Abstract:
The invention relates to a chisel ( 10 ) having a chisel head ( 11 ) and a chisel shaft ( 17 ), wherein near its end facing away from the chisel head the chisel shaft has a threaded portion having a thread ( 19 ), wherein the chisel head holds a chisel tip ( 20 ) made of a hard material, and wherein near the side facing the chisel shaft the chisel head is provided with a supporting surface ( 15 ). Especially when superhard hard materials are used for the chisel tip, a load-optimized chisel design is obtained by a domed supporting surface ( 15 ).

Description:
FIELD 
       [0001]    The invention relates to a chisel having a chisel head and a chisel shaft, wherein the chisel shaft has in the region of the end thereof facing away from the chisel head a thread portion having a thread, wherein the chisel head carries a chisel tip comprising a hard material and wherein the chisel head is provided with a support face in the region of the side facing the chisel shaft. 
       BACKGROUND 
       [0002]    Such cutting chisels are generally used on cutting rollers of ground processing machines, in particular road construction machines, mining machines or the like. 
         [0003]    The cutting rollers of road milling machines, mining machines or the like are usually provided with chisel holder changing systems. In this instance, base portions of the chisel holder changing systems can be connected to the surface of a cutting roller pipe, in particular welded or screwed thereto. In this instance, the base portions are positioned relative to each other so that helical loading members are produced on the surface of the cutting roller. Chisel holders are connected to the base portions, wherein the chisel holders may be screwed, welded or otherwise retained with respect to the base portion, for example, clamped. In the simplest case, the chisel holders may also be directly connected to the surface of a cutting roller pipe. The chisel holders have a chisel receiving member. The chisels described above can be mounted therein so as to be able to be replaced. During use of the machine, the chisels strike with the chisel tips thereof the substrate which is intended to be removed and cut into it. In this instance, the ground material is broken up. The material which has been removed in this manner can be transported, for example, via the helical broaching and loading members toward the center of the cutting roller and conveyed out of the operating region of the cutting roller at that location by means of ejectors. The material can then be transported away using appropriate devices, for example, transport belts. The chisels are provided with chisel tips, which comprise hard material and which bring about the cutting engagement. They are consequently subjected to an abrasive attack and must therefore comprise a suitable hard material in order to achieve the longest possible service-life. From the prior art there are known chisels in which the chisel tip comprises hard metal. In order to be able to generate uniform wear at the periphery with such chisels, the chisels are generally rotatably arranged in chisel receiving members of the chisel holders. 
         [0004]    There are also known chisels which are provided in the region of the chisel tips thereof with a “superhard material”. For example, the chisel tips have a coating of polycrystalline diamond or another material which has a hardness which is comparable with diamond. Such a chisel is known from US 2012/0080930 A1. Such chisel tips have an extraordinarily long service-life and exhibit hardly any wear during operational use. It is therefore not absolutely necessary to fix these chisels in a rotatable manner in the chisel holders. US 2012/0080930 A1 therefore proposes providing the chisel shaft of the chisel with a thread and clamping the chisel securely to the chisel holder by means of a nut. If after a specific operating time wear appears on the chisel, the nut can be released, the chisel can be rotated slightly and the nut can then be retightened. 
         [0005]    The chisel is supported with a support portion of the chisel head on a correspondingly formed counter-face of the chisel holder. In this instance, the support portion is constructed in a frustoconical manner and tapers from the chisel head in the direction toward the chisel shaft. During the cutting engagement of the chisel, the cutting force which acts on the chisel varies not only with regard to the value thereof, but also with regard to the force direction. In this instance, it may be the case inter alia that stresses which act in an impact-like manner act on the chisel in the case of uneven surface quality. Those loading situations may result in the support face of the chisel or the corresponding counter-face of the chisel holder being deflected and then the thread connection between the chisel and the chisel holder becoming loose. The chisel can then break or become lost. 
       BRIEF SUMMARY 
       [0006]    An object of the invention is to provide a chisel of the type mentioned in the introduction with which an improved operational reliability and service-life can be achieved. 
         [0007]    This object is achieved in that the support face of the chisel head is curved. 
         [0008]    The curvature of the support face allows an increased surface with respect to a frustoconical construction for the same construction space. This results in smaller surface pressures and therefore in a construction method which is optimized in terms of loading. Furthermore, in conjunction with a counter-face of the chisel holder, which counter-face is curved in accordance with the support face, a type of “ball-and-socket joint” can be constructed. Such a bearing can react particularly well to the changing force directions which occur during the cutting process and can discharge those forces uniformly and reliably into the chisel holder. Tension peaks which occur in particular in the case of impact-like loads are thereby minimized. The term “curved” is intended to be understood according to the invention to be support face geometries in which the support face is constructed to be spherically convex or correspondingly concave, in particular constructed to be spherical, ellipsoid-like, etc. Spherical or ellipsoid-like geometries can be readily produced and in particular allow the above-mentioned ball-and-socket type construction. 
         [0009]    According to a preferred construction variant of the invention, there may be provision for the chisel head to have a tool receiving member. By means of this tool receiving member, the chisel can be gripped from the front chisel side with a screwing tool and screwed to the chisel holder. The chisel is readily accessible in the region of the chisel head and has a diameter which is greater than the chisel shaft. In this instance, the tool receiving member can then also be constructed with a large effective cross-section in order to be able to better introduce the necessary tightening torques for clamping the chisel. 
         [0010]    In a particularly preferable manner, the tool receiving member is constructed as an outer polygonal member, in particular as an outer hexagonal member, so that screwing is possible with conventional fixing tools. It is also conceivable for one or more recesses which act as tool receiving members to be formed round the chisel head, such as, for example, bores. They can be orientated substantially axially, that is to say, therefore, parallel with the longitudinal center axis of the chisel according to the invention or substantially radially, that is to say, therefore, orthogonally to the longitudinal center axis. Tools can then be inserted therein and a rotation of the chisel brought about. An advantageous aspect of a through-hole in a radial direction is the fact that the tool receiving member is also retained when the chisel head is worn to a very great extent. 
         [0011]    A preferred variant of the invention is such that a concave discharge face of the chisel head directly or indirectly adjoins the tool receiving member and is arranged in the region between the chisel tip and the tool receiving member. The discharge face discharges the ground material which is substantially cut away by the chisel tip away from the tool receiving member and therefore prevents or at least reduces the wear in the region of the tool receiving member. In this regard, the discharge face forms a type of deflector. 
         [0012]    In that the discharge face discharges the cut material outward, wear to the chisel holder is also prevented. 
         [0013]    In a particularly preferred manner, there may also be provision for the maximum cross-section, in particular the diameter, of the chisel head to be greater than the cross-sectional region of the chisel holder adjoining the chisel head in order to protect it from wear. 
         [0014]    In order to be able to bring about a chisel construction which is as compact as possible, there may be provision for a receiving member in which the chisel tip is inserted to be formed in the region of the chisel head forming the discharge face. 
         [0015]    The chisel tip preferably has an operating portion which is formed from a superhard material. Such a material may be formed from a material which has a similar hardness to diamond. It is particularly conceivable to use polycrystalline diamond, natural diamond, synthetic diamond, vapor-deposition diamond, silicon-bonded diamond, cobalt-bonded diamond, thermally stable diamond, cubic boron nitride, a diamond-infiltrated material, a diamond-tipped matrix, a diamond-impregnated carbide or a similar material. This is not a conclusive listing and it is clear to the person skilled in the art that the advantages of the present invention are produced with a large number of different chisel tips and the materials used therein. 
         [0016]    A particularly preferred variant of the invention is such that the chisel shaft has an expansion portion in the region between the thread and the chisel head. That expansion portion is used to form relatively high resilient deformations during the tensioning of the chisel by means of the thread thereof and accordingly a pretension in the chisel shaft. Accordingly, the expansion portion acts as a type of spring. If the chisel strikes the hard substrate to be processed, as a result of the tension direction the pretensioning force is relieved and a residual clamping force is produced. The resilient deformation in the expansion portion ensures that the residual clamping force is not completely eliminated. If the chisel is then out of engagement with the ground again, the pretension in the expansion portion is again produced. The thread connection of the chisel is thereby prevented from becoming loose even in the event of impact-like loads. Furthermore, the resilient deformation in the expansion portion ensures that an adequate pretension and therefore also a residual clamping force is maintained in spite of the unavoidable setting losses. A durably reliable chisel fixing action is thereby achieved. This is particularly advantageous during the use of the above-mentioned superhard materials and the associated high running times of the chisels. 
         [0017]    In order to be able to form a sufficiently effective expansion portion in this instance with conventional road milling applications, the expansion portion is intended to extend at least 20 mm and a maximum of 50 mm in the direction of the longitudinal center axis of the chisel shaft. 
         [0018]    The expansion portion may have a portion with uniform cross-section, in particular a cylindrical cross-section and/or a cross-section which changes in the direction of the longitudinal center axis of the chisel shaft. In the case of changing cross-sections, the expansion rate of the expansion portion can be adjusted in a selective manner. 
         [0019]    A preferred variant of the invention is such that the shaft cross-section does not taper from the chisel head in the direction toward the thread portion and the thread portion does not have a substantially smaller diameter than the transition region which is formed between the chisel head and the thread portion, and such that a nut is retained on the thread. If the improbable case of a chisel breakage occurs, wherein the chisel head breaks off the chisel shaft, then the chisel shaft remaining in the chisel holder can be pulled backward out of the chisel holder with this construction. 
         [0020]    Another variant of the invention is such that the chisel head has a peripheral recess and/or a peripheral projection in the region of the support face. 
         [0021]    As already mentioned above, the forces acting on the chisel change during the cutting process. The curved support face of the chisel can react to those changing force directions particularly well, as explained above. The chisel is retained with the chisel shaft thereof in a chisel receiving member of the chisel holder or the like. If a particularly powerful pulse-like transverse force acts on the chisel, the axial portion thereof is discharged into the chisel holder via the support face. The radial portion instead attempts to pivot the chisel head with respect to the chisel holder; furthermore, the chisel shaft is thereby also stressed in terms of flexion. Finally, a tensile stress is also further introduced into the chisel shaft via the threaded connection. Consequently, a disadvantageous, multi-axis tension state can be produced in the region of the chisel shaft. In order to be able to achieve a configuration of the chisel which is optimized in terms of loading in this instance, there is provision according to a variant of the invention for the chisel head to have in the region of the support face a peripheral recess and/or a peripheral projection. Accordingly, a corresponding projection or a corresponding recess may be arranged in the region of the counter-face of the chisel holder. If, for example, a recess is arranged in the chisel head, a projection of the chisel holder engages therein. That engagement results in a connection geometry which allows improved discharge of forces and which reduces the tensions in the chisel shaft. 
         [0022]    Furthermore, such a construction of a chisel makes it possible to compensate for production tolerances between the curved face of the chisel and the chisel holder. If, for example, a recess is formed in the chisel head, there are formed at both sides of the recess defined abutment regions which always ensure a sufficiently reliable surface contact between the chisel and the chisel holder. For this functionality, there does not have to be provision, for example, for a projection of the chisel holder to engage in a recess of the chisel, or, if a projection is arranged on the chisel, for that projection to engage in a recess of the chisel holder. In order to compensate for the surface tolerances, it is instead simply necessary for a recess to be provided in the chisel and/or in the chisel holder. For example, it is also conceivable for the chisel holder and/or the chisel to be constructed so as to have recesses, in which a peripheral sealing element is introduced. That peripheral sealing element, for example, a copper ring, an O-ring or the like, then prevents introduction of dirt into the region of the chisel shaft. The above-mentioned tooth arrangement in which a projection and a recess of the chisel or the chisel holder engage in each other, may also perform such a sealing action to a given extent in the form of a labyrinth-like seal. 
         [0023]    There is provision in a particularly preferable manner for the recess and/or the projection to extend concentrically round the chisel shaft. 
         [0024]    As already mentioned, the thread of the chisel may carry a nut. That nut may be provided with a sealing portion. That sealing portion prevents dirt from being introduced into the chisel holder in the region of the chisel shaft. 
         [0025]    The nut preferably has a securing portion having blocking faces at the peripheral side. The securing portion adjoins with the blocking faces thereof support faces of the chisel holder and consequently forms in the peripheral direction of the thread a positive-locking fixing of the nut with respect to the chisel holder. When the chisel is tensioned, therefore, the nut does not have to be retained with a counter-tool. Furthermore, the nut is fixed to the chisel holder in a state protected from abrasive attack. A construction of the nut in a tension-optimized manner is produced when there is provision for the blocking faces to be constructed in a concave manner and preferably to merge into each other via convex transition portions. Such a geometry is further also simple to produce. 
         [0026]    A further preferred variant of the invention may be such that the chisel is constructed as a forged component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention is explained in greater detail below with reference to embodiments illustrated in the drawings, in which: 
           [0028]      FIG. 1  is a side view and a partially sectioned view of a chisel, 
           [0029]      FIG. 2  is a perspective view of the chisel according to  FIG. 1 , 
           [0030]      FIG. 3  is a plan view of the chisel according to  FIGS. 1 and 2 , 
           [0031]      FIGS. 4 and 5  are perspective views of a nut, 
           [0032]      FIG. 6  is a plan view of the nut according to  FIGS. 4 and 5 , 
           [0033]      FIG. 7  is a line of section indicated VII-VII in  FIG. 6 , 
           [0034]      FIGS. 8 and 9  are perspective views of a chisel holder, 
           [0035]      FIG. 10  is a side view of the chisel holder according to  FIGS. 8 and 9 , 
           [0036]      FIG. 11  shows a line of section indicated XI-XI in  FIG. 10 , 
           [0037]      FIG. 12  is an exploded view of a chisel holder changing system, 
           [0038]      FIG. 13  is a side view and sectioned view of the chisel holder changing system according to  FIG. 12 , 
           [0039]      FIG. 14  is a side view of a chisel, 
           [0040]      FIG. 15  is a perspective view of a milling roller of a road milling machine, 
           [0041]      FIG. 16  is a side view and partially sectioned view of a chisel, 
           [0042]      FIG. 17  shows a detail indicated in  FIG. 16 , 
           [0043]      FIG. 18  is a sectioned view of a chisel holder, 
           [0044]      FIG. 19  is a section detail taken from  FIG. 18 , 
           [0045]      FIGS. 20 and 21  show another alternative construction of a chisel, 
           [0046]      FIG. 22  is a section through a chisel holder changing system, 
           [0047]      FIG. 23  is a side view and partially sectioned view of a chisel holder according to  FIG. 22 , 
           [0048]      FIGS. 24 to 27  are side views of different versions of chisel holder changing systems. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]      FIG. 1  shows a chisel  10  having a chisel head  11  on which a chisel shaft  17  is integrally formed. The chisel head  11  has at the end thereof facing away from the chisel shaft  17  a receiving member  12  which is constructed in this instance in the form of a blind-hole-like bore. A chisel tip  20  is inserted into this receiving member  12 . The chisel tip  20  has a connection portion  23  which may comprise hard metal. The connection portion  23  has at the end thereof facing away from the chisel shaft  17  a receiving member in which a carrier member  22  is inserted. The carrier member  22  comprises a hard material, for example, hard metal. It is provided at the free end thereof with a hard material coating  21 . The hard material coating  21  is in this instance formed by a superhard material. In this instance, it is, for example, possible to use a material which has a similar hardness to diamond. In particular, the hard material coating  21  may comprise polycrystalline diamond. The carrier member  22  is connected to the connection portion  23  by means of a suitable connection. For example, a solder connection may be provided. The connection portion  23  may be connected to the chisel head  11  in the chisel receiving member  12  by means of a suitable connection. For example, a solder connection may be selected. The construction of the chisel tip  20 , comprising the connection portion  23  and the carrier member  22  which is connected thereto with a hard material coating  21  can be produced in a simple manner. The spatially smaller carrier member  22  may be coated in a suitable coating installation with the hard material coating. The connection portion  23  of wear-resistant material is structurally larger than the carrier member  22  and therefore has a high capacity for wear. 
         [0050]    It is also conceivable for the entire chisel tip  20  to be constructed integrally. The chisel tip could then comprise, for example, hard metal. It is further conceivable for the chisel head  11  itself to be provided with a hard material coating which forms the chisel tip and which is preferably of superhard material. The component complexity can thereby be considerably reduced. 
         [0051]    Alternatively, it is also conceivable for the hard material coating  21  to be applied directly to the connection portion  23  with the carrier member  22  being omitted. 
         [0052]    Alternatively, the connection portion  23  could also be constructed integrally with the carrier member  22 , which would lead to a similar chisel tip, as in the preceding example, only the interface would be different. 
         [0053]    The portion of the chisel head  11  forming the receiving member  12  has a discharge face  13  which expands from the chisel tip  20  in the direction toward the shaft  17 . That discharge face  13  may in particular be constructed in a concave manner, as clearly shown in  FIG. 1 . Adjacent to the discharge face  13 , the chisel head  11  forms a tool receiving member  14 . This is constructed in this instance as an external hexagonal member, as shown in  FIG. 3 . The external hexagonal member has a conventional wrench width for fitting a commercially available tool. Adjacent to the tool receiving member  14 , the chisel head  11  forms a support face  15 . The support face  15  is curved in a spherical manner. In the present embodiment, a simple-to-produce, convex ball contour is used as a spherical curvature. The chisel shaft  17  is formed centrally on the support face  15  so that the support face  15  extends in a uniform manner about the longitudinal center axis M of the chisel shaft  17 . The coupling of the chisel shaft  17  to the chisel head  15  is carried out in a tension-optimized manner via a transition  16  which is formed by a rounded portion. The chisel shaft  17  has a cylindrical region, which forms an expansion portion  17 . 1 . In the region of the free end of the chisel shaft  17 , a thread  19  is cut on the chisel shaft  17 . A recess  18  is provided between the thread  19  and the chisel shaft  17 . 
         [0054]    Via the thread  19 , the chisel can be screwed to the nut  30  shown in  FIGS. 4 to 7 . As these drawings show, the nut  30  has a sealing portion  31  in the form of a cylindrical attachment. In the outer periphery of the sealing portion  31  there is formed a groove which can clearly be seen in  FIG. 7 . This groove serves to receive a seal  32  which is constructed in this instance as an O-ring. A securing portion  33  adjoins the sealing portion  31 . The securing portion  33  has blocking faces  34  which are constructed in a concave-curved manner. The blocking faces  34  merge into each other via convex transition portions  35 . As shown in  FIG. 6 , the nut  30  has five blocking faces  34  which are arranged so as to be distributed in a uniform manner with the same angular spacing over the outer periphery of the nut  30 . The thread  36  extends through the nut  30 . In a state adjacent to the thread  36 , the nut  30  has in the region of the sealing portion  31  a radial impact face  37 . 
         [0055]      FIGS. 8 to 11  show a chisel holder  40  for receiving the chisel  10  shown in  FIGS. 1 to 3 . The chisel holder  40  has a base portion  41  which has a cylindrical outer contour. At the upper end thereof, the chisel holder  40  has a cylindrical attachment  42 . The cylindrical attachment  42  may include, in a non-limiting example, at least one surface contour  43  such as at least one of a peripheral projection and a peripheral recess arranged on the base portion  41 . In this instance, the diameter of the cylindrical attachment  42  is selected to be slightly larger than the diameter of the base portion  41 . The cylindrical attachment  42  forms a counter-face  44  which is constructed so as to be curved in a spherical manner and concave. The chisel holder  40  merges in a manner adjacent to the counter-face  44  into a chisel receiving member  45  which is constructed as a bore in this instance. In a state facing away from the counter-face  44 , the chisel receiving member  45  opens in a sealing portion  46  which is constructed in a bore-like manner as an inner cylinder. A seal receiving member is introduced in the wall region delimiting the sealing portion  46 . The seal receiving member may, as illustrated in this instance, be constructed as a peripheral groove  46 . 1 . 
         [0056]    The chisel holder  40  has at the end thereof facing away from the cylindrical attachment  42  a holder receiving member  47 .  FIGS. 8 and 11  allow the structure of the holder receiving member  47  to be seen more clearly. As can be seen from these illustrations, the holder receiving member  47  is constructed as an internal receiving member in the chisel holder  40 . It is delimited by five retention faces  47 . 1  which are curved in a convex manner. The retention faces  47 . 1  merge into each other via concave transition portions  47 . 2 . The curvature of the retention faces  47 . 1  and the transition portions  47 . 2  is constructed to be adapted to the curvature of the blocking faces  34  and the transition portions  35  of the nut  30 . Accordingly, the nut  30  can be guided from the rear end of the chisel holder  40  with the sealing portion  31  through the region of the holder receiving member  47  and pushed into the region of the sealing portion  46 . The insertion movement of the nut  30  is blocked by means of the impact face  37  which comes to rest on a stop  46 . 2  of the sealing portion  46 . In this assembly state, the seal  32  engages in the groove  46 . 1  of the sealing portion  46  so that the transition region between the outer contour of the nut  30  and the inner contour of the sealing portion  46  is sealed. The blocking faces  34  are arranged opposite the retention faces  47 . 1 . The transition portions  35  and  47 . 2  are also opposite each other. In this manner, a non-rotatable arrangement of the nut  30  in the holder receiving member  47  is achieved. Since the seal  32  is retained in a manner clamped between the nut  30  and the chisel holder  40 , the nut  30  is retained in a non-releasable manner. 
         [0057]      FIG. 12  is an exploded view of a chisel holder changing system in which the chisel holder  40  is secured in a suitable manner to a lower portion  50 , for example, welded. The lower portion  50  has for this purpose a securing portion  51  which in accordance with the cylindrical contour of the base portion  41  of the chisel holder  40  has a concave recess. The securing portion  51  is formed by a carrier portion  52  of the lower portion  50 . The carrier portion  52  is formed integrally on a base portion  54  by means of a transition portion  53 . The base portion  54  has a lower support face  55 . With the support face  55 , the chisel holder  40  can be placed on the outer face of a cutting roller pipe and can be secured thereto in a suitable manner, for example, welded. 
         [0058]      FIG. 13  shows the above-described assembly position of the nut  30  in the holder receiving member  47 . The chisel  10  can be inserted with the chisel shaft  17  thereof past the counter-face  44  into the chisel receiving member  45 . In this instance, the expanding counter-face  44  facilitates the introduction movement of the chisel  10 . When the thread  19  of the chisel  10  strikes the nut  30 , the chisel  10  can be screwed with the thread  19  thereof into the thread  36  of the nut  30 . This screwing-in movement can first be carried out by hand until the support face  15  comes to rest on the counter-face  44 . Subsequently, a suitable tool can be placed on the tool receiving member  14 . The chisel  10  can then be rotated with the tool and, in this instance, the threaded connection between the thread  19  and the thread  36  can then be tensioned. In order to ensure reliable fixing of the chisel  10  during the processing operations which are intended to be carried out, a high tightening torque has to be selected. In this instance, the support faces  15  and the counter-face  44  press each other. As a result of this pressing action, a seal between the chisel head  11  and the counter-face  44  is brought about in such a manner that no contamination can be introduced. Via the high torque, the expansion portion  17 . 1  of the chisel shaft  17  is resiliently deformed. This resilient deformation portion, in the event of loads acting on the chisel tip  20  in an impact-like manner, prevents the threaded connection between the nut  30  and the chisel shaft  17  from being able to be released. The selected geometry of the concave blocking faces  34  and the convex retention faces  47 . 1  enable increased force transmission regions with respect to conventional, elongate surface portions, as are conventional with nuts. Of course, the retention faces  47 . 1  may also be curved in a concave manner and the blocking faces  34  may accordingly be curved in a convex manner. 
         [0059]    The convex/concave pairings selected prevent for the selected high tightening torques a plastic deformation of the blocking faces  34  or the retention faces  47 . 1  from being able to be produced. Consequently, in particular the holder receiving member  47  remains in the desired form and during the chisel change a new nut  30  can be inserted in a reproducible manner. 
         [0060]    During the tool engagement, the chisel tip  20  strikes the substrate which is intended to be cut and cuts into it. In this instance, the material cut slides off the chisel tip  20 . As a result of the large forces present in the region of the chisel tip  20 , a great abrasive attack is brought about in this instance. This attack is taken into account by the structure of the chisel  10  with the connection portion  23 , which comprises hard material, for example, hard metal. After the material removed has passed the connection portion  23 , it reaches the region of the discharge face  13 . It has then already lost a large proportion of its abrasive nature and can be safely guided further by the discharge face  13 . In this instance, it is guided radially outward from the discharge face  13  and discharged from the tool receiving member  14  and the chisel holder  40  so that where possible it is not subjected to wear or is subjected only to slight wear. 
         [0061]    Since the chisel  10  cannot rotate, it is first worn away at one side. This is permissible up to a specific wear limit. Then, the chisel  10  is released by means of the appropriate tool which engages on the tool receiving member  14 . Subsequently, the nut  30  can be pulled from the holder receiving member  47  and inserted therein again in a rotated state. As a result of this rotation, the thread intake in the thread  36  is also arranged in a rotated position with respect to the chisel holder  40 . When the same chisel  10  is again screwed to the nut  30 , wherein the same tightening torque is again preferably intended to be selected, then the chisel head  11 , and consequently the chisel tip  20  opposite the chisel holder  40 , moves into abutment in a correspondingly rotated position. The processing side of the chisel  10  is then formed by a non-worn chisel tip location. 
         [0062]    In the present embodiment, 5 blocking faces  34  which are arranged in a state distributed in a uniform manner with respect to each other are provided on the nut  30 . Accordingly, the chisel  10  may also be secured at five mutually rotated locations to the chisel holder  40 . It has been found that such an arrangement is particularly advantageous when the chisel  10  is used for the purpose of fine-milling of road surfaces. When rotated by the extent of a blocking face  34 , the chisel  10  can then be worn in a manner optimized in terms of wear, wherein at the same time a high surface quality of the milled road surface is retained. When six blocking faces are used, optimized use of the chisel tip  20  in terms of wear is not achieved, as is possible with 5 blocking faces. When four blocking faces are used, there is an excessively high variance in the surface quality when the chisel tip  20  is intended to be used completely. Furthermore, when 5 blocking faces are used, that is to say, an uneven number of blocking faces  34 , it is also possible to operate in such a manner that the chisel  10  is always rotated to the extent of two blocking faces  34 . In this manner, a continuous uniform wear of the chisel for the purpose of high surface qualities of the milled surface can be achieved. 
         [0063]      FIG. 14  shows another construction variant of a chisel  10 . This chisel is constructed in an identical manner to the chisel  10  according to  FIGS. 1 to 3  with the exception of the structure of the chisel shaft  17 . Reference may therefore be made to the corresponding statements above. Furthermore, the nut  30  according to  FIGS. 4 to 7  can be screwed to the thread  19  of the chisel  10 , and it can accordingly be fitted in the chisel holder  40  according to  FIGS. 8 to 11 . 
         [0064]    The chisel shaft  17  of the chisel  10  according to  FIG. 14  has an expansion portion  17 . 1  which is constructed in the form of a cross-section reduction in order to achieve improved expansion behavior. 
         [0065]      FIG. 15  shows a milling roller  60  which has a milling roller pipe  61 . A large number of chisel holders  40  according to  FIGS. 8 to 11  are directly secured, for example, welded, to the surface  62  of the milling roller pipe  60 . The chisel holders carry the chisels  10 , for example, according to  FIGS. 1 to 3 . As described above, the chisel holder changing systems may accordingly also be fitted in accordance, for example, with  FIGS. 12 and 13  with the milling roller pipe  61 . To this end, the lower portions  50  are placed with the support faces  55  thereof on the surface  62  and welded to the milling roller pipe  60 . 
         [0066]      FIGS. 16 to 19  show an alternative construction of the invention to  FIG. 1 to 13 or 14 , wherein the chisel  10  and the chisel holder  40  are slightly modified. In order to prevent repetition, reference may therefore be made to the above statements and only the differences will be discussed below. As can be seen in  FIGS. 16 and 17 , in the region of the support face  15  a peripheral recess  15 . 1  is formed in a groove-like manner. It extends concentrically about the chisel axis M.  FIGS. 18 and 19  show the chisel holder  40  which in the region of the counter-face  44  has a peripheral projection  44 . 1 . It is constructed in a bead-like manner and also extends concentrically about the longitudinal center axis of the chisel holder  40 . The positioning of the projection  44 . 1  is selected in such a manner that, in the assembled state of the chisel  40 , it engages in the recess  15 . 1 . In this manner, a labyrinth-like seal is formed in the region of the support face  15 /counter-face  44 , and impedes the introduction of dirt into the region of the chisel receiving member  45 . Furthermore, the support face  15  is interrupted with the recess  15 . 1  so that reliable surface contact with respect to the counter-face  44  is always ensured, even with production-related deviations from the ideal shape. 
         [0067]    In place of the projection  44 . 1 , it is also possible to use a ring, for example, a sealing ring, in particular a commercially available O-ring or a copper ring or a similar metal ring. This may be laid in a peripheral groove of the chisel holder  40  in the region of the counter-face  44 . With the region thereof which protrudes over the counter-face  44 , this sealing ring then engages in the recess  15 . 1 . 
         [0068]      FIGS. 20 and 21  show another embodiment of a chisel  10 . This chisel is constructed in accordance with the chisel  10  according to  FIGS. 1 to 3 , for which reason, in order to prevent repetition, only the differences are intended to be discussed below. The chisel head  11  is provided with a plurality of tool receiving members  14  on an outer periphery. These may be formed as recesses in the outer contour of the chisel head  11 . The recesses are open in a radially outward direction and in an axially upward direction. Consequently, a tool can be readily fitted from the chisel tip  20 . Furthermore, the tool receiving members  14  cannot become clogged with waste material or are easy to clean where applicable. 
         [0069]      FIGS. 22 to 27  show various embodiments of chisel holder changing systems, in which the above-described chisels  10  can be used together with the nut  30  according to  FIGS. 4 to 7 . In these drawings, for the identification of identical or equivalent components, the same reference numerals as above are used. Reference may therefore be made in full to the statements above. 
         [0070]      FIG. 22  shows a tool holder changing system having a tool holder  40 , which carries at a base portion  41  an integrally formed plug type attachment  48 . A cylindrical attachment  42  is further formed on the base portion  41 . In the region of the cylindrical attachment  42 , a counter-face  44  corresponding to the counter-face  44  is again constructed in accordance with the chisel holder  40  according to  FIGS. 8 to 11 . In the base portion  41  and the cylindrical attachment  42 , there is formed a chisel receiving member  45  which terminates in a sealing portion  46 . The sealing portion  46  is again adjoined by the holder receiving member  47 , in which the nut  30  according to  FIGS. 4 to 7  is inserted. In this instance, the nut  30  again has a securing portion  33  with blocking faces  34 . The blocking faces  34  cooperate with retention faces  47 . 1  of the chisel holder  40  in order to secure the nut  30  in a rotationally secure manner. The nut  30  is again sealed with the sealing portion  31  thereof and the seal  32  on the sealing portion  46  of the chisel holder  40 . 
         [0071]    As can further be seen in  FIG. 22 , the chisel  10  with the thread  19  is screwed into the thread  36  of the nut  30  until the impact face  37  strikes the chisel holder  40 . 
         [0072]    The chisel holder  40  is inserted with the plug type attachment  48  thereof into a plug type receiving member of a lower portion  50 . The chisel holder  40  is supported with respect to the lower portion  50  and is retained in the lower portion  50  with a pressure screw  56  which acts on the plug type attachment  48 . 
         [0073]      FIG. 23  shows the combination of the chisel holder  40  with the chisel  10 , as described above with reference to  FIG. 22 . 
         [0074]      FIG. 24  shows another chisel holder changing system. Accordingly, there is again used a chisel holder  40  which receives the chisel  10  and the nut  30  in the manner described above. The chisel holder  40  is retained in a lower portion  50  with a plug type attachment which cannot be seen in  FIG. 24 . 
         [0075]      FIG. 25  shows a construction variant of a chisel holder changing system having a chisel holder  40  and a lower portion  50 . 
         [0076]      FIG. 26  shows another construction variant of a chisel holder changing system having a chisel holder  40  and a lower portion  50  which receives the chisel holder  40 . 
         [0077]      FIG. 27  discloses a tool system having a chisel holder  40 , in which the chisel  10  is inserted. The chisel holder  40  can be placed directly on the surface  62  of a milling roller pipe  60  and secured thereto, for example, welded.