Abstract:
An apparatus for extruding helical teeth in a gear blank includes a press including a die plate and a die base, the die plate being movable along an axis relative to the die base, the die base supporting the gear blank. A mandrel, aligned with the axis and moveable with the die plate along the axis, includes a surface that includes helical die teeth. A servo motor drives the mandrel in rotation about the axis to the required helix angle as the mandrel moves axially relative to the gear blank.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to forming spline teeth or gear teeth, and more particularly to cold extruding helical teeth in a gear blank workpiece. 
     2. Description of the Prior Art 
     Planetary gear units of the type used in automotive transmissions include ring gears having internal helical teeth rather than straight gears even though helical gear teeth are more difficult to form. The internal gear teeth must be formed with very precise dimensions and spacing in order to perform correctly. 
     The helical teeth may be formed by broaching, which is a cutting process in which a large broaching bar with cutting teeth is pulled through a gear blank to form the teeth. Broaching is a costly process, which requires a significant investment in dedicated machinery, lead bar, cutting tools and cutting oils. Broaching can only be applied to parts accessible in both axial directions since the long broach bar must be pulled through the inside of a gear blank to cut the teeth. 
     The helical teeth may be formed by gear shaping, another cutting process used to fabricate internal helical teeth. Although it is a slower process than broaching, it can be used to form blind end as well as through parts for high volume production. Even so, this process also requires an investment in expensive machinery and cutting tools. 
     Helical teeth may be formed by cold extrusion, in which the teeth are formed, rather than cut, into the part. A precision ground, hardened mandrel formed with external helical die teeth is forced into a workpiece, whose internal surface is formed with the negative contour of the die teeth. When helical teeth are being extruded, the mandrel must be guided in a helical path through the workpiece. This guidance combines axial translation and rotation about a central axis. 
     According to conventional practice, extrusion of helical ring gears requires a specific helical lead guide as part of each tool set to produce gear teeth at the proper helical angle. The lead guide is an expensive, large element of the die set and must be machined to precise dimensions. The lead mechanism requires a significant portion of the vertical dimension of the die set, and increases the total size of the hydraulic press. A lead guide and broach bar must be held in inventory for each product being made. 
     A need exists in the metal forming industry for an efficient, reliable technique for extruding internal and external helical gear teeth without using a lead guide to control the helical path of the mandrel through the material of the workpiece. 
     SUMMARY OF THE INVENTION 
     An apparatus for extruding helical teeth in a gear blank includes a press including a die plate and a die base, the die plate being movable along an axis relative to the die base, the die base supporting the gear blank. A mandrel, aligned with the axis and moveable with the die plate along the axis, includes a surface that includes helical die teeth. A motor of a programmed servo mechanism drives the mandrel in rotation about the axis as the mandrel moves axially relative to the gear blank, creating a helical path. 
     The servo motor and controller provides several advantages specific to the process of extruding helical gears including smaller size requires less die opening height; faster cycle time in extruding each gear, functional flexibility by programming the controller to control gear extrusion with many different helical gears and helix angles; fast die changing between different products, sensors for monitoring the extrusion process; and reduced the cost of the extrusion tooling and the hydraulic press. 
     The servo motor can be programmed to assist in the extrusion process by generating rotational torques while the hydraulic press actuates in the downward direction permitting better control of forces required to produce precision formed gear tooth profiles. 
     Moreover, the lead guide is replaced with a computer controlled electronic or hydraulic servo mechanism, which provides proper rotation of the mandrel to impart the exact helical gear geometry required for the gear being processed. The servo mechanism is much smaller than the fixed lead guide and is programmable for many different helical gear lead angles. 
     The servo controls are linked to a computer which controls axial movement and radial forces of the mandrel, thereby coordinating the press actuation sequence with the rotation and eliminating need for the mechanical lead guide. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a front view of an extrusion press equipped with a servo motor for forming internal helical gear teeth on a gear blank; 
         FIG. 2  is front view of a mandrel used in the extrusion press of  FIG. 1 ; and 
         FIG. 3  is front view of a mandrel and die base used to form external helical gear teeth in a gear blank. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIGS. 1 and 2 , an extrusion die assembly  12 , mounted in a hydraulic press  14 , includes a lower die plate  16 , resting on a base portion  18  of the press  14 , and an upper die plate  20 . Die guide posts  24  extend between upper die plate  20  and lower die plate  16 . One end of each die guide post  24  is fixed to upper die plate  20 ; the opposite end of each die guide post  24  has a ball bearing cage  26  attached to it. Affixed to lower die plate  16  are guide bushings  28 , with each guide bushing  28  aligned with one ball bearing cage  26 . Ball bearing cages  26  telescopically slide into their respective guide bushings  28  to allow axial movement of upper die plate  20  relative to lower die plate  16 , minimizing friction and maintaining the two die plates  16 ,  20  mutually parallel. The assembly  12  is concentric with and translates along an axis  29 . 
     A support plate  30 , guided on the guide posts  24 , is secured to the upper die plate  20  for movement with plate  20  along axis  29 . A mandrel  32  is fastened to support plate  30  by a bolt  34 , which slips through a bore  36  in the center of mandrel  32  and engages a tapped screw thread in support plate  30 . Dowels  38  mate both with dowel holes  40  in support plate  30  and corresponding dowel holes  42  in mandrel  32 . Mandrel  32  is formed with external die teeth  46 , a lead surface  47 , and a single step  48 , which is preferred to a multiple-step mandrel. The helix angle of die teeth  46  is the same as that desired in the gear to be formed from the workpiece. 
     A load cell  50 , mounted on lower die plate  16 , includes force sensors mounted within it and electrically connected to a controller. Load cell  50  senses the magnitude of load and torque applied to it during the forming operation. To control the forming process, force sensors are used to control both the downward press motion and the rotational torque provided by the servo mechanism. If the load is out of predetermined ranges of these parameters, then the press  14  will stop the forming operation so that the press equipment can be checked. Load cell  50  is optional, and the extrusion process can be conducted without this piece of equipment, if so desired. 
     Mounted on load cell  50  is a die base  52 . A retainer ring  54 , mounted on die base  50 , has a cylindrical central cavity. A hardened sleeve insert  56 , fitted within the retainer ring  54 , surrounds the workpiece gear blank  58 . The die base  50  supports the gear blank  58  axially during the forming process. Retainer ring  54 , sleeve insert  56  and gear blank  58  are located concentric with axis  29  and mandrel  32 . The gear blank  58  is formed with a cylindrical central cavity  53  that is aligned with axis  29 . 
     A gear blank  58  includes an annular, cylindrical surface of controlled diameter, in which the internal helical gear teeth will be extruded during the forming process.  FIG. 1  shows a ring gear blank  58  inserted into sleeve insert  56 . 
     A servo motor  90  is secured to upper die plate  20 , faces mandrel  32 , and has its shaft driveably connected to the mandrel, such that the armature of the servo motor and the mandrel rotate about axis  29  as a unit in response to control signals produced by a controller  92 . 
     Electronic signals  94 , produced by load cell  50  and representing the magnitude of the extrusion force and torque and the speed of press  14  are supplied to controller  92  as input. Electronic signals  96  produced by sensors  98  representing the angular displacement of mandrel  32  and the rotor of servo motor  90  from a reference position about axis  29 , and the speed of motor  90  are supplied to controller  92  as input. Electronic signals  100  produced by sensor  102  representing the angular displacement of workpiece  58  from a reference position about axis  29  are also supplied to controller  92  as input. 
     Controller  92  preferably includes an electronic microprocessor  104 , electronic memory  106 , and signal conditioning circuits, which communicate mutually and with an output  110  over a data bus  112 . The memory contains a control algorithm, which is executed using variables represented by the input signals and is programmed to produce many different helical gear lead angles and continually adjusts to deviation from expected behavior of the press  14 . 
     Control signals  114  are carried from the output  110  of controller  92  to a servo motor control (not shown), which actuates servo motor  90  to rotate about axis  29  in response to the control signals output by controller  29 . Similarly, controller  92  causes the assembly  12  to translate vertically along axis  29 . 
     The extrusion assembly  12  is used in a cold extrusion process for forming internal helical teeth in gear blanks  58 , with tight control of lead accuracy. 
     In operation, a gear blank  58  is inserted into ring insert  56 . Hydraulic press  14  is activated and forces the upper die plate  20  downward toward lower die plate  16 , guided by die guide posts  24 . 
     This axial translation carries mandrel  32  toward gear blank  58  such that the lead surface  47  enters the central opening  53  in the workpiece  58 . Servo motor  90  causes mandrel  32  to rotate about axis  29  to a desired angular position, at which the helical die teeth  46  on the external surface of mandrel  32  first contact the gear blank workpiece  58 . When the mandrel  32  is in its desired angular position, hydraulic press  14  is actuated to continue its axial path and servo motor  90  is actuated to rotate at a speed that is related to the speed of its axial path such that the internal gear teeth are formed on the workpiece  58  with the desired helix angle. 
     Die teeth  46  on mandrel  32  engage the inner surface of gear blank  58  and move downward into the material of the workpiece with a helical motion as they are forced into the gear blank, thereby forming helical gear teeth. When the predetermined depth of finished gear teeth is reached, hydraulic press  14  stops pressing on upper die plate  20  and retracts the upper die plate  20  and mandrel  32 . Servo radial forces are used to form the gear tooth flanks during the upper stroke of the press and die. 
     This movement causes mandrel  32  to withdrawal upward and to lift the workpiece  58  from the surface of the die base  50 . A box stripper  120 , secured to the die base  50 , contacts the upper surface  122  of the workpiece  58  forcing it from the mandrel  32  and allowing the mandrel to withdraw from the extruded gear. The motion of withdrawal will follow that of insertion. 
     The finished ring gear is then removed from press  14  and another gear blank  58  is inserted in its place preparatory to repeating the forming process. Because the travel distance of the press  14  is short, the length of the cycle period is short time and throughput is increased substantially over conventional techniques. 
     Although the extrusion method has been described with reference to external helical mandrel teeth  46  on the workpiece  58  being used to extrude internal teeth on the blank  58 , if external helical gear teeth are to be extruded on a workpiece  158 , as  FIG. 3  illustrates, a mandrel  132  is formed with a central cylindrical cavity  153 , which surrounds the outer surface of the workpiece  158  and is aligned with axis  29 . The inner surface of workpiece  158  is supported by a cylindrical plug  160  located in the cylindrical cavity  53  of the workpiece. The inner surface of mandrel  132  is formed with helical die teeth  146 . The servo motor  90  is driveably connected to mandrel  132  and rotates the mandrel as the press  14  forces the die teeth  146  axially into and through the wall of the workpiece  158 , thus forming external helical gear teeth on the outer surface of the workpiece or gear blank  158 . 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.