said ratchet means.

11. The invention of claim 1 wherein said fluid pressure means includes means for imposing a positive pressure on said wear means and means for actuating said pressure responsive means when said positive pressure is released to cause said pressure responsive means to restrict the flow of drilling fluid through said bit.

12. The invention of claim 11 including a negative pressure circuit with said positive pressure and switching means for switching said pressure responsive means from a positive pressure to a negative pressure to remove said restriction from the flow of drilling fluid through said bit.

13. The invention of claim 1, including:

14. The invention of claim 13, wherein said plug breaking means includes:

15. A wear detecting sensor for detecting wear in a rotating part, comprising:

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a device for sensing the extent of wear on various parts of a drill bit such as the external diameter or gauge of the bit, the bearings on which the moving bit parts rotate and the condition of the cutting or drilling edges of such bit together with means in the drill string for signaling to the surface when the wear on any of such portion of the bit reaches a predetermined amount.

2. Description of the Prior Art

In the drilling of well bores with well drilling bits it is very important to detect a predetermined degree of wear of the drill bit as soon as such wear occurs, although such wear occurs when the bit is several thousand feet below the surface of the earth. In the rotary method of drilling wells where a drill bit is advanced into the earth by rotation of the drill pipe and pressure applied downward on the bit and a drilling fluid is pumped in and out of the well bore, drill bits have been provided which will drill several hundred feet without being replaced, but wear occurs on the drill bit in three different parts, namely the gauge or hole size cutting edges, the bearings (in the case of a roller type drilling bit), and the bottom cutting or drilling edges.

When the bit gets out of gauge, it drills a tapered hole or what is known as a tight hole. When a new bit is run back in the hole, there is danger of pinching the bit and locking the cones or rollers so that they will not turn and drill. This requires another trip out of the hole and back to bottom again to replace the jammed bit with another new bit. If there is much tight hole, as such is usually the case, this second new bit will be damaged reaming the hole down to the bottom to which the hole was originally drilled, in which case the second bit will have to be pulled out of the hole and another new bit will have to be run to get back on bottom.

A large number of "green" or partially used or worn bits are pulled out of the hole because the driller has no way of knowing when the cutting or drilling edges of the drill bit are worn out. In the case of roller or cone bits, some times the space between the cutting teeth on the rollers or cones get filled and packed tight (commonly called break-up) with the formation through which it is drilling causing the drill bit to appear to be dull. When the drilling time slows down, the driller pulls the bit without really knowing what condition the bit is in. As a result, a green bit, which could make another 100 feet or more of hole, is pulled from the hole.

In those formations where the bit can be turned fast, the bearings (in the roller or cone type bit) will usually wear out before the bit teeth. When the bearings wear small enough, the rollers or cones drop off the bit, and require a costly fishing or retrieving job to remove them from the well hole. Sometimes it is impossible to continue drilling a well and it has to be abandoned because not all fishing jobs are successful in getting the junk iron out of the hole.

SUMMARY OF THE INVENTION

The present invention directs itself to a means for indicating to the driller that the drilling bit has worn away the maximum amount of wear either at the gauge or the well bore sizing portions of the drill bit, the cutting or drilling edges of the drill bit, and the bearings in the roller or cone type of drilling bits. In this manner the driller will be definitely advised that the maximum degree of wear has occurred and he can immediately take steps necessary to stop drilling and replace the bit.

It is, therefore, one of the objects of the invention to provide a drill bit with the gauge cutting portion of the bit with a core or drill hole that will act upon a control signal to indicate when the maximum wear has occurred on the gauge portion of the bit.

Another object of the invention is to provide a new and improved drill bit with means for indicating when teeth on the bit are worn to a predetermined degree. It is also an object to provide a new and improved drill bit having a device with the cutting teeth on the cones that will trip and act upon a ratchet spool to uncover a cored or drilled hole in the bearing spindle and actuate a control signal to indicate when the maximum wear has occurred on the teeth of the drill bit.

Another object of the invention is to provide a drill bit with the bearing spindles with a core or drilled hole that will act upon a control signal to indicate when the maximum wear has occurred on the bearings of the drill bit.

Another object of the invention is to provide a new and improved means of indication for each one of the drill bit cutting cones in the case of a bit having a plurality of cutting cones.

Another object of the invention is to convert the control signal to a discernible signal at the surface of the ground by the instrument drill sub by turning pre-sized orifices into the mud or air (in the case of using air for the drilling fluid medium) stream. By turning different pre-sized orifices into the drilling fluid stream, the pressures will vary from the normal, and these changes in pressure (one each for the gauge of the bit, the bearings that the cutting cones roll on, and the cutting teeth of the bit) can be detected at the surface of the ground in the pipe line from the mud pump by a pressure to current transducer and this signal amplified and sent to an audible horn and a strip chart recorder for a premanent record of the different events that gives a visible indication of what part of the bit has worn.

Another object of the invention is to provide the information immediately at the surface of the ground and to distinguish as to which part or parts have reached their maximum wear.

Still another object of the invention is to provide the information with enough time lag sufficient to let the driller continue to drill another foot or so if conditions would warrant doing so.

A construction designed to carry out the invention will be hereinafter described, and other and further objects of the invention will be shown when the following description is considered in connection with the accompanying drawings, and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a well bore showing therein a drilling apparatus constructed in accordance with this invention;

FIG. 2 is an enlarged, longitudinal view, partly in elevation and partly in section, showing the drill bit and details of wear sensors;

FIG. 3 is an enlarged cross-sectional view taken on line 3--3 of FIG. 2 showing the arrangement of the cone teeth wear indicating mechanism;

FIG. 3A is an enlarged cross-sectional view taken along line 3--3 of FIG. 2 illustrating an alternate embodiment for an arrangement for a wear sensor in the rotating cone;

FIGS. 4A and 4B are enlarged, longitudinal, sectional views of the upper and lower portions, respectively, of the instrument drill bit sub;

FIG. 5 is a cross-sectional view, taken on line 5--5 of FIG. 4-A;

FIG. 6 is an enlarged longitudinal view of the air cylinder and crank mechanism that moves the different size orifices in the mud or air stream;

FIG. 7 is a schematic of the above ground signal circuit and indicating instrument components;

FIG. 8 shows an example of a typical strip recording chart that shows how the different events are recorded; and

FIG. 9 is a schematic of the pneumatic control circuit of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Briefly, the drilling sensor apparatus of the present invention is designated generally A in FIG. 1 of the drawings wherein 9 designated a well bore drilled by rotary drilling equipment including a drill stem or pipe 8 extending downwardly through the well bore and supporting drill collars and the instrument sub 10 thereon. The drilling bit 11 is mounted at the lower end of the instrument sub. As will be described in detail hereinafter, the drill bit 11, as well as the instrument sub 10, include means for signaling to the surface when wear on various critical parts of the drill bit exceed a predetermined amount and thereby enable the driller at the surface to maximize the use or life of each drill bit and also minimize the risk of drilling either a tapered hole or having the bit fail in operation.

As shown in FIG. 2, the drill bit 11 may be of the type provided with a plurality of cutter cones 20 mounted upon spindles 18 carried on suitable antifriction bearings 19. The cutters are provided with cutting teeth 23 and have their heel portions projecting outwardly beyond the body 16 of the drill bit to insure the drilling of a full gauge hole through which the body of the bit and the related drilling equipment will pass as the drilling of the well progresses downwardly.

It is important that the operator of the drilling equipment known when the cutting diameter of the drilling bit has been reduced by wearing away of the heel portion of the cutter cones 20 to keep from drilling an undersized portion of the well bore, when the antifriction bearings 19 fail or become excessively worn and permit the cutter cones to slip off of the spindles 18 an be lost in the well bore, necessitating a cleaning or fishing operation in the well bore, and also when the cutting teeth 23 of the drill bit cutters 20 have become worn to the point at which they no longer cut efficiently.

In the present invention, the sensors for detecting wear in the drill bit include a plurality of pressurized fluid conduits 13, 14 and 15 having wear means adjacent to the critical wear points in the bit 11 which, when worn sufficiently, will open the pressurized conduits to allow a pressure drop therein and actuate signaling devices to indicate to the driller the fact that such wear has occurred. As best seen in FIGS. 4-A and 4-B of the drawings, a plurality of butterfly valves 33, 37 and 43 are positioned in the drilling mud conduit 29 of the instrument sub 10-A. As shown, each butterfly valve is provided with a different sized orifice and each such valve is connected to a fluid pressure cylinder which, in response to a drop in fluid pressure in one of the fluid pressure lines 13, 14 or 15 to which it is connected, rotates the butterfly valve and thereby causes a temporary restriction in the fluid passage 29 which in turn provides an audible as well as a visual signal to the driller at the surface in a manner to be described in detail hereinafter.

Considering now the apparatus of the present invention in more detail, as shown in FIG. 2 of the drawing, each of the fluid passages 13, 14 and 15 is provided with a circumferentially extending groove 13-A, 14-A and 15-A, respectively, formed in the upper shoulder 11-A of the drill bit 11. Corresponding fluid passages 13-B, 14-B and 15-B, respectively, are provided in the instrument housing 10-A for communicating fluid pressure from the sensors in the bit 11 to the actuator pistons 32, 36 and 41 as shown. A passage or orifice 31 is provided in the cylinder 32 for connecting a fluid pressure line thereto and similarly, orifices or passages 35 and 40 are provided in each of the piston cylinders 36 and 41, respectively, for connecting the fluid pressure lines 14 and 15 thereto.

As best seen in FIG. 6 of the drawings, a piston 32-A is provided in the cylinder 32 and is connected by means of a rod 32-B to a crank arm 42 which in turn is connected to a shaft 42-A on which is mounted the particular butterfly valve 33, 37 or 43 as the case may be. As shown, the shaft 42-A is mounted and suitable bearings 42-B to facilitate rotation of the shaft 42-A and the butterfly valve associated therewith.

As best seen in FIG. 2 of the drawings, the preferred embodiment of the present invention includes a bit 11 having a shank 16 thereon which is provided with a fluid passage 15 having a plug 17 positioned at the outer surface of such shank 16 in the terminal end of the fluid passage 15. When the heel portion of the cutting teeth 23 has worn and the drilled hole is undersized, the shank 16 of the bit 11 will wear away. When the shank 16 and the plug 17 wear away sufficiently, the fluid passage 15 will be opened to release fluid pressure therein and actuate the piston in the fluid cylinder 36 to rotate the butterfly valve 37 and restrict flow through the passage 29. As will be described in detail hereinafter, this will operate an audible as well as a visual signal at the surface to indicate to the driller that the shank portion 16 of the bit has worn sufficiently to wear away the plug 17 which will indicate to him that the bit has worn to the point that it is making an undersized or tapered hole and that therefore it is necessary that the bit be withdrawn from the bore hole and replaced with a new bit.

Similarly, the fluid passage 14 has a plug 21 at its terminal end, which, when the bearings 19 are worn and reduced in circumference, will be worn by the surface 23-A of the cone 23. When such plug 21 is worn away, it will release the fluid pressure in the passage 14 to actuate the piston and the cylinder 41 and in turn butterfly valve 43 so as to provide a restriction in the fluid passage 29 which will signal the driller at the surface in a manner to be described in detail hereinafter.

Also, the fluid passage 13 is connected to a passage 27 which, as best seen in FIG. 3 of the drawings, is positioned in the end of the spindle 18 on which the cutter cone 20 is mounted. A ratchet wheel or spool 26 is rotatably mounted about the end of the spindle 18 and is provided with a passage 27-A therein, which, when aligned with passage 27 will release the fluid pressure in the line 13 to actuate the piston in cylinder 32 and rotate the butterfly valve 33 to signal the operator at the surface that the tips 23 of the cone 20 have worn. Wear plugs 23' are provided in the ends of the passages 23-B in the cone 20. Such passages are arranged so as to communicate an internal passage 23-C which surrounds the end of spindle 18 and provides a housing in which the ratchet wheel 26 is free to turn. As shown, such wear plugs 23' are positioned in the cutting tips 23 so that as such tips wear, they necessarily wear the wear plugs 21'. As shown, the wear plugs 21' are mounted at the end of a shaft 25 which extends through a lock member 24 and through an inner pin guide 23-F. Such pin projects into the passage 23-C and, when moved longitudinally of the passage 23-B, will engage the teeth 26-A on the ratchet wheel 26 to rotate the ratchet counterclockwise as the roller cone is rotated in the normal drilling operations when the bit 11 is turned to the right. When the inner end of the pin 25 engages such teeth 26-A, it will rotate the ratchet counterclockwise and when the opening 27-A is aligned with passage 27 fluid pressure in the line 13 will thus be released to actuate the butterfly valve 33 in the instrument housing 10-A.

In the preferred form of this invention, pin 25 is urged inwardly toward the ratchet 26 by means of a compression coil spring 25-A that surrounds such pin the passage 23-B. The outer end of the pin extends through a suitable opening 24-A in the lock 24 and has a head 25-C thereon for holding the pin against longitudinal movement inwardly of the passage 23-B. After the wear plug 21 has worn sufficiently, the head 25-C will be worn off the pin 25 by continued rotation of the roller cone 20 and when such head is worn off, the compressed spring 25-A will be released and will move the pin 25 inwardly to engage the teeth of the ratchet 26, turn the ratchet and thereby release the pressure to signal that the teeth 23 have been worn to a predetermined amount.

It will be understood that as the bit 11 is rotated to the right, the roller cones 20 will rotate in a counterclockwise direction as viewed in FIG. 3 of the drawings and that the pin 25 will rotate the ratchet in a counterclockwise direction to align the passage 27-A with the passage 27.

An alternate embodiment for indicating wear of the teeth of the roller cone 24 is illustrated in FIG. 3A. In FIG. 3A, the spindle 18 again has passage 13 extending substantially axially therethrough in fluid communication with the radially extending passage 27 which terminates at the outer surface of the spindle 18 which is part of the internal passage 23-C between the end of the spindle 18 and the roller cone 23. A plugging element 126 is mounted by any suitable means such as sweat soldering into the passage 27 in order to allow the passages 27 and 13 to hold or contain fluid pressure. The plugging element 126 has a bore 126d therein that extends internally the length of the plugging element. The wall forming the plug bore 126d is weakened by circumferencial indentations at 126a to provide a weakened joint which allows the tip 126c of the plug to be broken off under the force of the pin 25, when such pin has been moved by spring 25a to its release position. Thus, after the head 25-C of the head 21 has been sufficiently worn off, the spring 25a moves the pin 25 to a release position in the internal passage 23-C. Thereafter, as the roller cone continues to rotate counterclockwise, the end of the released pin 25 engages the breakable tip 126c and, due to the weakened joint 126a, breaks the tip 126c off of the plug 126, thereby allowing the pressurized fluid in the passages 13 and 27 to escape so that such pressure loss may be used to indicate or provide a signal as discussed in greater detail hereinafter.

Considering now the control circuit of the present invention as shown in the schematic view of FIG. 9, a fluid pressure tank 54 provides fluid, such as compressed air, under of approximately 1,000 to 5,000 psi. A vacuum tank 51 is also provided with the control circuit for providing a negative pressure for use in operating control circuit as will be described in detail hereinafter. Also, as will be described in detail hereinafter, the tanks 54 and 51 as well as the valves shown in the schematic in FIG. 9 of the drawing are housed in the instrument control housing 10-A and are shown in more detail in FIG. 5 of the drawings. The valves 56 and 56-A which control the tanks 54 and 51, respectively, as well as valves 59 are accessible from the exterior of the housing 10-A.

With the tank 54 charged under suitable operating pressure, the valve 57 which had theretofore been closed, is opened and the valves 59, which are operably connected with each of the fluid sensor circuits 13, 14 and 15, respectively, are also opened to precharge such lines 13, 14 and 15 under positive pressure. With the valve 59 opened, the pilot pistons in the pneumatic valves 60, 67 and 73 which are each associated with the fluid pressure lines 15, 14 and 13, respectively, are also charged to move the pistons and such pilot valves to the right as shown in FIG. 9 of the drawings and thereby connect the vacuum line or conduit 78-A with each of the relay valves 62 in the lines 78-B. Each of the pilot valves 60, 67 and 73 is provided with a spring 61, 68 and 74, respectively, which is compressed by the pressure from the tank 54. After the pilot valves have been charged, the valves 59 are closed. Similarly, return pilot valves 69, 69-A and 75 are connected to the conduit 78-B via the conduits 78-C and 78-D which, as shown, connect opposite sides of valves.

When one of the wear sensors, such as the sensor 17, is worn away as mentioned above while the well is being drilled, this releases fluid pressure in the line 15 into the well bore 9 reducing the pressure in the pilot valve 60 and permitting the spring 61 to move the valve from a connection to lines 78-A to connect the positive pressure lines 57-A which will then be connected to the conduit 78-B. The positive pressure in 78-B will be communicated through the lines 78-B to lines 78-C and D. The pressure through 78-D will move the pilot valve 69 to the left to switch the line 78-C into operable connection with the line 78-E which passes positive pressure through the line 31 into the piston chamber 32 to rotate the butterfly valve 33 and restrict fluid flow through the passage 29. Also, positive pressure is introduced from the lines 78-E into the lines 78-F to actuate the valves 62-A and 62-B to close temporarily the fluid passage through the lines 88 and 98 to render the sensor apparatus, which actuates the other two butterfly valves 43 and 37, temporarily inoperative so as to give a clear indication to the driller at the surface that the wear plug 17 has been released and thereby avoid any confusion as to which critical portion of the bit is worn out.

As shown, a choke 64 is provided in the line 78-H that connects the opposite sides of the piston and the return valve 69. Also, an accumulator tank 65 is provided with the choke 64. After the volume tank 65 has filled sufficiently, the pressure will then equalize in the valve 69 on opposite sides of the piston therein, and the spring 69-B will switch the valve 63 back to a connection with the vacuum line 78-K that is connected to the vacuum tank 51. AS the vacuum reduces the pressure in the line 78-C and 78-E and line 31, the piston in the chamber 32 is moved back to its starting position to open the butterfly valve 33 and permit normal circulation of drilling fluid through the passage 29 in the manner which it occurred prior to signaling to the surface of the wear on the bit 11.

Thus, it will be appreciated that a temporary restriction in the mud circulation line will occur when a particular part of the bit has been worn and due to the differences in size of the orifice or openings 34, 38 and 44, pressure buildup in the mud line will vary according to the particular portion of the bit which has worn. Also, the chokes 64, 70 and 76 are of variable sizes so that the time required to return the particular butterfly valve with which they are associated to the original position, will vary so that not only will the signal at the surface vary according to the pressure buildup in the line, but also according to the length of time that such pressure buildup remains. In other words, in order to give a clear indication to the driller at the surface of which portion of the bit has worn, and to prevent more than one event or one worn portion of the bit to be signaled at a time, the different surges and duration of such surges avoids any confusion.

As shown in FIG. 7 of the drawings, the increase in pressure occurs in the mud flow line 79 and is sensed by the pressure-current transducer 80 and amplified by an amplifier 81 with the signal being sent to the relay 82. Such relay 82 is connected to an audible horn 83 to provide an audible signal of wear on the bit having occurred, and also a strip recorder 84 prints a record of the event on a chart 85. An enlarged view of the chart 85 is shown in FIG. 8 of the drawings to illustrate the different pen marks that are made as a result of different parts of the bit having become worn. For example, the pen traces a continuous line 86 to indicate minor variations in mud pressure in the mud line 79.

When the butterfly valve 33 having the largest diameter of orifice 34 restricts flow in the conduit 29, pressure in the mud line 79 and the choke 64 prevents the return of such butterfly valve 33 to its normal position for a time that is indicated by the length of the line 87 or which indicates the duration of the increased pump pressure while the valve 33 is restricting flow into line 29. Similarly, when the butterfly valve 37 having the intermediate sized orifice 38 therein restricts flow into line 29, the pump pressure is increased so that the line 89 is further to the right than the normal line 86 or the line 87 and, similarly, the length of the line 89 which indicates the duration of such elevated pump pressure is longer than line 87 because the choke 70 is sized so as to prevent the return of the butterfly valve 37 to its normal position a longer time than would be the case with the choke 64.

By the same token, when the butterfly valve 43, having the smallest diameter orifice 44 therein, is turned so as to restrict flow through the line 29, the readout on the strip chart 85 produces the line 91 which is the furtherest to the right on chart 85 indicating the highest pressure in the line 79 corresponding to the greatest restriction in the flow line. The length of the line 91 is determined by the choke 76 which holds the valve 43 in the closed position for a longer period of time than would be the case with the other chokes 70 or 64. Thus, it would be appreciated that the different trace lines 87, 89 and 91 indicate to a driller at the surface which portion of the drill bit as reached a critical wear point. Also, with the present invention, a date and time record may be put on the strip chart by means of a date-time stamp which will be actuated by movement of the tracing head some fixed distance beyond the normal mud pump pressure indicated by line 86. When the mud pump is shut down, the recording pen returns to a position indicated by the line 93.

As best seen in FIG. 5 of the drawing, the instrument housing 10-A includes a longitudinally extending cylindrical body 55 having a relieved portion 55-A for receiving the pressure tank 54. A longitudinally extending cap 52 is secured by a plurality of bolts 52-A to the housing 55. Such cap 52 has an arcuate outer surface corresponding to the outer surface of the housing 55 so that when the cap is positioned on the housing a longitudinally extending cylinder 10-A is formed. The longitudinally extending passage 29 is formed in the housing 55 and receives the butterfly valves 33, 37 and 43. Such housing 55 also includes an opening or relieved portion 55-B in which is positioned the actuating cylinders 32, 36 and 41 for rotating the butterfly valves 33, 37 and 43, respectively.

In operation, the apparatus of the present invention is connected to the lower end of a string of drill collars with the bit 11 secured at the lower end of such instrument housing 10-A. When the bit is in operation, the pressure in the fluid systems maintains a positive pressure on each of the wear plugs on sensors 17 and 21 and also maintains a positive pressure in the passage 27 in the spindle of the bit 11.

In the event sufficient wear occurs on the bit to wear away any of the wear plugs, pressure in the hydraulic system is released to signal to the surface the fact of a change in the mud pump pressure in the drill string. As described herein above, when pressure in the hydraulic system is released, the hydraulic cylinder connected to the wear plug which is released is caused to rotate to turn the butterfly valve attached thereto so as to restrict fluid flow to the passage 29. After a timed interval, depending upon the particular choke 64, 70 or 76, the fluid pressure in the hydraulic system causes such butterfly valve to be returned to a position extending longitudinally of the flow passage 29 to thereby relieve the restriction in the mud line and terminate the signal of the apparatus of this invention indicating that the bit is worn in a particular place.

After a signal of the worn condition of the bit hs been received at the surface, the drilling operations may be interrupted to pull the worn bit from the well bore and replace it with anew bit and continue drilling of the well bore. With the apparatus of this invention the maximum use from a particular bit may be obtained without running the bit beyond its maximum wear point so as to cause either a tapered hole or to loose a part of the bit in the well bore.

The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials as well as in the details of the illustrated construction may be made without departing from the spirit of the invention.