Abstract:
A turning machine having: a machine frame with a longitudinal axis; a rotary part driver for rotating a part about the longitudinal axis; three tools mounted on the frame, each tool having a radial tool axis transverse the longitudinal axis and each radial tool axis being disposed in a circumferential array spaced apart by substantially 120°; wherein each tool has an independent radial actuator and an independent longitudinal actuator, wherein each tool is movable radially and longitudinally relative to the part and relative to the other two tools.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates generally to a machine tool and turning machine. 
       BACKGROUND OF THE ART 
       [0002]    Turning machines such as lathes, specialized rotary cutters and metal machining tools for example, generally include means to rotate the workpiece or material removing cutters relative to each other. Typically, the workpiece is rotated about its axis while the cutting tools are moved relative thereto in order to remove material as required from the rotating outer surfaces of the part. However, in order to remove material from the workpiece, these cutters necessarily exert forces thereon, and are typically both radial and axially directed. Such forces tend to deflect the workpiece, making proper support of the workpiece essential and often making machining both difficult and time consuming. Ensuring adequate workpiece support is especially important for elongated workpieces, such as those used when machining shafts for example. The turning machine must therefore include steady rollers or supports, which also help provide reaction forces against the applied forces of the cutting tool. Of course, the more flexible the part is, the more critical the need to support the rotating workpiece or part to prevent dimensional inaccuracies and vibrational chatter, which can eventually result in tool wear and material waste. For example, in the case of highly complex geometries machined into the shafts of gas turbine engines, rejected components due to inaccuracies can result in significant expense. The high strength and high temperature resistant materials of which the shafts are constructed make dimensional accuracy and machining productivity extremely important. Therefore it is desirable to create a highly accurate turning machine which will permit at least one of reduced deflection of workpieces, reduced dynamic vibrations, enhanced reliability, increased productivity, lower manufacturing costs and higher finished part quality. 
       SUMMARY 
       [0003]    In accordance with one aspect, there is provided a turning machine tool comprising: a machine frame having a longitudinal axis; a rotating spindle to which a workpiece is fastenable for rotating said workpiece about a workpiece axis parallel to the longitudinal axis, said rotating spindle being engaged with said machine frame; and three displaceable tool holders mounted on the frame, each tool holder rigidly supporting a cutting tool having a radial tool axis transverse to the workpiece axis, and each radial tool axis being disposed in a circumferential array spaced apart by about 120°, each of said tool holders being independently radially displaceable along said radial tool axis and longitudinally displaceable relative to said workpiece. 
         [0004]    There is further provided a method of turning an elongated workpiece using a turning machine tool, comprising: providing the turning machine tool with three cutting tools each defining a radially extending tool axis spaced apart from each other by about 120°; rotating said elongated workpiece about a longitudinal workpiece axis transverse to each said radially extending tool axis; radially displacing said cutting tools along said tool axis towards said longitudinal workpiece axis until cutting surfaces of said cutting tools engage said workpiece; and displacing said cutting tools relative to said workpiece in a direction parallel to said longitudinal workpiece axis, thereby turning said workpiece using said cutting tools engaged thereto at three equally spaced points circumferentially thereabout. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0005]    In order that the invention may be readily understood, an embodiment is illustrated in the accompanying drawings, in which: 
           [0006]      FIG. 1  is a isometric view of a three-point turning machine; 
           [0007]      FIG. 2  is a schematic sectional view of the rotating workpiece with orientation of three cutting tool bits about its periphery; 
           [0008]      FIG. 3  is an isometric view of a cutting tool unit of a three-point turning machine; 
           [0009]      FIG. 4  is a schematic representation of a chip section of a single tool lathe of the prior art; 
           [0010]      FIG. 5  is a schematic representation of chip sections of the turning machine of  FIG. 1 , with zero offset between the tool bits; 
           [0011]      FIG. 6  is a schematic representation of chip sections of the turning machine of  FIG. 1 , with a radial and axial offset between the tool bits; 
           [0012]      FIG. 7  is a chart showing sample offset setting from the three cutters; 
           [0013]      FIG. 8  is a schematic perspective view of a tool holder with radial and longitudinal actuators for displacing the tool bit relative to the supporting carriage of the turning machine of  FIG. 1 ; 
           [0014]      FIGS. 9-11  show a tool probing system which may be employed; 
           [0015]      FIGS. 12-14  show a tool probing calibration system which may be employed; 
           [0016]      FIGS. 15 and 16  show another tool probing system; 
           [0017]      FIGS. 17-19  show a part probing system which may be employed; and 
           [0018]      FIG. 20  is an isometric view of a cutting tool unit of another three-point turning machine 
       
    
    
       [0019]    Further details will be apparent from the detailed description included below. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]      FIG. 1  shows a turning machine, or lathe  10 , having a frame  12  with a longitudinal axis  11  in a horizontal position. The rotating spindle  14  includes a jaw portion, which grasps (i.e. rigidly engages) one end of the workpiece  15 , such that the workpiece  15  is rotated by the spindle  14  around the longitudinal axis  13  of the workpiece, which is horizontal and parallel to the longitudinal axis  11  of the machine frame. The rotating spindle  14  is driven, by suitable motor  16 . 
         [0021]    The opposed end of the elongated workpiece  15  is supported by a tailstock  09 . The tailstock  09  is mounted on the frame  12  of the machine and is preferably longitudinally displaceable relatively thereto in longitudinal direction  08 , moving along guide rails  19  disposed on the frame  12 . The tailstock  09  may be located on the frame  12  by an interlocking mechanism  07 . 
         [0022]    The turning machine  10  further includes a tool carriage  20 , which is a close frame structure generally like a hexagon shaped yoke in the embodiment depicted in  FIG. 1 . The tool carriage  20  is displaceably mounted to the frame base  12  such that the carriage  20  can longitudinally slide in direction  21  relative to the stationary frame  12  and therefore at least relative to the workpiece  15  supported thereby. The direction  21  is parallel to the frame and workpiece axis  11 ,  13 . 
         [0023]    The turning machine  10  is particularly adapted for turning elongated workpieces, such as shafts used in gas turbine engines for example. Such parts require a high level of precision and often have complex shapes. Further, such elongated shafts are often hollow and thus relatively flexible. Accordingly, stability of the workpiece is important, as is balancing machining forces of the cutters acting on the part. Accordingly, the turning machine  10  includes three cutting tools, which are equally circumferentially spaced about the workpiece  15  (i.e. spaced 120 degree apart), as will be discussed in further detail bellow. 
         [0024]    The turning machine  10  of  FIG. 1  includes three cutting units  30  mounted to the carriage  20 , each activating a cutting tool (cutter)  32  for machining the workpiece  15 . The three cutters  32  are equally distributed circumferentially about the rotating workpiece  15 , and are therefore spaced 120 degree apart from each other as shown in  FIG. 2 . 
         [0025]    Preferably, each cutting tool unit  30  is also independently displaceable in the radial direction  23 , relative to the working axis  13 , ( FIG. 3 ). As best seen in  FIG. 8 , each tool unit  30  having cutting tool  32  fixed thereto is displaceable in the radial direction  23  with respect to the carriage  20  on rails  40  by a radial actuator  42  and in a axial direction  26  by a longitudinal actuator  44 , which translates the cutting tool  32  via cutter slide  25  on rails  45  disposed on the radial translating body  46  of the cutting unit assembly. To cut different diameters, the cutting units  30  move the cutters  32  along the axis  33  in a radial direction  23  with respect to the workpiece  15 , in such a way that all these three axis  33  (one of each cutting unit) intersect in a single point  22  which belong to the workpiece axis  13  (see  FIG. 2 ). This movement allows adjusting the radial position of each cutter  32  in the  23  directions in order to control the cutters radial position with respect to the workpiece axis  13 , thus, the part diameter. 
         [0026]    The cutters  32  are mounted on the cutting units  30  via a tool holder  24 , (see  FIG. 3 ), which is held by a locking mechanism on a cutter slide  25 . The said cutter slide  25  can move with respect to the cutting units  30  in the axial direction  26 , which is parallel to the workpiece axis  13 . This movement allows adjusting the axial position of the cutter  32  in order to control this relative axial position. Thus, the direction  26  is an axial cutter adjustment direction, allowing adjusting the axial offset between the three cutters. 
         [0027]    As per all lathes and other turning machines, in order to generate any surface of revolution, the workpiece executes a rotary movement  17  and the cutters execute the adjustment and the feed movements depending on the surface shape. 
         [0028]    For example, a cylindrical surface may be generated as follows: a) cutter axial adjustment (on  26  axis)—for zero axial offset between three cutters; b) cutter radial adjustment  23  (on  33  axis)—for the required part diameter; c) workpiece rotation  17  (around  13  axis)—to create the cutting speed [SFM]; and d) carriage  20  axial movement  21 —to generate the axial cutters feed [IPM]. 
         [0029]    In another example, a face normal to the part axis may be generated as follows: a) cutter axial adjustment (on  26  axis)—for zero axial offset between three cutters; b) axial carriage  20  positioning on  21  movement—for the required part length; c) workpiece rotation  17  (around  13  axis), to create the cutting speed [SFM]; d) carriage  20  axial movement  21 , to generate the axial cutters feed [IPM]. 
         [0030]    In another example, a conical shape may be generated as follows: a) cutter axial adjustment (on  26  axis)—for zero axial offset between three cutters; b) workpiece rotation  17  (around  13  axis), to create the cutting speed [SFM]; c) carriage  20  axial movement  21  combined with cutter radial movement  23 , to generate the linear cutter feed having with the axial direction  13  the same angle like the part taper shape. 
         [0031]    In another example, a surface of revolution of any shape may be generated as follows: a) cutter axial adjustment (on  26  axis), for zero axial offset between three cutters; b) the right combination between the carriage axial movement  21  and the cutters radial movement  23 , to generate the right part profile in an axial section. 
         [0032]    As mentioned, three cutters are simultaneously used. Depending on the relative cutter adjustment (in axial  26  and radial  23  directions) different cutting conditions are possible. This is shown in  FIGS. 5 ,  6  &amp;  7  for a very simple example of machining of a cylindrical surface, as will now be described further. 
         [0033]    For three point turning, keeping the load of each cutter constant (the same chip section (a 1  &amp; b 1 ) as for the simple cutter), there are two different scenarios:
       a. Zero cutter offset: (a) Zero radial offset for which the cutters are adjusted to the same diameter (same distance on radial direction  23  with respect to the part axis  13 ); and (b) Zero axial offset for which the cutters are adjusted to be in the same transversal plane (normal to the part axis  13 ), which intersects each other in the same point  22  on the part axis  13 .   b. Radial and axial offset: (a) Radial cutter offset for the cutter # 2  and # 3  with the same depth of cut (DOC) like the depth of cut of the single cutter (DOC 1 ); and (b) Axial cutter offset which depends of the DOC and the cutter geometry, more specifically, to the attack angle “κ” (ΔAx=a 1 /tg κ, (see  FIG. 7 ). For κ=90°, the axial cutter offset becomes zero.       
 
         [0036]    For zero cutter offset (for same cutter load) the productivity increases by a factor of 3 by increasing the feed (see  FIGS. 5 &amp; 7 ). The result is the reduction of the time per pass by a factor of 3, keeping the same number of passes. 
         [0037]    For radial and axial cutter offset, (for the same cutter load) the productivity increases by a factor of 3 by increasing the total depth of cut (DOC 3 =3*DOC 1 —see  FIGS. 6 &amp; 7 ). The result is the reduction of number of passes by a factor of 3, keeping the same time per pass. 
         [0038]    Depending on each application, the process planner can decide which strategy will be the best, and adopt the machining program to this best strategy. 
         [0039]    The movement of the carriage  20 , the cutting units  30 , the cutter slides  25  and all other parts of the turning machine  10  can be suitably controlled using numerical control system such as CNC type control system known in art for machine tools, and will as such not be described in further detail herein. 
         [0040]    By a good and accurate control of the cutters position (in axial and radial direction) it is possible to obtain a good dimensional control on the part diameter, length and profile (for complex shapes). 
         [0041]    By controlling the cutter offset (in axial and radial position), the load of each cutter is under control, so the radial component of the cutting force F Ri  (see  FIG. 2 ). To avoid the part deflection, the radial cutting force F Ri  of all three cutters have to be balanced (equals and at 120°). 
         [0042]    Accordingly, the turning machine  10  can be used to produce complex shaped surfaces on relatively flexible parts, especially elongated ones such as shafts, by balancing tool loading to reduce deflection of the workpiece and thereby enhance dimensional accuracy. Preferably, in order to control the radial cutting force F Ri  and to balance the loading imposed by the cutters  32  on the workpiece, each cutting unit  30 , includes a separate load sensor  48  which is operable to measure the load on the cutting tool  32  (see  FIG. 8 ). Each load sensor  48  communicates with a load balancing control system, which may be integrated within the numerical control system of the entire turning machine  10 , that controls the actuator  42  or  44  of the cutter slide  30  and  25  for radial and/or longitudinal displacement thereof. Therefore, radial loads applied by the cutters  32  can be measured and balanced accordingly by displacing the exact position of the cutting tools relative to the workpiece in order to be adequately balanced. 
         [0043]    The turning machine may further include additional features, such as a tool changing system, a part probing system and/or a tool probing and tool length compensation system. 
         [0044]    As schematically depicted in  FIGS. 9 to 11 , the machine  10  may be equipped with a tool probing system  40 , comprising for each cutting unit a tool probe  41  ( 41 L,  41 V &amp;  41 R— FIG. 11 ) capable to measure the actual tool length (see  FIG. 9 ) by touching the tool probe in the radial direction  42  ( 42 L,  42 V &amp;  42 R), and the actual axial tool position (see  FIG. 10 ) by touching the tool probe in the axial direction  43  ( 43 L,  43 V,  43 R). 
         [0045]    The actual length and axial position of each tool is communicated to the controller and automatic compensation will be applied for each of the three cutting units. 
         [0046]    The tool probing system may include a tool probing calibration system (see  FIGS. 12-14 ), which comprises a calibration tool  44  ( 44 L,  44 V,  44 R) for each unit installed directly into the spindle  59  of each cutting unit by the tool changing system of the machine (as any tool assembly of the tool magazine). This calibration tool  44  comprises a tool holder  56 , a bar  57  held into the tool holder  56  and ending in the opposite direction by a ball  58  ( FIG. 12 ). By touching the tool probe  41  in radial direction  45  ( 45 L,  45 V,  45 R) and in axial direction  46  ( 46 L,  46 V,  46 R), the system is calibrated. 
         [0047]    In order to protect the tool probe  41  during machining, this may be installed into a box  49  having a cover  50  which is closed during machining and is opened during the tool probing and calibration, giving access to touch the tool probe  41  by the cutting tool  32  or by the calibration arm  44 . 
         [0048]    The tool probe  44  also may be protected by installing it into a cavity in the machine body  51 , which may be covered by a cover  52  (see  FIG. 15 ). During machining, the tool probe is retracted in position SB ( FIG. 15 ) by the movement  53  and the cover  52  is closed. During the tool measurement, the cover is opened and the tool probe is out of the body cavity in the measurement position “M” ( FIG. 16 ) by the movement  53  in the opposite direction. 
         [0049]    As schematically depicted in  FIGS. 17-19 , the machine  10  may be equipped with a part probing system  60 , comprising for each cutting unit or just for one of them a part probe  61  capable to measure the part diameters by touching the part outside surface approaching by the radial movement  65  or the part axial lengths (distance between different part faces) by touching the part faces approaching by an axial movement  66 . 
         [0050]    The three part probes (one for each cutting unit) increases the accuracy of diameter measurement and in the fact that this makes possible the measurement of the position of the part in different sections—relative run out and concentricity (eccentricity). 
         [0051]    In order to calibrate the part probing system, calibration block gauges  62  are installed on the machine body  51 , in an accurate position. The calibration is performed by probing these block gauges in radial direction  63  and in axial direction  64  (see  FIG. 19 ). 
         [0052]    The turning machine  10  further may have one (or more) of the cutting units  30  modified into a turn-mill unit  70 , equipped with a motor  71  and the mechanisms capable to activate a milling or drilling cutter  72  (see  FIG. 20 ). This configuration allows the same set-up to be machine different part features (such slots, holes, etc.) by milling or by drilling. This unit may be designed to tilt with respect to the part axis, in order to produce holes or slots at a certain angle. 
         [0053]    The turning machine of the present invention can be used to produce highly complex shapes, particularly on elongated workpieces which are prone to deflection, such as flexible shafts or other parts which are usually difficult to manufacture due to deflection of the part during machining. As the cutting bits which simultaneously cut the workpiece, are disposed 120° apart from each other, the cutting forces on the workpiece can be balanced, thereby substantially preventing any force unbalance which may cause the workpiece to deflect during the machining process, negatively effecting productivity and cost as well as dimensional accuracy. The elimination of the deflection of elongated workpieces prevents productivity losses which are otherwise caused by traditional turning machines of the prior art, due to the need to reduce the depth of cut or reduce feed or speed to eliminate tool chattering and vibration using such prior art turning machines. Chattering also affects the tools life due to the loads imposed by impact and vibration. The turning machine of the present invention therefore provides higher accuracy due to load balancing between the three spaced apart tools bits, and use of three tool bits together simultaneously increases the productivity of the machining operations. The three tools bits can be displaced both radially and longitudinally substantially independently for more flexibility and accuracy during machining. 
         [0054]    Although the above description relates to a specific preferred embodiment, it will be understood that the invention in its broad aspect includes mechanical and functional equivalents of the elements described herein.