Abstract:
Besides the conventional process steps of manufacturing a valve-seat body, forming a through opening inside the valve-seat body, forming a valve-seat as a frustoconical section of the through opening, and forming a guide area, the proposed method for manufacturing a valve seat for a fuel injector includes the simultaneous fine-machining of all guide sections in the guide area and the valve-seat area using a master ball.

Description:
BACKGROUND INFORMATION 
     1. Field of the Invention 
     The present invention is based on a method for manufacturing a valve-seat body having a valve seat for a fuel injector and for manufacturing a fuel injector. 
     2. Background Information 
     German Application No. 40 37 952 already describes a fuel injector having a valve-seat body which, inter alia, has a guide bore and a valve seat. The guide bore serves for guiding an axially movable valve needle which is provided with a valve-closure member designed as a ball. This ball co-operates with the valve seat which tapers frustoconically in the downstream direction, forming a sealing-seat valve with it. In the guide bore located upstream of the valve seat, guide sections and fuel ducts alternate over the circumference of the guide bore. In the case of a conventional valve-seat body of this kind, both the guide sections and the valve seat are reworked upon reforming (massive forming, turning). In this context, the guide sections are machined separately from the machining of the valve seat in terms of time and tools by relatively inaccurate internal cylindrical grinding. In internal cylindrical grinding, an abrasive pencil is introduced into the guide bore and used for machining the guide sections in a rotational movement. The valve seat is also fine-machined by grinding, additional reworking by honing being necessary depending on the requirements. To achieve a high rotational accuracy combined with an optimum sealing behavior, several machining tools and sequential fine-machining steps are necessary. 
     Known from, for example, German Application No. 196 02 068, is a method for manufacturing rotationally symmetric valve-seat faces having a high surface finish at valves, in which method the valve seat is reworked as described above using a spherical tool body. In this context, the spherical tool body is designed with a diameter which is smaller than the cross-section of the guide opening in the valve-seat body to be machined so that only the immediate valve-seat face is fine-machined. The clearance of the guide opening must inevitably be greater than the diameter of the spherical tool since the tool could otherwise not immerse through the, in an axial direction, relatively long guide opening up the valve-seat face in the valve-seat body at all. Furthermore, there is such a great cutting volume for fine-machining in such a cylindrical guide opening that it is impossible to use a spherical tool body for the guide area. 
     Furthermore, German Patent Application No. 195 37 382 already describes a fuel injector which has a valve-seat body as well as a disk-shaped guide body lying upstream thereof. In this context, the guide body has an at least partially dome-shaped internal guide opening for a spherical valve member. The valve-seat body and the guide body are fine-machined, in each case, separately of each other at their internal openings to be made accurately. This is also carried out using different machining tools in different chucks. 
     SUMMARY OF THE INVENTION 
     The method according to the present invention for manufacturing a valve-seat body having a valve seat for a fuel injector has the advantage that, both guide sections and a valve-seat area in a valve-seat body are fine-machined most accurately in a simple fashion requiring little outlay of material, time, and tools. In this context, it is particularly advantageous that only one single machining tool, namely a very accurately formed “master ball”, is required for the fine-machining of the different areas, which, besides, is carried out simultaneously in an ideal manner. 
     It is particularly advantageous to fine-machine the guide sections and the valve seat by ball honing, or ball precision grinding or ball lapping. Using these processes, it is possible to remove minimal quantities of material at the desired locations in the valve-seat body so that, compared to known grinding methods, there are only very small cutting volumes resulting from the, in terms of the surface area, very small guide sections. 
     Using this machining technology, desired minimal curvatures are produced at the guide sections which have a radius which corresponds to the radius of the master ball. The guide sections, from the start, are advantageously formed narrow in the axial direction and in the circumferential direction, and have therefore a small surface area so that they can be accurately machined in an optimum fashion using the master ball. Thus, rotational accuracies are achieved in an advantageous fashion, which cannot be achieved in the case of conventional ball/conical sealing-seat arrangements with a comparably small outlay. 
     The fuel injector according to the present invention has the advantage that a valve-seat body having a valve-seat area and a guide area can be fine-machined in a particular simple and cost-effective manner, and, moreover, in an extremely high quality with regard to rotational accuracy and tightness. For that, the guide sections, from the start, are advantageously formed narrow in the axial direction and in the circumferential direction, and have therefore a small surface area so that they can be accurately machined in an optimum fashion using the master ball as machining tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a fuel injector having a specially formed valve seat which is manufactured according to the present invention. 
     FIG. 2 shows the valve-seat body including a “master ball”. 
     FIG. 3 shows a top view of the valve-seat body including a spherical valve-closure member which is located inside and cooperates with the valve seat. 
     FIG. 4 shows a top view of only the valve-seat body in which the contact spots of the valve-closure member in the valve-seat body are identified not to scale. 
    
    
     DETAILED DESCRIPTION 
     As an exemplary embodiment, an electromagnetically operated valve in the form of a fuel injector for fuel-injection systems of mixture-compressing internal combustion engines with externally supplied ignition is partially depicted in FIG.  1 . The valve has a tubular valve-seat body  1 , in which a longitudinal borehole  3  is formed concentrically to a longitudinal valve axis  2 . An axially movable valve needle  6  is arranged in longitudinal borehole  3 . 
     The electromagnetic operation of the valve is carried out in a conventional manner. For axially moving valve needle  6  and, consequently, opening the valve against the spring resistance of a return spring  8 , and respectively, closing the valve, an only partially shown electromagnetic circuit including a magnetic coil  10 , a core  11 , and an armature  12  is used. Valve needle  6  is formed of armature  12 , a spherical valve-closure member  13 , and a connecting part  14  connecting these two component parts, connecting part  14  having a tubular design. Return spring  8  supports itself against the upper end face of connecting part  14  with its bottom end. Armature  12  is connected to the end of connecting part  14  facing away from valve-closure member  13  via a welded seam  15 , and aligned with core  11 . On the other hand, valve-closure member  13  too is firmly connected to the end of connecting part  14  facing away from armature  12 , for example, via a welded seam  16 . Magnetic coil  10  surrounds core  11  which represents the end, enclosed by magnetic coil  10 , of a fuel inlet connection which is not further identified, and which serves for supplying the medium to be metered in by the valve, here fuel. 
     Concentrically to longitudinal valve axis  2 , a tubular metal intermediate piece  19  is joined to the bottom end of core  11  and to valve-seat support  1 , e.g., by welding, in a sealing fashion. In the downstream end of valve-seat support  1  facing away from core  11 , a cylindrical valve-seat body  25  is mounted by welding in a sealing manner in longitudinal borehole  3 , which runs concentrically to longitudinal valve axis  2 . The valve-seat body  25  designed according to the present invention has a fixed valve-seat area  26  facing core  11 . 
     Magnetic coil  10  is, at least partially surrounded in the circumferential direction by at least one conductive element  30  used as a ferromagnetic element which is designed, for example, as a bracket, and which engages on core  11  with its one end, and on valve-seat support  1  with its other end, and is connected to these by welding, soldering or bonding. 
     A guide area  31  of a through opening  32  of valve-seat body  25  serves for guiding valve-closure member  13  during the axial movement. Valve-seat area  26  also represents an area of through opening  32  which, for example, immediately adjoins guide area  31  in the downstream direction. At its one bottom end face  33  facing away from valve-closure member  13 , valve-seat body  25  is concentrically and firmly connected to a spray-orifice plate  34  having, for example, a pot-shaped design. The connection between valve-seat body  25  and spray-orifice plate  34  is made, for example, by a continuous and tight welded seam  45  which is made, for example, using a laser. By this method of assembly, the risk of an unwanted deformation of spray-orifice plate  34  in the area of its at least one, for example, four spray orifices  46  produced by erosive machining or punching is prevented. In an advantageous manner, spray-orifice plate  34  should be fixed to valve-seat body  25  prior to the fine-machining of valve-seat body  25  which will still be described in more detail in the following. 
     The insertion depth of the valve-seat part composed of valve-seat body  25  and spray-orifice plate  34  into longitudinal borehole  3  determines, inter alia, the adjustment of the stroke of valve needle  6  since the one end position of valve needle  6  is determined by the engagement of valve-closure member  13  on valve-seat area  26  when magnetic coil  10  is deenergized. The other end position of valve needle  6  is determined, for example, by the engagement of a top end face  22  of armature  12  on a bottom end face  35  of core  11  when the magnetic coil  10  is energized. The travel between these two end positions of valve needle  6  represents the stroke. 
     The spherical valve-closure member  13  co-operates with the, in the downstream direction, frustoconically tapering surface of valve-seat area  26  of valve-seat body  25 . The immediate valve seat can also be formed by a narrow annular seat area  26 ′ which is slightly raised compared to the frustoconically formed surface. In such a case, annular seat area  26 ′ projects from valve-seat area  26  by approximately 50 to 100 μm. Guide area  31  has a plurality of flow passages  27  allowing the medium to flow in a direction towards valve seat  26 ,  26 ′ of valve-seat body  25 . 
     FIG. 2 shows seat body  25  as individual component part together with a “master ball”  130  which is used as a machining tool for fine-machining in the practical application of the manufacturing process according to the present invention. In this context, master ball  130  is attached to, for example, a bar-shaped, rotating tool-holding body  129  which, in a comparable form, is known, for example, from German Application No. 196 02 068. Through opening  32  in valve-seat body  25  has a plurality of differently formed sections or areas which axially adjoin each other. In this context, the essential areas of through opening  32  are an inlet area  47  which tapers in the downstream direction, a middle opening area  48  which has a greater inside diameter than the diameter of spherical valve-closure member  13  or master ball  130  respectively, guide area  31 , valve-seat area  26  or annular seat area  26 ′ respectively, as well as an outlet area  49 . While areas  47 ,  48 ,  26  or  26 ′, and  49  have a uniform design over their circumference, guide area  31  is characterized by a sequence of web-type guide sections  51  and duct-type flow passages  27  alternating over its circumference. This above described contour of the inner through opening  32  as well as the otherwise substantially cylindrical outside contour are produced in known manner by appropriate creative forming and massive forming (e.g., cold working, cold pressing; optionally hardening). 
     According to the present invention, the final fine-machining of valve-seat area  26  and guide sections  51  in guide area  31  is carried out simultaneously using master ball  130 . In this context, the very hard master ball  130 , which can be produced very accurately and has an ideal spherical shape, has a slightly greater diameter than valve-closure member  13  which cooperates later with valve seat  26 ,  26 ′. The fine-machining of valve-seat body  25  using master ball  130  is a honing (ball honing), or precision grinding, or lapping in which finest-grained honing oils, lapping pastes or grinding pastes are used, making it possible to remove minimal quantities of material at the desired locations in valve-seat body  25 . Using this machining technology, desired minimal curvatures are produced at guide sections  51  which have a radius which corresponds to the radius of master ball  130 . Guide sections  51 , from the start, are advantageously formed very short and narrow in the axial direction and in the circumferential direction, so that they can be accurately machined in an optimum fashion using master ball  130 . 
     In this context, guide sections  51 , as viewed in an axial direction, are located in an ideal fashion in the area of ball equator  52 ,  52 ′ of master ball  130 , or respectively, of valve-closure member  13  which will later be arranged there, the guide sections  51  beginning minimally before ball equator  52 ,  52 ′ following opening area  48  as viewed, for example, in the downstream direction. Ball equator  52 ′ of valve-closure member  13  is indicated in FIG.  1 . Guide sections  51 , while facing away from valve-seat area  26 ,  26 ′, extend axially to the extent that they project beyond ball equator  52 ′ of valve-closure member  13  maximally by only 150 μm when valve-closure member  13  engages on valve-seat area  26 ,  26 ′. Flow passages  27  which, in each case, have a radially outer passage bottom  54  having, for example, the radius of opening area  48  as an extension thereof, extend between the individual guide sections  51 . As elucidated in FIGS. 3 and 4 which are top views of valve-seat body  25 , it is useful to provide five guide sections  51  and five flow passages  27  in guide area  31  in an alternating fashion over the circumference. However, designs using different numbers are also conceivable but at least three guide sections  51  should exist at all events. The top view of valve-seat body  25  shown in FIG. 4 is primarily intended to elucidate the places of contact of valve-closure member  13 , which is not shown here, in the area of valve seat  26 ,  26 ′, and at guide sections  51  in valve-seat body  25  respectively, the used blackenings not representing a true-to-scale marking. 
     Thus, areas  26 ,  26 ′,  51  of valve-seat body  25  which perform sealing and guiding functions are simultaneously fine-machined with the assistance of the very exactly formed master ball  130 . In the application of the ball honing, or precision grinding, or lapping using master ball  130 , the immediate sealing surface, which, in the exemplary embodiment shown in FIG. 2, is the slightly raised annular seat area  26 ′, as well as guide sections  51  are exactly adapted to the form of master ball  130 , or the minimally smaller valve-closure member  13  respectively. In the process, master ball  130  transfers its curvature to the axially very short guide sections  51 , the end result being slightly curved guide sections  51  at the end of guide area  31  facing away from valve-seat area  26 . Using the above described machining technology, rotational accuracies are achieved in an advantageous fashion, which cannot be achieved in the case of known ball/conical sealing-seat arrangements with a comparably small outlay. This manufacturing process guarantees nearly ideal roundnesses in valve-seat area  26 , having deviations (circularity tolerances) of only 0.5 μm or less.