TOOL CONDITION MONITORING

Abstract: A cutting tool (10) for a machine tool has a substrate (20) and one or more coating layers (12-18) of materials with different Raman spectra, e.g. Tin, TiA1N, TiA1XN (where X = Y, Cr, Nb, W, Mo or the like), AlN, TiC, ZrN, TiZrN, CrN or VN. A Raman spectroscopic probe (22) detects and distinguishes the Raman spectra. As the tool wears, different layers with different Raman spectra are detected, and a warning can be given when the tool needs replacing or recoating.

Description: TOOL CONDITION MONITORING This invention relates to methods and apparatus for monitoring the condition of a cutting tool of the type used in machine tools. Various methods for tool condition monitoring are already known. For example, a broken cutting tip of a tool can be detected and wear can be monitored by directly measuring its dimensions, e.g. using a metrological probe of the type sold by Renishaw pic. It is also possible to detect optically the shape of the tool tip. However, it would be useful to be able to supplement such direct methods, especially in an automated environment. Known indirect optical monitoring techniques include measurement of the infra-red reflectivity of the surface of the tool. However, for best results this requires that the detector be normal to the surface of the tool tip, giving rise to problems if tool tips of various different shapes are to be monitored. The presence of water on the tool tip can also affect its infra-red reflectivity. Other indirect techniques involve monitoring the vibration generated by the tool in use, or the acoustic emission which it produces in use. However, these are highly dependent upon the design of the tool, e.g. its shape and dimensions. As a result, it can be very difficult to interpret the results. Doing so requires the prior establishment of a history of the vibration or acoustic emission behaviour of tools of each particular design as they wear. The present invention seeks to provide alternatives to such methods. Broadly stated, the present invention provides apparatus and methods in which the condition of a cutting tool is monitored by illuminating its surface with light in order to generate light at one or more shifted wavelengths, e.g. by the Raman effect, and then detecting the shifted wavelength or wavelengths. In one aspect, the invention provides such a method, in which the tool to be monitored comprises at least one layer of a first material, lying over a second layer or a substrate of a second material, the first and second materials having different spectra of shifted wavelengths when illuminated; the method comprising the steps of: illuminating the surface of the tool with light, in order to generate a wavelength-shifted spectrum therefrom; analysing and detecting the resulting spectrum, to determine which of said first and second materials lies at the surface of the tool. Thus, when the first layer has worn off, such analysis will reveal the presence of the spectrum of the second material. Preferably, the spectrum is a Raman spectrum. A second aspect of the invention provides a cutting tool comprising at least one layer of a first material, lying over a second layer or a substrate of a second material, the first and second materials having different spectra of shifted wavelengths when illuminated. A third aspect of the invention provides a tool condition monitoring system, comprising a probe which is capable of being brought into proximity or contact with a tool to be monitored, a source of light for illuminating a surface of the tool via the probe, thereby generating a spectrum of shifted wavelengths, the probe collecting light of said spectrum and delivering it to an analyser, the analyser comprising means for determining the condition of the tool from its spectrum. Preferred embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, wherein: Fig 1 is a schematic diagram showing the tip of a cutting tool and a Raman probe, not to scale; Fig 2 is a schematic diagram of analysis and detection arrangements for a Raman spectrum acquired by the probe of Fig 1; Fig 3 is a graph illustrating how two Raman signals vary with depth d as the surface of the tool tip in Fig 1 is worn; Figs 4 and 5 illustrate two alternative Raman monitoring systems for a carousel of cutting tools; and Fig 6 illustrates a Raman system integrated with a tool setting probe on a machine tool ; and Fig 7 is a graph of the Raman spectra of suitable materials for tool tip coatings. In Fig 1, there is shown the tip 10 of a cutting tool. For example, this may be a tool particularly suitable for use in a fully automated high performance/high speed machining environment. It comprises a substrate 20, onto which are coated multiple hard coating layers 12-18. The layers 12- 18 may be superlattice coatings deposited by plasma vapour deposition, and may be based on titanium nitride (TiN) and related alloys such as TiAIN and TiAlXN (where X = Y, Cr, Nb, W, Mo and the like) . Other possible materials include A1N, TiCN, ZrN and TiZrN. Two or more such materials may be alternated in the various layers. Some suitable materials, such as a TiAlN/ZrN superlattice coating, are described in "Investigation of superlattice coatings deposited by a combined steered arc evaporation and unbalanced magnetron sputtering technique", L.A. Donohue et al, Surface and Coatings Technology, 76-77 (1995), pages 149-158. There may be more layers than the four shown, or less layers (including the possibility of just one or two layers over the substrate 20). For example, the respective layers 12-18 may be alternately TiN and TiAlN. The substrate 20 may be ferrous (e.g. high-speed steel or stainless steel) or it may be cemented tungsten carbide. The condition and state of wear of such a tool may be examined using a Raman probe 22. In the present example, this has an optical fibre 24 by means of which exciting laser light is delivered, and an optional narrow bandpass filter 28 to help render the laser light monochromatic (removing plasma lines and any fluorescence or Raman scattering generated in the fibre 24). A mirror 30 reflects the incoming laser light towards a notch filter 32. This is preferably a holographic or rugate notch filter at a low angle of incidence to the light, e.g. 10°. It reflects the light of the laser wavelength via an objective 34 towards the tool tip 10, while at the same time transmitting (and therefore removing) light of all other wavelengths. It therefore performs a similar function to the filter 28 in rendering the exciting light monochromatic. As a result of the exciting light, Raman scattering takes place at the tool tip 10. Since Raman scattering is a bulk effect, in which the re-emission occurs in all directions, adequate quantities of the Raman light are back-scattered via the objective 34 and passed to the filter 32. This in contrast to, for example, infra-red reflectivity measurements, where most of the light would be reflected as shown by the arrow 36. Thus, it is not necessary to ensure that the probe 22 is aligned normal to the surface of the tool tip 10. This means that any tool shape can be measured, and from any angle. Furthermore, unlike other optical techniques such as infra-red reflectivity, Raman is substantially unaffected by the presence of water on the tool tip. The Raman scattered light is filtered by the filter 32, which is highly effective to reject reflected and Rayleigh scattered light having substantially the same wavelength as the laser. The Raman scattered light then passes via an optical fibre 26, back to the analysis and detection arrangements seen in Fig 2. Fig 2 is an example of a simple (and therefore relatively cheap) analysis arrangement. The Raman scattered light which passes along the optical fibre 26 is split along two paths by a beamsplitter 38. One such path passes through a narrow bandpass filter 40, which selects for example Raman scattered light shifted by 550cm "* from the laser wavelength. This is detected by a photodetector 42. In the other path, a narrow bandpass filter 44 selects light shifted by (say) 640cm " from the laser wavelength, which is then detected by a detector 46. The beamsplitter 38 may be a conventional 50/50 beamsplitter, or it may split the light in other proportions e.g. to normalise the two selected wavenumbers relative to each other. Alternatively, the beamsplitter 38 may be dichroic, in the form of an edge filter, which reflects all light above a certain wavenumber, while transmitting all light below that wavenumber. This has the advantage of greater optical efficiency, since it significantly reduces the amount of the available signal lost in the beamsplitter. Of course, it is possible to swap the filters 40,44, using a dichroic beamsplitter 38 which transmits the wavenumbers above a certain value and reflects those below a certain value. Various other alternatives are possible, such as using a dichroic bandpass filter for the beamsplitter 38 (reinforcing or obviating the need for the filter 40), or a notch filter (reinforcing or obviating the need for the filter 44) . An alternative to the system shown in Fig 2 is to apply the incoming Raman scattered light to a diffraction grating. This disperses the various Raman wavenumbers at different angles. One or more photodetectors 42,46 may then be placed at appropriate positions (preferably behind slits and with appropriate focusing optics) to detect the particular Raman band or bands of interest. Further details of Raman analysis apparatus, and discussion of the arrangement of the filter 32, may be found in our European Patent No. EP 0543578. Experiments have revealed that the Raman spectrum of TiN has a significant peak near 550cm " . As the aluminium content of the alloy is increased, this peak moves to higher wavenumbers. When the TiAlN pure multi-lattice composition is formed, the peak is situated near 640cm " '. For intermediate compositions, the peak lies between these two values. Thus, where the layers 12-18 in Fig 1 are alternately TiN and TiAlN, the filters 40 and 42 may be tuned to 550cm " ' and 640cm " respectively, as mentioned above. Where the layers are of other materials, the tuning of the filters will of course be chosen accordingly. Fig 3 shows how the resulting signals detected by the photodetectors 42,46 vary in accordance with depth, as the tool tip wears and successive layers are exposed. At low depths d, where the outer TiN layer 18 is still intact, there are relatively high values of the 550cm " signal detected by the photodetector 42, and relatively low values of the 640cm " wavenumber detected by the detector 46. These are respectively shown by the lines 50,52 in Fig 3. As the tool tip wears, and the outer layer 18 is worn away, the next layer 16 becomes exposed. Thus, at greater depths d, the 550cm "" signal 50 drops, while the 640cm " signal 52 rises. At greater amounts of wear, where the TiAlN layer 16 is worn away and the TiN layer 14 is exposed, the signal 50 again rises, while the signal 52 drops; and so on. Preferably the bottom layer, closest to the substrate, is TiN. The outer layer could be TiAlN instead of TiN. The values of the peaks and troughs in the response curves 50,52 shown in Fig 3 have been normalised relative to each other, and the absolute values of the outputs of the detectors 42,46 in practice are unlikely to be exactly as shown. What can be more important in practice, however, is how these outputs vary relative to each other. One simple way of assessing this is shown in Fig 2, where the outputs of the two detectors are compared to each other in a differential amplifier 48. Where the tool tip 10 is in use on a machine tool, and subject to regular wear, its condition may be monitored by a series of regular inspections using the probe 22. Such regular inspections may be daily for example, or they may be programmed as part of the machining cycle in a CNC control, particularly where the control is directly controlled by an external computer. This computer may take a series of readings of the outputs of the photodetectors 42,46 (or of the output of the amplifier 48), counting the peaks and troughs in the curves 50,52 seen in Fig 3, and thereby counting the numbers of the layers 12-18 which have worn away. When a predetermined number of these layers have worn away, the computer control may be programmed to stop using that tool, and to use a different tool instead; or it may be programmed to trigger an alarm or generate a message indicating that the tool needs to be changed. The wavenumbers 550cm " and 640cm " have been given purely as examples. Other peaks in the Raman spectra of TiN and TiAlN may be used. For example, it has been found that TiN has a peak near 800cm " which is not present in TiAlN. Intermediate compositions between TiN and TiAlN produce corresponding signal levels at 800cm " . Thus, the height of this peak could be used to monitor the wear of the layers 12-18. In this case, the filter 40 would be tuned to 800cm " , while the filter 44 could be tuned to a background wavenumber which does not appreciably change as the aluminium content of the alloy varies. Other changes are apparent in the Raman spectra of TiN and TiAlN between 150 and 400cm " , and appropriate arrangements may monitor the peak heights, positions and/or half-widths in this region in order to analyse the Al composition. The invention is not restricted to tool tips have alternating layers of TiN and TiAlN. Characteristic peaks in the various materials of the composition TiAlXN (where X = Y, Cr, Nb, W, Mo, etc) may be used in a similar manner. One or more of the layers 12-18 may be of a material, or doped with a material (e.g. Cr) , specially chosen as a marker because it has a strong Raman peak at a particular characteristic wavelength, or a characteristic fluorescence, luminescence, etc spectrum, which may be detected by one of the filters 40,44. Just one or two layers of this material may be provided at or close to the substrate 20, so that the computer control which monitors the output of the detectors can detect this characteristic wavenumber and use it as an indication that the tool is now worn nearly to the substrate. Many other materials may of course be used, and there may only be one or two layers over the substrate 20, or there may be many more than the four shown m Fig 1. Where the material is of the form TiAlN, the element X selected will depend on the resulting multi-layer structure, and/or on the emission characteristics of the resulting alloy. A material may be chosen for the marker which exhibits resonance Raman scattering, i.e. it gives a very strong Raman signal when excited at a particular laser wavelength (which is then used for the laser excitation passing down the optical fibre 24) . Instead of a marker layer having a particularly strong Raman peak being provided, it is also possible to provide a layer of a material which has a marked absence of a Raman peak which is present in the other layer or layers of the tool tip 10. Particularly suitable materials for one of the layers are either CrN or VN. For example, there may be a layer of Cr or VN of, say, lμ thickness immediately over a high speed steel or cemented tungsten carbide substrate. Above this may be any conventional hard-wearing tool coating material having a detectable Raman signature, such as TiN, TiZrN or other materials discussed above. Fig 7 shows a Raman spectrum 90 for TiZrN, a spectrum 92 for CrN, and a spectrum 94 for VN . It can be seen that TiZrN has a pronounced peak between 200cm "" and 300cm " ", and another around 500cm "" , which are not present in CrN or VN. So TiZrN can be used as the outer wear layer, and an analyser such as in Fig 2 is used to detect one of these peaks. When the outer layer wears through, the Raman signal will disappear. It will do so while there still remains a hard-wearing coating of CrN or VN, without exposure of the substrate. Therefore, a warning is given that the tool needs to be changed or recoated, before the substrate can cause unacceptable surface finish to the workpieces being machined. An alternative to a marker layer of a different material is to use a layer having a greater or less thickness than the other layers 12-18. Computer monitoring of the Raman signals received periodically will then show that one of the peaks of the curves 50 or 52 will be much wider or narrower than the others, showing that the marker layer of a different thickness has been reached. It is also possible to use one or more marker layers of a material which exhibits another spectroscopic shifting property, rather than Raman, fluorescence or luminescence. The coolants commonly used on machine tools may themselves exhibit fluorescence. If so, the presence of such coolant on the tool tip 10 may mask the Raman signal which is to be measured and analysed. One solution to this problem is to use a coolant which does not fluoresce. Another solution is to wash the coolant away with a jet of water, as discussed below, prior to the measurement. Alternatively, the invention is particularly useful for use with the most modern non-lubricated cutting tools, where no coolant or lubricant is used. Various possible systems for measuring and analysing the wear in practice will now be described. Fig 4 shows a carousel 54 of cutting tools 56, for example in a high rpm machine tool. At a measuring station 58, there is located the fibre optic probe 22 according to Fig 1, and (optionally) a nozzle 60 for delivering a jet of water to clean coolant from a tool 56 located in the station 58. The optical fibre 26 brings the Raman scattered light back to a Raman system 62, which (in a simple version) may be as shown in Fig 2. In use, the carousel 54 is rotated in the direction of the arrow 64, indexing each tool 56 in turn to the measuring station 58. At that station, under computer control, the tool is cleaned by a jet of water from the nozzle 60 (if required) and a measurement is taken by the probe 22 and Raman system 62 in the manner described above. The result of this measurement is stored by the computer, giving a day-to-day record of the wear and condition of each individual tool 56. Should it be detected that a particular tool 56 is too worn for use, the computer can flag this fact, producing a list of those tools which need to be replaced. If the tool concerned is not to be replaced immediately, the computer system may be programmed not to use that tool but to use an alternative, identical tool in the carousel 54 instead. Fig 5 shows a similar arrangement to Fig 4. However, in place of the fibre optic connection 26, and the probe 22, a direct optical connection is used, and the Raman system 62 is similar to that described in EP 0543578. Thus, a microscope objective 65 is located at the measuring station 58. The laser illumination is injected into the optical path within the system 62. If desired, a beamsplitter or removable mirror 68 may be provided, enabling the tool 56 at the station 58 to be visually examined through the microscope objective by a video camera 70. Alternatively, if desired, a two-dimensional image of the surface of the tool may be formed on the CCD detector (not shown) which forms part of the Raman system 62 as described in EP 0543578. This two-dimensional image may be formed in white light, to aid visual inspection of the tool surface, or in the illumination provided by the laser. Alternatively, it may be formed in light of a selected Raman band, so that the image shows how the wear varies from one place on the tool to another. Visual monitoring by the video camera or the CCD may be used to scan the tool tip for breakage or other defects in its shape. In either of the systems of Figs 4 and 5, it is possible for the probe 22 or the microscope objective lens 65 (and the nozzle 60 if provided) to be indexed in sequence from one tool to another, the tools being kept stationary, as an alternative to indexing the tools in the carousel as indicated by the arrow 64. Also, in either of the systems of Figs 4 and 5, an auto focusing system may be provided to ensure that the tool tip is correctly focused by the probe 22 or the microscope objective lens. Where the Raman system 62 is relatively simple, e.g. as shown in Fig 2, it is possible to build the system into the tool carousel of an individual machine tool. However, where the present invention is to be used in a large manufacturing plant with many machine tools, and especially if the more expensive system according to EP 0543578 is to be used, it may be preferable to build the Raman system 62 and its associated components including the probe 22 or microscope objective lens 65 into a portable trolley. This can then be wheeled around the manufacturing plant and docked with a measuring station 58 on each machine tool carousel in turn. Appropriate docking connections may be provided both on the machine tool carousel and on the trolley (or on the fibre optic probe 22) to ensure correct alignment of the optical components with the measuring station 58. Fig 6 shows an alternative system which can be used for measurement and monitoring of the tool tip "in process", during a machining cycle. It is known to provide a tool setting probe on the bed of a machine tool such as a machining centre. Such tool setting probes may be electro- mechanical, and are sold commercially by Renishaw pic. Prior to machining, and optionally at other times during the machining cycle, the tool is brought into contact with the measuring tip of the tool setting probe, which generates a trigger signal upon contact. This trigger signal causes the machine control to take a reading of the instantaneous position of the tool, and thereby to generate offsets for future machining. Fig 6 shows such a tool setting probe 80, installed on the bed 82 of the machine tool. However, the tool setting probe 80 is modified in that it has an optical fibre 84 passing up its stylus stem 86 to the measuring tip 88. When the tool 56 is brought into contact with the measuring tip 88, as indicated by broken lines, the probe 80 generates a trigger signal in the usual way and sends it back to the machine control (not shown) . In addition to the usual generation of tool offsets, this causes the machine control to take a reading from a Raman system 62 connected to the optical fibre 84. For low cost, the Raman system 62 is preferably similar to that shown in Fig 2. Only one optical fibre 84 is shown in Fig 6. The laser input may pass down this optical fibre as well as the returning Raman scattered light, by folding the laser input light into the optical path within the system 62, using an appropriate beamsplitter (preferably dichroic) . Alternatively, a separate optical fibre may be used for delivery of the laser light, running parallel to the optical fibre 84 to the measuring tip 88 of the probe 80. As previously, the system shown in Fig 6 may include auto focus arrangements. The Raman measurement can be taken after the tool 56 has been backed a short distance away from the measuring tip 88 after contact, if desired. A further possibility for "in process" Raman measurement of the tip of the tool 56 is to provide a measuring station at a position on the bed of the machine tool which is separate from any tool setting probe 80. This measuring station may include a probe 22 such as shown in Fig 1. The tool 56 is then occasionally brought into the vicinity of the probe 22 at this station, and a Raman measurement made upon an instruction received from the machine computer control. A further possibility is to make Raman measurements of the tool in situ as it machines a workpiece. An appropriate optical fibre and a long focal length lens may be attached to non-rotating structure which moves with the movable spindle of a machining centre, or with the tool turret of a lathe. The lens is then focused on the tool tip as machining takes place. Raman tool condition monitoring systems as described above may be used in other ways than to study tool tips having multiple coating layers. For example, stress in a tool tip may be monitored, by monitoring a characteristic Raman peak of the material concerned which is wavenumber shifted or which is broadened or narrowed as a result of stress. Oxidation of the tool tip may also be monitored, by studying Raman bands which change as oxidation takes place. As an alternative to a marker layer amongst the layers 12- 18, the substrate 20 may alternatively be of a material having a characteristic Raman peak which can be detected when the overlying coatings wear away. The embodiments shown in Figs 4-6 enable lifetime forecasts to be made for each tool 56 on the basis of a quantitative measurement in real time.

Claims: CLAIMS 1. A method of monitoring the condition of a cutting tool, comprising illuminating the surface of the tool with light in order to generate light at one or more shifted wavelengths, and then detecting the shifted wavelength or wavelengths. 2. A method according to claim 1, in which the tool to be monitored comprises at least one layer of a first material, lying over a second layer or a substrate of a second material, the first and second materials having different spectra of shifted wavelengths when illuminated; the method comprising the steps of: illuminating the surface of the tool with light, in order to generate a wavelength-shifted spectrum therefrom; analysing and detecting the resulting spectrum, to determine which of said first and second materials lies at the surface of the tool. 3. A method according to claim 2, wherein there are a plurality of layers of different materials having different spectra of shifted wavelengths when illuminated. 4. A method according to claim 2 or claim 3, wherein at least one of said first and second materials is selected from TiN, TiAlN, TiAlXN (where X = Y, Cr, Nb, W, Mo or the like), A1N, Tie, ZrN, TiZrN and superlattice materials. 5. A method according to any one of claims 2 to 4 , wherein said second material is selected from CrN and VN. 6. A method according to any one of the preceding claims, wherein said wavelength-shifted light is Raman shifted light. 7. A cutting tool comprising at least one layer of a first material, lying over a second layer of a second material, the first and second materials having different spectra of shifted wavelengths when illuminated; wherein the second material is selected from CrN and VN . 8. A tool condition monitoring system, the tool comprising at least one layer of a first material, lying over a second layer or a substrate of a second material, the first and second materials having different spectra of shifted wavelengths when illuminated; the system including a probe which is capable of being brought into proximity or contact with a tool to be monitored; a source of light for illuminating a surface of the tool via the probe, thereby generating a spectrum of shifted wavelengths; and an analyser; the probe collecting light of said spectrum and delivering it to the analyser, the analyser distinguishing said first and second materials from their shifted spectra and thereby determining the condition of the tool . 9. A system according to claim 8, wherein the spectrum of shifted wavelengths is a Raman shifted spectrum.

Inventors: MUENZ WOLF-DIETER, PITT GILLIES DAVID, MUENZ, WOLF-DIETER, PITT, GILLIES, DAVID
Applicants: MUENZ WOLF DIETER, PITT GILLIES DAVID, RENISHAW PLC, MUENZ, WOLF-DIETER, PITT, GILLIES, DAVID
Application year: 1999
Main classification: G01B11
IPC codes: B23Q1, B23Q17, G01B11, G01N21