Abstract:
A drilling tool of the loose top type includes a basic body having two bendable branches having inner support surfaces that are resiliently pressable against side contact surfaces of a replaceable loose top. The mounting of the loose top is affected by turning-in from an initial position to an operative end position. An abutting edge along each side contact surface then bends out the branches and subjects the branches to a spring force that reaches a maximum in a dead position so as to then decrease somewhat up to the operative end position. During the final phase of the rotary motion, the operator obtains, in a tactile and/or auditory way, confirmation of the loose top indeed reaching its operative end position.

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
     This application claims priority to Swedish Application No. 0900847-5 filed Jun. 23, 2009, which is incorporated by reference herein. 
     TECHNICAL FIELD 
     This disclosure relates to a drilling tool intended for chip removing machining and of the type that includes a basic body having front and rear ends, between which a first center axis extends around which the basic body is rotatable in a given direction of rotation. Further, the drilling tool includes a loose top having front and rear ends, between which a second centre axis extends. The front end of the loose top includes one or more cutting edges. The front end of the basic body includes a jaw between two axially protruding, peripherally situated branches that are elastically bendable and have the purpose of resiliently clamping the loose top in the jaw. Specifically, a pair of inner support surfaces of the branches resiliently presses against a pair of external side contact surfaces of the loose top. Further, the branches have the purpose of transferring torque to the loose top via tangential support surfaces of the branches and cooperating tangential contact surfaces of the loose top. The inner support surface of the individual branch extends between first and second, tangentially separated side borderlines. The first tangentially separated borderline is heading and the second tangentially separated borderline is trailing during rotation of the tool. The individual side contact surface extends between first and second side borderlines. The second side borderline is rotationally trailing and is included in an edge to a trailing part surface, besides which the loose top is axially insertable into the jaw and turnable into and out of an operative engagement with the branches. 
     Drilling tools of the kind in question are suitable for chip removing or cutting machining, especially hole making of workpieces of metal, such as steel, cast iron, aluminium, titanium, yellow metals, etc. The tools may also be used for the machining of composite materials of different types. 
     BACKGROUND ART 
     In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art. 
     Drilling tools have been developed that, contrary to solid drills, are composed of two parts, including a basic or drill body and a head detachably connected with the same and thereby being replaceable. The head includes the requisite cutting edges. In such a way, the major part of the tool can be manufactured from a comparatively inexpensive material having a moderate modulus of elasticity, such as steel, while a smaller part, the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the cutting edges a good chip-removing capacity, a good machining precision and a long service life. The head forms a wear part that can be discarded after wear-out, while the basic body can be re-used several times, for example, 10 to 20 replacements. A now recognized term for these cutting edge-carrying heads is “loose tops”, which henceforth will be used in this document. 
     Loose top type drilling tools have a plurality of desired capabilities, one of which is that torque should be transferable in a reliable way from the rotatable, driven basic body to the loose top. Furthermore, the basic body should without problems be able to carry the rearwardly directed axial forces that the loose top is subjected to during drilling. Further, the loose top should be held centered in an exact and reliable way in relation to the basic body. Also, the loose top is clamped to the basic body not only during drilling of a hole, but also during retraction of the drilling tool out of the same. A user further desires that the loose top should be mountable and dismountable in a rapid and convenient way without the basic body necessarily having to be removed from the driving machine. In addition, the tool, and in particular the loose top manufactured from expensive materials, should be capable of low cost manufacture. 
     A loose-top tool intended for drilling and of the initially generally mentioned kind is previously known by EP 1013367. In this case, the two branches of the basic body are arranged to be turned into arched pockets, which are recessed in the rear part of convex envelope surfaces of two bars included in the loose top and separated by chip flutes, and which have a limited axial extension that in turn limits the maximally possible length of the branches. The internal support surfaces of the branches and the external side contact surfaces of the loose top, which are pressed against each other in order to resiliently and securely pinch the loose top in the jaw between the branches, have a rotationally symmetrical basic shape. The external side contact surfaces of the loose top generally have a larger diametrical dimension than the inner support surfaces of the branches in order to bend out the branches elastically or resiliently. In their angle-wise end position of turning, the rotationally heading, torque-transferring tangential support surfaces of the branches should be pressed into close contact against two tangential contact surfaces that form end surfaces in the two pockets in the loose top. 
     The tool of EP 1013367 is meritorious in several respects, one of which is that the axial support surface that is situated between the branches and forms a bottom in the jaw of the basic body does not need to be intersected by any slot or cavity in which chips could get caught. Another merit is that the loose top can be made fairly short in relation to its diameter, something that is material-saving and cost-reducing. In addition, the axial contact surface of the loose top as well as the axial support surface of the basic body extends between ends that are peripherally situated. In such a way, these surfaces become ample and thereby suitable to transfer great axial forces. 
     A disadvantage of the known tool is, however, that the mounting of the loose top in the jaw of the basic body risks becoming unreliable and cumbersome to carry out. Already when the two branches initially begin to be turned into the appurtenant pockets in the loose top, the branches are subjected to a clamping force that from then on becomes equally great during the entire rotary motion up to the end position in which the branches are pressed against the end surfaces of the pockets. Because the mounting is carried out in a manual way and the branches are held resiliently clamped against the side contact surfaces of the loose top by a force that is equally great during the entire rotary motion, it may become difficult for the operator to determine whether the loose top has reached its end position or not. This decision is made more difficult by the fact that the uniform clamping force has to be fairly great in order for the loose top to be clamped reasonably reliably. This means that the work with the turning-in becomes laborious, and therefore the operator, particularly when in a hurry, may unintentionally finish the turning-in too early, before the loose top has reached its end position in the jaw. Incorrect mounting of the loose top may, among other things, manifest itself in lost centering of the drilling tool in connection with the entering of a workpiece. 
     SUMMARY 
     The present disclosure aims at obviating the above-mentioned disadvantages of the known drilling tool and at providing an improved drilling tool. An object is accordingly to provide a drilling tool, in which the loose top and the cooperating jaw of the basic body are formed in such a way that the operator, in a tactile and/or auditory way, clearly perceives when the loose top reaches its end position during the turning-in. Another object is to provide a drilling tool, the loose top of which can be turned into the jaw of the basic body without the branches constantly subjecting the loose top to a great clamping force and thereby a great, uniform resistance during the entire turning-in operation. Still another object is to provide a drilling tool, the loose top of which is held reliably clamped in the jaw of the basic body, for example, by utilizing the inherent elasticity of the branches in such a way that an optimal grip on the loose top is provided. A further object is to provide a drilling tool, the loose top of which has a minimal length, and thereby a minimal volume, in relation to its diameter, all with the purpose of reducing the consumption of expensive material to a minimum in connection with the manufacture of the loose top. It is also an object to provide a drilling tool where the basic body can transfer great torques to the loose top. Still another object is to provide a drilling tool in which the loose top is centered and retains its centricity in an accurate way in relation to the basic body. 
     An aspect of the invention provides a drilling tool for chip removing machining, including a basic body having front and rear ends, between which a first center axis extends around which the basic body is rotatable in a given direction of rotation, and a loose top having front and rear ends, between which a second center axis extends, the front end including one or more cutting edges. The front end of the basic body comprises a jaw between two axially protruding, peripherally situated branches that are elastically bendable. The branches are capable of resiliently clamping the loose top in the jaw by inner support surfaces of the branches being resiliently pressed against external side contact surfaces of the loose top, and capable of transferring torque to the loose top via tangential support surfaces of the branches and cooperating tangential contact surfaces of the loose top. The inner support surface of the individual branch extends between first and second tangentially separated side borderlines, the first tangentially separated side borderline is heading and the second tangentially separated side borderline is trailing during rotation of the tool. The individual side contact surface extends between first and second side borderlines, the second side borderline that is rotationally trailing is included in an edge to a trailing part surface, besides which the loose top is axially insertable into the jaw and turnable into and out of an operative engagement with the branches. A second imaginary diametrical line, which extends perpendicular to the second center axis of the loose top between abutting edges that abut the second side borderline of each of the two side contact surfaces, has a length that is greater than the length of an analogous first diametrical line, which extends the shortest possible distance between the inner support surfaces when the branches are unloaded, and has opposite end points located at tangential distances from the first tangentially separated side borderline and the second tangentially separated side borderline of the respective inner support surface. 
     The side contact surfaces of the loose top with edges, in combination with a suitably selected distance between the inner support surfaces of the branches, upon the turning-in provides a successively increasing deflection of the branches up to a predetermined dead or intermediate position. At the predetermined dead or intermediate position the clamping force is maximal, so as to then decrease during the continued turning a short distance further until the end position is reached. During the final phase of the rotary motion between the dead position and the end position, the clamping force in the branches assists in rapidly bringing the loose top to the end position. This may manifest itself in either a tactile perception in the fingers of the operator or a click sound being audible to the ear, or a combination of these manifestations. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particular embodiments of the invention will be described in more detail below, reference being made to the appended drawings, on which: 
         FIG. 1  is a sectioned perspective view showing the basic body and loose top of an embodiment of the drilling tool in the composed state, 
         FIG. 2  is an exploded perspective view showing the drilling tool of  FIG. 1  where the loose top is separated from the basic body, 
         FIG. 3  is an enlarged exploded view showing the drilling tool of  FIG. 1  where the loose top is shown in a bottom perspective view and the front end of the basic body is shown in a top perspective view, 
         FIG. 4  is an exploded view showing the basic body and the loose top of  FIG. 1  in side elevation, 
         FIG. 5  is an end view V-V in  FIG. 4  showing the front end of the loose top, 
         FIG. 6  is an end view VI-VI in  FIG. 4  showing the basic body from the front, 
         FIG. 7  is an end view VII-VII in  FIG. 4  showing the loose top from behind, 
         FIG. 8  is an enlarged side view of the loose top of  FIG. 1 , 
         FIG. 9  is a cross section IX-IX in  FIG. 8 , 
         FIG. 10  is a partial perspective view showing the loose top of  FIG. 1  inserted into the jaw of the basic body of  FIG. 1  in a state when the turning-in of the same is to be started, 
         FIG. 11  is a section XI-XI in  FIG. 4 , 
         FIG. 12  is a cross section XII-XII in  FIG. 4 , 
         FIGS. 13-16  are a series of pictures showing the different positions of the loose top of  FIG. 1  in connection with the turning-in of the same into the jaw of the basic body, 
         FIG. 17  is a cross section XVII-XVII in  FIG. 4 , the loose top being shown in an intermediate position between the branches, 
         FIG. 18  is a cross section corresponding to  FIG. 17  in which the loose top is shown in its end position of turning, 
         FIG. 19  is an extremely enlarged, schematic picture showing different positions of the edge of the loose top of  FIG. 1  that bends out a cooperating branch, 
         FIG. 20  is an enlarged perspective view of the jaw of the basic body of  FIG. 1 , 
         FIG. 21  is a perspective exploded view illustrating an alternative embodiment of the invention, 
         FIG. 22  is a cross section showing the loose top according to  FIG. 21  in an initial position before turning-in into the jaw of the basic body, and 
         FIG. 23  is a cross section showing the loose top according to  FIG. 21  in its turned-in, operative position. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description and the claims, a number of cooperating pairs of surfaces of the basic body and the loose top, respectively, will be described. When these surfaces are present on the basic body, the surfaces are denominated “support surfaces”, while the corresponding surfaces of the loose top are denominated “contact surfaces” (for example, “axial support surface” and “axial contact surface”, respectively). Furthermore, it should be pointed out that the loose top includes a rear end in the form of a plane surface, which in the example serves as an axial contact surface for pressing against an axial support surface in the basic body. Depending on the context, this surface will be denominated either “rear end” or “axial contact surface”. Furthermore, an inner support surface of a branch and a side contact surface of the loose top are defined by a pair of side borderlines, one of which moves ahead of the other one during rotation. The borderlines are denominated “heading” and “trailing”, respectively, in order not to be mistaken for the concepts “front” and “rear”. In the drawings, the cooperating surfaces contacting each other in the operative state of the drilling tool are shown by similar surface patterns. 
     The drilling tool shown in  FIGS. 1 and 2  is in the form of a so-called twist drill and includes a basic body  1  as well as a loose top  2  in which the requisite cutting edges  3  are included. In its composed, operative state according to  FIG. 1 , the drilling tool is rotatable around a center axis indicated by C, more precisely in the direction of rotation R. 
     In  FIG. 2 , it is seen that the basic body  1  includes front and rear ends  4 ,  5 , between which a centre axis C 1  specific to the basic body extends. In the backward direction from the front end  4 , a cylindrical envelope surface  6  extends, in which two chip flutes  7  are countersunk that in this embodiment are helicoidal, but that also could be straight as in tap borers. In the example, the chip flutes  7  end in the vicinity of a collar  8  included in a rear part  9  that is intended to be attached to a driving machine (not shown). 
     Also the loose top  2  includes front and rear ends  10 ,  111  and a center axis C 2  with which two parts  12  of an envelope surface are concentric. The envelope part surfaces  12  are separated by two helicoidal chip flute sections  13  (see also  FIG. 3 ), which form extensions of the chip flutes  7  of the basic body  1  when the loose top is mounted onto the basic body. If the loose top  2  is centered correctly in relation to the basic body  1 , the individual centre axes C 1  and C 2  coincide with the centre axis C of the composed drilling tool. 
     Reference is now made to  FIG. 3  and other drawing figures. In  FIG. 3 , it is seen that the basic body  1  in the front end thereof includes a jaw  14  that is delimited between two identical branches or shanks  15  and an intermediate bottom that forms an axial support surface  16  for the loose top. Each branch  15  includes an inner support surface  17  that extends axially rearward from a front end surface  18  of the branch. Furthermore, the individual branch  15  includes a tangential support surface  19  that is facing forward in the direction of rotation, and thus is heading. An opposite, trailing tangential support surface  20   a  is included as a front part of the concave surface  20  that is present between two helicoidal borderlines  21 ,  22  and delimits the chip flute  7 . In a known way, the individual branch  15  is elastically bendable to be resiliently clampable against the loose top  2 . This is realized by the fact that the material in at least the front portion of the basic body  1  has a certain inherent elasticity, for example, lower modulus of elasticity than the material in the loose top  2 . The material in at least the front portion can include steel. The material in the loose top may, in a traditional way, include cemented carbide, which is hard carbide particles in a binder metal, cermet, ceramics or the like. Advantageously, the axial support surface  16  is plane and extends perpendicular to the center axis C 1 . In addition, the axial support surface  16  extends diametrically between the two part surfaces that together form the envelope surface  6 . Generally the axial support surface has a §-like contour shape. 
     As is further seen in  FIG. 3 , the rear end of the loose top is represented by an axial contact surface  11  that, like the axial support surface  16 , can be plane and extends perpendicular to the center axis C 2 . The axial contact surface  11  extends between diametrically opposed envelope part surfaces  12  and has a §-like contour shape. Furthermore, the loose top  2  includes a pair of external, diametrically opposed side contact surfaces  23 , against which the inner support surfaces  17  of the branches can be resiliently clamped. In certain embodiments, the contour shape of the surfaces  11  and  16  is identical, whereby complete surface contact is established in the operative state of the tool. 
     The front end  10  of the loose top  2 , in which the cutting edges  3  are included, is represented by an end surface that is composed of a plurality of part surfaces (see  FIGS. 5 and 8 ), which in this embodiment are identical in pairs and therefore not described individually. Behind the individual cutting edge  3 , as viewed in the direction of rotation, a primary clearance surface  24  is formed, which has a moderate clearance angle and transforms into a secondary clearance surface  25  having a greater clearance angle, via a borderline  26 . Via an additional borderline  27 , the secondary clearance surface  25  transforms into a third clearance surface  28 , which in turn, via an arched borderline  29 , transforms into a chip flute  13 . As may be best seen in  FIG. 8 , the concave surface  30  that delimits the chip flute section  13  extends partly up to the individual cutting edge  3  and forms a chip surface for the cutting edge  3 . In the chip surface of the cutting edge, also a convex part surface  31  is included. The design of the front end of the loose top may be modified in miscellaneous ways and is therefore incidental provided that the loose top can carry out chip removing machining. 
     Furthermore, it should be observed that adjacent to the envelope part surface  12 , a guide pad  32  (see  FIGS. 3 and 5 ) is formed, the main task of which is to guide the drilling tool. The diameter of the drilled hole is determined by the diametrical distance between the peripheral points  33  where the cutting edges  3  meet the guide pads  32 . Also the two cutting edges  3  converge into a tip  34 , which forms the very foremost part of the loose top, and in which there may be included a so-called chisel edge and a minimal centering punch (lack designations). 
     Reference is now made to  FIG. 8 , from which it is seen that the individual side contact surface  23  of the loose top  2  is laterally delimited between first and second side borderlines  35 ,  36 , the first side borderline  35  is heading and the second side borderline  36  is trailing during rotation of the tool. Rearward (downward in the drawing), the side contact surface  23  is delimited by a transverse, rear borderline  37 , while its front limitation includes two oblique borderlines  38 ,  39 , the first oblique borderline borders on the secondary clearance surface  25  and the second oblique borderline borders on the third clearance surface  28 . The second side borderline  36  is included in (or forms) an edge, designated  40 , that constitutes a transition between the side contact surface  23  and a rotationally trailing part surface  41 . Although it is feasible to form the edge  40  sharp, most embodiments manufacture the edge  40  as a radius transition, including, for example a convexly rounded, long narrow surface in the transition between the side contact surface  23  and trailing part surface  41 . Also, the trailing part surface  41  in this embodiment is wedge-shaped and borders on the trailing chip flute surface  30 . In particular, the trailing part surface  41  is delimited between the edge  40  and an acute borderline  42  that forms an acute angle with the edge  40 . The edge  40  and the acute borderline  42  diverge in the backward direction. 
     At the first side borderline  35  thereof, the side contact surface  23  transforms into a concave recess surface  43  that in turn borders on a tangential contact surface  44  (see  FIG. 3 ), against which the individual branch  15  is pressed, in order to transfer torque to the loose top. 
     In the illustrated embodiment, the side contact surfaces  23 , like the inner support surfaces  17  of the branches  15 , are essentially plane. As is further seen from the cross section in  FIG. 9 , the two opposite side contact surfaces  23  of the loose top  2  diverge at a certain angle α in the direction from the rear end toward the front end. In the reference plane RP 1 , which is situated on a level with the front end  40   a  of the edge  40 , the loose top has accordingly a width W 1  that is somewhat greater than the width W 2  in the reference plane RP 2 , which is situated on a level with the rear borderline  37  of the side contact surface. The difference between the width measures W 1  and W 2  can be very moderate, where the angle of divergence α is small. In the illustrated embodiment, W 1  is about 8.00 mm and W 2  is about 7.97 mm, the angle α is about 0.86° (α/2=0.43°). Although this angle of divergence is diminutive, the angle is, however, fully sufficient for bending out the branches  15  so much that the branches subject the loose top to a considerable clamping force. 
     In this connection, it should be pointed out that the angle of divergence α may vary upward as well as downward from about 0.86°. However, the angle of divergence α should amount to at least about 0.20° and at most about 2°. In certain embodiments, the angle of divergence α should be within the range of 0.60-1.20°. The size of the angle α depends on the axial length of the inner support surface  17  and side contact surface  23 . Specifically, the angle should be adapted to the length of the surfaces in such a way that surface contact is attained in the operative state of the loose top. 
     Because the side contact surfaces  23  diverge in the way described above, the front end  40   a  of the edge  40  is located at a greater radial distance from the center axis C 2  of the loose top than the rear end  40   b  of the edge  40 . In other words, the front end  40   a  will first contact the inner support surface  17  of the individual branch in connection with the turning-in of the loose top into the jaw  14 . Also, the edge  40  in this embodiment is straight. 
     Reference is now made to  FIG. 20 , which shows that the inner support surface  17  of the individual branch  15 , like the cooperating side contact surface  23  of the loose top, is delimited between first and second side borderlines  51 ,  52 . The first borderline  51  is heading and the second borderline  52  is trailing during rotation. Between the inner support surface  17  and the tangential support surface  19 , there is a concave clearance surface  53  having a radius that is greater than the radius of the recess surface  43 . 
     In  FIG. 10 , the loose top  2  is shown in an initial position in which the loose top has been inserted axially into the jaw between the branches  15 , but has not been turned into its operative end position. In order to coarse-center or provisionally retain the loose top in a reasonably, but not exactly, centered position during the subsequent turning-in, the rear part of the loose top and the inner parts of the branches are formed with cooperating guide surfaces. Each side contact surface  23  (see  FIGS. 3 and 4 ) transforms into a convex guide surface  45  being axially behind via an intermediate surface  46 . Between the guide surface  45  and the axial contact surface  11  of the loose top, a clearance surface  47  is present. As is seen in  FIG. 7 , the two guide surfaces  45 , which are formed on diametrically opposed sides of a central portion of the loose top, have a rotationally symmetrical shape. The surfaces follow a circle S 2 , the diameter of which is designated D 2 . The circle S 2 , and thereby also the surfaces  45 , are concentric with the centre axis C 2  of the loose top. In the illustrated embodiment, the surfaces  45  are cylindrical, although they could also be conical. 
     As is seen in  FIG. 3 , in combination with  FIGS. 6 and 20 , the two branches  15  are, at the rear ends thereof, formed with a pair of concave, internal guide surfaces  48 , which cooperate with the convex, external guide surfaces  45  of the loose top. Each such guide surface  48  transforms into an inner support surface  17  via an intermediate surface  49 , which is inclined in the inward/rearward direction from the inner support surface  17 . Also the two internal guide surfaces  48  are cylindrical, or alternatively conical, and are defined by an imaginary circle S 1  (see  FIG. 6 ), the diameter of which is designated D 1 . The diameter D 1  of the circle S 1  is somewhat greater than the diameter D 2  of the circle S 2 , which means that the guide surfaces  45 ,  48  do not contact each other when the loose top is operatively clamped in the jaw of the basic body. The difference in diameter may in practice amount to one or a few tenth of a millimeter. However, it is guaranteed that the loose top is coarse centered and retains an approximate intermediate position between the branches during the turning-in that is carried out from the initial position shown in  FIG. 10 . The fact that the diameters D 1  and D 2  are differently great means that the guide surfaces  45 ,  48  do not impose requirements of dimensional accuracy in connection with the manufacture. 
     The guide surface  45  (see  FIG. 8 ) is partially displaced in the tangential direction in relation to the side contact surface  23 , in such a way that the borderline  45   a  to the chip flute  13  is displaced rearward in the direction of rotation R in relation to the limiting edge  40  of the side contact surface, which is toward the left in  FIG. 8 . During the turning-in of the loose top in the turning direction V, the borderline  45   a  will therefore move before the limiting edge  40 . The practical consequence of this will be that the guide surfaces  45  can start to co-operate with the guide surfaces  48  with the purpose of provisionally coarse centering the loose top already before the limiting edges  40  get in contact with the inner support surfaces  17  of the branches  15 . 
     Reference is now made to  FIG. 11 , in which DL 1  designates a straight, first diametrical line that intersects the centre axis C 1  and extends the shortest possible distance between the inner support surfaces  17  of the branches  15  facing each other, and forms a right angle with the inner support surfaces. The ends of this shortest possible diametrical line DL 1  are designated Ea, Eb. It is evident that any other imaginary diametrical line (lacks designation) drawn between the inner support surfaces  17  and intersecting the centre axis C 1  becomes longer than the shortest diametrical line DL 1 . This applies irrespective of whether the imaginary, longer diametrical line is turned clockwise or counter-clockwise around C 1  in relation to the diametrical line DL 1  shown in  FIG. 11 . 
     In  FIG. 12 , DL 2  designates a second, likewise straight diametrical line that extends between the edges  40  of the two opposite side contact surfaces  23  and intersects the centre axis C 2  of the loose top. The diametrical line DL 2  extends between the front end points  40   a  of the edges  40  (see  FIG. 8 ). The individual side contact surface  23  forms an acute angle with the diametrical line DL 2 . In the example, the angle β amounts to about 85°. In certain embodiments, the angle β amounts to at least about 75° and at most about 88°. In yet more certain embodiments, the angle amounts are within the range of 80-86°. From the enlarged detailed section in  FIG. 12 , it is furthermore seen that the side contact surface  23  and the trailing part surface  41  form an obtuse angle γ with each other. In the example, γ amounts to about 152°. By the fact that the angle γ is obtuse, rather than acute, which would also be feasible, the portion of the loose top that surrounds the edge  40  will become robust and endure forces that act against the edge. 
     In certain embodiments, the diametrical line DL 2  is somewhat longer than the diametrical line DL 1 . Because the length difference is small, for example, some hundredths of a millimeter, and not visible to the naked eye, reference is now made to the series of pictures in  FIGS. 13-16 , as well as to the enlarged, schematic picture in  FIG. 19 . In  FIG. 13 , the loose top  2  is shown in an initial position P 1  according to  FIG. 10 .  FIG. 13  shows how the loose top in this position can be freely inserted axially into the jaw by the fact that the side contact surfaces thereof have no contact with the branches  15 . In this position, the convex guide surfaces  45  of the loose top are partially located between the concave guide surfaces  48  of the branches  15 . In a first step, the loose top is turned into the position P 2  according to  FIG. 14 , where the two opposite edges  40  get in contact with the inner support surfaces of the branches. After further turning, the loose top  2  reaches the position P 3  shown in  FIG. 15  where the diametrical lines DL 1  and DL 2  coincide. In this position, the edges  40  have reached a dead or intermediate position, in which the clamping force of the branches  15  is maximal. From this dead position P 3 , the loose top is turned further a short distance to reach its end position P 4  according to  FIG. 16 . In this position, the edges  40  have passed the dead position P 3  according to  FIG. 15 , but without the spring force or tensile capacity of the branches  15  having been exhausted. In the end position P 4  according to  FIG. 16 , the side contact surfaces  23  abut against the inner support surfaces  17  at the same time as the torque-transferring tangential support surfaces  19  of the branches are pressed in close contact against the tangential contact surfaces  44  of the loose top. 
     In  FIG. 19 , the different positions of the edge  40  in relation to the individual branch  15  are illustrated more clearly. In the position P 1  according to  FIG. 13 , the edge  40  lacks all contact with the inner support surface  17  of the branch. In the position P 2 , contact has been established with the inner support surface  17 . From this position and on, the edge  40  of the loose top starts to bend out the branch  15  while applying a successively increasing clamping force to the loose top. In the dead position P 3  according to  FIG. 15 , the clamping force in the branch has grown to a maximum, because here, the diametrical lines DL 1  and DL 2  coincide. In order to reach its end position P 4 , the loose top is turned further a short distance clockwise around the centre axis C 1 . During the comparatively short move between the positions P 3  and P 4 , when the edge  40  has passed the dead position P 3 , the continued turning of the loose top will entirely or partly be overtaken by the branches  15  as a consequence of the fact that the clamping force in the branches now aims to bring the loose top to the end position in which it no longer can be turned further as a consequence of the fact that the pairs of surfaces  23 ,  17  and  19 ,  44  are held pressed in close contact against each other. Practical tests carried out using the tool have shown that the concluding turning between the positions P 3  and P 4  is followed by a pronounced tactile sensation in the fingers of the operator, and at times an audible click sound, which confirms to the operator that the loose top has reached its operative end position. 
     The exact centering of the loose top in relation to the basic body is initiated in the position P 2 , when the edges  40  of the loose top first get in contact with the inner support surfaces  17  of the branches  15 . As the edges are turned toward their end position P 4 , the centering will become increasingly distinct and exact as a consequence of the increasing clamping force in the branches. The branches  15  retain an ample clamping force in the end position P 4 , even if the clamping force to a certain extent has been reduced in relation to the maximal clamping force in the position P 3 . By suitably adjusting such geometrical factors as the amount of rotary motion between P 3  and P 4  in relation to the selected difference in length between DL 1  and DL 2 , the clamping force in the operative end position can be predetermined. For instance, the clamping force in the end position P 4  can be determined to 50% of the maximal clamping force in the dead position P 3 . 
     In  FIGS. 17 and 18 , it is illustrated how narrow, although pronounced slits  50  arise between the pairs of cooperating guide surfaces  45 ,  48  of the loose top  2  and of the branches  15 , respectively, when the loose top is turned toward its operative end position. 
     Further, the drilling tool includes that the side contact surfaces  23  of the loose top  2  are situated near the front end  10  of the loose top, and that the corresponding inner support surfaces  17  of the branches  15  are situated far in front on the branches. Accordingly, the side contact surfaces  23  extend rearward from the two clearance surfaces  25 ,  28  that are included as part surfaces in the front end  10  of the loose top. In an analogous way, the inner support surfaces  17  of the branches extend rearward from the edge lines that form transitions to the front end surfaces  18  of the branches. By this location of the side contact surfaces and the inner support surfaces, respectively, there is provided a powerful grip or pinch along the front portion of the loose top adjacent to the cutting edges, because the branches have their greatest bending capacity, and thereby their optimal gripping capacity, in the area of the free, front ends thereof rather than in the vicinity of the rear ends. 
     In  FIG. 8 ,  44   a  designates the straight borderline that forms a transition between the part envelope surface  12  and the tangential contact surface  44  of the loose top (see also  FIG. 3 ). Said tangential contact surface  44  is inclined in relation to the axial contact surface  11  of the loose top at an angle δ, which in the example amounts to about 76°. The tangential support surface  19  (see  FIGS. 10 and 20 ) that cooperates with the individual tangential contact surface  44  is correspondingly inclined. By this inclination of the respective surfaces, a locking element is provided that, in combination with the pinching effect of the branches  15 , counteracts unintentional axial retraction of the loose top out of the jaw  14 , for example, in connection with the retraction of the drilling tool out of a drilled hole. The angle δ may vary upward as well as downward. In certain embodiments the angle amount is at least about 65° and at most about 85°. 
     In  FIGS. 4 and 5 , it is seen that the loose top  2  includes a key grip in the form of a pair of peripherally situated notches or seats  55 . 
     Embodiments of the invention enable the operator to obtain an apparent confirmation of the loose top having reached its operative end position during the turning-in. Further, the resistance of the bendable branches to the turning-in is not constantly great, but maximal only during the short moment when the edges are turned past the dead position. Furthermore, the inherent elasticity of the branches assumes entirely or partly the final turning-in from the dead position to the end position during the final phase of the rotary motion. In other words, the risk that the operator, for example, when in a hurry, unintentionally fails to finish the manual turning all the way up to the absolute end position is counteracted. Additionally, the loose top is securely pinched between the front ends of the branches, where the branches are most bendable and give an optimal clamping force. Furthermore, the loose top may be given a minimal volume in relation to its diameter, whereby the consumption of expensive material in the same is reduced to a minimum. Yet further, the basic body can transfer considerable torques to the loose top because the tangential support surfaces of the branches can be given an optimized length within the scope of the available axial length of the loose top. Furthermore, the loose top can be mounted and dismounted in a simple way with use of a simple key. In addition, the two side contact surfaces of the loose top are well exposed and easy to access if the two side contact surfaces would need to be ground in order to guarantee good centering. 
     Reference is now made to  FIGS. 21-23 , which illustrate an alternative embodiment in which the inner support surface  17  of each individual branch  15  is formed with a plurality of part surfaces or surface sections  17   a ,  17   b  and  17   c . The first surface section  17   a  extends between the rotationally trailing borderline  52  and the surface section  17   c , which is a concave radius transition to the second surface section  17   b , which in turn connects to the borderline  51 . Via a transition surface  53  that includes three facet surfaces, the surface section  17   b  transforms into the tangential support surface  19 . In the embodiment shown, the surface section  17   b  has a concave, more precisely part-cylindrical shape, while the surface section  17   a  is plane, or possibly slightly cambered. 
     Axially behind the inner support surface  17  there is, in the same way as in the previous embodiment, a cylindrical or otherwise rotationally symmetrical guide surface  48  that is included in a thickened, rear portion of the branch  15 , and is separated from the inner support surface  17  via an intermediate surface  49 . 
     In analogy with the inner support surface, the cooperating side contact surface  23  of the loose top  2  includes two surface sections  23   a ,  23   b , the first surface section  23   a  of which is rotationally trailing in relation to the second surface section  23   b . The surface section  23   a  extends between a borderline  23   c  to the surface section  23   b  and between the edge  40  that forms a transition to the rotationally trailing part surface  41 . The surface section  23   b  is convex and has the same rotationally symmetrical shape as the concave surface section  17   b  of the branch  15 . In certain embodiments, the surface section  23   b  has a cylindrical shape. Via the borderline  35 , the surface section  23   b  transforms into the recess surface  43 , which in turn transforms into the tangential contact surface  44 . The surface section  23   a  is plane, or slightly cambered, like the surface section  17   a  included in the inner support surface  17 . Axially behind the two surface sections  23   a ,  23   b , there is a convex, cylindrical or otherwise rotationally symmetrical guide surface for cooperation with the concave guide surface  48 . 
     In  FIG. 22 , the loose top  2  is shown in an initial position before turning-in (P 1 ) into the jaw between the branches  15 , which is similar to the position of the first embodiment shown in  FIG. 13 . The two plane surface sections  17   a  that are included in the two inner support surfaces of the branches are mutually parallel. A diametrical line DL 1  that intersects the center axis C and is perpendicular to the surface sections  17   a  represents the shortest distance between the surface sections  17   a . Said diametrical line DL 1  contacts the surface sections  17   a  in points that are situated between their side limitations  17   c  and  52 , respectively. DL 2  designates a second diametrical line that extends between the edges  40  along the surface sections  23   a  that are included in the two opposite side contact surfaces  23  of the loose top. The second diametrical line DL 2  is some hundredths of a millimeter longer than the first diametrical line DL 1 . However, the radial distance between the center axis C and the edge  40  that forms an end of the diametrical line DL 2  is somewhat smaller than the radial distance between the center axis C and the concave surface section  17   b . This means that the edges  40  of the loose top will clear the concave surface sections  17   b  when the turning-in of the loose top is started. 
     When the loose top  2  is turned in from its initial position (P 1 ) according to  FIG. 22  to the operative end position (P 4 ) according to  FIG. 23 , the following occurs. Initially, the edges  40  will freely pass the surface sections  17   b  without affecting the branches  15 . When the edges  40  have passed the radius transitions  17   c , the edges  40  will contact the surface sections  17   a  and successively start to bend out the branches. When the pair of edges  40  reaches the rotation angle position in which the diametrical lines DL 1  and DL 2  coincide with each other, which is similar to position P 3  in  FIG. 15  of the first embodiment, the deflection and thereby the spring forces becomes maximal, as a dead position is passed. In this state, the convex surface section  23   b  of the loose top has started to overlap the concave surface section  17   b  of the inside of the individual branch  15 . From said dead position, the turning-in of the loose top continues primarily by the spring force in the branches up to the operative end position, which is shown in  FIG. 23  and in which the tangential contact surfaces  44  of the loose top have been pressed against the tangential support surfaces  19  of the branches. In the final stage of the turning-in, which includes turning-in between the dead position and the end position, the convex surface sections  23   b  of the loose top will be located opposite the concave surface sections  17   b . The spring force in the branches will be transferred to the loose top by surface contact between the surface sections  17   b  and  23   b . Simultaneously, the plane surface sections  17   a  clear somewhat from the internal, plane surface sections  23   a  of the loose top. In other words, the fastening force that the branches exert will be located along an axial plane AP that extends diametrically between the surface pairs  17   b ,  23   b  according to  FIG. 23 . 
     The embodiment according to  FIGS. 21-23  includes an alternative type of axial locking element for the loose top that includes two seats  54  formed in the rear ends of the branches  15  and two male members  55  on the loose top. The seat  54  is, in this embodiment, a chute that is recessed in the individual branch  15  and situated between tangential support surface  19  thereof and the axial support surface  16  of the basic body. The individual male member  55  is in turn a ridge that is situated axially behind the tangential contact surface  44  of the loose top and connects to the axial contact surface  11 . In other words, the ridge  55  projects laterally in relation to the tangential contact surface  44 , the rear part thereof transforming into the axial contact surface  11 . When the loose top is turned into its operative position, the ridges  55  engage the chutes  54  without the ridges getting surface contact with the chutes. The ridges  55  are therefore activated only if the negative axial forces on the loose top overcome the spring force in the branches. 
     Further, the side contact surfaces of the loose top do not necessarily need to be plane. For instance, the side contact surfaces may be slightly cambered or markedly convex and arranged to cooperate with inner support surfaces that have been given a more or less markedly concave shape. 
     Although described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departure from the spirit and scope of the invention as defined in the appended claims.