Abstract:
A drilling head ( 1 ) with an axial oscillation generator, includes a mounting ( 2 ) for coupling the head to a motor for rotating the mounting about an axis, the mounting ( 2 ) having a longitudinal axis ( 5 ) coinciding with the axis of rotation of the motor, a tool holder ( 3 ), an elastically deformable element ( 6 ) for retaining the tool holder ( 3 ) in the mounting ( 2 ) and an element ( 7 ) for guiding the tool holder ( 3 ) in the mounting ( 2 ) along the longitudinal axis ( 5 ) of the mounting, wherein the mounting of the drilling head includes a controlled generator ( 8 ) of reciprocating movements in the direction of the aforementioned axis ( 5 ) positioned between the mounting ( 2 ) and the tool holder, the tool holder and the generator being connected by a longitudinal coupling member ( 9 ) suitable for damping the impacts withstood by the tool holder while enabling the transmission of the reciprocating movements.

Description:
The present invention relates to a drilling head fitted with a generator of axial oscillations (vibrations). 
     BACKGROUND OF THE INVENTION 
     A drilling machine has the disadvantage of producing long chips which present various problems in the drilling operation because they are difficult to move quickly away from the work zone and may impair the quality of the surface of the hole produced. 
     One remedy to this disadvantage is to cause these chips to break up by varying their thickness and by repeatedly interrupting the cutting operation. This is achieved by applying to the cutting tool, which is generally a drill bit, an axial vibration which makes the feedrate (the rate of penetration into the material) cyclically variable and therefore makes the thickness of the chip variable, with amplitudes and frequencies that cause the chips to break up. 
     At the same time, there are drilling operations in which it is beneficial to vibrate the tool only at certain times (when drilling multilayer materials for example) or to change the amplitudes and/or the frequencies of the oscillations in real time (in order to react and to adapt the cutting process to suit the material being drilled for example). It is therefore necessary to have a device that can be controlled, with sufficiently rapid reaction and the ability to determine the passage from one material to another. 
     Several devices that use axial oscillations in drilling have already been proposed. For example, mention may be made of the device described in document EP 994 758. The latter describes a coupling between a tool holder and its driving mount which constitutes a suspending of the tool holder from the mount in such a way that it can produce self-sustaining axial oscillations such that the rate at which the tool penetrates the workpiece varies cyclically around a mean rate which is of course non-zero. 
     Another document, DE 103 43 682, discloses a milling or drilling tool in which the rotary tool is subjected to a reciprocating axial movement by means of a piezoelectric or magnetic actuator housed in the tool holder, connected wirelessly to a power supply. The system is controlled remotely. This tool is intended for machining composite materials at very high (ultrasonic) frequencies and very low amplitudes of axial vibration. 
     Another document, US2008/041604, discloses a tool with an oscillating head in which the rotary tool is subjected to very high frequency oscillations which frequencies have to be equal to the natural frequency of the tool, so that the oscillation generated causes the tool to resonate axially. The actuators in the solutions described in document US2008/041604 are fully incorporated into the axial moving part of the devices, being incapable of generating movements and static or low-frequency loads at amplitudes high enough to break up the chips. In addition, because they involve the use of resonance, the characteristics of the oscillations obtained are very much dependent on the load, that is to say on the penetration force encountered during the machining operation. 
     OBJECT OF THE INVENTION 
     The known devices do not have all the qualities required of a machine for universal use with regard to the breaking-up of the chip produced during drilling by superposing axial oscillations on the advancing feed movement of the tool, namely a device that is robust, is able to generate axial oscillations of sufficient amplitude to break up the chips, that can be fitted to existing machines, and that has the ability to be adapted to suit different drilling programs, notably to take account of the various different materials through which the drill passes in one and the same operation. For example, it is known that the oscillations generated by the devices described in EP 994 758 and in US2008/041604 are sensitive to the load, which means to say that they are dependent on the cutting diameters and conditions (tools, cutting parameters etc.). It is also known that piezoelectric actuators are fragile and are unable to withstand stresses other than compressive loadings. 
     BRIEF DESCRIPTION OF THE INVENTION 
     To this end, the subject of the present invention is a drilling head with a controlled generator of axial oscillations, comprising a mount for coupling the head to a motor that drives the rotation of the mount about an axis, the mount having a longitudinal axis that coincides with the axis of rotation of the motor, a tool holder, elastically deformable suspension means for suspending the tool holder from the mount which are capable of transmitting torsion moments and guide means to guide the tool holder in the mount along the longitudinal axis of this mount, comprising a controlled generator of reciprocating movements (oscillations) in the direction of the abovementioned axis, interposed between the mount and the tool holder. 
     For preference the controlled generator (also known as the actuator) is tubular and of the piezoelectric actuator type, pressing against the tool holder mount via one of its ends, while the drilling head according to the invention comprises a sliding pivot, e.g. comprises a bearing in which is guided a shank of the tool holder, between the mount and the tool holder produced inside the tubular controlled generator, an axially elastic external sleeve of high torsional stiffness fixed by its ends to the mount and to the tool holder respectively, and an annular shim of determined axial stiffness and low flexural stiffness via which the tool holder presses axially against the end of the controlled generator. 
     The external sleeve and the annular shim serve to establish a preload on the controlled generator. The combined length of the controlled generator and of the annular shim in relation to the free length of the external sleeve and to its elastic property in the axial direction allows the preload on the controlled generator to be adjusted. The annular shim, which is axially stiff, allows the vibrations emanating from the controlled generator to be transmitted to the tool holder while its flexural flexibility safeguards the controlled generator against sudden variations in bending moments resulting from the drilling process and from any play there might be in the sliding pivot. 
     In an alternative form of embodiment of this example, the transmission of torque between the mount and the tool holder is performed by a guideway connection between the mount and the tool holder (a linear ball bearing on a profiled or splined shaft) so that the only remaining function of the external sleeve is to preload the controlled generator. 
     The structure of this external sleeve may be composite with wall portions that have high thermal conductivity or cutouts to encourage the dissipation of heat caused by the heating-up of the controlled generator. 
     In another embodiment, the drilling head comprises a sliding pivot between the mount and the tool holder inside the tubular controlled generator, an axially elastic external sleeve of high torsional stiffness fixed by its ends to the mount and to the tool holder respectively with, in parallel, at least one washer capable of transmitting radial load, which is fixed between the mount and the tool holder, and an annular shim of determined axial stiffness via which the tool holder presses axially against the end of the controlled generator. 
     As in the previous embodiment, the combined length of the controlled generator and of the annular shim in relation to the free length of the external sleeve and to its elastic property in the axial direction allows the preload on the controlled generator to be adjusted. The piezoelectric actuator is protected against torsion moments both by the external sleeve and by the aforementioned flexible element which together relieve the annular shim from the need to isolate the tool holder from the controlled generator torsionally; this measure makes it possible to react radial load that cannot be reacted by the sliding pivot connection when this connection has play, even if this play is small. It is possible to employ another washer for suspending the tool holder from the mount in parallel with the first one and therefore also with the external sleeve, this other washer being situated in the region of the pressing annular shim, which has reliefs passing through the washer via openings provided for that purpose. When there are two washers that are flexible in the axial direction co-existing, they form an axial guide of the guideway type suited to small axial movements and are able to react radial load, in which case the sliding pivot connection inside the tubular controlled generator, afforded by a ball-and-cage assembly for example, is no longer absolutely essential. The presence of such a connection (sliding pivot) does nonetheless make it easier for the tool holder and mount to be positioned accurately, complying with a set coaxiality tolerance, before the elastic guide elements are set in position during the assembly phase. If axial guidance is afforded only by elastic guide elements and the sliding pivot connection is abandoned, the tool holder will have to be positioned accurately with respect to the mount during assembly. 
     A contactless power transmission using electromagnetic transmitter-receiver systems (rotary transformer) is preferred. However, a contacting connection could be used via a slip-ring assembly. 
     In one particular alternative form of the embodiment of the invention, the controlled generator may incorporate a sensor capable of indicating the force that the drill bit is applying axially to the workpiece during machining. This sensor may take a direct measurement of this load, for example using the direct piezoelectric effect, with the aid of a few layers of piezoelectric material, or may take an indirect measurement, for example of the deformation of the controlled generator (deformation gauge for example), in which case the signal will need to be processed in order to determine the drilling load. It is thus possible to collect data regarding the variation of this force and use it to control the actuator or even to control the cutting parameters using the machine used (instantaneous cutting speed, feed rate). Specifically, this indication of the instantaneous value of the axial force involved in the drilling operation is a variable that is interpreted by a signal processing unit in order to modify the drilling parameters using a model contained in software, running on a data processing unit (computer for example). Typically, the variation in this axial load on the piezoelectric sensor-actuator can be interpreted as the passage from one material to another in a given assembly being drilled. Via this detection, the processing unit can modify the drilling parameters at the detected moment of passage from one material to another and can thus optimize the operation by reference to stored drilling programs. 
     Other features and advantages of the invention will become apparent from the description given hereinafter of one embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will be made to the attached drawings in which: 
         FIG. 1  is a schematic cross section of a device according to the invention, 
         FIG. 2  is an axial cross section through a first embodiment of the device according to the invention, 
         FIG. 3  is an exploded view of the components of  FIG. 2 , 
         FIG. 4  is an axial cross section through a second embodiment of the device according to the invention, 
         FIG. 5  is an exploded view of the device depicted in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , the drilling head depicted as  1  comprises a mount  2  and a tool holder  3  to keep a drill bit  4  coaxial with a longitudinal axis  5  of rotation of the mount  2 . 
     Via a second end  2   a , the mount is coupled to a drive motor M depicted schematically and an axis of rotation of which is coaxial with the longitudinal axis  5  which is also the longitudinal axis of the mount  2 . 
     The tool holder  3  is fixed to the mount  2  by a washer  6  which has the property of transmitting a torque between the mount and the tool holder an d which is elastically deformable in the direction of the longitudinal axis  5 . One example of such a washer is described in document EP 994 758. This washer  6  is fixed by peripheral screws  6   a  to the mount  2  while the tool holder  3  is fixed to the washer  6  by central screws  6   b . The tool holder  3  has a shank  3   a  which is guided with axial sliding in a bearing  7  of the mount. 
     The mount  2  is hollow and its central recess  2   b  contains a piezoelectric actuator  8  of which the oscillations, which are generated by a variable voltage applied to it, are directed along the longitudinal axis  5 . The upper end of this controlled generator, in which the piezoelectric elements are contained in a housing that preloads them, is fixed to the mount at its upper part  2   a . Its lower part itself is fixed to the part  3   a  of the tool holder. This fixing is performed using a longitudinal coupling member  9  having properties of reacting bending stresses (which have to exist because of the play in and imperfections of the bearing  7 ) while at the same time allowing the reciprocating movements to be transmitted. This member  9  connects the vibrating end  8   a  of the controlled generator to a pivot  10  which is wedged in terms of translation in the part  3   a  of the tool holder by means of two thrust ball bearings  11   a  and  11   b  that allow the controlled generator  8  to be torsionally decoupled from the tool holder  3  if the washer  6  breaks. 
     The power supply controlling the piezoelectric actuator has been depicted symbolically as  12 . Depending on circumstance, this may be of the contactless type (rotary transformer plus integrated electronics) or of the contacting type (of the slip-ring assembly type based on sliding contacts). 
     It is advantageous to use part of the active element of the controlled generator as a sensor of the axial loads to which it is subjected. The output from this sensor is denoted  13  in the figure. It will be of the contactless or of the contacting type depending on circumstance consistent with the supply of power to the controlled generator  8 . The signals emitted are directed to a processing unit  14  which at output  15  emits an actuator control signal that will be dependent on the input signal. Indeed it is known that the input signal is indicative of the axial load applied to the actuator; it is therefore representative of the resistance to penetration of the tool into the workpiece during machining and therefore of the nature of the material being worked. A variation in this signal corresponds, for example, in the case of a multilayer workpiece, to the crossing of an interface which may lead to a change in how the drilling operation progresses, not only in terms of the control of the actuator (via a transmission path  15   a  toward the controlled generator or the source of its power supply) but also in terms of the motor M that drives the rotation of the tool (via another transmission path  15   b  originating from the output  15  from the unit  14 ) and in terms of the rate at which the drill bit penetrates the workpiece, this being, for example, dependent on setpoint values stored in memory within the unit  14 . Just as was the case with control of the actuator, the output from the sensor will be processed preferably contactlessly, using any appropriate means (transponder or the like) for transferring the signals from a rotating sensor to a fixed processing unit and for powering the sensor where necessary. Incidentally, providing all or part of the signal processing unit within the tool holder would not constitute departure from the scope of the invention. 
       FIGS. 2 and 3  depict again most of the elements already described, using the same references. The mount  2  is coupled to the machine element M which may be a rotary shaft protruding from a motor or a fixed component if, for example, the workpiece is rotating about the axis  5 . The mount  2  accepts a shank  3   a  of the tool holder  3  via a ball-and-cage assembly or linear ball bearing  20  which provides axial guidance (sliding pivot) of the tool holder  3  in the mount  2 . This guidance is afforded inside the controlled generator  8  which in this instance is of tubular shape, without preload (raw piezoelectric material). The axial load measurement means (load sensor) here is represented in the form of a strain gauge  21  extending along an external power generator of the controlled generator  8 . 
     The essential difference between this embodiment and the diagram of  FIG. 1  lies in the means of coupling the tool holder  3  to the mount  2  which are able both to transmit the torque stemming from the drilling operation and the axial oscillations while at the same time safeguarding the controlled generator  8  against the torsional or bending forces that would impair its operation. 
     These means of coupling here comprise an external sleeve  22  screwed into the mount  2  by its second ends while its first end is coupled by screws  23  to the tool holder  3 . This external sleeve has the property of being torsionally very stiff but relatively flexible in the axial direction, having a set axial stiffness. For this purpose, it is cut circumferentially with a multitude of slots  24  which make it axially elastic while at the same time preserving its torsional stiffness. The number and the dimensions of the slots will be determined in such a way as to afford an axial elastic characteristic that will be calculated to achieve the preload to be applied to the controlled generator  8 . This characteristic will, however, be compatible with the energy of vibration in order not to dissipate this energy too much, for example by having too great an axial stiffness with respect to the loads. The slots may be filled with a material of high thermal conductivity in order to dissipate the heat given off by the actuator as it operates. The external sleeve  22  is used firstly to suspend the tool holder from the mount and secondly to transmit the torsion moments resulting from the cutting forces from the one to the other, while at the same time achieving the necessary preload that the actuator needs. Geometric shapes other than that described hereinabove can be used for the external sleeve  22 , provided that all of these functions are performed. Thus, this external sleeve may comprise an alternating succession of internal and external grooves which give it the appearance of a bellows of determined axial stiffness. 
     The coupling means also comprise an annular shim  25  which is interposed and trapped between one end of the controlled generator  8  and the tool holder  3 . The material and geometry of this annular shim  25  will be chosen so that the transmission of the axial oscillations generated by the controlled generator  8  is maintained while at the same time minimizing the bending stresses resulting from the cutting force which could pass via the shank of the tool holder and the bearing of the mount, forming the sliding pivot connection, that necessarily has to have a functional clearance, all be it minimal. As far as the geometry of this annular shim  25  is concerned, it will be noted that it is provided with cutouts in planes that are parallel and perpendicular to the axis  5  which define two parts of this annular shim which are joined together in the manner of a Cardan joint allowing micromovements, therefore movements of small amplitude, with flexural flexibility that protects the piezoelectric actuator. 
     It will be noted in this solution that the tool holder  3  has, passing through it, a channel C which is made along its axis, thus providing a passage for any coolants and lubricants used to cool and/or lubricate the cutting zone. These coolants may also serve to improve the dissipation of heat from the actuator. Various sealing means and orifices made in the surrounding components could also be produced in order to bring this coolant into contact with the actuator, to bring about forced cooling. 
     The embodiment depicted in  FIGS. 4 and 5  is very similar to the one just described. The difference is from the presence of a second washer in the means of coupling between the mount and the tool holder. This second washer  26  is similar to the washer  6  in  FIG. 1  in as much as, together with the external sleeve  22 , it serves to suspend the tool holder  3  from the mount  2  and to add radial stiffness to the connection, at the opposite end of the tool holder from the tool, thus contributing to avoiding the lateral micromovements that could be permitted by the play in the sliding pivot connection. The transmission of torque continues to be afforded by the external sleeve  22 , even if the second washer  26  when present contributes toward this. 
     It will be noted that there is a first washer  27 , similar to the second washer  26 , which has recesses  27   a  via which reliefs  25   a  belonging to the annular ring  25  pass through the second washer  26  to press against the tool holder  3 . The hollows separating the reliefs in the annular shim  25  are of large enough dimensions to exceed the amplitude of the vibrations of the controlled generator  8  which are transmitted to the tool holder  3  by the annular shim  25 , so as to prevent contact between the annular shim  25  and the first washer  27  at all times. The first washer  27  is fixed rigidly by the outer screws  23  to the external sleeve  22  and to the tool holder  3  and in an appropriate way (special screw  27   b ) to the spindle of the mount  2 . The role of the first washer  27  is likewise to add radial stiffness at the tool end of the tool holder so as, together with the first washer  26 , to create a guideway connection with flexible guidance suited to small-amplitude movements. 
     The axial stiffness of the external sleeve  22  is dependent on the preload that is to be imposed on the controlled generator  8 . This load in fact corresponds to the difference in position along the longitudinal axis  5  between that face of the tool holder that faces toward the annular shim  25  and toward the external sleeve  22  and the free end of this external sleeve, before its attachment using the screws  23 . This difference in position of the aforementioned two faces prior to assembly forms a mounting clearance which will need to be determined by the design and accurate production of the components. The fastening using the screws  23  will cause elastic elongation of the external sleeve  22  and, as a result, will cause compression (by an amount determined by the magnitude of the mounting clearance and by the axial stiffness of the external sleeve  22 ) of the controlled generator  8 /annular shim  25  combination. 
     The invention makes it possible in a simple way to solve the problem of breaking up the chips of a drilling tool and to slave this break up and, more generally, the parameters of the operation, to a detection of the nature of the material being processed, and makes it possible to do so, particularly in the case of a piezoelectric actuator, using the same active member.