Abstract:
A vibration assisted tapping device includes an elastic frame for translating axial vibration to a vibration having both axial and torsional components. The elastic frame has flexural members that connect an upper plate to a lower plate with a vibratory actuator preloaded therebetween. The flexural members are inclined relative to an axis of the frame in a direction calculated to result in translated vibration substantially aligned with the lead angle of the thread being cut. Also disclosed is an automated system for applying different vibration patterns to the tap/workpiece interface during tapping and for recording the tapping torque associated with each vibration pattern. The disclosed system permits identification of the vibration frequency and amplitude that results in the greatest reduction in tapping torque. The automated system may be configured as an adaptive machine tool to first identify and then apply the optimum vibration pattern.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to the field of vibration-assisted machining processes and more particularly to methods and devices for vibration-assisted tapping.  
           [0003]    2. Description of the Related Art  
           [0004]    Among the machining processes practiced in modern manufacturing, threading is one of the most time-consuming and, therefore, often results in a production bottleneck. Meanwhile, demand for high quality, high accuracy threaded holes, specifically those with a small diameter and a long depth has increased. Current threading technologies cannot fully satisfy this demand, which creates a need for an efficient and inexpensive threading technology.  
           [0005]    It is known that the application of vibration to the tap or workpiece during tapping can reduce the force required for tapping and increase the useful life of the tap. For example, ultrasonic vibration has been successfully applied to assist various machining processes for many years. Various experiments have tested axial vibration-assisted tapping. Axial vibration-assisted tapping experiments have generally concluded that, while tapping torque was reduced and tap life extended, the accuracy of the threads were unacceptably reduced when compared to threads tapped without vibration assistance. Further experiments have tested torsional vibration-assisted tapping, finding that tapping torque is reduced, tap life is extended and thread accuracy is improved relative to threads tapped without vibration assistance.  
           [0006]    Ultrasonic (&gt;20 KHz) screw-vibration has been applied to assist the tapping process on pure aluminum, and found that chip thickness and tapping torque were reduced, while the surface integrity and accuracy of the threads were significantly improved (Suzuki et al. (1991)). Shamoto et al (1994 &amp; 1996) applied a synchronized two-directional vibration to the machining of copper to form an eliptical vibration cutting pattern that reduces chip thickness, cutting forces and the specific energy of material removal while significantly improving the surface integrity of the machined surface.  
           [0007]    The results of vibration assisted machining and the above discussed experiments indicate that application of vibration to the tapping process may reduce tapping torque, lengthen tool life and improve the quality of the thread produced. There remains a need in the art for an empirically proven, reliable and inexpensive vibration-assistance device that can be incorporated into existing thread-tapping equipment. Preferably, such a vibration-assistance device would permit the resulting vibration to be tailored to a particular tapping process and will require little or no modification of the existing thread-tapping equipment.  
         SUMMARY OF THE INVENTION  
         [0008]    The torque required for tapping includes the torque required for material removal as well as relief face friction due to spring back (elastic recovery) of the machined surface. Vibration tapping can reduce tapping torque mostly because the workpiece surface is repeatedly cut, each successive cut removing a further portion of the material bearing on the relief face of the tap. The more times the workpiece surface is cut, the smaller the frictional force should be.  
           [0009]    Repeated cutting is a function of vibration direction, amplitude and frequency. Vibration above a certain frequency tends to reduce the amplitude of cutting face oscillation, thereby reducing the overlapping length of repeated cutting. Axial vibration adversely affects the tapping process by increasing thread error. Further axial vibration fails to improve the number of cutting times because the tap cutting takes place primarily in an axial direction with a small torsional element. Intuitively, torsional vibration should have a beneficial effect on thread tapping by increasing repeated cutting, as experimentation has proved. However, torsional cutting does not account for the axial component of a helix defined by the thread being cut.  
           [0010]    Briefly stated, an exemplary embodiment of a vibration-assisted tapping device comprises an elastic frame surrounding a source of axial vibration. The frame includes an upper plate and lower plate connected by angled flexural members. The axial vibrator is fixed to one of the upper or lower plates and biased against the other of the upper or lower plates to apply its axial vibratory force to the elastic frame. The flexural members are inclined relative to a longitudinal axis of the device at an angle equal to or greater than the lead angle of the thread being tapped.  
           [0011]    An exemplary embodiment of the frame is machined from a single piece of spring steel. A plurality of flexural members is arranged around the periphery of the frame to connect the upper plate to the lower plate. A piezoelectric vibratory actuator is aligned with the central axis of the frame and biased or preloaded relative to the frame such that the axial vibratory motion is efficiently transmitted to the frame. Axial stretching of the frame by the actuator is translated by the inclined flexural members into relative movement between the upper and lower plates having axial and torsional components. The configuration of the frame, i.e., the angle of the flexural members, the stiffness and elasticity of the frame, the frequency and amplitude of the vibration applied, combine to produce a relative movement between the upper and lower plates of the frame. This relative movement may be substantially aligned with a helix defined by the thread being tapped.  
           [0012]    A driver circuit operatively connected to the vibratory actuator allows the frequency of the vibration as well as the effective force, and hence the amplitude of the vibration to be varied. A torque sensor is arranged to measure the torque required for tapping over a range of vibration frequencies and amplitudes. This arrangement allows for testing to determine the most effective combination of vibration frequency and amplitude for a particular tapping process.  
           [0013]    An object of the invention is to provide a new and improved vibration assisted tapping device that is inexpensive, technologically simple and improves the speed and accuracy of thread tapping.  
           [0014]    Another object of the present invention is to provide a new and improved vibration-assisted tapping device that produces a screw vibratory motion substantially aligned with a helix defined by the thread being tapped.  
           [0015]    A further object of the present invention is to provide a new and improved vibration-assisted tapping device and a method for matching the vibration assistance to a particular thread tapping process. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    These and other objects, features and advantages of the invention will become readily apparent to those skilled in the art upon reading the description of the exemplary embodiments, in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is an elevational exterior view of an exemplary frame for translating axial vibration into vibration having an axial and torsional component in accordance with a first aspect of the present invention;  
         [0018]    [0018]FIG. 2 is an exemplary tapping arrangement incorporating an alternative exemplary frame supporting a workpiece;  
         [0019]    [0019]FIG. 3 is an exterior view of a vibration-assisted tap incorporating an alternative exemplary frame in accordance with an aspect of the present invention;  
         [0020]    [0020]FIG. 4 illustrates an exemplary screw thread for which a tapping operation may be carried out by a vibration-assisted tapping device in accordance with the present invention;  
         [0021]    [0021]FIGS. 4. 2 . 1 - 4 . 2 . 16  graphically illustrate tapping torque ranges as a function of driving frequency in Hz and driving voltage, while associated Tables  4 . 2 . 1 - 4 . 2 . 16  illustrate the experimental parameters and resulting data for the corresponding Figures;  
         [0022]    [0022]FIG. 5 is a sectional view through a workpiece holder for use in conjunction with the vibration-assisted tap holder shown in FIG. 3; and  
         [0023]    [0023]FIG. 6 is a functional block diagram of a system for applying the experimental parameters of Tables  4 . 2 . 1 - 4 . 2 . 16  to particular tapping operations and recording the resulting tapping torque data. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    With reference to FIG. 1, an aspect of the present invention relates to a frame  10  configured to translate an applied axial vibration into vibration having axial and torsional components. The exemplary frame  10  of FIG. 1 comprises a top plate  12  connected to a bottom plate  16  by a plurality of angled flexural members  14 . The exemplary frame  10  is machined from a single piece of spring steel, although alternative materials and methods of construction may occur to one of skill in the art. The bottom plate  16  is in the form of a ring integrally connected to the bottom of the flexural members  14 . This bottom plate configuration is a result of machining the frame  10  from a single piece of steel.  
         [0025]    The exemplary frame  10  incorporates six flexural members  14  connecting the upper plate  12  to the lower plate  16 . Each of the six flexural members  14  is inclined relative to an axis A of the frame  10 . The frame  10  and its angled flexural members  14  are configured such that, when exposed to that portion of the axial vibration which seeks to push the upper and lower plates apart, this axial spreading is translated into relative movement between the upper and lower plates having both an axial and torsional component. In the context of this application, this movement will be referred to as “screw vibration” and is illustrated as reciprocating movement generally along a path illustrated by arrow  40 . It can be seen from FIG. 1 that arrow  40  defines a path having both an axial component and a torsional component. The incline angle Øs, elasticity and stiffness of the flexural members  14  are calculated to produce a screw vibration  40  having an angle relative to a plane normal to the frame axis A that is substantially equal to or greater than the lead angle Ø L  of the thread being cut.  
         [0026]    [0026]FIG. 4 illustrates an exemplary screw  30  having a single-lead thread  32 . The illustrated single-lead thread  32  defines a helix around a central axis B. The thread  32  has a lead angle Ø L  relative to a plane P normal to the axis B of the helix defined by the thread  32 . According to an aspect of the present invention, the frame  10  is configured to translate axial vibration into screw vibration substantially aligned with the lead angle of the thread being cut. Experimentally, a flexural member angle Ø S  relative to the frame axis A equal to or greater than the lead angle Ø L  of the thread being cut has proven to produce screw vibration which produces a greater reduction in tapping torque than an equivalent torsional vibration. Screw vibration along path  40  is more closely aligned with the helical configuration of the thread being cut, thereby improving the accuracy of the resulting threads.  
         [0027]    The inventive frame may be incorporated into a work surface such as that of a work table and used to support the workpiece as shown in FIG. 2 or it may be incorporated into a tool holder, chuck or collet such as a tap holder  60  as shown in FIG. 3. The frame  10   a  illustrated in FIG. 2 incorporates four flexural members  14  connecting the top plate  12  to the bottom plate  16 . In the exemplary configuration shown in FIG. 2, a piezoelectric vibratory actuator  50  is arranged along the axis of the frame. The actuator/frame assembly is configured such that the actuator  50  is compressed or preloaded between the upper and lower plates  12 ,  16 . Each of the actuator  50  and frame  10   a  have an axial stiffness. In accordance with an aspect of the invention, the axial stiffness of the actuator  50  is approximately equal to the axial stiffness of the frame  10   a . This relationship has proven to result in an efficient translation of actuator vibration into screw vibration by the frame  10   a.    
         [0028]    [0028]FIG. 3 shows an exemplary vibration-assisted tap holder  60  incorporating a frame  10   b  and vibratory actuator  50 . It will be noted that the position of the upper and lower plates  12 ,  16  of the frame  10   b  are reversed with respect to the orientation of the upper and lower plates shown in FIG. 2. This frame  10   b  is reversed so that the resulting screw vibration path  40  is substantially aligned with the path of thread cutting. In FIG. 3, clockwise rotation of the tap  62  to cut a conventional right hand thread is reinforced by screw vibration along path  40  produced by the inventive frame  10   b  and vibratory actuator  50 . It is understood that a left-hand thread would be tapped with a vibration-assisted tap holder with flexural members inclined at an angle of—Ø S  relative to axis A. The opposite helical thread configuration of the left-hand thread would require a corresponding opposite angular inclination of the flexural members  14 .  
         [0029]    In FIG. 2, it is the workpiece to which the screw vibration is being applied. The illustrated orientation of frame  10   a  and vibratory actuator  50  produces a screw vibration along a path  40  substantially aligned with the path of thread cutting made by the tap  62 . It will be understood that the configuration illustrated in FIG. 2 is practical only if the workpiece  64  is small enough to vibrate. It will also be understood that larger workpieces and/or larger tap diameters will have greater tapping torque levels and greater masses to be vibrated and will likely require a vibratory actuator capable of producing a larger force to produce satisfactory results.  
         [0030]    [0030]FIGS. 2, 3,  5  and  6  illustrate a basic configuration for an adaptive vibration assisted tapping device in accordance with several aspects of the present invention. In FIG. 2, the workpiece holder  80  is mounted to a torque sensor  70  arranged to measure the tapping torque applied to the workpiece  64 . FIG. 5 illustrates an alternative workpiece holder  80   a  for use in conjunction with the vibration assisted tap holder of FIG. 3. The workpiece holder  80   a  is also supported on a torque sensor  70 . Of course, a torque sensor may also be incorporated into a vibration assisted tap holder such as that illustrated in FIG. 3. The piezoelectric vibratory actuators  50  of FIGS. 2 and 3 are responsive to oscillating signals that may take various forms such as a square wave, sine wave, or the like. The amplitude (in volts) and frequency of the driving signal determine the force and frequency of the vibration produced by the actuators.  
         [0031]    [0031]FIG. 6 illustrates a system for automating the application of different vibration patterns to a tapping operation and for collecting torque data associated with each vibration pattern. A computer  100  controls the piezoelectric driver  130  and receives torque sensor readings from a charge amplifier  120  via an interface box  110 . The computer  100  may be programmed to cycle through a range of vibration frequencies and driving voltages and record the resulting tapping torque for each frequency/driving voltage point. The resulting data can be used to determine the most effective vibration pattern for a particular tapping operation. This arrangement might be incorporated into a machine tool for the purpose of producing an adaptive machine tool. When the adaptive machine tool has cycled through the available range of frequencies and driving voltages, it may be programmed to return to the frequency/driving voltage combination that produced the greatest tapping torque reduction. Alternatively, the system of FIG. 6 might be used to configure a tapping machine to perform many substantially identical tapping operations. The tapping machine could be configured to produce vibration assistance at the frequency and amplitude that was determined experimentally to provide the greatest tapping torque reduction.  
         [0032]    Tables  4 . 2 . 1 - 4 . 2 . 16  and related FIGS. 4. 2 . 1 - 4 . 2 . 16  are constructed from data gathered from a series of tapping operations by a system such as that illustrated in FIG. 6. Tables  4 . 2 . 1 - 4 . 2 . 16  illustrate the tapping torque in Newton centimeters (N-cms) for a given driving frequency (200 Hz, 400 Hz, 600 Hz, and 800 Hz) and driving voltage (0V, 2V, 4V, 6V, 8V, and 10V) for four tap diameters (A=4-40, B=6-32, C=8-32, and D=10-32) and four materials (Aluminum Alloy 1100, Aluminum Alloy 6061, Stainless Steel 304 and Carbon Steel 1018) at a hole depth of {fraction (3/8)} inch and a spindle speed of 80 rpm. Corresponding FIGS. 4. 2 . 1 - 4 . 2 . 16  graphically illustrate the experimental results in terms of tapping torque ranges as a function of driving frequency in Hz and driving voltage. For the piezoelectric actuator used, the most effective vibratory frequency is generally in the range between approximately 400 Hz and 800 Hz. In other words, a driving frequency in this range produces the greatest reduction in tapping torque for a given drive voltage.  
         [0033]    The experimental results suggest that tapping operations involving different materials, tap sizes, thread types and tap rotational speeds are likely to require a different vibration frequency and/or amplitude to achieve the maximum available tapping torque reduction. An arrangement such as that disclosed which allows the tapping torque to be measured over a range of vibration frequencies and amplitudes will permit selection of the most effective combination for a given tapping operation. Generally speaking, the experimental results suggest that vibration frequencies above approximately 1000 Hz become less effective because the amplitude of the resulting screw vibration goes down, reducing the overlapping length of repeated cutting. Also generally speaking, larger taps require larger driving voltages to produce effective vibration amplitudes due to the increased mass of the tap and the relatively higher tapping torque.  
         [0034]    The illustrated embodiments incorporate a piezoelectric vibratory actuator although other actuators will occur to one of skill in the art and may be applicable to arrangements in accordance with the present invention.  
         [0035]    A screw vibration assisted tapping device in accordance with the present invention is advantageously compact and includes no moving parts. The simple configuration can be produced using known and well-established manufacturing techniques. The resulting assembly is extremely rugged and should have a long service life when incorporated into tapping equipment. The materials and configuration of the frame may be selected to produce a screw vibration tailored for the threads being tapped.  
         [0036]    While exemplary embodiments of the invention have been shown and described for purposes of illustration, the foregoing descriptions should not be deemed a limitation of the invention herein. Accordingly, various modification, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.