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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a construction machine for working pieces of ground, having a milling roller on which surface chisel holders are arranged, wherein a chisel, in particular a round shaft chisel, is exchangeably received in a chisel receiver of the chisel holder. 
   2. Discussion of Related Art 
   A construction machine designed as a road-milling machine is taught by German Patent Reference DE 39 03 482 A1. Road coverings can be cut off by road-milling machines. The chisels continuously wear out during operation of the machine. After the chisels have reached a certain wear state, they must be replaced. Thus it is necessary for a worker to approach the milling roller and there drive the chisels out of the chisel holders. For driving the chisels out, the worker uses a special ejection mandrel and a hammer. This can lead to injuries. Manipulation in the narrow milling roller area is extremely difficult and requires great care in order to reduce the risk of danger. After a chisel is removed from its chisel holder, it is necessary to insert fresh unworn chisels into the chisel holders. Replacement of the chisels is a very arduous and time-consuming job. 
   Manually operable exchangeable tools are known from German Patent Reference DE 32 23 761 C2 and from U.S. Pat. No. 3,342,531. They have a shoulder, which positively engages a circumferential groove in the chisel. The chisels can then be levered out of the associated chisel holder. Although the exchange process is easier with this, working on the milling roller is nevertheless dangerous and arduous. 
   SUMMARY OF THE INVENTION 
   It is one object of this invention to provide a road-milling machine of the type mentioned above but wherein the exchange of the chisels is simplified. 
   This object is achieved with a tool changing device assigned to the road-milling machine, and the tool changing device removes and/or mounts each chisel from or in the chisel holder. 
   Thus, in accordance with this invention a changing tool is proposed, which automatically removes the worn chisel and/or mounts an unworn chisel in the chisel holder receptacle of the chisel holders. Thus it is possible to reduce manual labor necessary for changing the chisels. Because the changing process is at least partially automated, it can be more rapidly performed, so that fewer machine outages are created. Also, with the device in accordance with this invention, the endangerment of the health and the stress on the body of the machine operator are reduced. 
   The tool changing device preferably is a mechanical tool device. It is arranged inside or outside of the milling roller. Different concepts can be used, depending on the intended use, during the technical layout of the tool changing device. 
   The tool changer can be positioned in relation to the chisel. The chisel can be positioned in relation to the tool changer. The tool changer and the chisel can be positioned with respect to each other. 
   In some embodiments, the tool changing device has at least one tool changer, which can be assigned to the individual chisel holders or groups of chisel holders by an actuating unit. It is also possible for a single tool changer to be mutually assigned to all chisels or chisel holders. It then removes or installs the chisels simultaneously. In an alternative embodiment of this invention, a tool changer of the tool changing device is respectively assigned to each chisel holder, and the tool changers are fixedly connected with the chisel holder. The tool changers can be connected with each other by a common control device. A machine operator can, for example, purposefully change individual chisels, groups of chisels, or all chisels together with this control device. 
   In another embodiment, the tool changing device imparts at least one dynamic pulse opposite the removal direction of each chisel to the milling roller, a portion of the milling roller, the chisel holder or a group of chisel holders. Thus, a pulse is generated by the tool changing device, which imparts an ejection force to the chisel because of the mass inertia of the chisel. The pulse can be built up, for example, by a vibration generated in the milling roller. It is also possible to provide one or several vibration devices. In a further embodiment, a pulse generator is employed on the milling roller. Thus it is possible, for example, to assign a stop to the milling roller, which has a contact face pointing in the work movement direction. A pulse generator creates a force on the contact face which is directed opposite the work movement direction. The pulse generator can be a mallet, which acts with its weight on the contact face. 
   As explained above, the tool changing device can be such that the chisel is positioned in relation to the tool changer. Positioning of the chisel can take place, for example, by a displacement device, which positions the milling roller in relation to the tool changer. In accordance with another embodiment of this invention, this can take place so that the milling roller is coupled with a drive motor of the construction machine by a drive train. A displacement device can have an auxiliary drive which can be coupled with the drive train and which turns the milling roller in the raised position by a predetermined or selectable angle of rotation. A torque of the auxiliary drive can be greater than the inertia of the milling roller and of the portion of the drive train moving together with the milling roller when the drive motor is switched off or uncoupled. During this it is possible to use the preset position pattern of the chisels and to store it in a control device. The actuating unit and/or the displacement device can have a position measuring system, and the actuating unit and/or the displacement device can be equipped with a numerical control. 
   In this case the layout of the tool can be such that the actuating unit positions the at least one tool changer in relation to the milling roller. During this the tool changer and the milling roller are brought into positions with respect to each other. 
   It is possible for tool changers to be arranged fixed in place on the machine. The chisels are then assigned to them by rotation of the milling roller. 
   The tool changer can be laid out so that it engages the chisel in a positive or non-positive manner and removes it from the chisel holder or installs it in the chisel holder. 
   The tool change can be further automated if the tool changing device conveys the removed chisels directly, or via a conveying device, to a container, or if a separating device is assigned to the tool changing device. The separating device conveys chisels from a storage unit to the tool changing device. 
   It is possible to optimize tool down time if a detection device is assigned to the milling roller, which checks the wear state of the chisels, or of a portion of the chisels, or of a single chisel, continuously, at intervals, or when directed, and if the detection device initiates or signals a tool change upon reaching a predetermined wear state. 
   For example, the wear detection can be designed so that at least one signal reception unit of the detection device is assigned to at least one structural unit of the machine which directly or indirectly participates in the working process. The signal reception unit detects an operational state of the structural unit of the machine, and the signal reception unit determines the wear state via a signal processing arrangement. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention is described in view of the drawings, wherein: 
       FIG. 1  is a lateral view and a partial representation of a milling roller of a road-milling machine with a chisel holder mounted thereon and with a tool changing device; 
       FIG. 2  is a lateral view and a partial representation of the milling roller in accordance with  FIG. 1 , with a tool changing device for installing unworn chisels; 
       FIG. 3  shows a milling roller with a chisel holder formed on it in one piece, in a sectional lateral view; 
       FIG. 4  shows a milling roller with a tool changing device in the milling roller interior, in a lateral view; and 
       FIG. 5  shows the representation in accordance with  FIG. 4 , in a changed work position. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   A rotary body of a road-milling machine, namely a milling roller  10 , is represented in  FIG. 1 . Base elements  20  are arranged in a systematic separation from each other on the roller surface  11  of the milling roller  10 . The base elements  20  are connected, preferably welded, to the roller surface  11 . The base elements  20  each has a plug-in receiver  21 . A plug-in shoulder of a chisel holder  23  can be inserted into the plug-in receiver  21 . The chisel holder  23  is fixed on the base element  20  by a pressure screw  22 . The chisel holder  23  has a chisel receiver  24 , which is embodied as a bore in the present case. A chisel  30 , here a round shaft chisel, can be inserted into the bore. The chisel  30  has a chisel head  31 , to the front of which a chisel tip  32 , comprising a hard alloy or a ceramic material, is fastened. A shaft  33 , on which a clamping sleeve  34  is drawn, adjoins the chisel head  31 . The clamping sleeve  34  is connected with the shaft  33  so that it is not axially displaceable, but rotatable in the circumferential direction. 
   The chisel head  31  rests on a counter-surface of the chisel holder  23 , with a wear-protection disk  35  placed between them. 
   As shown in  FIG. 1 , a tool changing device with a tool changer  40  is assigned to the chisel holder  23 . The tool changer  40  has an actuating motor  43  driving a transfer member  41 . In this case, the transfer member  41  is designed as a draw bar. On the end facing away from the actuating motor  43 , the transfer member  41  has an ejection mandrel  42 . The ejection mandrel  42  can be introduced into the chisel receiver  24  by the actuating motor  43 . Here, the mandrel penetrates the chisel receiver  24  through the rear bore opening  25  and then encounters the rear impact face formed by the shaft  33 . The actuating motor  43  pulls the ejection mandrel  42  into the chisel receiver  24 . In the process, the chisel  30 , together with its clamping sleeve  34 , is pushed out of the chisel receiver  24 . After the chisel  30  is moved out of the chisel receiver  24 , the actuating motor  43  pushes the ejection mandrel  42  out of the chisel receiver  24 , again. 
   The tool changer  40  can be displaced, for example linearly, in the direction of the center longitudinal axis of the milling roller  10  by an actuating unit  39 . It then can be assigned to the individual chisel holders  23  of the milling roller  10 , one after the other. Advantageously, the actuating motor  43  does not only move one ejection mandrel  42 , but moves several ejection mandrels  42  simultaneously, so that several chisels  30  can be pushed out of their chisel holders  23  in one actuating process. 
   It is also possible for the milling roller  10  to be rotated by an auxiliary drive mechanism of a displacement device  37 . The auxiliary drive mechanism can be operated when the milling roller  10  is lifted off the ground. It can then be displaced for a tool change by the auxiliary drive mechanism. A control unit can also be assigned to the auxiliary drive mechanism. It rotates the milling roller  10  in accordance with a preset program run, so that the chisels  30 , or a portion of the chisels  30 , can be oriented with respect to the tool changer  40 . 
   A tool changer  40 , which is used for installing an unworn chisel  30  into the chisel receiver  24 , is represented in  FIG. 2 . Again, the tool changer  40  has an actuating motor  43 , which linearly displaces the transfer member  41 . The transfer member  41  has an assembly bell  44  with a receiver  45 , in which the chisel head  31  of the chisel  30  to be installed is maintained. Accordingly, the tool changer  40  is assigned to the chisel holder  23  by an actuating unit. Thus, the chisel shaft is located opposite the bore entry into the chisel receiver  24 . Thereafter the actuating motor  43  is activated. The shaft  33  is then pushed into the chisel receiver  24 . The threading movement of the shaft  33  into the chisel receiver is made easier by a conical bore widening  26 . After the chisel  30  is installed in the chisel holder  23 , the chisel head  31  is released from the assembly bell  44 . The actuating motor  43  again moves into its initial position and is then available for the next installation process. 
   The tool changers represented in  FIGS. 1 and 2  can be used individually or together in a road-milling device. If they are used together, a fully automatic chisel change can be performed. 
   A portion of a milling roller  10  is represented in  FIG. 3 . The milling roller  10  has a milling roller tube, which forms the roller surface  11 . Chisel receivers  24  are directly cut into the milling roller tube, so that the chisel receivers  24  are connected in one piece with the milling roller tube. The chisel receiver  24  is formed by a bore having a bore end with a bore widening  26 , which makes the insertion of the chisel  30  easier. A tool changer  40  is arranged at the other end of the bore and can be embodied as a hydraulic or a pneumatic cylinder and can have a linearly displaceable ejection mandrel  42 . It is possible to employ the tool changing device represented in  FIG. 3  in any arbitrary, different chisel holder system, such as in a changer holder system as represented in  FIGS. 1 and 2 . A chisel  30  is inserted into the chisel receiver  24  and in its structural type, it corresponds to the chisels  30  represented in  FIGS. 1 and 2 . 
   The tool changer  40  is activated for removing the chisel  30  from its chisel receiver  24 . The ejection mandrel  42  then moves against the free end of the chisel shaft  33 . The ejection mandrel  42  ejects the chisel  30  in the direction of the center longitudinal axis of the chisel receiver  24 . The tool changer can also be used to again install a fresh unworn chisel  30  into the chisel receiver  24 . Thus, the chisel  30  can be connected with the extended ejection mandrel  42  and can be pulled into the chisel receiver  24  with the aid of the changing tool  40 . 
   A further embodiment variation of a milling roller  10  with a tool changing device is described in  FIGS. 4 and 5 . The tool changing device has a tool changer  40  housed in the interior of the milling roller  10 . The milling roller  10  is constructed similar to the milling roller  10  shown in  FIG. 3 . It has chisel holders  23  formed on it in one piece. It is possible to employ any arbitrarily differently designed chisel holder  23 . 
   The tool changer  40  has two articulated arms  47 ,  49 , which are connected with each other by a hinge  48 . The articulated arm  47  is fixed in place via a hinge  46 . A pulse generator  50  in the form of a weight is arranged at the free end of the second articulated arm  49 . On its interior circumference, the milling roller  10  has a stop  51  with a contact face  52 . On the side facing away from the contact face  52 , the stop  51  has an inclined deflection face  53 . 
   During normal milling operations, the tool changer  40  is maintained in the position represented in  FIG. 5 . If a chisel change is due, it is moved into the position shown in  FIG. 4 . Then the milling roller  10  is rotated in the circumferential direction until the pulse generator  50  impacts on the inclined deflection face  53  of the stop  51 . A pulse is thus generated, which acts opposite to the removal direction of the chisels  30 . Because of this pulse a force is introduced into the chisels  30  which pushes them out of the chisel receivers  24 . 
   After the pulse generator  50  has impacted the contact face  52 , it is deflected at the stop  51  and is again brought into its extended initial position via the inclined deflection face  53 . If needed, the process for generating a pulse can then be repeated. At the termination of the ejection process the tool changer  40  is again returned into the position represented in  FIG. 5 . A reversal of the action principle is also possible and the pulse generator can be rotated.

Summary:
A construction machine for machining floor surfaces, wherein the machine includes a milling roll having a plurality of tool holders on a surface thereof. A tool, especially a straight shank tool, is received in a tool receiving element of the tool holder in an exchangeable manner. With this invention it is possible to change the tool in one such construction machine in a simplified manner. Thus, the milling roll is associated with a tool changing device, and the tool changing device dismounts each tool from the tool holder and/or mounts each tool in the tool holder.