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CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/522,722 filed Nov. 1, 2004, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to an underreamer to be used in a bottom hole assembly of a drillstring. More particularly, the present invention relates to a underreamer having retractable blades or pads configured to retract or engage a borehole along field-replaceable guide inserts or pins in a substantially linear path. More particularly still, the retraction or engagement of the blades or pads results from decreases or increases in working fluid pressure flowing through the retractable assembly. 
         [0003]    Underreamers, in oilfield parlance, refer to downhole assemblies configured to enlarge existing boreholes. Underreamers function to enlarge smaller holes into larger-diameter boreholes. Often boreholes located below the lowest string of casing require bored diameters greater than the inner diameter of the next preceding string of casing. For these circumstances, an underreamer is installed behind a smaller drill bit and is run through the casing to the lower bore location. Once below the casing, the collapsible underreamer is expanded and a larger borehole is drilled. Once the larger bore is complete, the underreamer is retracted and the entire drilling assembly, bit, measurement equipment, and underreamer, is retrieved through the newly drilled borehole and casing thereabove. 
         [0004]    Additionally, if the retractable cutters of an underreamer are substituted with retractable stabilizer pads, a retractable stabilizer can be effective in numerous subterranean drilling situations to centralize the drill string during operation. A retractable stabilizer can be employed, as above, to stabilize a retractable underreamer drilling assembly or, in the alternative, can serve as an adjustable gauge stabilizer. An adjustable gauge stabilizer is capable of reconfiguring its outer diameter to create an underreamed borehole of a desired size. 
         [0005]    A recent exemplary expandable underreamer/stabilizer has been described in U.S. Pat. No. 6,732,817, issued on May 11, 2004 to Charles Dewey, et al., hereby incorporated by reference in its entirety. The invention disclosed in the Dewey patent relates to a three-bladed underreamer/stabilizer assembly wherein the three blades retract into and engage from a plurality of axial recesses having angled channels formed therein. The three blades of the Dewey patent engage the borehole by translating along the channels between a collapsed position and an expanded position in response to a differential pressure between an axial flowbore and the wellbore. The repetitive movement of the underreamer arms into and out of engagement in the presence of abrasive drilling fluids and cuttings can excessively wear the underreamer body thereby diminishing the useful life of the tool. 
         [0006]    Unlike the prior art, the present invention reliably provides for direct movement of the blades into the expanded position resulting from the increase in pump pressure. Because the arms of the present invention are moved into engagement with the adjoining bore wall by direct movement of a piston or mandrel down the underreamer body, the circuitous hydraulic path of prior art tools, which can become clogged preventing free movement of the activator ring driving the arms into and out of engagement, is avoided. The present invention avoids this problem. 
         [0007]    A hardfacing coating providing a low coefficient of friction of both the collapsible blades and the guides used to move these blades into and out of engagement with the bore wall additionally provides increased wear resistance and facilitates ready deployment under all well conditions. By utilizing a coating such as a QPQ nitride surface coating, the friction between the blades and guide inserts/pins is reduced. The hardfacing also makes the guide inserts/pins and blades more resistant the the abrasive drilling fluids present in a downhole environment. Because the guides and the blades can be replaced in the field when they become worn without the need to replace the entire underreamer body, the cost of using the underreamer with the present improvements is dramatically reduced over preexisting underreamer technology. The present invention constitutes a substantial improvement in the underreamer art by providing replaceable coated guides and blades. 
       SUMMARY OF THE INVENTION 
       [0008]    The underreamer of the present invention provides a tubular body having an axial flowbore and at least one longitudinal pocket formed therein; a pair of removable guide inserts installed longitudinally within said longitudinal pocket, each guide insert having at least one linear projection; a collapsible blade installed within the longitudinal pocket between the pair of guide inserts and having a linear groove corresponding to each linear projection on the guide inserts whereby each linear groove engagably contacts the corresponding linear projection; and thereby permits the collapsible blade to translate or move in a substantially linear path along the linear projection between an extended position and a retracted position in response to a change in the pressure within the axial flowbore. The body can be fitted with between three to five blades without departing from the spirit of this invention. The underreamer of the present invention can have a mandrel longitudinally disposed within the tubular body and having a plurality of load fingers engagably contacting the collapsible blade to manipulate the collapsible blades between the retracted and the extended positions by longitudinal translation of the load fingers in response to changes in flowbore pressure on the mandrel. The underreamer normally further provides a biasing spring opposably contacting the mandrel to maintain the collapsible blade in the retracted position when there is no pressure within the flowbore. 
         [0009]    The underreamer of the present invention provides the linear path of translation which is characterized by an acute angle departing from the central axis of the underreamer either upstream or downstream from said longitudinal pockets. The collapsible blade(s) and the guide inserts of the present invention can be QPQ nitride coated to provide wear resistance and to facilitate unrestricted movement of the blade out of and into the reamer body. These collapsible blade(s) can also include polycrystalline diamond cutter inserts, carbide buttons, or other hardened cutter elements, well known in the drilling industry. Furthermore, the blades can have cutting or hardened elements on a trailing face of each blade to allow the underreamer to operate coming out of the bore. The collapsible blade of the present invention can also be a stabilizer pad to allow this form of underreamer to be used as a stabilizer. 
         [0010]    A method of enlarging a borehole is also disclosed herein comprising the steps of installing at a distal end of a drillstring a collapsible underreamer having a tubular body, and an axial flowbore with a mandrel installed therein, and at least one longitudinal channel with removable guide inserts, a collapsible blade, and a guide insert lock installed longitudinally therein; pressurizing the bore of the underreamer to engage the collapsible blade with a guide insert and substantially linearly translate the collapsible blade to an extended position; and rotating the drillstring with the collapsible blade in the extended position to enlarge the borehole. The method further comprises changing the pressure through the axial flowbore to retract the collapsible blades, and retrieving the collapsible underreamer through an under gauge string of casing. 
         [0011]    Another method of using this underreamer comprises the steps of replacing the collapsible blade in the field by disconnecting the underreamer body from the drillstring; removing the mandrel; removing the guide insert lock; removing the used collapsible blade; inserting a replacement blade; reinstalling the guide insert lock; reinstalling the mandrel; and reinstalling the the underreamer body onto the drillstring. 
         [0012]    This method can further provide for replacing the collapsible blade in the field by:
   disconnecting the underreamer body from the drillstring; removing the mandrel; removing the guide insert lock; removing at least one guide insert; inserting a replacement replacement guide insert; reinstalling the guide insert lock; reinstalling the mandrel; and reinstalling the the underreamer body onto the drillstring. The method can further include replacing the collapsible blade in the field by disconnecting the underreamer body from the drillstring; removing the mandrel; removing the guide insert lock; removing the used collapsible blade; inserting a collapsible stabilizer pad in place of the blade; reinstalling the guide insert lock and the mandrel; and reinstalling the the underreamer body onto the drillstring.   
 
         [0014]    Similarly, this method can further include shortening the radial extension of the collapsible blade by disconnecting the underreamer body from the drillstring; removing the mandrel; removing the guide insert lock; removing the used collapsible blade; inserting a collapsible stabilizer pad in place of the blade; reinstalling a longer guide insert lock than the removed guide insert lock; reinstalling the mandrel; and reinstalling the the underreamer body onto the drillstring. 
         [0015]    The invention also includes a method to stabilize a drilling assembly in a borehole comprising the steps of installing above a drill bit at a distal end of a drillstring a collapsible stabilizer having a tubular body, an axial flowbore, and at least one longitudinal channel with removable guide inserts and a collapsible stabilizer pad installed longitudinally therein; pressurizing the axial flowbore of the collapsible stabilizer to engage the collapsible stabilizer pad with a guide insert and translate the collapsible stabilizer pad along a substantially linear projection of the guide insert to an extended position; and rotating the drillstring with the collapsible stabilizer pad in the extended position to stabilize the borehole. 
         [0016]    Another embodiment of the present invention is underreamer to be used within a wellbore drilling assembly, the underreamer comprising a tubular body providing an axial flowbore and at least one longitudinal pocket, said longitudinal pocket having at least one hole cut through the tubular body on each longitudinal side of the longitudinal pocket; a removable pin inserted through the hole on each longitudinal side of the longitudinal pocket; a collapsible blade installed longitudinally within the longitudinal pocket and having a linear groove corresponding to each pin wherein each linear groove engagably contacts the corresponding pin to retain said collapsible blade within said tubular body; and the collapsible blade translates along the pin between an extended position and a retracted position in response to a change in the pressure within the axial flowbore. 
         [0017]    A method of enlarging a borehole using this alternative embodiment comprises the steps of installing at a distal end of a drillstring a collapsible underreamer having a tubular body, an axial flowbore with a mandrel installed therein, and at least one longitudinal channel with removable pins securing a collapsible blade installed longitudinally therein; pressurizing the bore of the underreamer to engage a substantially linear groove formed in the collapsible blade with the removable pins and substantially linearly translate the collapsible blade to an extended position; and rotating the drillstring with the collapsible blade in the extended position to enlarge the borehole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]      FIG. 1A  is a schematic sectioned view drawing of a retractable downhole drilling assembly in an extended position in accordance with the present invention. 
           [0019]      FIG. 1B  is a schematic sectioned view drawing of the retractable downhole drilling assembly of  FIG. 1A  in a retracted position. 
           [0020]      FIG. 2  is a schematic representation of a section of a retractable downhole drilling assembly in a retracted position in accordance with a preferred embodiment of the present invention. 
           [0021]      FIG. 3  is a schematic representation of the retractable downhole drilling assembly of  FIG. 2  in an extended position. 
           [0022]      FIG. 4  is a schematic representation of a mandrel used to operate the retractable downhole drilling assembly of FIGS.  2  and  6 A/B. 
           [0023]      FIG. 5  is a schematic representation of a piston and through bore of the mandrel of  FIG. 4 . 
           [0024]      FIG. 6A  is a schematic representation of a retractable downhole drilling assembly with removable inserts for installing the collapsible blades. 
           [0025]      FIG. 6B  is a schematic representation of the retractable downhole drilling assembly of  FIG. 6A  with cutting surfaces on the collapsible blades. 
           [0026]      FIGS. 7A-C  depicts multiple representations of the guide inserts and collapsible blade to be used with the retractable downhole drilling assembly of FIGS.  6 A/B. 
           [0027]      FIGS. 8A-B  depict a top and side view of one embodiment of the guide insert lock used to hold the guide inserts and blades within the tool body shown in FIGS.  6 A/B. 
           [0028]      FIGS. 9A-B  depict a top and side view of one embodiment of a guide insert for guiding the motion of the retractable blade within the tool body shown in FIGS.  6 A/B. 
           [0029]      FIGS. 10A-B  depicts a top and side view of one embodiment of a matching guide insert for the guide insert shown in FIGS.  9 A/B. 
           [0030]      FIGS. 11A-C  depict a top view and a view from each side of the retractable blade that fits between the guide insert of FIGS.  9 A/B and  10 A/B. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    Referring initially to  FIGS. 1A and 1B , a retractable underreamer  100  is shown. Specifically,  FIG. 1A  shows underreamer  100  in an extended position while  FIG. 1B  shows underreamer  100  in a retracted position. Underreamer  100  is shown with a pin-end connection  102  on its downhole, or distal, end and a box-end connection  104  on its uphole, or proximal, end. A pin-end connection refers to male threads and a box-end connection refers to female threads. While underreamer  100  is shown as an assembly of three threaded subs  106 ,  108 ,  110 , it should be understood by one of ordinary skill in the art that multiple or single subs can be used to construct underreamer  100 . 
         [0032]    Underreamer  100  includes a plurality of longitudinal pockets  112  in which collapsible blades  114  are installed. Blades  114  are configured to extend ( FIG. 1A ) and retract ( FIG. 1B ) when a mandrel  116  is displaced. Mandrel  116  resides within a bore  118  of underreamer  100  and includes an engagement thruster  120  and a retraction thruster  121 . The engagement thruster  120  is affixed to mandrel  116  by a locking ring (not shown) within a locking groove (not shown) on mandrel  116 . The locking ring (not shown) is utilized to hold the engagement thruster  120  in place. This is shown in more detail in  FIG. 4 . Mandrel  116  also preferably includes a through bore  124  and a piston head  126 . In FIG.  1 A/B, a biasing spring  128  urges mandrel  116  in an upstream direction when no other loads are present upon mandrel  116 . Collapsible blades  114  slide linearly in and out of pockets  112  along a plurality of linear grooves  130  molded into the sides of blades  114 . Corresponding pins  132  are engaged into grooves  130  through main body  108  of underreamer  100  and are substantially perpendicular to pockets  112  and blades  114 . The ratio of mandrel bore  124  to drilling assembly bore  118  is such that increases in pressure therethrough act upon piston head  126  with force great enough to oppose biasing spring  128  and displace mandrel  118  thus extending blades  114 . 
         [0033]    In operation, underreamer  100  is preferably deployed to a location of interest in a retracted state, extended, used downhole, re-retracted, and then retrieved. Such operations are often performed when a section of wellbore requires underreaming at a location below a section having a smaller bore diameter, for example, below a string of casing. 
         [0034]    It should be understood by one ordinary skill that drilling assembly  100  can function either as an underreamer or as a stabilizer. An underreamer is designed to increase the diameter of a drilled wellbore while a stabilizer is used to contact a wellbore and stabilize the drillstring to prevent deviation of the drill bit. 
         [0035]    To use underreamer  100  in a wellbore, the assembly is preferably deployed downhole behind a smaller drill bit in a collapsed state. To extend blades  114 , the pressure of drilling fluid in the drillstring bore  124  is increased until the load upon piston head  126  is significant enough to displace mandrel  116  towards pin end  102 . With the displacement of mandrel, engagement thruster  120  loads blades  114  from behind. Because blades  114  are held within pockets  112  by pins  132  in grooves  130 , blades  114  slide outward and downhole (towards pin threaded end  102 ) from the loading of thruster  120 . The linear arrangement of grooves  130  enable blades  114  to extend outward such that an outer face  134  of blades is always substantially parallel to an axis of drilling assembly  110 . This parallel alignment helps ensure that blades engage the borehole in the best alignment possible, one that is substantially parallel to the path of the borehole to be stabilized or underreamed. With blades  114  extended drilling fluid is allowed to flow through bore  124  to lubricate a drill bit or operate any equipment farther downhole. 
         [0036]    When the retraction of blades  114  is desired, the pressure of drilling fluids through bore of drillstring  118  can be reduced to allow biasing spring  128  to move mandrel  116  away from pin end  102 . With mandrel  116  retracting, retraction thruster  121  can drive blades  114  upstream and towards box end  104 . Because pins  132  can engage grooves  130 , blades  114  can retract within pockets  112 , maintaining their substantially parallel alignment to the axis of main sub  108 . 
         [0037]    A simple “quick change” configuration is possible, whereby mandrel  116  is moved out of engagement with spring  128 , and alternate blades  114  are installed. This permits installation of replacement blades in the underreamer at a job site and avoids the need to send the entire underreamer body back to a shop for blade replacement. 
         [0038]    Several benefits of underreamer  100  over former retractable underreamers include the simplicity of operation, manufacture, maintenance, and repair. Main body  108  of drilling assembly  100  is constructed of a simple tubular design with a series of bores and simple cuts. Only a simple groove to retain the guide insert or several holes to insert pins is required within pockets  112 . No complex grooves or machined surfaces are required in pockets  112  or in bore  118 . Because only a small number of simple grooves to retain guide inserts  700 ,  750  are required within the body of the underreamer rather than multiple complex machined profiles within the walls of pockets  112 , manufacture, maintenance, and repair of drilling assembly  100  is relatively simple and quick. Alternatively, the blades  700 ,  750  can be retained by drilling standard holes into the longitudinal pockets  112  to insert pins  132 . Furthermore, the method for engaging or disengaging blades  114  is relatively simple compared to other solutions. Particularly, piston head  126  travels within a piston bore  140  that is somewhat larger than the ordinary flow bore  142  through the drillstring components thereabove. Furthermore, the diameter of bore  124  through mandrel  116  is substantially similar to the diameters of flow bores  142  before and  144  after mandrel  116 , resulting in negligible pressure drop across drilling assembly  100 . Because of the high cross-sectional area of the piston face between piston bore  140  and flow bore  124  through mandrel  116 , much higher loads can be transferred from the pressurized drilling fluid to blades  114 . As a result, drilling assembly  100  is capable of operating retractable blades  114  with much lower pressure drop than former devices. Lower pressure drop across drilling assembly  100  requires lower “activation” pressures to extend (or retract) blades  114 . The lowered pressures are beneficial in that that hydraulic seals and components of other drillstring devices are not susceptible to rupture. 
         [0039]    Referring generally to  FIGS. 2-5 , a drilling assembly  200  in accordance with one embodiment of the present invention is shown. Referring first to  FIG. 2 , a drilling assembly  200  is shown having a main sub  208 , and a plurality of collapsible blades  214  shown in a retracted state. Main sub  208  includes a plurality of longitudinal grooves  212  in which blades  214  are positioned and from which they extend. A plurality of pins  232  on opposite sides of each groove  212  retains each blade  214  in place. 
         [0040]    As depicted in  FIGS. 2-4 , drilling assembly  200  is constructed with 5 extendable blades  214 . It should be understood by one of ordinary skill in the art that any number of blades can be employed with the present invention, but 5 blades  214  are preferred. Typical underreamers only utilize 3 or fewer blades. This typical limitation is primarily a result of geometric limitations of the tools themselves. Because of the compactness of the drilling assembly and blade configuration of the present invention, additional blades are possible. For circumstances where drilling assembly  200  is to be used as an underreamer, additional blades translates to additional cutting surfaces, enabling the operator to enjoy longer cutter lifespan, or faster cutting rates. In circumstances where drilling assembly  200  is to be used as a stabilizer, it may be optimal to only employ 3 blades  214  in an effort to minimize any flow restrictions in the annulus between the drillstring and the wellbore. However, the use of 5 blades in place of 3 on a stabilizer makes for a more precisely centered drillstring, if desired. 
         [0041]    Referring now to  FIG. 3 , the drilling assembly  200  is shown with blades  214  in an extended position. Blades  214  have linear grooves  230  on either side for receipt of pins  232 . Drilling assembly  200  is preferably constructed such that blades  214  follow a substantially linear path from retraction to extension that maintains blades  214  substantially parallel to main sub  208  throughout the entire range of the extension motion. Furthermore, it is preferred that the path of extension for blades  214  be characterized by an acute angle with respect to the axis of the main sub  208 . Drilling assembly  200  is constructed such that the direction of that acute angle is towards the downhole end  202  of sub  208 , but uphole extension may be accommodated, if desired. Furthermore, if so desired, the present invention could be slightly modified to allow for a radial extension of blades  214  along a path substantially orthogonal to the axis of main sub  208 . No specific angle is required for the invention to function, and various angles can be utilized as desired. As can be seen from  FIG. 3 , each blade  214  of drilling assembly  200  is retained in place by 5 pins  232 , 3 on one side, and 2 on the other side. While this configuration is exemplary, it should be understood that various other configurations and quantities of pins  232  are possible and within the scope of the present invention. 
         [0042]    Referring now to  FIG. 4 , a mandrel assembly  201  to be used with drilling assembly  200  is shown. Mandrel assembly  201  includes a mandrel  216 , an engagement thruster  220  and a retraction thruster  222 . Engagement thruster  220  includes a piston head  226  upon which elevated pressure from drilling fluids acts to displace mandrel assembly  201  within drilling assembly  200 , extending (or retracting) blades  214 . The engagement thruster  220  is detachable from the mandrel  216 . The mandrel  216  includes a locking ring groove (not shown) on the end adjacent the engagement thruster  220 . A locking ring (not shown) can be installed in locking ring groove (not shown) on mandrel  216  to hold engagement thruster  220  in place. Additionally, load fingers ring  252  is moved on retraction thruster surface  222  on mandrel  216 . The load fingers ring  252  held in place by retraction thruster surface  222  on the mandrel  216  tapering on one end and the retractable blade  214  (not shown in  FIG. 4 ) on the other. 
         [0043]    Typically, the installation procedure consists of installing the blades  214  within the longitudinal pockets  112 . The blades  214  are retained in the extended position by clamps or other means after which the installation of the mandrel assembly  201  is accomplished. The mandrel assembly  201  is assembled by inserting the mandrel  216 , formed with retraction thruster surface  222 , into the bore  140  of the drilling assembly  200  or  600 . The blade  214  and guide inserts  700 ,  750  are released from their retained extended position or pins  232  (on the other embodiment) can then be installed. Then the engagement thruster  220  and locking ring (not shown) are installed. Once this is complete, the drilling assembly  100 ,  200 ,  600  is assembled and ready for use. 
         [0044]    Additionally, engagement thruster  220 , includes a plurality of load fingers  250  that correspond to each blade  214  of drilling assembly  200 ,  600 . Engagement thruster ring  252  carried on engagement thruster surface  222  also has load fingers corresponding to each blade  214  of drilling assembly  200 ,  600 . Load fingers  250 ,  252  engage longitudinal pockets (as indicated in  FIGS. 2-3 ) and thrust blades  214  into ( 250 ) and out of ( 252 ) the engaged position. The load finger  250  pushes the blade  214  upward and out as the mandrel  216  responds to changes in fluid pressure. As the mandrel  216  responds in the opposite direct, load fingers  252  retract the collapsible blades  214 . 
         [0045]    Referring briefly to  FIG. 5 , uphole end  204  of main sub  208  is shown. Mandrel  216  with piston head  226  is visible from this end and the ratio between bores  242  and  224  is visible. When pressures within the bore of the drillstring are elevated, hydraulic pressure exerts force upon piston head  226  as a result of the difference in diameter between bores  242  and  224 . By making ratio of bores  242  and  224  larger, more force upon mandrel  216  will result for incremental increases in bore pressure. 
         [0046]    Referring now to FIGS.  6 A/B, an alternate embodiment of the drilling assembly  200  is shown. Drilling assembly  600  has a main sub  608  with a plurality of longitudinal pockets  612 . Unlike the other embodiments, there are no holes in the main body  208  for pins. Instead, the collapsible blades  214  fit between a left guide insert  700  and a right guide insert  750 . The guide inserts  700 ,  750  have grooves that match the grooves on the corresponding collapsible blade  214 . The guide inserts  700 ,  750  and collapsible blade  214  are shown in more detail in  FIGS. 7A-C . The collapsible blades  214  of  FIG. 6  are substantially identical to the collapsible blades  214  of  FIG. 2 . The same collapsible blades  214  can be used with both a pin configuration as shown in drilling assembly  200  and a guide insert configuration as shown in drilling assembly  600 . 
         [0047]    The guide inserts  700 ,  750  have an outer surface  715 ,  765  that protrudes from the main insert body to engage with the sides of the longitudinal pockets  612  of main sub  608 . The inner surface of the guide inserts guide inserts  700 ,  750  have a plurality of raised surfaces  710 ,  760  to create a plurality of raised surfaces  710 ,  760  and grooves  705 ,  755 . The raised surfaces  710 ,  760  and grooves  705 ,  755  for each pair of guide inserts  700 ,  750  must match the configuration of linear grooves  230  for each collapsible blade  214 . 
         [0048]    Referring now to  FIGS. 7A-C , blade  214  for drilling assembly  200 ,  600  is shown. Blade  214  includes linear grooves  230  for engagement with pins  232  or guide inserts  700 ,  750  of drilling assembly  200 ,  600  respectively. Blades  214  are preferably constructed from machined tool steel and are configured with a leading surface  260 , a primary wear surface  262 , and a trailing surface  264 . Leading  260  and primary  262  wear surfaces are expected to carry the brunt of the wear of blades  214  during any underreaming or stabilizing operation. Trailing surface  264  is constructed to be used to drill out of a situation where the borehole collapses in behind drilling assembly  200 . 
         [0049]    Referring now to FIGS.  7 B/C, the outside surface of each guide insert is has a retaining projection  715 ,  765 . The retaining projection  715 ,  765  is designed to match a corresponding retaining groove (not shown) cut into longitudinal pockets  112 . The retaining projection  715 ,  765  fits into a mating groove on each side of the longitudinal pocket  112  to maintain the position of the guide inserts. Additionally, the retaining surface  805  on guide insert lock  800  also fits into the retaining groove (not shown). Once the retaining projection  715 ,  765  on guide inserts  700 ,  750  and the retaining surface  805  are locked into the corresponding groove (not shown) on the longitudinal passage  112 , the mandrel  216  can be installed and the load fingers  250 ,  252  engaged. The final assembly of this is demonstrated in  FIGS. 6A and 6B . 
         [0050]    Additionally, the guide insert lock  800  acts as a stop to prevent additional movement of the collapsible blades  214 . As the mandrel load fingers  250  force the collapsible blade  214  towards the guide insert lock  800  causing the collapsible blades to translate linearly along the raised sections  710 ,  760  of guide inserts  700 ,  750 . Once the leading edge  260  of the collapsible blade  214  reaches the guide insert lock  800 , the motion of the collapsible blade  214  is halted. No additional radial extension is possible without damaging the underreamer. By varying the length of the guide insert lock  800 , the radial extension of the collapsible blade  214  can be limited. This same process can be utilized to limit the radial extension when a stabilizer pad is utilized instead of the collapsible blade  214 . Additionally, guide insert lock  800  distributes excessive forces to the entire body of the underreamer rather than concentrating wear on the interior shoulder of the underreamer found in other prior art devices. 
         [0051]    For use with a drilling assembly such as shown by elements  100  and  200  if FIGS.  1 A/B and  2 , guide inserts  700 ,  750  are not required. However, for a typically more durable construction, guide inserts  700 ,  750  can be used. The collapsible blade  214  fits between the guide inserts  700 ,  750  by aligning the raised sections  710  of guide insert  700  with the grooves  230  in collapsible blade  700 ; similarly, the raised sections  760  of guide insert  750  are aligned with the grooves  231  in collapsible blade  214 .  FIGS. 7A-C  show various examples of how the guide inserts  700 ,  750  and the blade  214  interact. Once the guide inserts  700 ,  750  are assembled properly, they are placed within the channel  612  and held into place by guide insert lock  800 . This process will be described in more detail with regards to maintenance of drilling assembly  600 . 
         [0052]    Referring back to FIGS.  6 A/B, drilling assembly  600  functions in a manner similar to drilling assembly  200  in operation. The principal difference is when the mandrel  118  thrusts against the collapsible blade  214 , the blade  214  is forced outward in a linear path along the grooves of the guide inserts  700 ,  750 . This configuration is stronger than the pin configuration because there is a larger surface area in contact with the collapsible blade  214 , i.e. the grooves  230  in the blade  214  are generally in contact with the surface area of the raised sections  710 ,  755  of the guide inserts. This allows the drilling assembly  600  to last longer or accept more torque than drilling assembly  100  or  200 . While drilling assembly  600  is stronger than drilling assemblies  100  and  200 , drilling assemblies  100 ,  200 , and  600  are all advantageous in their ease of maintenance and manufacture. 
         [0053]    Maintenance of the drilling assembly  600  is also simplified over the prior art. The guide inserts  700 ,  750  and the collapsible blades  214  can be replaced in the field as they wear out. The process or replacing these components consists of removing any force causing the mandrel  118  to exert force on the guide inserts  700 ,  750  or the blades  214 . Once the force is released, the guide insert lock  800  can be removed from the recessed channel  612 . Once the guide insert lock  800  is removed, the guide inserts  700 ,  750  and blade  214  can be easily removed from the recessed channel  612 . This process can be repeated for each set of blade/guide inserts combination. The maintenance procedure for drilling assembly  200  is similar but requires removal of the pins  232  instead of the guide inserts  700 ,  750 . 
         [0054]    To replace any of these “wear” components, the operator can obtain replacement components as necessary and assemble a set consisting of a blade  214  and its corresponding guide inserts  700 ,  750  as shown in  FIG. 7C . Once the set is assembled, the set can be placed into a recessed channel  612  while the mandrel  118  force is released. The guide insert lock  800  is then slid into place and the mandrel force reapplied to hold the guide inserts  700 ,  750 , collapsible blade  214  and guide insert lock  800  in place. 
         [0055]    This ability to field-dress the drilling assembly  100 ,  200 ,  600  is advantageous because the main assembly  108 ,  208 ,  608  of the present invention will infrequently need service. The only parts that will be routinely replaced are the “wear” components such as the pins  232 , guide inserts  700 ,  750 , guide insert lock  800 , and the collapsible blades  214 . These components are much smaller to ship and much easier for an operator to maintain in inventory. Additionally, it makes it possible for an operator to keep multiple types of blades to be utilized for different formations or drilling situations. Some blades may contain carbide cutters, while others may use PDC cutting elements or other types of cutters/stabilizers. An operator can also easily change between a cutter blade and a stabilizer blade. This allows extreme flexibility to the operator in the field. An entire set of underreamer/stabilizer tools can be maintained in the field at a minimum of cost and space. 
         [0056]    Depending on the configuration of drilling assemblies  200 ,  600  different materials and configurations for surfaces  260 ,  262 , and  264  are possible. For underreamers, hardened cutting elements (not shown) are preferably placed on the periphery of surfaces  260 ,  262 , and  264 . For stabilizer purposes, hardened wear-resistant materials are preferred. The specific installations for materials and cutter elements upon surfaces  260 ,  262 , and  264  are well known to those skilled in the art, but specific materials and elements that are expected to be used include, but are not limited, to, polycrystalline diamond cutters (PDC), hardened metal cutter elements, carbide buttons, carbide inserts, hard metal overlays, flame-sprayed hard metal coatings, plasma-sprayed hardened coatings. 
         [0057]    Additionally, certain coatings such as QPQ nitride coating of both the guide inserts  700 , 750  and the blades  214  can be advantageous. While QPQ nitride coating of parts to increase durability is well known by one of ordinary skill in the art, QPQ nitride coating provides unexpected results in the present invention. By coating both the pins  232  or guide inserts  700 ,  750  with a QPQ nitride coating along with the cutter/stabilizer blades  214 , the friction between the two parts when expanding and retracting is thereby significantly reduced. This friction reduction can be advantageous and result in a longer useful life of both the guide inserts  700 ,  750  or pins  232  and the stabilizer/cutter blades  214 . While it is well known to coat the actual parts performing cutting operations such as the blades  214 , the coating of both the blades  214  and the guide inserts  700 ,  750  or pins  232  provides an increased service life of the components, thus making the drilling assembly  200 ,  600  have decreased maintenance costs and decreased downtime.

Summary:
The present invention discloses a collapsible drilling assembly having field-replaceable cutter or stabilizer blades ( 214 ) and method of using and installing new stabilizer blades while at a job location. The drilling assembly is deployed upon a distal end of a drillstring, expanded to a gauge size, and used as an underreamer or alternatively a stabilizer. The drilling assembly operates between retracted and extended positions through the increase in pressure of drilling fluid flowing therethrough.