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BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to a rotary drill for use in drilling. More particularly, the present invention relates to an earth drilling apparatus having extendable cutting blades for cutting or underreaming. 
     2. Summary of Related Art 
     Well drilling operations frequently require that the area being drilled be expanded at selected points. In many instances, where a pipe casing or lining is in place, the pipe requires clearing of debris or other material from deep inside the well. In each of these situations a special type of drill known as an underreamer is required. 
     Underreaming drills allow for the widening of a selected area in a well. In general, an underreaming drill includes a pilot bit portion and an underreaming tool attached upstream from the pilot bit portion. The underreaming tool includes plurality of cutting blades that are movable as a group between a retracted position and an extended position. In their retracted position, the blades are folded into the body of the underreaming tool, thus allowing insertion and extraction of the tool from the area being worked. In their extended position, the blades are pivoted to a position which is substantially perpendicular to the long axis of the tool. While extended, the blades are able to underream an area thus enlarging the region for a given purpose. 
     While the general object of blade retraction and extension has been achieved by many tools, movement of the blades has been effected by a variety of mechanisms. For example, underreamers which include rack-and-pinion like gear and driver arrangements are disclosed in U.S. Pat. No. 5,402,856 issued to Warren et al. on Apr. 4, 1995, for ANTI-WHIRL UNDERREAMER, U.S. Pat. No. 3,208,540 issued to Park on Sep. 28, 1965, for EXPANSIBLE ROTARY WELL DRILLING BIT, U.S. Pat. No. 2,872,160 issued to Barg on Feb. 3, 1959, for HYDRAULIC EXPANSIBLE ROTARY WELL DRILLING BIT, and U.S. Pat. No. 1,478,306 issued to Sweetman on Dec. 18, 1923, for UNDERREAMER. In general, in each of these patents one set of teeth are formed on a curved end-surface of each blade, while another set of teeth are formed on an axially-movable driver assembly. The blades are pivotably attached to the hollow drill body. 
     U.S. Pat. No. 4,431,065 issued to Andrews on Feb. 14, 1984, for UNDERREAMER and U.S. Pat. No. 5,010,955 issued to Springer on Apr. 30, 1991, for CASING MILL AND METHOD teach a gear arrangement similar to the patents mentioned immediately above, but further include a stop arrangement for halting the outward pivoting of the blades. While such a stop is arguably inherent in the patents mentioned immediately above, the arrangement is more positively defined in these latter patents. 
     Additional variations may be seen in U.S. Pat. No. 4,116,012, issued to Abe et al. on Sep. 26, 1978, for METHOD OF OBTAINING SUFFICIENT SUPPORTING FORCE FOR A CONCRETE PILE SUNK INTO A HOLE, U.S. Pat. No. 3,548,362, issued to Blank, Jr. on Dec. 15, 1970, for WELL CASING CONTACT TOOL, U.S. Pat. No. 2,756,968, issued to Emanuel et al. on Jul. 31, 1956, for EXPANSIBLE WELL SCRAPER, U.S. Pat. No. 2,124,663, issued to Wintemute on Jul. 26, 1938, for ROTARY UNDERREAMER, and U.S. Pat. No. 1,667,155 issued to Higdon on Apr. 24, 1928, for DRILLING BIT, which teach various arrangements of slots alone or slots and driving pins which are engaged to selectively effect outward or inward pivoting movement of cutting blades with respect to the drill shaft for underreaming. 
     While arguable providing improvements in the art of underreamer drills, the art still suffers from a variety of problems. The environment in which these drills operate causes frequent problems to the underreaming assembly, such as debris entanglement and premature wear. The prior art mechanisms, while perhaps well-engineered, are overly complex and require for too much “down time” for the tool and, hence, the drilling operation. Disassembly and reassembly are difficult and time consuming. Hours are lost when a tool fails and has to be withdrawn, repaired and reinserted. 
     By way of example, cutting blades of known underreamers are pivotably retained to the assembly by set-screw arrangements. High rotational speed and hard materials frequently cause these screws to break, thus allowing parts of the tool to drop into the well, requiring removal of the tool and an extensive fishing expedition to recover the dropped parts. The heads of the set screws themselves, being externally exposed, are subject to wear, further complicating disassembly and reassembly. 
     In addition, known drilling apparatus frequently suffer from premature wear on not only the drilling assembly but also on the connection between the cutting blades and the body of the underreamer due to the tortional force being concentrated at the connection where the cutting blades are extended. 
     Accordingly, a simple yet effective underreaming apparatus remains wanting. 
     SUMMARY OF THE INVENTION 
     It is the general object of the present invention to provide an underreaming assembly to overcome the problems of the prior art. 
     More particularly, it is an object of the present invention to provide an underreaming apparatus which is relatively easy to operate and which provides for minimal operational “down time.” 
     It is a further object of the present invention to provide such an apparatus which may be readily disassembled and reassembled for changing the cutting blades. 
     Still an additional object of the present invention is to provide such an apparatus which has a minimal number of parts and threaded elements. 
     A further object is to provide a dynamic fluid chamber that is able to axially drive a mandrel on the build-up of a selected fluid pressure to effect extension of the cutting blades, thereby providing an efficient and positive engagement mechanism. 
     An additional object of the present invention is to provide such an apparatus which is capable of transferring torque to the entire underreaming when the cutting blades are extended. 
     These and other objects are accomplished by the provision of a drilling apparatus having extendable cutting blades for cutting or underreaming. The drilling apparatus of the present invention includes a top sub, a pressure sleeve mated to the top sub, and a blade body mated to the pressure sleeve. A pilot bit is mated to the lower end of the blade body. A plurality of movable cutting blades are pivotably attached to the blade body by pivot pins. Each of the pins is inserted through the blade body and the cutting blade. Slidable covers, held in place by the pressure sleeve, are used to retain the pins. A mandrel is centrally positioned with respect to the pressure sleeve and the blade body and is axially movable therein. Each of the cutting blades includes an end which is operatively associated with channels transversely formed on one end of the mandrel. 
     Movement of the mandrel in a first axial direction effects retraction of the cutting blades. Movement of the mandrel in a second axial direction effects extension of the cutting blades. The mandrel is held in its blade-retracting position by the biasing force of a spring. A build-up of hydraulic fluid, selectively delivered from the operator, in a pressure chamber formed between the pressure sleeve and the mandrel causes the mandrel to be moved to its blade-extending position when a critical mass of fluid pressure is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood by reference to the following detailed description of the preferred embodiments of the present invention when read in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout the views, and in which: 
     FIG. 1 is an environmental view of the drilling apparatus of the present invention shown relative to a drilling operation platform; 
     FIG. 1 a  is a view taken along line  1   a  of FIG. 1 which illustrates the operative portion of the drilling apparatus of the present invention in greater detail; 
     FIG. 2 is perspective view of the drilling apparatus of the present invention, including the top sub, the pressure sleeve, and the blade body; 
     FIG. 3 is an exploded view of the drilling apparatus of the present invention, illustrating internal as well as external components; 
     FIG. 4 is an exploded view of the blade body relative to the blade, the blade pin, and the dove-tail cover; 
     FIG. 5 is a perspective view of the mandrel of the drilling apparatus; 
     FIG. 6 is a partially-sectioned side view of the drilling apparatus of the present invention with the cutting blades in their retracted positions; 
     FIG. 6 a  is a view taken along line  6   a  of FIG. 6 which details the arrangement between the mandrel and the retracted cutting blade; 
     FIG. 7 is a view taken along line  7 — 7  of FIG. 6 which details the relationship between the mandrel and the pressure sleeve; 
     FIG. 8 is a view taken along line  8 — 8  of FIG. 6 which details the relationship between the mandrel, the blade body, and the cutting blades in their retracted positions; 
     FIG. 9 is a partially-sectioned side view of the drilling apparatus of the present invention with the cutting blades in their extended positions; 
     FIG. 9 a  is a view taken along line  9   a  of FIG. 9 which details the arrangement between the mandrel and the extended cutting blade; and 
     FIG. 10 is a view taken along line  10 — 10  of FIG. 10 which details the relationship between the mandrel, the blade body, and the cutting blades in their extended positions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawings disclose the preferred embodiment of the present invention. 
     While the configurations according to the illustrated embodiment are preferred, it is envisioned that alternate configurations of the present invention may be adopted without deviating from the invention as portrayed. The preferred embodiment is discussed hereafter. 
     The drilling apparatus of the present invention may find applications in a variety of operations. For example, the apparatus may find utility in many drilling applications including well drilling, boring, and coring operations. However, the present invention finds particular application in the removal of obstructions from piping, as generally illustrated in FIG.  1 . Furthermore, operations may be on dry ground or, as illustrated in FIG. 1, on ocean-bound derricks or rigs. 
     With reference to FIG. 1, a drilling rig, generally illustrated as  10 , is positioned roughly over an existing pipe or well  12 . The well  12  has one or more integral narrowed regions, generally illustrated as  14 . As may be understood by reference to the figure, the regions  14  act to restrict passage of a bit and limit the bit&#39;s outer diameter. 
     Occasionally obstructions appear in one or more ares of the well  12 . Such obstructions may be composed of a variety of materials, but typically include paraffin, rock, or shale. Sometimes the obstruction is intentionally placed during the formation of the well  12  so as to halt or limit the unwanted flow of fluids into or out of the well  12 . Such an obstruction is formed by the insertion of, for example, cement into the well at a certain point. Later during well construction the cement will have to be removed to allow the continuation of well construction. In any event, an obstruction is illustrated in FIG. 1 as material  16 . 
     In all such instances it will be necessary for the obstruction, whatever the material or its source, to be cleared to allow proper operation of the well  12 . A drill motor with bit and bit extension attached will normally be employed for such purposes. However, in the event that a restriction such as region  14  exists, a bit which can pass the restriction and thereafter be extended to undercut the area is required. 
     As generally set forth above, the drilling apparatus of the present invention provides a solution to this problem by employing a drilling apparatus, generally illustrated as  18 . The apparatus is connected to a drill motor (not illustrated) which is located on or about the drilling rig via an extension shaft  20 . As is known in the art, the length of the extension shaft may be lengthened at intervals during the drilling process by adding more individual shaft elements (drill pipe). This procedure is reversed to remove the drilling apparatus  18 , also as is known in the art. Those skilled in the art will also recognize that continuous milled tubing having various diameters may also be used as opposed to the use of standard drill pipe. 
     At the lowermost part of the drilling apparatus  18  is removably provided a pilot bit  22 . The pilot bit  22  is threadably attached to the drilling apparatus  18  and acts to properly center the drilling apparatus  18  as it enters the material it is drilling. 
     The drilling apparatus  18  and its general environment is illustrated in greater detail in FIG. 1 a  which is a view taken along line  1   a  of FIG.  1 . The drilling apparatus  18  includes a pressure sleeve  24  and a blade body  26  which is threadably mated to the pressure sleeve  24 , as will be discussed in more detail below. The pilot bit  22  is threadably mated to the lower end of the blade body  26 . Defined through the approximate center of the pilot bit  22  and fanning out therefrom are a series of fluid passageways  28  shown in phantom. (The external openings of the fluid passageways  28  are more clearly shown in FIG. 3.) Hydraulic fluid is injected from the approximate area of the motor through the extension shaft  20 , through the drilling apparatus  18  as will be described in greater detail below, and through the fluid passageways  28  for lubricating, cutting and cooling purposes. 
     Underreaming of the obstructed area (or the subject material) is accomplished via the selected extension of a plurality of cutting blades  30  which are illustrated in FIGS. 1 and 1 a  as being extended from the blade body  26 . A plurality of slots  32  are formed in the wall of the blade body  26  to receive the cutting blades  30  when in their retracted positions. Each of the slots  32  includes a first cutting blade stop edge  35  and a second angled stop edge  37 , which is more clearly shown in FIG.  4  and further described herein. A cutting surface  33  is formed on each of the cutting blades  30 . The cutting surface  33  would ordinarily be provided with a cutting material such as carbide (not shown). 
     In general operation, the cutting blades  30  are in their retracted positions relative to the blade body  26  when the drilling apparatus  18  is inserted into and withdrawn from the well  12 . Once having passed the restricted region  14 , the cutting blades  30  are extended to open the obstructed area. 
     FIG. 2 is a perspective view of the drilling apparatus  18 . As illustrated in this figure, the drilling apparatus  18  includes the pressure sleeve  24 , the blade body  26  and a top sub  34 . The top sub  34  is substantially hollow to allow the flow-through of hydraulic fluid, as is known in the art. A threaded end  36  is formed on one end of the top sub  34  for attachment to other shafts to extend the assembly to its desired length. 
     In addition to providing more detail as to the arrangement and construction of the cutting blades  30  in relation to the blade body  26 , FIG. 2 also illustrates a dove-tail cover of pivot pin retaining cover  38 . The cover  38  and its functions will be described in further detail as this description proceeds. 
     FIG. 3 illustrates an exploded view of the drilling apparatus  18  in relation to the pilot bit  22 . While the external components of the drilling apparatus  18  have been generally introduced, the primary internal component, a mandrel  40 , is shown initially in FIG.  3 . The mandrel  40  is movable in both directions along the long axis of the drilling apparatus  18  and includes external threads  41 . (Both the pressure sleeve  24  and the blade body  26  are substantially hollow.) It is the movement of the mandrel  40  which effects selective movement of the cutting blades  30  between their extended and retracted positions, as will be discussed in further detail below. 
     The top sub  34  includes a series of internal threads  42  which are matable with the external threads  41  of the mandrel  40  This arrangement allows for selective attachment of the top sub  34  with the mandrel  40 , such mated arrangement being best illustrated in FIGS. 6 and 9. The pressure sleeve  24  includes a series of internal threads  43  which are matable with a series of external threads  44  formed at one end of the blade body  26 . The threads  44  of the blade body  26  terminate at a shoulder  45  formed on the blade body  26 . This arrangement allows for selective attachment of the pressure sleeve  24  with the blade body  26 . The blade body  26  has a series of internal threads  46  formed at its opposite end which are matable with a series of external threads  49  formed at one end of the pilot bit  22 . This arrangement allows for selective attachment of the blade body  26  with the pilot bit  22 . 
     While illustrated in FIG. 3, the construction and arrangement of the blade body  26  and its accompanying cutting blades  30  is more clearly illustrated in FIG.  4 . With reference then to FIG. 4, the profile of the cutting blade slot  32  is readily seen. This profile roughly corresponds to the external profile of the illustrated cutting blade  30 . Such a configuration having a relatively close tolerance is beneficial in resisting the possible infiltration of debris into the interior of the blade body  26  when the cutting blades  30  are in their retracted positions. 
     For purposes of the immediate discussion to explain the blade arrangement and to aid in simplicity of understanding, reference will only be made to a single blade arrangement, although each of the three cutting blades  30  are arranged identically. The cutting blade  30  includes a blade body attachment end  48  for pivotable attachment to the blade body  26 . The attachment end  48  is pivotably attached to the blade body  26  by a pivot pin  50 . The pin  50  is slidably insertable through a channel or bore  52  defined completely through the attachment end  48  of the cutting blade  30 . The pin  50  is also slidably insertable through a pair of opposing channels or bores  54 ,  54 ′ defined through the blade body  26  adjacent to the cutting blade slot  32 . 
     The pivot pin retaining cover  38  is slidably fitted into a pivot pin retaining cover slot  56 . A pair of opposed walls  58 ,  58 ′ are formed on the slot  56  and are bevelled inwardly to mate with a pair of opposed edges  60 ,  60 ′ formed on the pivot pin retaining cover  38 . The walls  58 ,  58 ′ and the edges  60 ,  60 ′ mate in a dove-tail configuration, thus allowing only for sliding insertion or sliding removal from the blade body  26  along the external threads  44 . 
     The pivot pin retaining cover  38  is held in place by the pressure sleeve  24  after the latter is threadably fitted to the blade body  26 . (The attachment of the pressure sleeve  24  to the blade body  26  is illustrated in FIGS. 2 and 3.) The pivot pin retaining cover  38  includes a pressure sleeve abutment wall  61  which is canted or angled. The leading edge of the pivot pin retaining cover  38  which is defined by the cant or angle extends somewhat beyond the shoulder  45  of the blade body  26 . This configuration assures a press fit within the pivot pin retaining cover slot  56  when the pressure sleeve  24  is threadably mated to the blade body  26  by a biasing force. 
     Because the outer side of the pivot pin retaining cover  38  is generally flush with the outer side of the blade body  26  after insertion and because the tolerances of the walls  58 ,  58 ′ and the edges  60 ,  60 ′ are relatively close, debris is substantially kept out of the pivot pin retaining cover slot  56 . In addition, the presence of the pivot pin retaining cover  38  eliminates the risk of shearing of the pins  50 , thus overcoming a significant problem of the prior art. Furthermore, this arrangement, coupled with the relatively simple means of cover retention as described above, allows for relatively easy disassembly and assembly of the drilling apparatus  18  at such times as it may be necessary to change the cutting blade  30 . 
     To assemble the blade body  26 , once the cutting blade  30  is positioned in its cutting blade slot  32  and with the pivot pin retaining cover  38  removed, the pin  50  is inserted through one of the pair of opposing channels  54 ,  54 ′ to pass through the channel  52 , and into the other of the pair of opposing channels  54 ,  54 ′. As illustrated, the ends of the pin  50  are bevelled such that, once in its assembled position, the ends lie substantially flush with the floor of the pivot pin retaining cover slot  56 . The pivot pin retaining cover  38  is then slid into place into the pivot pin retaining cover slot  56 . Once this procedure has been undertaken for each of the cutting blades  30 , the pressure sleeve  24  is threaded to the blade body  26 , thus locking the pivot pin retaining covers  38  in position. Disassembly is made by reversing each of these steps. 
     FIG. 5 is a perspective view of the mandrel  40  of the drilling apparatus  18 . As noted above, the mandrel  40  includes external threads  41  which threadably mate with the internal threads  42  of the top sub  34 . The external threads  41  are formed on the end of a shaft  62  which, at its distal end, includes a reduced diameter portion  64  having an O-ring  66 . The proximal end of the shaft  62  includes a first smooth portion  68 . The smooth portion terminates at a shoulder area  70  having an O-ring  72  positioned within a mating groove  73  thereupon. The first smooth portion  68 , in combination with the shoulder area  70 , forms the mandrel part of a fluid pressure chamber, to be further described below with respect to FIG.  6 . (It should be noted that the O-rings are suggested and may be substituted with T-seals.) 
     On the side of the shoulder area  70  opposite the first smooth portion  68  is a second smooth portion  74  which terminates at a hex portion  76 . The hex portion  76  terminates at a first triangular portion  78  which itself terminates at a recessed triangular portion  80 . Between the first triangular portion  78  and the recessed triangular portion  80  is a first canted wall  81 . The recessed triangular portion  80  terminates at a second triangular portion  82  which itself forms the terminal part of the mandrel  40 , Between the recessed triangular portion  80  and the second triangular portion  82  is a second canted wall  83 . The first triangular portion  78  is smaller than the second triangular portion  82 . Finally, a third canted wall  85  is formed at the end of the mandrel  40 . 
     Defined axially along the center of the mandrel  40  is a fluid channel  100  which is illustrated in FIGS. 6 and 9. The fluid channel is provided to deliver hydraulic fluid between the two opposite ends of the mandrel  40 . Illustrated in FIG. 5 is a pilot bit hydraulic fluid outlet port  84  which feeds fluid into the fluid passageways  28  of the pilot bit  22  for lubrication. In addition to the pilot bit hydraulic fluid outlet port  84  are provided a pressure chamber radial port  86  which selectively delivers fluid to the pressure chamber as will be further described below and a series of cutting blade ports  88  (of which only one is shown) which selectively delivers lubricating and debris-clearing fluid to the cutting blades  30 . 
     The cutting blade ports  88  enable fluid to be directed to and around the cutting blades  30 , thus enabling both lubrication of the cutting surface  33  (similar to the lubricating features of a milling machine) and to clear the slots  32  thus preventing cuttings from jamming the cutting blades  30  in their extended positions. In this way, the cutting blade  30  may be retracted without being jammed by debris. 
     FIG. 6 is a partially-sectioned side view of the drilling apparatus  18  of the present invention illustrating the cutting blades  30  in their retracted positions. In addition, this figure is useful in its clear presentation of the relationship of the mandrel  40  to the top sub  34 , the pressure sleeve  24 , and the blade body  26  as well as to their miscellaneous attached elements. 
     The hex portion  76  of the mandrel  40  allows for axial movement within the blade body  26 . This arrangement is best illustrated in FIG. 7 which is taken along the line  7 — 7  of FIG.  6 . As is shown, the interior surface of the blade body  36  is partially configured so as to substantially mate with the external hexagonal configuration of the hex portion  76 . This arrangement not only provides a positive rotational lock between the mandrel  40  and the blade body  26 , but also provides a method of allowing the entire drilling apparatus  18  to substantially absorb the torque produced during the drilling operation. 
     FIG. 8 is a sectional view taken along lines  8 — 8  of FIG.  6  and shows the blades  30  in their retracted positions. In such a position, and as will be further discussed below, a portion of each of the blades  60  positively rest against the second triangular portion  82  while retracted. 
     With the configurations of FIGS. 7 and 8 as background, reference will be made back to FIG. 6 for a more complete understanding of the drilling apparatus  18  and its operation. FIG. 6 illustrates a spring chamber  90  formed substantially between the inner wall of the pressure sleeve  24  and the second smooth portion  74  of the mandrel  40 . A spring  92  is provided within the spring chamber  90 . One end of the spring rests against the shoulder area  70  of the mandrel  40  while the other end of the spring  92  positively engages and rests against one end of the blade body  26  to provide a biasing force therebetween. This biasing force maintains the drilling apparatus  18  in its hole insertion/withdrawal configuration in which the blades  30  are retracted. 
     As noted above, the fluid channel  100  is defined axially along the complete length of the mandrel  40  to provide fluid to the pilot bit hydraulic fluid outlet port  84  at one end of the mandrel as well as to the pressure chamber radial port  86  and to the cutting blade ports  88 . 
     The relationship between the mandrel  40  and the cutting blades  30  is more clearly understood by reference to FIG. 6 a  which is a view taken along line  6   a  of FIG.  6 . This figure details the arrangement between the mandrel  40  and one of the retracted cutting blades  30 , although each of the remaining cutting blades  30  is formed with the same configuration and operates in an identical manner as the illustrated cutting blade  30 . With reference to this exemplary cutting blade  30  (the remaining cutting blades  30  being configured in a like manner), the cutting blade  30  includes a first positive stop contact surface  93 , a second position stop contact surface  95 , a first flat surface  94 , a second flat surface  96  adjacent to the first flat surface  94 , a third flat surface  97  adjacent to the second flat surface  96 , and a fourth flat surface  98  adjacent to the third flat surface  97 . An edge  99  is formed at the junction of the first surface  94  and the second flat surface  96 . Each of these features serves a function in the retraction and extension of the cutting blade  30 . 
     As is illustrated in both FIGS. 6 and 6 a , in its retracted position, the cutting blade  30 , which is pivotably mounted on the pivot pin  50 , rests substantially on the second triangular portion  82  of the mandrel  40 . More specifically, the second flat surface  96  of the cutting blade  30  positively engages and rests against the second canted wall  83 , while the third flat surface  97  positively engages and rests against an outer wall of the second triangular portion  82  to provide a substantially planar contact surface to allow the transfer of torsional force from the cutting blade  30  to the mandrel  40 . 
     To be moved to its extended position, the mandrel  40  must be moved relative to the pressure sleeve  24  and the blade body  26 . This movement effects rotation of each of the cutting blades  30  upon their pivot pins  50 , thus causing extension of the cutting blades  30 . However, the mandrel  40  is retained in its resting, cutting blade-retracted position of FIG. 6 by the biasing force of the spring  92 . To overcome the biasing force of the spring  92  and to thereby effect movement of the mandrel  40  relative to the pressure sleeve  24  and the blade body  26 , hydraulic fluid (not shown) must be introduced within the pressure sleeve  24  via the pressure chamber radial port  86 . The hydraulic fluid is introduced into a variably-sized pressure chamber  102  which is between an O-ring  104  that is provided substantially within the pressure sleeve  24  and the O-ring  72  of the mandrel  40 . 
     On introduction of sufficient fluid into the chamber  102 , the biasing force of the spring  92  is eventually overcome and the mandrel  40  is axially moved to its cutting blade expanding position, as illustrated in FIG.  9 . Details of the assembly are shown in FIG. 9 a . While values vary depending on the conditions of the underreaming task involved, generally pressures above between about 150 to 200 p.s.i. are required to effect movement of the mandrel  40 . (Pressures below these values still result in fluid flowing through the fluid channel  100  and out of the pilot bit hydraulic fluid outlet port  84  as well as out of the cutting blade ports  88 .) 
     As the mandrel  40  is shifted from its cutting blade-retracted position of FIG. 6 to its cutting blade-extended position of FIG. 9, the third canted wall  85  of the mandrel  40  engages the fourth flat surface  98  of the cutting blade  30 , initially urging the end of the cutting blade  33  having the cutting surface  33  formed thereon to be pivoted outward. As the mandrel  40  continues in its axial movement, the first canted wall  81  of the mandrel  40  engages the first surface  94  of the cutting blade  30 , further urging the cutting surface  33  to be pivoted outward. The cutting blade  30  continues to be pivoted until the first surface  94  of the cutting blade  30  positively engages and rests against the flat portion of the first triangular portion  78  as the mandrel  40  moves to its maximum position as shown in FIG.  9 . The fully extended position of the cutting blades  30  is illustrated in FIG. 10 which is a cross-section of the drilling apparatus  18  taken along lines  10 — 10  of FIG.  9 . This arrangement provides for a planar contact surface that evenly transfers torsional forces from the cutting blades  30  to the mandrel  40 . 
     The cutting blade  30  achieves a positive stop in its extended position through three surface-to-surface contacts between the portions of the blade  30 , the blade body  26 , and the mandrel  26 . These stop arrangements are illustrated clearly in FIG. 9 a . When extended, the first positive stop contact surface  93  and the second positive stop contact surface  95  of the blade  30  abuts the cutting blade stop edges  35  and  37  of the blade body  26 , respectively. This forms two positive stops. The other positive stop is formed between the planar surface of the first triangular portion  78  of the mandrel  40  and the first flat surface  94  of the cutting blade  30 . By these features a positive lock is achieved and the risk of damaging the operative elements (such risk being well-known in the gear tooth designs of the prior art) is significantly reduced if not eliminated. In this regard, the tortional force from the extended cutting blades  30  is absorbed by both the mandrel  40  and the blade body  26  and transferred to the entire underreaming assembly  18 . Therefore, tortional force is not merely concentrated at the connection point of the cutting blades  30 . 
     Retraction of the blades  30  to their resting positions shown in FIG. 6 is enabled by release of the fluid pressure from the fluid chamber  102 . As the spring  92  acts to return the mandrel  40  to its cutting blade-retracted position, the edge  99  formed at the junction of the first surface  94  and the second flat surface  96 , is acted upon by the second canted wall  83  of the mandrel  40 . (It is to be recalled that the first triangular portion  78  of the mandrel is smaller than the second triangular portion  82 .) This movement initiates inward pivoting of the cutting blade  30  at the end defined by the cutting surface  33  which, as the edge  99  slides inwardly (relative to the mandrel  40 ) along the second canted wall  83 , is completed when the second flat surface  96  positively engages and rests upon the second canted wall  83  and the third flat surface  97  positively engages and rests upon a flat surface of the second triangular portion  82 . 
     Preferably each of the metal components of the present invention would be composed of case-hardened steel. 
     The present invention provides a drilling apparatus which is simple in construction and is easily disassembled and reassembled. The drilling apparatus of the present invention allows for easy changing of the cutting blades, prevents shearing of the cutting blades, and resists invasion by debris. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.

Summary:
A drilling apparatus for underreaming includes a top sub, a pressure sleeve mated to the top sub, and a blade body mated to the pressure sleeve. A pilot bit is mated to the lower end of the blade body. A plurality of movable cutting blades are pivotably attached to the blade body by pivot pins. Each of the pins is inserted through the blade body and the cutting blade. Slidable covers, held in place by the pressure sleeve, are used to retain the pins. A mandrel is centrally positioned with respect to the pressure sleeve and the blade body and is axially movable therein. Each of the cutting blades includes an end which is operatively associated with channels transversely formed on one end of the mandrel. Movement of the mandrel in a first axial direction effects retraction of the cutting blades. Movement of the mandrel in a second axial direction effects extension of the cutting blades. The mandrel is held in its blade-retracting position by the biasing force of a spring. A build-up of hydraulic fluid, selectively delivered from the operator, in a pressure chamber formed between the pressure sleeve and the mandrel causes the mandrel to be moved to its blade-extending position when a critical mass of fluid pressure is achieved.