Abstract:
The wear status of a micro-endmill tool may be inferred by monitoring the chip production rate of the tool in operation. Chips may be extracted from a work area, captured on an adhesive surface, imaged, and counted to determine the chip production rate. When the rate of chip production falls, the feed rate of the micro-endmill may be increased to a level suitable for the current state of tool wear. In this manner, costly and inconvenient work stoppages to evaluate the wear status of a tool are eliminated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of PCT Application No. PCT/US2015/014848 filed on Feb. 6, 2015 and entitled “SYSTEMS AND METHODS FOR REAL-TIME MONITORING OF MICROMILLING TOOL WEAR”. PCT Application No. PCT/US2015/014848 claims priority to, and the benefit of, U.S. Provisional Application Ser. No. 61/936,607 filed on Feb. 6, 2014 and entitled “SYSTEMS AND METHODS FOR REAL-TIME MONITORING OF MICROMILLING TOOL WEAR.” Both of the above applications are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to micromilling, and in particular to monitoring of tool wear in connection with the same. 
       BACKGROUND 
       [0003]    Micromilling is a material-removal manufacturing process for producing parts that typically are less than 1 mm in size, with features measured in microns, and sub-micron manufacturing tolerances. A micromilling machine typically consists of a tool known as a micro-endmill placed in a spindle. The micro-endmill contains one or more teeth. The spindle typically rotates the micro-endmill at speeds exceeding 50,000 RPM while advancing the tool through the material. Each time a tooth of the micro-endmill passes through the material, a chip is produced and removed from the material. As the micro-endmill removes chips, the teeth become dull. Dull micro-endmill teeth cause a manufacturing defect known as “burring” and eventually cause the micro-endmill to break. 
         [0004]    Measuring the state of wear of a micro-endmill is typically done by halting the cutting process, removing the micro-endmill from the machine, and examining it under a microscope. The radius of the cutting edge of the teeth is measured visually, and is used as a quantification of tool wear. This process is cumbersome and time-consuming; accordingly, improved systems and methods for assessing tool wear are desirable. 
       SUMMARY 
       [0005]    In an exemplary embodiment, a tool wear monitoring system comprises a skirt couplable to a micro-endmill to contain chips produced during operation of the micro-endmill. The system further comprises a tube coupled to the skirt, the tube configured to extract air from the skirt and carry the chips to a nozzle. The system further comprises an adhesive tape disposed at an outlet of the nozzle to catch chips exiting the nozzle, a conveyor belt to move the adhesive tape into the field of view of a camera, the camera operative to obtain images of the chips on the adhesive tape, and a software program operative on a computing device to count the chips from the images. 
         [0006]    In another exemplary embodiment, a method for monitoring wear of a micromilling tool comprises determining an initial chip production rate; extracting, via a skirt, tube, and pump, chips produced during operation of the micromilling tool; depositing, from a nozzle coupled to the tube, the chips on adhesive tape; moving, via a conveyor belt, the adhesive tape to bring the chips into the field of view of a camera; acquiring, via the camera, an image of the chips; counting, by an image processing system, the chips in the image to determine a current chip production rate; and calculating, using the initial chip production rate and the current chip production rate, the wear status of the micromilling tool. 
         [0007]    In another exemplary embodiment, a method of counting chips produced by a micro-endmill comprises obtaining, by a system for monitoring tool wear, an image of chips captured by adhesive tape; converting the image to grayscale; thresholding the image to remove excessive lustre; equalizing a histogram of the image to improve contrast; thresholding the image to identify a background grayscale level; converting the image to black and white; eroding the image to reduce pixelated errors; performing edge detection on the image to form edges therein; performing dilation on the image to connect at least a portion of the edges; filling the image components arising from the edge detection; and counting the chips appearing in the image. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    With reference to the following description and accompanying drawings: 
           [0009]      FIG. 1  illustrates a block diagram of an exemplary micromilling tool wear monitoring system in accordance with an exemplary embodiment; 
           [0010]      FIG. 2  illustrates a block diagram of exemplary components of a micromilling tool wear monitoring system in accordance with an exemplary embodiment; 
           [0011]      FIG. 3  illustrates an exemplary micromilling tool wear monitoring system in accordance with an exemplary embodiment; 
           [0012]      FIG. 4A  illustrates an exemplary method for processing images in connection with use of a micromilling tool wear monitoring system in accordance with an exemplary embodiment; 
           [0013]      FIGS. 4B through 4J  illustrate image processing steps in an exemplary method for processing images in connection with use of a micromilling tool wear monitoring system in accordance with an exemplary embodiment; 
           [0014]      FIG. 5A  illustrates chip production over time in accordance with an exemplary embodiment; and 
           [0015]      FIG. 5B  illustrates a method for monitoring micromilling tool wear in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following description is of various exemplary embodiments only, and is not intended to limit the scope, applicability or configuration of the present disclosure in any way. Rather, the following description is intended to provide a convenient illustration for implementing various embodiments including the best mode. As will become apparent, various changes may be made in the function and arrangement of the elements described in these embodiments without departing from the scope of the present disclosure. 
         [0017]    For the sake of brevity, conventional techniques for machining, micromilling, microscopy, and/or the like may not be described in detail herein. Furthermore, the connecting lines shown in various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical tool wear monitoring system. 
         [0018]    In accordance with principles of the present disclosure, tool wear may be monitored during the cutting process via measuring the rate of chip production. The “minimum chip thickness” principle indicates that a tooth cannot produce a chip that is thinner than ⅓ of the cutting edge radius of that tooth. Accordingly, principles of the present disclosure contemplate real-time monitoring of the state of wear of a micro-endmill; this may be accomplished by recognizing the existence of a relationship between the state of tool wear and a discrepancy between the number of chips that should be produced (based, for example, on machining parameters) and the number of chips that are actually produced. An exemplary tool wear monitoring system may be configured to measure this discrepancy and thus infer the state of tool wear. 
         [0019]    As opposed to prior approaches, principles of the present disclosure enable monitoring of micro-scale tool wear without halting the cutting process, and thus enable measurement of micro-scale tool wear rates. Machining parameters may be adjusted according to the amount of tool wear. Additionally, principles of the present disclosure enable prevention of micro-scale tool wear-related defects such as burring and tool breakage. 
         [0020]    A tool wear monitoring system may be any system configured to facilitate detection or inference regarding the wear state of a milling tool. In an exemplary embodiment, with reference to  FIGS. 1, 2, and 3 , a tool wear monitoring system  100  comprises a pneumatic component  120 , an adhesion component  140 , an imaging component  160 , and a control component  180 . Pneumatic component  120  is configured to extract chips created during a milling process from the work area, and deliver the chips elsewhere for evaluation. Adhesion component  140  is configured to secure and/or deliver the chips for evaluation. Imaging component  160  is configured to assess and evaluate the chips, and consequently determine and/or infer the wear state of a tool. Control component  180  is configured to provide feedback and/or control to a system or portions thereof, for example a micromilling machine, responsive to the determination of the wear state of a tool. Components of tool wear monitoring system  100  may be physically, electrically and/or communicatively coupled to one another, for example at least partially via wired or wireless communication links  190 . 
         [0021]    In various exemplary embodiments, tool wear monitoring system  100  is configured to allow for real-time assessment of the rate of chip production in a micromilling machine  101  as operative on a workpiece  105 . Using information from the minimum chip thickness effect, tool wear can be inferred. 
         [0022]    In an exemplary embodiment, with reference to  FIGS. 2 and 3 , pneumatic component  120  comprises skirt  122 , tubing  124 , pump  126 , and nozzle  128 . In various exemplary embodiments, skirt  122  is configured to be fitted around and/or coupled to micro-endmill  102 , for example as illustrated in  FIG. 3 . Skirt  122  functions as a physical containment or funneling element to contain chips as they are produced and cause the chips to be caught in an airflow stream through tubing  124 . In an exemplary embodiment, skirt  122  comprises a 0.75 inch diameter transparent plastic suction cup having a height of about 0.5 inch. However, skirt  122  may comprise any suitable material and dimensions configured to contain chips produced by micro-endmill  102 . A hole of suitable diameter, for example between about 0.05 inches and about 0.15 inches, and preferably about 0.08 inches, is punched through the side of skirt  122  to facilitate coupling with tubing  124 . Skirt  122  is coupled to micro-endmill  102  such that the bottom of the skirt is positioned from about 0.5 mm to about 1 mm above the tip of the endmill; in other words, when micro-endmill  102  is operative, a gap of between about 0.5 mm and about 1 mm exists between the bottom of skirt  122  and the surface of workpiece  105 . This provides sufficient clearance for air to be pulled into the interior of skirt  122 , but maintains a sufficient pressure differential between the exterior and the interior of skirt  122  so that chips are not allowed to leave the interior of skirt  122 . 
         [0023]    Tubing  124  is coupled to skirt  122 . Tubing  124  is configured to provide vacuum extraction of the air and chips contained by skirt  122 . Tubing  124  may comprise polyethylene or other suitable strong and/or flexible material. Additionally, polyethylene provides resistance to static electricity and thus reduces the likelihood of chips becoming stuck in tubing  124 ; moreover, polyethelyne is sufficiently hard enough to prevent chips from becoming embedded in the interior wall of the tube and is also resistant to kinking. 
         [0024]    In various exemplary embodiments, tubing  124  is configured with an inner diameter of between about 0.1 inches and about 0.25 inches, and preferably about 0.125 inches, in order to provide a suitable airflow velocity and room for movement of chips therethrough. 
         [0025]    Tubing  124  is coupled to pump  126 . Pump  126  supplies vacuum pressure to extract chips from the interior of skirt  122  by way of tubing  124 . In order to ensure accurate chip rate production calculations, pump  126  is desirably selected such that no chips can be lodged in the interior during operation and dislodged at a later time during operation. Thus, in various exemplary embodiments, pump  126  comprises a vacuum pump operative on the venturi principle and having no moving parts in the interior of the pump. Rather, the vacuum force is created by a pressurized air input. In one exemplary embodiment, pump  126  comprises a Vaccon DF 1-3 venturi suction pump. In this exemplary embodiment, pump  126  provides a static vacuum of approximately 12″ Hg (400 mbar) at 100 psi (7 bar) supply pressure; corresponding air consumption is approximately 100 lpm (liters per minute). However, any suitable pump  126  may be utilized, as desired. 
         [0026]    Tubing  124  is coupled to nozzle  128 . Tubing  124  and nozzle  128  may be separate components; alternatively, tubing  124  and nozzle  128  may be monolithic; i.e., tubing  124  may widen at one end into a section considered to be nozzle  128 . In various exemplary embodiments, nozzle  128  has an inner diameter twice that of tubing  124 . In other exemplary embodiments, nozzle  128  has an inner diameter four times that of tubing  124 . Moreover, nozzle  128  may be configured with any suitable inner diameter configured to provide sufficient airflow slowing as compared to the flow speed in tubing  124 , in order to ensure adhesion of chips in connection with adhesion component  140 . This relationship in tubing diameters is important in order to increase the air velocity where the chips are being pulled into the air stream at skirt  122 , and to decrease the air velocity where the chips are being pushed out of the air stream via nozzle  128  onto tape  144 . Increasing the air speed at tubing  124  inlet helps to prevent chip loss at the inlet due to chip scatter from the spindle, while decreasing air speed at nozzle  128  outlet helps prevent chip loss due to air dispersion. A four-fold increase in the inner diameter between tubing  124  and nozzle  128  results in an approximately sixteen-fold decrease in airflow velocity at the outlet of nozzle  128 . 
         [0027]    Nozzle  128  disperses chips from tubing  124  onto tape  144 . Nozzle  128  functions to prevent chips from being lost (that is, blown into the environment rather than adhered to tape  144 ) and to roughly equally disperse chips across the width of tape  144  within the field-of-view of camera  162 . In order to minimize chip dispersion, the end of nozzle  128  may desirably be placed between about 0.1 inches and about 0.25 inches, and preferably about 0.125 inches, above the surface of tape  144 . 
         [0028]    Pneumatic component  120  may be powered as suitable, for example by a single pressured airline that provides pressure for pump  126  and for the cooling system of the spindle of micromilling machine  101 . During operation of pneumatic component  120 , airstream velocity at the inlet of tubing  124  is desirably between about 150 meters per second (m/s) and about 250 m/s, and preferably about 210 m/s, when tubing  124  is configured with an inner diameter of 0.125 inches. Additionally, airstream velocity at the outlet of nozzle  128  is desirably below 20 m/s when nozzle  128  is configured with an inner diameter of about 0.5 inches. In this manner, chips are effectively collected from within skirt  122  and delivered and adhered to tape  144 . 
         [0029]    In various exemplary embodiments, adhesion component  140  comprises belt  142  and tape  144 . Belt  142 , for example a conveyor belt, operates as a base to move a strip of tape  144  material past nozzle  128  and thereafter past camera  162 . Tape  144  operates to collect and secure chips exiting nozzle  128 . 
         [0030]    Belt  142  may comprise any suitable conveyor belt or similar device. In various exemplary embodiments, belt  142  may be configured with a suitable and/or adjustable belt speed, for example a speed range of between about 0.5 meters per minute (m/m) to about 20 m/m. Additionally, belt  142  may be configured with a suitable belt color to reduce image glare and provide contrast for chips (for example, for dark chips, a matte white belt color is desirable, while for lighter colored chips such as aluminum, a matte black belt color may be desirable). Belt  142  may be configured with a multicolor or striped belt in order to allow tape  144  to be moved back and forth thereon to a suitable background color for the currently produced chips, as desired. 
         [0031]    Tape  144  may comprise any suitable tape configured to be deliverable via belt  142  and capable of retaining chips. In various exemplary embodiments, tape  144  has a weak adhesive on one side and a strong adhesive on the other side. The roll of tape  144  is positioned so that the weak adhesive makes contact with belt  142  and the strong adhesive faces towards nozzle  128 . As belt  142  moves, the friction force of the weak adhesive against belt  142  pulls tape  144  off of the tape sourcing roll onto belt  142 . Tape  144  may be configured with any suitable dimensions; however, in various exemplary embodiments, tape  144  is configured with a width approximately twice that of the inner diameter of nozzle  128  so that chips may be fully captured on tape  144 . Tape  144  may be selected to be generally transparent, translucent, and/or opaque, as desired, depending on the color of belt  142  and in order to provide suitable imaging contrast with chips. 
         [0032]    Once chips are secured on tape  144 , the chips are advanced via belt  142  to camera  162 . Camera  162  may comprise any suitable image capture device. In one exemplary embodiment, camera  162  comprises a Dino-Lite brand digital universal serial bus (USB) microscope. Camera  162  may be mounted on an adjustable mounting bracket to allow for greater functionality of the camera as well as initial focusing. Camera  162  may be configured with a field of view at least as wide as the width of tape  144  in order to obtain suitable images for evaluation (i.e., in order to ensure that chips captured on tape  144  do not fall outside the field of view). Camera  162  takes images of tape  144  and chips captured thereon, for example at regular intervals or on demand. When camera  162  is acquiring an image, belt  142  is desirably paused, for example via a signal from image processing system  166 , in order to minimize motion blurring. Belt  142  thereafter returns to motion. 
         [0033]    Lighting system  164  provides illumination to chips captured on tape  144  as they pass through the field of view of camera  162 . Lighting system  164  may comprise any suitable component or component for providing bright diffuse lighting as is known in the art. 
         [0034]    Image processing system  166  receives images from camera  162  and processes them to identify and count individual chips. Image processing system  166  may comprise any suitable hardware and/or software components. In one exemplary embodiment, image processing system  166  comprises a laptop personal computer having technical computing software such as Matlab operative thereon. Additionally, it will be understood that in certain exemplary embodiments, image processing system  166  and tool control component  180  may all be operative on and/or comprise hardware and/or software components of a single system, for example a laptop personal computer, desktop computer, tablet, smartphone, and/or the like. 
         [0035]    In various exemplary embodiments, control system  180  is configured to control one or more of micromilling machine  101 , pump  126 , belt  142 , camera  162 , and/or image processing system  166 . For example, responsive to image processing system  166  determining that the rate of chip production has fallen below a threshold, control system  180  may send a signal to micromilling machine  101  to increase the feed rate. Moreover, control system  180  may control the interval of image acquisition by camera  162 , the speed and/or starting/stopping of belt  142 , and any other suitable aspects of tool wear monitoring system  100 . 
         [0036]    With reference now to  FIGS. 4A-4J , in various exemplary embodiments a method  400  for counting chips comprises acquiring a digital image from camera  162  (step  405 , illustrated in  FIG. 4B ). The image is converted to grayscale (step  410 ). Thresholding is performed to remove excessive lustre (step  415 , illustrated in  FIG. 4C ). Histogram equalization is performed to improve contrast (step  420 , illustrated in  FIG. 4D ). Thresholding is performed to find the grayscale background level (step  425 , illustrated in  FIG. 4E ) and the image is converted to black and white (step  430 , illustrated in  FIG. 4F ). Erosion is performed to reduce pixelated errors (step  435 , illustrated in  FIG. 4G ). Edge detection is performed (step  440 , illustrated in  FIG. 4H ), and dilation is performed to connect edges and close components (step  445 , illustrated in  FIG. 4I ). Components are filled (step  450 , illustrated in  FIG. 4J ) and then counted (step  455 ). 
         [0037]    Turning now to  FIG. 5B , in an exemplary embodiment a method  500  for monitoring micromilling tool wear comprises obtaining a chip count resulting from operation of a micro-endmill (step  510 ). The chip count is compared to a target chip count (step  520 ), for example a target chip count equal to a chip count that would be expected if the micro-endmill were operating at the expected tooth-passing rate. If the difference between the chip count and the target chip count exceeds a threshold, the feed rate may be adjusted (step  530 ), for example increased, in order to increase the chip production rate. The process is repeated, as desired, in order to regularly monitor the chip count and/or adjust the feed rate. In one exemplary embodiment, the threshold for the difference between the target chip count and the chip count is a decrease of between about 40% and about 60%. Moreover, a suitable threshold may be selected based on the understanding that chip production typically goes from a level X to about a level X/2 as the tool wears, representing slippage of approximately every other tooth in the micro-endmill tool. 
         [0038]    The foregoing exemplary embodiments have presented airflow-driven extraction of chips. It will be appreciated that principles of the present disclosure are also applicable to fluid-driven extraction of chips, for example via cutting fluid. In these exemplary embodiments, cutting fluid may be directed over and/or around workpiece  105  and then extracted together with the resulting chips; the cutting fluid and chip mixture may be thereafter passed through a generally planar plastic enclosure to permit photographing/counting of the chips therein. 
         [0039]    In tool wear monitoring system  100 , the rate at which chips pass by camera  162  may be compared to the expected tooth-passing rate of micro-endmill  102 . If the chip production rate is equal to the tooth-passing rate, then the tool cutting-edge radius is less than that calculated by the minimum chip thickness equation. Similarly, if the chip production rate drops below the tooth-passing rate, then the tool cutting-edge radius is known to be equal to that calculated by the minimum chip thickness equation. 
         [0040]    In an exemplary embodiment, the feed rate and spindle speed of micromilling machine  101  are initially set so that the chip production rate is equal to the tooth passing rate. The chip production rate is then observed until the chip production rate drops below the tooth passing rate.  FIG. 5A  shows an example plot of chip production rate with time as will be observed in this process. Times and chip production rate values are labeled, and will be referred to in the following equations. 
         [0041]    At the beginning of the tool-wear measuring operation (prior to time T 1 ), the feed rate f0 is set according to Eq. (1), where re0 is the initial cutting-edge radius of the cutting tool, before cutting begins, n is the number of teeth on the cutter, N is the spindle speed, and δ is a small value, perhaps 1% of the value calculated if δ is zero. 
         [0000]        f 0=0.3* re 0* n*N+δ   (Equation 1)
 
         [0042]    The initial chip production rate C 0  will be equal to the tooth passing rate, as shown in Eq. (2). 
         [0000]        C 0= n*N   (Equation 2)
 
         [0043]    The chip production rate is measured by tool wear monitoring system  100 . At some future time T 2 , the chip production rate is observed to decrease. At that time, the amount of tool wear re1 at the previous time T 1  can be calculated according to Eq. (3). 
         [0000]        re 1= f 0/(0.3* n*N )  (Equation 3)
 
         [0044]    At time T 3 , the feed rate is increased to the value f1, calculated as in Eq. (4). 
         [0000]        f 1=0.3 *re 1* n*N+δ   (Equation 4)
 
         [0045]    When the feed rate is increased to value f1, the chip production rate will rise back to C 0  and become constant. The tool&#39;s continued wear will cause the chip production rate to drop again, detected at time T 5 . At that time, the amount of tool wear re2 at time T 4  can be calculated according to Eq. (5). 
         [0000]        re 2= f 1/(0.3* n*N )  (Equation 5)
 
         [0046]    At time T 6 , the feed rate is increased to the value f2, calculated as in Eq. (6). 
         [0000]        f 2=0.3* re 2* n*N+δ   (Equation 6)
 
         [0047]    Generalizing, at each future time T i+1  that the chip production rate is observed to drop below the value C 0 , the amount of tool wear rei at the previous time T i  is calculated according to Eq. (7), and the feed rate is increased to the value fi as in Eq. (8). 
         [0000]        rei=fi− 1/(0.3 *n*N )  (Equation 7)
 
         [0000]        fi= 0.3* rei*n*N+δ   (Equation 8)
 
         [0048]    This process may be continued until the tool breaks and/or is otherwise replaced. The tool wear rate is given by the values of rei at the times T i  as i varies from 0 until the tool breaks. In various exemplary embodiments, C 1  is approximately half the value of C 0 , representing slippage of approximately every other tooth in micro-endmill  102 . 
         [0049]    While the principles of this disclosure have been shown in various embodiments, many modifications of structure, arrangements, proportions, the elements, materials and components, used in practice, which are particularly adapted for a specific environment and operating requirements may be used without departing from the principles and scope of this disclosure. These and other changes or modifications are intended to be included within the scope of the present disclosure. 
         [0050]    The present disclosure has been described with reference to various embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure. Likewise, benefits, other advantages, and solutions to problems have been described above with regard to various embodiments. However, benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element. 
         [0051]    As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. §112(f), unless the element is expressly recited using the phrase “means for.” Also, as used herein, the terms “coupled,” “coupling,” or any other variation thereof, are intended to cover a physical connection, an electrical connection, a magnetic connection, an optical connection, a communicative connection, a functional connection, and/or any other connection. 
         [0052]    When language similar to “at least one of A, B, or C” or “at least one of A, B, and D” is used in the claims, the phrase is intended to mean any of the following: (1) at least one of A; (2) at least one of B; (3) at least one of C; (4) at least one of A and at least one of B; (5) at least one of B and at least one of C; (6) at least one of A and at least one of C; or (7) at least one of A, at least one of B, and at least one of C.