Lap structure of a lapping tool and lapping process

Abstract: To provide a lap structure of a lapping tool which is superior in wear and abrasion resisting property and maintains a lap shape for a long period of time without requiring any correction working. The lap structure of a lapping tool of the wet type wherein lapping is performed for a surface of a work using abrasion grain to finish the surface of the work is constructed such that a particle holding layer 11 wherein a particle holding member 12 softer than the abrasion grain 14 embeds and holds hard particles 13 in a substantially uniformly dispersed condition is formed on a lap surface, that a surface of the particle holding layer 11 is polished so that flat surfaces of the hard particles 13 polished together are exposed to the surface of the particle holding layer 11, and that the abrasion grain is held on the surface of the particle holding member 12 between the exposed hard particles 13.

Description: This invention relates to a lapping tool of the wet type for lapping a surface of a workpiece using polishing grain material, and more particularly to a lap structure of a lapping tool and a lapping process. A lapping tool of the wet type performs surface treatment with a mixture of abrasion grain and lap liquid interposed between a workpiece and a lap. Conventionally, a lapping tool of the wet type has a lap structure which has a suitable hardness like that of cast iron and has polishing grain material bitten in the surface of the lapping tool itself. However, since such a holding member as cast iron for holding abrasion grain is made of a material softer that polishing grain material, also the holding member itself is abraded by lapping and cannot maintain a predetermined shape. Accordingly, where a high surface accuracy is required, correction working to correctly work the tool is necessitated as occasion calls. Accordingly, a high cost is required when lapping for a special shape with which a high cost is required for correction working is performed. JP-A-63251170 shows a lap structure and a process for lapping according to the preamble of claims 1 and 4. There, the hard particles have no flat surface, and no additional polishing grain material is used. The present invention has been made in view of such a situation as described above, and it is an object of the present invention to provide a lap structure of a lapping tool which is superior in wear and abrasion resisting property and maintains a lap shape for a long period of time without requiring any correction working. In order to attain the object described above, according to the present invention, there is provided a lap structure according to claim 1 and a lapping process according to claim 4. Since the flat polished surfaces of the dispersedly embedded hard particles are exposed discretely to the surface of the particle holding member and the particle holding member is softer than the polishing grain material to be used for lapping, the polishing grain material abrades the particle holding member other than the hard particles at an initial stage so that recessed portions are formed between the hard particles, and the polishing grain material is admitted in the recessed portions and bites into and is held by the particle holding member. Consequently, the surface of the workpiece is lapped by the polishing grain material thus held in the particle holding member. Since the hard particles are exposed at the flat faces thereof to the surface of the lap, the lapping tool is superior in wear and abrasion resisting property and the lap shape is maintained for a long period of time. Accordingly, also in working for which a high surface accuracy is required, correction working, which is working in which correction of the tool is performed as occasion calls midway of working, is not required, and reduction of the cost can be achieved. Where nickel is used for the particle holding member and diamond is used for the hard particles, the lapping tool can be suitably used for works made of iron, particularly works made of a base material of an SUS material to which Cr is plated or works made of an SUS material processed by nitriding or other cast ion works. Where the lapping tool laps a surface of a non-circular work, conventional reciprocating lapping while the lap is turned as occasion calls is impossible, since, according to the present invention, the lap shape is always maintained by the hard particles, working with a high degree of accuracy can be performed only by reciprocating movements of the lapping tool which does not involve any turning motion, and no correction working is required, either. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a lapping tool according to an embodiment of the present invention. FIG. 2 is a vertical sectional view of the lapping tool. FIG. 3 is an enlarged sectional view showing a structure of a surface of a lap of the embodiment. FIG. 4 is an enlarged sectional view of the surface of the lap illustrating a process of forming the lap surface structure. In the following, an embodiment of the present invention shown in FIGS. 1 to 4 will be described. In the present embodiment, the present invention is applied to a lapping tool 1 for finishing an outer peripheral face of a piston ring, and as shown in FIGS. 1 and 2, an inner peripheral face of a lapping machine 2 in the form of an elliptic cylinder forms a lap surface 3. A holder 5 in the form of a flattened elliptic column is fixedly mounted at a lower end of a support shaft 4 which is inserted from above in the lapping machine 2, and piston rings 6 each of which is a work are fitted and held in three grooves formed along a circumferential direction on an outer peripheral face of the holder 5. Each of the piston rings 6 is made of wear and abrasion resisting cast iron formed as an elliptic ring, and an elliptic outer peripheral face of it is to be lapped. Also other members such as a member formed from a base material of an SUS material and plated with Cr or a member of an SUS material processed by nitriding are suitable as such works. Some gap is left between the outer peripheral face of the holder 5 and the inner peripheral face of the lapping machine 2, and the piston rings 6 fitted on the outer peripheral face of the holder 5 extend outwardly from the outer peripheral face of the holder 5 and are held in pressure contact with the inner peripheral face of the lapping machine 2, that is, the lap surface 3 under a predetermined lapping pressure. The holder 5 having a center axis aligned with the lapping machine 2 is moved upwardly and downwardly so that the outer peripheral faces of the piston rings 6 are slidingly contacted with the lap surface 3 to lap the outer peripheral faces. The lap surface 3 of the lapping machine 2 is shown in an enlarged scale in FIG. 3. A particle holding layer 11 is formed on the surface of a base member 10 of the lapping machine 2, and in the present embodiment, the particle holding layer 11 is constructed such that diamond particles 13 which serve as hard particles are embedded and held in a substantially uniformly dispersed condition in nickel 12 which serves as a particle holding member softer than SiC (normally called carborundum) 14 serving as abrasion grain. The diamond particles 13 are polished at surfaces thereof into flat faces and exposed, and the exposed faces of the diamond particles 13 lie substantially in a plane and the SiC 14 is received in abraded recessed portions between the diamond particles 13 and bites in and is held by the nickel 12. Here, the particle size of the diamond particles 13 is 150 to 250 µm and is approximately 200 µm in average. The SiC 14 as abrasion grain has a diameter around or less than 30 µm. The particle size of the abrasion grain is selected in accordance with a step stage of lapping working. A process of forming the lap surface 3 of such a structure as described above is illustrated in FIG. 4. First, as seen in (1) of FIG. 4, soft nickel 12 in which diamond particles 13 are dispersed and mixed substantially uniformly is electro-deposited on the surface of a base member 10 using a popularly industrialized technique. Then, the surface of the particle holding layer 11 is polished as seen in (2) of FIG. 4 until it has the thickness of approximately 100 µm. By the polishing, also the diamond particles 13 are polished so that flat faces of them are formed and exposed to the surface. The ratio (diamond density) of the total area of the exposed flat faces of the diamond particles 13 to the overall surface area of the particle holding layer 11 is approximately 60 to 80 percent. Lapping working is started with such a surface condition of the lap as described above. Piston rings 6 fitted with and held by the holder 5 are inserted into the cylinder of the lapping machine 2, and the piston rings 6 are moved back and forth in upward and downward directions to start lapping working with a mixture of SiC 14 and lapping liquid interposed between the piston rings 6 and the lap surface 3. Then, at an early stage after the lapping working is started, the nickel 12 is abraded by the SiC 14 so that recessed portions between the diamond particles 13 are formed, and the SiC 14 is admitted into and held in the recessed portions as seen in (3) of FIG. 4 while the SiC 14 simultaneously bites into and is then held by the nickel 12 which forms the bottoms of the recessed portions. Thereafter, the lap surface 3 keeps its condition shown in (3) of FIG. 4, and the outer peripheral faces of the piston rings 6 are lapped by the SiC 14 admitted in the recessed portions between the diamond particles 13. Since the flattened faces of the diamond particles 13 are exposed to the lap surface 3, the lap surface 3 is superior in wear and abrasion resisting property, and the lap shape is maintained for a long period of time. Accordingly, in finishing the outer peripheral surfaces of the piston rings 6, no later correction working need be performed additionally. Consequently, improvement in productivity can be achieved. And, the lapping tool is suitable for mass production and can be produced at a reduced cost. By the way, if a surface of a work to be worked has a circumferential face, then the plane pressure accuracy can be maintained readily by performing reciprocating lapping working while the lap is turned as occasion calls as in the prior art. However, if the piston rings 6 as works have an elliptic shape as in the present embodiment or have some other special shape, then reciprocating lapping working which involves turning motion of the lap cannot be performed, and consequently, it is difficult to maintain the plane pressure accuracy. In particular, when lapping working which does not involve turning motion of the lap is performed, if the lapping plane pressure is locally high, then where the tool is low in wear and abrasion resisting property, abrasion proceeds only at the location of the tool in a short time and the shape of the tool is varied. Besides, even for reworking to an over-size, the working cost is increased. The lapping tool 1 of the present embodiment can perform surface working of such a non-circular work as described above with a high degree of accuracy only by reciprocating movements of the lap since the diamond particles 13 exposed to the lap surface 3 maintains the lap shape and is superior in wear and abrasion resisting property. And, where the work has an elliptic shape like the piston rings 6 in the preset embodiment, the working time can be reduced and the productivity can be enhanced by lapping the entire elliptic profile at a time as in the case of the lapping tool 1 in the present embodiment. The present invention seeks to provide a lap structure of a lapping tool which is superior in wear and abrasion resisting property and maintains a lap shape for a long period of time without requiring any correction working.

Claims: A lap structure of a lapping tool (1) of the wet type wherein for lapping a surface of a workpiece (6), said structure comprises: a particle holding layer (11) formed on a lap face (3), wherein a particle holding member (12) embeds and holds hard particles (13) in a substantially uniformly dispersed condition, wherein the hard particles (13) are exposed to the surface of said particle holding layer (11), characterized in that an exposed surface of said particle holding layer (11) together with the exposed surfaces of said hard particles (13) are provided in a polished condition so that the respective exposed polished surfaces of the particle holding layer (11) and of the hard particles (13) lie substantially in a common plane, and in that the particle holding member (12) is made of a relatively soft material so that polishing grain material (14) used to finish the surface of the workpiece is able to form recesses on the surface of said particle holding member (12) between the exposed hard particles (13) for holding the polishing grain material (14). A lap structure of a lapping tool (1) according to claim 1, characterized in that nickel is used for said particle holding member (12), and diamond is used for said hard particles (13). A lap structure of a lapping tool (1) according to claim 1, characterized in that said lapping tool (1) laps a surface of a non-circular workpiece (6). Process for lapping a surface of a workpiece (6) with a lap structure of a lapping tool (1) of the wet type, said structure comprising: a particle holding layer (11) formed on a lap face (3), wherein a particle holding member (12) embeds and holds hard particles (13) in a substantially uniformly dispersed condition, characterized in that a surface of said particle holding layer (11) is polished so that flat surfaces of said hard particles (13) polished together are exposed to the surface of said particle holding layer (11), and that polishing grain material (14) used to finish the surface of the workpiece is held on the surface of said particle holding member (12) between the exposed hard particles (13), wherein the particle holding member (12) is softer than the polishing grain material (14).

Inventors: KUDOU OSAMU, MIURA SEISHI, ONIMARU MASAYUKI, SATOU ISAO, KUDOU, OSAMU, MIURA, SEISHI, ONIMARU, MASAYUKI, SATOU, ISAO
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1996
Main classification: B24B19
IPC codes: B24B33, B24B19, B24B37, B24D99