Patent Publication Number: US-11378492-B2

Title: System for estimating the state of wear of a cutting tool during machining

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to French patent application FR 2001061 filed Feb. 3, 2020, the entire disclosure of which is incorporated by reference herein. 
     TECHNICAL FIELD 
     The disclosure herein relates to the field of estimating the state of wear of a cutting tool during machining. 
     BACKGROUND 
     Industrially and in particular in the aerospace industry, specialized, numerically controlled machine tools (boring-drilling machines, milling machines) are used which make it possible to machine complex and highly precise shapes, potentially without disassembling the part, according to the numerical definition of this part. 
     These machine tools are fitted with various cutting tools (drill bits, milling cutters), the state of wear of which has to be monitored in order that the machining is always performed with a high degree of precision and that it meets the tolerance and surface state requirements. 
     In general, the state of wear of a cutting tool is estimated by direct means by using measurement tools such as, for example, stereoscopic microscopes, profilometers, three-dimensional scanners, lasers, cameras, etc. The measurements taken by this type of tools are precise and repeatable but have the drawback of having to be taken outside of the machining process and therefore necessitate a non-negligible production downtime. 
     There are indirect measurement means which consist in producing an estimate of a level of wear of a cutting tool according to a count of the machining time on the basis of the predetermined service life of the cutting tool. However, this estimate is imprecise given the fact that the rate of wear is a variable phenomenon that depends on numerous factors. Additionally, the service life defined prior to machining is specified according to a conservative criterion which leads to wastage of cutting tools. 
     An object of the disclosure herein is therefore to propose an indirect system or method automating the estimation of the state of wear of a cutting tool during machining which exhibits a high degree of precision, thereby avoiding wastage of cutting tools while necessitating no machining downtime. 
     SUMMARY 
     The disclosure herein relates to a system for estimating the state of wear of a cutting tool mounted on a machine tool, the system comprising:
         an acquisition module configured to acquire, over a determined duration of machining, values of an operating signal specific to the cutting tool mounted on the machine tool, and   a microprocessor configured to:   calculate current values of a set of wear indicators on the basis of the values of the operating signal, and   determine the state of wear of the cutting tool according to the current values of the set of indicators using a predetermined wear model modeling the state of wear of the cutting tool according to the training values for the set of wear indicators.       

     This system allows precise, rapid, repeatable and real-time estimation of the level of wear of a cutting tool during machining. Thus, this system necessitates no machining downtime and makes it possible to avoid wastage of cutting tools. 
     Advantageously, in a training phase:
         the acquisition module is configured to acquire, over a training machining operation, a set of values of the training operating signal specific to the cutting tool and a set of training wear measurements corresponding to the cutting tool, and   the microprocessor is configured to:   calculate a series of training values for the set of wear indicators on the basis of a first portion of the set of values of the training operating signal,   construct the wear model by applying a regression technique configured to calibrate the series of training values for the set of wear indicators to a first portion of the set of training wear measurements corresponding to the first portion of the set of values of the training operating signal, and   validate the wear model by using a second portion of the set of values of the training operating signal.       

     Thus, the system makes it possible to construct, in a straightforward and precise manner, a wear model that is able to estimate the level of wear in accordance with the actual wear of the cutting tool. 
     Advantageously, during the validation of the wear model, the microprocessor is configured to:
         use the wear model to estimate test wear values for the cutting tool on the basis of the second portion of the set of values of the training operating signal,   compare the test wear values estimated by the wear model with a second portion of the set of training wear measurements corresponding to the second portion of the set of values of the training operating signal, and   validate the wear model when the difference between the test wear values and the corresponding second portion of the set of training wear measurements does not cross a predetermined threshold.       

     Thus, the wear model may be validated according to the desired estimation threshold. 
     Advantageously, the operating signal specific to the cutting tool is a signal that comes from the machine tool during machining, the signal being selected from among the following signals: power, torque, and current intensity. 
     These signals come from sensors which are already present at the spindle of the machine tool and thus it is not necessary to install new sensors. 
     As a variant, the operating signal is a vibration signal that comes from the machine tool during machining. 
     The vibration signal gives a precise indication of the wear of the cutting tool but potentially requires the installation of a vibration sensor at the spindle of the machine tool. 
     Advantageously, the set of wear indicators comprises:
         a first indicator corresponding to the determined duration of machining,   a second indicator corresponding to a standard deviation of the operating signal,   a third indicator corresponding to a kurtosis of the operating signal,   a fourth indicator corresponding to a skewness of the operating signal.       

     These indicators exhibit an optimal correlation with the level of wear of the cutting tool. 
     According to one embodiment of the disclosure herein, the system comprises a measurement device used to take the set of training wear measurements for the cutting tool corresponding to the set of values of the training operating signal. 
     Advantageously, the measurement device is an instrumented tool holder comprising a set of sensors that are configured to measure the wear during machining in the training phase. 
     According to another embodiment of the disclosure herein, the measurement device is an optical device suitable for measuring the wear of the cutting tool at the end of each machining pass in the training phase. 
     Advantageously, the microprocessor is configured to access predetermined tuning data comprising cutting condition parameters and an experimental plan defined on the basis of the cutting condition parameters. 
     Advantageously, the cutting condition parameters comprise depths of pass, widths of pass, feed rates, critical rotational speeds, radii of curvature and cutting plans. 
     Another subject of the disclosure herein is a machine tool comprising the system for estimating the state of wear of a cutting tool according to any one of the preceding features. 
     A further subject of the disclosure herein is a method for estimating the state of wear of a cutting tool mounted on a machine tool, the method comprising the following steps:
         acquiring, over a determined duration of machining, values of an operating signal specific to the cutting tool mounted on the machine tool,   calculating current values of a set of wear indicators on the basis of the values of the operating signal, and   determining the state of wear of the cutting tool according to the current values of the set of indicators using a predetermined wear model modeling the state of wear of the cutting tool according to training values for the set of wear indicators.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other particularities and advantages of the device and of the method according to the disclosure herein will become more clearly apparent from reading the description that is given below, by way of non-limiting indication, with reference to the appended drawings, in which: 
         FIG. 1  schematically illustrates a system for estimating the state of wear of a cutting tool, according to one embodiment of the disclosure herein; 
         FIG. 2  is a flowchart schematically illustrating the production of the wear model in a training phase, according to one preferred embodiment of the disclosure herein; 
         FIGS. 3A and 3B  are graphs illustrating regression algorithm optimization techniques, according to one embodiment of the disclosure herein; 
         FIG. 4  is a graph illustrating the comparison between estimated wear values and actual wear measurements, according to one embodiment of the disclosure herein; and 
         FIG. 5  is a flowchart schematically illustrating a method for estimating the state of wear of a cutting tool, according to one preferred embodiment of the disclosure herein. 
     
    
    
     DETAILED DESCRIPTION 
     The principle of the disclosure herein consists in or comprises estimating the state of wear of a cutting tool during machining by interpreting the signals that come from the machine tool. 
       FIG. 1  schematically illustrates a system for estimating  1  the state of wear of a cutting tool  3 , according to one embodiment of the disclosure herein. 
     The cutting tool  3  is mounted on a spindle  5  of the numerically controlled machine tool  7  allowing programmed shapes to be machined on a part  9  to be machined. In general, the machine tool  7  is fitted with a magazine in which various cutting (boring-drilling, milling, etc.) tools are located. Thus, the term “cutting too” refers to any type of cutting tool that may be mounted on the machine tool  7 . 
     According to the disclosure herein, the estimation system  1  comprises an acquisition module  11 , a microprocessor  13 , a storage unit  15 , an input interface  17  (a keyboard, for example) and an output interface  19  (a screen, for example). Advantageously, all of these hardware elements of the estimation system  1  are already integrated within a control device  20  for the machine tool  7 . 
     The acquisition module  11  is configured to acquire, at successive times and over a determined duration of machining, values of an operating signal specific to the cutting tool  3  mounted on the machine tool  7 . 
     The operating signal specific to the cutting tool  3  is a signal that comes from the machine tool  7  during the machining operation. Advantageously, this operating signal is a signal which comes from sensors that are already present at the spindle  5  of the machine tool  7 . In particular, the operating signal may be a signal of the electrical power of the spindle  5  of the machine tool  7 , a torque signal, or a signal of the intensity of electric current flowing in the machine tool. Specifically, the more the cutting tool is worn, the greater the force exerted by the machine tool, thus entailing an increase in the torque, the current draw, power consumption and vibration, etc. 
     Other types of operating signals that come from the machine tool  7  may be used such as, for example, a vibration signal. In this case, a vibration sensor is installed at the spindle  5  of the machine tool  7  if it is not already equipped with such a sensor. 
     In addition, the microprocessor  13  is configured to process the values of the operating signal specific to the cutting tool  3  acquired from the machine tool  7 . More particularly, the microprocessor  13  is configured to calculate current characteristic values of a set of wear indicators on the basis of the values of the operating signal and then to determine the state of wear of the cutting tool  3  according to these current characteristic values of the set of indicators by using a predetermined wear model  21  stored in the storage unit  15 . The wear model  21  is produced beforehand in a training phase for modeling the state of wear of the cutting tool  3  according to training values for the set of wear indicators. 
       FIG. 2  is a flowchart schematically illustrating the production of the wear model in a training phase, according to one embodiment of the disclosure herein. 
     The flowchart describes the training steps carried out during the machining operations of a cutting tool  3  of interest in order to construct a wear model  21  relating to this tool  3 . Of course, a wear model  21  is constructed for each type of cutting tool. 
     Steps E 1  and E 2  relate to the preparation of a machining and cutting condition program for the selected cutting tool  3  and the setting up of this tool  3 . 
     Step E 1  relates to the tuning of a test program for predetermined data comprising cutting condition parameters for the tool  3  and machining condition parameters for this tool  3 . Specifically, from a database relating to the cutting tool  3 , the cutting condition parameters for this tool  3  and the machining condition parameters are downloaded. These parameters comprise depths of pass “ap” (i.e. the recess that has to be created in the material of the part  9  by the cutting tool  3 ); widths of pass “ae” (i.e. the width of the recess); feed rates “f” in mm/min (i.e. the distance traveled by the tool in one minute of removing material); critical rotational speeds “Vc” between the cutting tool  3  and the material; and radii of curvature “R” and cutting plans. In general, for each cutting tool  3  and for each of the cutting condition parameters, an operating interval between a minimum value and a maximum value is defined. 
     Additionally, an experimental plan is defined on the basis of these cutting condition and machining condition parameters by taking at least the minimum and maximum values of each of these parameters. By defining a set of machining configurations, the experimental plan aims to ensure that the cutting tool  3  is used under its typical conditions, i.e. within a consistent range of values of each of these parameters. 
     Step E 2  relates to choosing and setting up the cutting tool  3  of interest used in the tuning test (i.e. the training phase). Thus, the selected cutting tool  3  is arranged on its tool holder, the tool holder is arranged on the spindle  5  of the machine tool  7  intended for the tuning test and the part  9  to be cut is put in place. Additionally, the previously established tuning program is downloaded. This program will allow the cutting tool  3  to check all of the predefined configurations in the experimental plan. 
     Step E 3  relates to the start of machining (milling, drilling, turning, etc.) and as soon as machining has started, the acquisition of values x i  of the training operating signal that come from the machine tool  7  is launched. 
     Specifically, the acquisition module  11  is configured to acquire, at predetermined time intervals (for example, at regular intervals of a few minutes), a set of values x i  of the training operating signal specific to the cutting tool  3 . By way of example, the operating signal is the power of the spindle  5 . Advantageously, the acquisition is carried out according to a sampling N having a frequency higher than or equal to 1 Hz. For example, for a frequency of 100 Hz, 100 values x 1 , x 2 , . . . x 100  of the operating signal are acquired per second. The set of values x i  of the training operating signal is stored in the storage unit  15 . The duration Δt for which the cutting tool  3  has machined is also stored. This duration is equal to the sum of the predetermined time intervals. 
     Acquisition of a set of training wear measurements M k  corresponding to the set of values x i  of the training operating signal of the same cutting tool  3  is also carried out. 
     According to a first embodiment, the estimation system  1  comprises a measurement device (not shown) used to automatically acquire the set of training wear measurements M k  for the cutting tool  3  corresponding to the set of values of the training operating signal. Each wear measurement corresponds, for example, to the average of the maximum values in mm of the wear of the edges or sides of the cutting tool  3 . 
     Advantageously, the measurement device is integrated into the machine tool in the form of an instrumented tool holder comprising a set of sensors that are configured to measure the wear during machining in the training phase. In this case, the acquisition module  11  automatically retrieves the set of training wear measurements M k . 
     According to a second embodiment, the measurement device is a device which is not integrated into the machine tool  7  and may be an optical tool such as a stereoscopic microscope or camera, or a measurement tool such as a profilometer, three-dimensional scanner, etc. This measurement device is used to directly measure the wear of the cutting tool  3  at the end of each machining pass of the experimental plan defined for training. 
     In steps E 4 -E 7 , the microprocessor  13  is configured to construct the wear model  21 . 
     More particularly, in step E 4 , the microprocessor  13  is configured to clean up the data by deleting, for example, inconsistent data. Additionally, the microprocessor  13  is configured to contextualize the set of training wear measurements M k  with the set of values x i  of the training operating signal that comes from the machine tool  7 . 
     In step E 5 , the microprocessor  13  is configured to subdivide the set of values x i  of the training operating signal and the corresponding set of training wear measurements M k  into first P 1  and second P 2  portions. The first portion P 1  (for example about 70%) is used to calibrate the wear model  21  while the second portion P 2  (the remaining 30%, for example) is used to test the model  21 . 
     In step E 6 , the microprocessor  13  uses the first portion P 1  of the set of values x i  of the training operating signal to calculate a series of training values relating to the set of wear indicators. This set of wear indicators comprises a first indicator K 1  corresponding to the determined duration of machining, a second indicator K 2  corresponding to a standard deviation of the operating signal, a third indicator K 3  corresponding to a kurtosis of the operating signal, and a fourth indicator K 4  corresponding to a skewness of the operating signal. These indicators aim to transform the values of the operating signal over a predetermined time interval into a single value representative of the operating signal over this interval. The predetermined time interval may be a few minutes, for example five minutes. 
     The first indicator K 1  is simply the duration Δt of machining stored during the acquisition, in step E 3 , of the set of values x i  of the training operating signal. Thus, this first indicator K 1  indicates the total time that the cutting tool  3  has spent in the material:
 
 K   1   =Δt   (1)
 
     The second indicator K 2  corresponding to the standard deviation a of the operating signal is defined according to the values x i  of the operating signal, the sampling N which represents the number of acquisitions per second and the average  x  of these N values x i . The second indicator K 2  (i.e. the standard deviation σ) is thus defined as follows: 
     
       
         
           
             
               
                 
                   
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     The third indicator K 3  is a kurtosis K of the operating signal defined according to the standard deviation σ (i.e. the second indicator K 2 ) in addition to the values x i  of the operating signal, the sampling N and the average  x . The third indicator K 3  (i.e. the kurtosis K) is thus defined as follows: 
     
       
         
           
             
               
                 
                   
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     The fourth indicator K 4  is a skewness Sk of the operating signal defined according to the same variables as the third indicator K 3  as follows: 
     
       
         
           
             
               
                 
                   
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     In step E 7 , the microprocessor  13  is configured to construct the wear model  21  by applying, for example, a regression technique to the set of indicators K 1 -K 4 . Specifically, the regression technique is used to calibrate the series of training values for the set of wear indicators K 1 -K 4  on the first portion P 1  of the set of training wear measurements M k  corresponding to the first portion P 1  of the set of values x i  of the training operating signal. 
     Various regression algorithms may be used such as, for example, linear regressions using penalties such as ridge or lasso or regressions based on decision trees such as XGBoost, etc. It is possible to use standard rules and/or hyperparameter optimization methods known to those skilled in the art to calibrate the regression algorithms. 
     By way of non-limiting example, the wear may be expressed as a superposition of various indicators K i  assigned weighting coefficients α i  generated by the regression algorithm. Thus, the estimated wear may be expressed as follows:
 
 U=Σ   i=1   4   K   i α i   (5)
 
     It should be noted that the estimate given by formula (5) above is just an example specific to some types of regression algorithms. 
     Additionally, in order to optimize the regression algorithm with the desired degree of precision, it is possible to adjust (in a manner known to those skilled in the art) the hyperparameters specific to the algorithm. For example, each weighting coefficient α 1  associated with each indicator K i  of formula (5) is generated by hyperparameters. 
     Specifically,  FIGS. 3A and 3B  are graphs illustrating regression algorithm optimization techniques. More particularly,  FIG. 3A  illustrates the variations in a maximum error “Max error” according to the values of the hyperparameters “alpha” and “L1 ratio” of the regression algorithm.  FIG. 3B  illustrates the variations in an average prediction error “RMSE” according to the values of the hyperparameters “alpha” and “L1 ratio” of the regression algorithm. 
     The maximum error “Max error” and the average error “RMSE” are used as performance metrics, the aim being to make these metrics tend toward zero in order to obtain the best possible precision. In other words, the pair (alpha, L1 ratio) is sought which minimizes these errors “Max error” and “RMSE” for each point (i.e. for each indicator K i ). 
     In steps E 8 -E 10 , the microprocessor  13  is configured to validate the wear model  21  by a validation test by using a second portion of the set of training operating signals. 
     More particularly, in step E 8 , the microprocessor is configured to test the wear model  21  constructed previously by using the second portion P 2  of the set of values x i  of the training operating signal in order to estimate test wear values T j  for the cutting tool  3 . 
     In step E 9 , the microprocessor  13  is configured to compare the test wear values T j  estimated by the wear model  21  with the second portion P 2  of the set of actual training wear measurements M k  corresponding to the second portion P 2  of the set of values x i  of the training operating signal. 
     By way of example,  FIG. 4  is a graph illustrating the comparison between the estimated wear values T j  and the actual wear measurements M k . 
     According to this example, the estimated wear values T j  are evaluated by a wear model  21  constructed by using a linear regression algorithm. This graph has the wear in mm on the ordinate and the machining number on the abscissa. The dots represent actual wear measurements M k  while the squares represent the wear values T j  estimated by the wear model  21 . This graph shows quite a good correlation between the actual values and the estimated values. 
     In step E 10 , the microprocessor  13  is configured to validate the wear model  21  when the difference between the test wear values T j  and the corresponding second portion P 2  of the set of actual training wear measurements M k  does not cross a predetermined threshold S. The threshold S may be determined according to the precision desired for the wear estimate and the type of operation. In general, an error in the estimate of about 10% is considered to be acceptable. 
     In step E 11 , the wear model  21  is stored in the storage unit  15  and may then be used as a wear model  21  to estimate, in real time and automatically, the wear of a cutting tool  3  in a machining operation. It should be noted that a wear model  21  is stored in the storage unit  15  for each type of cutting tool. 
       FIG. 5  is a flowchart schematically illustrating a method for estimating the state of wear of a cutting tool, according to one preferred embodiment of the disclosure herein. 
     In step E 21 , the wear model  21  developed for the type of cutting tool  3  of interest, which is mounted on the spindle  5  of the machine tool  7 , is downloaded. 
     In step E 22 , as soon as machining has started, the acquisition module  11  is configured to regularly acquire, at successive times and over a determined duration of machining, the values x i  of the operating signal (for example, the power of the spindle) specific to the cutting tool  3  of interest. 
     In step E 23 , the microprocessor  13  is configured to calculate the current characteristic values of the set of indicators K 1 -K 4 , defined by formulas (1)-(4), on the basis of the values x i  of the wear operating signal. 
     In step E 24 , the microprocessor  13  is configured to use the wear model  21  downloaded in step E 21  to determine the state of wear U of the cutting tool  3  according to the current characteristic values of the set of indicators K 1 -K 4 . The result U of the estimation may be displayed on the output interface  19  (for example, the screen of the machine tool). The regularity of the estimation of the state of wear may be predefined according to the type of machining. For example, the microprocessor  13  may be configured to calculate ten estimates for each milling sequence. 
     Thus, the estimation system and method according to the disclosure herein make it possible to indirectly measure the level of wear of a cutting tool of interest during machining and make it possible to warn in real time of premature wear. The disclosure herein also makes it possible to optimally manage the service life of the cutting tool. Additionally, the estimation system consumes very little computing power and requires very little additional hardware, facilitating its integration into all sorts of industrially used machine tools. 
     The subject matter disclosed herein can be implemented in software in combination with hardware and/or firmware. For example, the subject matter described herein can be implemented in software executed by a processor or processing unit. In one exemplary implementation, the subject matter described herein can be implemented using a computer readable medium having stored thereon computer executable instructions that when executed by a processor of a computer control the computer to perform steps. Exemplary computer readable mediums suitable for implementing the subject matter described herein include non-transitory devices, such as disk memory devices, chip memory devices, programmable logic devices, and application specific integrated circuits. In addition, a computer readable medium that implements the subject matter described herein can be located on a single device or computing platform or can be distributed across multiple devices or computing platforms. 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a”, “an” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.