The invention relates to a bit holder for an earth working machine, in particular a road milling machine, that comprises a bit receptacle in the region of a working side of a support member and that indirectly or directly carries an insertion projection on an insertion projection side of the support member, the support member comprising two stripping surfaces [first or second stripping surfaces] that form a stripping surface pair and are at an angle to one another. In order to achieve a stable and long-lived configuration with such a bit holder, provision is made according to the present invention that the support member comprises at least one further stripping surface that is at an angle to the two stripping surfaces of the stripping surface pair.

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a bit holder for an earth working machine, in particular a road milling machine, that comprises a bit receptacle in the region of a working side of a support member and that indirectly or directly carries an insertion projection on an insertion projection side of the support member, the support member comprising two stripping surfaces that form a stripping surface pair and are at an angle to one another. 
     2. Description of the Prior Art 
     U.S. Pat. No. 3,992,061 discloses a bit holder that forms a support member having an integrally shaped-on insertion projection. The support member is penetrated by a cylindrical bore embodied as a bit receptacle. A working tool, in the present case a round-shank bit, can be inserted into the bit receptacle. The support member comprises two stripping surfaces, at an angle to one another, that serve for bracing against corresponding support surfaces of a base part. The base part comprises an insertion receptacle into which the bit holder can be replaceably inserted with its insertion projection. In the installed state, the stripping surfaces of the bit holder abut against the support surfaces of the base part. A clamping screw that clamps the insertion projection in the insertion receptacle of the base part is used in order to maintain a fixed correlation of surfaces. 
     During working utilization, the working tool engages into the substrate to be worked, in which context large working forces are transferred and are dissipated from the bit holder in the base part. The direction and also the magnitude of forces varies, under otherwise identical conditions, simply because of the fact that the working tool forms a chip that becomes thicker from the entry point to the exit point (comma-shaped chip). In addition, the force direction and force magnitude vary as a function of different parameters such as, for example, the milling depth, advance, material being worked, etc. 
     The configuration of a bit holder shown in U.S. Pat. No. 3,992,061 cannot discharge the working forces with a sufficiently good service life, especially at high advance speeds. In particular, the stripping surfaces quickly become deflected. In addition, the insertion projection is also exposed to large flexural stresses, creating the risk that an insertion projection breakage will occur after component fatigue. 
     DE 34 11 602 A1 discloses a further bit holder. This comprises a support member that is braced via projections against a base part. Shaped onto the support member is a clamping part that can be secured to the base part via key connections. 
     A further bit holder is known from U.S. Pat. No. 4,828,327. Here the bit holder is configured as a solid block that is penetrated by a bit receptacle. The bit holder furthermore comprises a threaded receptacle that is in alignment with a screw receptacle of a base part. A fastening screw can be passed through the screw receptacle and screwed into the threaded receptacle of the bit holder. Upon tightening of the fastening screw, the bit holder is pulled into an L-shaped recess of the base part and braced there against bracing surfaces. The bit holders are usually arranged protrudingly on the surface of a tubular milling drum. During working utilization, transverse forces also occur that act transversely to the tool advance direction. These transverse forces acting in the direction of the longitudinal center axis of the tubular milling drum cannot always be absorbed in sufficiently stable fashion with the bit holders described in U.S. Pat. No. 4,828,327. In particular, these transverse forces are transferred into the fastening screw, which is then highly loaded in shear. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to create a bit holder of the kind mentioned previously that is notable for an extended service life. 
     This object is achieved in that the support member comprises a further stripping surface that is at an angle to the two stripping surfaces of the stripping surface pair. 
     According to the present invention, three stripping surfaces that are used to discharge loads into the base part are made available on the bit holder. The three stripping surfaces are at an angle to one another and thus form a three-side bracing member similar to a pyramid having a triangular base surface. This bracing member ensures that the bit holder is fixedly seated on the base part even when the direction of the working force changes. In addition, the three stripping surfaces also act to reduce the load on the insertion projection. 
     In the context of the invention, one or more additional stripping surfaces can also be added in combination with the three stripping surfaces in order to adapt the bit holder to a specific operational task. For example, four stripping surfaces that are all at an angle to one another can be used. 
     According to a preferred configuration of the invention, provision can be made that the two stripping surfaces of the stripping surface pair are arranged at least locally in front of the insertion projection in the advance direction of the bit holder, and a further stripping surface is arranged at least locally behind the insertion projection oppositely to the advance direction. Alternatively, provision can also be made that the two stripping surfaces of the stripping surface pair are arranged at least locally behind the insertion projection oppositely to the advance direction, and a further stripping surface is arranged at least locally in front of the insertion projection in the advance direction. The distribution of the stripping surfaces and the further stripping surface onto the regions of the bit holder in front of and behind the insertion projection optimally takes into account the force situation during working engagement. As explained above, a chip that thickens from the entry point to the exit point of the working tool forms. The working forces at the beginning of tool utilization are, in terms of their direction, more such that a load on the bit holder in front of the insertion projection occurs. The direction of the working force then changes, so that the regions behind the insertion projection are also increasingly loaded. The above-described arrangement of the stripping surfaces optimally takes into account the resulting load situation. 
     A load-optimized design results from the fact that the two stripping surfaces of the stripping surface pair and the at least one further stripping surface diverge from the insertion projection side toward the working side. The diverging stripping surfaces also form a prism-shaped bracing member in the region of the insertion projection side, and make possible here a reliable outward discharge of force. 
     To allow the bit holder to be installed on a tubular milling drum at different positions as both a left-hand and a right-hand part, a particularly preferred configuration of the invention provides that the at least one further stripping surface is embodied substantially symmetrically with respect to the center transverse plane extending in the direction of the longitudinal center axis of the insertion projection. Because the bit holder is configured symmetrically at its surface regions of the stripping surfaces that come into contact with the base part, identical load situations are achieved in the different installation positions. 
     Provision can preferably be made that a further stripping surface at least locally forms the underside of a front-side skirt of the bit holder. The front-side skirt usually covers a frontal region of the base part and thus protects it from wear. The fact that the front-side skirt is now also used to mount the stripping surfaces yields a compact design, and the bit holder is easy to produce. 
     Provision can also be made that a further stripping surface at least locally forms the underside of a rearward support projection. In certain utilization conditions, a large portion of the forces are transferred via the rearward support projection. The planar further stripping surface offers reliable bracing here. 
     As has already been mentioned above, the stripping surfaces of the stripping surface pair and the further stripping surface can form a three-surface bracing guide. The three stripping surfaces correspondingly form a pyramid having a triangular base surface as a bracing guide. 
     To allow reliable interception of the transverse forces occurring during working utilization, provision is made according to a variant of the invention that the lines normal to the stripping surfaces of the stripping surface pair point respectively to their bit holder side, viewed in the tool advance direction. The stripping surfaces of the stripping surface pair are thus correspondingly arranged, for example in the context of utilization of the bit holders on a tubular milling drum, with an inclination with respect to the rotation axis of the tubular milling drum. As a result of this arrangement, the transverse forces can reliably be intercepted. This arrangement may also be described as a configuration of the stripping surfaces so as to support the support member against forward and rearward forces and side to side forces orthogonal to the insertion direction. 
     Reliable installation of the bit holder in a base part is possible, even in austere construction-site service and at poorly visible locations, when provision is made that the stripping surfaces of the stripping surface pair enclose an obtuse angle, in particular in the range between 100° and 140°. This design moreover prevents jamming from occurring even after extended utilization when the stripping surfaces may wear away a little farther with respect to the support surfaces. The bit holder can thus always be replaced easily. In addition, this angled incidence of the stripping surfaces guarantees dependable discharge of working forces. In particular, the variation in working forces during tool engagement is taken into account. 
     A bit holder according to the present invention can be such that the stripping surfaces of the stripping surface pair and/or the at least one further stripping surface are connected to one another at least locally in the region of the insertion projection side via a transition segment. The stripping surfaces accordingly do not meet one another at the apex of the angle, so that a sharp-edged angular transition that can be damaged is not produced. In addition, a resetting region can also be created with the transition segment and in interaction with the base part. The bit holder can accordingly reset continuously into this resetting space when the stripping surfaces and/or support surfaces of the base part become worn, in which context the stripping surfaces always remain set against the support surfaces. In particular, planar abutment is maintained even if the bit holder needs to be exchanged for a new one, even repeatedly, on an existing base part. 
     Particularly preferably, the insertion projection is attached onto the insertion projection side at least partly in the region of the stripping surfaces of the stripping surface pair and/or of the at least one further stripping surface. A direct association between the stripping surfaces and the insertion projection thereby becomes possible, resulting in a smaller component size and moreover an optimized force path. 
     A bit holder according to the present invention can be characterized in that the longitudinal axis of the insertion projection and the longitudinal center axis of the prism formed by the stripping surfaces of the stripping surface pair enclose an angle in the range between 100° and 130°. Here as well, this configuration feature results in an optimized force path. 
     In a design that provides on the bit holder a bit receptacle, for example a bore, to receive a working tool, in particular a round-shank bit, provision is optimally made that the longitudinal center axis of the bit receptacle is arranged at least locally between the stripping surfaces of the stripping surface pair. The result is on the one hand that a good division of the working forces introduced via the working tool onto both stripping surfaces can be achieved. Furthermore, the bit holder can also be positioned in a different orientation with respect to a tubular milling drum, while reliable force transfer is still maintained. 
     It has been found that an optimum division, into longitudinal and transverse forces, of the forces to be discharged can be achieved if provision is made that the angle between the longitudinal center axis of the prism formed by the stripping surfaces of the stripping surface pair and the longitudinal center axis of the bit receptacle is in the range between 40° and 90°, particularly preferably between 50° and 80°. These angular positions also ensure that because of the incidence of the stripping surfaces of the stripping surface pair, the overall width of the bit holder does not become too great, thus guaranteeing a material-optimized design. 
     According to a further variant embodiment of the invention, provision can be made that the bit receptacle transitions into a flushing conduit, and that the flushing conduit emerges at least locally in the region between the stripping surfaces of the stripping surface pair. The flushing conduit is thus arranged so that the stripping surfaces do not meet one another at a sharp point. 
     If provision is made, according to a variant of the invention, that a first stripping surface of the stripping surface pair and the at least one further stripping surface are respectively incident to one another at an angle preferably in the range between 100° and 140° and form a support region, the bit holder can then be inserted into a likewise correspondingly configured angled bit holder receptacle of the base part and braced in stable fashion therein. The opening angle reflects a wide spectrum of directions from which forces can act in the course of tool engagement and as a result of changes in other parameters. 
     A particularly preferred variant of the invention is such that a plane receiving the angle bisector is arranged between the stripping surfaces of the stripping surface pair, and that the longitudinal axis of the insertion projection is arranged symmetrically with respect to that plane. As a result of this symmetrical configuration, the bit holder can also be installed at different installation positions on a tubular milling drum or the like, and this has the advantage that only one variant is needed and it is not necessary to work with left and right bit holders. 
     Additionally or alternatively, provision can be made that the longitudinal center axis of the insertion projection is at an angle in the range from −10° to +10° with respect to the angle bisector that is formed between the longitudinal center axis of the stripping surface of the stripping surface pair and the further stripping surface. A uniform preload is thus applied when the bit holder is secured to the base part. Provision is particularly preferably made in this context that this angle is in the range from −2° to +2°. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further explained below with reference to an exemplifying embodiment depicted in the drawings, in which: 
         FIG. 1  is a perspective side view of a combination of a base part and a bit holder; 
         FIG. 2  is an exploded view of what is depicted in  FIG. 1 ; 
         FIG. 3  is a front view of the bit holder according to  FIGS. 1 and 2 ; 
         FIG. 4  is a rear view of the bit holder according to  FIGS. 1 to 3 ; 
         FIG. 5  is a side view from the left of the bit holder according to  FIGS. 1 to 4 ; 
         FIG. 6  is a vertical section, through the central transverse plane of the bit holder, of what is depicted in  FIG. 5 ; 
         FIG. 7  is a side view from the right, partly in section, of the bit holder according to  FIGS. 1 to 6 ; 
         FIG. 8  shows a section marked VIII-VIII in  FIG. 5 ; 
         FIG. 9  shows a section marked IX-IX in  FIG. 7 ; 
         FIG. 10  shows a section marked X-X in  FIG. 7 ; 
         FIG. 11  is a plan view of the tool combination according to  FIG. 1 ; 
         FIG. 12  shows a section marked XII-XII in  FIG. 11 ; 
         FIG. 13  is a view from the front of the bit holder according to  FIG. 5 ; 
         FIG. 14  is a view from behind of the bit holder; and 
         FIG. 15  is a rotated side view of the bit holder. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a tool combination made up of a base part  10  and a bit holder  20 . Bit holder  20  is connected replaceably to base part  10 . Base part  10  comprises a solid basic member  13  that comprises a lower attachment side  11 . This attachment side  11  is concavely curved, the curvature being selected in accordance with the outside diameter of a tubular milling drum. Base part  10  can thus be placed with its attachment side  11  onto the outer side of the tubular milling drum and welded in place onto it. Basic member  13  comprises on the front side a projection that is demarcated laterally by oblique surfaces  14  and at the front side by inclined surfaces  15 . Inclined surfaces  15  are incident at an angle to one another, and oblique surfaces  14  adjoin inclined surfaces  15  at an angle. This results in an arrow-shaped geometry of base part  10  at the front, leading to better clearing action by base part  10 . 
     As  FIG. 2  illustrates, a bit holder receptacle  16  having an insertion receptacle  16 . 7  is recessed into base part  10 . Insertion receptacle  16 . 7  penetrates entirely through basic member  13 , and thus opens into attachment side  11 . A threaded receptacle  18  that opens into insertion receptacle  16 . 7  (see  FIG. 12 ) is recessed into base part  10 . Bit holder receptacle  16  comprises first support surfaces  16 . 1  and second support surfaces  16 . 2 . First support surfaces  16 . 1  form a first support surface pair, and second support surfaces  16 . 2  form a second support surface pair. In each support surface pair, the respective support surfaces  16 . 1 ,  16 . 2  are arranged at an angle to one another. Support surfaces  16 . 1  are furthermore respectively incident at an angle to support surfaces  16 . 2 , resulting in a frustoconical bit holder receptacle  16 . Resetting spaces  16 . 3 ,  16 . 4 ,  16 . 5  in the form of recesses are provided respectively in the transition region between the individual support surfaces  16 . 1  and  16 . 2 . A cutout  16 . 6  that creates a transition from bit holder receptacle  16  to threaded receptacle  18  is furthermore provided in the region of resetting space  16 . 5 . 
     As is further evident from  FIG. 2 , a surface  17  that is demarcated laterally by oblique surfaces is formed around the entrance into threaded receptacle  18 ; the oblique surfaces open divergently toward the back side of base part  10 . This creates a capability for easy cleaning of surface  17 , and thus of a tool receptacle  43  of a compression screw  40 . Compression screw  40  comprises a threaded segment  41  with which it can be screwed into threaded receptacle  18 . Compression screw  40  is furthermore embodied with a compression extension  42  in the form of a frustoconical stem that is shaped integrally onto threaded segment  41 . 
     As  FIG. 2  further shows, bit holder  20  can be connected to base part  10 . Bit holder  20  possesses a support member  21  that is equipped on the front side with a skirt  22 . Skirt  22  carries an integrally shaped-on web  22 . 1  that rises upward proceeding from skirt  22 . An extension  23  that terminates in a cylindrical segment  24  is also integrally coupled onto support member  21 . Cylindrical segment  24  is provided with wear markings that are embodied in the present case as circumferential grooves  26 . Cylindrical segment  24  terminates in a support surface  25  that concentrically surrounds the bore entrance of bit receptacle  27 . Bit receptacle  27  transitions via a bevel-shaped introduction segment  27 . 1  into support surface  25 . 
     As  FIG. 4  shows, bit receptacle  27  is embodied as a passthrough bore. Support member  21  is provided with a back-side cutout that serves as a flushing conduit  28 . Flushing conduit  28  consequently opens bit receptacle  27  radially outward in the region of its bore exit. Removed particles that have entered bit receptacle  27  during utilization of the tool can thus be conveyed radially outward through flushing conduit  28 . 
     It is evident from  FIG. 3  that support member  21  comprises first stripping surfaces  29 . 1  in the region of skirt  22 . These stripping surfaces  29 . 1  are at an oblique angle ε 1  to one another (see  FIG. 13 ), and are connected to one another via a transition segment  29 . 2 . The angle ε 1  between first stripping surfaces  29 . 1  corresponds to the angle between first support surfaces  16 . 1  of base part  10 . 
     It is evident from  FIG. 4  that support member  21  possesses, on the back side, downward-pointing second stripping surfaces  29 . 4 . Second stripping surfaces  29 . 4  are at an angle ε 2  to one another (see  FIG. 14 ); here as well, the angle ε 2  between second stripping surfaces  29 . 4  corresponds to the angle between second support surfaces  16 . 2  of base part  10 . While first stripping surfaces  29 . 1  transition into one another by means of transition segment  29 . 2 , a transition region between the two stripping surfaces  29 . 4  is formed by flushing conduit  28  and a transition segment  29 . 5 . Stripping surfaces  29 . 1  and  29 . 4  may also be referred to as bearing surfaces  29 . 1  and  29 . 4 . 
     Stripping surfaces  29 . 1  and  29 . 4  each form stripping surface pairs in the shape of a prism. These prisms have a longitudinal center axis MLL that is formed in the angle bisector plane between the two first stripping surfaces  29 . 1  and second stripping surfaces  29 . 4 , respectively. These angle bisector planes are labeled “WE” in  FIG. 14 . The longitudinal center axis is indicated there as MLL; in principle, longitudinal center axis MLL can be located at any position within the angle bisector plane. 
       FIGS. 3 and 4 , in conjunction with  FIGS. 13 and 14 , show that first stripping surfaces  29 . 1  and also second stripping surfaces  29 . 4  diverge proceeding from the insertion projection side toward the working side. In the present example, the lines normal to stripping surfaces  29 . 1 ,  29 . 4  correspondingly converge from the insertion projection side toward the working side. The surface normal lines consequently converge in the region of the tool engagement point at which working forces are introduced into the tool system. 
     For purposes of the present invention, for example, the first stripping surfaces  29 . 1  can be interpreted as stripping surfaces of the stripping surface pair, and one or both of the second stripping surfaces  29 . 4  as (a) further stripping surface(s). Conversely, the two second stripping surfaces  29 . 4  can also form the stripping surfaces of the stripping surface pair, and one or both first stripping surfaces  29 . 1  then form the further stripping surface(s). The “first/second stripping surfaces  29 . 1 / 29 . 4 ” terminology will continue to be used hereinafter. 
     The use of two stripping surface pairs having the respective first and second stripping surfaces  29 . 1  and  29 . 4  takes optimally into account the variation in working forces during tool engagement. A comma-shaped chip is produced during tool engagement. Not only the force magnitude but also the force direction changes as this chip is formed. Correspondingly, at the beginning of tool engagement the working force acts in such a way that it is dissipated more via the stripping surface pair formed by first stripping surfaces  29 . 1 . As tool engagement progresses, the direction of the working force rotates and it is then dissipated increasingly via the stripping surface pair formed by second stripping surfaces  29 . 4 . The angle γ′ (see  FIG. 5 ) between the stripping surface pairs must therefore be embodied so that the variation in working force is taken into consideration, and so that this working force always acts into the prisms formed by the stripping surface pairs. This arrangement of the stripping surfaces or bearing surfaces  29 . 1  and  29 . 4  can also be described as a configuration of the stripping surfaces or bearing surfaces so as to support the support member  21  against forward and rearward forces and side to side forces orthogonal to the insertion direction of the insertion projection  30 . 
     The central transverse plane MQ of bit holder  20  is labeled in  FIGS. 3 and 9 . The bit holder is constructed mirror-symmetrically with respect to this central transverse plane MQ, so that it can be installed on a milling drum as a right-hand or left-hand part. 
     The advance direction is characterized in  FIGS. 3 and 4  with usual arrow indications. The bit holder sides are arranged transversely to the advance direction. The lines normal to stripping surfaces  29 . 1  and  29 . 4  thus each point downward and toward their side (viewed in the tool advance direction) of the bit holder, as is clear from  FIGS. 3 and 4 . This situation is shown again in  FIG. 5  in a side depiction. 
     The working force acts, however, not only in the direction of the image plane according to  FIG. 5 , but also in a transverse direction. These transverse force components are then ideally intercepted by the angled incidence (ε 1 , ε 2 ) of stripping surfaces  29 . 1 ,  29 . 4 . Because the working forces exhibit less variation in the transverse direction at the beginning of tool engagement, angle ε 1  can also be selected to be smaller than ε 2 . 
       FIG. 5  further shows that an insertion projection  30  is shaped integrally onto support member  21  and transitions via a fillet transition  29 . 3  into first stripping surfaces  29 . 1  and second stripping surfaces  29 . 4 . Insertion projection  30  is arranged so that it adjoins support member  21  substantially (at a proportion of approximately 90% in the present case) in the region of first stripping surfaces  29 . 1 . Insertion projection  30  carries two abutment surfaces  31 . 1  on the front side. As is evident from  FIG. 3 , these are embodied as convexly curved cylindrical surfaces. Abutment surfaces  31 . 1  extend along and parallel to longitudinal center axis M (see  FIG. 5 ) of insertion projection  30 . Abutment surfaces  31 . 1  are thus also parallel to one another. Abutment surfaces  31 . 1  are arranged at a distance from one another in the circumferential direction of insertion projection  30 . They have the same radius of curvature and are arranged on a common reference circle. The radius of curvature corresponds to half the reference circle diameter. A recess  31 . 2  is provided in the region between abutment surfaces  31 . 1 , and abutment surfaces  31 . 1  extend parallel to recess  31 . 2 . The recess can have a wide variety of shapes; for example, it can be simply a flat-milled surface. In the present exemplifying embodiment, recess  31 . 2  forms a hollow that is hollowed out in concave fashion between abutment surfaces  31 . 1 . The concavity is designed so that a partly-cylindrically shaped geometry results. Recess  31 . 2  extends not over the entire length of insertion projection  30  but instead only over a sub-region, as is evident from  FIG. 13 . Recess  31 . 2  is open toward the free end of insertion projection  30 , i.e. in the insertion direction. Recess  31 . 2  also opens up radially outward with no undercut. Insertion projection  30  comprises on the back side, located opposite abutment surfaces  31 . 1 , a compression screw receptacle  32  that is equipped with a pressure surface  32 . 1 . 
       FIGS. 6 and 9  illustrate that recess  31 . 2  has a concavely inwardly curved geometry between the two abutment surfaces  31 . 1 , and in particular can form a partly-cylindrically shaped cross section. 
       FIGS. 7 to 10  depict in more detail the configuration of insertion projection  30 .  FIG. 9  clearly shows the concave inward curvature of recess  31 . 2  that adjoins the convex abutment surfaces  31 . 1 . It is clear from  FIG. 10  that insertion projection  30  has, in its region adjoining abutment surfaces  31 . 1 , a substantially circular or oval cross-sectional conformation.  FIG. 8  illustrates the region of compression screw receptacle  32 , pressure surface  32 . 1  being incident at an angle δ to longitudinal center axis M of insertion projection  30 . This angle of incidence δ is preferably in the range between 20° and 60° in order to achieve an optimum draw-in effect for bit holder  20 . 
       FIG. 7  furthermore shows that pressure surface  32 . 1  is arranged at a distance equal to distance dimension A from the attachment region of insertion projection  30  onto support member  21 . 
     Abutment surfaces  31 . 1  are arranged at a distance equal to distance dimension B from the attachment region of insertion projection  30  onto support member  21 . The surface centroid of abutment surfaces  31 . 1  is arranged at a distance equal to distance dimension C from the surface centroid of pressure surface  32 . 1 . 
     For installation of bit holder  20  into base part  10 , insertion projection  30  is inserted into insertion receptacle  16 . 7 . The insertion motion is limited by the first and second stripping surfaces  29 . 1 ,  29 . 4  that come to a stop against first and second support surfaces  16 . 1 ,  16 . 2 . 
     As may be gathered from  FIGS. 1 and 12 , the correlation here is such that transition segment  29 . 2  extends beyond resetting space  16 . 4 , resetting space  16 . 5  is spanned by transition segment  29 . 5 , and the lateral resetting spaces  16 . 3  are spanned by the angled region that is formed between first and second stripping surfaces  29 . 1 ,  29 . 4 . The result of the fact that bit holder  20  is distanced in the region of these resetting spaces  16 . 3 ,  16 . 4 ,  16 . 5  is that during working utilization, bit holder  20  can reset into resetting spaces  16 . 3 ,  16 . 4 ,  16 . 5  when stripping surfaces  29 . 1 ,  29 . 4  and/or support surfaces  16 . 1 ,  16 . 2  wear away. This is the case in particular when worn bit holders  20  are to be replaced with new ones, on an existing base part  10 . To fix in place the installation state described above, compression screw  40  is screwed into threaded receptacle  18 . Compression extension  42  thereby presses with its flat end surface onto pressure surface  32 . 1  and thus produces a draw-in force that acts in the direction of longitudinal center axis M of insertion projection  30 . This draw-in force may also be referred to as a tension loading in the insertion projection  30  parallel to the insertion direction. At the same time, however, compression screw  40  is incident at an angle to longitudinal center axis M of insertion projection  30  such that a clamping force acting toward the front side is also introduced into insertion projection  30 . This clamping force is transferred via abutment surfaces  31 . 1  into the corresponding concave counter-surface of the cylindrical segment of insertion receptacle  16 . 7 . The fact that abutment surfaces  31 . 1  are distanced via recess  31 . 2  guarantees that insertion projection  30  is reliably immobilized by way of the two bracing regions formed laterally by abutment surfaces  31 . 1 . The result is, in particular, that the surface pressures which occur are also kept low as a result of the two abutment surfaces  31 . 1 , leading to reliable immobilization of insertion projection  30 . 
     Effective wear compensation can be implemented by the fact that bit holder  20  can reset into resetting spaces  16 . 3 ,  16 . 4 ,  16 . 5  in the event of wear; stripping surfaces  29 . 1 ,  29 . 4  extend beyond support surfaces  16 . 1 ,  16 . 2  at every point, so that in the event of erosion, support surfaces  16 . 1 ,  16 . 2  are in any case eroded uniformly without producing a “beard” or burr. This configuration is advantageous in particular when, as is usually required, base part  10  has a service life that extends over several life cycles of bit holders  20 . Unworn bit holders  20  can then always be securely fastened and retained even on a base part  10  that is partly worn. It is thus also simple to repair a machine in which the tool system constituted by base part  10  and bit holder  20  is used. It is usual for a plurality of tool systems to be installed on such a machine, for example a road milling machine or surface miner, the base part usually being welded onto the surface of a tubular milling drum. When all or some of bit holders  20  are then worn, they can easily be replaced with new unworn or partly worn bit holders  20  (which can be used e.g. for rough clearing operations). 
     For replacement, firstly compression screw  40  is loosened. The worn bit holder  20  can then be pulled with its insertion projection  30  out of insertion receptacle  16 . 7  of base part  10 , and removed. The new (or partly worn) bit holder  20  is then inserted with its insertion projection  30  into insertion receptacle  16 . 7  of base part  10 . Compression screw  40  can then be replaced, if necessary, with a new one. It is then screwed into base part  10  and secured to bit holder  20  in the manner described. 
     It is evident from  FIG. 12  that base part  10  carries a projection  50  that protrudes into insertion receptacle  16 . 7 . This projection  50  is constituted in the present case by a cylindrical pin that is driven from attachment side  11  into a partly-cylindrical recess  19 . Partly-cylindrical recess  19  surrounds the cylindrical pin over more than 180° of its circumference, so it is retained in lossproof fashion. That region of the cylindrical pin which protrudes into bit receptacle  27  engages into recess  31 . 2  between abutment surfaces  31 . 1 . Upon insertion of insertion projection  30  into insertion receptacle  16 . 7 , protrusion  50  threads reliably into recess  31 . 2  that is open toward the free end of insertion projection  30 . Alignment of bit holder  20  with respect to base part  10  is thereby achieved. This alignment ensures that first and second stripping surfaces  29 . 1 ,  29 . 4  now come into accurately fitted abutment against support surfaces  16 . 1 ,  16 . 2  so that incorrect installation is precluded. In addition, the lock-and-key principle of projection  50 , and of recess  31 . 2  adapted geometrically to it, prevents an incorrect bit holder  20  from inadvertently being installed on base part  10 . 
     The angular correlations of bit holder  20  according to the present invention will be discussed in further detail below. 
     It is evident from  FIG. 5  that longitudinal center axis  24 . 1  of bit receptacle  27  is at a respective angle α and φ to the longitudinal orientations of transition segments  29 . 2  and  29 . 5 , and thus also to longitudinal center axis MLL of the prisms formed by first stripping surfaces  29 . 1  and by second stripping surfaces  29 . 4 , respectively. The angle α can be between 40° and 60°, and the angle φ in the range between 70° and 90°. 
       FIG. 5  further shows that in a projection of stripping surfaces  29 . 1  and  29 . 4  into a plane perpendicular to the advance direction (said projection corresponding to  FIG. 5 ), stripping surfaces  29 . 1  and  29 . 4  are angled with respect to one another at an angle γ in the range between 40° and 60°, and that the opening angle between transition segments  29 . 2  and  29 . 5  in the longitudinal orientation according to  FIG. 5  is between 120° and 140°. The angle γ′ between longitudinal center axes MLL of the two prisms formed by stripping surfaces  29 . 1  and  29 . 4  (stripping surface pairs) is correspondingly in the range between 120° and 140°. Furthermore, in a projection of this kind of stripping surfaces  29 . 1 ,  29 . 4 , first stripping surfaces  29 . 1  are at an angle β, and second stripping surfaces at an angle μ, to longitudinal center axis M of insertion projection  30 . The same also applies here to longitudinal center axes MLL of the prisms. The angles β and μ can be in the range between 100° and 130°, preferably in the range between 110° and 120°. 
       FIG. 13  shows that first stripping surfaces  29 . 1  enclose an angle ε 1 . This angle ε 1  should preferably be in the range between 100° and 120°. The angle bisector of this angle ε 1  is located in a plane, and  FIG. 13  illustrates that insertion projection  30  is arranged symmetrically with respect to that plane. 
     In the same manner, the rear second stripping surfaces  29 . 4  are correspondingly also incident to one another at an angle ε 2 , as shown in  FIG. 14 . The angle ε 2  can, however, differ from angle ε 1 , and in the present exemplifying embodiment can be between 120° and 140°, and insertion projection  30  is also arranged and equipped symmetrically with respect to the angle bisector plane of said angle ε 2 . 
       FIG. 15  shows that a first stripping surface  29 . 1  of the first stripping surface pair and a second stripping surface  29 . 4  of the second stripping surface pair are respectively incident to one another at an angle co, and form a support region.