Abstract:
A multi-axis computer controlled machine tool for relief grinding of single and multiple start threaded hobs ( 46 ). The machine comprises an angular oriented spindle ( 65 ) that can be angularly (swivel) positioned thereby providing the flexibility to utilize both cup-shaped grinding wheels ( 55 ) and pencil-shaped grinding wheels ( 52 ) for grinding of hob and milling cutter relieved tooth geometry without exchanging grinding spindles assemblies or modifying the machine construction to accommodate two types of grinding methods or utilizing an additional machine axis for the spindle re-orientation.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to machine tools, such as tool grinding machines, and in particular to multi-axis, high precision, computer controlled machine tools for grinding of relieved tooth profiles of rotary cutting tools. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the grinding of tooth profiles of cutting tools, such as gear hobs, worm gear hobs, constant pitch normal base hobs, and milling cutters with module sizes greater that 8 module, a multitude of grinding wheels are use to complete the entire tooth profile on both flanks and tip features. As the module size increases from 8 to approximately 16, different approaches to grinding can be used depending on the desired finished tooth re-sharpening life and the width of the tooth space at the root of the profile. The tooth re-sharpening life or length of the cam relieved tooth surface can be maximized using pencil-shaped tapered cone grinding wheels having relatively small diameters in comparison to length. The relieved length of the tooth surface is dependent on the interference point of the grinding wheel with the next adjacent tooth when radial cam grinding the relief along the tooth helix of the hob thread or the index position in the case of a milling cutter. 
         [0003]    Dependent of geometry of the tooth space there is a practical surface speed limit for grinding and bending/shear strength weakness of the tip of the pencil-shaped wheel that must be considered. When approaching the pencil-shaped wheel grinding practical application limitations, a different process of grinding utilizing cup-shaped grinding wheels with relatively larger diameters compared to width can be employed. Surface speed of grinding and strength of the portion of the wheel grinding the root portion of the tooth are overcome. However, a disadvantage of cup-shaped grinding wheels is that their larger diameters limit the length of the cam relieved tooth surface to the interference point as described previously. As the module size increase above 16 module and up to 50 module, the use of a cup-shaped wheel is restricted because of the wheel to adjacent tooth interference described. In most cases, a pencil-shaped tapered cone grinding wheel must be used for module hob tooth sizes over 16 module in order to provide an adequate tooth relieved length for re-sharpening life. 
         [0004]    Current tool manufacturing practices use the blending of profiles produced from multiple grinding wheels and wheel shapes to grind the tooth flanks, tooth tip radii, and tooth tip outside diameter separately in multiple (e.g. up to five) setups. With CNC machine motion technology and rotary truing/dressing devices it is possible to contour the above mentioned pencil-shaped or cup-shaped grinding wheels to incorporate multiple features, for example, the tooth bottom radius/ramp, tooth pressure angle flank, tooth tip radius, and tooth tip outside diameter. In the example mentioned, the finish grinding process may be reduced to blending profiles of just two pencil-shaped or cup-shaped grinding wheels. Probing of profiles, utilizing probes such as the Renishaw 3-D with acoustical touch sensing, assists in the relative positioning of the grinding wheel to the left and right tooth reference points and blending of profiles to achieve the required tooth profiles and tooth thickness. 
         [0005]    Given the above, many tool manufacturing facilities employ a plurality of grinding machines dedicated to either pencil-shaped wheels or cup-shaped wheels. A few incorporate a machine tool design to allow the exchange of spindle assemblies and drive mechanisms to accommodate the physical orientation for either pencil-shaped wheels or cup-shaped wheels. Most require the use of dedicated machines with physical orientation as capable for only pencil-shaped wheel or only cup-shaped wheel grinding. In most if not all cases, the cam relief motion on the machine tools use one axis to provide the radial motion for the cam relief which limits the machine&#39;s flexibility. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is directed to a machine tool with an angular oriented spindle that can be angularly positioned (swivel) thereby providing the flexibility to utilize both cup-shaped and pencil-shaped grinding wheels for grinding of hob and milling cutter relieved tooth geometry without exchanging grinding spindles assemblies or modifying the machine construction to accommodate two types of grinding methods or utilizing an additional machine axis for the spindle re-orientation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1(   a ),  1 ( b ) and  1 ( c ) illustrate the lead setting angular relationships of a cup-shaped grinding wheel to a hob tooth profile of universal module size. 
           [0008]      FIGS. 2(   a ),  2 ( b ) and  2 ( c ) illustrate the lead setting angular relationships of a pencil-shaped grinding wheel to a hob tooth profile of universal module size. 
           [0009]      FIG. 3  shows the design planes relative to the radial cam relieving motion from the front to back profiles of the hob along the involute helicoids. 
           [0010]      FIG. 4  illustrates the change in pressure angle from front to back of the constant base pitch hob tooth profile. 
           [0011]      FIG. 5  illustrates the relationships and direction of cam relief for the machine major components, hob workpiece and grinding wheel spindle assembly orientated for using pencil-shaped grinding wheels. 
           [0012]      FIG. 6  illustrates the relationships and direction of cam relief for the machine major components, hob workpiece and grinding wheel spindle assembly orientated for using cup wheels 
           [0013]      FIG. 7  is an enlarged view of the inventive grinding spindle assembly in its operative position in a machine. 
           [0014]      FIG. 8  is an enlarged view of an automatic wheel exchange unit attached to the machine base for exchanging grinding wheel packs and coolant manifolds between the grinding spindle mounting interface and storage stations. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    Before any features and at least one construction of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other constructions and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purposes of description and should not be regarded as limiting. 
         [0016]    The use of “including”, “having” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The use of letters to identify elements of a method or process is simply for identification and is not meant to indicate that the elements should be performed in a particular order. 
         [0017]    Although references may be made below to directions such as upper, lower, upward, downward, rearward, bottom, top, front, rear, etc., in describing the drawings, there references are made relative to the drawings (as normally viewed) for convenience. These directions are not intended to be taken literally or limit the present invention in any form. In addition, terms such as “first”, “second”, “third”, etc., are used to herein for purposes of description and are not intended to indicate or imply importance or significance. 
         [0018]      FIGS. 1(   a ),  1 ( b ) and  1 ( c ) show the angular relationships of a grinding wheel in a coordinate system for a cup-shaped grinding wheel  1  in contact along a involute helicoid  2  of one of a sequential series of hob teeth  3  along a thread path  4  with single or multiple starts with a front profile  5  at the cutting face defined by the intersection of the thread involute helicoid  2  and a straight or helical flute  6  and with a relieved back profile  7  near the end of the sharpening life of the tooth position that is restricted in length by the interference point  8  of the grinding wheel  1  with the adjacent hob tooth  9  along the involute helicoid  2 . Relationships defined in  FIGS. 1(   a ),  1 ( b ) and  1 ( c ) are the fixed angle HA (angle  10 ) of the grinding spindle assembly, the grinding wheel pressure angle PA (angle  11 ), and the axial pressure angle APA (angle  12 ) of the hob tooth, the lead angles LAS 1  and LAS 2  of the involute helicoids  13  as viewed from the front, and the swivel setting angle setting  14  of the grinding head assembly as viewed from the front, and the relieving direction  15  relative to the coordinate system. 
         [0019]      FIGS. 2(   a ),  2 ( b ) and  2 ( c ) show the angular relationships of the grinding wheel in a coordinate system for a pencil-shaped or cone-shaped grinding wheel  16  in contact along a involute helicoid  17  of one of a sequential series of hob teeth  18  along a thread path  19  with single or multiple starts with a front profile  20  at the cutting face defined by the intersection of the thread involute helicoid  17  and a straight or helical flute  21  and with a relieved back profile  22  near the end of the sharpening life of the tooth position that is restricted in length by the interference point  23  of the grinding wheel  16  with the adjacent hob tooth  24  along the involute helicoid  17 . Relationships defined in the  FIGS. 2(   a ),  2 ( b ) and  2 ( c ) are the fixed angle  25  of the grinding spindle assembly, angle B (angle  26 ) which is difference between pressure angle PA trued/dressed on the grinding wheel and the axial pressure angle APA (angle  27 ) of the hob tooth, the lead angles LAS 1  and LAS 2  of the involute helicoids  28  as viewed from the front, and the swivel setting angle setting  29  of the grinding head assembly as viewed from the front, and the relieving direction  30  relative to the coordinate system. 
         [0020]      FIG. 3  defines the design planes used for optimizing the grinding wheel geometry to reduce deviation from the theoretical tooth profiles with or without involute modifications and the inherent error of the hob tooth grinding wheel to tooth pressure angle conjugant contact points. Design planes are defined at the front  31 , midpoint of cam relief  32  and back  33  sections. Also shown are the related nominal hob outside radius  34 , nominal hob midpoint radius  35 , and end of sharpening life nominal hob radius  36 . 
         [0021]      FIG. 4  shows the characteristic error for a hob tooth of an involute profile produced by ordinary machine tool manufacturing methods. Theoretically, a new and different hob is produced every time the hob is sharpened from front to back. A sharpened hob will cut a gear only to the approximate form as the new hob. In addition, the more the hob is sharpened back, the worse the error becomes.  FIG. 4  shows two axial sections of the hob tooth involute helicoid profile, without involute modifications for the illustration, representing the theoretically correct front position of a new hob  37  and the sharpened back position  38  near the end of the sharpening life along the tooth involute helicoid. The inherent error  39  (error A and error B) occurs when a grinding wheel or form tool, which by nature has a fixed geometric profile, attempts to contact this helicoid surface at both sections  37  and  38  when driven in a fixed path by the machines relieving motions. 
         [0022]    The recognition of the inherent error problem discussed above and attempts to solve are known from “Buckingham, Earl,  Spur Gears , McGraw-Hill Book Co. Inc., NY, 1928”, wherein it is discussed relieving hob tooth surfaces utilizing special characteristic of the involute helicoids. With the disclosed method, contact between the relieved surface and the grinding wheel will be the straight line generatrix of the involute helicoids and the relieved surface itself will be an involute helicoid. However this method makes no provision for modifications such as semi-topping ramps, involute modifications, and protuberance. For fine pitch hobs the inherent error is negligible and for coarse pitch hobs of low quality the magnitude of the error is unimportant. For accurate hobs the inherent error point of can be identified with the following equation: 
         [0000]    
       
         
           
             
               
                 
                   Q 
                   = 
                   
                     
                       ( 
                       
                         Hob 
                          
                         
                             
                         
                          
                         Outside 
                          
                         
                             
                         
                          
                         
                           Diameter 
                            
                           
                             ( 
                             mm 
                             ) 
                           
                         
                       
                       ) 
                     
                     
                       
                         ( 
                         
                           Normal 
                            
                           
                               
                           
                            
                           
                             Module 
                              
                             
                               ( 
                               mm 
                               ) 
                             
                           
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           Number 
                            
                           
                               
                           
                            
                           of 
                            
                           
                               
                           
                            
                           Hob 
                            
                           
                               
                           
                            
                           Threads 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0023]    When the Q factor is greater than 20, it marks the approximate region where hobs with a straight-line axial profile will cut gears which, for all practical purposes, have true involute profiles. As the Q factor decreases, the hob tooth axial profile becomes more curved and the potential for inherent error increases. 
         [0024]    Computer analysis of the grinding wheel contact pattern with modified tooth involute helicoid, setup angles and offset of the grinding wheel can optimize the grinding process to reduce the inherent error but can not eliminate it. Corrective machine motions to change the pressure angle relations during the relieving process can minimize the inherent error to acceptable level to provide a more accurate, longer life, hobs. The grinding wheel head assembly  40  ( FIG. 6 ) of this invention can provide the grinding wheel oscillating motion synchronized with the relieving motion to minimize the inherent error. 
         [0025]    The CNC Controller (such as a Fanuc 160iB computer control) of the inventive grinding machine is operable to provide the radial or offset radial cam relieving motion either from a single axis moving in a horizontal plane when the grinding head assembly is positioned about a horizontal plane for using cup wheels for grinding, or at a compound angle about the vertical plane using multiple axes to provide the cam relieving motion when the grinding head assembly is positioned about a vertical plane for using pencil wheels for grinding. In addition, the grinding head assembly is capable of imparting an oscillating motion to the grinding wheel superimposed on the radial or offset radial cam relieving motion which acts to change the swivel orientation of the grinding wheel relative to the pressure angle of the hob tooth profile thereby reducing the tooth pressure angle at the back of the hob tooth relative to the front and midpoint of the relieved tooth profile. This motion makes it possible to manufacture constant normal base pitch hobs which could not be produced by hob grinding prior to the present invention. 
         [0026]      FIG. 5  illustrates the inventive machine having a table  41 , direct motor driven head stock  42 , live center tailstock  43 , steady rests  44  to assist loading arbors assemblies, work holding arbor  45  ( FIG. 6 ), hob work piece  46 , linear motor driven drive longitudinal axial slide assembly  47 , linear motor driven vertical slide assembly  48 , linear motor driven horizontal radial in-feed slide assembly  49 , grinding spindle swivel assembly  50  orientated for pencil wheel grinding, the slanted (preferably 25 degrees) grinding spindle housing  51  with spindle  65 , and pencil grinding wheel  52  with spindle mounted adapter  66 . Vector  53  illustrates the relative compound angle radial relieving motion (i.e. cam relieving motion). Five axes synchronization is needed for the head stock  42  and the longitudinal axial slide  47  to generate the hob thread lead, the vertical slide assembly  48  and horizontal slide assembly  49  combined to generate the cam relieving motion  53 , and the optional oscillation of the grinding wheel swivel assembly  50  superimposed on the cam reliving motion  53  to minimize the inherent pressure angle profile errors. 
         [0027]      FIG. 6  illustrates machine table  41 , direct motor driven head stock  42 , live center tailstock  43 , the steady rests  44  to assist loading arbors assemblies, work holding arbor  45 , hob work piece  46 , linear motor driven drive longitudinal axial slide assembly  47 , linear motor driven vertical slide assembly  48 , linear motor driven horizontal radial in-feed slide assembly  49 , grinding spindle swivel assembly  50  orientated for cup-shaped wheel grinding, slanted grinding spindle housing  51  (preferably 25 degrees) with spindle  65 , cup-shaped grinding wheel  55  with spindle mounted adapter  66 . Vector  56  illustrates the relative compound angle radial relieving motion (i.e. cam relieving motion). Three axes synchronization is needed to produce the cam thread relieving positioning the head stock  42  and the longitudinal axial slide  47  to generate the hob thread lead, the horizontal slide assembly  49  to generate the cam relieving motion  56 . The offset setting angle of the grinding wheel swivel assembly  50  and vertical axes  48  height are held at a fixed position during the cam relieving process during cup wheel grinding. 
         [0028]    The grinding wheel head assembly  40  mounted for vertical motion preferably comprises a variable speed high frequency grinding spindle and capable of being automatically swung through a vertical plane arc of at least plus or minus 120 degrees to a desired setup compound angle dependent on the profile of grinding wheel, thread lead of the cutter, and orientation of the grinding spindle. The grinding spindle housing  51  is orientated from a vertical swivel plane of the grinding wheel head  40  and in a fixed angular position, preferably 25 degrees from the vertical plane, with the grinding wheel position closer to the work piece and the direct drive motor of the spindle  65  away from the work piece. 
         [0029]    The preferred 25 degree orientation of the grinding spindle  65  provides additional clearance between the grinding spindle motor housing  51  and the outside diameter of the work piece when using cup shaped grinding wheels  55  ( FIG. 6 ), with grinding wheel head  40  positioned approximately plus or minus 30 degrees from a horizontal plane, and with relief grinding using a quick horizontal motion combined with rotary and longitudinal motions. The preferred 25 degree orientated grinding spindle can also facilitate pencil shaped grinding wheels  52  ( FIG. 5 ) with the grinding wheel head  40  positioned approximately plus or minus 40 degrees or thereabout from a vertical plane resulting in a relief grinding motion using both quick vertical and horizontal motions combined with rotary and longitudinal motions. In addition to the motions described, the grinding wheel head assembly can be oscillated approximately plus or minus 3½ degrees from the setup angular position during the relief motions to produce coarse module normal base pitch hobs which will have corrected tooth profile pressure angle at the back as well as at the front relieved hob tooth flank. While the fixed 25 degree orientated of the grinding spindle is preferred, the present invention is not limited thereto. Other angular orientations are contemplated and are within the scope of the present disclosure. 
         [0030]      FIG. 7  illustrates additional machine construction including a grinding wheel exchanger cabinet  57 , the grinding wheel swivel assembly  50  that includes the slanted grinding spindle  51 , a probe  58  (e.g. Renishaw 3D) for setup and inspection functions and a table mounted rotary dresser (wheel truing) assembly  59 . The automatic grinding of hobs and blending of multiple wheel profile preferably includes the in-cycle exchange of grinding wheel packs mounted on spindle adapters, determination of the rough grinding wheel locations relative to pitch points on respective tooth flanks of a tooth space by use of the probe  58 , use of acoustical touch sensing to verify the contact position of the grinding wheel profile to the hob existing tooth profile, contouring using the machine axes with acoustical touch sensing for generating grinding wheel profiles on a multitude of grinding wheel technologies (including super abrasives like CBN) with the use of the table mounted dresser  59 , measuring of ground profile for blending and error correction with the probing system  58 , and the automatic analysis and feedback for error correction of profiles. 
         [0031]      FIG. 8  illustrates an example of an automatic wheel exchanger  60  with multiple stations mounted on a rotary carousel  61  that stores wheel pack assembled to grinding spindle adapters (collectively  62 ) and associated coolant manifolds  63 . A programmable slide assembly  64  facilitates the simultaneous exchange of wheel packs  62  and coolant manifolds  63  between the grinding spindle  51  and the multiple stations on the carousel  61 . The automatic wheel and coolant manifold exchange device  60  (located in cabinet  57 ) is attached to the machine tool to facilitate complete automatic grinding of a hob or milling cutter. The automatic wheel changer device can stage a multitude of grinding wheels mounted on spindle adapters with associated coolant manifolds and automatically exchange the grinding wheel pack with coolant manifolds between the device and grinding spindle thus automatically providing the multitude of wheels needed to complete the entire hob or milling cutter tooth profile. The standard number of stations in the wheel changer device is preferably 8 but magazine storage devices can be interfaced to the wheel exchanger to increase the number of available wheel packs with coolant manifolds available or only limited by space restrictions. A probing system (e.g. Renishaw 3D) is incorporated to provide the tooth space setup positioning to multiple grinding wheels as well as post-grinding inspection with automatic profile path correctional feedback for truing/dressing. Measuring and reporting of ground profile quality according to current AGMA and DIN internationally accepted hob and milling cutter standards is also contemplated. 
         [0032]    The inventive machine is capable of grinding single and multiple start hobs preferably greater than 200 mm outside diameter and in the range of about 8 to about 50 module tooth sizes with tooth cutting faces defined by multiple straight or helical flutes (gashes). The machine can also relief grinding large milling cutters preferably greater than 200 mm with axial indexing of teeth and tooth cutting faces defined by multiple straight or helical flutes. The machine preferably incorporates programmable quick response direct and linear motor driven axes with precision glass scale position feedback in a defined combination of rotational, vertical, horizontal, longitudinal, and grinding head swivel motions to relief grind each tooth profile of a hob or milling cutter. Tapered or contoured outside diameter hobs and milling cutters may also be ground. Multiple grinding wheels may be required for grinding each tooth space to complete the left and right flanks, tip radii, and tip diameter. 
         [0033]    While the invention has been described with reference to preferred embodiments it is to be understood that the invention is not limited to the particulars thereof. The present invention is intended to include modifications which would be apparent to those skilled in the art to which the subject matter pertains without deviating from the spirit and scope of the appended claims.