Abstract:
In this process for drilling a bore ( 61 ), a drilling tool ( 11 ) is driven simultaneously in rotation about an axis (A) and in translation along the axis (A) in accordance with an advance movement. The advance movement is combined with an oscillating movement of the drilling tool ( 11 ) along the axis (A) of an amplitude sufficient to break up the swarf formed, while at the same time keeping the drilling tool ( 11 ) in the bore ( 61 ) which is in the process of being drilled. In certain exemplary embodiments, the application may be relevant to the aircraft construction industry.

Description:
PRIORITY CLAIM TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to French Patent Application No. 06/09484 filed on Oct. 27, 2006. 
     TECHNICAL FIELD 
     The present invention relates to a process for drilling a bore, in which process a drilling tool is driven simultaneously in rotation about an axis and in translation along the axis in accordance with an advance movement. 
     The invention is applicable, for example, to aircraft construction. 
     BACKGROUND TO THE INVENTION 
     In order to implement such a drilling process in this type of application, use is generally made of a machine tool comprising: 
     a casing; 
     a tool-holder spindle extending along an axis; 
     a mechanism for driving the tool-holder spindle, the mechanism comprising:
         a first member for driving the spindle in rotation about its axis relative to the casing;   a second member for driving the spindle in translation along its axis relative to the casing, the second member for driving in translation being screwed onto a threaded portion of the spindle so that the spindle advances or returns along the axis as a function of the relative rate of rotation of the drive members.       

     A tool of that type is known, for example, from U.S. Pat. No. 5,351,797. The drive mechanism of such a tool is referred to as “having positive feed” in English or “à avance mécanique” in French. 
     A single motor thus ensures, via the drive mechanism, that the spindle is driven in rotation about its axis and, at the same time, that it is advanced or returned by translation along its axis. 
     Since the translation and rotation drives of the spindle are connected mechanically, the advance of the spindle per revolution is constant. Thus, the variations in the speed of the motor have no effect on the rate of advance per revolution. The thickness of the swarf formed therefore remains constant and promotes the surface quality and the precision of the bores drilled by such a tool. 
     Although such a tool is found to be generally satisfactory, its use for drilling deep bores or materials renowned for being difficult, such as composite materials, may prove to be tricky. 
     For example, when a deep bore is drilled, the swarf accumulates in the flutes of the drill bit carried by the spindle until the motor of the tool is caused to stall. It is then necessary to proceed in several stages or to start fresh drilling cycles for the same bore to be drilled. 
     The drilling quality may also be degraded if the drill bit is repositioned incorrectly relative to the bore which has been started. The pressure of the swarf inside the bore also causes the deterioration of the surface thereof. 
     In order to solve that problem, French Patent No. 2,873,315 has proposed a tool provided with means for displacing the second member for driving in translation along the axis of the spindle between an advanced position and a returned position. Thus, in the course of drilling, the spindle is removed completely from the bore under the action of the displacement means in order to evacuate the swarf and to ensure that the bore is cleared. Although such a withdrawal of the spindle can be effected far more rapidly than by the conventional return movement of the spindle, it increases substantially the time necessary to drill a bore. 
     In addition, the performance of the clearing operation requires a control system in order to observe a predetermined clearing frequency or to meet a specific clearing requirement. Such a control system is relatively complex. 
     SUMMARY OF THE INVENTION 
     An object of the invention is therefore to solve those problems by providing a process which permits the drilling of deep bores or materials renowned for being difficult in a reduced time and which can be implemented by simpler tools. 
     To that end, the invention relates to a process for drilling a bore, in which process a drilling tool is driven simultaneously in rotation about an axis and in translation along the axis in accordance with an advance movement, wherein the advance movement is combined with an oscillating movement of the drilling tool along the axis of an amplitude sufficient to break up the swarf formed, while at the same time, keeping the drilling tool in the bore which is in the process of being drilled. 
     The invention relates also to a tool for drilling a bore for the implementation of a process such as defined above. 
     According to particular embodiments, the tool may have one or more of the following features, taken in isolation or in accordance with any technically possible combination: 
     the tool comprises:
         a casing;   a tool-holder spindle which extends along an axis and which is to carry a drilling tool;   a mechanism for driving the tool-holder spindle, which mechanism is capable of driving the drilling tool simultaneously in rotation about an axis and in translation along the axis in accordance with an advance movement, and the drive mechanism is suitable for combining the advance movement of the spindle with an oscillating movement along the axis;       

     the drive mechanism comprises:
         a first member for driving the spindle in rotation about its axis relative to the casing;   a second member for driving the spindle in translation along its axis relative to the casing, the second drive member being screwed onto a threaded portion of the spindle, so that the spindle advances or returns along the axis as a function of the relative rate of rotation of the drive members;       

     the oscillating movement has an amplitude greater than or equal to the advance of the spindle in the course of one revolution of the second drive member about the axis relative to the first drive member; 
     the drive mechanism comprises a cam/cam follower unit for ensuring the oscillating movement of the spindle; 
     the cam/cam follower unit comprises at least one element forming a cam and one element forming a cam follower, and a first of those elements is fixed for rotation with the spindle and the second of those elements is fixed for rotation with the second drive member; and 
     the second element of the unit forming the cam/cam follower is formed by the second drive member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description which is given purely by way of example and with reference to the appended drawings in which: 
         FIG. 1  is a diagrammatic view in lateral section of a tool for implementing a process according to the invention, and 
         FIGS. 2 and 3  are partial diagrammatic views, in lateral and enlarged section, illustrating a different position of the cam/cam follower system of the drive mechanism of the tool of  FIG. 1 . 
     
    
    
     Throughout the following, the terms “right”, “left”, “vertical”, “horizontal”, “lower”, “upper”, “top” and “bottom” are to be understood in relation to the position of the tool in the Figures. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates diagrammatically a portable machine tool  1  which comprises principally: a casing  3 ; a tool-holder spindle  5  extending along a vertical axis A; a motor  7 , for example a pneumatic motor; and a mechanism  9  which drives the spindle  5  and which connects the motor  7  and the spindle  5  mechanically. The spindle  5  is received in the casing  3  in such a manner that it is mobile in rotation about its axis A and in translation along that axis A. 
     In order to be able to drill bores, a drilling tool  11 , in the form of a drill bit, is mounted in a removable manner at the lower end  13  of the spindle  5 . The output shaft  15  of the motor  7  carries an output gear wheel  17  which meshes with the drive mechanism  9 . In the example shown, the motor  7  is disposed parallel with the spindle  5 . However, it could be disposed substantially at right-angles relative to the latter, as explained, for example, in French Patent No. 2,829,952, the contents of which are incorporated herein by reference. As illustrated by  FIG. 1 , the drive mechanism  9  comprises the following elements, so that it constitutes a mechanism referred to as “having positive feed” in English or as “à avance mécanique” in French: a first gear wheel/positive clutch  19  meshing with the output gear wheel  17 ; a first gear wheel  21  meshing with the first gear wheel/positive clutch  19 ; a second gear wheel/positive clutch  23  surmounting the first gear wheel/positive clutch  19 ; a second gear wheel  25  which surmounts the first gear wheel  21 , and a fixed positive clutch  27  which surmounts the second gear wheel/positive clutch  23  and which is, for example, fixedly joined to the casing  3 . 
     The first gear wheel  21  is slipped onto the spindle  5  and is fixed for rotation with the latter. The spindle  5  is mobile in translation relative to the gear wheel  21  along the axis A. This connection between the spindle  5  and the first gear wheel  21  is, for example, obtained by means of splines. 
     In a conventional manner, the second gear wheel/positive clutch  23  is carried by a slide  29  in order to be mobile under the action of a piston  31  between a lowered position ( FIG. 1 ), in which the second gear wheel/positive clutch  23  is form-fitted to the first gear wheel/positive clutch  19  and is therefore fixed for rotation with the latter, and a raised position in which the second gear wheel/positive clutch  23  is form-fitted to the fixed positive clutch  27  and is therefore fixed in rotation relative to the casing  3 . 
     In a conventional manner, the numbers of teeth of the gear wheels/positive clutches  19  and  23  and of the gear wheels  21  and  25  are adapted in such a manner that, when the two gear wheels/positive clutches  19  and  23  are form-fitted to each other, the gear wheel  25  rotates at a slightly higher rate than that of the gear wheel  21  in order to drive the spindle  5  in translation towards the bottom, in an advance movement, as will be described hereinafter. 
     Unlike conventional positive-feed mechanisms, the second gear wheel  25 , in the example described, is free to rotate relative to the spindle  5  and is therefore not screwed onto a threaded portion of the latter. 
     The second gear wheel  25  is extended towards the top by a rotary positive clutch  33  which is, for example, integral with the second gear wheel  25 . By way of variation, the positive clutch  33  can be secured to the latter. 
     The positive clutch  33  is therefore fixed for rotation with the second gear wheel  25  and can rotate freely relative to the spindle  5  about its axis A. 
     The drive mechanism  9  also comprises, as can be seen more clearly in  FIG. 2 , a dog/positive clutch  35 , which surmounts the positive clutch  33 , and a system  37  for displacing the dog/positive clutch  35  relative to the casing  3 . 
     The dog/positive clutch  35  is screwed onto a threaded portion  39  of the spindle  5 . The front or lower face of the dog/positive clutch  35  comprises coupling teeth enabling it to be form-fitted to the positive clutch  33 . The height of those teeth and of those of the positive clutch  33  are such that they enable the form-fit to be maintained when the dog/positive clutch  35  is displaced axially between its advanced position and its returned position described hereinafter, it being observed that the gear wheels  21  and  25  are fixed axially relative to the casing  3 . 
     The displacement system  37  comprises a cam/cam follower unit  41  and a mechanism  43  for the axial bearing of the unit  41  on the dog/positive clutch  35 . 
     In the example shown, the unit  41  comprises a cam  45  and a cam follower  47 . The cam follower  47  is formed by the rear or upper end of the dog/positive clutch  35 . To be more precise, and as illustrated by  FIG. 2 , a relief  49  in the form of a recess is formed in the rear face  51  of the dog/positive clutch  35 . That recess  49  has, for example, an elongate shape extending at right-angles, along a diameter of the dog/positive clutch  35 , to the axis A. 
     The cam  45  is slipped onto the spindle  5  behind the dog/positive clutch  35 . The front or lower face  53  of the cam  45  is provided with a cam ramp  55  which, in the example shown, is a projection which has a shape substantially complementing that of the recess  49 . This can be seen in  FIG. 2 , where the reliefs  49  and  55  have the same angular orientation relative to the axis A. In  FIG. 3 , those reliefs  49  and  55  have orientations arranged at 90° to each other. 
     The cam  45  is fixed for rotation with the spindle  5 . This fixing is effected, for example, by means of splines. 
     The mechanism  43  for axial bearing comprises, for example, damping means  56 , especially in the form of a thrust spring. Those resilient means  56  bear, at the rear, on a wall  57  of the casing  3  and, at the front, by way of a roller bearing  59 , on the rear end of the cam  45 . The roller bearing  59  enables the friction between the damping means  56 , which are fixed in rotation about the axis A, and the cam  45 , which is driven in rotation about the axis A by the spindle  5 , to be limited. 
     The mechanism  43  which is fixedly joined to the casing  3  restrains the resulting thrust of the tool  11  by holding the front face  53  of the cam  45  against the rear face  51  of the dog/positive clutch  35  and therefore it restrains the dog/positive clutch  35 . The spindle  5 , which is fixed for translation with the dog/positive clutch  35  owing to the helical connection between the dog/positive clutch  35  and the spindle  5 , is therefore held towards the front against the thrust of the spindle  5 . 
     It will be appreciated that the rate of relative rotation between the dog/positive clutch  35  and the cam  45  is the same as that between the gear wheels  21  and  25 . 
     The dog/positive clutch  35  is mobile axially relative to the casing  3  between an advanced position and a returned position. 
     The returned position is illustrated by  FIG. 2 . In that position, the projection  55  and the recess  49  have the same angular orientation relative to the axis A and are therefore engaged one inside the other. The stack formed by the dog/positive clutch  35  and the cam  45  therefore has a lower height, as viewed along the axis A. 
     In the advanced position, illustrated by  FIG. 3 , the reliefs  49  and  55  are arranged angularly at 90° to each other. The projection  55  is therefore not engaged in the recess  49  so that the stack formed by the dog/positive clutch  35  and the cam  45  has a height which is greater by e than that which it has in the returned position of the dog/positive clutch  35 . 
     The operation of the tool will now be described. 
     If the second gear wheel/positive clutch  23  is in the lowered position and if the motor  7  is supplied with pressurized air, the second gear wheel  25  and the dog/positive clutch  35  rotate about the axis A at a rate slightly higher than that of the first gear wheel  21 . The drive mechanism  9  therefore ensures: the driving in rotation of the spindle  5  via the gear wheel/positive clutch  19  and the first gear wheel  21 ; and, at the same time, the advance of the spindle  5 , that is to say, its displacement in translation towards the bottom along the axis A, this advance being due to the helical connection between the spindle  5  and the dog/positive clutch  35  and to the difference in the rates of rotation between the dog/positive clutch  35  and the gear wheel  21  and therefore the spindle  5 . 
     If the spindle  5  is driven in rotation in the clockwise direction, the thread of the helical connection between the spindle  5  and the dog/positive clutch  35  is left-handed in order to ensure the described advance movement. 
     As indicated above, the dog/positive clutch  35  and the cam  45  will rotate relative to each other about the axis A at a relative rate equal to that existing between the gear wheels  21  and  25 . Owing to that relative rotation, the dog/positive clutch  35  will pass alternately from its advanced position to its returned position. 
     The general advance movement of the spindle  5  described above, which is produced owing to the positive-feed mechanism  9 , is therefore combined with an oscillating translation movement of the spindle  5  along the axis A. That oscillating movement has an amplitude of travel substantially equal to e. 
     The first drive mode described above enables a bore  61  to be drilled in a workpiece  63  by means of the drill bit  11  carried by the spindle  5 . The oscillation amplitude of the spindle  5  is relatively low so that the spindle  5  remains inside the bore  61  in the course of the drilling operation. 
     Typically, the oscillation amplitude is substantially equal to the advance of the spindle  5 , that is to say, to the advance of the latter along the axis A in the course of one revolution of the gear wheel  25  relative to the gear wheel  21 . 
     The tool  1  also offers a second drive mode, which is described below. 
     If the second gear wheel/positive clutch  23  passes into its raised position, the drive mechanism  9  ensures: the driving in rotation of the spindle  5  via the first gear wheel/positive clutch  19  and the first gear wheel  21  and, at the same time; the return of the spindle  5 , that is to say, its displacement in translation towards the top along the axis A, the second gear wheel/positive clutch  23 , the second gear wheel  25 , the positive clutch  33  and the dog/positive clutch  35  then being fixed in position, the rotation of the spindle  5  about its axis A ensuring the return movement owing to the helical connection between the dog/positive clutch  35  and the spindle  5  owing to the left-handed thread. 
     This second drive mode enables the spindle  5  to be returned when the drilling of the bore has been completed. 
     As indicated above, during the drilling operation, the spindle  5  will perform a general translation movement towards the front combined with an oscillating translation movement of lesser amplitude. At each return of the spindle  5 , this oscillating movement will bring about the breaking up of the swarf formed. This fractionation of the swarf formed will facilitate, on the one hand, its evacuation and, on the other hand, the production of a bore  61  of very great precision with a high-quality surface state, even in the case of very great depths. 
     In addition, the oscillating movement of the spindle  5  is obtained owing to the cam/cam follower unit  41  and therefore, does not require the use of a pneumatic or electronic control system. The tool  1  is therefore simple. 
     Therein, the time taken to drill such a bore is reduced, since it is not necessary to withdraw the spindle  5  completely from the bore  61  in the process of being formed. 
     It will also be appreciated that the oscillation frequency of the spindle  5  is proportional to the relative speed between the dog/positive clutch  35  and the cam  45 . This oscillation frequency is therefore reduced, which limits undesirable vibration and noise pollution. 
     It will be further appreciated that the profiles of the reliefs  49  and  55  do not have sharp edges, so as to avoid shock and impact. The absence of shock enables the tip and the cuffing edges of the drill bit  11  to be preserved intact. 
     More generally, the oscillating travel of the spindle  5  may be greater than a fraction of the advance of the spindle  5  but is kept sufficiently low so as not to affect the drilling operation. It is, in particular, such that the drill bit  11  remains in the bore  61  for most of the drilling operation. It is preferably of the order of magnitude of the advance of the spindle  5  and, for example, less than three times the advance of the spindle  5 . 
     Likewise, other types of cam/cam follower units  41  may be used. These may comprise, for example, several cam followers. They may also comprise ramp surfaces provided on the opposing faces  51  and  53  of the dog/positive clutch  35  and the cam  45  with balls interposed between those two faces. 
     In addition, the driving in rotation of the elements of the cam/cam follower unit  41  may be different from that described above. Thus, the second gear wheel  25  may be screwed onto the spindle  5  and subjected directly to an oscillating movement without it being necessary to provide a dog/positive clutch  35 . 
     It will also be noted that the oscillating movement of the spindle  5  may be obtained by means other than a cam/cam follower unit  41 . 
     The linking of the first drive mode, that is to say, the driving in rotation and, simultaneously, the advance of the spindle  5 , and the second drive mode, that is to say, the driving in rotation and, simultaneously, the return of the spindle  5 , then the stoppage of the supply to the motor  7 , can be controlled in a classical manner, for example, automatically by the pneumatic circuit for supplying the motor  7 , after operating the button for starting the tool  1 . 
     More generally, the displacement of the first member  21  for driving the spindle  5  in rotation and the displacement of the second member  35  for driving the spindle  5  in translation can be effected by two separate motors which control each other, for example, electronically.