Abstract:
A method for machining a workpiece using a programmable, numerically controlled machining system by calculating or retrieving a compensated toolpath based on comparing contact position from monitoring a vibration signal from a vibration sensor during probing of workpiece with rotating tool during relative motion therebetween. Contact position is compared to position from predetermined toolpath and wherein the predetermined toolpath extends between initial machining point and end machining point. Machining the workpiece is done along compensated toolpath. The method may be done for repeated passes of machining. The compensated toolpath may include an angle offset to a machining path coordinate system of the predetermined toolpath. Workpiece may be mounted in a multi-axis manipulator of machining system for the probing and machining Multi-axis manipulator may be computer controlled and may be part of a robot.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    This invention relates to numerically controlled machining systems, adaptive machining and, in particular, to a system and method for machining a workpiece using vibration sensing. 
         [0003]    Description of Related Art 
         [0004]    Precision machining is known for manufacturing parts with general-purpose, numerically controlled machining systems. Various automated methods and systems have been developed for cutting or other removal of material and for making cutter adjustments using an ordinary numerical control cutter offsets. These adjustments of the cutting tool are necessary to take into account a large number of variables, such as wear of the cutting tool, repositioning and/or replacement of the cutting tool, as well as dimensional changes of the cutting tool, of the workpiece and of the machining apparatus itself due to such factors as heating, deflection under load, and the like. 
         [0005]    It is known to use tool offsets with numerically controlled machine tools after the machine is set up for the manufacture of a particular workpiece or part. Closed loop machining apparatus and methods have been developed to automate the tool offset or compensation process. Computer-operated numerical control systems carry out the computations of offsets. It is also known to mount a tool sensor such as a touch trigger probe or vibration sensor on the bed of the machining apparatus or on a pivotal arm that can be swung out of the way when desired. 
         [0006]    The position of the cutting tool can be calibrated against such a probe by noting the tool position when contact with the probe occurs. From the observed deviations between the programmed and the actual positions, a compensating offset may be determined and stored in the memory associated with the computer numerical control means. The offset compensates for the difference between the programmed contact position and the actual contact position. Such a system and method is disclosed in U.S. Pat. No. 4,382,215. 
         [0007]    U.S. Pat. No. 4,428,055 discloses a system and method for automatically machining a workpiece under computer numerical control in which the cutting tool is set automatically. Vibration sensing detects contact between a cutting edge of the cutting tool and position reference surfaces and the cutting edge is calibrated against these surfaces under closed loop control. The deviation between the programmed and the actual positions of the cutting edge at each reference surface is determined and an initial compensating tool position offset is automatically provided in each axis and is entered into numerical control means. 
         [0008]    The initial compensating tool position offset is used to determine where a tool starts cutting along a predetermined toolpath. A toolpath is a path through space that the tip of a cutting tool follows on its way to producing the desired geometry of the workpiece. The toolpath has an initial machining point and an end machining point in space. The initial compensating tool position offset is an offset to the initial machining point but this compensation does not effect or change the toolpath between the initial and end machining points. 
         [0009]    There is need for a more accurate, better, faster, and less expensive method for CNC machining There is also a need for a more efficient CNC machining method that is highly adaptive to workpiece variation. There is need for a more accurate, method for CNC machining that takes into account workpiece variation from piece to piece. There is need for a more accurate determination of the precise position and machining of the part with the rotating tool regardless of abrasive wear of the tool and/or workpiece deformation. 
       SUMMARY OF THE INVENTION 
       [0010]    A method for machining a workpiece with a programmable, numerically controlled machining system, the system including numerical control means adapted to operate under the direction of a predetermined machining program including a predetermined toolpath extending between an initial machining point and an end machining point and at least one rotating cutting tool powered by a computer numerically controlled machine. The method includes the following steps: (1) probing the workpiece with the rotating tool during relative motion therebetween, (2) monitoring a vibration signal from a vibration sensor mounted in the machining system, (3) recording a contact position of the workpiece when the monitored vibration signal indicates contact between the rotating tool and the workpiece, (4) comparing the recorded contact position to a predetermined initial machining point of the predetermined toolpath and calculating or retrieving a compensated toolpath based on the comparison wherein the predetermined toolpath extends between the initial machining point and the end machining point, and (5) machining the workpiece along the compensated toolpath. 
         [0011]    The method may further include machining the workpiece along multiple passes wherein steps (1) through (5) are repeated for two or more of the passes. The method may further include calibrating the machine by probing a calibration part having known dimensions with the cutting tool while monitoring a vibration signal from the vibration sensor during the probing with the cutting tool. The calibrating may further include determining tool parameters of the cutting tool during the calibrating that are influenced by at least one of a tool diameter, a tool geometry, and distance between the tool and a workpiece guide mounted to the machine. 
         [0012]    The compensated toolpath may include an angle offset to a machining path coordinate system of the predetermined toolpath. The workpiece may be machined with multiple passes wherein steps (1) through (5) are repeated for two or more of the passes. 
         [0013]    The workpiece may be mounted in a multi-axis manipulator of the numerically controlled machining system before step (1). During step (1), the probing includes moving the multi-axis manipulator holding the workpiece to provide the relative motion between the rotating tool and the workpiece, and during step (5), the machining the workpiece along the compensated toolpath includes moving the multi-axis manipulator holding the workpiece. 
         [0014]    The machine being a grinder and the rotating tool a grinding wheel. 
         [0015]    One embodiment of the method is for repairing an airfoil, tip using the programmable, numerically controlled machining system, including a grinding wheel powered by a computer numerically controlled machine. Before machining repair material is added at the tip of the airfoil. The grinding wheel powered by the machine makes machining passes to machine away excess repair material and blend the airfoil surface of the airfoil at different chordwise locations along the airfoil between leading and trailing edges of the airfoil. At least one of the machining passes includes the following steps: 
         [0016]    (1) probing the airfoil with the rotating grinding wheel during relative motion therebetween, 
         [0017]    (2) monitoring a vibration signal from a vibration sensor mounted in the machining system, 
         [0018]    (3) recording a contact position of the airfoil when the monitored vibration signal indicates contact between the rotating grinding wheel and the airfoil, 
         [0019]    (4) comparing the recorded contact position to a predetermined initial machining point of the predetermined toolpath and calculating or retrieving a compensated toolpath based on the comparison wherein the predetermined toolpath extends between the initial machining point and the end machining point, and 
         [0020]    (5) machining the airfoil along the compensated toolpath. 
         [0021]    A gas turbine engine blade containing the airfoil may be mounted in a multi-axis manipulator of the numerically controlled machining system before step (1) and the probing the airfoil with the rotating grinding wheel during relative motion therebetween includes moving the multi-axis manipulator holding the blade relative to the machine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings where: 
           [0023]      FIG. 1  is a diagrammatical view illustration of a cutting machine with a rotatable tool and a vibration sensor for adjusting a predetermined toolpath to machine a workpiece. 
           [0024]      FIG. 2  is a diagrammatical view illustration comparing an exemplary predetermined toolpath to a compensated toolpath for machining the workpiece illustrated in  FIG. 1 . 
           [0025]      FIG. 3  is a diagrammatical view illustration of steps of the machining using the compensated toolpath illustrated in  FIG. 2 . 
           [0026]      FIG. 4  is a diagrammatical view illustration of an angle offset between the predetermined and compensated toolpaths and predetermined and compensated tool coordinate systems of the predetermined and compensated toolpaths illustrated in  FIG. 2 . 
           [0027]      FIG. 5  is a diagrammatical view illustration of the CNC machine and a calibration part in a manipulator for calibrating the grinding machine and the manipulator illustrated in  FIG. 1 . 
           [0028]      FIG. 6  is a flow chart of the exemplary toolpath compensation method illustrated in  FIG. 1 . 
           [0029]      FIG. 7  is a diagrammatical view illustration of an airfoil repair with repair material on the airfoil of the exemplary workpiece illustrated in  FIG. 1 . 
           [0030]      FIG. 8  is a diagrammatical view illustration of machining multiple passes of the airfoil repair illustrated in  FIG. 7 . 
           [0031]      FIG. 9  is a diagrammatical view illustration of a polished the airfoil repair of airfoil illustrated in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    Illustrated in  FIG. 1  is an exemplary embodiment of a CNC (computer numerically controlled) machining system  8  including a cutting machine  10  illustrated herein as a grinder having an exemplary cutting tool  12  illustrated herein as a grinding wheel  14 . The CNC machining system  8  includes a multi-axis manipulator  16  which holds and moves a workpiece  18  with respect to the cutting tool  12 . The multi-axis manipulator  16  is articulated. One example of such a multi-axis manipulator is a LR Mate 200iC Series electric servo-driven mini-robot sold by FANUC Robotics. In the case of the machine  10  illustrated in  FIG. 1 , the workpiece  18  is a gas turbine engine blade  19  and the exemplary method illustrated herein is grinding away excess metal  20  on a surface  22  of an airfoil  21  of the blade  19  for the purpose of blending the airfoil surface  22 . A multi-axis manipulator is disclosed in United States Patent Application 20140316573 entitles “ROBOT SYSTEM HAVING A ROBOT FOR CONVEYING A WORKPIECE” to Takahiro Iwatake published Oct. 23, 2014 and incorporated herein by reference. The multi-axis manipulator  16  is CNC controlled and may be a robot controlled by a robot controller such as one available from FANUC Robotics. 
         [0033]    A vibration sensor  28  such as an accelerometer located on the CNC machining system  8  detects a vibration when the grinding wheel  14  contacts the workpiece  18  which is held, moved, and positioned by the manipulator  16 . The vibration sensor  28  is illustrated as being mounted on the machine  10  but may be alternatively mounted on the manipulator  16 . The vibration sensor  28  is monitored until the contact vibration is detected. The detection triggers the manipulator  16  to capture its current position, which represents the precise contact position of the workpiece  18  at the time and point of actual contact and may be referred to as an actual contact position  29 . 
         [0034]    Referring further to  FIGS. 2-4 , the cutting tool  12  illustrated herein is the grinding wheel  14  guided through a toolpath  34 . The multi-axis manipulator  16  of the CNC machining system  8  holds and moves a workpiece  18  with respect to the cutting tool  12 . The toolpath  34  is a path through space that the tip of the cutting tool  12  follows to produce the desired geometry of the workpiece  18 . The toolpath  34  has an initial machining point  36  and an end machining point  38  in space. 
         [0035]    An initial compensating tool position offset has and may be used to determine where a tool starts cutting along a predetermined toolpath  34  or where the actual or compensated initial machining point  36  is placed. The initial compensating tool position offset is an offset to the initial machining point  36  but this compensation does not effect or change the toolpath  34  between the initial and end machining points  36 ,  38 . During machining, a workpiece guide  30  mounted on the machine  10  helps guide the workpiece  18  through the toolpath  34  during each of one or more passes  40  of the grinding wheel  14  against the workpiece  18 . 
         [0036]    Referring to  FIG. 1 , a compliance means  32  acting on the machine  10  for urging the grinding wheel  14  against the workpiece  18  insures the complex-geometry workpiece  18  stays in contact with the grinding wheel  14  and the workpiece guide  30  throughout the machining process. The compliance means  32  may be an air pressure or spring powered device operable to urge the guide  30  and the grinding wheel  14  against the workpiece  18 . An air pressure powered compliance means  32  may be with the use of a rotary air cylinder to ensure constant pressure on the workpiece  18  during probing and during machining. 
         [0037]    The CNC machining system  8  is programmed or operable to adjust or compensate a predetermined toolpath  34  so that the CNC system  8  causes the cutting tool  12  (illustrated herein as the grinding wheel  14 ) to follow a compensated toolpath  35  from an initial machining point  36  to an end machining point  38  as illustrated herein in  FIGS. 3 and 4 . The predetermined toolpath  34  follows a nominal part shape stored in the CNC machining system  8  as a 3D model from which the predetermined toolpath  34  may be extracted. The compensated toolpath  35  is different from the predetermined toolpath  34  at least between the initial and end machining points  36 ,  38 . The predetermined toolpath  34  includes a predetermined initial machining point  36  or position which may be different from the actual contact position  29 . A comparison is made of the recorded contact position  29  to a predetermined contact position such as the initial and end machining points  36 ,  38  from a predetermined toolpath  34  stored in a computer. If the comparison indicates a difference, then the workpiece is machined along the compensated toolpath  35 . 
         [0038]    One exemplary machining path compensation can be an angle offset  48  to a machining path coordinate system  46  as illustrated in  FIGS. 2-4 . An angular difference of Theta2-Theta1, illustrated in  FIG. 2 , may be used to provide an angle offset  48  on the machining path coordinate system  46 . Basically, the angular difference tells you how much to rotate the theoretical or predetermined coordinate system  46  to a compensated coordinate system  47  along the compensated toolpath  35  and, consequently, the toolpath itself to accommodate the variation in the part or workpiece  18 . Respective first through fourth positions P 1 -P 4  along both the predetermined and compensated toolpaths  34 ,  35  are indicated in  FIG. 4 . The angle offset  48  is illustrated at each of the positions P 1 -P 4  along the compensated toolpath  35 . The compensated coordinate system  47  along the compensated toolpath  35  is rotated by the angle offset  48  at each of the first through fourth positions P 1 -P 4  along both the predetermined and compensated toolpaths  34 ,  35  are indicated in  FIG. 4 . 
         [0039]      FIG. 5  illustrates an exemplary calibration part  60  or workpiece mounted on the multi-axis manipulator  16  of the CNC machining system  8  for calibrating the machine  10  with the cutting tool  12  illustrated herein as the grinding machine and the grinding wheel  14 . Probing by vibration sensing is used to measure the angle Theta between the calibration part which has known dimensions and the abrasive grinding wheel  14  when they are in contact. Calibration is used to determine tool parameters of the abrasive grinding wheel  14  that are influenced by one or more of tool diameter, tool geometry and distance between tool and part guide. 
         [0040]    Illustrated in  FIG. 6  is a flow chart of an exemplary method of determining an exemplary compensated toolpath as disclosed above. The flow chart includes: 
         [0041]    1) Mount a workpiece in a manipulator. 
         [0042]    2) Using the manipulator, move the workpiece towards a rotating tool operably mounted in a machine. 
         [0043]    3) Monitor a vibration signal from a vibration sensor mounted on the machine. 
         [0044]    4) Stop moving the workpiece when the monitored vibration signal indicates contact between the rotating tool and the workpiece and record workpiece contact position. 
         [0045]    5) Compare the recorded contact position to a predetermined contact position. 
         [0046]    6) Adjust or modify a predetermined toolpath or retrieve or calculate a compensated toolpath based on the comparison of the recorded contact position to a predetermined contact position from the predetermined toolpath stored in a computer. 
         [0047]    7) Then machine the workpiece along the compensated toolpath. 
         [0048]    A new compensated toolpath may be calculated and used to machine the same workpiece for each of multiple passes of the tool along the workpiece. The control of the machine, tool, and monitoring of the manipulator may be controlled by a computer in the machine and the manipulator may be a multi-axis manipulator. 
         [0049]    The machining illustrated in  FIGS. 7-9  is an airfoil  21  blending method. The blending method includes grinding away excess metal  20  on a surface  22  of an airfoil  21  of the blade  19  for the purpose of blending the airfoil surface  22 . The airfoil  21  extends radially between a base  50  and a tip  52  and has a chord C extending between leading and trailing edges LE, TE of the airfoil. The method includes adding repair material  54  at the tip  52  of the airfoil  21  as illustrated in  FIG. 7 . The method includes then making machining passes  40  at different chordwise locations  42  along the airfoil  21  between leading and trailing edges LE, TE of the airfoil  21  as illustrated in  FIG. 8 . The machining passes  40  machine away excess repair material and blend an airfoil surface  22  of the airfoil  21 . The machining passes  40  are preferably made outwardly from the actual contact position  29  during probing through the tip  52  of the airfoil  21 . The airfoil  21  is then polished smooth as illustrated in  FIG. 9 . 
         [0050]    While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims.