Abstract:
A method of manufacturing a transmission case housing is provided wherein a minimum quantity of lubrication as a compressed air/oil mist is supplied as the housing is rough bored and face milled. The transmission case defines a plurality of transmission fluid drainage holes for draining transmission fluid from the transmission when installed in a vehicle. The housing is positioned with the fluid drainage holes below a central axis of the housing and a plurality of internal bores and faces are bored and face milled on the housing. The compressed air/oil mist is sprayed from the cutting head to cool and lubricate the boring and face milling tools. Machining chips are blown off the rough bored housing through the fluid drainage holes.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to a method and machining line for manufacturing a transmission case. 
       BACKGROUND 
       [0002]    Large housings for transmissions are cast and then machined to close tolerances required to mount transmission gears, clutches and other critical components. Conventional machining lines for transmission housings cool and lubricate the housing with a flood of a water/oil emulsion coolant in both the rough and finish machining steps. A considerable amount of heat is created during the rough boring and face milling operation that builds up in the housing and in the chips machined from the housing. The flood of the water/oil emulsion coolant was formerly thought to be essential to cool the housing and wash large volumes of the hot chips from the part and tool. 
         [0003]    Large amounts of water/oil emulsion coolant necessitate large coolant circulation systems for removing chips and cooling the recirculated coolant. The coolant and chips add to the waste disposal load of a plant and increase processing costs. Water/oil emulsion coolants can be recycled. Metallic cutting chips reclaimed from the coolant can be salvaged but at a reduced value compared to chips produced in a dry metal cutting operation. Coolant circulation systems are costly and require valuable manufacturing floor space. Operation of the water/oil emulsion coolant systems uses substantial energy. 
         [0004]    Minimum quantity lubrication (MQL) systems have been developed that provide mist lubrication in air for milling, drilling, and tapping relatively small features that are less than 200 mm in diameter of a transmission housing. The bottom area of the transmission housing that is enclosed by the transmission fluid pan is processed through milling, drilling, and tapping operations with the bottom area inverted to provide easier access. MQL systems were not developed for rough boring and finish boring machines because of the heat generated by the boring and face milling operations. The hot chips removed from the housing contribute to the heat load. Flood cooling was previously thought to be the only way to provide cooling to the housing and lubrication to the boring and face milling machines. 
         [0005]    Dedicated boring machines having large boring bars equipped with multiple cutting tools are used to machine large housings having inner diameters to be bored and faces to be milled that are more than 200 mm in diameter, such as a rear wheel drive transmission cases, and the like. Dedicated boring bar machines are expensive to purchase and require long purchasing lead times. Dedicated boring machines require substantial time for changeover to a different part or style of part and tooling changes result in long periods of line downtime. Dedicated boring machines on a machining line are normally part of a single path line so that if the boring machine requires servicing, the entire line is shut down. 
         [0006]    The above problems and other problems are addressed by this disclosure as summarized below. 
       SUMMARY 
       [0007]    According to one aspect of this disclosure, a method of manufacturing a transmission case from an “as cast” housing is provided wherein a minimum quantity of lubrication as an oil mist in compressed air is supplied as the housing is rough bored and face milled. The transmission case is a cast housing that defines a plurality of transmission fluid drainage holes for draining transmission fluid from the transmission when installed in a vehicle. The housing is positioned with the fluid drainage holes below a central axis of the as cast housing and a plurality of internal bores and faces are bored and face milled on the housing to form a rough bored housing. Machining chips are blown off the rough bored housing through the fluid drainage holes and through the bell-shaped end of the housing. Machining chips are also blown off by the turbulent air flow created by rotation of the tool as the tool is retracted from the housing. 
         [0008]    According to other aspects of this disclosure, the step of boring and face milling the internal bores and faces may further comprise supplying a machine tool cutting head with a flow of compressed air and an oil mist through an internal passage in the cutting head. The compressed air and an oil mist are sprayed from the cutting head to cool and lubricate the boring and face milling tools. Compressed air supplied through the cutting head without the oil mist is also used to cool the housing. 
         [0009]    The cast housing may include a bell-shaped end and a rear end. The compressed air and oil mist are allowed to flow when the cutting head is within the cast housing and are inhibited during a tool change operation when the cutting head is outside the housing. Chips formed during the boring and face milling operations are blown off the machined housing through the bell-shaped end and the fluid drainage holes in the housing during and after boring and face milling a plurality of internal bores and a plurality of faces on the cast housing. 
         [0010]    The method may further comprise machining a plurality of datums and locating the rough machined housing on the datums with the drainage holes above a central axis of the cast housing. The rough machined housing is further bored and milled with finish boring and face milling tools that are also provided with the compressed air and an oil mist through the machine tool cutting head. 
         [0011]    The cast housing may include a bell-shaped end and a rear end. The cast housing is initially positioned with the bell-shaped end facing a machine tool arbor and then is repositioned with the rear end facing the machine tool arbor. A rear bore of the cast housing is then bored and face milled. 
         [0012]    The step of boring and face milling a plurality of internal bores and a plurality of faces of the cast housing to form a rough machined housing may be performed by a computer numerically controlled machining center with a tool magazine. 
         [0013]    The step of boring and face milling a plurality of internal bores and a plurality of faces of the cast housing is performed by boring tools and interpolated face milling tools that perform the facing milling operations. 
         [0014]    According to another aspect of this disclosure, a machining center is disclosed for machining a housing that comprises a fixture, a plurality of interchangeable tools and a compressed air/oil mist lubrication system. The fixture holds the housing that defines a plurality of fluid drainage holes disposed below a central axis of the housing. Interchangeable tools are provided for boring and face milling a plurality of bores and faces of the housing. The air/oil mist lubrication system lubricates and cools the interchangeable tools while boring and face milling the housing and also blows machining chips off the housing through the fluid drainage holes. 
         [0015]    According to other aspects of this disclosure as it relates to a machining center, the air/oil mist lubrication system may include flow channels defined by the interchangeable tools. The air/oil mist lubrication system may include a controller that allows the air/oil mist to flow when the interchangeable tool is inside the housing and inhibits the flow of air/oil mist during a tool change. The air/oil mist lubrication system blows machining chips off the housing after the interchangeable tool is retracted from the housing. The air/oil mist flow rates are adjustable and could be varied for each different type of cutting tool used to machine the cast housing. 
         [0016]    The housing may include a bell-shaped end and a rear end, and the machining center may further comprise a positioner, or trunnion, adapted to change the orientation of the housing. The positioner holds the bell-shaped end of the housing facing the machine tool arbor when a plurality of internal bores and faces are bored and face milled, and positions the rear end facing the machine tool arbor when the rear bore of the housing is bored and face milled. 
         [0017]    The above aspects of this disclosure and other aspects are described below with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a side elevation view partially exploded away of a rear wheel drive transmission housing and a boring tool. 
           [0019]      FIG. 2  is a diagrammatic view of a portion of a machining line for machining a transmission housing such as that shown in  FIG. 1 . 
           [0020]      FIG. 3  is a flow chart showing the steps of a method of machining a transmission housing such as that shown in  FIG. 1 . 
           [0021]      FIG. 4  is a perspective view of a trundle of a machining center holding a transmission housing in a bottom down orientation. 
           [0022]      FIG. 5  is a side elevation view of the trundle of a machining center holding a transmission housing in a bottom down orientation and horizontally pivoted 180 degrees. 
           [0023]      FIG. 6  is a perspective view of a trundle of a machining center holding a transmission housing in a bottom up orientation. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The illustrated embodiments are disclosed with reference to the drawings. However, it is to be understood that the disclosed embodiments are intended to be merely examples that may be embodied in various and alternative forms. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed are not to be interpreted as limiting, but as a representative basis for teaching one skilled in the art how to practice the disclosed concepts. 
         [0025]    Referring to  FIG. 1 , a rear wheel drive (RWD) transmission housing  10  is illustrated with a boring tool  12 , which may also be referred to herein as a boring and milling tool or an interchangeable tool. The tool  12  has a larger length to diameter ratio of up to 2.5/1 to facilitate reaching into the housing  10 . In addition, the spindle of the CNC machining center is used to reach into the housing  10  from the bell end  16  to minimize the overall length of the boring tools  12 . The housing  10  has a bell-shaped end  16 , or bell end, and a back end  18 . The gear sets, clutches and other components of the transmission (not shown) are assembled into the bell end  16  and the drive shaft (not shown) is assembled to the back end  18 . A bottom side  20  of the housing  10  defines a plurality of fluid drainage holes  22  and other openings. The fluid drainage holes are provided to allow for circulation of transmission fluid in the completed transmission. A transmission fluid pan  24  covers and encloses the bottom of the housing  10 . 
         [0026]    The housing has a central axis indicated by “X” in  FIG. 1  that corresponds to the axis of the main shaft of the transmission (not shown). 
         [0027]    The boring and face milling tool  12  is adapted to be attached to an arbor of a machining center (shown in  FIG. 2 ) by a quick connect fitting  26 . The quick connect fitting  26  defines a concentric fluid port  28  that is aligned with the axis X. A compressed air source and an oil source are connected to the tool  12  by the quick connect fitting  26  to provide a compressed air/oil mist  34  through internal passages  36  in the tool  12 . The flow of air and oil mist maybe separately controlled by an air valve  38  and an oil valve  40 . Compressed air supplied through the tool  12  without oil may also be used to cool the part. The fitting  26 , internal passages  36 , air source  30 , oil source  32 , air valve  38  and oil valve  40  may be generally referred to as a lubrication system. 
         [0028]    The tool  12  is provided with a plurality of cutter inserts  42  that are used to bore and face mall the housing  10 . The cutter inserts  42  cut into the housing  10  and create machining chips  44 . The lubrication system cools the housing and machining chips  44  by directing the compressed air/oil mist  34  through nozzles  46  and onto the housing  10  in the area where the cutter inserts  42  are used to machine the housing  10 . The compressed air/oil mist  34  cools the housing  10  and machining chips  44  during the machining operation. The compressed air/oil mist  34  also functions to blow the machining chips  44  out of the housing  10  and through the fluid drainage holes  22  and also through other openings, such as the bell-shaped end  16  of the housing  10 . During a tool change the oil valve  40  and air valve  38  may be separately controlled so that both may be closed to stop spraying the air/oil mist  34 . Alternatively, only the oil valve  40  may be closed to reduce or eliminate oil from the air/oil mist  34  while the tool  12  is retracted from the housing  10 . In this way, the compressed air can be used to remove machining chips  44  from the housing  10  without wasting oil or spraying oil inside the machining center. Machining chips  44  are also blown off the housing  10  by the turbulent air flow created by the propeller-like action of the rotating tool  12 . 
         [0029]    Referring to  FIGS. 2 and 3 , a machining line is illustrated in  FIG. 2  and a flowchart describing the steps of the method is provided in  FIG. 3 . The first part of the machining line is a loading station  52  where an “as cast” transmission housing is provided to the machining line. The transmission housing  10  is transferred to a datum machining station  54  where datum surfaces are machined on the housing at locations where the transmission housing is to be fixtured as it proceeds through the machining line  50 . 
         [0030]    In the next step, at  56 , the housing is rough bored and face milled in the position shown in  FIG. 4  with the bottom side  20  of the housing  10  facing downward. The fluid drainage holes  22  in the bottom side  20  of the housing  10  receiving the machining chips  44  and the air/oil mist  34  falls through the holes  22  as a result of the pressure from the compressed air and as a result of the force of gravity. Several boring and face milling tools may be sequentially inserted into the bell-shaped end  16  of the housing  10  to complete rough boring and rough face milling of the housing  10 . The transmission housing has internal bores and faces to be milled that are between 205 mm and 295 mm. Boring and milling such large surfaces was previously thought to only be possible with flood cooling due to the heat generated by the large amount of material removed. 
         [0031]    The housing is then repositioned inside the machining center by rotating 180° in a horizontal plane “H” (shown in  FIG. 5 ) to the position shown in  FIG. 5  to provide access to the back end  18  of the housing  10 . Rough boring and face milling tools are sequentially inserted into the housing  10  through the back end  18  while the air/oil mist is sprayed onto the housing through the internal passages in the tools  12 . Again, a large volume of chips  44  are removed from the housing  10  and a substantial amount of the heat created by the process and retained in the machining chips  44  is removed through the fluid drainage holes  22  in the bottom side  20  of the housing  10 . 
         [0032]    The housing is then transferred to a plurality of machining centers, at  58 , and repositioned by being pivoted in a vertical plane “V” (shown in  FIG. 6 ) to the position shown in  FIG. 6  with the fluid drainage holes  22  in the bottom side  20  of the housing  10  facing upward. The bottom side  20  of the housing  10  is milled, drilled and tapped to form ports, passageways and other features of the housing  10  in several machining centers. The machining centers used to mill, drill and tap the housing remove only a limited volume of chips and can utilize MQL systems because less material is removed. 
         [0033]    The next step is to transfer the housing to a finish boring and face milling station at  60 . The housing is pivoted in a vertical plane “V” to the position shown in  FIG. 4  with the fluid drainage holes on the bottom side  20  of the housing  10  and boring and face milling tools are used to machine smooth surfaces on the inside bores and faces of the housing  10 . The air/oil mist  34  is directed onto the housing as the tools machine the bell-shaped end of the housing  10 . 
         [0034]    The housing  10  is again pivoted 180° in a horizontal plane “H” to the position shown in  FIG. 5  within the machining station. Finish boring and face milling tools  12  are sequentially inserted into the housing  10  through the back end  18  while the air/oil mist is sprayed onto the housing through the internal passages in the tools  12 . A large volume of chips is again removed from the housing but the volume of chips is less than what was removed in the rough boring and face milling operation. The chips  44  and air/oil mist  34  are directed by the compressed air/oil mist and the force of gravity through the fluid drainage holes in the bottom side  20  of the housing  10 . 
         [0035]    After the finish boring and face milling operation the housing  10  is transferred to a high pressure wash operation at  62  to clean the housing  10  and remove any residue of the air/oil mist  34  and machining chips  44 . 
         [0036]    The housing is then dehydrated and leak tested at  64  and is unloaded from the line  50  at an unloading station  66 . 
         [0037]    Referring to  FIG. 4 , a trunnion  68  is illustrated that is part of the machining centers making up the machining line  50 . The housing  10  in  FIG. 4  is held on the datums  76  used to locate the housing in a fixture  70 , or positioner, with the bottom side  20  facing downward. In this position, the fluid drainage holes  22  are in a position to allow the machining chips  44  and excess air/oil mist  34  to fall through the fluid drainage holes  22  with the fluid drainage holes  22  being disposed below the central axis “X.” In  FIG. 4 , internal bores  72  and faces  74  are illustrated that are formed by the boring and face milling tools (as shown in  FIG. 1 ). 
         [0038]    Referring to  FIG. 5 , the trunnion  68  is shown with an arcuate arrow “H” to illustrate the horizontal pivoting motion in which the housing is pivoted to allow access to either the bell-shaped end or the back end of the housing  10 . The housing  10  is retained in the fixture  70  with the bottom side  20  facing downward. 
         [0039]    Referring to  FIG. 6 , the trunnion  68  is shown with the housing  10  shown with the bottom side  20  facing upward and disposed above the central axis “X.” The housing  10  is retained in the fixture  70  and the bores  72  and faces  74  are also shown. The trunnion  68  pivots the housing in the vertical plane indicated by the arcuate arrow “V.” 
         [0040]    The embodiments described above are specific examples that do not describe all possible forms of the disclosure. The features of the illustrated embodiments may be combined to form further embodiments of the disclosed concepts. The words used in the specification are words of description rather than limitation. The scope of the following claims is broader than the specifically disclosed embodiments and also includes modifications of the illustrated embodiments.