Abstract:
A rotatable tool for chip removing machining including a basic body and a loose top; the body includes two torque-transferring drivers and an intermediate bottom in which a rear coupling part of the loose top is receivable, and a center hole that mouths in the intermediate bottom. A centric pin is insertable into the center hole, and a screw is accessible from the outside of the basic body. The coupling part includes a pair of side contact surfaces which cooperate with a pair of side support surfaces on the drivers of the basic body for the transfer of torque and negative axial forces between the coupling part and the drivers.

Description:
This application claims priority under 35 U.S.C. §119 to Swedish Patent Application No. 0900845-9, filed on Jun. 23, 2009, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates generally to a rotatable tool intended for chip removing machining and of the type that includes, on one hand, a basic body having front and rear ends between which a first center axis extends, with which an envelope surface is concentric and around which the basic body is rotatable, and, on the other hand, a loose top that includes front and rear ends between which a second center axis extends with which an envelope surface is concentric. The front end of the basic body includes a jaw which is delimited by two torque-transferring drivers and an intermediate bottom, in which a rear coupling part of the loose top is receivable. A centric pin, which protrudes axially rearward from the coupling part, is insertable into a center hole that mouths in the bottom of the jaw. The center hole has a hole wall, which extends axially inside the basic body and in which a threaded hole being concentric with a third center axis, mouths, the hole being intended for a screw, which co-operates with the pin and is accessible from the outside of the basic body. The coupling part of the loose top and the drivers of the basic body include pairs of co-operating contact surfaces and support surfaces for the transfer of torque and axial forces between the same. The invention also relates generally to a loose top and a basic body as such. 
     Tools of the kind in question are suitable for chip removing or cutting machining 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 OF THE INVENTION 
     For more than a decade, drilling tools as well as milling tools have been developed, e.g., in the form of shank-end mills, that, contrary to integral solid tools, are composed of two parts, viz. a basic body and a head being detachably connected with the same and thereby being replaceable, and in which head the requisite cutting edges are included. 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, viz. the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the requisite cutting edges a good chip-removing capacity, a good machining precision and a long service life. In other words, the head forms a wear part that can be discarded after wear-out, while the basic body can be re-used several times (e.g., 10 to 20 replacements). A now recognized denomination of such cutting edge-carrying heads is “loose tops”, which henceforth will be used in this document. 
     On rotatable tools of the loose top type, a plurality of requirements are put, one of which is that torque should be transferable in a reliable way from the rotatable, driven basic body to the replaceable loose top. Furthermore, the basic body should, without problems, be able to carry the rearwardly directed, positive axial forces that the loose top may be subjected to during operation, e.g., drilling. Another requirement is that the loose top should be held centered in an exact and reliable way in relation to the basic body. Accordingly, each unintentional eccentricity between the center axis of the loose top and the center axis of the basic body should not be more than 0.01 mm. Most preferably, it should be smaller than 0.005 mm unless exact centricity can be achieved. A further requirement that is of particular importance as for drilling tools is that the loose top should be fixedly held to the basic body not only during drilling of a hole, but also during retraction of the drilling tool out of the same when the loose top is subjected to negative axial forces that aim to pull the same out of the jaw of the basic body. Yet another requirement or desire from the users&#39; side is 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 an expensive material, should be possible to be manufactured at low costs. 
     Drilling tools as well as milling tools (shank-end mills) of the loose top type are widely known and may be divided into a number of different categories depending on the ideas on which the designs are based. Accordingly, there is a dividing line between a first category of tools that are based on the use of flexible, elastically bendable branches as drivers in the jaw of the basic body, and a second category of tools that utilize fixed, non-compliant lugs as drivers. In the first-mentioned category of tools, the branches are brought to be bent outward in connection with the mounting of the loose top, so as to then resiliently clamp the same in the jaw of the basic body. A general merit of such tools is that the clamping of the loose top in all essentials can be provided with the inherent elasticity of the branches. In other words, no separate tightening or lock features, e.g., in the form of screws are required. 
     A disadvantage of the fact that the drivers consist of flexible branches is, however, that the inherent elasticity of the branches runs the risk of being reduced after a certain time of use of the tool. When a number of loose tops have been replaced, the branches may accordingly start to be deformed plastically, rather than elastically, and in such a way successively lose their clamping force. There are many disadvantages with this. Above all, the loose top risks losing its exact centering in relation to the basic body. Furthermore, the ability of the drivers to counteract axial retraction of the loose top out of the jaw may be considerably impaired. This is particularly disadvantageous in connection with drilling tools, where the loose top risks remaining in the drilled hole in connection with the tool being pulled out of the same. 
     To the second category of tools, i.e., tools the torque-transferring drivers of which are non-compliant, belongs among others the drilling tool that is disclosed in U.S. Pat. No. 6,012,881. In this case, a rear coupling part of the loose top is inserted axially in a jaw between two non-compliant drivers, the insides of which include axially running, torque-transferring ridges that engage corresponding chutes in the coupling part of the loose top, besides which a centric pin protruding rearward from the loose top is inserted into a center hole that is formed in the basic body and mouths in the bottom of the jaw. With the pin, a radial screw mounted in a radial hole in the basic body co-operates with has the purpose of locking and centering the loose top in relation to the basic body. 
     A disadvantage of the drilling tool according to U.S. Pat. No. 6,012,881 is, however, that certain requirements on the tool are so conflicting that practical production of the same becomes difficult, not to say impossible. Accordingly, a fine fit is required between the concave and convex contact surfaces of the drivers and the coupling part, respectively, which have the purpose of centering the loose top, in order for the centering to become acceptable. The extremely fine fit between the contact surfaces means, however, that mounting as well as dismounting will be difficult to carry out. Another disadvantage is that all negative (retractive) axial forces that act on the loose top have to be carried by the radial screw alone via the pin. In order to manage this task, the radial screw has to be sturdy. What is more, also the fit between the centering pin of the loose top and the hole in the basic body has to be fine. Namely, if the fit is large (e.g., 0.1 mm), the radial screw will upon tightening apply radial forces to the pin and thereby the loose top that aim to space apart the loosetop from the centered position. If it is fine (e.g., about 0.01 mm), a frictional resistance difficult to overcome arises when the pin is to be pushed into and pulled out of, respectively, the hole. 
     The present invention aims at obviating the above-mentioned disadvantages of the known tool and at providing an improved loose-top tool, in particular a drilling tool. Therefore, an object of the invention to provide a loose-top tool in which the loose top in a reliable way can be retained in the jaw of the basic body even if the loose top is subjected to considerable negative axial forces. Furthermore, the invention aims at allowing an exceptionally exact centering of the loose top in relation to the basic body. 
     Another object of the invention is to provide a loose-top tool in which the loose top can be mounted in a simple and convenient way without taking into account any disadvantageous frictional resistance. Furthermore, the centering should be provided via the rear pin of the loose top, whereby the centering is made independent of other surfaces than co-operating contact surfaces on the pin and the receiving hole. 
     SUMMARY OF THE INVENTION 
     In an embodiment, the invention provides a rotatable tool for chip removing machining, including a basic body having front and rear ends between which a first center axis (C 1 ) extends, with which an envelope surface is concentric and around which the basic body is rotatable; and a loose top that includes front and rear ends between which a second center axis (C 2 ) extends with which an envelope surface is concentric. The front end of the basic body includes a jaw which is delimited by two torque-transferring drivers and an intermediate bottom in which a rear coupling part of the loose top is receivable, and a center hole that mouths in the intermediate bottom. The center hole has a hole wall which extends axially inside the basic body and in which a threaded hole being concentric with a third center axis (C 3 ) mouths for a screw to co-operate with a centric pin which protrudes axially rearward from the coupling part. The centric pin is insertable into the center hole, and the threaded hole is accessible from the outside of the basic body. The coupling part of the loose top includes a pair of axial contact surfaces which cooperate with a pair of axial support surfaces on the drivers of the basic body for the transfer of positive axial forces between the coupling part and the drivers. The coupling part of the loose top includes a pair of side contact surfaces which cooperate with a pair of side support surfaces on the drivers of the basic body for the transfer of torque and negative axial forces between the coupling part and the drivers. The pair of side contact surfaces define two laterally projecting male members arranged to engage two corresponding seats defined by the pair of side support surfaces of the drivers in an axially locking position. The pin of the loose top includes a surface for turning the pin and the loose top by cooperation with the screw. 
     In another embodiment, the invention provides a loose top for rotatable tools for chip removing machining, including front and rear ends between which a center axis (C 2 ) extends, with which a pair of envelope part surfaces are concentric, and a centric pin protruding from a rear coupling part. The coupling part separates two axial contact surfaces and includes two laterally protruding male members which are axially separated from the axial contact surfaces and prevent retraction of the coupling part out of a jaw in a tool basic body. The pin includes a surface for turning the pin and the loose top when the surface cooperates with a screw. 
     In yet another embodiment, the invention provides a basic body for rotatable tools for chip removing machining, including front and rear ends between which a center axis (C 1 ) extends around which the basic body is rotatable, and with which an envelope surface is concentric. The front end includes a jaw which is delimited by two torque-transferring drivers, and an intermediate bottom in which there mouths a center hole. The center hole has a hole wall which extends axially inside the basic body and in which there mouths a radial hole that includes a female thread for a screw and that extends between the center hole and the envelope surface. Free, front ends of the drivers form axial support surfaces for a co-operating loose top, and in the insides of the drivers, seats are recessed, which open inward toward the center of the jaw. 
     The invention is based on the idea of isolating the necessary centering function and axial locking function from each other and simultaneously ensuring convenient mounting and dismounting, respectively, of the loose top. This is effected by allowing two opposite male members on the rear coupling part of the loose top to co-operate with two corresponding, female-like seats in the insides of the drivers, with the purpose of locking the loose top axially in the jaw between the drivers. In doing so, the centering of the loose top is in all essentials provided only by the rear pin that by a unique design allows an extremely exact centering of the loose top in relation to the basic body, without because of this giving rise to difficulties of mounting and dismounting, respectively, the loose top. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention. 
         FIG. 1  is a perspective view of a loose-top tool in the form of a drilling tool, the basic body and the loose top of which are shown in an assembled state; 
         FIG. 2  is an exploded perspective view showing the loose top separated from the basic body; 
         FIG. 3  is an enlarged exploded view showing a jaw included in the basic body from a top perspective view and a loose top from a bottom perspective view; 
         FIG. 4  is a top perspective view of the loose top; 
         FIG. 5  is an end view of the loose top as viewed from the front; 
         FIG. 6  is a side view of the loose top; 
         FIG. 7  is an end view of the loose top as viewed from behind; 
         FIG. 8  is an additional side view of the loose top as viewed at an angle of 90° to  FIG. 6 ; 
         FIG. 9  is an exploded top perspective view of the basic body and the loose top; 
         FIG. 10  is an end view of the basic body as viewed from the front; 
         FIG. 11  is an enlarged detailed section in the assembled tool; 
         FIG. 12  is an analogous detailed section showing the loose top separated from the basic body; 
         FIG. 13  is a partial side view of the front part of the basic body; 
         FIG. 14  is section A-A in  FIG. 13  showing a screw separated from the basic body; 
         FIG. 15  is section A-A in  FIG. 13  showing the screw inserted into a radial hole in the basic body; 
         FIG. 16  is an enlarged and schematic picture that illustrates the geometrical design of a centering pin included in the loose top; and 
         FIG. 17  is an analogous picture showing an alternative embodiment of the centering pin; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a number of co-operating 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 same are denominated “support surfaces”, while the corresponding surfaces of the loose top are denominated “contact surfaces” (e.g., “axial support surface” and “axial contact surface”, respectively). 
     In the drawings, the loose-top tool according to the invention has been exemplified in the form of a drilling tool, more precisely in the form of a twist drill, i.e., a drill the chip flutes of which are helicoidal. As is seen in  FIGS. 1 and 2 , the tool includes a basic body  1  and a loose top  2  in which the requisite cutting edges  3  are included. In its assembled, 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 center 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 case are helicoidal (the invention is also applicable to so-called tap borers having straight chip flutes). In the example, the chip flutes  7  end in a collar included in a rear part  8  that is intended to be attached to a driving machine (not shown). 
     Also the loose top  2  includes front and rear ends  9 ,  10  and an own center axis C 2  with which two convex envelope part surfaces  11  are concentric. Between these surfaces  11 , two helicoidal chip flutes or chip flute sections  12  are countersunk, 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, the individual center axes C 1  and C 2  coincide with the center axis C of the assembled drilling tool. 
     Since the major part of the basic body  1  lacks interest in connection with the invention, henceforth only the front end portion thereof will be illustrated together with the loose top  2 , more precisely on an enlarged scale. 
     Now reference is made also to other drawing figures,  FIGS. 3 and 9  of which illustrate how the chip flutes  7  of the basic body  1  are delimited by concave surfaces  13  that extend between helicoidal borderlines  14 ,  15 . In an analogous way, each chip flute  12  in the loose top  2  includes a concave limiting surface  13 . 
     The front end  9  of the loose top (see  FIGS. 4 and 5 ), in which the cutting edges  3  are included, is represented by an end surface that is composed of a plurality of part surfaces, which are identical in pairs and therefore not described individually. Behind the individual cutting edge  3 —as viewed in the direction of rotation R—a primary clearance surface  16  is formed, which has a moderate clearance angle and transforms into a secondary clearance surface  17  having a greater clearance angle. The clearance surface  17  transforms into a third clearance surface  18 , which in turn transforms into the chip flute  12  via a concave, arched borderline  19 . In this case, in each secondary clearance surface  17 , a flushing-medium duct  20  mouths. The concave limiting surface  13  of the individual chip flute  12  extends axially not only up to a borderline  19 , but also up to an individual cutting edge  3 , where it forms a chip surface adjacent to the same. In the transition between each surface  13  and the (rotationally) trailing envelope surface  11 , a guide pad  21  is formed, the primary purpose of which is to guide the drilling tool in connection with hole making. Also the guide pad  21  may be utilized to surface-finish the generated hole wall. The diameter of the generated hole is determined by the diametrical distance between the peripheral points  3   a ,  3   b  where the cutting edges  3  meet the guide pads  21 . It should also be mentioned that the cutting edges  3  converge into a front tip  3   c  and form an obtuse nose angle, e.g., within the range of 120-160°, with each other. 
     In  FIG. 3 , it is furthermore seen that the loose top  2  includes a coupling part generally designated  22 , which is situated between two diametrically opposed axial contact surfaces  23 . A centering pin  24  protrudes axially rearward from a plane, rear end surface  25  of the coupling part  22 . 
     In the basic body  1 , a jaw  26  (see  FIG. 3 ) is formed, which is delimited by two diametrically separated drivers  27   a ,  27   b  as well as by an intermediate bottom surface  28 . In the bottom surface  28 , a center hole  29  mouths into which the pin  24  is insertable. The drivers  27   a ,  27   b  are essentially non-compliant lugs (contrary to elastically compliant branches). As may be best seen in  FIG. 14 , a through hole  30  extends radially between the center hole  29  and the envelope surface  6  of the basic body. In order to distinguish the hole  30  from the center hole  29 , henceforth the same is denominated “radial hole”. In the radial hole  30 , the center axis of which is designated C 3 , a female thread is included that can co-operate with the male thread of a screw  31 , which is arranged to co-operate with the pin  24 . 
     In contrast to the radial screw that is included in the known tool, and that has the purpose of locking the loose top axially, the radial screw  31  of the tool according to the invention only has the purpose of turning the pin  24 —and thereby the loose top in its entirety—and thereafter retain the same in a given position of turning. The axial locking of the loose top is, on the contrary, ensured by other more reliable means, viz. co-operating male and female members included in the coupling part and the drivers, respectively, as will be seen more in detail below. 
     The coupling part  22  of the loose top has a long narrow, rail-like basic shape and extends diametrically between the two opposite envelope part surfaces  11 . On opposite sides, the coupling part has a pair of laterally projecting male members  32   a ,  32   b , which in the example are in the form of long narrow ridges. Each such ridge is delimited by a torque-carrying side contact surface  33  (see  FIG. 12 ), which in this case is plane and intersected by a reference plane RP that is spaced-apart from the axial contact surface  23  and extends perpendicular to the center axis C 2  of the loose top. Each side contact surface is inclined in relation to the reference plane RP, more precisely in the rearward/outward direction from a concave radius transition  34  which extends from the individual axial contact surface  23 . At the rear end thereof, the side contact surface  33  transforms into a convex radius transition  35  to the plane, rear end surface  25  of the coupling part. The angle (lacks designation) between the individual side contact surface  33  and the reference plane RP amounts in the example to approx. 80°, but may vary upward as well as downward. 
     In  FIG. 7 , it is seen that one ridge  32   a  runs from the peripheral end  36   a  adjacent to one of the envelope part surfaces  11  to an inner end  37   a  in the vicinity of the center of the loose top, while the other ridge  32   b  extends from a peripheral end  36   b  to an inner end  37   b  in the area of the center of the loose top. It should be noted that the ridges individually extend only along approximately half the length of the coupling part  22  and end in a chip flute surface  13 . 
     The axial contact surfaces  23  (see  FIG. 3 ) of the loose top co-operate with axial support surfaces  38  of the free ends of the drivers  27   a ,  27   b . In the example, the axial support surfaces  38  as well as the axial contact surfaces  23  are plane. In the insides of the drivers, chutes  39  (see also  FIG. 12 ) are formed, which serve as female members and co-operate with the male members  32   a ,  32   b  on the coupling part  22  of the loose top. As may be best seen in  FIG. 12 , the individual chute  39  in the driver  27   a  is formed axially behind a bulge  40  that is delimited by a side support surface  41  included in the chute as well as by a chamfer surface  42  that forms a transition between the side support surface  41  and the axial support surface  38 . Via a concave radius transition  43 , the side support surface  41  transforms into the bottom surface  28  of the jaw. In the same way as the side contact surface of the loose top  33 , the side support surface  41  is plane and inclined in relation to an intersecting reference plane RP spaced-apart from the axial support surface  38  and perpendicular to the center axis C 1  of the basic body. In practice, the side support surface  41  and the co-operating side contact surface  33  may have approximately the same angle of inclination (e.g., 10°) in relation to the respective reference plane. The level difference between the surfaces  23  and  25  of the loose top  2  (see  FIG. 12 ) is somewhat smaller than the level difference between the surfaces  38  and  28  of the basic body. When the loose top, in its operative state, is mounted to the coupling part in the jaw of the basic body, the axial contact surfaces  23  will therefore be pressed against the axial support surfaces  38  without the rear surface  25  of the coupling part contacting the bottom surface  28  of the jaw (see  FIG. 11 ). 
     In order to ensure that the loose top  2  is always mounted in a correct, predetermined way in relation to the screw  31 , the ridges  32   a ,  32   b  have, like the bulges  40 , been given unsymmetrical contour shapes. As is seen in  FIG. 10 , where the basic body  1  is shown axially from the front, the bulge  40  (on the driver  27   a ) shown to the left is broken into two sections  40   c ,  40   d , a first  40   c  one of which extends inward from a peripheral end  43  to a breaking point  44 , where the same transforms into a second section  40   d , the sections  40   c ,  40   d  forming an obtuse angle (in the example, approx. 172°) with each other. It should also be observed that the inner end  45  of the bulge is situated in the vicinity of the center hole  29 . More precisely, the bulge  40  runs up to one of the chip flutes  7 . The other bulge  40  (on the driver  27   b ) is straight along its entire length from a peripheral end  46  to an inner end  47  adjacent to the other chip flute  7 . As is seen in  FIG. 7 , the ridge  32   a  on the coupling part  22  of the loose top is in an analogous way angularly broken into two sections  32   c ,  32   d , the first one  32   c  of which extends from a peripheral end  36   a  to a breaking point  48 , where it transforms into the other section  32   d  while forming an obtuse angle (172°) with the same. The inner end  37   a  of the ridge  32   a  is situated in the vicinity of the pin  24 . The opposite ridge  32   b  is generally straight, but transforms at the inner end thereof into a short, angled section  32   e  that only has the purpose of allowing the axial insertion of the coupling part into the jaw of the basic body. 
     It should be evident that that the asymmetry described above ensures that the coupling part  22  of the loose top only can be inserted axially into the jaw  26  in a single predetermined way. The reason therefore will be clearer below. 
     In the shown, preferred embodiment of the tool according to the invention, the centering of the loose top  2  is provided by a unique design of the pin  24  of the loose top in co-operation with the radial screw  31 . A first embodiment of the pin is illustrated in the schematic picture in  FIG. 16 , where the hole wall of the hole  29  is assumed to be cylindrical and is shown with a dash-dotted circle S 1 , the center of which coincides with the center axis C 1  of the basic body. The circle S 1  forms simultaneously a circle circumscribed around the pin, the center of which coincides with the center axis C 2  of the loose top. When the loose top is centered, the specific center axes C 1  and C 2  coincide with the center axis C of the composed tool, and therefore only the center axis C is shown in  FIG. 16 . The pin  24  is formed with two diametrically opposed part surfaces, a first part surface  50  which forms a contact surface, which in this case has a cylindrical basic shape and is pressable against the hole wall approximately in the surface field that is designated  52  in  FIG. 3 . This surface field  52  is situated opposite the radial hole  30 . The contact surface  50  (see  FIG. 16 ) extends tangentially between two axially running boundary generatrices  53   a ,  53   b , which are situated along the circumscribed circle S 1 . The tangential extension of the contact surface  50  is determined by an arc angle α that according to the invention is less than 180°. In the example according to  FIG. 16 , a amounts to 170°. At the two boundary generatrices  53   a ,  53   b , the contact surface transforms into two clearance surfaces  54   a ,  54   b , which are included in a second part surface designated  51 , in which also a plane shoulder surface  51   a  is included. The two clearance surfaces  54   a ,  54   b  are cylindrical and tangent to an imaginary, common circle S 2 , the center M of which is eccentric in relation to the center of the circumscribed circle S 1 , i.e., the center axis C. As is clearly seen in  FIG. 16 , the two clearance surfaces  54   a ,  54   b  are spaced-apart from the circular line S 1 , i.e., the hole wall in the hole  29 , so far that the radial distance between the circle S 1  and the respective clearance surface successively increases in the direction from the boundary generatrices  53   a ,  53   b  toward the boundary generatrices  55   a ,  55   b  in which the clearance surfaces transform into the plane shoulder surface  51   a . By this geometry, it is ensured that the two clearance surfaces  54   a ,  54   b  do not contact the hole wall in the operative end position of the loose top. Furthermore, the geometry results in that the radial distance R between the center axis of the pin  24  and arbitrary points along the contact surface  50  delimited between the boundary generatrices  53   a ,  53   b  always is equally great and coincides with the corresponding radial distance between the hole wall and the center axis of the basic body. This means that the pin for certain is located with the center axis C 2  thereof in a position coinciding with the center axis C 1  of the hole when the contact surface  50  (by means of the screw  31 ) is pressed against the hole wall. In order to ensure an exact centering, the contact surface  50  of the pin may be made with extraordinary high dimensional accuracy, e.g., by precision grinding. In such a way, a dimensional deviation of 0.01 mm (or smaller) can be ensured. 
     In  FIG. 16 , the center axis C 3  of the radial hole  30  is illustrated by a dash-dotted line. In relation to the center axis, the pin  24  assumes an initial position in the jaw  26  of the basic body, in which the plane part surface  51   a  is inclined at an angle β, which in the example amounts to 115°. 
     From what has been described above, it should be axiomatically clear that the pin  24  has a cross-sectional area that is less than the cross-sectional area of the hole  29 . 
     In  FIG. 17 , an alternative embodiment of the pin  24  is illustrated. Also in this case, the hole wall in the hole  29  is cylindrical and has a cross-sectional area that is larger than the cross-sectional area of the pin  24 . Instead of being cylindrical, in this case the contact surface  50  of the pin  24  is elliptic between the two boundary generatrices  53   a ,  53   b  that determine the tangential extension of the contact surface. In doing so, a crescent-shaped gap  56  arises between the contact surface and the hole wall, the pin obtaining line contact with the hole wall in two separated places, viz. along the boundary generatrices  53   a ,  53   b . By imparting the contact surface  50  a high dimensional accuracy, the radial distances or the radii R 1  between the center axis of the pin (and of the loose top) and the boundary generatrices  53   a ,  53   b  may also in this case be made exactly as large as the radius R 2  between the hole wall and the center axis of the basic body. When the screw is pressed against the inclined shoulder surface  51 , accordingly an exact centering of the loose top is obtained regardless into which position the pin is turned. 
     In practice, the arc angle α between the generatrices  53   a ,  53   b  should amount to at least 90° and at most 175°. 
     When the loose top is to be mounted in the jaw  26  of the basic body, the screw  31  is held in an initial position in which the inner end thereof is removed from the center hole  29  of the basic body. In this state, the coupling part  22  of the loose top  2  is inserted axially between the drivers  27   a ,  27   b  and with the pin  24  sticking into the center hole  29 . The insertion of the pin into the center hole can be made without any difficulties because the cross-sectional area of the pin is less than the cross-sectional area of the center hole. During this initial insertion of the coupling part and the pin, the axial contact surfaces  23  of the loose top will be pressed and rest against the axial support surfaces  38  of the drivers  27   a ,  27   b  (without the surfaces  25  and  28  contacting each other). Next, the loose top is turned toward its operative end position. This can initially be made manually, whereupon the screw  31  is tightened. In doing so, the screw will be pressed against the shoulder surface  51   a  of the pin (see  FIG. 15 ), and by the fact that this is inclined in relation to the center axis C 3  of the radial hole  30 , the pin will be turned until the side contact surfaces  33  of the coupling part  22  are pressed in close contact with the corresponding side support surfaces  41  of the respective driver  27   a ,  27   b . In such a way, the loose top  2  is locked axially in an extraordinarily efficient way, more precisely via the ridges  32  serving as male members on the coupling part  22  and the chutes  39  serving as female members in the drivers  27   a ,  27   b  of the basic body. In this axially locking position, the loose top is retained in a reliable way as long as the screw  31  locks the pin in the assumed angle position. In connection with the turning-in of the loose top, the inclination of the co-operating side contact surfaces  33  and side support surfaces  41  entails that the same is subjected to a positive axial force that aims to press the axial contact surfaces  23  against the axial support surfaces  38 . 
     A fundamental advantage of the invention is that the negative (retractive) axial forces that act on the loose top are carried by the co-operating male and female members, i.e., the ridges and chutes, and not by the centering pin. If the unique design of the centering pin is applied, an utmost accurate centering of the loose top in relation to the basic body is furthermore obtained. In addition, the pin—in spite of the exact centering—can be inserted into and removed out of the receiving hole without difficulties as a consequence of the fact that the cross-sectional area of the pin is smaller than the cross-sectional area of the hole. 
     While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims and their equivalents thereof. Accordingly, the general idea of the invention, i.e., the idea of locking a turnable loose top by means of co-operating male and female members on the coupling part of the loose top and in the drivers of the basic body, respectively, may be applicable regardless of the design of the pin. However, the exemplified pin is preferred in order to provide an exact centering as well as a convenient mounting and dismounting, respectively. The invention is also applicable to other rotatable tools than exactly drilling tools, in particular milling cutters, such as shank-end mills. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.