Abstract:
A spindle apparatus for receiving and driving a tool holder, comprises a drive, a shaft coupled to the drive with a forward end of the shaft having a receptor for affixation of a tool holder, and a clamping device supported at the forward end of the shaft for integral driven rotation therewith. The clamping device has a clamping element disposed within the receptor and selectively actuable between an active position for retaining the tool holder and an inactive position for inserting and releasing the tool holder. An actuator is operatively connected with the clamping device for controlling movement of the clamping element between the active and inactive positions. The actuator is disposed adjacent the forward end of the shaft in surrounding relation thereto, which facilitates a particularly compact, efficient and reliable design.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is entitled to the benefit of, and claims priority from, U.S. Provisional Patent Application Ser. No. 61/283,488, filed Dec. 4, 2009, and entitled “COMPACT INDUSTRIAL ROTATING SPINDLE AND TOOL CHANGING SYSTEM,” the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains generally to machine tools, and more specifically to methods and apparatus for selectively clamping and releasing a tool or similar device, for example, but without limitation, apparatus for receiving and driving a tool holder via an electric machine spindle. 
       BACKGROUND OF THE INVENTION 
       [0003]    In various manufacturing industries, electrically operated spindle apparatus are utilized for holding and driving differing machine tools to perform differing machining operations. In such applications, a variety of tools may be needed to transform a raw material workpiece into a finished product, e.g., via cutting, boring, trimming, shaping, polishing, engraving, and/or other manners of machining. Hence, it is desirable if not essential that the spindle apparatus provide for convenient, simplified and quick interchange of differing tools. Often, such spindle apparatus are utilized in a computer controlled machine, such as a robotic apparatus, wherein the spindle apparatus is adapted for the interchange of tools automatically without operator intervention, e.g., via pneumatic, electrical or hydraulic actuation, under a preprogrammed computer control such as a so-called “computer numerically controlled” or “CNC” machine control system. For example, in the woodworking industry, such spindle apparatus may be utilized in a robotic apparatus wherein the spindle apparatus is supported on a robotic arm to be manipulated in relation to a workpiece to be cut, bored, profiled or otherwise machined by one or more tools driven by the spindle apparatus, and also to be manipulated in relation to a tool support carousel or rack for automated interchange of one tool for another. 
         [0004]    While conventional spindle apparatus perform satisfactorily in such automated computer controlled machine environments, the known spindle apparatus suffer a number of recognized disadvantages. A representative conventional electric motor spindle apparatus comprises a spindle unit and an actuator unit in combination, in which the spindle unit has an electric motor driven shaft fitted at one end with a clamping assembly to interchangeably accept multiple differing tools or implements, with the actuator unit mounted to the opposite end of the spindle unit from the clamping assembly to open and close the clamping assembly via linear reciprocation of a spring-loaded drawbar extending through the length of the shaft. This form of spindle apparatus is relatively large, bulky and heavy, owing to the assemblage of the spindle and actuator units, making the spindle apparatus unsuitable for use in some installations and applications wherein limited space is available for manipulation of the apparatus. Even in installations in which the known type of spindle apparatus is suitable, the size and weight of the apparatus contributes to slow the motions of the apparatus which must be executed in performing an automated tool exchange procedure. Such spindle apparatus also have a substantial number of moving parts which necessarily present a correspondingly increased need for regular adjustments, tuning and other maintenance, and a commensurate risk of misadjustment of parts and incidence of part failures. 
         [0005]    There according exists a recognized need within the machine tool industry for an improved form of spindle apparatus suitable for use in CNC and other machines to perform automated interchange of multiple tools. 
       SUMMARY OF THE INVENTION 
       [0006]    It is accordingly an object of the present invention to improve upon the known forms of spindle apparatus, both as conventionally used for automated tool change operations and non-automated machine tool applications. A further object of the present invention is to address the recognized disadvantages of such known spindle apparatus. Another object of the invention is to provide a spindle apparatus having a smaller dimensional size and lesser bulk and weight, yet with comparable or superior performance, to that of known spindle apparatus A related object of the invention is to provide a spindle apparatus which can be deployed in installations in which conventional spindle apparatus could not heretofore be utilized. A still further object of the invention is to provide a spindle apparatus with a reduced number of operational parts, less frequent required adjustments and other maintenance steps, and enhanced reliability with a lesser incidence of failures and downtime than with known spindle apparatus. A more specific object of the invention is to provide a spindle apparatus which eliminates the use of a hollow shaft, a spring-loaded drawbar and a distal-mounted actuator unit. It is another object of the invention to provide a spindle apparatus that may be adapted for uses in other machines than electric spindles, such as standard electric motors, belt driven spindles, or robotic arms. 
         [0007]    Briefly summarized, the present invention provides a spindle apparatus for receiving and driving a tool holder, which basically comprises a drive, a shaft coupled to the drive with one end of the shaft (herein referred to as the forward end) having a receptor configured for affixation of a tool holder, and a clamping device supported at the forward end of the shaft for integral driven rotation therewith. The clamping device has a clamping element disposed within the receptor and selectively actuable between an active position for retaining the tool holder and an inactive position for inserting and releasing the tool holder. An actuator is operatively connected with the clamping device for controlling movement of the clamping element between the active and inactive positions. According to the present invention, the actuator is disposed in surrounding relation to the shaft adjacent the same forward end of the shaft as the clamping device. 
         [0008]    in accordance with a preferred embodiment of the invention, the shaft has an axial bore in its forward end in communication with the receptor, and the clamping device comprises a clamp housing disposed within the axial bore and a reciprocable bolt disposed within the clamp housing and operatively connected with the clamping element for moving the clamping element between the active and inactive positions via reciprocation of the bolt. The shaft includes a radial opening therethrough, and the actuator includes a connecting pin extending through the radial opening into engagement with the reciprocable bolt of the clamping device. The pin is disposed for access from exteriorly of the apparatus for selective disconnection of the pin from the clamping device to permit removal of the clamping device outwardly through the axial bore in the shaft at its forward end. 
         [0009]    The shaft is preferably supported at its forward end by a front bearing arrangement and at the opposite end of the shaft by a rear bearing arrangement, with the actuator being disposed adjacent the front bearing arrangement. The shaft may preferably be a substantially solid shaft. 
         [0010]    In a preferred embodiment, the actuator comprises a first piston disposed coaxially about the shaft in fixed relation thereto and a second piston disposed coaxially about the shaft for axial movement toward and away from the first piston, with the second piston being connected to the clamping device through a radial opening in the shaft. The actuator may include a connecting pin extending through the radial opening into engagement with the clamping device. A spindle housing preferably contains the drive, the shaft, the clamping device and the actuator, with the housing having an opening therein for access to the connecting pin for disconnection of the pin from the clamping device to permit removal of the clamping device outwardly through the forward end of the shaft. 
         [0011]    As will thus be understood, the spindle apparatus of the present invention is designed to be used in a machine in a manufacturing or industrial environment wherein an automated tool change process is utilized to interchange different tools used with the machine rapidly and without human intervention. A computer controlled machining center or a robotic arm is a typical form of such a manufacturing or industrial setting for the present apparatus, in which the machining center or robot moves a workpiece and/or the spindle apparatus relative to the other to form the workpiece into a desired product, e.g., by cutting, engraving, trimming, shaping, profiling, grinding, polishing, and in many cases a combination of such operations. A variety of forming tools are usually needed to accomplish such a manufacturing operation. In a preferred embodiment, the spindle apparatus may be powered by compressed air and electricity, controlled via sensors operating in conjunction with a Computer Numerically Controlled (CNC) machine control system. Additional variations of the design can eliminate one or more of these requirements. The present apparatus may also be used in non-automated installations and in applications outside of machining centers or robotics. One example of such an alternative installation would be a through feed cutting machine such as a molder or tenoner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross-sectional view of a prior art electric motor spindle apparatus, taken through the lengthwise axis of the apparatus; 
           [0013]      FIG. 2  is an enlarged cross-sectional view of the actuator unit of the electric motor spindle apparatus of  FIG. 1 , also taken through the lengthwise axis thereof; 
           [0014]      FIG. 3  is a front end elevational view of one preferred embodiment of electric motor spindle apparatus according to the present invention; 
           [0015]      FIG. 4  is a cross-sectional view of the entire electric motor spindle apparatus of  FIG. 3 , taken along line  4 - 4  thereof through the full lengthwise extent of the axis of the apparatus; 
           [0016]      FIG. 5  is a more enlarged cross-sectional view of the forward clamping end of the electric motor spindle apparatus of  FIG. 3 , taken along line  5 - 5  thereof through a tool-clamping sensor of the apparatus; 
           [0017]      FIG. 6  is another enlarged cross-sectional view, similar to  FIG. 5 , of the forward clamping end of the electric motor spindle apparatus of  FIG. 3 , taken along line  6 - 6  thereof through a spindle rotation sensor of the apparatus; 
           [0018]      FIG. 7  is another enlarged cross-sectional view of the forward clamping end of the electric motor spindle apparatus of  FIG. 3 , also taken along line  4 - 4  thereof with the clamping assembly in an extended non-clamping position for releasing a tool holder and for receiving a new tool holder; 
           [0019]      FIG. 8  is another enlarged cross-sectional view, similar to  FIG. 7 , showing the forward clamping end of the electric motor spindle apparatus of  FIG. 3  with the clamping assembly in a retracted clamping position for grasping and retaining a tool holder; 
           [0020]      FIGS. 9A and 9B  are side elevational and end elevational views, respectively, showing typical overall outer dimensions of a conventional prior art spindle apparatus of the type shown in  FIGS. 1 and 2 ; 
           [0021]      FIGS. 10A and 10B  are side elevational and end elevational views, respectively, showing typical overall outer dimensions of a spindle apparatus according to the embodiment of the present invention shown in  FIGS. 3-8 ; 
           [0022]      FIG. 11  is a vertical cross-sectional view of an alternative design of actuator piston components for the embodiment of the present spindle apparatus of  FIGS. 3-8 ; 
           [0023]      FIG. 12  is a vertical cross-sectional view of an alternative design of sensor disc for the embodiment of the present spindle apparatus of  FIGS. 3-8 ; 
           [0024]      FIG. 13  is another enlarged cross-sectional view of the forward clamping end of the electric motor spindle apparatus of  FIG. 3  depicting the process of removal of the clamping assembly thereof; 
           [0025]      FIG. 14  is an enlarged cross-sectional view of the forward clamping end of the electric motor spindle apparatus of another preferred embodiment of electric motor spindle apparatus according to the present invention, with the clamping assembly in an extended non-clamping position for releasing a tool holder and for receiving a new tool holder; and 
           [0026]      FIG. 15  is another enlarged cross-sectional view, similar to  FIG. 14 , showing the forward clamping end of the electric motor spindle apparatus of  FIG. 14  with the clamping assembly in a retracted clamping position for grasping and retaining a tool holder; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Referring now to the accompanying drawings and initially to  FIGS. 1 and 2 , a typical conventional prior art electric motor spindle apparatus of the above-described type equipped for selective automated tool changing operations basically comprises an electric motor spindle  10  in combination with an actuator  12 . The spindle  10  includes a shaft  14  rotatably supported at its opposite ends via front bearings  15  and rear bearings  16 , with an electric motor comprised of a stator  18  and a rotor  20  surrounding the shaft  14  between the bearings  15 ,  16  to drive rotation of the shaft  14 . A main housing  22  encloses and supports the assembly of the shaft  14 , bearings  15 ,  16 , and the electric motor  18 ,  20 . A working end  14 A of the shaft  14  opens outwardly from one end of the housing  22  (at the leftward end of the housing as viewed in  FIG. 1 ) and is configured to receive a tool (not shown) to be driven via the shaft  14 . For automatic tool change applications of the spindle/actuator apparatus, the tapered end opening  14 B in the working end  14 A of the shaft  14  is configured, e.g., in the form of a tapered end opening  14 B in the shaft  14 , to mate with the uniform geometry of an interchangeable tool holder (not shown). 
         [0028]    The shaft  14  is hollow and interiorly supports a selectively actuable clamping assembly  24  adjacent the working end  14 A of the shaft  14  for grasping the interchangeable tool holder in order to provide a secure operational connection between the tool holder and the shaft  14 . Several types of clamping assemblies are commercially available to grasp the interchangeable tool holder. The particular structure and method of operation for such clamping assemblies vary, but have in common the capability to peripherally grasp and pull the tool holder axially into the working end  14 A of the shaft  14  so as to securely retain the tool holder for unitary rotation with the shaft  14 .  FIG. 1  depicts one of the most common types of such a clamping assembly  24  in the form of a stub shaft  24 A having an axial recess  24 B in its forward end with a set of engagement balls  24 C supported in radial bores surrounding the recess for movement into and out of the recess  24 B. The stub shaft  24 A is affixed integrally to the forward end of a drawbar  25 , essentially a slender rod that is supported within the spindle shaft  14  to extend through the full length of its hollow interior of the spindle shaft  14  for slidable axial movement relative to the shaft  14 . The distal end of the drawbar extends outwardly from the distal end of the shaft  14 . A spring, or set of springs,  27  encircle the drawbar  25  within the shaft  14  to extend between an abutment against the shaft  14  adjacent the clamping assembly  24  and an abutment against a flange  30  adjacent the distal end of the drawbar  25  opposite the clamping assembly  24 . In this manner, the spring  27  normally acts axially against the flange  30  of the drawbar  25  to urge the drawbar  25  rearwardly within the shaft  14  away from its working end  14 A and, in turn, to pull the clamping assembly  24  inwardly within the working end  14 A of the shaft  14 . Alternatively, the slidable disposition of the drawbar  25  within the shaft  14  enables the application of an axial sliding force against the exposed distal end of the drawbar  25  and against the biasing force of the spring  27  thereby to compress the spring  27  and extend the drawbar  25  forwardly within the shaft  14  so as to extend the clamping assembly  24  into the end opening  14 B of the shaft  14 . As seen in  FIG. 1 , the end opening  14 B is formed with an annular expansion zone  14 C forwardly of the disposition of the clamping assembly  24  as normally withdrawn under the force of the spring  27  which permits the clamping balls  24 C to move radially outwardly from the recess  24 B when the drawbar  25  and the clamping assembly  24  are thusly extended. 
         [0029]    Such forward opening movements and rearward closing movements of the drawbar  25  and the clamping assembly  24  are controlled by the actuator  12 , indicated only generally in  FIG. 1 , but in a more detailed cross-section in  FIG. 2 . The actuator  12  is mounted to the spindle housing  22  at the distal end of the spindle  10  opposite the clamping assembly  24  at the working end  14 A of the shaft  14 . The actuator  12  basically is comprised of one or more reciprocable pistons  26  connected to a bolt  28  extending axially through the pistons  26  to reciprocate integrally with the pistons  26 . In the embodiment of the actuator  12  shown in  FIGS. 1 and 2 , the actuator  12  is adapted to be operated pneumatically to reciprocate the assembly of the pistons  26  and the bolt  28 , but it is also known to employ actuators powered by electricity or hydraulic pressure. A leading end of the bolt  28  projects forwardly from the actuator  12  in axial alignment with the exposed rearwardly-extending end of the drawbar  25 . The pistons  26  are supported to travel within the actuator  12  between a fully retracted inactive position, shown in  FIGS. 1 and 2 , wherein the leading end of the bolt  28  is spaced rearwardly out of contact with the drawbar  25 , thereby permitting the spring  27  to fully retract the drawbar  25  and the clamping assembly  24  within the shaft  14 , and a forwardly-extended position wherein the bolt  28  contacts and slidably advances the drawbar  25  forwardly within the shaft  14  against the biasing force of the spring  27  sufficiently to move the retaining balls  24 C of the clamping assembly  24  into the expansion zone  14 C. 
         [0030]    As previously noted, the tool holder is an interchangeable part which has a geometric configuration that mates with the geometric configuration of the tapered end opening  14 B of the working end  14 A of the shaft  14  and the geometric configuration of the recessed end opening  24 B of the clamping assembly  24 . The tool holder is itself configured to rigidly and integrally hold a tool or like working implement, e.g. a cutting tool element, whereby the term ‘tool’ is often used to identify the subassembly of the tool element and the tool holder collectively. The mated tapering configuration of the shaft opening  14 B and the tool holder facilitate accurate, repeatable, and secure location and seating of the tool holder. Typically, the tool holder is formed with a drive key configured to be engaged by the balls  24 C of the clamping assembly  24 . Thus, when the bolt  28  and, in turn, the drawbar  25 , are advanced by the actuator  12  to extend the clamping assembly  24  into expansion zone  14 C of the working end  14 A of the shaft  14 , the engagement balls  24 C are unconstrained to move radially outwardly whereby a tool holder may be inserted into or released from the clamping assembly  24 . Then, when the bolt  28  is withdrawn allowing the springs  27  to retract the drawbar  25 , the engagement balls  24 C are forced to move radially inwardly into engagement with the drive key of the tool holder, thereby grasping the tool holder and pulling it into the working end  14 A of the shaft  14 . In this manner, the clamping assembly  24 , together with the frictional engagement between the mating surfaces of the shaft and the tool holder, is effective to transmit the rotational torque of the shaft  14  generated by the electric motor  18 ,  20  to the tool holder. 
         [0031]    The positions of the assembly of the actuator pistons  26  and bolt  28 , and in turn the positions of the clamping assembly  24 , are monitored by electrical sensors  32  which communicate with the CNC machine control system. For example, typical sensors  32  may be commercially available, non-contact electronic devices capable of detecting the presence or absence of a material within a predetermined recognition range of the sensor. Generally, one sensor  32 A is disposed to monitor a rotating component of the spindle  10  to determine if the spindle  10  is rotating or stopped. One or two sensors  32 B are disposed to monitor the end of the drawbar  25  to determine the presence or absence of a tool as indicated by the extended or retracted position of the drawbar  25 . One or more additional sensors may be disposed within the actuator  12  to monitor the extended or retracted disposition o the pistons  26  and the bolt  28 , e.g., a sensor  32 C disposed to detect the location of the rearward end of the bolt  28 , e.g., via a sensor disc  34  affixed to the end of the bolt  28 , to determine whether the pistons  26  and bolt  28  are in their forward or rearward position. 
         [0032]    The conventional operation of the spindle and actuator apparatus to accomplish a tool change process in a typical CNC machine may thus be understood. First, the CNC control system deactuates the electric motor  18 ,  20  to bring the spindle  10  to a standstill, which is confirmed and communicated to the CNC control system via the spindle rotation sensor  32 A. The machine then manipulates the spindle and actuator apparatus  10 ,  12  into a tool exchange position relative to a storage magazine, carousel, or rack that holds multiple interchangeable tools, whereupon the actuator  12  is energized to begin the tool change process by extending the actuator pistons  26  forwardly and in turn extending the drawbar  25  against the biasing force of the spring  27 , thereby extending the clamping assembly  24  to relieve the engagement of the balls  24 C on the tool holder so as to release the tool onto an empty storage location on the magazine/carousel/rack. After depositing the released tool, the machine moves the spindle and actuator apparatus  10 ,  12  away from the released tool and into position to accept and engage a different tool. The pneumatic force acting on the actuator pistons  26  is released to allow the pistons  26  and the drawbar  25  to return to their respective retracted positions under the biasing force of the spring  27 . In turn, the clamping assembly  24  is retracted within the working end  14 A of the shaft  14 , thereby grasping and drawing the new tool into clamped position within the clamping assembly  24 . The sensor  32 B confirms when the drawbar  25  has returned to its fully retracted position, indicating that the tool holder is correctly clamped, whereupon the CNC control system re-energizes the electric motor  18 ,  20  to resume driven rotation of the spindle, thereby completing the tool change sequence. 
         [0033]    Turning now to  FIGS. 3-8 , an electric motor spindle apparatus according to one preferred embodiment of the present invention is indicated generally at  50  and will be recognized to differ substantially in both structure and operation to the known prior art automatic tool change spindle apparatus of  FIGS. 1 and 2 . The spindle apparatus  50  basically comprises an elongate main spindle housing  52  to opposite ends of which are affixed a front bearing housing  54  and a rear bearing housing  56  having front and rear ball bearing sets  58 ,  60 , respectively, for rotationally supporting a longitudinal shaft  62  extending centrally through the interior of the main housing  52 . The shaft  62  is a solid body except only that the forward end of the shaft  62  has an axial cavity  62 A serving as a receptor in which is affixed a clamping device, such as clamping assembly  64 , selectively actuable for clamped engagement and unclamped release of a mating tool holder  65 , all as more fully described hereinafter. An actuator, such as the actuator assembly generally indicated at  66 , is supported within the front bearing housing  54  to surround the forward end of the shaft  62  immediately rearwardly of the front bearing set  58  and is connected with the clamping assembly  64  for actuating clamping and unclamping thereof, as also more fully described hereinafter. Rotational driving force is imparted to the shaft  62  by an electric motor comprised of a rotor  68  fixed about the shaft  62  and a stator  70  fixed stationarily within the main housing  52  in surrounding relation to the rotor  68 . A connector unit  72  is affixed to the exterior of the housing  52  adjacent its rearward end to supply operating electrical power to the electric motor and compressed air for pneumatic operation of the actuator assembly  66 . The distal rearward end of the main housing is enclosed by a cover plate  74 . 
         [0034]    The clamping assembly  64  comprises a hollow clamping housing  76  fitted within the cavity  62 A of the spindle shaft  62  and secured in place by mating threads formed on the inserted end of the clamping housing  76  and on the interior end on the cavity  62 A, whereby the clamping housing  76  rotates integrally with the shaft  62 . The forward end of the clamping housing  76  faces outwardly from the forward end of the shaft  62  and is formed with an axial opening into the hollow interior of the clamping housing  76 , through which a clamping bolt  78  extends slidably into the clamping housing  76 . The clamping bolt  78  is surrounded by a coil spring  80  extending between an interior shoulder within the clamping housing  76  and a tensioning nut  82  threaded about the bolt  78 , whereby the spring  80  urges the bolt  78  inwardly within the clamping housing  76 . The interior end of the clamping bolt  78  has a bore formed diametrically therethrough which receives a high strength pin  84  extending radially outwardly through aligned slots  76 A,  62 B in the clamping housing  76  and in the shaft  62 , respectively, for connecting the clamping bolt  78  with the actuator assembly  66  to transmit clamping and unclamping movements to the clamping assembly  64 , as more fully described below. 
         [0035]    The forward end of the clamping housing  76  is formed as a narrowed neck portion  76 B of reduced diameter about which are formed a plurality of circular openings each containing a clamping ball  75  movable radially inwardly and outwardly within the respective opening. The forward end of the clamping bolt  78  radially inwardly adjacent the neck portion  76 B is formed with an annular recess  77  disposed to allow the balls  75  to move radially inwardly into the recess  77  when the bolt  78  slides forwardly in the clamping housing  76 . The forward end of the recess  77  is formed with an outwardly tapering surface disposed to direct the balls  75  radially outwardly within their openings when the clamping bolt  78  slides rearwardly within the clamping housing  76 . 
         [0036]    This arrangement of the clamping assembly  64  is thereby effective for selectively grasping and releasing the tool holder  65  under the selective actuation of the actuator assembly  64  as described below. The tool holder  65  is formed in its forward end with a recess  65 A configured to receive any of a plurality of like-formed interchangeable tool elements (not shown), as is conventional. The opposite rearward end of the tool holder  65  is formed with a peripheral external taper mated to the internal taper of the cavity  62 A of the shaft  62 . The rearward end of the tool holder  65  is formed interiorly with a recess  65 B encircled by an annular lip  65 C adapted to be engaged and grasped rigidly by the clamping balls  75  when moved and held in their radially outward position and to be disengageable from the clamping balls  75  when moved into their radially inward position relative to the clamping housing  76 . 
         [0037]    The actuator assembly  66  comprises annular front and rear pistons  85 ,  86  disposed within the front bearing housing  54  in surrounding relation to the spindle shaft  62 . The rear piston  86  is closely fitted around the shaft  62  immediately forwardly of the electric motor  68 ,  70  and is fixed stationarily in place to the front bearing housing  54  by retaining rings  88 . The rear actuator piston  86  has a forwardly projecting hub portion about which the front piston  85  is fitted for sliding axial movement toward and away from the rear piston  86  whereby the front piston  85  is isolated from the rotating shaft and also can be sealed relative to the rear piston. A port (not shown) is machined in the front bearing housing  54  to connect a source of compressed air, delivered via the connector  72 , to the spacing between the front and rear pistons  85 ,  86  for actuating movement of the front piston  85  away from the rear piston  86 . Sealing rings  90  are fitted about the inward and outward surfaces of the front piston  85  and about the outer surface of the rear piston  86  to seal the pistons relative to each other and to the front bearing housing  54 , whereby the spacing between the pistons  85 ,  86  is airtight to contain the pressurized air. Several dowel pins  92  extend axially between bores formed in the forward face of the front piston  85  and the front bearing set  58  to serve as guides for reciprocating movement of the front piston  85 , and coil springs  94  surround the dowel pins  92  to urge the front piston  85  toward the rear piston  86  into a home position adjacent thereto. An annular sensor disc  96  surrounds the shaft  62  in slidable relation thereto forwardly of the front piston  85  and is fixed to the connecting pin  84  preferably by threaded engagement of the pin  84  in the body of the sensor disc  96  to connect the actuator assembly  66  to the clamping assembly  64 . 
         [0038]    Thus, the actuator assembly  66  operates the clamping assembly  64  in the following manner, best understood with reference to  FIGS. 7 and 8 . In normal operation of the spindle apparatus  50 , the actuator assembly  66  is deactuated, whereby the front piston  85  resides in its home position immediately adjacent the rear piston under the influence of the springs  94 , as depicted in  FIG. 8 . The clamping assembly  64  is therefore disposed with its clamping bolt  78  withdrawn rearwardly relative to the clamping housing  76 , thereby causing the clamping balls to be engaged and moved outwardly by the tapered forward end surface of the clamping bolt recess  77  so as to engage the tool holder  65  with radial and axial force inwardly of its rearward lip  65 C to grasp it rigidly as a unit with the shaft  62  to rotate integrally therewith under the driving force of the motor  68 ,  70 . When it is desired to exchange the tool holder  65  for another tool holder, the motor  68 ,  70  is denergized to bring the shaft  62  to a standstill, and thereupon the actuator assembly  66  is actuated by delivering compressed air into the space between the front and rear pistons  85 ,  86  causing the front piston  85  to move forwardly away from the rear piston  86  and, in turn, compressing the springs  94 , as depicted in  FIG. 7 . As the front piston  85  moves forwardly, the sensor disc  96  also slides forwardly, in turn moving the connecting pin  84  and the clamping bolt  78  forwardly. When the annular recess  77  in the forward end of the clamping bolt  78  advances into adjacency to the clamping balls  75 , the clamping force exerted by the balls  75  against the tool holder  65  is relieved and the balls  75  are permitted to move radially into the annular recess  77  sufficiently that the tool holder  65  may be withdrawn forwardly from the shaft  62  and a new tool holder inserted into the tapered front end of the shaft  62 . Thereupon, the source of the compressed air is deactivated, allowing the compressed spring force of the springs  94  to return the front piston  85  to its home position and simultaneously allowing the clamping bolt  78  to retract within the clamping housing  76  as the compressed spring force of the clamping spring  80  is relieved. The clamping balls  75  are thereby engaged again by the tapered surface at the forward end of the recess  77  in the clamping bolt  78 , causing the balls  75  to be forced radially outwardly into clamping engagement with the replacement tool holder. 
         [0039]    The actuator assembly  66  preferably includes safety features to control the limits of the forward and rearward movements of the front piston  85 . First, a retaining ring  98  is fitted within the front bearing housing  54  to define an engagement stop to be contacted by the front piston  85  at the forward limit of its travel and thereby to prevent excessive spring force from the spring  80  from being transmitted to the front bearing set  58  which could potentially damage the front bearing set  58  under the significant forces created by the actuator assembly  66 . In addition, a first electronic proximity sensor  100  (see  FIG. 5 ) is fitted radially through the spindle body  52  and the front bearing housing  54  in a position to detect whether the sensor disc  96  is in its normal rearward clamping position and to generate a corresponding control signal which may be transmitted via the connector  72 , e.g., to the CNC machine in which the spindle apparatus  50  is installed. A second electronic proximity sensor  101  (see  FIG. 6 ) is similarly fitted radially through the spindle body  52  and the front bearing housing  54  in a position to detect whether the sensor disc  96  is in its forward position indicating that the tool holder has been released for replacement and to generate a corresponding control signal for transmission via the connector  72 . Another sensor (not shown) may be provided at a suitable position, e.g., in the rear bearing housing  56  behind the rear bearing set  60  to detect whether the shaft  62  is under rotation. 
         [0040]    The manner in which the present spindle apparatus  50  accomplishes the connection between the clamping assembly  64  and the actuator assembly  66  via the connecting pin  84  provides a significant and distinct improvement over prior art spindle designs by allowing the entire clamping assembly  64  to be selectively removed from the front end of the spindle apparatus  50  without disassembling or disturbing any other components. As shown in  FIG. 13 , an opening  102  is provided radially through the side of the spindle body  52  at the axial location of the pin  84  when actuator assembly is in its retracted home position, whereby the pin  84  can be unthreaded from the sensor disc  96  and removed outwardly from the spindle apparatus  50  through the opening  102  when desired. When the pin  84  is removed, the clamping housing  76  can be unthreaded from the spindle shaft  62  whereupon the entire clamping assembly  64  can then be quickly and easily withdrawn and replaced as a unit, enabling the spindle apparatus  50  to be promptly returned to operation, with minimal downtime and minimal impact to production efficiency. An assembly tool  104  matable with the forward end of the clamping housing  76  may be provided to accomplish insertion and removal of the clamping assembly into proper positioning within the cavity  62 A in the shaft  62  so as thereby to aid in insertion and removal of the connecting pin  84 . 
         [0041]    Another significant feature of the present spindle apparatus  50  is its ability for fine adjustments to be made to the clamping assembly  64 . The threaded mounting of the clamping housing  76  to the spindle shaft  62  enables the axial positioning of the clamping assembly  64  relative to the stationary components of the spindle apparatus  50  to be finely set upon installation of the clamping assembly  64 . To facilitate the necessary connection of the clamping assembly  64  to the actuator assembly  66  via the connecting pin  84  at any such axial position of the clamping assembly  64 , the spindle shaft  62  is formed with two or more slots  62 B circumferentially spaced about the shaft  62  and the slots  62 B are elongated in the axial direction. In this manner, following the threaded adjustment of the positioning of the clamping assembly  64 , the connecting pin  84  can be installed through the slots  62 B which are exposed at the opening  102 . An additional adjustment to the clamping assembly  64  is possible by adjusting the threaded disposition of the spring preload nut  82  along the clamping bolt  78  to selectively increase or decrease the biasing force exerted by the spring  80 . 
         [0042]    The spindle apparatus of the present invention will thus be recognized to provide a number of significant advantages over conventional spindle apparatus. The disposition of all mechanical components of the actuator assembly  66  at the forward end of the apparatus immediately surrounding the clamping assembly  64  and the forward working end of the shaft  62  substantially reduces the overall size of the spindle apparatus in substantially all dimensions, reduces the number of mechanical components thereby simplifying and reducing manufacture and assembly time and cost, and improves the operational reliability with commensurate reduction of subsequent maintenance cost, as compared to comparable conventional spindle apparatus. 
         [0043]      FIGS. 9A ,  9 B,  10 A and  10 B illustrate the comparative difference in overall physical size of a typical conventional spindle apparatus ( FIGS. 9A ,  9 B) to a comparable embodiment of the spindle apparatus  50  of the present invention belonging to the same size and class of spindles within the industry. As will be seen, the present spindle apparatus  50  is smaller in every dimension but most notably in the length of the spindles. Specifically, the overall length of the present spindle apparatus is reduced by more than 10% (e.g. from 420 mm to only 377 mm). In general, it is expected in the industry that an increase in spindle power (within the same size range and method of cooling) requires a longer stator, increasing the length of the spindle apparatus. However, the design of the present spindle apparatus of  FIGS. 10A and 10B  produces 46% more power than the conventional spindle apparatus of  FIGS. 9A and 9B , and is still able to achieve more than a 10% (e.g. 43 mm) reduction in overall length. The reduction in overall length is especially significant for applications of the spindle apparatus in certain robotic and advanced CNC machining applications, as a shorter spindle enables the size of part being manufactured to be increased. Additionally, a shorter spindle apparatus is capable of functioning within more restricted dimensions when performing certain machining operations. For example, machining applications that require internal operations (such as the manufacture of housings, gear cases, boat hulls, bath tubs, or similar objects) can be problematic because they require the spindle to perform work on the inside surface of the part, where space is frequently at a premium. The spindle apparatus according to the present invention permits much greater versatility for manufacturing these types of components. 
         [0044]    The design of the present spindle apparatus in situating the actuator assembly at the forward end of the apparatus advantageously eliminates the need to employ a hollow spindle shaft with an internal drawbar to actuate a clamping assembly for a tool holder. Spindle drawbar and spring arrangements present inherent difficulties not only in the manufacturing process, but throughout the entire life cycle of the spindle apparatus, and are recognized in the industry to cause a high frequency of spindle failures. Such arrangements require close tolerances to be maintained between the spring, drawbar, and shaft throughout the entire length of the shaft. Especially in the case of the shaft, the high tolerance machining required to create a through hole for a drawbar is expensive and requires specialized boring and internal grinding machinery. Maintaining the proper alignment, clearances, and runout increases the complexity of the initial assembly of the apparatus, and complicates later repair of the spindle apparatus. Spindle balance is also affected by the inherent movement of these large components inside the shaft. In sum, the forward disposition of the actuator assembly and the ability to use a solid spindle shaft in the present apparatus results in simplified and less costly manufacture, better balancing and more reliable operation with less frequent and less costly maintenance of the apparatus in use, and greater productivity through extended cutting tool life and improved work piece surface qualities. 
         [0045]    Another advantage accruing from the ability to use a solid spindle shaft is to enable the creation of unique spindle variations that were not possible with the traditional hollow shaft design. For example, a double-ended automatic tool change spindle may be manufactured as one contemplated alternative embodiment of the present spindle apparatus. Such a double-ended spindle design is not possible with existing spindle constructions because the drawbar actuator design requires that an actuator be situated at one end of the hollow spindle shaft. 
         [0046]    Spindle maintenance is also considerably improved by the present spindle apparatus by making it possible to remove and replace the clamping assembly without disassembling the spindle or removing it from the CNC machine or robot, as described above. By contrast, in known spindle apparatus designs, it is necessary to disassemble the spindle at least to the point at which the rear bearings may be accessed, so that the drawbar and surrounding spring may be removed through the rear of the spindle. This operation generally requires removing the spindle apparatus from the robotic or CNC machine, removing the actuator, and disassembling any sensors necessary to expose the end of the drawbar. Since the clamping assembly  64  of the present invention can be removed from the forward end of the apparatus, many of the wear prone components that must be periodically replaced can be more quickly and easily exchanged with less apparatus downtime. 
         [0047]    While the present invention has been described hereinabove in relation to one preferred embodiment of the present spindle apparatus, those persons skilled in the relevant art will readily recognize and understand that various alternative embodiments and other variations are possible. For example, the present spindle apparatus may utilize other forms of tool holder clamping assemblies than the ball-type clamping assembly  64  in the embodiment of  FIGS. 3-8 . Specifically, the spindle apparatus may equally well employ other commercially available clamping systems which utilize fingers or other gripping elements instead of balls, as is depicted as an additional exemplary embodiment in  FIGS. 14-15 . In  FIGS. 14-15 , like components that correspond to the components of the spindle apparatus of  FIGS. 3-8  are identified by corresponding reference numerals in the 100 series of numerals. 
         [0048]    The spindle apparatus of  FIGS. 14-15  is designated overall at  150  and includes a main housing  152 , front and rear bearing housings with respective bearing sets (only front housing  154  with front bearing set  158  being shown), a solid spindle shaft  162 , and an actuator assembly  166  (comprised of front and rear pistons  185 ,  186 , piston retaining rings  188 , a sensor disc  196 , piston return springs  194 , and connecting pin  184 ), substantially as in the embodiment of  FIGS. 3-8 . The spindle apparatus  150  differs primarily only in the structure of its clamping assembly  164 , specifically in the configuration and arrangement of the clamping housing  176 , the clamping bolt  178  and the provision of clamping fingers  175  instead of clamping balls  75 . In this embodiment of clamping assembly  164 , the clamping housing element  176  is a hollow body of reduced lengthwise dimension fitted rigidly into the forward end of the cavity  162 A in the spindle shaft  162  and carries internally a plurality of the clamping fingers  175  in an annular array projecting forwardly through an opening in the front end of the housing  176 . The clamping bolt  178  extends slidably through the clamping housing  176  and its clamping fingers  175  and is formed of two bolt elements  178 A,  178 B threaded together in axial alignment. The rearward bolt element  178 B is formed at its rearward end with an enlarged shoulder  182  through which is formed a radial bore to receive the connecting pin  184 . The forward end of the bolt element  178 B is threaded into a bore formed in the rearward end of the forward bolt element  178 A. A spring  180  encircles the clamping bolt  178  between the clamping housing  176  and the shoulder  182 , to urge the clamping bolt  178  rearwardly into a home position retracted relative to the clamping housing  176 . The threaded connection between the bolt elements  178 A,  178 B enables a selective preloading of the spring  180 . The forward end of the bolt element  178 A is formed with a radially enlarged tapering cam surface  177  which acts on the clamping fingers  175  to move them radially outwardly into clamping engagement interiorly of the tool holder  165  when the bolt  178  is withdrawn rearwardly into its home position by the actuator assembly  166  ( FIG. 14 ) and relieves clamping force on the fingers  175  to allow radially inward movement for disengagement of the tool holder  165  when the bolt  178  is extended forwardly by the actuator assembly  166  ( FIG. 15 ). 
         [0049]    Many other variations of the present spindle apparatus are also possible. For example, it is contemplated that springs  94  may not be required to return the front piston to its home position, but instead the pneumatic actuation of forward piston movement via application of compressed air between the actuator pistons may be switched to application of a vacuum in order to execute rearward return piston movements. Additionally, although compressed air is used in the described embodiments to operate the actuator assembly, hydraulic or electrical power may alternatively be used to operate the actuator assembly. It is additionally contemplated that other configurations of the actuator pistons may be utilized. For example, as illustrated in  FIG. 11 , the rear actuator piston  286  may be of a configuration to wrap around the inside and outside diameters of the front piston  285  and may then be utilized to perform the additional function of the front bearing housing  54 . In such an embodiment, the rear actuator piston  286  can be held in place by fasteners connected to the spindle housing  52 . 
         [0050]    An alternative arrangement of the actuator assembly and sensor disc is shown in  FIG. 12 . In this variation, the sensor disc  296  extends through the rear piston  286  toward the rear of the spindle apparatus. The sensor disc  296  could also be located between the rear piston  286  and the shaft  62 , which could be advantageous if it is necessary to have additional electrical sensors or an encoder behind the actuator assembly and also could facilitate even more compact spindle designs. 
         [0051]    The present invention also is not limited to electric motor spindle apparatus. Since the shaft is solid and all components (actuator, sensors, etc.) necessary to execute automatic tool change operations are consolidated to a forward location relative to the shaft, a much broader range of applications is conceivable. Thus, the invention may be adapted to operate with other machines and power sources by fitting a machine coupling behind the rear piston. For example, the present apparatus could be fitted onto 50/60 Hz standard industrial motors, servo motors, or air motors. 
         [0052]    Those persons skilled in the art will thus recognize and understand that the invention is susceptible of broader utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, it is to be understood that the foregoing disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof.