Abstract:
A method includes selecting a first gear ratio and a second gear ratio. A first hypoid gear set defines the first gear ratio and a second hypoid gear set defines the second gear ratio. The first hypoid gear set includes a first ring gear that is formed with at least one first inside blade and at least one first outside blade coupled to a first gear cutter. The second gear set includes a second ring gear that is formed with at least one second inside blade and at least one second outside blade coupled to a second gear cutter. The method also includes identifying parameters of the first inside blade and the second inside blade, commonizing at least a portion of the respective identified parameters and forming at least one of a common inside blade and a common outside blade for forming a first modified ring gear and a second modified ring gear.

Description:
TECHNICAL FIELD 
     The disclosure generally relates to gear cutting and forming. 
     BACKGROUND 
     Hypoid gears are generally formed with a cutting machine that rotates both a cutter tool and a stock piece of metal while the axes of rotation of the cutter tool and the stock are orientated at an angle. The cutter tool and/or the stock are advanced toward one another, generally along the axes of rotation as the blades of the cutter tool shave material from the stock to form gear teeth on the stock. Some machines will vary the orientation of the cutter tool and/or stock perpendicular to the axis of rotation during cutting to form a desired hypoid tooth shape. Both pinion and ring gear of a hypoid gear set are cut in this manner. Typical cutting machines are disclosed in U.S. Pat. Nos. 5,116,173 to Goldricil, and 5,662,514 to Masseth, the disclosures of which are hereby incorporated by reference in their entireties. 
     Typically, a single cutter tool contains blades that are dimensioned to form a single gear for a single gear ratio (i.e. number of gear teeth/number of pinion teeth). That is, a cutter tool assembled with blades designed for cutting a ring gear with a gear ratio of 4.11 to 1 can not be used to cut a ring gear of a different gear ratio, and cannot be used to cut a pinion gear. In the example of a gear ratio of 4.11 to 1, a typical pinion for a vehicle differential has 9 teeth and the ring gear has 37 teeth. Many cutter tools may be dimensioned such that different blades may be used to form different gears of different gear ratios, but typically, the blades for forming the different gears are not common. That is, typically, a gear cutting tool includes a plurality of inside blades and a plurality of outside blades extending therefrom for forming the teeth of a hypoid gear. Typically, the inside blade forms the drive side of a hypoid ring gear tooth, and the outside blade forms the coast side of a hypoid ring gear tooth. 
     With continual development in blades, the life of a blade is extended due to, for example, tip coatings and blade materials and treatments. These developments permit blades to last longer and permit cutter tools to be used for longer periods of time between blade replacement. 
     SUMMARY 
     An illustrative embodiment includes a method includes selecting a first gear ratio and a second gear ratio. A first hypoid gear set defines the first gear ratio and a second hypoid gear set defines the second gear ratio. The first hypoid gear set includes a first ring gear that is formed with at least one first inside blade and at least one first outside blade coupled to a first gear cutter. The second gear set includes a second ring gear that is formed with at least one second inside blade and at least one second outside blade coupled to a second gear cutter. The method also includes identifying parameters of the first inside blade and the second inside blade, commonizing at least a portion of the respective identified parameters and forming at least one of a common inside blade and a common outside blade for forming a first modified ring gear and a second modified ring gear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, preferred illustrative embodiments are shown in detail. Although the drawings represent some embodiments, the drawings are not necessarily to scale and certain features may be exaggerated, removed, or partially sectioned to better illustrate and explain the present invention. Further, the embodiments set forth herein are not intended to be exhaustive or otherwise limit or restrict the claims to the precise forms and configurations shown in the drawings and disclosed in the following detailed description. 
         FIG. 1  is a schematic perspective view of a gear cutter system. 
         FIG. 2  is a perspective view of an exemplary gear cutter tool. 
         FIG. 3  is a side view of an exemplary inside blade. 
         FIG. 4  is a side view of an exemplary outside blade. 
         FIG. 5  is a partial sectional schematic view of a hypoid gear set. 
         FIG. 6  is an enlarged partial sectional view of a gear tooth. 
         FIG. 7  is a partial sectional view of a ring gear. 
         FIG. 8  is a partial perspective view of a gear. 
         FIG. 9  is a partial end view of a gear during forming. 
         FIG. 10  is a schematic view of the orientation of a cutting tool to a gear stock. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an embodiment of a gear cutting system  20 . The system  20  includes a tool support  22 , and a work support  24  supported by a base  26 . The tool support  22  includes a carriage  30 , a tool head  32 , and a tool spindle  34 , and a cutting tool, illustrated generally at  36 . The carriage  30  is moveable relative to the base  26  generally in the spatial direction X. The tool head  32  is moveable relative to the carriage  30  generally in the spatial direction Y. The tool spindle  34  may be moveable relative to the tool head  32  generally in the spatial direction Z while rotating the tool head  36 . Accordingly, the tool head  36  may be rotated in the rotational direction T and moved in any of the spatial directions X, Y, Z simultaneously. Further, these movements are controlled by a device such as a computer numeric control (CNC) machine that may be capable of very fine adjustments on the order of millionths of an inch. 
     The work support  24  includes a table  40 , a work head  42 , and a work spindle  44 . The work spindle  44  is illustrated with a gear stock  50  attached thereto. The work table  40  is moveable relative to the base  26  generally in the spatial direction Z. The work head  42  is moveable relative to the table  40  generally in the rotational direction B. The work spindle  44  is moveable relative to the work head  42  generally in the rotational direction W. 
     Accordingly, the system  20  may form at least hypoid teeth on the gear stock  50  as the cutting tool  36  is rotated relative to the gear stock  50 . Generally, the axes of the gear stock and the cutter tool do not intersect, as illustrated in  FIG. 1 . 
       FIG. 2  illustrates the cutting tool  36  to include a plurality of inside blades  60  and a plurality of outside blades  62 . While the tool  36  is illustrated with 11 pairs of blades  60 ,  62 , other suitable numbers of blade pairs, such as 17 pairs, may be used. Further, either the inside blade  60  or the outside blade  62  may be adjusted as necessary to form the desired gear tooth profile. 
       FIG. 3  generally illustrates an inside blade  60  to show the details for the discussion herein. The inside blade  60  includes a root end  70  and a tip end  72 . The inside blade  60  also includes an inside cutting blade pressure angle φcI, an inside blade distance BdI, and an inside blade point width WtI. 
       FIG. 4  generally illustrates an outside blade  62  to show the details for the discussion herein. The outside blade  62  includes a root end  80  and a tip end  82 . The outside blade  62  also includes an outside cutting blade pressure angle φcO, an outside blade distance BdO, and an outside blade point width WtO. 
       FIG. 5  illustrates a partial view of a hypoid gear set  90 . Hypoid gear set  90  includes a ring gear  92  and a pinion  94 . The ring gear  92  includes a plurality of ring gear teeth  98 . The pinion includes a plurality of pinion gear teeth  100  and is defined generally by a pinion outside gear radius PGR. 
       FIG. 6  illustrates an exemplary tooth profile of a pinion gear tooth  100  for a hypoid tooth of the pinion  94 . The pinion tooth has a coast side pressure angle, or first pressure angle, α c  and a drive side pressure angle, or second pressure angle α d  that are measured relative to the pitch plane of the pinion  94 . 
       FIGS. 7-9  illustrate portions of the ring gear  92 .  FIG. 7  illustrates the pitch angle Θp and the ring gear radius RGR.  FIG. 8  illustrates a toe  110 , a heel  112 , a top land  114 , a mean slot width Sw  116  a drive side  118  of each tooth  98 , and a coast side  120  of each tooth  98 .  FIG. 9  illustrates the spiral angle φs for an exemplary tooth  98 . 
       FIG. 10  illustrates an exemplary relative orientation of the tool  36  to the gear stock  50  during forming of an exemplary ring gear. 
     To form a first hypoid ring gear, the tool  36  is rotated relative to a gear stock as the blades  60 ,  62  cut into the gear stock. In the exemplary embodiment illustrated, the gear stock is also rotated and the system  20  will vary the spatial orientation of the tool  36  relative to the gear stock. The relative movement of the tool  36  to the gear stock during each cutting stroke ( FIG. 10 ) includes movements in the X and Y directions ( FIG. 1 ). Accordingly, the Cuts made in the gear stock are not defined solely by the rotation of the tool  36  and the rotation of the gear stock  50 , but also by the movements in the X and Y directions to form a desired tooth profile, such as the tooth profile illustrated schematically in  FIG. 8 . The speed of rotation of the tool  36  and the stock  50  may be several hundred or several thousand rotations per minute (rpm). 
     To form a second hypoid ring gear, the tool  36  is used while the movements in the X and Y directions of the system  20  are changed to form the desired tooth profile of the second hypoid gear. For example, the first hypoid ring gear may have  39  teeth while the second hypoid ring gear may have  41  teeth. In this example, the first hypoid gear may mesh with an eleven-tooth pinion gear to define a gear ratio of 3.55 (39/11), and the second gear may mesh with an eleven-tooth pinion gear to define a gear ratio of 3.73 (41/11). While each meshing gear set includes an eleven-tooth pinion, the pinions must have a different tooth profile to mesh correctly with its corresponding ring gear. As an additional example, a third gear ratio may include 43 ring gear teeth and 13 pinion teeth to define a gear ratio of 3.31 (43/13). In all of the above examples, the ring gear has an outer diameter of about 9.75 inches (24.77 centimeters). 
     To communize the blades  60 ,  62  for cutting each of the first ring gear (39 teeth), the second ring gear (41 teeth), and third ring gear (43 teeth), a new profile may be selected for the blades  60 ,  62 , or an existing profile may be selected. That is, the profiles illustrated in  FIGS. 3 and 4  for existing blades that cut one of the exemplary ratios may be selected and the other ratios formed with the same blades. As an example, the blades  60 ,  62  for forming the second ring gear (41 teeth) will be selected. 
     Once the blades  60 ,  62  are selected, the other ring gears are “designed around” the blades  60 ,  62 . That is, corrections are made to the system  20 , including the adjustable parameters discussed above, to form a ring gear that may have a different number of teeth than the selected blades were intended to form. One parameter is the relative speed of the tool  36  to the speed of the stock. 
     By way of further explanation, reference will be made to the drawings and the following paragraphs to illustrate various steps in at least one non limiting example of a method to face hob hypoid gear teeth with common blades. 
     Match the Sums of Gear Blade Pressure Angles 
     As seen in  FIGS. 3 and 4 , the inside blade pressure angle Φ c I, is added to the outside blade pressure angle Φ c O for the selected ratio. Next, the inside blade pressure angle Φ c I, is added to the outside blade pressure angle Φ c O for the target ratio. Then, the sum of the pinion tooth pressure angles of the pinion of the target ratio are increased or decreased until the corresponding sum of the inside blade pressure angle Φ c I and the outside blade pressure angle Φ c O for the target ratio is equal to the sum of the inside blade pressure angle Φ c I and the outside blade pressure angle Φ c O for the selected ratio. 
     Match the Gear Pressure Angles 
     Turning to  FIG. 7 , the gear pitch angle Θp for the target ratio may be increased or decreased as necessary until the inside blade pressure angle for the target ratio is identical to the inside blade pressure angle for the selected ratio and outside blade pressure angle for the target ratio is identical to the outside blade pressure angle for the selected ratio. 
     Match the Gear Blade Distances 
     Referring again to  FIGS. 3 and 4 , the inside blade distance B d I and the outside blade distance B d O are matched for the target ratio and the selected ratio. To accomplish this, the mean tooth slot width Sw and/or the spiral angle Φ (as seen in  FIG. 9 ) of the target ratio are increased or decreased as necessary to provide a resulting inside blade distance and an outside blade distance that is about identical to the selected ratio. 
     Verify the Gear Blade Point Widths 
     Turning again to  FIGS. 3 and 4 , the inside blade point width W t I and the outside blade point width W t O of the selected ratio are considered to determined whether the resulting target ratio will have proper rootline cleanup without interference or clipping of the tooth profile. That is, if the blade point widths of the selected ratio are less than desired, the resulting ring gear formed for the selected ratio may have a ridge (not shown) within the tooth slot. Further, if the blade point widths of the selected ratio are more than desired, the resulting ring gear formed for the selected ratio may have portions of the tooth profile clipped as the inside blade cuts into the profile formed by the outside blade and/or the outside blade cuts into the profile formed by the inside blade. Adjustment of the blade point width of either the inside blade or the outside blade may be necessary. 
     Match the Pinion Blade Pressure Angles Φ c I and Φ c O 
     Continuing with reference to  FIGS. 3 and 4 , the blades for the pinion are considered. Importantly, the blades for the ring gear are not the same blades for the pinion of the selected ratio. That is, the selected ratio requires a tool  36  for the ring gear having blades of a predetermined profile, and a separate tool having pinion blades of a predetermined profile. To form the desired target pinion of the target ratio using the selected tool of the selected ratio, adjustments are made to design input parameters (to system  20 ) such as the contact length factor, the tooth profile change, the bias change. 
     Match the Pinion Blade Distance 
     As seen in  FIGS. 3 and 4 , the pinion blade distances for the inside blade B d I and the outside blade B d O may be compensated for when forming the target pinion with the selected pinion blades by adjusting the cutter radius change. 
     Verify the Pinion Blade Point Widths 
     Continuing again with reference to  FIGS. 3 and 4 , the inside blade point width W t I and the outside blade point width W t O of the selected ratio are considered to determine whether the resulting pinion of the target ratio will have proper rootline cleanup without interference or clipping of the tooth profile. That is, if the blade point widths of the selected ratio are less than desired, the resulting pinion gear formed for the selected ratio may have a ridge (not shown) within the tooth slot. Further, if the blade point widths of the selected ratio are more than desired, the resulting pinion gear formed for the selected ratio may have portions of the tooth profile clipped as the inside blade cuts into the profile formed by the outside blade and/or the outside blade cuts into the profile formed by the inside blade. Adjustment of the blade point width of either the inside blade or the outside blade may be necessary. 
     If the blade point widths of the selected ratio is adequate to not introduce any undesired interference and will provide adequate rootline cleanup, then the selected ratio blades may be used to form the target ratio. However, some adjustment of the blades may be necessary. 
     After cutting either the selected ratio or target ratio (ring gear or pinion) the teeth are measured using a coordinate measuring machine (CMM) to determine whether the actual tooth profile is within acceptable tolerances of the desired tooth profile. Further adjustments to the system  20  may be necessary for any of the selected ratio ring rear or pinion or the target ratios ring gears or pinions. 
     In the embodiment illustrated the system  20  includes a microprocessor that will accurately control the movement of the tool in all parameters described above while the system is operating. Generally, this accuracy is within thousandths of an inch. Since the system  20  is intended to correct minor variations in the resulting gears, the flexibility to form multiple gear sets of multiple gear ratios with a pair of tools (one for the ring gear and one for the pinion) is afforded. 
     Many, if not all, dimensions of the first modified ring gear (target gear cut with common blades) are about identical to the dimensions of the first ring gear (target gear Cut with prior art dedicated blades), although some dimensions of the resulting gear may be slightly different from the original design without undesirable effects to strength and noise, vibrations, and harshness (NVH) characteristics. 
     The preceding description has been presented only to illustrate and describe exemplary embodiments of the methods and systems of the present invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. It will be understood by those skilled in the art that various chances may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. The invention may be practiced otherwise than is specifically explained and illustrated without departing from its spirit or scope. The scope of the invention is limited solely by the following claims.