1 A Method of Determining the Wear State When stripping road surfaces by means of milling and when removing mineral deposits with surface miners likewise by means of milling, the tools used, in 5 particular milling chisels, are subject to a continuous wear process. When the tools reach a certain wear state, it is advisable to replace them since otherwise, the efficiency of the remainder of the process decreases. In this connection, it is necessary to differentiate among different wear states that lead to the replacement of a milling chisel or chisel holder: 10 1. Replacement of the chisel since there is no longer sufficient wear material (particularly hard metal at the tip). The penetration resistance becomes too great, causing the efficiency to decrease (excessive frictional loss); the wear is for the most part rotationally symmetrical. 15 2. Replacement of the chisel holder because the wear limit has been reached (the holder begins to wear at the contact surface between the chisel and the holder). This wear is usually symmetrical. 20 3. Non-rotationally symmetrical wear on the chisel tip and/or on the chisel head due to insufficient rotational motion of the chisel during the milling process. This results in a poor milling pattern as well as the risk of tool breakage since the supporting action of the chisel head is lost. 25 4. The chisel holder can also be subject to additional non-rotationally symmetrical wear. 5. Chisel breakage. 5555420_1 (GHMatters) P81651.AU.4 2 Furthermore, worn and/or broken chisels can lead to secondary damage to the chisel holders and worn chisel holders can lead to secondary damage to the milling roller. A prompt replacement of the chisels and/or chisel holders is therefore necessary and reduces costs. By contrast, it is likewise not optimal 5 from a cost standpoint for the chisels and/or chisel holders to be replaced too early because the chisels and chisel holders, as wearing parts, are very cost intensive. This results in failure to properly utilize remaining wear potential. Up to now, the wear state of chisels and chisel holders has been ascertained through visual inspection by the machine driver. In order to do so, the machine driver 10 must shut down the machine (switch off the engine and decouple the roller from the drive train). The driver must then open the rear roller flap in order to visually inspect the milling roller. The milling roller is then rotated by means of a second drive unit in order to 15 permit inspection of the entire milling roller. The task of roller inspection can also be performed by a second operator. In this case, the wear state of the chisel holders is usually determined by means of so-called wear markings. The wear state of the chisels is determined by means of longitudinal wear and the rotational symmetry of the wear pattern. 20 Inspecting the wear state of the chisels and holders is very time-consuming and is also unproductive since no production can take place during this time. The entire process is interrupted, thus additionally reducing availability. Furthermore, due to the fact that the assessment is very prone to subjectivity, there is the risk 25 of failure to optimally utilize the wear potential of the holders and chisels. DE 102 03 732 Al has disclosed an apparatus in which operation can be optimized by monitoring operating states of machine components that are directly or indirectly involved in the milling process. Thus among other things, the 5555420_1 (GHMatters) P81651.AU.4 3 wear state of the chisels is assessed by evaluating various machine parameters and characteristic values. During operation of the milling machine, the problem arises that the milling process and the subsurface itself - whose properties fluctuate widely - have a large influence on the evaluation of the operating state 5 of components. Because of the widely fluctuating properties of the subsurface to be milled, it is not easily possible to differentiate between changing operating states of machine components directly or indirectly involved in the milling process that are attributable to changing surface properties and those that are attributable to changing tool properties. 10 AT 382 683 B has disclosed a mining machine in which the cutting roller is monitored in a contactless fashion. In this case, photoelectric detectors are used, which detect the presence of the chisels. This method is unable to provide a quantitative wear evaluation. 15 It would be advantageous if embodiments of the present invention would provide a method of the type mentioned at the beginning with which it is possible to carry out a quantitative wear detection. 20 The present invention provides in a first aspect a method of determining a wear state of a milling tool of a surface miner, the milling tool comprising at least one of: a chisel, a chisel holder and a chisel holder replacement system that is equipped with a chisel and chisel holder, the method comprising the steps of: determining a position value associated with a position of at least one 25 point of that milling tool by means of a contactless measurement method; and comparing the determined position value to at least one reference value stored in a memory device to provide quantitative information associated with the wear state of the milling tool. 5555420_1 (GHMatters) P81651.AU.4 4 According to embodiments of the invention, the comparison of the position value to the reference value is used to establish a reference value that includes quantitative information about the wear on the milling tool, in particular on the chisel and/or chisel holder. It is then possible to evaluate the extent of wear, 5 which assists the user with decision-making. It is thus possible for the user to decide whether a tool replacement must be carried out or whether the wear state permits continuation of a current milling task. This makes it possible to achieve a significant improvement in efficiency. 10 The reference value may be a value that is known in advance, i.e. a value that is determined and stored, or may be a characteristic field or a functional correlation with which one or more position values or calculations of them is/are compared. The step of comparing the determined position value to at least one reference 15 value may comprise use of a switching unit, According to one possible embodiment of the invention, the position of the chisel tip is detected as a position value. 20 In one example, the position of the chisel tip relative to the center of motion of a milling roller is measured and compared to a reference value determined, for example, when the holder and chisel are new. This directly yields the longitudinal wear on the chisel and holder. When a certain limit wear length is reached, the chisel is replaced. 25 Alternative to an absolute measurement relative to the center of motion of the milling roller or another point of origin, it is also possible to perform a relative measurement based on at least two position values, e.g. on the chisel. If the points are selected so that one position value in the vicinity of the chisel tip is 5555420_1 (GHMatters) P81651.AU.4 5 established and the at least one other position value is situated in a region that is subject to only slight wear or to no wear at all, then the longitudinal wear can be directly determined as the difference between the two position values in comparison to the reference value that corresponds to the difference between 5 these two points when the chisel is new. If the reference value for the chisel tip is established again after each chisel replacement, then it may also possible to determine the holder wear based on this. In this context, however, it may be necessary to use chisels whose lengths 10 are known in advance (for example due to prior measurement) or lie within a certain tolerance range (for example the production tolerance range in new chisels). By taking into account the holder wear thus determined, it may then also be possible to determine the exact wear on the new chisel on an already worn holder. The wear on a holder is typically negligibly low over the service life of a 15 chisel. If the volume of the chisel head or a part of the chisel head, in particular the chisel tip or a part of the chisel tip, is to be determined based on the determined position value, then it may be possible based on the positions of the points that 20 represent the position value, to produce a contour line, for example, that can be used to determine an average wear volume of the chisel tip and/or the chisel head. This wear volume may then be compared to a reference value that describes, for 25 example, the contour line of an unworn tool. It is also possible to compare the wear volume to previously measured contour lines, which describe certain wear states and function as reference values. 5555420_1 (GHMatters) P81651.AU.4 6 A further specification of the wear state may be achieved in that in order to establish the reference value, the position of at least one point of the chisel, chisel holder, and/or chisel holder replacement system is determined in the unworn new state. 5 Because it can be derived, for example, from the geometric interrelationships, the reference value may be a theoretical value that is preset in a fixed way (for example the length of the chisel head used as the chisel parameter) or else it is possible for the reference value and position value to be determined in the worn 10 or partially worn state. This method may be suitable if, for example, the current wear situation is not known or else because an entire roller is being replaced and is equipped with already worn chisel holders and chisels. According to a preferred embodiment of the invention, it is also possible for a 15 wear parameter to be correlated with the at least one position value as a reference value by means of a characteristic field or a functional correlation; the functional correlation or the characteristic field can be determined in advance. The characteristic field may, for example, contain experimentally determined correlations. 20 If the process guidance is carried out in such a way that the position of the central longitudinal axis of the chisel head and/or chisel tip is established as a position value and this position value is compared to a reference value that contains the ideal position of the central longitudinal axis, then this may permit 25 an asymmetrical wear on the chisel to be detected in a simple way. A particularly preferable method is characterized in that the wear on the chisel and the wear on the chisel holder are determined separately from each other in an additional calculation step. This makes it possible to obtain detailed system 5555420_1 (GHMatters) P81651.AU.4 7 information that gives the user an exact picture of the wear on the system components. This may be carried out in a particularly simple way if the method is embodied so 5 that first, the total wear is determined based on the addition of the wear on the chisel holder and the wear on the chisel. Then a tool replacement is performed. As the method continues, the actual position of at least one point of the chisel mounted on the worn or partially worn chisel holder must be determined in the new state. Then this measurement result is compared to a reference value of this 10 at least one point in the new state of the chisel and chisel holder and then the wear on the chisel holder is determined through subtraction. The chisel wear in turn is calculated from the difference between the total wear and the holder wear. If one assumes that the holder wear between two chisel replacements 15 approaches zero, then the chisel wear may be determined at any time according to this method by using its subtraction of the holder wear from the measurement respectively preceding it. Another method may be embodied as follows. A first reference value is 20 established in the new state of the chisel and holder and is stored until the holder is discarded. A second reference value is established and then after each chisel replacement, is established again by means of a reference measurement and then stored. Then at any time, the chisel wear is determined from a measurement with a subsequent subtraction using the second reference value, 25 the total wear is determined from a measurement with a subsequent subtraction using the first reference value, and the holder wear is determined from the difference between the second and first reference value. Alternatively, an embodiment of the method can be selected as follows: 5555420_1 (GHMatters) P81651.AU.4 8 First, the total wear is determined by comparing at least one position of the chisel tip to a reference position that was previously established in the new state. Based on this, the total wear is determined by adding the chisel holder wear and 5 the chisel wear. Then, the position of a second point on the chisel is compared to a corresponding reference position in the new state of the chisel. This second point is closer to the holder than the first point. The chisel holder wear is determined from it. The difference between the shift of the second point and the overall shift yields the chisel wear. 10 If, in order to determine the wear on the chisel holder, at least one point of the chisel shaft, which is subject to only slight wear or to no wear at all, is established as a position value and compared to the reference value of this point in the unworn or partially worn state, then the wear state of the holder may be 15 determined directly in the tool system, without having to carry out a reference measurement on a replaced tool. Preferably at least one point of an extraction groove of the chisel head may be established as a position value/reference value. 20 The extraction groove typically is subject to only slight wear or to no wear at all. In one embodiment, an exact determination of the wear is possible because the position value and/or the reference value is determined, for example, by means of runtime measurement, phase shifting, or triangulation. In this context, the 25 physical effect on which the measurement principle is based is immaterial, i.e. it can be acoustic, optical, magnetic, or mechanical. If the method is carried out in such a way that it determines the rotational position of a milling roller that supports the chisel, chisel holder, and/or chisel 5555420_1 (GHMatters) P81651.AU.4 9 holder replacement system, it may then be possible to ascertain the rotational position of the roller and based on it, the exact position of the respectively measured tool. It may then be possible to specifically locate the respective chisel and to replace it as needed, for example. 5 The present invention provides in a second aspect a method of determining a wear state of a chisel of a milling tool of a surface miner, the surface miner comprising a milling roller that supports a plurality of milling tools, each milling tool comprising a chisel and a chisel holder, or a chisel holder replacement 10 system that is equipped with a chisel and a chisel holder, the method comprising the steps of: determining a position value of at least one point of the chisel of the milling tool relative to an origin by means of an absolute measurement method; and 15 comparing the determined position value to a reference value stored in a memory device; wherein the method is conducted such that a result of the comparison of the determined position value with the reference value is indicative of quantitative information associated with the wear state of the chisel. 20 In the first and second aspect of the present invention the step of determining the position value may comprise use of an optical measurement method. In the method in accordance with the second aspect of the present invention the 25 step of determining a position value may comprise determining the position value of at least one point of the chisel of the milling tool relative to a center of motion of the milling roller. 5555420_1 (GHMatters) P81651.AU.4 10 The position value may be a distance and the step of determining the position value may comprise use of a distance sensor. The distance may be a distance between a free end of a chisel tip and the origin. 5 The present invention provides in a third aspect an apparatus arranged for determining the wear state in accordance with the method of the second aspect of the present invention. 10 The invention will be explained in greater detail below in conjunction with exemplary embodiments shown in the drawings. Fig. 1 shows a side view of a chisel, namely a round-shafted chisel, which is for a road surface milling machine, a mining 15 machine, a surface miner, or the like, and is inserted into the holder of a replaceable holder tool for such machines; Fig. 2 shows the chisel head of the chisel from Fig. 1, depicted in a graph; 20 Figs. 3 through 7 show various wear states of the chisel head, depicted in graphs; Fig. 8 is a schematic side view of a road surface milling machine; 25 Fig. 9 shows a milling roller of the road surface milling machine according to Fig. 8, with a measuring device that functions based on the principle of triangulation; and 6137886_1 (GHMatters) P81651.AU.4 JMEAD 11 Fig. 10 shows the milling roller according to Fig. 9, with an alternative embodiment of the measuring device that functions based on a shadow-casting method. 5 Fig. 1 shows an example of a chisel 10 of the type known from the prior art and described, for example, in DE 38 18 213 Al. The chisel 10 has a chisel head 12 and a chisel shaft 15 integrally formed onto the chisel head. The chisel head 12 supports a chisel tip 11 composed of a hard material, for example hard metal. 10 This chisel tip 11 is usually soldered to the chisel head 12 along a contact surface. A circumferential extraction groove 13 is provided in the chisel head 12. This extraction groove 13 serves as a tool receptacle in that a removing tool is placed onto it, enabling removal of the chisel 10 from a chisel holder 70. 15 The chisel shaft 15 supports a longitudinally slit, cylindrical clamping sleeve 21. This sleeve is secured so that it cannot be detached in the direction of the longitudinal span of the chisel 10, but is able to freely rotate in the circumference direction on the chisel shaft 15. In the region between the clamping sleeve 21 and the chisel head 12, a wear-prevention disk 20 is provided. When installed, 20 the wear-prevention disk 20 rests against a counterpart surface of a chisel holder 70 and, on the side oriented away from the chisel holder 70, rests against the underside of the chisel head 12. The chisel holder 70 is equipped with an attachment 71 in which a chisel 25 receptacle 72 in the form of a cylindrical bore is provided. In this chisel receptacle 72, the clamping sleeve 21 is secured with its outer circumference clamped against the inner wall of the bore. The chisel receptacle 72 opens out into the ejection opening 73. An ejecting pin (not shown) can be inserted through this opening in order to remove the chisel 10. It acts on the end of the chisel 5555420_1 (GHMatters) P81651.AU.4 12 shaft 15 in such a way that by overcoming the clamping force of the clamping sleeve 21, the chisel 10 is slid out from the chisel receptacle 72. As is clear from Fig. 1, the attachment 71 is provided with two circumferential 5 grooves in a cylindrical region below the wear-prevention disk 20. These grooves serve as wear markings 74. During operational use, the wear prevention disk 20 rotates and in so doing, can produce wear (chisel holder wear) on the supporting surface of the attachment 71. When the supporting surface abraded down until it reaches the second wear marketing, the chisel holder 70 is worn down to the 10 point that it must be replaced. The chisel holder 70 has an insertion attachment 75 that can be inserted into an insertion receptacle 82 of a base part 80 and can be clamped there by means of a clamping screw 83. 15 The clamping screw 83 in this case presses a contact surface 76 of the chisel holder 70 against a supporting surface 84 of the base part 80. In a manner not shown in detail in Fig. 1, the underside 81 of the base part 80 itself is welded to the milling roller tube of a milling roller. 20 To determine the wear state of the chisel 10 mounted on the chisel holder, a contactless measurement of the chisel head 12 is performed. In this instance, a specified point or a plurality of points of the chisel head 12 is/are measured/determined as position values. Fig. 2 shows the contour of an unworn 25 chisel head 12, depicted in a side view. This contour is depicted in a graph, with the span of the chisel head 12 in the direction of the longitudinal axis of the chisel 10 (x-axis) plotted over the span in the width direction (perpendicular to the central longitudinal axis, i.e. the y-axis). The contour of the chisel head 12 5555420_1 (GHMatters) P81651.AU.4 13 (including the chisel tip 11) is composed of a multitude of measured points; the linear border (contour line) represents an interpolation of these position values. The measurement on a milling roller 35 (see Figs. 9 and 10) can be performed 5 when the chisel 10 is in the unworn state or in a partially worn state. Then these points, i.e. the calculated chisel head contour, or also only a single measurement point can be determined and stored as reference values/a reference value in a memory device. 10 Fig. 3 shows various measurement images of a chisel 10. The chisel head 12 here is shown in the unworn state and in three measured wear states V1 through V3. The wear state V3 represents the state in which the chisel 10 is no longer suitable for further operation and must be replaced. The wear can be determined by comparing the reference value (contour in the unworn state) to the 15 respectively measured current chisel head contour (wear states V1, V2, or V3) that represent the position value. The contours must be aligned with one another. Due to holder wear or other error sources, unworn contour regions are not necessarily situated at the same 20 absolute positions (i.e. relative positions in relation to the rotational axis of the roller). Setting the contours into relation with one another once again requires reference features that are clearly identifiable and appear in both the unworn and worn state, thus permitting an alignment and an exclusive evaluation. The extraction groove 13, the wear markings 74, the wear-prevention disk 20, or 25 other prominent regions that are subject to only slight wear or to no wear at all can be used as reference features. As depicted in Fig. 3, the length of the chisel head 12 decreases during operational use. Through subtraction of the contour lines in a switching unit, it is 5555420_1 (GHMatters) P81651.AU.4 14 possible to obtain information about the wear state, which is visually depicted for the user on a display unit, for example. Instead of measuring the entire contour of the chisel head 12, it is also possible for only a part of the contour or a single point, in particular the front end of the chisel tip 11, to be detected as a position 5 value. More detailed information about the wear state can be obtained if the wear on the chisel tip 11 and on the chisel head 12 (without the chisel tip 11) are determined separately. This is easily possible with the previously known position of the attachment 10 (attachment point TP/attachment line 18) of the chisel tip 11 to the chisel head 12 in the unworn state. If the measured wear states V1 through V3 are superimposed on the unworn contour of the chisel head 12, as shown in Fig. 2, then the total wear volume can 15 be determined through integration of the measured contours along the x-axis. In Fig. 4, the wear volume is depicted as a shaded region. Consequently, the wear volume can be used as a position value and compared to a reference value. The reference value in this case can be comprised of a 20 functional correlation or a characteristic field; different wear volumes are correlated with associated wear states (for example a mm 3 of wear volume corresponds to b% wear). Based on the previously known position of the attachment point TP/attachment line 18, the tip wear 16 and the head wear 17 can then be separately determined through subtraction. This wear determination 25 gives the user qualitative information, for example, about whether the chisel 10 is still suitable for certain milling tasks. Thus a chisel that has not in fact reached its wear limit yet, but does have a certain degree of tip wear, can no longer be used for fine milling work, for example. 5555420_1 (GHMatters) P81651.AU.4 15 In the method according to Figs. 3 and 4, the measured position values are superimposed with the reference values. During the machining process, wear occurs not only on the chisel head 12, but also on the chisel holder. The chisel holder wears down in the direction of the central longitudinal axis of the chisel 5 10. The isolated determination of the longitudinal wear on the chisel holder in this direction then succeeds simply because the degree of shifting of the position value toward the reference value required to reach the superimposed positions shown in Figs. 3 and 4 (shifting of the chisel head 12 in the direction of the x-axis until, for example, the extraction grooves of the individual wear states are 10 congruent) constitutes an absolute wear value of the chisel holder. The total shifting of the chisel tip 11 minus this wear value is then a separate indication of the chisel wear. Fig. 5 shows a method variant in which information about wear is obtained based 15 on measuring a clearly identifiable feature on the chisel. The feature and the surroundings of this feature should be subject to only slight wear or to no wear at all. According to Fig. 5, the extraction groove 13, a part of the extraction groove 13, or a point (for example the groove bottom) of the extraction groove 13 is used as a definite feature and reference criterion. 20 Furthermore, this feature is then used for determining the distance to the position of the free end of the chisel tip 11 (position value). This makes it easily possible to determine the longitudinal wear in a simple way. If the position of the chisel tip 11 is known, then as described above, it is once again possible to determine the 25 chisel holder wear. Alternatively or in addition, as specified in conjunction with Fig. 4, it is also possible to determine the volume wear and the associated wear lengths (X = total wear). 5555420_1 (GHMatters) P81651.AU.4 16 The relative measurement through the use of a definitely identifiable feature as a reference point has the advantage that a wear determination can also be made in the absence of a reference measurement because for example, the length of the replaced chisel 10 or the wear state of the chisel 10 or chisel holder is not 5 known. Fig. 3 shows the change in the chisel head contour as the wear increases. The decreasing length of the chisel head 12 is clearly visible. If the chisel head contours are respectively shifted toward the point of the maximum length of the 10 chisel 10 (Fig. 6), then on the one hand, through the changing head angle (the angle becomes flatter as chisel wear increases) and on the other hand, through the increasing volume of the chisel head 12, the increasing wear state of the chisel 10 becomes evident when the area under the contour curve, starting from the respective tip over a certain length, is integrated. It is thus possible to 15 establish a functional correlation between the head volume/area and the absolute chisel length for a certain chisel type, for example by means of experiments carried out in advance to determine reference values. If this correlation is known, then the length of the chisel 10 can be determined 20 through measurement/integration of the respective head volume currently present. This method, however, is based on the assumption that the chisel tip 11 has a cross-section that increases in the longitudinal direction of the chisel. The integration method is preferably established so that even with completely worn chisels 10, the integration does not extend into the head region since erosion in 25 the head region distorts the result. This method has the advantage of not requiring a reference point that is subject to little wear. Sometimes such a reference point is not present or is very difficult to identify. 5555420_1 (GHMatters) P81651.AU.4 17 In order to determine the asymmetry factor of the wear, it is possible, for example, to calculate the average between the upper and lower contour lines. Fig. 7 shows the contour lines and their averages for a new chisel tip 11, for a chisel tip 11 with rotationally symmetrical wear (R1), and for a chisel tip 11 with 5 non-rotationally symmetrical wear (R2). It is clear that with the asymmetrically worn chisel 10, the average line M2 of the two contour lines has a certain slope in relation to the longitudinal axis of the chisel. The angular position could be easily evaluated in order to determine the asymmetry factor of the wear. It is also possible, however, to directly ascertain/determine the position/deviations of 10 several contour lines. According to the same method, it is possible to determine an asymmetrical wear on the chisel head 12. Usually, however, an asymmetrically worn chisel head 12 is accompanied by an asymmetrically worn chisel tip 11. It is therefore sufficient 15 to analyze the chisel tip 11. Fig. 8 symbolically depicts a road surface milling machine 30 in which a machine body 32 is supported by four column-like traveling units 31. The road surface milling machine 30 can be controlled from a control center 33. A milling roller 35 20 is mounted in a milling roller housing 34. In order to measure the wear state using one of the above-described methods, a light source 50 and a camera 40 are associated with the milling roller 35. The milling roller 35 is shown in greater detail in Fig. 9. Numerous chisel holder 25 replacement systems, each equipped with a respective chisel holder 70, are fastened to the surface of a milling roller tube 35.1 of the milling roller 35. A chisel 10 is mounted in each chisel holder 70. In the current example, the chisel holders 70 are welded directly to the milling roller 35. 5555420_1 (GHMatters) P81651.AU.4 18 It is also conceivable, however, to use the chisel holder replacement system according to Fig. 1. Then, the base part 80 is welded to the milling roller 35. As the apparatus for measuring the chisel contours, an optical system is used in 5 which a type of "scanning operation" is employed to measure a contour line of the roller surface. The measurement principal used here can, for example, be a triangulation method in which the roller surface is illuminated for example by means of a light source 50 such as a laser line. The laser line produced in this way is observed by a camera 40 at a different angle, as a result of which height 10 differences on the roller surface (for example produced by the chisels 10) produce shifts in these projection lines. With a known differential angle between the camera 40 and the light source 50, it is possible to calculate the height coordinates. By rotating the milling roller 35, it is thus possible to establish a height profile of the circumference surface of the roller, from which the contour 15 line of the individual chisels 10 can be read. Another optical measurement principal for measuring the height and/or geometry of the chisels 10 is the shadow-casting method according to Fig. 10. This method takes advantage of the fact that as they move through a light curtain produced by 20 a light source 50, the chisels 10 produce a shadow contour that can be observed by a camera 40 and evaluated. The significant advantage of this method lies in the fact that it is possible to work with a single camera line using the same principle as a document scanner. It is thus possible, even for measurements of milling rollers 35 with large diameters and high rotation speeds, to be carried out 25 with a sufficient resolution. According to an alternative embodiment version of the invention, it is possible to modify the method described in relation to Fig. 10. In this case, a light curtain in the form of a light plane is once again produced by the light source 50. This light 5555420_1 (GHMatters) P81651.AU.4 19 plane extends parallel to the central longitudinal axis of the milling roller 35 and tangential to the roller surface. In this case, the light plane is oriented so that when the milling roller 35 rotates, the chisel tips of the chisels 10 initially pass through the light plane. 5 They then throw shadows that can be detected by the camera 40. The chisels 10 are conveyed across the light plane over a certain rotation angle of the roller until they dip below the light plane again. 10 A reference measurement can be performed with unworn chisels 10/chisel holders 70. In this case, the time at which the chisel 10 passes through the light plane (entry or exit) is detected and the associated rotation angle of the milling roller 35 is detected. After an operational use, a corresponding measurement is carried out on the partially worn (worn) chisel 10. Due to the reduction in length, 15 the chisel 10 breaks through the light plane later and dips below it earlier in comparison to an unworn system. The corresponding rotation angles of the milling roller 35 can then be determined as position values. These rotation angles are then compared to the rotation angles of the unworn system (reference values). It is then possible to calculate the wear state through subtraction using 20 the differential angle or else the differential angle is used directly as a wear criterion. During the milling process, the measurement system is suitably stowed in a corresponding protective device during phases in which no measurement is 25 performed. When a second camera 40 is used, it is possible, for example, to perform a direct measurement of the height geometry without an additional light source. 5555420_1 (GHMatters) P81651.AU.4 20 Alternatively, through appropriate positioning of a second camera, additional measurements of the contours can be performed, thus increasing the overall information density and increasing the probability of detecting asymmetrical wear states. 5 Alternatively, the position of the chisel tip 11 and the position of the chisel head contour can also be detected in at least one step by means of other distance measuring sensors (for example ultrasonic sensors, proximity switches). 10 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. In the claims which follow and in the preceding description of the invention, 15 except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 20 5555420_1 (GHMatters) P81651.AU.4