Patent Publication Number: US-2023150038-A1

Title: High-speed spindle with forced mechanical vibratory assistance

Description:
PRIOR ART 
     The applicant has developed numerous solutions for vibration drilling, in which a cutting tool is subjected to axial oscillations while it rotates. 
     These oscillations make it possible to break up chips and improve the drilling performance. 
     To bring about the axial movement of the tool, numerous existing solutions are based on the use of rolling bearings, one or more raceways of which have an undulating surface. 
     The patents EP2790860 B1 and EP 2501518 B1 describe examples of vibration machining devices. 
     The rolling bearings are typically made up of balls which are held in angular position with respect to one another by a rotary cage during rotation. 
     In the known solutions, the frequency of the axial oscillations depends on the rotational speed and on the number of undulations experienced by the rolling bearings while they rotate. 
     The rotational speed of the tool depends on its cutting speed and on its diameter. Thus, the more the diameter of the holes to be drilled decreases, the more the rotational speed needs to increase to maintain an equivalent cutting speed. However, the frequency of the axial oscillations cannot exceed a threshold, which is around 300 Hz, without generating excessive mechanical loads, on account in particular of the inertia of the moving pans. The rotational speed of the known vibration drilling spindles, based on a mechanical conversion of the rotational movement into an axial vibrating movement, is thus generally limited to 10 000 rpm. 
     In some applications, numerous small-diameter holes need to be produced very rapidly, for productivity reasons. It is thus common to drive small-diameter drill bits, in conventional non-vibration drilling, at rotational speeds much greater than 10 000 rpm, for example around 20 000 rpm, in order to adhere to their cutting speed. For some materials, the chips generated during cutting at these rotational speeds, in conventional drilling, have a short length and are evacuated easily. 
     However, other materials generate longer chips during conventional drilling, and these cannot be evacuated as easily without involving chip-clearing cycles. Thus, the choice of materials that are able to be machined in conventional drilling remains limited without losing productivity, this proving to be a drawback in certain applications. 
     Although there exist purely mechanical vibration drilling solutions in which the choice of the frequency of the oscillations is decoupled from the rotational speed of the shaft, these being based on the use of electromechanical or piezoelectric elements, these solutions are much more expensive and complex than the purely mechanical solutions, and their implementation, if mechanically possible, remains economically unviable in many applications, in particular when it is desired to minimize the changes made to the pool of existing machine tools during the implementation of the vibration drilling solution. 
     DE102005002460 presents a drilling tool comprising an oscillating unit incorporating a rolling bearing of the “thrust ball bearing” type having a single ball rolling between a first and a second ring. Such a rolling bearing is not designed to operate at high rotational speeds on account of the centrifugation of the ball. A calibration spring produces a forward movement which keeps the rolling bearing under compression. 
     U.S. Pat. No. 3,088,342 describes an oscillating drilling tool having a rolling bearing of the “thrust ball bearing” type. The oscillating movement is achieved with a split ring which, given its arrangement and the step imposed on the ball, brings about an oscillating movement which exhibits a significant discontinuity. Therefore, such a device cannot operate at high rotational speeds on account of the mechanical wear and the vibrations that are brought about. 
     There therefore remains a need that has not yet been met, as far as the applicant is aware, to benefit from a compact spindle that is capable of rotating at a high rotational speed while subjecting a cutting tool to axial oscillations with a frequency suitable for reducing the length of the chips that are formed. 
    
    
     SUMMARY OF THE INVENTION 
     The invention aims to meet this need, and achieves this aim by virtue of a spindle for a machine tool, having
         a housing,   a shaft for driving a cutting tool, mounted rotatably inside the housing so as to be able to move axially relative to the housing,   a single ball, interposed axially between a rolling bearing ring that is fixed relative to the housing and a rolling bearing ring that is movable with the shaft, one of these rings defining an inclined rolling bearing surface that is not perpendicular to the axis of rotation of the shaft, such that the rotation of the ball brings about an axial oscillation of the shaft.       

     The use in the invention of a single ball for bringing about the axial oscillating movement of the shaft makes it possible to keep the frequency of the axial oscillations at a value compatible with the inertia of the parts to be moved, including for rotational speeds greater than 10 000 rpm. Moreover, the absence of a cage that rotates with the ball reduces the heating of the rolling bearing at high rotational speeds. The invention makes it possible, if desired, to create a compact vibration drilling spindle that is capable of replacing a conventional spindle without otherwise modifying the machine tool. 
     Preferably, the ball is partially fitted in an annular groove formed in the shaft. This makes it possible to reduce the distance from the axis of its center of gravity and therefore the imbalance associated with its rotation, and the bending moment exerted by the ball on the shaft. 
     Preferably, the fixed rolling bearing ring is the one that defines the inclined rolling bearing surface. The inclined rolling bearing surface is advantageously planar, making it possible to create it very easily with high precision and a good surface state, this being advantageous for minimizing friction between the ball and the rolling bearing rings. 
     Such an inclined rolling bearing surface does not have a step. The lack of a step limits the generation of vibrations and mechanical wear. 
     Advantageously, the axial cutting loads are at least partially reacted at the rolling bearing ring that is fixed relative to the housing. 
     Preferably, the ball is made of ceramic, making it possible to optimize the strength, density ratio. 
     It is preferred for the ball to be situated at the rear of the spindle. This limits the effect of the bending moment on the quality of guidance of the shaft at the tool. 
     The spindle preferably has two sets of ball bearings, respectively at the front and at the rear of the spindle. These rolling bearings are preferably angular contact, and flanged rolling bearings. The ball is disposed preferably behind the rear set. 
     The rolling bearings are preferably kept centered so as to be able to move axially by elastic strips with oriented deformation and of annular overall shape. The latter preferably have, on their outer circumference, fixing tabs that are fixed relative to the housing and, between these fixing tabs, tabs for retaining the rolling bearings, the flexibility of the portions of the strips extending between the fixing tabs and the tabs for retaining the rolling bearings allowing the rolling bearing to move axially during the axial oscillations of the shaft. The use of the strips provides an elegant solution to the problem of ensuring radial stiffness while allowing the axial movement that is necessary for the shaft to be able to oscillate axially. The strips have a high stiffness in the radial direction, but their small thickness allows them to bend in order to follow the axial movement of the rolling bearings. The strips may be superposed to increase radial stiffness, while maintaining axial flexibility. 
     The rolling bearings may be mounted on bearings that are rotationally indexed relative to the strips, preferably by pins passing through the strips, the bearings having sectors forming a protrusion on their end edge, against which the sectors the strips res, the strips being in contact with the outer rings of the rolling bearings via their retaining tabs. These sectors make it possible to immobilize the retaining tabs of the strips relative to the bearings while maintaining an axial clearance between the immobilized zones in order to allow the portions of the strips extending therebetween to bend in order to allow the axial movement of the rolling bearings relative to the housing during the axial oscillations of the shaft. 
     Flat springs may be present for pressing the strips against the outer rings of the rolling bearings. These flat springs may be left out, apart from that or those serving, where appropriate, as an elastic member for applying the axial preload on the shaft, as explained below. 
     The spindle has an elastic return member which returns the shaft toward the rear, during the rotation of the ball. This axial preloading of the shaft toward the rear is advantageously effected by at least one flat spring. The spindle may thus have at least one flat spring, or even a single flat spring, which exerts a return force toward the rear. This flat spring may be situated at the front or at the rear of the spindle. Placing it at the rear make it possible to avoid the introduction of a compressive force along a significant rotor length. The return force toward the rear of such an elastic member is advantageously at a maximum during non-zero cutting forces and relieved when the cutting forces are greater than 0. 
     The axial immobilization of the strips with respect to the housing may be effected in various ways, but very preferably, the strips are held at the fixing tabs with the aid of a series of spacers. Preferably, the spindle thus has a main tubular spacer, fixed relative to the housing, and fixed positioning rings disposed on either side of the main spacer, the strips having their fixing tabs gripped between the main spacer and the positioning rings. 
     The spindle preferably has bearing end rings on either side of the bearings, in which end rings the abovementioned pins are fitted, the flat spring or springs pressing against one end of these end rings, the other end bearing against a surface that is fixed relative to the housing. 
     The housing is preferably closed at the rear by an end piece against which the rolling bearing ring defining the inclined rolling bearing surface rests. 
     Preferably, the spindle has a peripheral rolling bearing ring, coaxial with the shaft, for reacting the centrifugal forces of the ball. Reacting centrifugal forces is particularly advantageous for drilling holes at rotational speeds greater than 10 000 rpm. 
     The ratio d ball /d path  is preferably between 0.4 and 0.6 where d ball  denotes the diameter of the ball and d path  that of the contact point of the ball with the inclined rolling bearing surface. 
     A further subject of the invention is a machining method, in particular a drilling method, in which the shaft of a spindle according to the invention is driven at a rotational speed of at least 10 000 rpm, for example between 15 000 and 30 000 rpm, in particular around 15 000 to 20 000 rpm. 
     A further subject of the invention is a machining method, in particular a drilling method, in which the shaft of a spindle according to the invention oscillates axially with a vibration frequency of between 0.4 and 0.6 axial oscillations per revolution, in particular around 0.5. 
     The spindle may undergo an advancing movement during the rotation of the shaft, in a conventional manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood better from reading the following description of nonlimiting implementation examples thereof and from examining the appended drawing, in which: 
         FIG.  1    schematically shows a perspective view of an example of a spindle according to the invention, 
         FIG.  2    is a longitudinal section through the spindle in  FIG.  1   , 
         FIG.  3    shows the rear of the spindle in more detail, 
         FIG.  4    shows the front of the spindle in more detail, 
         FIG.  5    shows a perspective view of an elastic strip on its own, 
         FIG.  6    shows a perspective view of a bearing end ring on its own, 
         FIG.  7    shows a perspective view of a bearing on its own, 
         FIG.  8    shows the ring having an inclined rolling bearing surface in axial section, and 
         FIG.  9    shows an embodiment variant of the spindle in longitudinal section. 
     
    
    
     DETAILED DESCRIPTION 
     The spindle  1  according to the invention, shown in particular in  FIGS.  1  to  4   , has a housing  10 , preferably metallic, with the overall shape of a cylinder of revolution about a longitudinal axis X. 
     The casing  10  is mounted in a guiding and advancing mechanism (not shown), known per se, of the machine tool. A support  11  fixed to the housing allows said mechanism to move the spindle  1  axially by the distance necessary to produce the drilled hole. 
     The spindle  1  has a shaft  20  which is intended to carry at the front a tool such as a drill bit (not shown) and which is coupled at the rear to a pulley  21  for driving it in rotation. 
     The drill bit has for example a diameter less than or equal to 2.5 mm. 
     The rotational speed of the shaft  20  is for example between 10 000 and 20 000 revolutions per minute. 
     The invention is not limited to a particular tool, or to the creation of the drilled hole. It may in particular prove useful to carry out machining operations such as milling operations, counterboring operations, etc. 
     The shaft  20  is guided in rotation about the axis X relative to the housing  10  by sets of front rolling bearings  30  and rear rolling bearings  40 . 
     The set of front rolling bearings  30  has two angular contact ball bearings  31 , the angle of contact of which is for example 15°, each having an inner ring  32 , in contact with the shaft  20 , balls  33 , an outer ring  34  and flanges  35 . The rolling bearings  31  bear against one another and are fitted in a front bearing  50 . 
     The set of rear rolling bearings  40  is embodied in a similar way, with two angular contact rolling bearings  41 , the angle of contact of which is for example 15°, each having an inner ring  42 , in contact with the shaft  20 , balls  43 , an outer ring  44  and flanges  45 . The rolling bearings  41  bear against one another and are fitted in a rear bearing  51 . 
     The inner ring  42  of the rearmost rolling bearing comes to bear axially against a shoulder  23  of the shaft  20 , as can be seen in particular in  FIG.  3   . 
     A tubular inner spacer  24  is mounted on the shaft  20  between the sets of front rolling bearings  30  and rear rolling bearings  40 , and comes to bear at its ends against the inner rings  32  and  42  of the corresponding rolling bearings. 
     A blocking ring  70  is fixed to the shaft  20  at the front and immobilizes the inner rings  32  of the rolling bearings  31 , the inner spacer  24  and the inner rings  42  of the rolling bearings  41  and causes them to axially preload the shaft. 
     The ring  70  is fixed to the shaft in the example illustrated with the aid of three cone-point set screws  71  which make it possible to correct an out-of-roundness, if necessary. 
     An O-ring seal  72  is accommodated in a groove  73  in the shaft  20  and presses against the blocking ring  70 . 
     The housing  10  is closed at the front by a front nut  90 , screwed therein, and at the rear by a rear closure part  95 , which can be held in various ways on the housing  10 , for example with the aid of a nut %, as illustrated in  FIG.  2   . 
     The front nut  90  has a forwardly directed collar  190 , which forms a chicane  192  with a rearwardly directed collar  191  of the blocking ring  70 . 
     An inner ring  195  is mounted on the shaft  20  at the rear, and has a forwardly directed collar  196 , which forms a chicane  198  with a collar  197  of the closure part  95 . 
     The chicanes  192  and  198  form a contactless sealing system at the front and the rear of the spindle  1  while providing a clearance allowing the rotation and translational movement without friction between the facing rotating and fixed parts. 
     An O-ring seal  199  is accommodated in a groove  27  in the shaft  20  and presses against the facing surface of the inner ring  195 . 
     A series of spacers are disposed in the housing  10  in contact with its inner surface, being immobilized between the front nut  90  and the rear closure part  95 , namely, from the front to the rear; a ring forming a front spacer  91 , a ring forming a front bearing spacer  92 , a main tubular spacer  93  and a ring forming a rear bearing spacer  94 . 
     Four stacks  100 ,  101 ,  102  and  103  of elastic strips  110  are interposed axially between the spacers  91  and  92 , between the spacers  92  and  93 , between the spacers  93  and  94 , and between the spacers  94  and  95 , respectively. 
     Each stack  100 ,  101 ,  102  or  103  has, in the example in question, at least two strips  110 , for example five, one of which is shown on its own in  FIG.  5   . 
     Each strip  110  has an annular overall shape and has fixing tabs  111 , distributed regularly at its periphery, of which there are three in the example in question, which are directed radially toward the outside and which bear with their radially outer edge against the inner surface of the housing  10 . The height of the fixing tabs  11   l  is slightly greater than the thickness of the bearing spacers  92  and  94 . 
     The circular-arc portions  112  connecting the fixing tabs  111  carry, half-way along their length, other tabs  113 , which are directed radially toward the inside. These tabs  113  each have a radial slot  114  which opens, at one end, onto the radially inner free edge of the tabs  113  and, at the opposite end, into a circular hole  115  formed in the circular-arc portion  112 . 
     The front bearing  50  is disposed between two bearing end rings  121  and  122 . Pins  130  are fitted in drilled holes  140  and  141  corresponding to these end rings  121  and  122  and the front bearing  50 , in order to keep the rings  121  and  122  in a predetermined angular orientation with respect to the front bearing  50 . 
     These pins  130  pass through the strips  110  by virtue of the holes  115 . The slots  114  make it easier to fit the pins  130 . Thus, the stacks  100  and  101  are kept angularly in a predefined position with respect to the bearing  50  and to the end rings  121  and  122 . 
     The bearing  50  and the rings  121  and  122  have, on their facing faces, protruding sectors  143 , as can be seen in  FIGS.  6  and  7   , the angular extent of which corresponds substantially to that of the of the tabs  113 , and which enclose the latter between one another. 
     The rear bearing  51  is similarly disposed between bearing end rings  120  and  121 , and pins  130  angularly immobilize the strips  110  disposed therebetween, as in the case of the front bearing  50 . 
     The tabs  113  of the strips  110  come axially into contact with the outer rings  32  and  42  of the rolling bearings  31  and  41 . 
     This assembly allows a certain freedom of movement in the axial direction of the sets of rolling bearings  30  and  40 , while keeping them centered as a result of the stiffness of the strips  110  in the radial direction, as will be described in detail below. 
     The main spacer  93  is formed with a shoulder  171  at each of its ends, set back from an end portion  172  surrounding the corresponding end ring  121  or  122 . 
     A flat spring  170  is mounted inside each end portion  172  and is interposed axially against the shoulder  171  and this end ring  121  or  122 . 
     At the front, two superposed flat springs  170  are mounted around the blocking ring  70  and are interposed axially between the front nut  90  and the end ring  121 , as can be seen in  FIG.  2   . 
     The closure part  95  is formed with a shoulder  176  and an end portion  177  in front of the latter, which extends around the adjacent end ring  122 . 
     A flat spring  170  is mounted inside the end portion  177  and is interposed axially between the closure part  95  and the adjacent end ring  122 . 
     The flat springs  170  grip the elastic strips around the front and rear rolling bearings via the end rings  121  and  122 , by way of the tabs  113  bearing on the outer rings of the rolling bearings. 
     The presence of an additional flat spring  170  at the front, between the nut  90  and the adjacent end ring  121 , creates permanent elastic loading of the shaft  20  toward the rear in order to press the ball  200  onto the rings  201  and  202 . 
     According to the invention, the spindle  1  has a mechanism for generating axial oscillations of the shaft  20  while it rotates. 
     This mechanism has a single ball  200  which rolls between a rotating rolling bearing ring  201 , which is mounted on the shaft  20  and rotates therewith, and a fixed rolling bearing ring  202 , which is carried by the closure part  95 . 
     A peripheral rolling bearing ring  203  is inserted into the closure part  95 , after the shoulder  176 , and extends around the path followed by the ball  200  while it rotates. This peripheral ring  203  makes it possible to react the centrifugal forces during the rotation of the ball  200 . 
     The rotating ring  201  is held against a shoulder  28  of the shaft  20 , which borders an annular groove  29 , the concavity of which substantially conforms to the path followed by the ball  200 . 
     The rolling bearing ring  202  has a rear face  230 , which is planar and perpendicular to its axis, and a front face  231 , which is planar and extends obliquely, as can be seen in  FIG.  8   , the normal to this face making an angle g with the axis of the rolling bearing ring  202 , which is a few degrees, for example around 0.3° in the example in question. The formula for g is: g=Arctan(amplitude/d path ), with “amplitude” corresponding to the total peak/trough variation of the vibration oscillation, d path  being the diameter of the path of the contact point. 
     As can be seen, the rolling bearing surface  231  does not have a step. 
     Thus, during its rotation about the axis X, the ball  200  carries out a periodic and sinusoidal axial movement which is caused by the inclination of the front face  231 . The ball  200  is only in contact, during its high-speed rotation, with the fixed rolling bearing ring  202 , the rotating rolling bearing ring  201  and the peripheral rolling bearing ring  203 . On account of the use of a single ball, the rolling of the latter induces bending stresses on the shaft, but this remains controlled and with an acceptable amplitude on account of the relatively small distance between the ball  200  and the spindle axis  20 . 
     The fact that there are no undulations on the rolling bearing surface  231  but rather a flat surface makes it possible to produce the latter very easily, with a very good surface state. 
     Preferably, the ball  200  is made of ceramic. Its diameter is preferably greater than or equal to 5 mm, making it possible to reduce the Hertz pressure at the contacts. Its diameter is for example 6 mm. 
     To mount the spindle  1 , all the internal constituent elements can be disposed on the shaft  20 , and the assembly can be inserted via the front end of the housing  10 , the closure pan  95  already being in position, and then the front screw  90  can be fixed. 
     The spindle  1  operates as follows. 
     The shaft  20  is driven in rotation by the pulley  21 , for example by a belt. 
     The ball  200  rolls between the rolling bearing rings  201  and  202  and in doing so moves the shaft  20  forward counter to the preload associated with the presence of an additional flat spring  170  at the front. 
     The movement of the shaft  20  is possible on account of the presence of the strips  110 , the arced portions  112  of which can bend on account of the clearance provided next to them by the presence of the sectors  143 . This bending allows the front bearing  50  and rear bearing  51  to move axially so as to follow the oscillations brought about by the movement of the ball  200 . 
     Axial oscillations of the shaft  20  are thus obtained, the frequency of which is given both by the rotational speed of the shaft  20  and by the Willis formula applied to this rolling bearing with three contact points. 
     The axial travel of the shaft  20  during the oscillating movement is for example between 0.02 mm and 0.15 mm. The shaft of the spindle oscillates with a vibration frequency of between 0.4 and 0.6 oscillations per revolution, for example around 0.5. 
     The presence of the groove  29  in the shaft  20 , in which groove the ball  200  is partially inscribed, reduces the distance  200  from the axis X and therefore the phenomenon of imbalance linked to the use of a single ball  200 . Moreover, the distance traveled by the ball  200  and the resultant wear are reduced. Lastly, the bending moment induced by the asymmetric loading with a single ball is reduced. 
     Of course, the invention is not limited to the example that has just been described. 
     It is possible for example to close the housing  10  differently at the rear, as illustrated in  FIG.  9   . In this figure, it is apparent that the rear closure part  95  is retained in the housing by an elastic ring  300  mounted in a corresponding groove in the housing  10 , thereby reducing the axial and radial space requirement of the housing. 
     It is possible to do away with the flat springs, apart from the one used to provide the axial preload on the shaft. The flat spring used to provide the axial preload on the shaft may be disposed at the rear, i.e. at the position in which, in  FIG.  2   , the flat spring  170  is located between the rear shoulder  171  and the ring  121  adjacent to the rolling bearing  41  radially on the inside of the bearing  51 .