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TECHNICAL FIELD 
     This invention relates generally to drilling tools used in the drilling of oil and gas wells, or similar drilling operations, and in particular to a tool that can drill a hole diameter larger than the inside diameter (ID) or the drift diameter (DD) of the casing or pipe installed in the well above the hole being drilled. Such tools are commonly known as Underreamers. Additionally this tool relates to drilling tools known as expandable stabilizers. 
     BACKGROUND 
     When drilling through subterranean formations in the exploration for oil and gas, it is common practice to drill larger diameter holes at the surface, and successively smaller diameter holes as the well is drilled deeper. When the desired depth is reached for a given wellbore diameter, a tubular casing is cemented in place. This practice allows for the protection of water tables from drilling and production fluids, improves drilling and production efficiency, and protects the wellbore. It is often desirable to drill a hole larger than the inside diameter of the last casing that was set, at some known depth below the surface. This may be desirable, for example, for setting additional casing below this known depth, which will require drilling an annular well bore diameter sufficient for cementing of the lower casing. This creates a special drilling situation since conventional drill bits of the size needed to generate the desired well bore diameter will not fit inside the casing that has already been set. Tools used for these applications are commonly known as underreamers. Other applications for underreamers include enlarging zones for gravel pack completions or to compensate for plastic flow of salt and shale formations. 
     Two principal tools are commonly used in the drilling industry to achieve the objective of drilling the well bore diameter larger than the drift diameter of the casing. The first tool used for this purpose is known as the “Bi-center bit.” The Bi-center bit is an undersized drill bit with a large eccentric cutting structure located off-center above a smaller pilot drill bit that is centered axially with the drill collars. Fielder discloses such a device in U.S. Pat. No. 5,678,644. The Bi-center bit is sized so that while running it into the hole, the smaller pilot bit will be pushed to one side to allow the tool to pass through the inside of the casing. When the Bi-center bit reaches the bottom of the hole, the pilot bit acts as a centered pivot point for the eccentric cutting structure above it. When drilling, the eccentric cutting structure will then rotate around the pilot bit and generate a larger hole than the inside (or drift diameter) of the casing. 
     Problems are frequently associated with the use of the Bi-center bit. For example, when drilling a soft formation, the pilot bit will be forced the one side of the hole opposite the larger eccentric cutting structure and the resultant hole drilled will be smaller than required. The offset design of the Bi-center bit results in uneven wear of the cutting structure and lower rates of penetration. Furthermore, the torque generated during the use of Bi-center bits fluctuates and can have a damaging effect on the drill string, and the tool is unreliable in controlling the angle of the hole. An additional limitation is the inability of the Bi-center bit to drill out cement or a casing shoe. Due to this limitation, an extra trip is required to drill out cement or a casing shoe when using Bi-center bits. 
     The second tool used for the purpose of drilling a section of the well bore diameter larger than the drift diameter of the casing is an underreamer. A typical underreamer includes extendable arms pivotally mounted in a housing on hinge pins for movement between a retracted position and an extended position. The underreamer may be hydraulically or mechanically actuated. While the underreamer is being lowered into the hole, the arms will be in the collapsed or retracted position to permit the tool to pass through the inside diameter of the casing. When the underreamer reaches the depth at which it is desirable to increase the well bore diameter, the arms of the underreamer are hydraulically or mechanically actuated into the extended position. 
     In the past, most underreamers utilized roller cone type cutters. Weber discloses such a device in U.S. Pat. No. 4,064,951. These devices were limited in their effectiveness in many formations, and unreliable as a result of the numerous moving components and sealing systems required for their construction. Roller cone type cutters require bearing systems. The most reliable roller cone cutters also required a lubrication and sealing system. Furthermore, the component parts were subject to breakage that resulted in costly operations to remove the debris from the bottom of the hole. More recently, attempts have been made to build underreamers which utilize synthetic diamond material. Simpson discloses such a device in U.S. Pat. No. 4,589,504. These underreamers were also prone to breakage of the support arms and cutting elements that resulted in costly operations to remove the debris from the bottom of the hole. 
     Other underreamer designs have been designed to perform only reaming operations to prevent bit sticking, and are of a type that includes a long conical tapered body attached by splines in a conical shell. Deely discloses such a device in U.S. Pat. No. 3,051,255. These tools were designed to improve wellbore concentricity and ensure that the drill bit does not get stuck. Such devices have suffered from difficulties in their manufacture as related to the design, as well as operational limitations. In particular, underreamers that incorporate long tubular sections which are internally tapered are extremely difficult to manufacture with quality tolerances. The inner surface of the cutting lugs cannot mate uniformly with the length of the conical surface traversing the inner surface of the cutting lugs. Another disadvantage of this design is that the radial forces imparted to the cutting lugs generate resultant forces that remove weight from the bit and urge the tool to disengage and return to the retracted position. The result is an unstable tool that cannot tolerate the shock and vibration associated with simultaneous drilling. Another disadvantage of these tools is that they are severely limited in their total expansion capability and are not capable of enlarging the wellbore by any significant amount. 
     A primary limitation of past underreamer designs has been the necessity to first drill a pilot hole with a conventional drill bit, then remove the entire drill string, assemble the underreamer onto the drill string, and then begin the underreaming operation. This two step drilling process is slow and costly. 
     Expandable stabilizers may be used during operations designed to increase the hole diameter. The principals and design solutions known to the industry for the construction of underreamers also apply to the construction of expandable stabilizers. The primary difference is that cutting elements are replaced with wear elements. 
     It is seen that there is a need for a tool with greater strength and reliability to overcome the disadvantages and limitations commonly associated with conventional bi-center bits, underreamers, and expandable stabilizers of the general configurations described above. 
     SUMMARY OF THE INVENTION 
     In the preferred embodiment, the invention provides a tool that is capable of enlarging the hole while drilling, that overcomes many of the deficiencies of past underreamer designs and Bi-center bits as previously noted. It is on the basis of this unique ability that the inventor has designated the tool as the “EWD” which is an acronym for “Enlarging-while-drilling.” In an alternative embodiment, the tool operates as an expandable stabilizer. 
     Configured as an enlarging-while-drilling tool, the current design provides an extremely stable configuration with far fewer moving parts and a higher degree of reliability than previous designs. As a result of the stability, and unique internal assembly of the invention, the risk of breakage and disaster is substantially reduced, since it is impossible for a cutting lug to fall from the body of the enlarging-while-drilling tool. Additionally, the stability of the preferred embodiment allows for a greater extension of the cutting lugs and enables the operator of the invention to expand the well bore in a greater amount than previous designs. For example, a 12¼″ wellbore can be enlarged to 14¾″ with this tool. The greater strength and stability of this tool permits the operator to continue to drill a deeper hole while simultaneously enlarging the wellbore. This eliminates the need to drill a pilot hole with a conventional drill bit, then remove the entire drill string, assemble the underreamer onto the drill string, and then begin the underreaming operation. 
     The drilling tool with extendable elements accomplishes the above noted operating features. The tool is an assembly of tubular sections that are attached to each other by threaded connections in a vertical alignment. As assembled, there is a continuous path for the flow of drilling fluid through the tool. 
     The tool is delivered into the casing in the retracted position. A shear pin holds the tool in the retracted position while drilling out cement and the casing shoe. When the tool reaches the desired depth in the well bore, the tool may be extended by application of the required weight known to shear the pin. Actuation of the tool results in extension of a plurality of cutting lugs sliding radially outward from the tool. When the tool is actuated, a mandrel assembly moves downwardly relative to a housing assembly. A tapered shaft portion engages matchingly tapered inner surface portions of the cutting lugs, and forces the cutting lugs outward through longitudinal slots in the housing assembly. The cutting lugs move radially outward until a cylindrical shaft portion engages a matchingly curved inner surface portion of the cutting lugs. With the cutting lugs extended, the tool will enlarge a hole to a substantially larger diameter than the drift diameter of the casing through which it passed. When configured for extended stabilization, the cutting lugs are simply replaced with stabilizing lugs, and the nozzle receptacles are plugged to increase the flow of drilling fluid through the tool. The stabilizing lugs each have a wear surface attached to their exterior surface. The wear surface may be a hardmetal or attachment of wear elements such as diamonds or tungsten carbide inserts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a half-section of the tool in the retracted position. This is the position in which the tool would normally be lowered into the well. In the retracted position, the cutting elements are positioned radially at a distance that is less than the inside diameter of the casing. 
     FIG. 2 is a cross section as indicated in FIG.  4 . 
     FIG. 3 is a cross section as indicated in FIG.  1 . 
     FIG. 4 is a half-section view of the tool in the extended position. In this position the cutting lugs will act to enlarge the hole diameter. 
     FIG. 5 is a cross section as indicated in FIG.  1 . 
     FIG. 6 is a cross section as indicated in FIG.  4 . 
     FIG. 7 is a partial, enlarged cross section as indicated in FIG.  4 . 
     FIG. 8 is a half-section of the tool configured as an expandable stabilizer, the alternate embodiment of the tool, in the retracted position. This is the position in which the tool would normally be lowered into the well. In the retracted position, the stabilizer elements are positioned radially at a distance that is less than the inside diameter of the casing. 
     FIG. 9 is a half-section of the tool configured as an expandable stabilizer in the extended position. In this position the stabilizer lugs will act to stabilize the tool and the drill string by engagement with the hole diameter. 
    
    
     DESCRIPTION 
     Referring to FIGS. 1 and 4, the reference numeral  10  generally designates the enlarging-while-drilling (EWD) tool, embodying features of the preferred embodiment. The EWD tool  10  includes a tubular drive shaft  12  which has at its top end a threaded upper drive shaft connection  14  configured for connection to a drill string component (not shown). Referring to FIG. 1, drive shaft  12  has a longitudinal drive shaft central bore  16  throughout for the passage of drilling fluid. A plurality of drive lugs  18  extend downwardly from drive shaft  12 . An externally splined shaft  20  extends below drive lugs  18 . A threaded lower drive shaft connection  22  is below splined shaft  20 . 
     Located directly beneath, and in vertically slidable relationship with drive shaft  12  is a tubular drive cap  24  having an internally splined center section  26  which engages splined shaft  20  to provide a vertically slidable connection between drive shaft  12  and drive cap  24  to transmit rotation of the drill string through tool  10  when tool  10  is in a retracted position. Downwardly facing drive lugs  18  on drive shaft  12  align with and engage complimentarily opposing drive slots  28  on top of drive cap  24 . This occurs upon axial movement of drive shaft  12  toward drive cap  24  during actuation of tool  10 , as will be described hereinafter. At the bottom of drive cap  24  is a threaded drive cap connection  30 . 
     Directly beneath drive cap  24 , is a tubular upper housing  32  having a longitudinal upper housing central bore  34 . On top of upper housing  32  is a threaded upper housing top connection  36 , which is threadedly connected to drive cap connection  30 . Upper housing  32  has a plurality of longitudinal slots  38  that intersect upper housing central bore  34  and extend to the exterior of upper housing  32 . Referring to FIG. 5, a tapered perimeter  40  is formed along each of longitudinal slots  38 . Tapered perimeter  40  is outwardly reducing, such that the perimeter opening longitudinal slots  38  form at the interior surface of upper housing central bore  36  is greater than the perimeter opening longitudinal slots  38  form at the exterior surface of upper housing  32 . Referring to FIG. 1, at the bottom of upper housing  32  is a threaded upper housing bottom connection  42 . 
     Attached to the bottom of drive shaft  12  is a tubular upper mandrel  44  having a threaded upper mandrel connection  46  at its top end and being threadedly connected to lower drive shaft connection  22 . An upwardly facing mandrel shoulder  48  is formed at the upper end of mandrel connection  46 . Likewise, an opposing downwardly facing drive cap shoulder  50  is formed between upper housing top connection  36  and splined center section  26 . Mandrel shoulder  48  and drive cap shoulder  50  engage to carry tensile force through tool  10  when tool  10  is suspended by upper drive shaft connection  14 . Upper mandrel  44  is vertically slidable within upper housing central bore  34 . Upper mandrel  44  has a longitudinal upper mandrel central bore  52 , and has an externally tapered bottom  54 . 
     A tubular lower mandrel  56  is located vertically slidable within upper mandrel central bore  52 , and has at its top end external circumferential grooves  58  for sealing against upper mandrel central bore  52 . Seals  60  effect a fluid tight seal between lower mandrel  56  and upper mandrel  44 . Lower mandrel  56  has a longitudinal lower mandrel central bore  62  for passage of drilling fluid, and a lower mandrel threaded connection  66  on its bottom end. 
     A tubular lower housing  68  has a longitudinal lower housing central bore  70 . At the top of lower housing  68  is an external-internal double thread connection  72  for complementary threaded connection to upper housing bottom connection  42 , and simultaneous complementary connection to lower mandrel connection  66  at the bottom end of lower mandrel  56 . Lower housing  68  has a lower housing threaded connection  74  on its bottom end for attachment of a drill bit or other drill string portion (not shown). Lower housing  68  has at least one fluid course  76  intersecting lower housing central bore  70 , and directed upwards. A nozzle  80  is located at the radial end of each fluid course  76 . 
     A cutting lug  82  is located in each of longitudinal slots  38 . Each of cutting lugs  82  must be positioned in longitudinal slots  38  from the inside of tool  10 , and are slidable radially of tool  10  between the retracted and extended positions as will be described further. As seen in FIG. 1, cutting lugs  82  have a conically tapered surface  85  that matches externally tapered bottom  54  of upper mandrel  44 . As seen in FIG. 6, the inner most surface of cutting lugs  82  has a curved inner surface  86  which matches the cylindrical outside surface of upper mandrel  44 . As can be seen in FIG.  5  and FIG. 6, cutting lugs  82  have an exterior taper  84 , such that exterior taper  84  permits cutting lugs  82  to extend a limited distance through longitudinal slots  38  before matchingly engaging tapered perimeter  40  of longitudinal slots  38 . As can be seen in FIG. 4, the bottoms of cutting lugs  82  are downwardly angular in the direction of the central axis of tool  10 , and in angular matching and sliding contact with tapered perimeter  40  of longitudinal slots  38 , such that without other forces in effect, gravity will force cutting lugs  82  to slide downwardly and radially inward, towards the center of tool  10 . The angular relationship between tapered perimeter  40  and exterior taper  84  is designed to stabilize cutting lugs  82  in the extended position, prevent entrance of drilling debris into the interior of tool  10  in both extended and retracted positions, and to eliminate any possibility of cutting lugs  82  falling into the hole. 
     Cutting lugs  82  have a plurality of cutting elements  88  which may be polycrystalline diamond compact (PDC) cutters, natural diamonds, tungsten carbide inserts or other wear resistant material mounted to engage and enlarge the well bore as tool  10  is rotated and progresses downwardly through the well bore in the extended position. 
     One or more shear pins  90  are placed between drive shaft  12  and drive cap  24  to keep tool  10  in the retracted position until the predetermined weight required to sever shear pins  90  is applied. In this manner, tool  10  can remain in the retracted position while all of the weight necessary is applied to a conventional drill bit to drill out the cement and casing shoe. 
     Referring to FIG. 4, an end  91  of shear pin  90  has been sheared off, and drive shaft  12  and upper mandrel  44  have moved downwardly to position upper mandrel  44  inside cutting lugs  82  to extend cutting lugs  82  to the desired diameter to underream the well bore as tool  10  rotates and progresses down the hole. 
     Pressure indicator  92  is supported on the top end of lower mandrel  56 , and has openings to allow passage of mud during drilling operations. 
     Referring to FIG. 7, tool  10  is shown in the extended position, and a hydraulic lock pin assembly  120  is illustrated. A lock pin  94  is incorporated in drive shaft  12 . Piston  96  is integral with lock pin  94  and positioned in a bore  98  in communication with a radial hole  100 . An O-ring  102  seals between piston  96  and bore  98 . A spring  104  is positioned between a retainer  108  and piston  96  and urges piston  96  to a retracted position. The drilling fluid circulating inside tool  10  passes through radial hole  100  and bore  98  to exert pressure on piston  96 , compress spring  104  and to extend pin  94 . When extended, pin  94  engages a pin hole  110  in drive cap  24  to lock tool  10  in an extended position as shown in FIGS. 4 and 7. 
     In this embodiment, a mandrel assembly  112  comprises a drive shaft  12  and an upper mandrel  44 , and a housing assembly  114  comprises a drive cap  24 , upper housing  32 , lower housing  68 , and lower mandrel  56 . Mandrel assembly  112  and housing assembly  114  are located in longitudinally slidable and sealing relation. It can be seen that the numerous connections between the tubular sections in this embodiment may be arranged differently to accomplish the same result. For example, in an alternative embodiment, not shown, mandrel assembly  112  includes lower mandrel  56 . 
     In an alternative embodiment, shown in FIG.  8  and FIG. 9, drilling tool with extendable elements  10  is shown configured to operate as an expandable stabilizer by replacing internally assembled cutting lugs  82  with internally assembled stabilizer lugs  182 . Stabilizer lugs  182  have an external wear surface  187  which may be created by coating the external surface of stabilizer lugs  182  with a hardmetal, or by attachment of a plurality of wear elements  188  to each stabilizer lug  182 . Additionally, when tool  10  is configured as an expandable stabilizer, flow course  68  and nozzle receptacle  80  are unnecessary and may be plugged if desired to increase the flow of drilling fluid to the drill bit (not shown). 
     OPERATION 
     In the operation of the preferred embodiment, the EWD tool  10  is configured as an enlarging-while-drilling tool. Tool  10  is connected to a drill string (not shown). Rotation of the drill string rotates the drilling tool with extendable elements  10 . In the retracted position, cutting lugs  82  are supported at conically tapered surface  85  by engagement with externally tapered bottom  54  of upper mandrel  44 , and at curved inner surface  86  by engagement with lower mandrel  56 . When the predetermined weight required to shear pin  90  is applied, tool  10  is actuated. When tool  10  is actuated, drive shaft  12  and upper mandrel  44  are forced downwardly relative to drive cap  24 , upper housing  32 , lower housing  68 , and lower mandrel  56 . In the downward movement, externally tapered bottom  54  of upper mandrel  44  traverses conically tapered surface  85 , forcing cutting lugs  82  radially outward until the outside surface of upper mandrel  44  engages curved inner surface  86 . As shown in FIG. 6, in the fully extended position, curved inner surface  86  is matchingly supported by the outside surface of upper mandrel  44 , positioning cutter lug  82  in full extension to cut the well bore to the larger diameter desired. As seen in FIG. 6, in the fully extended position, tapered exterior perimeter  84  of each cutting lug  82  is in substantially full perimeter engagement with tapered perimeter  40  of longitudinal slots  38  so as to prevent the complete passage of cutting lugs  82  through longitudinal slots  38 , and to securely support cutting lugs  82  while preventing intrusion of drilling fluid debris into the interior of tool  10 . As seen in FIG. 6, the vertical portion of tapered exterior perimeter  84  of each cutting lug  82  is tapered, as is the vertical portion of longitudinal slots  38  to maximize the surface contact area and stability between cutting lugs  82  and longitudinal slots  38  when tool  10  is in the extended position. 
     Drive lugs  18  are engaged with drive slots  24  to provide high torque, increased wear area, and increased strength of engagement. In this position splined shaft  20  and splined center section  26  do not have to carry all the drilling torque which is subject to great vibration and variation. 
     As drive shaft  12  is forced downwardly to actuate tool  10  into the extended position, drive shaft central bore  16  moves into close proximity with pressure indicator  92 , restricting the flow area for the drilling fluid, as shown in FIG.  4 . This restricted flow area causes a relative increase in the fluid circulating pressure at the surface, which is an indication that tool  10  is in the fully extended position. Pressure indicator  92  may be attached to either lower mandrel  56  or drive shaft  12  to achieve the same result. 
     In the extended position, there remains a continuous flow path through tool  10 . As drilling fluid passes through lower housing  68  a portion of the drilling fluid will enter flow course  76  and exit nozzle receptacle  80  which is aligned in the direction of cutting elements  88  to help cool and clean cutting elements  88  during expanding while drilling operations. 
     When tool  10  has been moved into the extended position and drilling operations have begun, the drilling fluid pressure internal to tool  10  will increase as the flow rate is increased, imposing pressure on piston  96  through radial hole  100  and bore  98 . Piston  96  will extend into bore hole  110  to lock tool  10  in the extended position. If a soft formation is encountered, and little weight is required to operate tool  10 , pin  94  engaged in bore hole  110  will prevent tool  10  from returning to the retracted position. When both weight and circulating pressure are removed from tool  10 , tool  10  may be retracted. With tool  10  in the retracted position, circulation of drilling fluid will not be similarly restricted by pressure indicator  92 , and the same flow rate will result in a relative decrease in the fluid circulating pressure at the surface, indicating that tool  10  is in the retracted position. 
     EWD tool  10  can be configured to operate as an expandable stabilizer by simply replacing internally assembled cutting lugs  82  with internally assembled stabilizer lugs  182 , and by optionally plugging nozzle receptacle  80 . Stabilizer lugs  182  are then actuated in the same manner as cutting lugs  82  in the Preferred Embodiment. The operation of tool  10  is precisely the same as the preferred embodiment, except that tool  10  will act as a drill string stabilizer. 
     Although an exemplary embodiment of the invention has been disclosed for purposes of illustration, it will be understood that various changes, modifications, and substitutions may be incorporated into such embodiment without departing from the spirit of the invention as defined by the claims appearing hereinafter.

Summary:
A tool for expanding a hole size while drilling tool is disclosed. The tool utilizes a mechanically actuated mandrel and extendable cutting lugs designed in a configuration with fewer moving parts than previous designs. The cutting lugs have multiple cutting elements attached. The cutting lugs are driven between two cylindrical diameter surfaces to achieve a large differential between the retracted and extended positions of the tool, and to provide sufficient stability to allow the tool to be used in the extended position while drilling forward. The lugs are internally assembled and tapered to closely match the slots in the body of the tool for additional stability and to prevent complete passage through the tool for prevention of catastrophic failure. The tool has a locking mechanism to allow the tool to remain in the extended position when the weight is removed from the tool. In an alternate embodiment, the cutting lugs may be replaced with stabilizing lugs having a wear resistant surface or mounted with wear resistant elements to form an expanding drill string stabilizer.