A chisel-holder changing system for a road milling machine, a coal cutting machine or the like to which a chisel-holder is fastened in an exchangeable manner by means of a connecting mechanism. The chisel-holder accommodates at least one chisel that can be exchanged. The middle longitudinal axis of the chisel is set at an operating angle of less than 90° with respect to the surface to be worked. In order to be able to obtain varied removal results using such a chisel-holder changing system, the chisel-holder can be fixed in two or more configuring positions to the base part and in different attachment positions of the chisel, the middle longitudinal axis of the chisel is slanted at various working angles.

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a chisel-holder changing system for a road milling machine, a coal cutting machine, or the like, to which a chisel holder is fastened in an exchangeable manner by a connecting mechanism, wherein the chisel holder accommodates at least one chisel that can be exchanged, and wherein a central longitudinal axis of the chisel is set at a working angle of less than 90° relative to the surface to be worked. 
     2. Description of Related Art 
     A chisel-holder changing system is known from German Patent Reference DE 43 22 401 C2. The chisel-holder changing system features a base part that is welded onto a surface of a roller body that projects outwardly. The base part has a plug receiver, in which a plug connector of the chisel holder can be inserted. The chisel holder has a receiving hole, in which the chisel shaft of a circular-shaft chisel is inserted. The chisel holder can be fixed to the base part using a fastening screw. In the assembled state, the central longitudinal axis, the axis of rotation, of the circular-shaft chisel is slanted with respect to the surface to be worked. 
     SUMMARY OF THE INVENTION 
     One object of this invention is to provide a chisel-holder changing system of the initially mentioned type, with which different removal results can be obtained with regard to the quality of the worked surface. 
     This object is achieved with a chisel holder that can be fixed in two or more attachment positions to a base part, and a central longitudinal axis of the chisel is slanted at different working angles in the various attachment positions. 
     Due to different settings of the chisel, a single chisel-holder changing system can realize different removal results. For example, for road milling, through variation of the working angle, either coarse or fine milling can be produced. 
     According to one preferred embodiment of this invention, the chisel holder is supported by a support surface on a contact surface of the base part, and for implementing the different attachment positions, the chisel holder can be moved on its support surface relative to the contact surface. 
     In order to be able to change the chisel position quickly, it is possible to adjust the chisel holder stepwise relative to the base part. In particular, the individual steps correspond to determined, preferred working angles. For road milling, these angles are, for example, 35°, 40°, and 45°. 
     The step-wise adjustment can be implemented, for example, such that the chisel holder has toothing that engages counter-toothing of the base part, and such that the toothing and the counter-toothing have flat support edges that extend perpendicular to the movement direction of the chisel holder relative to the base part. With the support edges, the forces occurring during operation can be reliably transferred into the base part. 
     Normally, the chisel-holder changing system is mounted on a roller-shaped base that can move about an axis of rotation. So that the forces acting in the direction of the axis of rotation can also be reliably received in the working insert, a chisel-holder changing system according to this invention has the counter-toothing of the base part with two sets of partial toothing. The sets of partial toothing are slanted for receiving transverse forces in the direction perpendicular to the direction of advance of the chisel. 
     An alternative embodiment of the chisel-holder changing system can be implemented, for example, so that the base part has a contact part that is configured with a plurality of concentrically arranged tooth elements of the counter-toothing. The chisel holder carries a gripping plate that has toothing adapted to the counter-toothing, and the chisel holder can rotate relative to the base part about an adjustment axis that is perpendicular to the direction of advance of the chisel. Such a connecting mechanism can also be configured as Hirth-type serrations. Due to the plurality of tooth elements, extremely high forces can be transmitted by this connecting mechanism. 
     In order to create a fixed connection of the chisel holder to the base part, which simultaneously allows simple and quick changing of the working angle, in one embodiment of this invention the chisel holder has a base carrying a holder attachment for receiving the chisel. The base features an oblong hole that is aligned with a threading receiver of the base part. A fastening screw can be inserted into the oblong hole and the screw can be screwed into the threading receiver, and the chisel holder can be adjusted when the fastening screw is removed. 
     A screw head of the fastening screw can be housed for protection in a recess that expands and connects to the oblong hole. 
     An especially good and quick attachment of the screw head to the chisel holder can be achieved if the screw head is designed as an oval-head screw head. 
     Wear or excessive stress can damage a chisel holder. In order to prevent the chisel holder from breaking uncontrollably and damaging the base part, the chisel holder carries a holder attachment on a base. The holder attachment has a receiving hole for the chisel stem, and the holder attachment is connected to the base through a set breaking point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention is explained in greater detail with reference to embodiments shown in the drawings, wherein: 
     FIGS. 1A and 1B show a chisel-holder changing system in two different views and in a first setting position; 
     FIGS. 2A and 2B show the chisel-holder changing system from FIGS. 1A and 1B but in a second setting position; 
     FIGS. 3A and 3B show the chisel-holder changing system from FIGS. 1A and 1B but in a third setting position; 
     FIG. 4 shows one embodiment of a base part for a chisel-holder changing system, in a perspective view; and 
     FIG. 5 shows a side view of another embodiment of a chisel-holder changing system. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a chisel-holder changing system that has a base part  40  and a chisel holder  20 . The base part  40  has a lower connection surface  41 , with which the base part  40  can be set on a roller-shaped base, for example, a road milling machine. The connection to the roller-shaped base is by welded joints. On its side facing upwards, the base part  40  has a curved contact surface. The contact surface has counter-toothing  42 . The individual teeth of the counter-toothing  42  are designed with a triangular cross section, as shown in FIG. 1A, and thus form two contact edges. The teeth of the counter-toothing  42  extend over the entire width of the base part  40 . A threaded receiver  43  is worked into the contact surface of the base part  40  at the center of the width of the teeth. In this way, the threaded receiver is arranged so that it occupies the greatest space possible in the base part  40  and therefore provides a sufficient thread length for a fastening screw. As shown in FIG. 1A, the threaded receiver  43  is formed slanted in the contact surface. Due to this space-saving arrangement, a small total height of the base part  40  can also be realized. This has special advantages because the entire construction of the chisel-holder changing system can thus be kept small. 
     The chisel holder  20  can be installed on the base part  40 . The chisel holder  20  has a base  25  that has an oblong hole  28 . The oblong hole  28  extends to the outer side of the base  25  by a recess  27 . The fastening screw, which represents the connection mechanism  30 , can be inserted through the oblong hole  28  and screwed into the threaded receiver  43 . In this way, the screw head  32  is then housed protected in the recess  27 . Tightening a fastening screw presses the screw head  32  against the chisel holder  20  in the transition region between the oblong hole  28  and the recess  27 . Here, toothing  26  of the base  25  can be maintained in tight engagement with the counter-toothing  42 . Due to the plurality of teeth in engagement, a large connecting surface is formed, wherein the chisel holder  20  can be pressed against the base part  40  and large forces can be transferred during operation through this connecting surface. 
     The base  25  features a projecting holder attachment  21 , in which a receiving hole  22  is formed. The central longitudinal axis of the receiving hole  22  is thus perpendicular to the curved contact surface of the base part  40 . A discharge channel  23  is bored in the chisel holder  20  from the back side of the chisel-holder changing system. The discharge channel  23  crosses the receiving hole  22 . A chisel  10  can be inserted in the receiving hole  22 . The chisel  10  features a chisel head  17  that carries a chisel joint  11 . The chisel joint  11  is designed as a hard metal element. A chisel stem is connected to the chisel head  17  and an adapter sleeve  12  is pulled onto the shaft. The adapter sleeve  12  is braced in the receiving hole  22 . Simultaneously, the adapter sleeve  12  retains the chisel stem of the chisel  10  and thus prevents it from falling out. In the inserted state, the chisel head  17  lies on a wear protection disk  14 . The wear protection disk  14  is supported on the chisel holder  20  around the receiving hole  22 . The chisel  10  can be pulled from the receiving hole  22  by two different methods. In one method, a discharge pin can be inserted through an inlet opening  24  into the discharge channel  23  until the pin contacts a front surface  13  of the chisel stem. Then, with the blow of a hammer, the chisel stem can be removed from the receiving hole  22 . In the other method, a circumferential removal groove  15  is formed in the chisel head  17 , and a removal tool is inserted into the groove  15 . By means of a lever, the chisel  10  can then be removed from the receiving hole  22 . 
     As shown in FIGS. 1A and 1B, the fastening screw is arranged in the middle in oblong hole  28 . This results in a working angle α that is used to adjust the central longitudinal axis  16  of the chisel  10  with regard to the surface to be worked. The complementary angle is shown in FIG. 1A with α′. When this working angle α is to be changed, merely the fastening screw must be loosened. The chisel holder  20  can then be moved relative to the base part  40 . Subsequently, the fastening screw is tightened again so that the toothing  26  can be braced against the counter-toothing  42 . Such a changed setting position is shown in FIGS. 2A and 2B. At this position, the working angle α is 45° (compared with the previous setting of 40°). In the setting position according to FIGS. 2A and 2B, the fastening screw and the screw head  32  contact one end of the oblong hole  28  and the recess  27 , respectively. 
     The other maximum working angle α is illustrated in FIGS. 3A and 3B. Here, the fastening screw and the screw head  32  contact the other end of the oblong hole  28  and the recess  27 , respectively. In FIGS. 3A and 3B, a working angle α of 35° is realized. Independently of the setting position of the chisel holder  20 , the central longitudinal axis  16  of the chisel  10  is normal to the contact surface of the base part  40 . Here, an optimum transfer of forces generated during operation is possible. 
     In the embodiment shown in FIGS. 1A-3B, a cylindrical geometry is chosen for the screw head  32  of the fastening screw. For improving the contact pressure, an oval-head geometry can also be used. The screw head  32  includes a tool receiver  31  for loosening and tightening the fastening screw. 
     One embodiment of the base part  40  is shown in a perspective view, in FIG.  4 . As shown, the counter-toothing  42  is divided into two sets of partial toothing  42 . 1  and  42 . 2 . The two sets of partial toothing  42 . 1  and  42 . 2  are slanted in the direction perpendicular to the direction of advance of the cylindrical stem chisel  10 . In this way, a serrated-tooth shaped edge  42 . 3  is formed in the transition region between the two sets of partial toothing  42 . 1  and  42 . 2 . The toothing  26  of the chisel holder  20  is also realized corresponding to this formation of the counter-toothing  42 . Due to this configuration, the forces acting perpendicular to the direction of advance of the chisel can be transmitted by a positive fit through the toothing  26  and the counter-toothing  42  into the base part  40 . 
     Another embodiment of a chisel-holder changing system is shown in FIG.  5 . As shown in this representation, the base part  40  with its connecting surface  41  is set on the surface of a roller body  50 . The base part  40  has a contact part  44 . The contact part  44  has a star-shaped counter-toothing  42 . In this way, the individual teeth of the counter-toothing  42  project radially outwards from a common center. The central longitudinal axis of the fastening screw, of which the screw head  32  is shown in FIG. 5, is also arranged in this common center. A gripping plate  25 . 1  of the base  25  of the chisel holder  20  can be set on the contact part  44 . The gripping plate  25 . 1  has a toothing  26  that is formed adapted to the counter-toothing  42  of the base part  40 . The toothing connection shown in FIG. 5 is usually designated as Hirth-type serrations. The bracing of the toothing  26  with the counter-toothing  42  is done by means of the fastening screw. When the fastening screw is loose, the chisel holder  20  can be moved in steps, corresponding to the toothing division relative to the base part  40 . Thus, the working angle α of the chisel  10  is also changed. 
     With the chisel-holder changing systems shown in the drawings, the working angle α can be changed quickly and without great effort. Accordingly, the removal tool, for example a road milling machine, can be easily adapted to different applications. For example, if a larger working angle a is chosen, then a fine surface roughness can be created during road milling. However, small working angles a lead to a large waste volume and simultaneously greater roughness.