Patent ID: 6910451

Claim:
A method of producing a variable valve timing control system which comprises: a tubular housing comprising: a housing body having a density, and a sprocket portion adapted to be linked to an engine crankshaft for receiving a drive force transmitted from the crankshaft by way of a chain, the sprocket portion and the housing body being integrally formed with each other; a camshaft having an external periphery formed with a drive cam for operating an engine valve, the tubular housing being mounted to the cam shaft and making a rotation relative to the camshaft when required, the camshaft receiving the drive force transmitted from the sprocket portion, to thereby rotate as a follower; a phase variation mechanism disposed in the tubular housing, and varying a rotational phase of the sprocket portion relative to the camshaft in accordance with oil pressure supplied to the phase variation mechanism; and an oil pressure control device that controls the oil pressure supplied to the phase variation mechanism, wherein the method comprises the following sequential operations of: (a) sintering the housing body of the tubular housing and the sprocket portion of the tubular housing, to form an integrated sintered body; (b) recompressing the integrated sintered body; (c) mounting the recompressed sintered body on a jig, so that the jig is engaged inside of the housing body; (d) disposing the sprocket portion of the recompressed sintered body between a drive die and a follower die; (e) form-rolling the sprocket portion of the recompressed sintered body by rotating the drive die while pressing the drive die and the follower die on the sprocket portion, to produce the sprocket portion having a relatively higher density than a density of the housing body of the recompressed sintered body, and to prevent and straighten deformation of the housing body; and (f) finally subjecting the recompressed sintered body to heat treatment after form-rolling.