Abstract:
A method of joining multiple powder metal components to form a powder metal component assembly using an adhesive is disclosed. By machining at least one of the powder metal components prior to the adhesive joining, otherwise difficult to machine features can be more easily machined for less cost and at higher production rates. Unlike high temperature joining techniques, the adhesive joins the powder metal components at room temperature. This room temperature adhesive joining eliminates the thermal distortions in pre-joined machined features common to high temperature joining techniques such as brazing or welding that bring these features out of specification during joining.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This represents the national stage entry of PCT International Application No. PCT/US2009/046423 filed Jun. 5, 2009 which claims the benefit of U.S. Provisional Application 61/175,154 filed May 4, 2009 which is hereby incorporated by reference for all purposes. 
    
    
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     This invention relates to the joining of powder metal components. In particular, this invention relates to the joining of power metal components, such as rotors and adaptors, using adhesive. 
     BACKGROUND OF THE INVENTION 
     Powder metallurgy provides a cost effective way of producing components having relatively complex shapes at an acceptable cost. By compacting a powder metal into a preform and then sintering the preform into a sintered part, a porous body can be formed that is dimensionally close to the final desired component. Secondary operations, such as machining, grinding and the like can be used, and in some cases may be necessary, to bring the sintered part within dimensional specifications. The near net shape fabrication techniques used in powder metallurgy reduce the amount of time needed to complete these secondary operations and also minimize the scrap produced as the sintered parts are dimensionally very close to the final desired size of the part. However, some complex parts have dimensional requirements that make their fabrication difficult, even using powder metallurgy. 
     For example, a variable valve timing (VVT) rotor has many features and dimensional requirements that make fabrication difficult. The VVT rotor has a flat surface that must be extremely smooth (total tolerance of 15 microns) and that has an adjacent surface that is perpendicular to and that extends outwardly from this flat surface. During part fabrication, this flat surface can not be easily ground because of the adjacent surface. Bringing this flat surface into an acceptable range might be done in a number of ways. 
     Elaborate machining techniques could be used to finish this flat surface. However, these techniques are very expensive, require skilled labor, and take time to perform. Moreover, the initial compaction of a part having this kind of complex shape may require the use of high technology presses that are costly and difficult to operate and maintain. 
     Alternatively, a VVT rotor could be fabricated as two separate components, the flat surface ground within the specification, and then the two components could be brazed or welded together. However, even this process requires additional finishing once the parts are brazed or welded together, because thermally joining the components together induces stresses and creates distortions in the components that bring the flat surface out of the acceptable range. 
     Hence, there is a need for an improved method for making powder metal components having difficult to achieve dimensional requirements and a need for the powder metal components themselves. 
     SUMMARY OF THE INVENTION 
     The present invention uses adhesive to join multiple powder metal components together. By fabricating the powder metal components separately and then adhesively joining them together to form an assembly, there is an opportunity before joining the components to machine some of the surfaces that would be difficult to machine if the part was formed as a unitary body instead of as an assembly (e.g., grinding a flat surface that has a feature extending therefrom). Furthermore, the powder metal components are joined at or near room temperature and so, unlike brazing or welding, the adhesive joining of the powder metal components does not induce stress or thermal distortions in the powder metal components that brings their pre joined dimensions out of an acceptable range. 
     Rib structures can be applied at the interface surfaces to assure adhesion and/or sealing at the interface surfaces, and to position the components relative to one another and aid in assembly. 
     Adhesive joining of powder metal components could be used in any one of a number of types of powder metal assemblies. In one particularly advantageous form, a rotor assembly for a VVT engine is formed using the adhesive joining methods described herein. This rotor assembly has a flat surface formed on the rotor with an adaptor extending therefrom. The flat surface on the rotor can be ground within an acceptable range and then the adaptor can be adhesively joined such that the flat surface does not go outside of the acceptable range during the joining step. Moreover, once the rotor and the adaptor are joined to form the rotor assembly, an inner diameter of both can be turned to produce a finished rotor assembly having acceptable concentricity in the through hole. 
     The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front exploded perspective view of two powder metal components to be assembled according to the invention; 
         FIG. 2  is a rear exploded perspective view of the components of  FIG. 1 ; 
         FIG. 3  is a front perspective view of the components assembled; 
         FIG. 4  is a rear perspective view of the assembled components; 
         FIG. 5  is a flow chart of the process used to make the finished rotor assembly; 
         FIG. 6  is a perspective view of a part to which the invention can be applied; and 
         FIG. 7  is a perspective view of the part of  FIG. 6  separated into components for application of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIGS. 1-2 , a blank adaptor  10  and a rotor  12  are shown that can be adhesively joined to one another to form a rotor assembly  14  which undergoes finishing operations to form a finished rotor assembly. The adaptor  10  and the rotor  12  are both formed using powder metallurgical processes. Typically, this includes uniaxially compacting a powder metal and binder material in a tool and die set to form a powder metal preform and then sintering the powder metal preform to form a sintered part. Other steps known to those skilled in the art may also be used during this forming process including, but not limited to, burning off some of the binder material prior to sintering to reduce carbon content, forging the sintered part, coining the sintered part, heat treating the sintered part, and the like. The hole  58 , made up of holes  58 A and  58 B would typically not be formed during compaction or sintering, but would typically be drilled either after sintering or after both parts are assembled together, but the holes  58 A and  58 B are illustrated in  FIGS. 1-4  for clarity and ease of description. 
     Referring now to  FIGS. 1-4 , the details of the blank adaptor  10  are shown. The blank adaptor  10  has a generally cylindrical body  18  with an axially-extending through hole  20  having a radially inward facing surface  21  or inner diameter. On a mating end  22  that mates with the rotor  12 , the blank adaptor  10  has a plurality of radially outward facing interface surfaces  24  and recesses  26  formed therebetween. The recesses  26  are radially offset from the radially outward facing interface surfaces  24 . 
     The details of the rotor  12  are also shown in  FIGS. 1-4 . The rotor  12  has a generally cylindrical body  28  extending from a first flat surface  30  to a second flat surface  32 . The first flat surface  30  and the second flat surface  32  are essentially parallel to one another and are both perpendicular to an axis of rotation A-A. Angularly spaced about a radially outward facing surface  34  of the body  28 , a plurality of teeth  36  are formed. 
     The body  28  also has an axially-extending through hole  38  with a portion for mating with the mating end  22  of the blank adaptor  10  and with a portion for defining a segment of the through hole after the rotor  12  and the blank adaptor  10  are joined and finished. A ledge  40  that extends perpendicular to the axis of rotation A-A separates these two portions. On one side of the ledge  40 , the portion of the axially-extending through hole  38  for mating with the mating end  22  of the blank adaptor  10  has a plurality of radially inward facing interface surfaces  42  with a plurality of recesses  44  formed therebetween. The plurality of recesses  44  are radially offset from the plurality of interface surfaces  42 . 
     On the other side of the ledge  40 , a radially inward facing surface  46  defines a through hole  48  which, after the rotor  12  and the blank adaptor  10  are adhesively bonded and joined, will be turned to form a portion of the finished through hole  56 . 
     The rotor  12  and the blank adaptor  10  are joined in the manner outlined in  FIG. 5  to form the unfinished rotor assembly  14  and then, after step  118 , the finished rotor assembly. The finished rotor assembly would look like the unfinished assembly in  FIGS. 3 and 4 , except the surfaces that are finished after assembly, like the ID of the bore, would be machined. First, the blank adaptor  10  and the rotor  12  are separately compacted in a compaction step  110  and are separately sintered in a sintering step  112  to form the components shown separated in  FIGS. 1 and 2 , respectively. Next, the first flat surface  30  and the second flat surface  32  of the rotor  12  are ground in a pre joining machining step  114  prior to joining the rotor  12  and the blank adaptor  10 . In the form shown, the first flat surface  30  and the second flat surface  32  are finished using fine grinding and brush deburring to obtain a total tolerance of less than 15 microns. However, where other types of powder metal components are being joined, the pre-joining machining step  114  may include other types of machining operations known to those skilled in the art. 
     Next, in a joining step  116 , the mating portions of the rotor  12  and the blank adaptor  10  are adhesively joined to one another to form a rotor assembly  14  as seen in  FIGS. 3 and 4 . First, a bead of an adhesive  50  is applied to the radially inward facing interface surfaces  42  of the rotor  12  as shown in  FIG. 2 . The mating end  22  of the blank adaptor  10  is then telescopically slid into the mating portion of the axially-extending through hole  38  of the rotor  12  until the blank adaptor  10  abuts or approaches the ledge  40 . During this insertion, the adhesive  50  wets the interface surfaces  42  and  24  between the rotor  12  and the blank adaptor  10 , respectively, such that the adhesive  50  forms a seal between the rotor  12  and the blank adaptor  10 . This seal prevents the leakage of hydraulic fluid at the adhesively joined interface in the finished rotor assembly  16  through the oil feed hole  58  as will be described below in more detail. The adhesive  50  is left to cure at or near room temperature (e.g., typically at or below 120 degrees Fahrenheit), preferably without the addition of external heat in addition to ambient, to join or bond the rotor  12  to the blank adaptor  10 . 
     The adhesive  50  could also be applied in a variety of ways other than by the above-described application of a bead to one of the interface surfaces. The adhesive  50  could also be applied to the radially outward facing surfaces  24  of the blank adaptor  10  in addition to, or instead of, applying the adhesive  50  to the radially inward facing interface surfaces  42  of the rotor  12 . Further, the adhesive  50  could be spread over the interface surfaces  24  and  42  in a number of other ways including, but not limited to, brushing it on the interface surfaces, spraying it on the interface surfaces, and the like. It is contemplated that the adhesive  50  may in part seep into the pores of the rotor  12  and the blank adaptor  10  to further strengthen the bond between the two components. 
     Various types of adhesives may be used. The adhesive  50  is preferably an anaerobic adhesive that does not need to be exposed to oxygen to cure. The adhesive may also be an epoxy that is activated upon the mixing of two or more constituents. However, any adhesive suitable for use with metal could be used to join the components. 
     Notably, the use of the adhesive  50  that cures at room temperature (without the addition of external heat) as a means of joining the rotor  12  and the blank adaptor  10  does not induce stresses or thermal distortions that would bring the first flat surface  30  or the second flat surface  32  out of an acceptable dimensional range. In contrast, if the rotor  12  and the blank adaptor  10  were joined by brazing, welding, or another high temperature joining process commonly used to join metallic components, then thermal distortions would be highly likely to occur that would bring the flat surfaces  30  and  32  back out of the acceptable range achieved during the pre joining machining. 
     It is contemplated that interlocking features could be formed in the blank adaptor  10  and the rotor  12  along the interface. In one form, these interlocking features are formed where recesses  26  and  44  are located. These interlocking features supplement, but do not replace, the adhesive joining of the components. When the rotor assembly is subjected to high rotational stresses about the axis of rotation A-A during service, such interlocking features may provide an interference that prevents the adaptor from shearing from the rotor along the adhesive interface. 
     In one form as illustrated in  FIGS. 1 and 2 , one or more radially-extending interlocking ridge and groove sets are formed along the interface to interlock the two components. The adaptor  10  has a ridge  27  that fits closely in one of the recesses  44 . Upon telescopically inserting the mating end  22  of the blank adaptor  10  into the axially-extending through hole  38  of the rotor  12 , the ridge  27  formed on the blank adaptor  10  slides into a matching recess  44  formed in the rotor to interlock the components against relative rotation. Of course, multiple sets of ridges and grooves could be formed along the interface. Further, a first component could have both ridges and grooves formed thereon that interlock with a second component have mating grooves and ridges. 
     Although ridges and grooves are described as one example of the interlocking features, it is contemplated that other geometries may be suitable to interlock the components. Any geometry in which a portion of a first component interlocks with a portion of a second component to prevent shearing of the components relative to one another under extreme stress is suitable. It is further contemplated that, in some forms, a key could be placed between a recess formed in the first component and a recess formed in the second component to perform the same function as interlocking features. 
     It should be appreciated that the blank adaptor  10  can be inserted into the rotor  12  in a number of ways. However, in a preferred form, the dimensions of the mating portions of the blank adaptor  10  and the rotor  12  will be close enough in size that a press fit is required to snuggly force the blank adaptor  10  and the rotor  12  together. 
     After the rotor  12  and the blank adaptor  10  are adhesively joined to form the unfinished rotor assembly  14 , the unfinished rotor assembly  14  can undergo various post-joining machining operations in a post-joining machining step  118  to form the finished rotor assembly  16  ( FIG. 4 ), which is the adhesively joined assembly after the post joining machining operations. 
     In the form shown, these post-joining machining operations include turning outer diameter of the blank adaptor  10  to form a radially outward facing finished surface  57  and turning the inner diameter of a through hole  56 , which is made up of the combination of the through hole  20  of the blank adaptor  10  and the through hole  48  of the rotor  12 . By turning these diameters after the blank adaptor  10  and the rotor  12  have been adhesively joined, very tight concentricity requirements are virtually guaranteed in the finished rotor assembly  16 . 
     Further, post-joining, an oil feed hole  58 , made up of outer hole  58 A in the rotor  12  and inner hole  58 B in the adaptor  10 , can be machined that extends from an outer surface  60  of the finished rotor assembly  16  to the through hole  56 . As the oil feed hole  58  extends through the area of the adhesive interface, the previously mentioned seal formed across the interface by the adhesive  50  is important to ensure that any oil does not leak at the interface between the finished adaptor  52  and the finished rotor  54 . Any leakage of oil along at the interface would be detrimental to the performance of the finished rotor assembly  16 . 
     Surface  42  includes rib structure  70  which projects above the surrounding parts of surface  42  and includes spaced legs  72  joined at their inner ends by circumferentially extending base rib  76 . Hole  58  is between the legs  72 , and the ends of the legs  72  fit between legs  78  and abut against the side of base rib  80  of rib structure  82  that is included as part of each outwardly facing surface  24  and projects from the surrounding parts of surface  24 . When assembled, the hole  58  is surrounded by the rib structures  70  and  82 , and the legs  72  abutting against the side of base rib  80  encloses the hole  58  on four sides to cut off the leak paths around the ends of the ribs  72 . Ideally, the sides of the legs  72  also abut against the sides of the legs  78  to further insure against leakage around the ends of the legs  72 . 
     Adhesive may be applied as a bead on the surface  42  extending along the edge between the surface  42  and the flat surface  30  from one recess  44  to the next recess  44 . When the adaptor is inserted into the rotor, the adaptor wipes the bead of adhesive inwardly along the surface  42 , including along the axially extending legs  72  to wet the overlapping portions of both surfaces  24  and  42 , behind the leading edge of the adhesive being wiped inwardly along the surface  42 . When insertion reaches the laterally extending base rib  76 , the edge of base rib  76  between legs  72 , which runs laterally relative to the direction of adaptor insertion into the rotor, wipes adhesive upon further rotor insertion and inhibits adhesive from sagging onto ledge  40 . This helps to help retain the adhesive between the surfaces  24  and  42 , especially in the area between the legs  72 , and helps to assure an adequate quantity of adhesive bonding the interface surfaces  24  and  42 . 
     The rib structures  70  and  82  project above the adjacent portions of the respective interface surfaces  42  and  24  so as to create a small interference (e.g. 90 microns on the diameter) to a small clearance (e.g., 140 microns on the diameter) between the rib structures and the respective facing surface  24  and  42 . The rib structures make leakage of the hole  58  at the interface between the rotor and the adaptor less likely, and serve to radially locate the rotor and adaptor relative to each other. In addition, they preserve a small clearance between the adjacent surfaces  24  and  42 , that are spaced apart from one another by the rib structures, in which adhesive can reside. This insures that adhesive is not wiped completely clean off the interface surfaces when the parts are assembled, as could occur if there was no clearance between the interface surfaces. Also, if there is a clearance between the rib structures and the surfaces they face, adhesive will fill in those spaces as well. The rib structures also reduce the insertion forces necessary to assemble the rotor and the adaptor. For example, if there was interference between the interface surfaces themselves, without rib structures, the insertion forces may be several thousand pounds, whereas with rib structures they can be reduced to 150-200 pounds. 
     Although the embodiment described above describes the joining of the rotor  12  and the blank adaptor  10  to form a finished rotor assembly  16 , it is contemplated that this method could be used to form any number of powder metal component assemblies formed from a first powder metal component and a second powder metal component. Further, it is contemplated that the male/female joining portions could be switched on the components (e.g., the rotor could have a portion that extends into the adaptor during joining). 
       FIG. 6  illustrates an example  100  in which two generally cylindrical parts  102  and  104  are joined by a smaller portion  106  between them. When the surfaces  108  and  110  adjacent to the smaller portion are precision surfaces, or if the spacing between those surfaces must be precise, or if the individual lengths of the parts  102  and  104  must be precise while maintaining the overall length of the entire part, machining presents a challenge. The challenge is exacerbated even further if the smaller portion  106  is non-round. 
     To solve this problem, the invention can be applied and the portions split as shown in  FIG. 7 . The parts are made of sintered powder metal and the difficult to machine surfaces  108  and  110  are machined precisely prior to assembly, using ordinary machining such as fine grinding or lapping. The parts can then be joined adhesively according to the invention, and any subsequent machining or finishing processes can be performed. 
     As described above, using the method of adhesive joining components to form a powder metal component assembly offers many distinct benefits over traditional methods of joining multiple powder metal components using high temperature joining methods such as brazing or welding. The powder metal components in the assemblies made from the above-described method will retain their pre joined dimensions much better than those that are made using high temperature joining methods. Thus, assemblies can be formed that have complex geometries, but that also meet strict dimensional requirements that would be time consuming and/or costly to achieve in a part that is integrally formed. 
     A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiment described.