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CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application, and thus claims benefit pursuant to 35 U.S.C. §120 of U.S. patent application Ser. No. 11/175,565, filed Jul. 6, 2005, now U.S. Pat. No. 8,186,458, issued May 29, 2012, which is related to U.S. patent application Ser. No. 11/175,567, filed Jul. 6, 2005, now U.S. Pat. No. 7,753,139, issued Jul. 13, 2010, both of which are incorporated by reference in their entireties. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     FIELD OF THE INVENTION 
     The present invention relates generally to methods and apparatus for drilling an enlarged sidetracked well bore from an existing primary well bore in geologic formations, and more particularly, to methods and apparatus for milling a window through casing lining a primary well bore, and drilling an enlarged sidetracked well bore through the casing window, all in one trip into the primary well bore. 
     BACKGROUND 
     Once a petroleum well has been drilled and cased, it may be desirable to drill one or more additional sidetracked well bores that branch off, or deviate, from the primary well bore. Such multilateral well bores are typically directed toward different targets within the surrounding formation, with the intent of increasing the production output of the well. 
     Multilateral technology provides operators several benefits and economic advantages, such as tapping isolated pockets of hydrocarbons that might otherwise be left unproduced, and improving reservoir drainage so as to increase the volume of recoverable reserves and enhance the economics of marginal pay zones. By utilizing multilateral technology, multiple reservoirs can also be drained simultaneously, and thin production intervals that might be uneconomical to produce alone may become economical when produced together. Multiple completions from one well bore also facilitate heavy oil drainage. 
     In addition to production cost savings, development costs also decrease through the use of existing infrastructure, such as surface equipment and the primary well bore. Multilateral technology expands platform capabilities where slots are limited and eliminates spacing problems by allowing more drain holes to be added within a reservoir. In addition, by sidetracking damaged formations or completions, the life of existing wells can be extended. For example, sidetracked well bores may be drilled below a problem area once the casing has been set, thereby reducing the risk of drilling through troubled zones. Finally, multilateral completions accommodate more wells with fewer footprints, making them ideal for environmentally sensitive or challenging areas. 
     To maximize the productivity of multilateral completions, it is desirable to enlarge at least some of the sidetracked well bores to thereby increase the production flow area through such boreholes. By drilling a sidetracked well bore through a casing window, and then enlarging the sidetracked well bore beyond the casing window, the far reaches of the reservoir can be reached with a comparatively larger diameter borehole, thereby providing more flow area for the production of oil and gas. 
     However, conventional methods for drilling an enlarged sidetracked well bore require multiple trips into the primary well bore. For example, a first trip may be made into the primary well bore to run and set an anchored whipstock comprising an inclined face that guides a window mill radially outwardly into the casing to cut a window in the casing. The window mill is then tripped out of the primary well bore, and a drill bit is lowered in a second trip to drill the sidetracked well bore through the casing window. The diameter of the sidetracked well bore is thereby limited by the diameter or gauge of the drill bit that can extend through the casing window. Once the sidetracked well bore has been drilled, the drill bit is then tripped out of the primary well bore, and another drilling assembly, such as a drill bit followed by a reamer, for example, is lowered in a third trip into the primary well bore to extend and enlarge the sidetracked well bore. It is both expensive and time consuming for an operator to make multiple trips into a primary well bore to drill and enlarge a single sidetracked well bore, and such concerns are only compounded when drilling more than one sidetracked well bore in a multilateral completion. 
     Thus, in recent years, a window milling bit comprising diamond cutters has been developed that is operable to mill a window through a standard metal casing and drill a sidetracked well bore through the casing window in a single trip into the primary well bore. This window milling bit with diamond cutters thereby eliminates one trip into the primary well bore, but at least another trip is still required to enlarge the sidetracked well bore. Therefore, a need exists for apparatus and methods that enable milling a window through a casing in a primary well bore, and drilling an enlarged sidetracked well bore through the casing window in one trip into the well bore. 
     To perform such a sidetracking operation, it would also be advantageous to provide a single cutting device capable of both milling the casing and drilling an enlarged sidetracked well bore. Such a device is desirable to provide a more compact drilling assembly for increased maneuverability and control while drilling the enlarged sidetracked well bore through the casing window. 
     Further, when operating a window milling bit to mill casing and drill formation, whether drilling an enlarged borehole or not, the cutting structures on such a bit may be worn down during operation. Thus, a need exists for a cutting device with multiple cutting structures adapted to recover gauge as the device is used to mill through casing and/or drill into formation. In addition, it may be desirable for the window milling bit to have at least a first cutting structure to perform the milling operation, and at least a second cutting structure to perform the drilling operation. Thus, a need exists for a cutting device with multiple cutting structures wherein at least one of the cutting structures is selectively presented when desired by the operator. Such a cutting device would be useful for many other purposes, including drilling through different types of formation rock, or replacing worn cutting structures when drilling a lengthy borehole, for example. 
     The present invention addresses the deficiencies of the prior art. 
     SUMMARY 
     In one aspect, the present disclosure relates to a method of milling a window through a casing in a primary well bore and drilling an enlarged sidetracked well bore. In an embodiment, the method comprises running into the primary well bore a drilling assembly comprising at least one cutting apparatus adapted to drill an enlarged borehole, milling a window through the casing, and drilling the enlarged sidetracked well bore, wherein the milling and drilling steps are performed in one trip into the primary well bore. The method may further comprise steering the drilling assembly and/or stabilizing the drilling assembly. 
     In another aspect, the present disclosure relates to a drilling assembly comprising at least one cutting apparatus operable to drill an enlarged borehole, wherein the drilling assembly is operable to mill a window through a casing in a primary well bore and drill an enlarged sidetracked well bore through the window in one trip into the primary well bore. In various embodiments, the drilling assembly may further comprise a bent housing motor, a rotary steerable system, and/or a stabilizer. In one embodiment, the at least one cutting apparatus comprises an expandable window milling bit having at least a collapsed position and an expanded position, and the expandable bit may comprise on/off control and/or diamond cutters operable to mill the window in the collapsed position and drill the enlarged sidetracked well bore in the expanded position. In another embodiment, the at least one cutting apparatus comprises a window milling bit and a reamer. The window milling bit may comprise a stationary cutting structure and a movable cutting structure. Further, an original operable gauge of the moveable cutting structure may substantially equal an original gauge of the stationary cutting structure. In yet another embodiment, one or both of the window milling bit and the reamer are expandable, and at least one expandable component may comprise on/off control. In still another embodiment, the at least one cutting apparatus comprises a bi-center bit. 
     In another aspect, the present disclosure relates to a method of milling a window through a casing in a primary well bore and drilling an enlarged sidetracked well bore into a formation comprising running into the primary well bore a system comprising a reamer and a mill with diamond cutters, milling a window through the casing with the diamond cutters, and drilling the enlarged sidetracked well bore, wherein the milling and drilling steps are performed in one trip into the primary well bore. The method may further comprise steering the system and/or stabilizing the system. In an embodiment, the drilling step comprises operating at least one of the mill and the reamer in an expanded position. The method may further comprise controlling whether an expandable component is on or off. In an embodiment, drilling the enlarged sidetracked well bore comprises creating an initial sidetracked well bore with the mill and enlarging the initial sidetracked well bore with the reamer. The method may further comprise using a first cutting structure of the mill during the milling step and using a second cutting structure of the mill during the drilling step. In an embodiment, the first cutting structure protects the second cutting structure during the milling step. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more detailed description of the present invention, reference will now be made to the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional side view depicting one embodiment of method for milling a casing window and drilling an enlarged sidetracked well bore, with a representative drilling assembly shown connected to a whipstock and an anchor being run into a primary cased well bore; 
         FIG. 2  is a cross-sectional side view of the method of  FIG. 1  showing the drilling assembly drilling an enlarged sidetracked well bore through a casing window that was milled by a lead cutting device of the drilling assembly; 
         FIG. 3  is a cross-sectional side view of one embodiment of a cutting device with multiple cutting structures, wherein the device is shown in a collapsed position; 
         FIG. 4  depicts an end view of the cutting device of  FIG. 3  in the collapsed position; 
         FIG. 5  is a cross-sectional side view of the cutting device of  FIG. 3 , wherein the device is shown in an expanded position; 
         FIG. 6  depicts an end view of the cutting device of  FIG. 3  in the expanded position; 
         FIG. 7  is a cross-sectional view of another embodiment of a cutting device with multiple cutting structures, wherein a moveable cutter block is shown in a first position; and 
         FIG. 8  is a cross-sectional side view of the cutting device of  FIG. 7 , wherein the moveable cutter block is shown in a second position. 
     
    
    
     NOTATION AND NOMENCLATURE 
     Certain terms are used throughout the following description and claims to refer to particular assembly components. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. 
     Reference to up or down will be made for purposes of description with “up”, “upper”, or “upstream” meaning toward the earth&#39;s surface or toward the entrance of a well bore; and “down”, “lower”, or “downstream” meaning toward the bottom or terminal end of a well bore. 
     DETAILED DESCRIPTION 
     Various embodiments of methods and apparatus for milling a casing window and drilling an enlarged sidetracked well bore in one trip into a primary well bore, and various embodiments of a cutting device comprising multiple cutting structures, will now be described with reference to the accompanying drawings, wherein like reference numerals are used for like features throughout the several views. There are shown in the drawings, and herein will be described in detail, specific embodiments of drilling assemblies and cutting devices with the understanding that this disclosure is representative only, and is not intended to limit the invention to those embodiments illustrated and described herein. The embodiments of the apparatus disclosed herein may be utilized in any type of milling, drilling or sidetracking operations. It is to be fully recognized that the different teachings of the embodiments disclosed herein may be employed separately or in any suitable combination to produce desired results. 
       FIG. 1  and  FIG. 2  depict two sequential, cross-sectional side views of a method for milling a window  35  through a casing  30  lining a primary well bore  20 , and drilling an enlarged sidetracked well bore  25  into the surrounding formation  10 . As used herein, an enlarged sidetracked well bore  25  is a sidetracked well bore with a diameter greater than the diameter of a window milling bit  110  or other tool used to mill the casing window  35 . 
     Referring first to  FIG. 1 , the method comprises lowering a bottomhole drilling assembly  100  connected to a whipstock  200  and an anchor  300  into the primary well bore  20  via a drill string  50  using conventional techniques. In one embodiment, the drilling assembly  100  comprises a window milling bit  110  at its lower end that is capable of milling through the casing  30  and drilling into the formation  10 . One example of such a window milling bit  110  is depicted and described in U.S. Pat. No. 6,648,068, hereby incorporated herein by reference for all purposes. 
     The drilling assembly  100  may further comprise various other components  120 ,  130 ,  140 ,  150 ,  160 ,  170  and  180 . For example, in addition to the window milling bit  110 , the drilling assembly  100  may comprise a directional device  120 , a measurement-while-drilling (MWD) tool  130 , a logging-while-drilling (LWD) tool  140 , one or more additional mills  150 , a borehole enlarging device  160 , one or more drill collars  170 , and a stabilizer  180 , for example. Although components  120 ,  130 ,  140 ,  150  and  170  may be provided in the drilling assembly  100 , such apparatus are entirely optional and would not be required to perform any of the methods disclosed herein. Further, in some embodiments of the methods of the present invention, the bore hole enlarging device  160  and/or the stabilizer  180  may not be required. 
     When the drilling assembly  100 , whipstock  200  and anchor  300  have been lowered to a desired depth in the primary well bore  20  by the drill string  50 , the whipstock  200  is angularly oriented so that an inclined surface  210  of the whipstock  200  faces in the desired direction for drilling the enlarged sidetracked well bore  25 . Once the whipstock  200  is oriented, it is then set into place via the anchor  300  disposed at the lower end thereof, as shown in  FIG. 1 . The anchor  300  engages the surrounding casing  30  to lock the whipstock  200  into place against both axial and rotational movement during operation. 
     When the whipstock  200  has been angularly oriented and set into place by the anchor  300  in the primary well bore  20 , the drilling assembly  100  disconnects from the whipstock  200  and proceeds to mill the window  35  through the casing  30 . Specifically, the window milling bit  110  is rotated and lowered while engaging the inclined surface  210  of the whipstock  200 , which acts to guide the window milling bit  110  radially outwardly into cutting engagement with the casing  30  to mill a window  35  therethrough. 
     As depicted in  FIG. 2 , the method further comprises extending the drilling assembly  100  through the casing window  35  and drilling into the formation  10  to form an enlarged sidetracked well bore  25 . The various embodiments of the method for forming the enlarged sidetracked well bore  25  depend, in part, upon which components comprise the drilling assembly  100 . For example, in one embodiment, the drill string  50  comprises standard jointed pipe and conventional drilling is performed wherein the entire drill string  50  and drilling assembly  100  are rotated from the surface of the primary well bore  20 . In another embodiment, the drill string  50  may comprise either jointed pipe or coiled tubing, and the drilling assembly  100  comprises a directional device  120 , such as a bent housing motor or a rotary steerable system, for example, operably connected to the window milling bit  110  to rotate and/or steer the bit  110  during operation. When using a bent housing motor system as the directional device  120 , drilling into the formation  10  is achieved by sliding the drill string  50 , whereas a rotary steerable system would allow the drill string  50  to continue to rotate while steering the window milling bit  110 . Therefore, it may be advantageous to use jointed drill pipe  50  and a rotary steerable system as the directional device  120  when drilling a long borehole into the formation  10 . 
     In one embodiment of the method for forming an enlarged sidetracked well bore  25 , the drilling assembly  100  comprises at least the window milling bit  110 , which is adapted to drill an initial sidetracked well bore, and a well bore enlarging device  160 , such as a reamer, for example, that follows behind the window milling bit  110  to expand the initial borehole and thereby form the enlarged sidetracked well bore  25 . The window milling bit  110  can drill the initial sidetracked well bore at the same time as the reamer  160  enlarges the borehole to form the enlarged sidetracked well bore  25 . 
     In one embodiment, the reamer  160  is expandable and has basically two operative states—a closed or collapsed state, where the diameter of the reamer  160  is sufficiently small to allow it to pass through the casing window  35 , and an open or partly expanded state, where one or more arms with cutters on the ends thereof extend from the body of the reamer  160 . In this latter position, the reamer  160  expands the diameter of the initial sidetracked well bore to form the enlarged sidetracked well bore  25  as the reamer  160  is rotated and advanced in the borehole. 
     As one of ordinary skill in the art will readily recognize, there are a wide variety of expandable reamers  160  capable of forming an enlarged sidetracked well bore  25 . For purposes of example, and not by way of limitation, one type of expandable reamer  160  is depicted and described in U.S. Pat. No. 6,732,817, hereby incorporated herein by reference for all purposes. Such a reamer  160  comprises moveable arms with borehole engaging pads comprising cutting structures. The arms translate axially upwardly along a plurality of angled channels disposed in the body of the reamer  160 , while simultaneously extending radially outwardly from the body. The reamer  160  alternates between collapsed and expanded positions in response to differential fluid pressure between a flowbore in the reamer  160  and the wellbore annulus. Specifically, fluid flowing through the flowbore enters a piston chamber through ports in a mandrel to actuate a spring-biased piston, which drives the moveable arms axially upwardly and radially outwardly into the expanded position. When the fluid flow ceases, the differential pressure is eliminated, and the reamer  160  returns to the collapsed position. 
     In a first embodiment, the ports into the piston chamber remain open, so the reamer  160  expands and contracts automatically in response to changes in differential pressure. In a second embodiment, the reamer  160  includes on/off control. For example, the reamer  160  may comprise an internal stinger biased to block the ports into the piston chamber to prevent the piston from actuating in response to differential pressure between the flowbore and the wellbore annulus. This internal stinger may be aligned using an actuator, such as the flow switch depicted and described in U.S. Pat. No. 6,289,999, to open the ports into the piston chamber. Once these ports are open, differential pressure between the flowbore and the wellbore annulus will actuate the piston. Thus, this second embodiment of the reamer  160  is selectively actuatable, thereby providing the operator with on/off control. 
     Another representative type of expandable reamer  160  is depicted and described in U.S. Patent Publication No. US 2004/0222022-A1, hereby incorporated herein by reference for all purposes. This type of reamer  160  comprises moveable arms that are radially translatable between a retracted position and a wellbore engaging position, and a piston mechanically supports the moveable arms in the wellbore engaging position when an opposing force is exerted. The piston is actuated by differential pressure between a flowbore within the reamer  160  and the wellbore annulus. This type of reamer  160  may also include on/off control. For example, in one embodiment, the reamer  160  may comprise a sliding sleeve biased to isolate the piston from the flowbore, thereby preventing the moveable arms from translating between the retracted position and the wellbore engaging position. A droppable or pumpable actuator may be used to align the sliding sleeve to expose the piston to the flowbore and actuate the piston. Thus, this embodiment of the reamer  160  is selectively actuatable to provide the operator with on/off control. 
     Another representative type of expandable reamer  160  utilizes swing out cutter arms that are hinged and pivoted at an end opposite the cutting end of the arms, which have roller cones attached thereto. The cutter arms are actuated by mechanical or hydraulic forces acting on the arms to extend or retract them. Typical examples of this type of reamer  160  are found in U.S. Pat. Nos. 3,224,507; 3,425,500 and 4,055,226, hereby incorporated herein by reference for all purposes. As one of ordinary skill in the art will readily understand, while specific embodiments of expandable reamers  160  have been explained for purposes of illustration, there are many other types of expandable reamers  160  that would be suitable for use in forming an enlarged sidetracked well bore  25 . Therefore, the methods and apparatus of the present invention are not limited to the particular embodiments of the expandable reamers  160  discussed herein. 
     In another embodiment of the method for forming an enlarged sidetracked well bore  25 , the well bore enlarging device  160  that follows the window milling bit  110  is a winged reamer. A winged reamer  160  generally comprises a tubular body with one or more longitudinally extending “wings” or blades projecting radially outwardly from the tubular body. Once the winged reamer  160  has passed through the casing window  35 , the window milling bit  110  rotates about the centerline of the drilling axis to drill an initial sidetracked borehole on center in the desired trajectory of the well path, while the eccentric winged reamer  160  follows the bit  110  and engages the formation  10  to enlarge the initial borehole to the desired diameter of the enlarged sidetracked well bore  25 . Winged reamers  160  are well known to those of ordinary skill in the art. 
     Yet another method for milling the casing window  35  and drilling the enlarged sidetracked well bore  25  comprises replacing the standard window milling bit  110  with a bi-center bit, which is a one-piece drilling structure that provides a combination reamer and pilot bit. The pilot bit is disposed on the lowermost end of the drilling assembly  100 , and the eccentric reamer bit is disposed slightly above the pilot bit. Once the bi-center bit passes through the casing window  35 , the pilot bit portion rotates about the centerline of the drilling axis and drills an initial sidetracked borehole on center in the desired trajectory of the well path, while the eccentric reamer bit portion follows the pilot bit and engages the formation  10  to enlarge the initial borehole to the desired diameter of the enlarged sidetracked well bore  25 . The diameter of the pilot bit is made as large as possible for stability while still being capable of passing through the cased primary well bore  20 . Examples of bi-center bits may be found in U.S. Pat. Nos. 6,039,131 and 6,269,893. 
     Another method for milling the casing window  35  and drilling the enlarged sidetracked well bore  25  comprises replacing the standard window milling bit  110  with an expandable cutting device. One embodiment of such an expandable device is the cutting device  300  shown in  FIGS. 3-6 . The cutting device  300  is adapted to mill the casing window  35  and drill the enlarged sidetracked well bore  25  therethrough. In particular,  FIGS. 3-4  depict a cross-sectional side view and an end view, respectively, of the cutting device  300  in a collapsed position for milling the casing window  35 , and  FIGS. 5-6  depict a cross-sectional side view and an end view, respectively, of the cutting device  300  in an enlarged position for drilling the enlarged sidetracked well bore  25 . The collapsed diameter D C  of the cutting device  300  shown in  FIGS. 3-4  is smaller than the expanded diameter D E  of the cutting device  300  shown in  FIGS. 5-6 . In one embodiment, the collapsed diameter D C  may be 12¼ inches, and the expanded diameter D E  may be 14¾ inches to 15 inches, for example. 
     The cutting device  300  comprises an upper section  310  with an internal flow bore  315 , a body  320  with angled tracks  322  and an internal chamber  325 , one or more stationary cutting structures  330  disposed on the lower end of the body  320 , one or more moveable cutter blocks  340 , a moveable piston  370  with an internal flowbore  375 , and one or more links  380  that connect the moveable cutter blocks  340  to the piston  370 . Thus, at least one and any number of multiple moveable cutter blocks  340  may be connected to the piston  370 . In the embodiments shown in  FIGS. 3-6 , three stationary cutting structures  330  are disposed 120 degrees apart circumferentially, and three moveable cutter blocks  340  are disposed 120 degrees apart circumferentially. Thus, the stationery cutting structures  330  alternate with the moveable cutter blocks  340  such that cutters are positioned 60 degrees apart circumferentially, as best depicted in  FIGS. 4 and 6 . The stationary cutting structures  330  and the moveable cutter blocks  340  may comprise the same or different types of cutters, such as diamond cutters and/or tungsten carbide cutters, for example. 
     A threaded connection  312  is provided between the upper section  310  and the lower section. The piston  370  extends into both the upper section flowbore  315  and the internal chamber  325 , and seals  372 ,  376  are provided between the piston  370  and the body  320 , and between the piston  370  and the upper section  310 , respectively. An upper end  374  of the piston  370  is in fluid communication with the primary well bore  20  via a port  324  in the body  320 , and a lower end  378  of the piston  370  is in fluid communication with the internal chamber  325  of the body  320 . 
     In operation, the cutting device  300  is run into the primary well bore  20  in the collapsed position shown in  FIGS. 3-4 . In this configuration, the piston  370  is pushed axially forward toward the downstream direction, which thereby causes the moveable cutter blocks  340  to be pushed axially forward in the downstream direction via link  380 . Disposed in a counter-bore  360  in the upper section  310  is a shear screw  350  that engages a shear groove  355  in the piston  370  to maintain the piston  370  in the position shown in  FIGS. 3-4 . In other embodiments, the piston  370  may be spring-loaded to bias to the collapsed position. 
     As shown in  FIGS. 3-4 , the cutting device  300  has a first collapsed diameter D C , and the moveable cutter blocks  340  are positioned axially forward, or downstream, of the stationary cutting structures  330 . Because the moveable cutter blocks  340  are positioned ahead of the stationary cutting structures  330 , they will perform most of the cutting required to mill the window  35  through the casing  30 . However, the stationary cutting structures  330  may also assist in milling the casing window  35 . 
     When the casing window  35  is complete, the cutting device  300  continues to drill ahead into the formation  10  at least until the upper section  310  is clear of the window  35 . Then the cutting device  300  may be actuated to the expanded position shown in  FIGS. 5-6  to drill the enlarged sidetracked well bore  25 . In the embodiments shown in  FIGS. 3-6 , the cutting device  300  is actuated hydraulically, but one of ordinary skill in the art will recognize that such actuation can be performed by any means, including mechanically, electrically, chemically, explosively, etc. or a combination thereof. 
     To actuate the cutting device  300  to the expanded position, the piston  370  must be released from the position shown in  FIGS. 3-4  and then retracted to the position shown in  FIGS. 5-6 . In particular, the drilling fluid in the internal chamber  325  acting on the lower end  378  of the piston  370  must be pressured up to exceed the pressure in the primary well bore  20  that acts on the upper end  374  of the piston  370  through port  324 . This differential pressure must be sufficient to shear the shear screw  350  and retract the released piston  370  until it engages a shoulder  314  within the flowbore  315  of the upper section  310 , as best depicted in  FIG. 5 . As the piston  370  retracts in response to this differential pressure, the moveable cutter blocks  340  will also be retracted since they are connected to the piston  370  via links  380 . As the moveable cutter blocks  340  retract in the axially upward, or upstream, direction, they are simultaneously directed radially outwardly along the angled tracks  322  in the body  320 , such as tongue-and-groove tracks  322 . Thus, the moveable cutter blocks  340  are expanded radially outwardly to an enlarged diameter D E  as shown in  FIGS. 5-6 . As one of ordinary skill in the art will appreciate, the size of the enlarged diameter D E  is based, in part, on the length of the piston  370  and the angle of the tracks  322  in the body  320 . 
     In other embodiments, the cutting device  300  may include on/off control. For example, the cutting device  300  may comprise a slideable sleeve capable of blocking the port  324  that provides fluid communication between the piston  370  and the primary well bore  20 . In this blocked configuration, the cutting device  300  would be “off” since there would be no differential pressure acting on the piston  370  to make it retract or extend. However, selectively moving the slideable sleeve to open the port  324  would turn the cutting device  300  “on” since the piston  370  could then actuate in response to differential pressure as described above. 
     In the expanded position, the cutting device  300  will drill the enlarged sidetracked well bore  25 . In the embodiments shown in  FIGS. 3-6 , the moveable cutter blocks  340  and the stationary cutting structures  330  will drill the face portion (i.e. end) of the enlarged sidetracked well bore  25 , and the moveable cutter blocks  340  will drill the gauge portion (i.e. diameter) of the enlarged sidetracked well bore  25  substantially alone, without the stationary cutting structures  330 . Thus, in one embodiment, the apparatus comprises a one-trip milling and drilling assembly  100  with a single expandable cutting device  300  disposed at an end thereof for milling a window  35  through casing  30  in the primary well bore  20  and drilling an enlarged sidetracked well bore  25 . In another aspect, the apparatus comprises a cutting device  300  comprising multiple cutting structures  330 ,  340  wherein at least one of the cutting structures is selectively presented. 
     Referring again to  FIGS. 1-2 , in drilling operations, and especially when drilling an enlarged borehole, it is advantageous to employ a stabilizer  180 , which may be positioned in the drilling assembly  100  above the reamer  160 , separated by one or more drill collars  170 . Alternatively, if the expandable cutting device  300  is used to form the enlarged sidetracked well bore  25 , the reamer  160  may or may not be provided, and the stabilizer  170  could be positioned where the reamer  160  is shown. The stabilizer  170  provides centralization and may control the trajectory and the inclination of the window milling bit  110  or the cutting device  300  as drilling progresses. The stabilizer  170  may be a fixed blade stabilizer, or an expandable concentric stabilizer, such as the expandable stabilizers described in U.S. Pat. Nos. 5,318,137; 5,318,138; and 5,332,048, for example 
       FIGS. 7-8  depict an alternative embodiment of a cutting device  400  comprising multiple cutting structures  330 ,  340  having many of the same components as the cutting device  300  shown in  FIGS. 3-6 . However, the alternative cutting device  400  comprises tracks  422  having a much smaller angle than the tracks  322  depicted in  FIGS. 3-6 . In various embodiments, the tracks  422  may have only a slight angle, or the tracks  422  may be substantially parallel to a longitudinal axis  405  of the alternative cutting device  400 . 
       FIG. 7  depicts one embodiment of the alternative cutting device  400  comprising tracks  422  having a slight angle in the collapsed position (corresponding to  FIG. 3  for cutting device  300 ), and  FIG. 8  depicts the alternative cutting device  400  in the expanded position (corresponding to  FIG. 5  for cutting device  300 ). In this embodiment, the alternative cutting device  400  is operable to recover gauge that is worn away during milling or drilling. In more detail, when the alternative cutting device  400  is in the position shown in  FIG. 7 , the moveable cutting structures  340  are positioned axially forward, or downstream of, and radially inwardly of, the stationary cutting structures  330 . Thus, whether milling a casing window  35  or drilling into the formation  10  in the position shown in  FIG. 7 , the moveable cutter blocks  340  will mill or drill the face portion of the window  35  or borehole, whereas the stationary cutting structures  330  will substantially mill or drill the gauge portion. As such, the stationary cutting structures  330  will lose gauge over time. By way of example, the initial gauge of the stationary cutting structures  330  may be 12¼ inches, but after milling or drilling, the gauge may be reduced to 12 inches. Therefore, to recover the lost ¼ inch gauge, the alternative cutting device  400  is actuated to the position shown in  FIG. 8 . When actuated, the moveable cutter blocks  340  are retracted axially by the piston  370  via link  380  while simultaneously traversing radially outwardly along the slightly angled tracks  422 . This slight expansion of the moveable cutter blocks  340  is designed to recover the gauge lost by the stationary cutting structures  330  so that milling or drilling may continue at the same original gauge. For example, the moveable cutter blocks  340  in the position shown in  FIG. 8  may have a gauge of substantially 12¼ inches. 
     In another embodiment, the alternative cutting device  400  may comprise tracks  422  that are substantially parallel to the axis of the cutting device  400 . In this embodiment, the cutting device  400  may comprise, for example, a first cutting structure presented for milling and a second cutting structure selectively presented for drilling. For example, if the cutting device  400  of  FIGS. 7-8  comprised tracks  422  that were substantially parallel to the axis of the cutting device  400 , the moveable cutter blocks  340  would be positioned axially forwardly of, and at a slightly greater radial expansion as the stationary cutting structures  330  in the position of  FIG. 7 . Thus, the moveable cutter blocks  340  would mill the casing window  35  while protecting the stationary cutting structures  330 . Also in this embodiment, when the cutting device  400  is actuated to the position shown in  FIG. 8 , the moveable cutter blocks  340  would be retracted directly axially upstream to thereby reveal the stationary cutting structures  330 , which would perform the drilling operation in conjunction with the moveable cutter blocks  340 . 
     As one of ordinary skill in the art will readily appreciate, such a cutting device  400  with substantially parallel tracks  422  could comprise multiple cutting structures of various types, such as PDC cutters and tungsten carbide cutters, for example, wherein each type of cutting structure is designed for a specific purpose. Such a cutting device  400  could also be used for a variety of different purposes. For example, the cutting device  400  could be used to drill any type of borehole into the formation  10 , with each of the multiple cutting structures being presented as necessary due to a change in the type of rock comprising the formation  10 , or due to a shift in the integrity of the formation  10 , for example. It may also be advantageous to provide multiple cutting structures of the same type so that as one cutting structure becomes worn, another cutting structure can be presented. One of ordinary skill in the art will readily understand that many other variations are possible and are well within the scope of the present application. 
     The foregoing descriptions of specific embodiments have been presented for purposes of illustration and description and are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many other modifications and variations are possible. In particular, the specific type and quantity of components that make up the drilling assembly  100  could be varied. Further, the quantity of cutting structures  330 ,  340  provided on the cutting devices  300 ,  400  could be varied, as well as the specific means by which such cutting structures  330 ,  340  are presented. For example, instead of retracting the piston  370 , in other embodiments, the piston  370  may be advanced to actuate the cutting devices  300 ,  400 . In other embodiments, the piston  370  may be retracted and extended multiple times. In addition, the materials comprising the cutting structures  330 ,  340  could be varied as required for the milling or drilling operation. Further, the tracks  322 ,  422  may have any angle, including a reverse angle, such that the moveable cutter blocks  340  are moved radially inwardly when the piston  370  retracts. In addition, the expandable cutting device  300  may be expanded at different times in the method, such as during milling of the casing window  35 , for example. 
     While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims which follow, the scope of which shall include all equivalents of the subject matter of the claims.

Summary:
A method of milling a window through a casing in a primary well bore and drilling a sidetracked well bore into a formation including running a drilling assembly including a body having an axis defined therethrough, a piston that is movable within a cavity formed in the body, a stationary cutting structure coupled to the body, and a movable cutting structure coupled to the body, milling a window through the casing in a first trip into the primary well bore, drilling the sidetracked well bore in the first trip into the primary well bore, applying a differential pressure across the piston, and moving the movable cutting structure from the collapsed position to the expanded position. The movable cutting structure is coupled to the piston and is movable between a collapsed position and an expanded position.