Abstract:
A method of manufacturing a cylinder head and crankcase for a small engine. A crankcase and a cylinder head are cast to close tolerances and include as-cast mounting flanges, which are assembled in face-to-face contact by employing self-threading screws. Bearing recesses are cast into the crankcase. The cylindrical sidewalls of the bearing recesses are provided with as-cast flutes and roller bearings are press-fitted into the bearing recesses.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to single-piston, two-cycle gasoline engines and more particularly, techniques for eliminating certain prior art machining operations performed on cylinder head and crankcase castings. 
   Current manufacturing techniques involve casting a cylinder block and a crankcase using a die-casting process utilizing standard casting tolerances that are relatively broad. The cast cylinder and crankcase go through numerous machining steps to arrive at the finished product, ready to be assembled together, and with additional engine parts, into a completed engine. 
   Traditionally, a typical die casting process employs “standard casting tolerances”, which are known as “steel safe”. “Steel safe” means that the core pins that are used to produce holes in a part are on the high side of broad tolerances so that as wear occurs on them, they would nevertheless remain in tolerance. Die details that create the outside surface of the casting are dimensioned on the low side of the broad tolerance so that wear on the die allows the resultant part to remain in print tolerance. This allows a die to produce large quantities of parts with little attention paid to the dimensional integrity of the parts, resulting in a low maintenance cost. 
   At least in the manufacture of cylinder blocks and crankcases for single-piston, two-cycle gasoline engines, these savings are illusory in that mating surfaces, such as the mating surface between the block and the crankcase, must be machined. Also, the broad tolerance core pin openings must be drilled and tapped to receive the fasteners for these parts. Further, the crankshaft bearing portal must be machined to a press tolerance and machined to accommodate bearing locator snap rings. All of these machining operations require labor and equipment costs, which negate any savings in employing standard casting tolerances. 
   In addition to the cost factors involved in machining the foot area of the cylinder head and the mating area of the crankcase to ensure a proper seal, the machining operation itself contributes to exhaust gas leaks in the casting. All aluminum die castings are inherently porous. However, the initially chilled surface of the casting provides a dense skin, which seals the porous interior of the casting. When this skin is machined to provide precise gasket mating surfaces between the cylinder block and crankcase, the dense skin is removed and exhaust leakage is permitted through the gasket area. 
   Analyzing the costs of the traditional machining operations, including the costs of the machine tools, the labor involved in operating the machine tools, the time loss due to the number of steps involved, and the risks of poor quality due to potential errors that the large number of operations required can cause led to the realization that by requiring tighter tolerances on the die mold and its components, one could decrease the total cost of the manufacturing process despite the increased die mold and maintenance costs and the decreased die mold life. 
   SUMMARY OF THE INVENTION 
   According to this invention, no machining operations are required in the foot flange area between the cylinder block and the crankcase. The die caster is required to hold tighter tolerances in respect to flange flatness and surface finish, as well as the fastener hole diameters and true positional location of those diameters. 
   The preferred tolerances are: 
   Flange flatness=0.006 inch over the entire surface of the flange 
   Perpendicularity of flange holes to the flange=0.002 inch 
   True positional location of the flange holes=0.006 inch 
   The cylinder block flange mates with a crankcase flange, which also is die-cast to the same tight tolerances, and an O-ring is provided in a groove in the crankcase flange. The O-ring and the unmachined flange surfaces provide a reliable seal between the flange surfaces and, since the fastener openings or holes are cast to tight tolerances, self-tapping screws may be used to attach the cylinder block to the crankcase, thus eliminating the need for drill and tap operations. 
   This invention also provides for an improved bearing mount for the crankshaft. The crankcase is die-cast, with bearing seats having a plurality of radially inwardly directed flutes. The bearings are press fitted into the seats. Even though press fit tolerances are not as precise as machined tolerances, the as cast flutes create spaces for material displacement during the bearing pressing operation. The flutes also allow for a radial bending of the surrounding casting material during the pressing operation rather than a circumferential stretch, as occurs when the casting is machined for a press fit. 
   Since a pair of roller bearing units are provided for the crankshaft, a pair of bearing seats are provided with each bearing seat extending inwardly from each end of the crankshaft portal in the crankcase casting. The base of each bearing seat is defined by an annular seat, which locates the bearing during the press fitting operation. This eliminates the need for machined grooves and locating clips in the driveshaft portal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a cylinder block according to this invention; 
       FIG. 2  is a plan view of the cylinder block shown in  FIG. 1 ; 
       FIG. 3  is an elevational view of the cylinder block, viewed from the air-fuel intake side; 
       FIG. 4  is an elevational view of the cylinder block viewed from the exhaust port side; 
       FIG. 5  is a cross-sectional view, the plane of the section being indicated by the line  5 — 5  in  FIG. 2 ; 
       FIGS. 6-9  are cross-sectional views that progressively illustrate various machining operations performed on a cylinder block according to prior art practices; 
       FIG. 10  is a flow chart illustrating the progression of various prior art machining operations; 
       FIG. 11  is a flow chart illustrating the progression of various machining operations according to this invention; 
       FIG. 12  is a perspective view of the crankcase according to this invention; 
       FIG. 13  is a side elevational view of the crankcase; 
       FIG. 14  is an elevational view of the other side of the crankcase; 
       FIG. 15  is a top plan view of the crankcase; 
       FIG. 15A  is a cross-sectional view, the plane of the section being indicated by the line  15 A— 15 A in  FIG. 15 ; 
       FIG. 16  is an elevational view of one of the crankshaft bearings of the invention; 
       FIG. 17  is an elevational view of one side of the crankshaft portal; 
       FIG. 18  is an elevational view of the other side of the crankshaft portal; and 
       FIG. 19  is a view similar to  FIG. 17  but showing the flutes on the other side of the portal in phantom outline. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, and particularly to  FIGS. 1-5 , there is illustrated a cylinder block  10  according to this invention. The cylinder block  10  has an intake port flange  14 , an exhaust port flange  12 , and a foot flange  16  at the bottom of the cylinder block  10 . The foot flange  16  is adapted to be connected to a crankcase connecting flange, as will become apparent. First and second fastener openings  18  and  19  are die-cast in the cylinder block  10  under close tolerances. Fins  22  are provided on the cylinder block  10  to cool the block during operation. 
   The cylinder block  10  is cast with a flange mounting surface  20  having an as cast flatness of approximately 0.006 inches. As will become apparent, this provides a sealing surface that eliminates the prior art machining step. Elimination of the machining step on the surface  20  also eliminates the removal of the as-cast skin, which serves as a seal against leakage through the relatively porous interior of the casting. 
   The cylinder block  10  also is provided with axially aligned openings  24  through the fins  22  to provide tool access to the fastener openings  18  and  19 . The openings  24  are preferably as-cast openings formed by core pins in the mold. Still further, the cylinder block  10  is provided with a piston cylinder chamber  26 , a threaded spark plug opening  28 , and scavenging ports  27 . An exhaust port  42  extends from the cylinder chamber  26  to a face  46  of the exhaust port flange  12  of the block  10 . Fastener openings  44  are cast into the face  46  by mold core pins (not shown). The opposite side of the cylinder block  10  is provided with an intake port  32  extending from the cylinder  26  to a face  36  of the intake port flange  14  of the block  10 . Fastener openings  34  are cast into the face  36  by mold core pins (not shown). 
   Referring now to  FIGS. 6-9 , a series of prior art machining operations that are accomplished at three separate machining stations are illustrated. In  FIG. 6 , a die-cast engine block  10   a  is die-cast to broad tolerances and positioned at a first machining station. The piston block  10   a  is cast with a plurality of cooling fins  22   a , a piston chamber  26   a , scavenging ports  27   a , an intake port  32   a  (FIG.  8 ), and an exhaust port (not shown). At the first machining station, a flange mounting surface  20   a  of a foot flange  16   a  is machined to close tolerances as is indicated by the phantom line in FIG.  6 . 
   After the mounting surface  20   a  is machined at the first machining station, the cylinder block  10   a  is transferred to a second machining station ( FIG. 7 ) where fastener openings  18   a  and  19   a  are drilled in the flange  16   a  and axially aligned access openings  24   a  are drilled through the fins  22   a . The fastener openings  18   a  and  19   a  are tapped for fastening bolts (not shown). Mounting holes  34   a  ( FIG. 8 ) and mounting holes (not shown, but corresponding to the holes  44 ) are drilled and tapped to accommodate screws so that the intake manifold and the exhaust manifold, respectively, can be mounted on the cylinder block  10   a . Further at the second machining station, a spark plug opening  28   a  is drilled and tapped. 
   The cylinder block  10   a  is moved to a third machining station ( FIG. 9 ) where the piston chamber  26   a  is subjected to a boring operation. 
   The sequence of the foregoing operations is illustrated in FIG.  10 . It should be appreciated that even though casting costs are relatively low as a result of wide as cast tolerances, the material handling and machining costs combine to eliminate any savings in the casting operation. By requiring the die caster to hold tighter tolerances, particularly with respect to the flatness of the foot flange mating surface  20  and the fastener apertures, a net savings results, even though casting costs are relatively high. 
   The process according to this invention is illustrated in the flow chart of FIG.  11 . Initially, a die casting is produced having tight tolerances, particularly with respect to flange flatness and surface finish as well as fastener hole diameters and true positional location of the diameters. The preferred tolerance is approximately 0.006 inch for the mounting surface  20 . The perpendicularity of the fastener openings  18 ,  19 ,  34  and  44  to the surfaces  20 ,  36  and  46  is approximately 0.002 inch. The true positional location of the fastener openings  18 ,  19 ,  34  and  44  is approximately 0.006 inch. 
   The casting is positioned at a single machining station where the piston chamber  26  is subjected to a boring operation. The spark plug hole or opening  28  is drilled and tapped and the axially aligned fin openings  24  are drilled. The spark plug opening  28  is substantially formed during the molding as is indicated in phantom outline  28   b  in FIG.  5 . To simplify the problem of a through core pin in the mold, a thin web of material closes off the opening  28  in the as cast condition. It is this thin web that is removed during the drilling step as indicated in FIG.  11 . It is contemplated that the drilling step may be eliminated by the use of a through core pin, i.e., a core pin entering the mold surface, which forms a top side  30  of the cylinder block. Similarly, the fastener openings  18  and  19  are cast with thin webs of material  18   b  and  19   b , which are removed by a drilling operation as indicated in FIG.  11 . Further, the exhaust port  42  and the intake port  32  have as cast thin webs adjacent the cylinder chamber  26 . A separate machining operation is not required since these webs are removed during the boring operation. Additionally, it is contemplated that the fin holes  24  need not be machined but may be provided in the casting. Again, casting the holes  24  requires complicated core pin placement in the mold. 
   Note that there has been a reduction in a number of machining steps over the prior art. By comparing FIG.  10  and  FIG. 11 , it can be seen that the flange surface machining step of the prior art has been eliminated, and the fifth and sixth steps are simplified, because only the fins need be drilled and the thin web  49  of the first and second openings  18  removed. Also, by utilizing self-tapping screws in the installation of the intake and exhaust manifolds onto the intake port structure  14  and exhaust port structure  12 , respectively, there is no need to drill those holes as in the fifth or to tap those holes as represented by the sixth step. Further, the process is simplified by using only a single machine where three had previously been employed. 
   The second aspect of the invention eliminates even more machining steps by further increasing the features provided by the casting process over that disclosed for the first aspect of the invention. The casting process of the second aspect of the invention adds the following features, in addition to those listed for the first aspect hereinabove. 
   The spark plug chamber  28  is cast fully open to the top side  30  of the cylinder. The fin holes  24  are formed by using pins in the die casting process. In addition, first and second openings  18  through the flange  16  are completely open, so no web  49  is formed. The tolerances on the flange surface  20  and the first and second openings are the same as those identified above in the first aspect of the invention. 
   By providing the aforementioned additional features during the casting process, the machining steps shown in  FIG. 11  can be further reduced, so that the steps indicated by broken lines are eliminated. This leaves only the steps described by solid lines still necessary, as described below. 
   Referring now to  FIGS. 12-19 , there is illustrated a crankcase  100 , which is adapted to be attached to the cylinder block  10 . The crankcase  100  is cast to tight tolerances, particularly in areas that are required to be machined according to prior art practices. According to this invention, no machining operations are required and the crankcase is assembled to the cylinder block  10 . 
   The crankcase  100  includes a crank chamber  102  into which a piston rod (not shown) extends to drive a crank (not shown), which converts the reciprocating motion of the piston rod to the drive shaft (not shown) of a powered tool such as a chainsaw. The crankcase  100  further includes a crankcase connecting flange  104  defining an opening  105  to the crank chamber  102  and having a flange mounting surface  106  provided with first and second fastener openings  108  and  110 , which are adapted to be aligned with the first and second fastener openings  18  and  19 , respectively, which are die-cast in the cylinder block foot flange  16 . The openings  108  and  110  are also cast under the same tight tolerances as the openings  19  and  20  so that the cylinder block  10  may be assembled to the crankcase  100  by self-tapping fasteners (not shown) rather than by threaded fasteners entering machined and tapped apertures according to prior art techniques. 
   The crankcase  100  is cast so that its flange mounting surface  106  has an as cast flatness of about 0.006 inches. This provides a sealing surface that eliminates the prior art machining step. Elimination of the machining step on the surface  106  also eliminates the removal of the as-cast skin, which serves as a seal against leakage through the relatively porous interior of the casting. 
   A perimeter groove  112  is cast into the surface  106  and is provided with an O-ring  114  ( FIGS. 15 and 15A ) preformed to the outline of the groove  112 . The O-ring  114  seals against the flange mounting surface  20  of the cylinder block  10  when the cylinder block  10  is assembled to the crankcase  100  as previously described. To aid in this assembly step and to retain the O-ring  114  in place during this operation, a tab  116  is provided on the O-ring  114  that is received in a notch  118 . 
   A bearing assembly is provided for the drive shaft, which eliminates prior art machining steps in this area. Referring to  FIGS. 12-14  and  16 - 19 , first and second bearing recesses  120  and  122  are cast at one end of the crank chamber  102 . Each recess  120  and  122  is defined by cylindrical sidewalls  124  and  126  and by toroidal bases  128  and  130 , respectively. Each cylindrical sidewall  124  and  126  is provided with a plurality of rounded, radially inwardly directed flutes  132  and  134 , respectively. The flutes  132  and  134  are evenly spaced about the sidewalls  124  and  126  and are separated by arcuate sidewall portions  136  and  138 , each having an arcuate dimension corresponding to the arcuate dimension of each flute  132  and  134 . As may be noted with reference to  FIGS. 17-19 , however, the flutes  132  and  134  are mutually offset at a distance corresponding to the aforementioned arcuate dimension. 
   A roller bearing  140  ( FIG. 16 ) is press fitted into each bearing recess  120  and  122 . The provision of the flutes  132  and  134  allows for radial bending to occur between the contact areas of the flutes, as opposed to circumferential stretch of the casting under a heavy press fit. Also, the flutes allow for material flow between the flutes during the pressing operation. The toroidal bases  128  and  130  form seats for the bearings  140  during the pressing operation, thus eliminating the need for machined grooves and locating clips in the drive shaft portal. The offset relationship of the flutes  132  and  134  helps to minimize noise and vibration. Also, to that end, the number of ball bearings in each bearing  140  is not equal to the number of flutes  132  or  134 . In the illustrated embodiment, there are eight ball bearings in each bearing  140  and seven flutes  132  or  134  in each bearing cavity. 
   While the invention has been shown and described with respect to particular embodiments thereof, those embodiments are for the purpose of illustration rather than limitation, and other variations and modifications of the specific embodiments herein described will be apparent to those skilled in the art, all within the intended spirit and scope of the invention. Accordingly, the invention is not to be limited in scope and effect to the specific embodiments herein described, nor in any other way that is inconsistent with the extent to which the progress in the art has been advanced by the invention.