Abstract:
A tappet made of Al alloy is used in a valve operating mechanism of an internal combustion engine of a vehicle. On the whole outer circumferential surface of the tappet, a helical projection and a helical groove are formed alternately to form an uneven surface. A plurality of uneven surfaces are formed on a ridge of the projection at regular intervals. The outer circumferential surface is coated with a wear resistant Fe film by thermal spraying. The uneven surfaces on the projection provide high adhesion strength of the film to the outer circumferential surface of the tappet to improve peel resistance, thereby increasing durability and reliability of the Al alloy tappet.

Description:
This is a divisional of Ser. No. 08/901,982, filed Jul. 29, 1997, now U.S. Pat. No. 5,839,402. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a wear resistant cylinder such as an Al alloy tappet in which the outer circumferential surface is coated with a hard film, and a method of manufacturing it. 
     A tappet body used in a direct acting type valve to operating mechanism in an internal combustion engine is generally formed from Fe material such as steel and cast iron. Recently, in order to lighten the valve operating system, it is inclined to be made of Al alloy. 
     However, Al alloy tappet provides lower mechanical strength and lower wear resistance than Fe material. Thus, when the tappet is inserted in a cylinder head made of the same material, Al alloy, the sliding surface is likely to wear earlier or to cause scoring. 
     To solve the problem, the outer circumferential surface of Al alloy tappet may be coated with wear resistant material which is different from a base metal. If the outer circumferential surface of the base metal is flat, adhesion strength of the wear resistant material is low, and high peel resistance is not available, so that it is necessary to perform preliminary treatment for the base metal to make the whole outer circumferential surface of the tappet to a rough surface. 
     To make the rough surface, blasting is generally applied, but for the treatment, it is necessary to employ a specialized blasting apparatus, which takes a long time to involve low productivity and high consumption of blasting material, thereby increasing manufacturing cost. 
     To overcome the problem, U.S. Pat. No. 5,605,102 issued to Nobuo Hara et al. discloses a tappet in which the outer circumferential surface is made to a rough surface by simple means and is coated with wear resistant material. 
     Describing the invention in the U.S. Patent, as shown in FIG. 10 of an attached drawing, the whole outer circumferential surface of Al alloy cylindrical tappet is made to a threaded uneven surface 2 which comprises a projection 2a and a groove 2b, and is thermally sprayed to form a film 3. 
     The whole outer circumferential surface of the tappet body comprises the uneven surface 2 and the film 3 thereon. The projection 2a and groove 2b provide high adhesion strength of the film 3 to the base metal to improve peel resistance in an axial direction. But, in a circumferential direction, there is neither adhesiveness nor frictional resistance by the projection and groove, so that high peel resistance is not obtained, thereby providing low durability or reliability of the tappet. 
     SUMMARY OF THE INVENTION 
     To overcome the foregoing problem, it is an object of the present invention to provide a wear resistant cylinder such as an Al alloy tappet and a method of manufacturing it in which a projection and a groove of the outer circumferential surface of the cylinder is modified in form, thereby improving peel resistance of a film to a base metal in both axial and circumferential directions. 
     According to one aspect of the present invention, there is provided a wear resistant cylinder which comprises a cylinder body; a projection on an outer circumferential surface of the cylinder body; a groove which is formed adjacent to the projection on the outer circumferential surface of the cylinder body; and a wear resistant film with which the projection and the groove on the outer circumferential surface are coated, a recess being formed on a ridge of the projection. 
     Not only in the axial direction of the cylinder body but also in the circumferential direction, peel resistance of the film is improved, thereby increasing durability and reliability of the cylinder. 
     According to another aspect of the present invention, there is provided a method of manufacturing a cylinder, the method comprising the steps of forming a projection and a groove alternately on an outer circumferential surface of a cylinder body, an uneven surface being formed on a ridge of said projection at the same time, and coating the outer circumferential surface of the cylinder body with a wear resistant film. 
     It avoids conventional blasting, thereby facilitating manufacturing of the cylinder, such as a tappet, and decreasing cost. 
     According to a further aspect of the present invention, there is provided a method of manufacturing a wear resistant cylinder, the method comprising the steps of forming a projection and a groove on an outer circumferential surface of a cylinder body; cutting off a ridge of the projection by a suitable length in a circumferential direction to form an uneven surface; and coating the outer circumferential surface of the cylinder body with a wear resistant film. 
     According to a still further aspect of the present invention, there is provided a method of manufacturing a cylinder, the method comprising the steps of forming an optional groove on an outer circumferential surface of a cylinder body; pressing a sharp cutting tool against the outer circumferential surface of the cylinder body to move the cutting tool in an axial direction to form a helical groove and a helical projection which has an uneven surface on a ridge while the cylinder body is rotated; and coating the outer circumferential surface of the cylinder body with a wear resistant film. 
     The recess is formed without fail, thereby increasing reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more apparent from the following description with respect embodiments as shown in the drawings wherein: 
     FIG. 1 shows an embodiment of the first method in order of steps, (A) being a partially vertical sectioned front view of a tappet body which has a helical projection and groove on the outer circumferential surface, (B) being a partially vertical sectioned front view which shows the outer circumferential surface onto which a film is thermally sprayed; 
     FIG. 2 is an enlarged view of a portion (X) in FIG. 1; 
     FIG. 3 is an enlarged view of a portion (Y) in FIG. 1; 
     FIG. 4 is a horizontal sectional plan view taken along the line IV--IV in FIG. 3; 
     FIG. 5 is an enlarged sectional view which shows how to form a projection, a groove and an uneven surface; 
     FIG. 6 is an enlarged view of a portion (Z) in FIG. 1; 
     FIG. 7 is a sectional view similar to FIG. 5, showing an embodiment of the second manufacturing method; 
     FIG. 8 is a perspective view of another embodiment of the second manufacturing method; 
     FIG. 9 is a perspective view of an embodiment of the third manufacturing method; and 
     FIG. 10 is a partially vertical sectional view which shows a conventional method of manufacturing a tappet. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 (A) and (B) illustrate a method of the present invention in order of steps. 
     A cylindrical tappet body 1 is made of conventional Al alloy such as Al--Si--Cu, and has a bore la in which a shim (not shown) made of wear resistant metal fits, on the upper surface. 
     As shown in FIG. 1(A), the outer circumferential surface of the tappet body 1 comprises an uneven surface 2 which comprises a helical projection 2a and groove 2b having a predetermined pitch, and as shown on an enlarged scale in FIGS. 2 to 4, uneven surfaces 2c are formed on the projection 2a at regular intervals such as 0.1 to 3 mm in a circumferential direction. 
     To form the projection 2a, groove 2b and uneven surfaces 2c, there is a method as follows. 
     The first method is a method of forming the projection 2a, groove 2b and uneven surface 2c simultaneously. As shown on an enlarged scale in FIG. 5, the tappet body 1 is held horizontally by a chuck (not shown) and is rotated at fixed speed, and a sharpened threading tool 4 is pressed against the outer circumferential surface of the tappet body 1. The threading tool 4 is moved in a direction of an arrow, i.e. in an axial direction, so that the surface of the tappet body 1 is cut to form the helical projection 2a and groove 2b. 
     To overlap a circumferential cutting width &#34;W&#34; partially formed by the cutting tool 4, or to form an overlapping portion &#34;L&#34; as shown in FIG. 5, by suitably determining depth &#34;H&#34; of cut on the surface of the tappet body 1 by the cutting tool 4, an opening angle &#34;θ&#34; of the cutting tool 4, and an axial feed rate of the cutting tool 4 per one rotation of the tappet body 1 or a pitch &#34;P&#34; of the projection 2a or groove 2b, a ridge of the projection 2a is discontinuously chipped in a direction contrary to a feed direction of the cutting tool 4 to form a plurality of discontinuous uneven surface 2c. 
     To form the overlapping portion &#34;L&#34;, the relationship among the depth &#34;H&#34; of cut, the opening angle &#34;θ&#34; and the pitch &#34;P&#34; may be set to H tan θ/2≧P/2. 
     The inventors of the present invention confirmed that the uneven surface was formed on the outer circumferential surface of the tappet body 2 having a diameter of 20 to 50 mm to form the uneven surface 2c easily by setting the opening angle θ of the cutting tool 4 to 30 to 90°, the overlapping portion &#34;L&#34; to 0.05 to 0.35 mm, a rake angle to 20°, a back clearance angle to 25°, the pitch &#34;P&#34; of the projection and groove to 0.10 to 1.00 mm and the depth of cut to 0.25 to 0.80 mm. 
     The following is presumed. As mentioned above, the pitch decreases compared with conventional threading, a suitable overlapping portion &#34;L&#34; is provided between the cutting width &#34;W&#34; of the cutting tool 4, so that axial thickness of the ridge 2d of the projection 2a decreases. Further, the ratio of the depth &#34;H&#34; of cut to the pitch &#34;P&#34; increases and the diameter of the ridge of the projection 2a is made to be smaller than the diameter of the tappet body 1, so that reaction force in a contrary direction to feed by cutting resistance is applied to the cutting tool 4. Thus, in the tappet body 1 made of low toughness Al alloy, the ridge 2d is pressed by the reaction force, and the uneven surface 2c is formed by being cut off in a circumferential direction 
     The outer circumferential surface of the tappet body 1 is formed as a roughened surface, and as shown in FIG. 1(B), wear resistant metal such as Fe is thermally sprayed to the outer circumferential surface which comprises a roughened surface to form a film 3. The film 3 is formed as above, thereby making a tappet which provides not only axial but also circumferential high peel resistance of the film 3. There is a problem that adhesion strength of the film 3 is conventionally low in a circumferential direction, but as shown in FIGS. 4 and 6, according to the present invention, thermal spraying material is coated onto the plurality of uneven surface 2c formed on the projection 2a, thereby providing high adhesion effect of the film 3 in the circumferential direction to cause high peel resistance. 
     The uneven surfaces 2c which are formed by cutting the ridges 2d are not as smooth as mechanically processed surfaces, but are irregularly roughened, thereby providing suitable peel resistance to the film 3 thermally sprayed onto the surface, which is advantageous. 
     The second method comprises the steps of forming the projection 2a and the groove 2b and, thereafter, cutting off the ridge 2d of the projection 2a at a suitable length in a circumferential direction to form the uneven surfaces at regular intervals. 
     The second method comprises the steps of forming the helical projection 2a and groove 2b on the outer circumferential surface of the tappet body 1 by pressing and moving the cutting tool 4 on the outer circumferential surface of the tappet body 1 in an axial direction of the tappet body 1 while the tappet body 1 is rotated at fixed speed, and thereafter by moving the cutting tool 4 in a contrary direction (as shown by a solid arrow in FIG. 7) to a formerly moving direction (as shown in a broken arrow in FIG. 7) to form uneven surfaces (not shown) at regular intervals on the ridge 2d of the projection 2a by the cutting tool 4. 
     The pitch in returning the cutting tool 4 may be equal to the pitch &#34;P&#34; in going forth, but preferably may be significantly larger than it. 
     When the relationship among the depth of cut &#34;H&#34;, the opening angle &#34;θ&#34; and the pitch &#34;P&#34; is set to H tan θ≧P/2 in a forwarding path, the uneven surfaces 2c as above are formed on the ridge 2d of the projection 2a with cutting of the projection 2a. Further, when the cutting tool 4 returns, deeper recesses are formed on the ridge 2d by the cutting tool 4. Two kinds of shallower and deeper recesses improve circumferential peel resistance of the film thermally sprayed thereafter. 
     As another embodiment of the second method, while the tappet body 1 is rotated at fixed speed similar to the above, the sharp cutting tool 4 is pressed against the outer circumferential surface of the tappet body 1 to move the tappet body 1 in an axial direction, thereby forming the helical projection 2a and groove 2b on the outer circumferential surface of the tappet body 1. Thereafter, as shown in FIG. 8, onto the outer circumferential surface having unevenness, a pressing roller 5 on which a plurality of protrusions 5a extends axially (or in a direction crossed to the projection) is put in parallel with the tappet body 1 and pressed onto it. The pressing roller and the tappet body 1 are rotated at the same circumferential speed in a contrary direction, recesses (not shown) are formed at regular intervals on the ridge 2d of the projection 2a by the protrusions 5a of the pressing roller 5. 
     According to this method, the ridge 2d of the projection 2a are pressed by the protrusions 5a of the pressing roller 2a to form the uneven surfaces. Burrs (not shown) which are formed at the edges of the uneven surfaces to prevents a film thermally sprayed thereafter from peeling off, thereby increasing peel resistance of the film, which is advantageous. 
     As a method of forming uneven surfaces on the ridge 2d of the projection 2a, in addition to the method which uses the pressing roller 5, there is a method of moving a thinner grinding wheel or a milling tool in an axial direction which is perpendicular to the projection 2a. Any of the methods may be applied. 
     In the third method, an optional groove is formerly formed on the outer circumferential surface. Thereafter, while the tappet body 1 is rotated around its axis, a sharp cutting tool is pressed against the outer circumferential surface to move the tappet body 1 axially, thereby forming a groove and a projection which has recesses at regular intervals on its ridge. 
     For example, as shown in FIG. 9, on the outer circumferential surface of the tappet body 1, a plurality of V-sectioned grooves which extend axially are formed by a rotary grindstone or a milling machine (not shown) at suitable intervals in a circumferential direction. Thereafter, similar to what is shown in FIG. 5, while the tappet body 1 is rotated around its axis, a sharp cutting tool 4 is pressed against the outer circumferential surface of the tappet body 1 to form uneven surfaces at regular intervals on a ridge 2d of a helical projection 2a. 
     Thereafter, similarly, a film is formed on the outer circumferential surface of the tappet body 1 by thermal spraying. Similar advantages to the second method can be achieved according to the third method. The present invention is not limited to the foregoing embodiments. For example, in the foregoing embodiments, the projection 2a and the groove 2b are helical, but a plurality of annular projections and grooves spaced in parallel to each other may be formed, and a plurality of uneven surfaces may be formed on the annular projections 2a. Instead of the above thermal spraying, the film 3 may be formed by plating or coating means. The present invention may be applied to an air cylinder, a piston of a hydraulic cylinder, a piston of an internal combustion engine, etc. in addition to an Al alloy tappet. 
     The foregoings merely relate to preferred embodiments of the present invention. Various changes and modifications may be made by person skilled in the art without departing from the scope of claims wherein: