Abstract:
A machine tool assembly includes a high speed rotating spindle for forming extremely precise surface contours on a work piece. The spindle is supported on a spherical air bearing about its center of gravity. The spindle can be rotated through an air turbine drive system incorporated into the spherical air bearing to eliminate any undesirable moments about an axis perpendicular to the long spindle axis. The spindle is adjustable in pitch and yaw directions through the influence of X-Y actuators. The X-Y actuation system is preferably electromagnetic and provides a non-contact method of displacing the spindle shaft in a controlled manner at any speed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/666,674 filed Mar. 30, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     This invention relates generally to high-speed spindle assemblies for shaping a non-circular hole in a work piece.  
         [0004]     2. Related Art  
         [0005]     Some production applications require the formation of non-circular holes. For example, in the manufacture of pistons for an internal combustion engine, the so-called pin holes formed for the wrist, or gudgeon, pin often have a slight trumpet shape opening toward the center for accommodating flex in the wrist pin. Additionally, this trumpet shape of the pin hole is optimally designed with a slight ovality to further account for distortion in the wrist pin as the piston moves through its various cycles. This trumpet shape and non-circular cross section of the pin hole must be formed to exacting tolerances. For example, tolerances in the range of 3 to 5 microns are often required for these types of applications.  
         [0006]     There are industrial methods for creating such non-circular holes to exacting tolerance specifications, including hydraulically actuated milling tools and electro-chemical machining techniques. The prior art has also suggested boring non-circular shapes using a special machine tool spindle equipped with active magnetic bearings. Active magnetic bearings act upon the principle that the rotating spindle is formed of, or with, a ferromagnetic material that is supported in a magnetic field generated by an electromagnet stator. A control system, with appropriate power amplifiers, modulates the magnetic field to maintain the spindle in the desired radial position while it rotates. This radial position can be maintained even under changing load conditions.  
         [0007]     While active magnetic bearing systems provide exceptional spindle mobility, they can be expensive to produce and difficult to control. In order to achieve high-dynamic performance and acceptable levels of tool acceleration, it is necessary to provide very large, powerful magnetic bearing actuators. Furthermore, active magnet bearing systems can be difficult to dampen properly in some cutting conditions and if not properly sized and controlled.  
       SUMMARY OF THE INVENTION  
       [0008]     According to the subject invention, a machine tool assembly of the type having a pitch-and-yaw adjusting spindle is provided for forming high precision surface contours on a work piece. The assembly comprises a spindle defining a long axis and having a shaping tool extending from one end thereof. The spindle includes a bearing journal. A journal box at least partially envelopes the bearing journal for rotatably supporting the spindle. A drive motor operatively interacts with the spindle for forcibly rotating the spindle about its long axis. At least one X-Y actuator is provided for controlling pitch and yaw of the spindle to move the shaping tool in a non-circular orbital path. The bearing journal and the journal box have concentric, generally spherically opposing surfaces centered about a center point intersecting the long axis of the spindle.  
         [0009]     According to another aspect of the subject invention, a method is provided for magnetically manipulating a high speed spindle assembly for forming an irregular hole with a dimensionally varying axial trajectory in a work piece. The method comprises the steps of affixing a radially extending shaping tool to one end of a spindle having a long axis, supporting the spindle for rotation about the long axis, rotating the spindle about the long axis, creating a magnetic field that influences at least part of the rotating spindle, and adjusting the yaw and pitch angulations of the long axis during rotation of the spindle by manipulating the magnetic field to thereby move the shaping tool in a predetermined, non-circular orbital path. The step of supporting the spindle for rotation about the long axis further includes confining the spindle in a spherical bearing centered about a center point intersecting the long axis of the spindle.  
         [0010]     The subject invention therefore comprises a hybrid between the prior art active magnetic bearing systems and the prior art fixed bearing designs that allows free form holes to be machined and provides a cost benefit and improved performance over the prior art systems. The subject orbiting spindle is based, preferably, on a spherical air bearing that allows an X Y actuator system to introduce off-center motions in the spindle about its center of gravity. This provides a cost advantage over electromagnetic bearings as electromagnetic spindles have typically 10 axes to control. However, the subject hybrid orbiting spindle has only two axes to control, yet it is capable of producing substantially the same motions.  
         [0011]     The subject invention allows the spindle to rotate about its axis and to also pitch and yaw in a highly controlled manner. The spherical bearing&#39;s center of rotation may be placed at the center of gravity for the spindle to insure that the mass of the spindle is reacted by the air bearing and not by the X Y actuation system, thus providing more dynamic force capabilities for following complex precise orbits. The pitch and yaw of the spindle can be controlled with respect to its rotational position. By changing the X Y actuator position, the tool tip of the spindle can be orbited to produce any free form shape.  
         [0012]     In the preferred embodiment, the electromagnetic X Y actuation system provides a non-contact method of displacing the spindle shaft at any speed. There are several methods of providing the rotation to the spindle, including an air turbine, an electric motor, or a non-contact rotary coupling, to name a few. In the example of the air turbine embodiment, the air turbine may be incorporated into the spherical air bearing and thus eliminate any moment about the axis perpendicular to the spindle long axis. If an air turbine is used, the mass of the spindle can be reduced, thereby allowing for better profile performance and better acceleration time to achieve the desired rotational velocity. Displacement sensors provide feedback for the position of the X-Y actuation system and allow the orbit to be measured.  
         [0013]     The hole forming assembly of this invention overcomes the disadvantages and shortcomings of the prior art by expanding the available range of hole shapes and configurations, particularly in a three-dimensional sense, which can be formed with great accuracy at high speeds.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:  
         [0015]      FIG. 1  is a simplified front elevation of a spindle assembly according to the subject invention and an exemplary piston work piece prepared for a hole forming operation;  
         [0016]      FIG. 2  is a simplified cross-section of an orbiting spindle assembly according to the subject invention;  
         [0017]      FIG. 3  is a cross-sectional view taken along lines  3 - 3  of  FIG. 2  depicting the X-Y actuation system;  
         [0018]      FIG. 4  is an alternative view of the spindle assembly wherein the drive motor is spaced from the center of gravity;  
         [0019]      FIG. 5  is an alternative view of the spindle assembly wherein the X-Y actuator system is incorporated into the spherical air bearing;  
         [0020]      FIG. 6  is a cross-section taken along lines  6 - 6  of  FIG. 5  showing a companion X-Y actuation system;  
         [0021]      FIG. 7  is an alternative view of the assembly wherein the X-Y actuator system is incorporated into the air turbine drive system;  
         [0022]      FIG. 8  is a cross-section along lines  8 - 8  of  FIG. 7 ;  
         [0023]      FIG. 9  is a simplified perspective view illustrating an exemplary hole geometry in a work piece in which a continuously axially varying trajectory is created by the shaping tool;  
         [0024]      FIGS. 10 and 11  illustrate exemplary non-circular hole geometries, each having an irregular load configuration of which the subject invention is capable of producing;  
         [0025]      FIGS. 12 and 13  illustrate alternative shaping tool configurations.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     Referring to the Figures, wherein like numbers indicate like or corresponding parts throughout the several views, a high-speed spindle assembly is generally shown at  20  in  FIGS. 1 and 2 . The spindle assembly  20  is of the type for forming non-circular holes  22  in a work piece  24 . In  FIG. 1 , the work piece  24  is shown for purposes of example only comprising a piston for an internal combustion engine. The non-circular hole  22  is illustrated as the pin hole for containing the so-called wrist pin (not shown). The work piece  24 , however, may comprise any component and is not limited to pistons, engines or even automotive applications. Rather, any field of endeavor in which a non-circular hole  22  of high precision tolerance may benefit from the subject invention.  
         [0027]     The assembly  20  includes a rigid shaft-like spindle, generally indicated at  26 , extending along an long axis A between a rear end  28  and a shaping end  30 . A shaping tool  32  extends radially outwardly from the spindle  26  adjacent it&#39;s shaping end  30 , and terminates in a cutting edge  34 . The point-like shape of the cutting edge  34  is an exemplary depiction only, however, as the actual cutting edge  34  of the shaping tool  32  could comprise a removable carbide (or other material) tip of any commercially available shape. In another variation, the shaping tool  32  may be held in a removable tool holder which is secured to the spindle  26  via a taper and bolt arrangement. The spindle  26  is provided with wrench flats  40  adjacent its rear  28  and shaping  30  ends to facilitate removal of the shaping tool  32  for maintenance and tool holder  36  interchanges.  
         [0028]     The assembly  20  further includes a housing  42  through which the rear  28  and shaping  30  ends of the spindle  26  extend. The spindle  26  is rotatably supported in the housing  42  by a spherical air bearing assembly, generally indicated at  120 . The spherical bearing assembly  120  includes an enlarged, spherical bearing journal  126  attached to the spindle  26 , preferably coincident with the center of gravity of the rotating spindle assembly. In any event, the center of the spherical surface which forms the bearing journal  126  lies along the long axis A. The bearing journal  126  is contained within a journal box taking the form of spherical bearing cups  128 . Pressurized air is pumped into the interface between bearing journal  126  and the journal box  128  to levitate the spindle  26  for high-speed rotation, e.g., on the order of 100,000 RPM.  
         [0029]     Bearing air is introduced to the interface between individual bearing cups in the journal box  128  and the bearing journal  126  through one or more inlets  132  as shown in  FIGS. 1 through 5 . While discrete inlets  132  are illustrated, acceptable results can be achieved by the use of porous ceramic bearing cups. In this configuration, air permeates through the bearing cups to levitate the journal bearing  126 . Another example of air introduction for the bearing feature includes the use of hydrostatic pockets, or depressions, formed in the surface of the journal box  128  into which air from the inlets  132  is directed. Regardless of the method used to introduce air to the interface between the bearing journal  126  and the journal box  128 , the spindle  26  is supported by a cushion of air within the spherical bearing assembly  120  and allowed to wobble as it rotates at high speeds.  
         [0030]     A drive motor, generally indicated at  44 , is disposed within or outside of the housing  42  and operates to forcibly rotate the spindle  26  about its long axis A. The drive motor  44  may be of any known variety, operating on either AC or DC current, fluid, air or any other type of energy source. In the examples shown, the drive motor  44  comprises an air turbine. A turbine air inlet  122  directs a controlled stream of pressured air at an impeller  124 . The impeller  124  can be integrally formed about the equator of the spherical bearing journal  126  as shown in  FIGS. 2 and 7 . When the air turbine is integrated into the bearing journal  126 , undesirable moments are eliminated to help stabilize spindle rotation at high speed. Pressurized air flowing through the inlet  122  acts upon the impeller  124  and causes the spindle  26  to rotate within the housing  42  about the long axis A. Air is exhausted from any convenient location, such as via outlet  130  shown in  FIG. 2 .  
         [0031]      FIG. 4  illustrates an alternative drive motor arrangement, generally indicated at  44 ′. Here, an air turbine assembly having an inlet  122 ′ acts upon the impeller  124 ′ which is formed about the spindle  26  at a location remote from the spherical bearing assembly  120  and remote from the center of gravity of the spindle. In this example, the hybrid orbiting spindle assembly  20  functions in the same manner, however, the drive mechanism for rotating the spindle  26  at high speeds is different. Those skilled in the art will appreciate other arrangements for the drive motor, which may include an electric motor or other devices.  
         [0032]     Referring again to  FIGS. 1 through 5 , the assembly  20  is shown including a pair of Y-axis actuators  134  positioned on opposing sides of the spindle  26  at a distance from the spherical bearing assembly  120 . Similarly, X-axis actuators  136 , oriented perpendicular to the Y-axis actuators  134 , are also positioned on opposite sides of the spindle  26 . These actuators  134 ,  136  are oriented in a common plane and can be selectively energized through an appropriate control mechanism to urge the spindle  26  with an electromagnetic attractive force. By intentionally and variably energizing the actuators  134 ,  136 , acting like moment arms, the shaping tool  34  can be forced to move in a controlled path, either circular or non-circular. Although the figure suggests that the Y-axis actuators  134  are located in a generally vertical plane, whereas the X-axis actuators  136  are located in generally horizontal plane, this is not necessary. In some cases, it may be desirable to orient the respective axes at an approximate 45 degree angle relative to the horizontal. Also, although four actuators  134 ,  136  are shown, it is possible to accomplish the desired articulation of the shaping tool  34  using three actuators spaced in generally equal arcuate increments around the long axis A.  
         [0033]     Position sensors  138 ,  140  are associated with the Y-axis actuators  134  and X-axis actuators  136 , respectively. The position sensors  138 ,  140  operate by feeding information about the position of the spindle  26  in the form of an electrical voltage. Normally, these position sensors  138 ,  140  are calibrated so that when the spindle  26  is in a neutral position, the sensor produces a null voltage. When the spindle  26  moves above the neutral position, a positive voltage is produced. When the spindle  26  moves below the neutral position, a negative voltage results.  
         [0034]     A controller (not shown) independently controls each of the actuators  134 ,  136  to adjust the radial position of the spindle  26  as it is levitated on a cushion of air within the spherical bearing assembly  120 . By controlling the spindle  26  position through the actuators  134 ,  136 , the shaping end  30  can be articulated and caused to scribe a highly controlled, non-circular orbital path. The controller may be of the centralized type coordinating inputs from all of the sensors  138 ,  140  and issuing outputs to all of actuators  134 ,  136  to achieve the desired articulation of the shaping end  30 . Alternatively, the controller may include separate components independently controlling for the X-axis and the Y-axis. In this latter configuration, one controller for the X-axis actuators  136  would receive voltage signals from the X position sensors  140 , process this information with a mathematical model including dimensional relationships such as axial distance to the cutting edge  34  and the tool radius measured from long axis A to the cutting edge  34 , and send current (or voltage) requests to an integrated or stand-alone amplifier. Thus, the controller would receive multiple inputs, i.e., inputs from every sensor in the X-plane, and issue multiple outputs to all of the actuators  136  in the X-plane to dynamically control the spindle  26 .  
         [0035]     The X-axis controller may include anti-aliasing filters, analog-to-digital converters, a digital signal processor, and pulse-width modulation generators. Voltage from the position sensors  136  would be passed through the anti-aliasing filters to eliminate high frequency noise from the signal. After the high frequency content is removed, the position signal is sampled by an analog-to-digital converter which converts the voltage signal to a form that can be processed by a digital signal processor. The digital information is then passed through a digital filter and produces an output proportional to the amount of current (or voltage) required to correct or adjust the position of the spindle  26  according to a predetermined value. The requested current is compared to the actual current supplied to the actuators  136 , which is also sent, filtered, and sampled with an analog-to-digital converter. The error between the actual and requested current is used to characterize the pulse-width modulation signal sent to the amplifiers. This information is then sent to the pulse-width modulation generators which creates the pulse-width modulation wave forms sent to the amplifiers. The Y-axis controller would work in a similar fashion receiving multiple input signals from the Y-position sensors  138  and issuing multiple corrective actions via outputs to the Y-axis actuators  134 . A detailed description of a suitable control system may be found in U.S. Ser. No. 11/065,618 filed Feb. 24, 2005 and assigned to the assignee of the subject application, the entire disclosure of which is hereby incorporated by reference.  
         [0036]     A rotary position sensor (not shown), in the form of a rotary encoder, would be incorporated into the assembly  20  for determining the angular position of the spindle  26 , and thus the angular position of the shaping tool  32  about the long axis A. The rotary encoder would communicate with the controller(s) to enable coordinated adjustments of the actuators  134 ,  136 .  
         [0037]     An axial motion controller is schematically represented at  104  in  FIG. 1 . The axial motion controller  104  moves the shaping tool  32  relative to the work piece  24  in directions generally parallel with the long axis A while the shaping tool  32  simultaneously forms the non-circular hole  22 . At the same time, the X-Y controllers manipulate the actuators  134 ,  136 , thus dimensionally varying the axial trajectory of the hole  22  in the work piece  24 . The axial motion controller  104  can operate by holding the work piece  24  stationary and translating the spindle assembly  20 , or as shown in  FIG. 1  may include a work piece holder  106  which is moved relative to a stationary spindle assembly  20 . Alternatively, both components can be moved at the same time relative to a fixed point of reference.  
         [0038]      FIGS. 5 and 6  illustrate an alternative configuration of the actuators  134 ′,  136 ′ wherein they are integrated into the spherical bearing assembly  120 . These integrated actuators  134 ′,  136 ′ are responsive to a permanent ring magnet  142  (or steel ring) embedded in the bearing journal  126 . The actuators  134 ′,  136 ′ thus introduce torques within the bearing journal  126  at precisely controlled times and in precisely controlled amounts to accomplish a predetermined orbital path in the cutting tool  32 .  
         [0039]      FIGS. 7 and 8  illustrate yet another variation of the actuators  134 ″,  136 ″ wherein the air turbine  124 ″ is enlarged and formed of a magnetically reactive material. Here, the X-Y actuators  134 ″,  136 ″ are arranged on opposing sides of the rim-like turbine  124 ″ and react therewith to introduce controlled wobble in the spindle  26 . Those skilled in the art will likewise appreciate other alternative configurations of the actuator and drive motor features.  
         [0040]     This combination of axial, or longitudinal, movement coupled with a continuously varying orbital path enables creation of geometrically complex shapes such as that depicted in  FIG. 9 . Here, an exaggerated hole  22  is shown having a generally elliptical profile at the opening of the work piece  24 , wherein the ellipse has a generally vertical major axis  108 . As the hole  22  extends deeper into the work piece  24 , the size of the ellipse reduces, while the major axis  108  is rotated in a clockwise direction. This is represented by the phantom elliptical cross-section at the midpoint  110 . As the hole  22  continues deeper into the work piece  24 , the shape of the hole  22  enlarges while the major axis  108  continues to rotate clockwise until reaching a termination point  112  wherein the major axis  108  of the elliptical shape is now generally horizontal. The complex, dimensionally varying axial trajectory of the hole  22  in the work piece  24  is not limited to the exemplary configuration shown in  FIG. 0 .  FIG. 10  illustrates a non-elliptical, irregular hole  22 ′ of a 2-lobed cam-like shape.  FIG. 11  illustrates a multi-lobed shape of the hole  22 .″ Those skilled in the art will appreciate that nearly endless variety of shapes can be produced using the subject spindle assembly  20  and the multiple input-multiple output control strategy of the controllers.  
         [0041]      FIGS. 12 and 13  illustrate various alternative arrangements of the shaping tool  32 ′ wherein two or more cutting edges  34 ′ may be used. Alternatively, the cutting edge  34 ″ may repeat continuously around the shaping end  30 .″ In yet another embodiment, the cutting edge may take the form of a grinding disk or abrasive wheel (not shown).  
         [0042]     Although the exemplary embodiments of this invention have been described in connection with hole formation in the more traditional sense, those skilled in the art will appreciate that these novel techniques can be carried out on an external surface. Thus, shaping of the non-circular surface can be carried out on an exterior surface with only straight-forward modifications to the shaping tool  34 . Therefore, the invention contemplates a surface shaping methodology and device which can be used with equal effectiveness on holes and external features requiring non-round shapes with dimensionally varying trajectories.  
         [0043]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, using the above teachings in an inverse manner, it should be also possible to generate exceptionally round, highly cylindrical holes over relatively large axial travels. In a further example of the use of these teachings, the cutting tool can be replaced by a grinding tool thereby creating a novel precision grinder. It is, therefore, to be understood that the invention may be practiced otherwise than as specifically described.