Abstract:
A drill countersinking and seal groove machining tool to accommodate an accurate drill countersinking and seal groove machining operation. The linear compensator tool applies sufficient force to react to the drilling or seal groove machining process, but not so much force as to distort the work piece being drilled or machined. The tool ensures that the reactant force does not exceed the machine force override allowances. Varying spring rates and/or air pressures on the linear compensator system will accommodate most applications. The tool absorbs over travel of the machining tool, in order to ensure that the surface to be machined is always in contact with the machining tool.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to machining applications, and in particular to a linear compensator tool for drill countersinking and seal groove machining. The linear compensator tool ensures accurate drill countersinking and seal groove machining capabilities without control system feedback. 
     BACKGROUND OF THE INVENTION 
     Current fabrication processes for trimming and drilling, and machining fuel seal grooves in composite and metallic aircraft panels utilize standard 3, 4, and 5 axis Numerically Controlled (NC) machine tools. Numerous machines of this type exist at aerospace companies which do not have integrated capabilities for machining operations to achieve specific seal groove widths/depths. Furthermore, these machines typically do not have integrated capabilities for performing drill countersinking operations to achieve specific countersink sizes/depths, and are relegated to drill-only operations which do not require specific depth control. 
     NC machines required to perform these types of processes are forced to integrate a complicated, expensive, and time consuming process of measuring and recording surface profile variations prior to actual machining and drilling. These recorded part surface variations are then used to adjust, or offset, the NC program to account for the deviations from the engineered nominal surface. NC Machines outfitted with the capability to perform these types of processes are substantially more expensive and complicated due to the added components and control hardware and software to operate the system. The lack of viable low-cost drill countersinking tools forces companies to convert these machines into accurate drill countersinking machines with expensive modifications and/or total machine replacement. This situation is prevalent throughout the aircraft industry, both in the commercial and military sectors. 
     Numerous machines exist today in production throughout the world without the capability to accurately machine seal grooves and drill countersink without substantial additional processes to accommodate the variations seen in composite and/or metallic panels, including surface profile variations. Numerically Controlled machines are programmed to move to a specific point in space without regard to where the actual part might be located. It is assumed that the part is located within a specific tolerance within the machine&#39;s work cell to achieve the desired level of accuracy during processing. Very small variations in machine accuracy and part location (i.e., as small as 0.001″—smaller than the thickness of a human hair) will result in seal groove widths and depths, and countersink diameters out of tolerance. 
     The primary issues with accurate seal groove machining and drill countersinking of composite or metallic parts is knowing or being able to reference the part&#39;s surface profile that will be machined, or the part&#39;s surface that will be drilled. All seal grooves and countersinks are referenced by this surface. There is currently no Commercial-Off-The-Shelf (COTS) seal groove machining system available in industry which can accurately machine a seal groove to a specified width and depth while adjusting to varying part surface profiles real time without some type of control system feedback or extensive measurement operations to identify the actual part surface profile. 
     In an expensive and complicated Automated Drilling Machine or Intelligent Drilling System the capability of sensing this surface location is incorporated into the machine and control system. This allows the machine to countersink to a depth relative to the sensed part surface. When the surface is located physically, or by non-contact methods, the drill countersink tool is fed a specific distance into the part relative to that surface to achieve the desired countersink diameter/depth. 
     Retrofitting existing machines without the specific designed-in countersinking and seal groove machining capabilities is very expensive and results in substantial machine downtime during retrofit. Most NC Machines have no or limited available control lines to die spindle for intelligent drilling systems. Integration costs for intelligent drilling systems are extremely costly and impact machine operations during installation/debugging. 
     SUMMARY OF THE INVENTION 
     The drill countersinking and seal groove machining tool proposed in this patent application precludes having to implement substantial changes to the machine and/or additional processes to accommodate an accurate drill countersinking or seal groove machining operation. The functionality of the linear compensator tool allows it to be used like any other standard tool which does not require any interface to the control system or special NC Programming allowances. This tool can be setup and adjusted off-line of the machine, unlike many of the specially designed drill countersinking machines. This tool can be stored as a standard tool in the machine&#39;s automated tool storage/retrieval system. 
     This tool effectively turns an ordinary NC milling machine into an automated drilling machine at a much lower cost and allows the use of existing machines without upgrading or replacing the equipment. This tool effectively turns an ordinary NC milling machine into an accurate seal groove milling machine without the need for elaborate measurements of the part surface profiles. 
     The seal groove machining and drill countersinking tool incorporates a linear compensator design which applies sufficient force to react to the drilling or seal groove machining process, but not so much force as to distort the work piece being drilled or machined. Additionally, the linear compensator design ensures that the reactant force does not exceed the machine force override allowances. Varying spring rates and/or air pressures on the linear compensator system will accommodate most applications. The tool is designed to absorb over travel of the machining tool, in order to ensure that the surface to be machined is always in contact with the tool. Incorporation of the linear compensator system provides countersinking and seal groove machining capabilities that do not require some form of control system feedback. 
     The linear compensator design can be adapted to virtually any numerical control machine spindle interface (i.e., HSK Holders, CAT Tapered Holders, etc.) with very minor modifications to the machine. A variety of adjustable micro-stop countersinking and seal groove machining assemblies can be adapted to the linear compensator system, enabling reaction to part surface profile variations and producing an accurate countersink or seal groove real time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of a standard tool holder connected to a NC machine. 
         FIG. 2  is a schematic sectional view of a hollow shaft air cylinder linear compensator tool attached to a standard tool holder and NC machine. 
         FIG. 3  is a schematic side view of a micro-stop nose piece. 
         FIG. 4  is an exploded isometric view of the micro-stop nose piece of  FIG. 3 . 
         FIG. 5A  is a schematic sectional view of the linear compensator tool of  FIG. 2  at the beginning of a machining operation of a maximum thickness panel. 
         FIG. 5B  is a schematic sectional view of the linear compensator tool of  FIG. 2  at the beginning of a machining operation of a nominal thickness panel. 
         FIG. 5C  is a schematic sectional view of the linear compensator tool of  FIG. 2  at the beginning of a machining operation of a minimum thickness panel. 
         FIG. 6A  is a schematic sectional view of the linear compensator tool of  FIG. 2  when first contacting the maximum thickness panel. 
         FIG. 6B  is a schematic sectional view of the linear compensator tool of  FIG. 2  when first contacting the nominal thickness panel. 
         FIG. 6C  is a schematic sectional view of the linear compensator tool of  FIG. 2  when first contacting the minimum thickness panel. 
         FIG. 7A  is a schematic sectional view of the linear compensator tool of  FIG. 2  after drill countersinking a maximum thickness panel. 
         FIG. 7B  is a schematic sectional view of the linear compensator tool of  FIG. 2  after drill countersinking a nominal thickness panel. 
         FIG. 7C  is a schematic sectional view of the linear compensator tool of  FIG. 2  after drill countersinking a minimum thickness panel. 
         FIG. 8  is a schematic sectional view of a linear bearings with springs linear compensator tool. 
         FIG. 9  is a schematic sectional view of the linear compensator tool of  FIG. 8  after absorbing over travel. 
         FIG. 10  is a schematic sectional view of a mechanical sleeve with spring linear compensator tool. 
         FIG. 11  is a schematic sectional view of the linear compensator tool of  FIG. 10  after absorbing over travel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , standard tool holder  21  has a shaft  25  with a splined receptacle capable of receiving and rotating a tool. In this instance, the tool is a countersinking drill bit  24 . Tool holder  21  may hold one of a number of machining tools, including a seal groove cutter. 
     Referring to  FIG. 2 , a linear compensator tool (LCT) is connected to a standard tool holder  21  to ensure accuracy in machining processes. The standard tool holder  21  is connected to a spindle of an NC machine  19 . The LCT can exist in a number of embodiments including a linear bearing with springs LCT  121  ( FIGS. 8 and 9 ) and a mechanical sleeve with spring LCT  141  ( FIGS. 10 and 11 ). In this example, the LCT is a hollow shaft air cylinder LCT  31 . As illustrated by  FIG. 2 , LCT  31  is connected to tool holder  21  by means of a clocking ring  51  and a bearing (not visible). The bearing (not visible) is connected to clocking ring  51  by means of connector snap  55 . The bearing (not visible) and clocking ring  51  are connected to tool holder  21  by means of connector snap  53 . Locking pin  57  extends vertically from the top surface of ring  51 , and slides into a bushing (not visible) on the face of NC machine  19 . Clocking ring  51 , the bearing (not visible), and locking pin  57  ensure that the body of LCT  31  is fixed and does not rotate with tool holder  21  and countersinking drill bit  24 . 
     Attached to the bottom of clocking ring  51  is outer casing  33  of LCT  31 . Casing  33  is generally cylindrical in shape with the exterior portion of casing  33  being smooth. In an alternate embodiment, casing  33  could take other forms such as a square or rectangle. The interior surface of casing  33  is machined in a manner to enable it to receive inner casing  37 . The upper interior surface of casing  33  forms a downward facing shoulder  34 . 
     Inner casing  37 , generally cylindrical in shape, slidingly engages outer casing  33 . In an alternate embodiment, casing  37  could take other forms such as a square or rectangle. The outer surface of casing  37  forms a flange section  38 . Flange section  38  and shoulder  34  limit the movement of casing  37  within casing  33 . O-ring seals  39 ,  41  ensure that the contact surfaces between outer casing  33  and inner casing  37  are properly sealed. A cap  35  is placed around inner casing  37 , on the bottom of LCT  31 , and is secured to outer casing  33 . Cap  35  is generally circular in shape and has a T-shaped cross section that produces a small annulus between inner casing  37  and cap  35 . O-ring seal  43  ensures that the contacting surfaces between inner casing  37  and cap  35  are sealed. Inner casing  37  is free to telescope in and out of casing  33 , but is limited in range by cap  35  and shoulder  34 . 
     A spring  45  surrounds inner casing  37 , and is located in the annulus between inner casing  37  and cap  35 . Spring  45  acts to compress inner casing  37  as far as possible towards clocking ring  51 . Air ports  47 ,  49  are located in outer casing  33 . Port  47  is connected to a compressed air line, whereas port  49  is open to the atmosphere. 
     Micro-stop nose piece  61  is attached to the bottom of inner casing  37  by way of mounting flange  77 . As illustrated by  FIGS. 3 and 4 , micro-stop nose piece  61  is comprised of various components. These components include nose piece  63 , locking collar  65 , locking ring  67 , threaded fixture  69 , shaft  79 , and tool casing  85 . Locking ring  67  is threaded, and is screwed on to threads  71  on fixture  69 . Locking collar  65  slides onto fixture  69 , and is positioned around threads  71 . A pin (hot visible) is located on the inner surface of collar  65 , and slides into slot  75  on fixture  69 . The pin (not visible), captured in slot  75 , ensures that collar  65  can not rotate around fixture  69 . One end of collar  65  is saw tooth patterned. Nose piece  63  is threaded on one end  63   b , and is screwed onto the threads  73  on fixture  69 . End  63   b  of nose piece  63  has teeth that align with the teeth on collar  65 , preventing rotation of nose piece  63 . End  63   a  of nose piece  63  has an aperture that extends from the main body of the nose piece  63 , and allows a machining tool to pass through the aperture, forming a shoulder. 
     One end of shaft  79  is splined, and the other extends through fixture  69 , where tool collar  85  surrounds it. Just above collar  85 , a thrust bearing  83  is placed on shaft  79 . Pinned collar  81 , located just above bearing  83 , holds bearing  83  in place on shaft  79 . The shaft assembly is inserted into nose piece fixture  69 . Section  87  of the nose piece assembly  61  contains a close tolerance pilot that controls the center line of shaft  79 . Just above section  87  is a locking ring  89  which locks the pilot in place. Just above the locking ring  89  is another locking ring  91  which locks shaft  79  into the micro-stop nose piece assembly  61 . Once mounted to inner casing  37 , the splined end of shaft  79  is connected to the tool holder shaft hub  25 . Shaft  79  can move axially within LCT  31  due to the splined end and hub. 
     As illustrated by  FIGS. 5A ,  5 B, and  5 C, hollow shaft air cylinder LCT  31  is connected to a standard tool holder  21  for countersinking. Standard tool holder  21  is connected to a spindle of NC machine  19 . LCT  31  is connected to tool holder  21  as previously discussed. 
     A countersinking drill bit  24  is inserted into the micro stop nose piece assembly  61 . Bit  24  has a counterbore portion  24   a  at its upper end that extends below end  63   a  of nose piece  63 . Referring back to  FIGS. 3 and 4 , nose piece  63  is adjusted to ensure the desired countersink depth. The desired depth is determined by the extent that bit  24 , and in particular counterbore portion  24   a  extends below the aperture on end  63   a  of nose piece  63 . Nose piece  63  is adjusted by screwing ring  67  toward connector flange  77 . Locking collar  65  is then free to move up or down on fixture  69 . Nose piece  63  is then rotated on threads  73  in order to control the extent that counterbore portion  24   a  of bit  24  extends below end  63   a . Once the desired depth is set, locking collar  65  is positioned to lockingly engage the teeth on end  63   b  of nose piece  63 . Locking ring  67  is then tightened securely against collar  65 , locking the nose piece  63  in position and ensuring the desired drill depth of bit  24 . 
     NC machine  19  is programmed to lower tool holder  21  from a starting point  106  to a point  107  based on the thickness of the minimum thickness panel  105 . Programming will ensure that counterbore portion  24   a  of bit  24  cuts to the proper depth of the panel regardless of whether the panel is one of maximum thickness  101 , nominal thickness  103 , or minimum thickness  105 . Typical variations in panel thickness are illustrated by  109 , and in one embodiment, may be less than 0.020 inches. 
     The programmed point  107  is the same point in space regardless of the thickness of panels  101 ,  103 ,  105 . Programmed point  107  is determined by measuring the amount of travel it takes for end  24   a  to form the counterbore in minimum thickness panel  105  to the correct depth. The travel of tool holder  21  to point  107  should equal the distance d in  FIG. 5C . The traveled distance of tool holder  21  to point  107  will be slightly greater than the distance d′; which is the distance counterbore end  24   a  travels to cut the counterbore to the proper depth in medium thickness panel  103  ( FIG. 5B ). The traveled distance of LCT  31  to point  107  will be even greater than the distance d″, which is the distance counterbore end  24   a  travels to cut the counterbore to the proper depth in maximum thickness panel  101  ( FIG. 5A ). 
     Referring hack to  FIG. 2 , pressure is supplied to LCT  31  by an air source (not shown), which pumps air into LCT  31 . Air enters LCT  31  through port  47  and fills the annulus between outer casing  33  and inner casing  37 . As LCT  31  is pressurized, inner casing  37  fully extends outwards from casing  33 . As inner casing  37  extends outwards from casing  33 , port  49  ensures that any air trapped below flange  38  in the annulus between casing  37  and casing  33  is vented to the atmosphere. When casing  37  is fully extended, a gap  112  exists between shoulder  34  and flange  38 . Gap  112  is designed to absorb over travel of tool holder  21 , and in one embodiment, gap  112  is designed to absorb up to 0.100 inches of over travel. The air pressure is sufficient so that drill bit  24  will not cause shoulder  34  to move toward flange  38  as it drills. However, when nose piece end  63   a  contacts the surface of one of the panels  101 ,  103 ,  105  it will stop downward travel of flange  38  ( FIG. 7 ). 
     As illustrated by  FIGS. 5A ,  5 B, and  5 C, the tool holder  21  starts at the same elevation  106  and ends at the same elevation  107 . NC machine  19  rotates countersinking drill bit  24  and begins lowering tool holder  21  and bit  24  toward programmed point  107 . Given the different thicknesses of panels  101 ,  103 ,  105 , tool holder  21  is at a different distance from the panel depending on the panel thickness. 
     Considering minimum thickness panel  105 , when bit  24  first contacts panel  105 , the pressure of LCT  31  is such that bit  24  will penetrate the panel surface and continue toward the desired point  107  without any change in the position of flange  38 , as illustrated by  FIG. 6C . As NC machine  19  continues to lower tool holder  21 , bit  24  rotates and continues downwards until it has penetrated the panel and drill bit counterbore portion  24   a  has eat the proper counterbore depth in panel  105 . The pressure of LCT  31  is regulated such that the once the shoulder on end  63   a  contacts the panel surface, the force acting upwards against nose piece  63  is greater than the force acting downwards on inner casing  37 . However, when machining the minimum thickness panel  105 , end  63   a  contacts the surface when tool holder  21  is at point  107 , as illustrated by  FIG. 7C . The NC machine  19  stops drilling once tool holder  21  has reached point  107 . 
     Considering nominal thickness panel  103 , bit  24  starts drilling sooner than with panel  105  because it contacts panel  103  at a lesser distance d′. The pressure of LCT  31  is such that bit  24  will penetrate the panel surface and continue toward the desired point  107  without any change in the position of flange  38 , as illustrated by  FIG. 6B . When nose piece  63   a  contacts panel  103 , tool holder  21  is not yet at point  107 . The resistance of nosepiece  63   a  overcomes the air pressure, causing shoulder  34  to advance toward flange  38 . As shoulder  34  advances toward flange  38 , shaft  79  advances further into receptacle  25  ( FIG. 2 ). Drill bit  24  does not move further downward, however, as it has fully cut the counterbore and nosepiece  63   a  prevents further downward movement. 
     For maximum thickness panel  101 , the same occurs as with nominal thickness panel  103 . Bit  24  starts drilling sooner than with panels  105 ,  103  because it contacts panel  101  at a lesser distance d″. The pressure of LCT  31  is such that bit  24  will penetrate the panel surface and continue toward the desired point  107  without any change in the position of flange  38 , as illustrated by  FIG. 6A . Drill bit counterbore  24   a  will have cut to the full depth before LCT  31  has reached point  107 . As LCT  31  moves further downward, nosepiece  63   a  prevents further downward movement of drill bit portion  24   a , causing shoulder  34  to advance toward flange  38 . As shoulder  34  advances toward flange  38 , shaft  79  advances further into receptacle  25  ( FIG. 2 ). 
     As illustrated by  FIGS. 7A ,  7 B, and  7 C, LCT  31  continues downward until reaching point  107 . The amount of over travel absorbed by LCT  31  varies with the panel thickness. As illustrated by  FIG. 7C , when drilling a panel of minimum thickness  105 , LCT  31  absorbs the least amount or no over travel. Due to the thickness of panel  105 , the shoulder formed by the aperture on end  63   b  of nose piece  63  contacts the panel surface when tool holder  21  reaches point  107 , which is programmed for the minimum thickness panel  105 . In one example, there is no over travel to be absorbed. Accordingly, at the end of the machining operation, the original gap  112  between flange  38  and shoulder  34  remains. 
     As illustrated by  FIG. 7B , when drilling a panel of nominal thickness  103 , LCT  31  absorbs over travel. Due to the thickness of panel  103 , the shoulder formed by the aperture on end  63   b  of nose piece  63  contacts the panel surface before tool holder  21  reaches point  107 , which is programmed for the minimum thickness panel  105 . As a result, LCT  31  must absorb the over travel distance  110 , which is equal to the difference between d′ and d ( FIGS. 5B and 5C ). In one example, shoulder  34  has advanced towards flange  38 , leaving a gap  113 . 
     Referring to  FIG. 7A , when drilling a panel of maximum thickness  101 , LCT  31  absorbs the greatest amount of over travel. Due to the thickness of panel  101 , the shoulder formed by the aperture on end  63   b  of nose piece  63  contacts the panel surface before tool holder  21  reaches point  107 , which is programmed for the minimum thickness panel  105 . As a result, LCT  31  must absorb the over travel distance  111 , which is equal to the difference between d″ and d ( FIGS. 5A and 5C ). In one example, the over travel distance  111  is equal to original distance  112  that LCT  31  was designed to absorb. Accordingly, when tool holder  21  reaches point  107 , flange  38  is in contact with shoulder  34 . 
     LCT  31  operates as previously discussed when connected to a standard tool holder  21  for seal groove machining. The only change in regard to the operation of LCT  31  when seal groove machining is countersinking drill bit  24  is replaced with a seal groove cutting tool. As explained above, the gap between flange  38  and shoulder  34  allows LCT  31  to absorb over-travel by the tool holder, which guarantees nosepiece  63  contacts the panel surface resulting in a consistent seal groove width/depth regardless of the panel thickness. The variations in panel thickness illustrated above may be present over the surface profile of a single panel sought to be machined. During the seal groove machining process, LCT  31  responds to variations in the surface profile of a panel by compressing (absorbing over travel) or extending depending on the panel thickness at a given point. 
     Referring to  FIGS. 8 and 9 , an alternate embodiment LCT is illustrated in the form of linear bearings with spring LCT  121 . LCT  121  is connected to tool holder  21  by means of clocking ring  51  and a bearing (not visible). Bearing (not visible) is connected to clocking ring  51  by means of connector snap  55 . Bearing (not visible) and clocking ring  51  are connected to tool holder  21  by means of connector snap  53 . Locking pin  57  extends vertically from the top face of ring  51 , and slides into abashing (not visible) on the face of the NC machine. Clocking ring  51 , bearing (not visible), and locking pin  57  ensure that the body of LCT  121  is fixed and does not rotate with tool holder  21  and drill countersinking bit  24 . 
     A plurality of flanged linear bearings  127  are attached to the bottom of clocking ring  51 . Bearings  127  extend downward towards mounting plate  123 . Mounting plate  123  is circular in shape, but in an alternate embodiment could take other forms such as a square or rectangle. A rod  129  travels through each linear bearing  127  and extends downward before connecting to mounting plate  123 . Locking nuts  130  are attached to the end of rods  129  opposite mounting plate  123 . Nuts  130  ensure that rods  129  are fixed between clocking ring  51  and mounting plate  123 . Rods  129  can move axially in linear bearings  127 , but are limited in range of movement due to nut  130  on one end and linear bearing  127  on the other. 
     Surrounding each rod  129  and linear bearing  127  is a spring  131 , which is connected between clocking ring  51  and mounting plate  123 . Spring  131  acts to ensure that LCT  121  is fully extended in its natural state, ensuring a maximum gap between clocking ring  51  and mounting plate  123 . Plate  125  is connected to the bottom of mounting plate  123 . 
     Micro-stop nose piece  61  is attached to the bottom of plate  125  by way of mounting flange  77 . Once micro-stop nose piece assembly  61  is mounted to plate  125 , the splined end of shaft  79  is connected to tool holder shaft hub  25 . Shaft  79  can move axially within LCT  121  due to the splined end and hub. 
     Linear bearings with spring LCT  121  performs just as LCT  31 .  FIG. 8  illustrates LCT  121  in a natural state, prior to contacting a workpiece. Gap  133  between linear bearings  127  and mounting plate  123  is the largest when plate  123  is fully extended.  FIG. 9  illustrates LCT  121  absorbing over travel, as indicated by the decreased size of gap  133 . 
     Referring to  FIGS. 10 and 11 , an alternate embodiment LCT is illustrated in the form of spring actuated cylinder LCT  141 . LCT  141  is connected to tool holder  21  by means of clocking ring  51  and a bearing (not visible). Bearing (not visible) is connected to clocking ring  51  by means of connector snap  55 . Bearing (not visible) and clocking ring  51  arc connected to tool holder  21  by means of connector snap  53 . Locking pin  57  extends vertically from the top face of ring  51 , and slides into a bushing (not visible) on the lace of the NC machine. Clocking ring  51 , bearing (not visible), and locking pin  57  ensure that the body of LCT  141  is fixed and does not rotate with tool holder  21  and drill countersinking bit  24 . 
     Attached to the bottom of clocking ring  51  is mounting plate  143 . Mounting plate  143  is generally cylindrical and flat, with a T-shaped cross section  144  on each side. Outer casing  145  is machined to slide over and connect securely to mounting plate  143  of LCT  141 . Casing  145  is generally cylindrical in shape with the exterior portion of casing  145  being smooth. In an alternate embodiment, casing  145  could take other forms such as a square or rectangle. The interior surface of casing  145  is machined in a manner to enable it to receive inner casing  147 . The lower interior surface of casing  145  forms an upward facing shoulder  146 . 
     Inner casing  147 , generally cylindrical in shape, slidingly engages outer casing  145 . In an alternate embodiment, casing  147  could take other forms such as a square or rectangle. The outer surface of casing  147  forms a flange section  148 . Flange section  148  of casing  147 , shoulder  146  of casing  145 , and T-cross section  544  of plate  143  limit the movement of casing  147  within casing  145 . Plate  143 , outer casing  145 , and inner casing  147  are machined to connect to one another with extremely close tolerances to form a mechanical sleeve. A small annulus if formed between the inner casing  147  and T-shaped cross section  144  of plate  143 . Inner casing  147  is free to telescope in and out of casing  145 , but is limited in range by section  144  of plate  143  and shoulder  146  of outer casing  145 . 
     A spring  149  surrounds inner casing  147 , and is located in the annulus between inner casing  147  and outer casing  145 . Spring  149  acts to ensure that LCT  141  is fully extended in its natural state, ensuring a maximum gap between flange  148  and T-section  144 . Air ports  151  are located on the exterior of outer casing  145 . Airports  151  are open to the atmosphere and ensure that LCT  141  does not become pressurized with the telescoping movement of inner casing  147 . 
     Micro-stop nose piece  61  is attached to the bottom of inner casing  147  by way of mounting flange  77 . Once micro-stop nose piece assembly  61  is mounted to casing  147 , the splined end of shaft  79  is connected to the tool holder shaft hub  25 . Shaft  79  can move axially within LCT  141  due to the splined end and hub. 
     Spring actuated cylinder LCT  141  performs just as LCT  31  and LCT  121 .  FIG. 10  illustrates LCT  141  in a natural state prior to contacting a workpiece. Gap  153 , between flange  148  and T-section  144  is the largest when casing  147  is fully extended.  FIG. 11  illustrates LCT  141  absorbing over travel, as indicated by the decreased size of gap  153 . 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the Invention. For example, linear compensator tool could be used in a number of various machining applications requiring material surface accuracy.