Abstract:
A machine for machining a workpiece having a first central longitudinal axis passing through a workpiece plane that is disposed orthogonally relative to the first central longitudinal axis is provided. The machine includes a chuck or fixture on which the workpiece is disposable, a grinding spindle having a body, a wheel supporting an abrasive and an insulator electrically isolating the wheel from the body, the wheel being operable to remove material from the workpiece, the grinding spindle having a second central longitudinal axis about which the grinding spindle rotates, the second central longitudinal axis of the grinding spindle passing through the workpiece in the workpiece plane so as to create a continuous gear tooth on the workpiece and an electrochemical grinding (ECG) element configured to execute ECG processing on the grinding spindle and the workpiece to soften the workpiece as the gear tooth is being created by the grinding spindle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 14/158,097, filed on Jan. 17, 2014, the contents of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to a machine and a machining method and, more particularly, to a machine for machining or grinding gear teeth and to a gear teeth grinding method. 
         [0003]    Gears are used in various industrial and technological applications to permit power transmission from one rotating or translating element to another. Each gear generally includes an array of gear teeth that mesh with the gear teeth of another gear so that the rotation or translation of the first gear can be transmitted to the second. The shapes of the gear teeth can be varied with some gear teeth being linearly shaped, some being helically shaped and others being provided as double-helical or herringbone shaped, and still others being provided as arcuate shaped (or C-Gear) gear teeth. 
         [0004]    Gears having gear teeth that are double helically (or herringbone) shaped include a side-to-side (not face to face) combination of two helical gears of opposite hands and, from a top-wise viewpoint, the helical grooves form a V formation with an apex in the middle. Whereas helical gears tend to produce axial loading, a side-thrust of one half of each gear is balanced by that of the other half. This means that gears having double helical or herringbone shaped gear teeth can be used in torque gearboxes without requiring a substantial thrust bearing. Gears having arcuate shaped teeth may also have self-aligning characteristics, which eliminate axial loads with the added benefit of reducing gear tooth end loading due to their inherent ability to adapt to axis misalignment. 
         [0005]    However, while these shape gears are desired, due to manufacturing limitations, such gears can only be partially formed. Specifically, current manufacturing techniques use a large grinding wheel which forces a gap to be designed at the apex of the V formation since, when forming one tooth of the V formation, the grinding wheel would otherwise collide with the other tooth of the V formation. Thus, when using a grinding wheel, a true V formation is not formed since a space is required between adjacent teeth to allow for the size of grinding wheel. Further, as these wheels only provide straight line grooves, the resulting teeth are limited to linear shapes. Conversely, while non-wheel precision grinding shapes might allow more complex shapes such as curved lines, these non-wheel shapes do not allow for teeth production at a speed to be economical to create gears in a manufacturing setting. As such, there is a need for a grinding methodology which allows for the creation of gapless double helical/herringbone gear shapes and is sufficiently robust to be used in a manufacturing setting. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    According to one aspect of the invention, a machine for machining a workpiece having a first central longitudinal axis passing through a workpiece plane. The workpiece plane is disposed orthogonally relative to the first central longitudinal axis is provided. The machine includes a chuck or fixture on which the workpiece is disposable, a grinding spindle having a body, a wheel supporting an abrasive and an insulator electrically isolating the wheel from the body, the wheel being operable to remove material from the workpiece, the grinding spindle having a second central longitudinal axis about which the grinding spindle rotates, the second central longitudinal axis of the grinding spindle passing through the workpiece in the workpiece plane so as to create a continuous gear tooth on the workpiece and an electrochemical grinding (ECG) element configured to execute ECG processing on the grinding spindle and the workpiece to soften the workpiece as the gear tooth is being created by the grinding spindle. 
         [0007]    According to another aspect of the invention, a gear including at least one of apex gap-less double-helical shaped teeth, apex gap-less herringbone shaped teeth and c-shaped teeth is provided and is machined by a process. The process includes disposing a workpiece having a first central longitudinal axis passing through a workpiece plane disposed orthogonally relative to the first central longitudinal axis on a chuck, disposing a grinding spindle having a second central longitudinal axis such that the second central longitudinal axis of the grinding spindle passes through the workpiece in the workpiece plane, executing ECG processing on the grinding spindle and the workpiece and using the grinding spindle to remove material from the workpiece to form the at least one of the apex gap-less double-helical shaped teeth, apex gap-less herringbone shaped teeth and c-shaped teeth. 
         [0008]    According to yet another aspect of the invention, a method of machining a gear is provided and includes disposing a workpiece having a first central longitudinal axis passing through a workpiece plane disposed orthogonally relative to the first central longitudinal axis on a chuck, disposing a grinding spindle having a second central longitudinal axis about which the grinding spindle rotates, the second central longitudinal axis of the grinding spindle passing through the workpiece in the workpiece plane, executing ECG processing on the grinding spindle and the workpiece to soften an area of the workpiece and using the grinding spindle to remove material from the area of the workpiece to create a continuous gear tooth having one of apex gap-less double-helical shaped gear teeth, apex gap-less herringbone shaped gear teeth and c-shaped gear teeth. 
         [0009]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0011]      FIG. 1  is a circumferential view of a double helical gear with an apex gap; 
           [0012]      FIG. 2  is a schematic illustration of a machine for machining a gear in accordance with embodiments; 
           [0013]      FIG. 3  is a perspective view of a machine for machining a gear in accordance with embodiments; 
           [0014]      FIG. 4  is an enlarged perspective view of a component for machining a gear in accordance with embodiments; 
           [0015]      FIG. 5  is an enlarged perspective view of a component for machining a gear in accordance with alternative embodiments; 
           [0016]      FIG. 6  is a circumferential view of an apex gap-less double helical gear (or an apex gap-less or herringbone gear) in accordance with embodiments; 
           [0017]      FIG. 7  is a perspective view of the apex gap-less double helical gear of  FIG. 6 ; 
           [0018]      FIG. 8  is an enlarged version of a portion of  FIG. 6 ; 
           [0019]      FIG. 9  is a circumferential view of an apex gap-less c-shaped gear in accordance with embodiments; 
           [0020]      FIG. 10  is a flow diagram illustrating a method of machining gear teeth; and 
           [0021]      FIG. 11  is a perspective view of the machine for machining a gear in accordance with another aspect of an exemplary embodiment. 
       
    
    
       [0022]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    In helicopter transmission design, transmission weight reduction is of considerable importance. Thus, since the gears inside a transmission are normally the heaviest components in a drive system, reducing gear size and numbers of gears can be useful in reducing transmission weight and volume. As will be described below, gear size reductions can be achieved by eliminating extraneous gear features, such as apex regions in a double helical (or herringbone) gear. Normally, such extraneous gear features are forced into use by manufacturing limitations. 
         [0024]    With reference to  FIG. 1 , a conventional double helical gear  1  is provided. The conventional double helical gear  1  includes a first side  2  having a helical gear pattern of a first hand, a second side  3  having a helical gear pattern of a second hand opposite the first hand and an apex gap  4  defined axially between the first and second sides. The double helical gear  1  has a relatively high gear contact ratio owing to the presence of the helical gear patterns of the first and second sides  2  and  3 . As a result, a gear mesh of the double helical gear exhibits increased strength and reduced noise signature as compared to that of a straight spur gear. The apex gap  4  is formed as a result of processes used to shape and precision grind the gear flanks and roots of the helical gear teeth. The apex gap  4  may add a considerable weight and size penalty to an overall transmission system in which the double helical gear  1  resides. 
         [0025]    As will be described below, a gear grinding machine is provided and incorporates the use of a high speed grinding spindle with its center axis intersecting a center axis of the gear. Electrochemical grinding (ECG), and super abrasives, such as cubic boron nitride (CBN), may be utilized in a creep feed, deep cut, grinding process allowing for almost any conceivable gear flank design. The gear grinding machine produces hyper smooth ground surfaces of less than 1 micro inch Ra, burr free edges, with low heat generation and has the ability to grind exotic high hardness conductive materials. ECG allows for a very small grinding wheel with extremely low tool wear. 
         [0026]    With reference to  FIGS. 2 and 3 , a machine  10  is provided for machining a workpiece  11 . The workpiece  11  may have a substantially cylindrical initial shape with a first central longitudinal axis  110 . The machine  10  includes a chuck or fixture  20  on which the workpiece  11  is disposable, and a grinding spindle  30 . The grinding spindle  30  is configured to remove material from the workpiece  11  and has an elongate shape with a second central longitudinal axis  300 . The grinding spindle  30  is disposable relative to the chuck  20  and the workpiece  11  such that the first and second central longitudinal axes  110  and  300  may or may not intersect one another. The machine  10  further includes an electrochemical grinding (ECG) element  40 , which is configured to execute ECG processing on the grinding spindle  30  and the workpiece  11 . 
         [0027]    As shown in  FIG. 2 , the grinding spindle  30  may include a wheel  31 , a spindle body  32  and an insulator  33 . The wheel  31  is disposed to be rotatable about the second central longitudinal axis  300  of the grinding spindle  30  and includes a main wheel portion  310 , which extends axially outwardly from an end of the spindle body  32 , and a tip  311  defined at a distal end  312  of the main wheel portion  310 . Abrasive  34  may be attached to the tip  311 . The spindle body  32  is disposed to drive rotation of the wheel  31  about the central longitudinal axis  300  of the grinding spindle  30  and the insulator  33  is disposed to electrically insulate the wheel  31  from the spindle body  32 . 
         [0028]    In accordance with embodiments, the abrasive  34  may include a super abrasive, such as cubic boron nitride (CBN), diamond, etc. In addition, the tip  311  may be pencil-shaped or substantially conical and may have an involute profile  313 . That is, an outer surface of the tip  311  may curve inwardly from an edge of the main wheel portion  310  with a radius of curvature that decreases with increasing axial distance from the edge of the main wheel portion  310 . At the axial end of the tip  311 , the radius of curvature may flip direction such that the end-most portion of the tip  311  has a blunt, rounded surface. 
         [0029]    The ECG element  40  includes a first electrical lead  41 , a second electrical lead  42  and a dispenser  43 . The first electrical lead  41  is configured to positively charge the workpiece (anode)  11 , the second electrical lead  42  is configured to negatively charge the grinding spindle (cathode)  30  and the dispenser  43  is configured to dispense electrolytic fluid  430  toward the workpiece  11 . The opposite electrical charging of the workpiece  11  and the grinding spindle  30  in combination with the dispensation of the electrolytic fluid  430  toward the workpiece  11  causes a material of the workpiece  11  to soften by a substantial degree. This softening permits the grinding spindle  30  to remove material from the workpiece  11  in various forms or configurations. In some cases, the softening facilitates removal of material from the workpiece  11  by the grinding spindle to a desired depth in only a single pass and more rapidly than could be done without the softening. 
         [0030]    The machine  10  further includes a machine body  50  and a controller  51 . The machine body  50  may be provided, for example, as one or more support structures  500  and robotic arms  501  that are coupled to the chuck  10 , the grinding spindle  30  and the ECG element  40  to position the various elements with respect to one another for grinding internal or external gears. The controller  51  may be provided as a computer numerical control (CNC) element. Where the controller  51  is provided as the CNC element, the machine body  50  is formed to define four axes (e.g., rotational axis B and spatial axes X, Y, Z, as shown in  FIG. 3 ) and is capable of performing multi-axis synchronous motion. The axes may include the rotary axis B for indexing the workpiece  11 , the vertical axis Y running parallel to the first central longitudinal axis  110  of the workpiece  11  (i.e., a cutter path), the horizontal axis X for centrality adjustments between the wheel  31  of the grinding spindle  30  and the workpiece  11  and the fore and aft axis Z to control a cutting depth of the grinding spindle  30 . In accordance with the embodiments, the ECG element  40  may be integral the machine body  50  and the controller  51 . 
         [0031]    With reference to  FIGS. 4 and 5 , it is to be understood that the machine  10  can be employed to machine a gear with outwardly facing gear teeth (see  FIGS. 3 and 4 ) or inwardly facing gear teeth (see  FIG. 5 ). In the latter case, as shown in  FIG. 5 , the robotic arms  501  may include a hook structure  502 . The hook structure  502  extends forwardly along the fore and aft axis Z (see  FIG. 3 ) from the robotic arm  501 , downwardly along the vertical axis Y (see  FIG. 3 ) and then reversely along the fore and aft axis Z. The grinding spindle  30  is disposed at the distal end of the hook structure  502 . 
         [0032]    With the machine  10  provided as described above, the workpiece  11  may be ground or cut by the grinding spindle  30  in various forms and configurations. For example, the grinding spindle  30  may provide the workpiece  11  with gear teeth in one or more of an apex gap-less double-helical shape (or an apex gap-less herringbone shape) and a c-shape. 
         [0033]    An example of a gear  60  that can be formed by the machine  10  to have gear teeth in an apex gap-less double-helical shaped formation is shown in  FIGS. 6, 7 and 8 . The gear  60  includes a body  61  defining a central longitudinal axis that would be aligned with the second central longitudinal axis  300 , first and second opposite axial faces  62 ,  63  and a circumferential face  64 . The circumferential face  64  is formed by the machine  10  and includes a first annular array  65  of helical gear teeth  651  and helical gear lands  652  of a first hand and a second annular array  66  of helical gear teeth  661  and helical gear lands  662  of a second hand, which is oppositely oriented with respect to the first hand. The first and second annular arrays  65  and  66  converge such that each helical gear tooth  651  abuts a corresponding helical gear tooth  661  and each helical gear land  652  abuts a corresponding helical gear land  662 . 
         [0034]    As shown in  FIGS. 6, 7 and 8 , the abutment of each helical gear tooth  651  with the corresponding helical gear tooth  661  and of each helical gear land  652  with the corresponding helical gear land  662  may be achieved with little to no apex region defined between the first and second annular arrays  65  and  66  and without an interruption in the respective shapes of the helical gear teeth  651 ,  661  or the helical gear lands  652 ,  662  in the region of the abutment (see  FIGS. 6 and 8 ). Also, a shape of the helical gear teeth  651 ,  661  may be reflective of the tip  311  of the grinding spindle  30  (see  FIG. 7 ). In addition, it is to be understood that although the gear  60  is illustrated in  FIGS. 6 and 7  with the region of the abutment being linear and axially centered, other embodiments exist. For example, the region of the abutment may be offset from an axial center of the gear  60  and the abutment itself may be staggered relative to the axial center of the gear  60 . 
         [0035]    With reference to  FIG. 9 , another example of a gear  70  that can be formed by the machine  10  is provided. The gear  70  has gear teeth in an apex gap-less c-shaped formation. The gear  70  includes a body  71  defining a central longitudinal axis that would be aligned with the second central longitudinal axis  300 , first and second opposite axial faces  72 ,  73  and a circumferential face  74 . The circumferential face  74  is formed by the machine  10  and includes an annular array  75  of c-shaped gear teeth  751  and c-shaped gear lands  752 . Each c-shaped gear tooth  751  has an arcuate face with nearly constant involute transverse profiles corresponding to the shape of the tip  311  of the grinding spindle. In practice, this configuration would be expected to provide a nearly constant pressure angle across the length of the c-shaped gear tooth  751 . Like the gear  60 , the gear  70  would possess a self-alignment characteristic but with the added benefit of reducing gear tooth end loading due to their inherent ability to adapt to multiple axis misalignment. 
         [0036]    With reference to  FIG. 10 , a method of machining gear teeth such as the gear teeth described above is provided. As shown in  FIG. 10 , the method first includes a rough grinding of the gear teeth from a solid, such as a workpiece, using the ECG pencil grinding method described above (operation  100 ). Once the rough grinding is fully or partially completed, the method further includes a carburization and hardening of the workpiece (operation  101 ) and a finishing grind of the gear teeth using the ECG pencil grind method described above (operation  102 ). Of course, it will be understood that the ECG grinding method of operations  100  and  102  need not be limited to the ECG pencil grinding method and can be replaced by an ECG grinding method designed to form any tooth shape (e.g., a tooth shape that is reflective of the tip  311  of the grinding spindle  30  having an involute profile  313 ). 
         [0037]    In a case in which first longitudinal axis  110  and second longitudinal axis  300  do not intersect, second longitudinal axis  300  passes through workpiece  11  at a position that is off-sect from first longitudinal axis  110  such as shown in  FIG. 11 . For example, robotic arms  501  may be selectively shifted along the x-axis to a position that causes second axis  300  of spindle  11  to be off-set relative to first axis  110  of workpiece  11 . By shifting spindle  30  along the x-axis more complex gear tooth profiles may be achieved. The more complex gear profiles may provide desired operational enhancements to the gearbox. 
         [0038]    With the machine  10 , gearboxes for helicopters and other weight limited applications, may be designed with higher power densities. This is due to the fact that every pound of weight that is removed from a transmission design as a result of using the machine  10  to fashion gears with apex-less configurations translates into better performance characteristics. 
         [0039]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. By way of example, while described in the context of gearboxes used in power dense environments, aspects of the invention can be used to create intermeshing gears in other contexts, such as clock machinery, elevator machinery without limitation. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.