Abstract:
A method of machining a multi-layer workpiece includes the steps of: rotating a cutting tool at an operating speed; contacting the cutting tool against the workpiece; moving the cutting tool through one layer and into another layer within the workpiece; and detecting at least one vibration characteristic associated with the cutting tool during the contacting step and/or moving step.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention.  
           [0002]    The present invention relates to a method of machining, and, more particularly, to a method of machining a multi-layer workpiece  
           [0003]    2. Description of the Related Art  
           [0004]    When machining a workpiece in the form of a material sheet, it is important to know the position of the workpiece surface relative to a cutting tool used to machine the workpiece. The workpiece may be in the form of a multi-layer workpiece including multiple layers of material such as aluminum, titanium, stainless steel and fiber-reinforced composite materials. Multi-layer workpieces may be particularly useful in the aerospace industry since they provide high strength, light weight structures. In addition to determining the surface position of the workpiece, it is also important to know the location of the interfaces between the different layers of the multi-layer workpiece. Since the cutting tool may cut differently within the different materials, it is important to know the boundary layers of the different layers as the cutting tool progresses through the workpiece.  
           [0005]    For many applications, an opening which is machined into the workpiece receives a fastener for fastening the workpiece to a structural member, another workpiece, etc. To ensure that a proper length fastener is utilized, it is necessary to determine the thickness of the workpiece.  
           [0006]    It is known to use various types of detectors for detecting the exterior surface of a workpiece to be machined. For example, edge finders utilize a measuring probe with a ball at the tip of the probe. The ball makes contact with the exterior surface of the workpiece and a microswitch is made to provide an output signal to a controller. Conductivity probes are similar to edge finders, except that the sensing element measures the conductivity of the workpiece. It is also known to utilize lasers which reflect a light beam from the surface of the workpiece. Lasers tend to be very costly, and are subject to dirt, etc. which scatters the light beam projected upon the workpiece surface.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method of machining a multi-layer workpiece, in which the vibration amplitude and vibration frequency of the cutting tool are utilized to determine contact between the cutting tool and the workpiece, as well as the interface location between adjacent layers in the workpiece.  
           [0008]    The invention comprises, in one form thereof, a method of machining a multi-layer workpiece including the steps of: rotating a cutting tool at an operating speed; contacting the cutting tool against the workpiece; moving the cutting tool through one layer and into another layer within the workpiece; and detecting at least one vibration characteristic associated with the cutting tool during the contacting step and/or moving step.  
           [0009]    An advantage of the present invention is that contact between the cutting tool and the workpiece is accurately detected.  
           [0010]    Another advantage is that the interface between adjacent layers is also accurately detected.  
           [0011]    Yet another advantage is that existing machines may be easily retrofitted by simply adding one or more accelerometers at selected locations without modifying the existing structure of the machine.  
           [0012]    A further advantage is that the wear state of the cutting tool may be accommodated in the calculation techniques utilized for detection of contact with the workpiece or the interface between adjacent layers.  
           [0013]    A still further advantage is that the thickness of the workpiece may be determined utilizing the machining method of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 is a sectional view of an embodiment of a machine utilized for carrying out a method of machining of the present invention;  
         [0016]    [0016]FIG. 2 is a graphical illustration of the vibration amplitude of the cutting tool as it contacts and passes through the layers of the multi-layer workpiece; and  
         [0017]    [0017]FIG. 3 is a graphical illustration of the vibration frequency of the cutting tool within a composite layer and an aluminum layer. 
     
    
       [0018]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring now to the drawings, and more particularly to FIG. 1, there is shown a machine  10  used for machining a multi-layer workpiece  12  in accordance with a method of machining of the present invention. Machine  10  generally includes a spindle motor  14 , radial offset mechanism  16 , axial feed mechanism  18  and eccentric rotation mechanism  20 , each carried by a frame  22 . Machine  10  may be stationarily mounted or may be mounted in a mobile fashion such as to a robot arm.  
         [0020]    Spindle motor  14  includes a body  24  and a rotatable tool holder  26  configured for holding a cutting tool  28  during rotation. Cutting tool  28 , which defines a tool axis  30 , can be designed for producing a hole (not shown) in workpiece  12 . Cutting tool  28 , such as a drill bit, milling tool, etc. is moved toward and into workpiece  12  so as to form a hole in workpiece  12  which is the same diameter as cutting tool  28  (such as in a simple drilling operation) or larger than the diameter of cutting tool  28 . For further details of the general operation of machine  10 , reference is hereby made to U.S. Pat. No. 5,971,678 (Linderholm), which is assigned to the assignee of the present invention. For details concerning the use of such a machine to form a hole which is larger than the diameter of the cutting tool, reference is hereby made to U. S. Pat. No. 5,641,252 (Eriksson et al.), which is also assigned to the assignee of the present invention.  
         [0021]    Accelerometer  32  is mounted to frame  22  of machine  10  at a location which is sufficient to receive vibrational energy transmitted from cutting tool  28 . Accelerometer  32  provides an output signal to a controller (not shown) used to detect the position of cutting tool  28  relative to workpiece  12 , as will be described in more detail hereinafter. Alternatively, accelerometer  32  may be placed directly upon workpiece  12  for receiving vibrational energy transmitted therefrom such as indicated by accelerometer  32 A.  
         [0022]    Workpiece  12  includes a plurality of layers, with each layer being in the form of a laminae having a metallic or composite structure. In the embodiment shown, workpiece  12  includes three laminae  34 ,  36  and  37  with laminae  34  having a composite structure, laminae  36  having a metallic structure, and laminae  37  having a composite structure. More particularly, in the embodiment shown, lamina  34  and  37  have a fiberglass structure and laminae  36  has an aluminum structure.  
         [0023]    A digital encoder  38  is positioned relative to tool holder  26  to sense the rotational speed of tool holder  26  and cutting tool  28 . Encoder  38  provides an output signal to the controller for use in the machining method of the present invention, as will be described in more detail hereinafter. Alternatively, a tachometer rather than a digital encoder may be positioned relative to tool holder  26  for sensing the rotational speed thereof.  
         [0024]    According to an embodiment of a method of the present invention, machine  10  is used for forming a hole  40  in multi-layer workpiece  12 . More particularly, cutting tool  28  is rotated at an operating speed. When rotating, cutting tool  28  transmits vibrations to accelerometer  32 , which in turn provides an output signal corresponding to at least one vibration characteristic associated with cutting tool  28 . The vibration characteristic is in the form of an amplitude and/or a frequency, as will be described in more detail hereinafter. As cutting tool  28  is contacted with and moved through multi-layer workpiece  12 , the vibration amplitude and/or vibration frequency change. The changes in the vibration characteristics may be used to determine when cutting tool  28  contacts upper laminae  34 , when cutting tool  28  moves through laminae  34  and contacts laminae  36 , when cutting tool  28  moves through laminae  36  and contacts laminae  37 , and when cutting tool  28  exits from the bottom of workpiece  12 . Referring to Table 1 below and FIG. 2, conjunctively, the vibration amplitude of cutting tool  28  as it passes through workpiece  12  will be described in more detail.  
                                             TABLE 1                       No.   Time (s)   Event   Characteristics                                1   0-2   Tool in air   Low amplitude. Vibrations comes from the machine       2   3   Contact   Amplitude rises gently when impact occurs       3   3-11.5   Composite   Amplitude remains on the same level       4   11.5   Transition/Aluminum   A transient marks the impact with aluminum       5   11.5-26.5   Aluminum   Amplitude slightly higher than composite. Random                   transients       6   26.5   Transition/Composite   Amplitude goes down. No distinct variation in                   amplitude       7   26.5-35   Composite   Amplitude levels out. No transients       8   35   Breakthrough   Tool breaks through composite with a small transient.       9   35-37   Overdrill   Tool is scraping the edge of the hole, causing some                   transients       10   37-41.3   Return   Tool goes back. Amplitude is decreasing to air (1) level.                  
 
         [0025]    When cutting tool  28  is brought up to operating speed, a certain amount of low amplitude vibrations occur simply as a result of imbalances etc. of the rotating parts within machine  10 . The time interval  1  between 0-3 seconds shown in FIG. 2 thus has a low vibration amplitude. At time interval  2 , cutting tool  28  contacts upper laminae  34  of workpiece  12  which causes the vibration amplitude to increase. The vibration amplitude does not spike, but rather increases at a relatively slow amplitude rise as cutting tool  28  enters laminae  34 . The vibration amplitude remains relatively constant during time interval  3  extending between 3 and 11.5 seconds. As cutting tool  28  passes through laminae  34  and enters aluminum laminae  36 , the vibration amplitude rapidly spikes and remains at a higher vibration amplitude level through time interval  5  as cutting tool  28  passes through aluminum laminae  36 . At time interval  6  corresponding to proximately 26.5 seconds, cutting tool  28  leaves laminae  36  and enters composite laminae  37 . The vibration amplitude decreases a noticeable extent, and transient spikes are reduced. During time interval  7 , cutting tool  28  passes through third laminae  37  and the vibration amplitude remains relatively constant with few transient spikes. At time interval  8  corresponding to approximately 35 seconds, cutting tool  28  breaks through third laminae  37  at the bottom of work piece  12 , thereby causing a small but noticeable transient spike in the vibration amplitude. By knowing the feed rate of cutting tool  28  through workpiece  12 , the time difference between time interval  8  at which cutting tool  28  breaks through laminae  37  and time interval  2  at which cutting tool  28  contacts laminae  34 , the thickness of workpiece  12  may be determined. Cutting tool  28  continues to be moved in an axial direction to ensure that cutting tool  28  passes through workpiece  12 . Cutting tool  28  scrapes the sidewall edges of hole  40  to some extent, thereby causing some transient vibration amplitude spikes. During time interval  10 , extending from approximately 37-41.3 seconds, cutting tool  28  is moved in an opposite axial direction to return to a home position. During the return movement of cutting tool  28 , the vibration amplitude again decreases to a level which generally only corresponds to small vibrations caused by machine  10 .  
         [0026]    From the foregoing, it is apparent that the vibration amplitude of cutting tool  28  may be easily used to detect when cutting tool  28  contacts workpiece  12 . By simply setting a threshold value for the vibration amplitude, contact between cutting tool  28  and laminae  34  may be easily detected. Moreover, in a case where cutting tool  28  moves from a composite to an aluminum laminae, such as when cutting tool  28  moves through laminae  34  and into aluminum laminae  36 , the vibration amplitude again provides a noticeable spike which may be used to detect the interface between adjacent laminae. However, it may also be noted that when cutting tool  28  moves through aluminum laminae  36  into composite laminae  37 , the vibration amplitude does not change a significant extent. Moreover, in the case where adjacent layers are formed from different metallic materials or different composite materials, the vibration amplitude may not change to an appreciable extent. Accordingly, although the vibration amplitude provides a good indicator of contact between tool  28  and laminae  34 , it may not provide a good indicator of the interface location between adjacent layers of workpiece  12 .  
         [0027]    It will also be noted from FIG. 1 that the vibration amplitude rise of cutting tool  28  is much lower when cutting tool  28  enters a composite layer, as compared to when cutting tool  28  enters a metallic layer. More particularly, as cutting tool  28  enters a metallic layer, the vibration amplitude spikes quite rapidly. Thus, it is possible to use various numerical analysis techniques to determine the vibration amplitude rise and thereby infer whether cutting tool  28  is entering a composite or a metallic layer.  
         [0028]    To determine the interface between adjacent layers of multi-layer workpiece  12 , it has been found that the vibration frequency rather than the vibration amplitude tends to be more accurate. Referring to FIG. 3, a frequency spectrum for a composite layer and an aluminum layer are illustrated. When cutting tool  28  is cutting an aluminum layer, the peaks drop in frequency. The simple explanation for this is that the spindle speed drops when entering the aluminum layer as a result of the higher cutting forces required to machine the aluminum layer when compared to the composite layer, and the lack of feed back control for the spindle motor. The peaks drop in frequency to even a greater extend for a titanium layer.  
         [0029]    Various numerical analysis techniques may be utilized to determine frequency values and frequency changes of cutting tool  28  when cutting multi-layer workpiece  12 . For example, a Fast Fourier Transform technique has been found to provide suitable calculation results to determine the interface between adjacent layers within acceptable error limits. Digital filtering techniques may also be utilized to reduce unwanted noise from the output signal provided by accelerometer  32 .  
         [0030]    The two primary factors which have been found to effect the vibration amplitude are the type of material which cutting tool is cutting as well as the wear state of cutting tool  28 . The principal effect of an advanced wear state of cutting tool  28  is that the vibration amplitude is increased. This can be accommodated through tuning of the controller to adjust the amplitude of the signal received from accelerometer  32 . In addition, it may be necessary to filter the signal to remove transients caused by the advanced wear state of cutting tool  28 .  
         [0031]    While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.