Abstract:
An exemplary embodiment of a high speed spindle for rotating a tool at high rotational rates includes a hollow spindle body and a cylindrical spindle shaft configured for rotation within the spindle body and having a front end and a rear end. The spindle shaft includes a front hollow region and a rear hollow region. A motor is configured to impart rotational drive forces to the spindle shaft during operation. The spindle shaft includes a thrust bearing flange disposed intermediate the front end and the back end of the spindle shaft. A front air bearing and a rear air bearing are configured to radially support the spindle shaft for high speed rotational movement. A thrust air bearing is disposed between the front air bearing and the rear air bearing within the spindle housing and configured to act on the thrust bearing flange of the spindle shaft to constrain axial movement of the spindle shaft within the spindle housing. A collet is disposed within the front hollow region of the spindle shaft. An exemplary embodiment of a collet involves a centrifugal collet of unitary sleeve structure and having a plurality of flexures and weights.

Description:
BACKGROUND 
   There are two types of spindles on the market, i.e. conventional and reciprocating spindles. Conventional spindles cover a wide range of revolutions per minute (RPM) up to 300,000 RPM, and provide only rotation to the drill bit, not a feeding motion into the work piece, which is provided by a system to move the spindle. These spindles are typically relatively heavy, e.g., weighing between 4 and 7 kilograms, and require considerable energy to accelerate and decelerate in a feeding movement. The present trend towards small drill bit diameters demands good machine stability which is very difficult to obtain with high weight spindles. Reaction forces from accelerating and decelerating a heavy spindle generate vibrations on the machine which are detrimental to small-hole drilling. 
   Reciprocating spindles provide both rotation to the drill bit as well as a feeding motion. The reciprocating spindle typically has a very low moving mass since only the shaft and driving coil are moving, e.g., in the neighborhood of 0.45 kilograms. This is well suited for small-hole drilling because it generates very small forces during drilling; however it has drawbacks. One is that the stroke of the spindle is limited in contrast to the conventional spindle where it can be as large as required. Secondly it is difficult to run high RPMs with any practical stroke. 
   The majority of PCB drilling machines use spindles with air actuated spring loaded tapered collets. This solution is very complex, contains 20 to 35 parts, and is difficult to balance, expensive to manufacture and hard to maintain low run-out. In addition the taper collet configuration is affected by centrifugal force which causes it to reduce the grip on the drill bit as RPMs increase. 
   There are many existing centrifugal collets. Examples of collets are disclosed in U.S. Pat. No. 6,443,462B2, and U.S. Pat. No. 5,997,223. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view, taken along line  1 - 1  of  FIG. 2 , of an exemplary embodiment of a spindle assembly. 
       FIG. 1A  is a cross-sectional end view taken along line  1 A- 1 A of  FIG. 1 . 
       FIG. 2  is an end view of the spindle of  FIG. 1 . 
       FIG. 3  is a cross-sectional view, taken along line  3 - 3  of  FIG. 2 , of the exemplary embodiment of the spindle assembly of  FIG. 1 . 
       FIG. 4  is a cross-sectional view, taken along line  4 - 4  of  FIG. 2 , of the exemplary embodiment of the spindle assembly of  FIG. 1 . 
       FIG. 5  is an isometric view of a centrifugal chuck. 
       FIG. 6  is an end view of the chuck of  FIG. 5 . 
       FIG. 7  is a cross-sectional view of the chuck of  FIG. 5 , taken along line  7 - 7  of  FIG. 6 . 
       FIGS. 8 and 9  are diagrammatic views conceptually illustrating motion of collet weights and clamping forces exerted by an exemplary collet weight and flexure of an exemplary embodiment of a collet. 
   

   DETAILED DESCRIPTION 
   In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. The figures may not be to scale, and relative feature sizes may be exaggerated for illustrative purposes. 
   An exemplary embodiment of a spindle is miniaturized, to reduce the weight and to facilitate use of non-exotic materials for the non-magnetic spindle shaft. Suitable materials for the spindle shaft which are non-magnetic materials, for the case in which the spindle shaft is driven by a permanent magnet DC motor, include non-magnetic steel or an INCONEL.® Other materials which could be used for the shaft are ceramics, as described in U.S. Pat. No. 5,997,223, and beryllium, although these materials may be more expensive and have other drawbacks. By decreasing the shaft diameter from larger diameters typically used for conventional spindle shafts, to smaller diameters, such as 10 mm, for example, standard materials may be used for the spindle shaft. The air bearing configuration of this exemplary embodiment provides improved load distribution by locating thrust bearings together with the motor in between radial air bearings, instead of locating a thrust bearing on the back of the spindle. This reduces the cantilever effect. The cantilever effect may be further reduced by eliminating a protrusion of the spindle shaft from the spindle body. 
   In an exemplary embodiment, a front radial air bearing is combined with a thrust bearing, and can be assembled with the shaft and tested as a unit and later inserted into the spindle body for final assembly. In an exemplary embodiment, the spindle may be configured in such a way that wires from the motor rotating the spindle are brought out directly through the rear radial bearing without any need of manipulating them by winding them through the body of the spindle. This simplifies the assembly greatly and decreases the cost of building spindles. 
   An exemplary embodiment of a spindle utilizes a permanent magnet DC motor, with a permanent magnet mounted in a bore of the spindle shaft. An advantage of this approach is that centrifugal forces of the rotor are dissipated by the spindle shaft in contrast to other designs where the rotor is attached to the outer diameter of the spindle shaft and special devices need to be employed to retain centrifugal forces during high RPM operation. Another advantage of a DC permanent magnet motor is that the motor RPM can be derived from the internal motor windings, without the need of a special RPM sensor. Alternatively, the spindle shaft may be driven by an AC motor, or by other drive systems, such as a pneumatic system. 
   Turning now to  FIGS. 1-4 , an exemplary embodiment of a spindle  50  is illustrated. FIGS.  1  and  3 - 4  are longitudinal cross-sections of the spindle  50 , showing features of the spindle. The spindle includes a hollow outer spindle body  52 , a rear end cap  54 , a front air bearing  70  and a rear air bearing  72 . A spindle shaft  60  is mounted for rotation in the spindle body. 
   The spindle shaft  60  has a rear interior bore  60 A; the permanent magnet  92  of the DC motor is mounted in the middle of the bore, e.g. by cementing it in place. In an exemplary embodiment, the magnet is a rare earth magnet. Exemplary dimensions of the magnet include an outer diameter 6.5 mm and a length of 12.5 mm. It is noted that rare earth magnet DC brushless motors are generally known in the art, as well as techniques for driving the motors. 
   On the front end of the spindle shaft  60 , a drill bit depth adjustment screw  62  is installed between the rear bore  60 A and a front bore  60 C. In the front bore  60 C of the spindle shaft, a centrifugal collet  150  is installed and secured by a shaft cap  66  which is inserted into the end of the front bore  60 C, i.e. at the spindle nose. The shaft cap  66  may be fabricated of a material such as tool steel, and may be used also for balancing the front of the rotor. Adjustments in the dimensions of the bore  60 A at the rear of the spindle shaft may be used to balance the other end of the shaft; e.g., material can be removed from the bore  60 A to balance the rear end of the shaft. Material can be removed from the outer surface of the center of the spindle shaft to balance the middle of the shaft. By doing this, three plane balancing may be achieved. 
   A function of the shaft cap  66  is to guide the tool  10 , e.g. a drill bit or router tool having a tool shank  12  and a working end  14 , being inserted during a tool changing operation by a tool changer into the collet  150  without damaging it and center the drill shank  12  in respect to the spindle bore. In an exemplary embodiment, the shaft cap  66  has a push-fit into the front bore  60 C of the spindle shaft and is secured in place with an adhesive. 
   The spindle shaft  60  is supported for rotation within the spindle body  52  by front and rear radial air bearings  70  and  72 . The rear air bearing  72  provides a hole  54 - 1  ( FIG. 2 ) for the spindle motor wire  96  passage from motor stator windings  94  to a motor drive circuit (not shown). The air bearings are mounted in the outer spindle housing  52 , and air and water passages are sealed and separated by O-rings  76 . The air inlet port  120  ( FIGS. 1 and 2 ) for the air bearings is connected to a source of pressurized air (not shown). 
   A rear thrust bearing structure  84  of the thrust bearing  80  works in conjunction with a thrust bearing flange  60 B on the spindle shaft  60 . The working gap of the thrust bearing is established by a thrust bearing spacer  82 . The radial bearings and thrust bearings include orifices  70 - 4  and  70 - 5  for air distribution. Brass plugs  88  are used to close excess holes to create proper air and water distribution. The front air bearing  70  is secured to the face of the spindle housing  54  by four screws  70 - 1 . 
   Exemplary length and diameter dimensions for the spindle shaft for an exemplary embodiment are 75 mm in length and 10 mm in diameter, a thrust bearing flange outer diameter of 19 mm, diameter, and a flange thickness of 1.9 mm. In an exemplary embodiment, the thrust bearing flange is located intermediate the front and rear ends of the shaft, in close proximity to the center of gravity of the spindle shaft. 
   In between the front and rear air bearings  70  and  72 , a stator and cooling jacket assembly  90  for the permanent magnet DC motor is mounted inside the outer housing  52 . O-rings  76  seal the water passages  106 ,  108 ,  110 ,  114  ( FIG. 4 ). Water distribution is built into the spindle housing through the holes  106 ,  108  (see  FIG. 4 ) drilled longitudinally along the wall of housing  52  and enter and exit through holes  102 ,  104  (see  FIGS. 2 and 4 ) in the rear radial bearing  72  and end cap  54 . Air distribution is built into the spindle housing through the hole  122  (see  FIG. 1 ) drilled longitudinally along the wall of housing  52 , with pressurized air entering through hole  120  (see  FIGS. 1 and 2 ) in the rear radial bearing  72  and end cap  54 , and being exhausted to the atmosphere through the air bearings. The passages are sealed by O-rings  76 . The end cap  54  houses the air and water fittings and also has an opening  54 - 1  ( FIG. 2 ) for bringing motor wires  96  outside the spindle from the stator coils  94 . The end cap is secured to the spindle housing by screws  54 A through the flange of the rear air bearing. 
   In an exemplary embodiment, the air bearings  70 ,  72  and the cooling jacket assembly  90  are fabricated of bronze, and the spindle outer housing  52  is fabricated of stainless steel. Other materials suitable for the purpose may alternatively be employed. The inner diameters of the air bearings and cooling jacket are sized in conjunction with the outer diameter of the spindle shaft to provide working clearances. 
   Collet 
   An exemplary embodiment of a collet  150  is illustrated in  FIGS. 5-7 . This embodiment may be manufactured separately by an electrical discharge machining (EDM) process, and all accuracy pertaining to run-out may be built into the collet before installing it into the spindle nose. The collet features are defined in a sleeve structure, fabricated from a cylindrical blank, which is inserted into the spindle nose, i.e. in the bore at the spindle shaft front end. The collet  150  has a plurality of flexures  170  defined by cut-outs of the sleeve blank from the inside diameter (ID) surface and the outside diameter (OD) surface of the sleeve, creating conditions which allow the flexures  170  to expand and allow the weights  160  to move outward during rotation. The maximum clamping force is controlled by the flexure and weight geometry when the moving weights of the collet reach the ID surface of the spindle shaft and cannot expand anymore. In addition the installation of the collet is facilitated because the weights can collapse with the flexures to conform to the ID surface of the spindle shaft front end during installation. 
   An exemplary embodiment of the blank of the collet sleeve is made from tool steel which has been heat treated and tempered. After the heat treatment, the outer surface of the collet sleeve blank is ground to a precise diameter and the interior bore defining an ID surface  152 A is ground and honed to ensure proper size and concentricity. This sleeve with its accurately determined ID and OD provide the finished accuracy of the collet. After the heat treatment and grind and hone operations, the EDM process may be carried out. 
   The geometry of the collet  150 , in an exemplary embodiment, has three weights  160  spaced symmetrically around the rotating axis of the spindle; the axis is indicated as  156  in  FIG. 7 . The weights are connected by flexures  170  which are defined by material removed from the sleeve blank by an EDM fabrication process. During the EDM process, in an exemplary embodiment, the OD and ID of the sleeve blank are not modified to maintain the integrity and symmetry of the collet. The weights and flexures are defined during the EDM process by material removed from the sleeve blank to provide the functionality of the collet. 
   An exemplary embodiment of the collet may have the characteristic that the drill bit or tool  10  may be held in the spindle shaft end, i.e. the nose of the spindle, without the need of providing a special O-ring to hold the drill bit in place before rotating the spindle. The geometry of the weights and flexures may be designed in such a fashion that the weights expand in the bore of the spindle nose and clamps the drill bit shank at the same time. In an exemplary embodiment, the OD of the tool shank is larger than the ID of the collet. The collet retains the shaft of the tool at zero RPM, even without the use of an O-ring. With material removed both from the inner surface as well as the outer surface of the sleeve blank to fabricate the collet, the inner surfaces of the collet will be deflected outwardly as the tool shank is inserted into the collet, thus providing a clamping force on the tool shank. The clamping force increases with RPM; therefore the strongest clamping is on the highest RPM, until the weights move outwardly enough to contact the interior surface of the spindle shaft in which the collet is installed. 
   The geometry of the weights  160  is designed in such a way that the clamping force is multiplied by leverage. The leverage is illustrated in  FIGS. 8 and 9 . Here, circles  202  and  204  represents the outer and inner surfaces of the sleeve blank from which the collet is to be fabricated. Line  206  represents a line along which material is removed by the fabrication process, e.g. by an EDM process, defining a single weight  160  and a flexure  170 .  FIG. 8  represents the collet at rest, and  FIG. 9  represents the collet in rotational motion;  FIG. 9  is exaggerated to visibly illustrate deflections. The center of gravity of the weight  160  is shown at  160 - 1 . As the collet is rotated, the center of gravity of the weight will move outwardly, as depicted conceptually in  FIG. 9 . The point  154  on the outer surface of the collet acts as a response point, and is typically in contact with the inner surface of the bore in the spindle shaft. The motion of the weight  160  outwardly tends to cause region  152 A- 1  to be deflected inwardly, thus applying a clamping force on the tool shank. The leverage results from the distance of the center of gravity from the response point  154 . The thickness of the flexures  170  controls the stiffness of the device and the shape of the weights  160  control the clamping force. By varying these two parameters, the collet can be designed to clamp on lower RPMs or on higher RPMs. The clamping forces are exerted along longitudinally extending lines or surfaces  152 A ( FIGS. 5 and 6 ) which extend along the entire longitudinal length of the collet, not just on points or small areas of the tool shank, and this contributes to reduction in runout errors. 
   In an exemplary embodiment, the collet  150  is mounted inside the front bore  60 C of the spindle shaft  60 , and a shaft cap  66  is installed at the front of the spindle shaft bore to protect the collet during insertion of drill bit by the tool changer. On the back of the collet in the spindle shaft there is a precise bore  60 D to accept the drill bit shank  12  after it passes the full collet length. On the back of the bore to accept the drill bit shank there is an adjustable stop screw  62  to establish the proper protrusion of the drill bit point from the spindle shaft end. The shaft cap  66  is also used for final balancing purposes; material can be removed from its face for final balancing. 
   An exemplary embodiment of the collet  150  may be field exchangeable, i.e. replaceable, in the spindle  50 , by first removing the cap  66 , and withdrawing the collet  150  from the bore in the shaft. Another collet may be assembled to the spindle by reversing the procedure. The collet is assembled into the spindle shaft with a slip interference fit; post assembly grinding or machining is not performed. In an exemplary embodiment, the collet is small and of relatively small mass, and so replacement of the collet with another collet will not appreciably affect the balance. An exemplary collet embodiment may have OD and ID dimensions of 6.35 mm and 3.175 mm, respectively, and a length of 7.6 mm, with a total mass of 1.41 grams. Of course, other embodiments of a collet may have different dimensions and mass characteristics. 
   Benefits of exemplary embodiments of the spindle architecture may include one or more of the following: very low weight, high RPM, low weight and small size, economical to manufacture, single part centrifugal collet, small size centrifugal collet to minimize centrifugal forces, run-out control built into collet—not machined on assembly, replaceable collet, large Z-axis stroke to accommodate all drilling machine functions, minimization of shock waves on drilling machine during Z operation, and very low run-outs on drill bits. An exemplary embodiment of the spindle is configured for high speed use, e.g., at least 400,000 RPM, and may have a weight of 1000 grams, e.g. about 930 grams in one exemplary embodiment. Exemplary applications for the spindle include drilling and micromachining in the printed circuit board industry. 
   Although the foregoing has been a description and illustration of specific embodiments of the subject matter, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims.