Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
2000-02-28
2002-07-16
Meeks, Timothy (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S237000, C427S238000, C427S239000, C427S249100, C427S249700
Reexamination Certificate
active
06419997
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a guide bush included in an automatic lathe to support a rodlike workpiece for rotation and axial sliding at a position near a cutting tool (cutter), and a method of forming a hard carbon film over the inner surface of the guide bush to be in sliding contact with the workpiece.
BACKGROUND TECHNOLOGY
Guide bushes mounted on the column of an automatic lathe to hold a rodlike workpiece for rotation at a position near a cutting tool are classified into rotary guide bushes and stationary guide bushes. A rotary guide bush rotates together with a workpiece and holds the workpiece for axial sliding. A stationary guide bush remains stationary and holds a workpiece for rotation and axial sliding.
A guide bush of either type has a portion having a taper outer surface provided with slits to make the same portion elastic, a threaded portion to hold the guide bush on the column, and an inner surface for holding a workpiece. The inner surface always in sliding contact with a workpiece is liable to be worn and, particularly, the inner surface of a stationary guide bush is worn rapidly.
A guide bush proposed in JP-A No. 4-141303 has an inner surface to be in sliding contact with a workpiece, which slides and rotates on the inner surface, attached with a cemented carbide (super hard alloy sleeve) or a ceramic material attached to the inner surface by brazing or the like.
When the inner surface of a guide bush is attached with a cemented carbide or a ceramic material excellent in wear resistance and heat resistance, the wear of the inner surface of the guide bush can be reduced to some extent.
However, when the workpiece is subjected to heavy machining on an automatic lathe, in which the depth of cut is large and the cutting speed is high, the workpiece is damaged or seizing occurs due to decrease in the diametrical clearance between the guide bush and the workpiece even if the inner surface of the guide bush is attached with a cemented carbide or a ceramic material, because the cemented carbide and the ceramic material have comparatively a large coefficient of friction and a low thermal conductivity. Therefore, it has been difficult to increase the depth of cut and cutting speed.
The stationary guide bush has advantages that a workpiece can be accurately machined in a high roundness because the workpiece can be held so that its axis may not run out, less noise is generated, the automatic lathe may be of a simple, compact construction.
However, the inner surface of the stationary guide bush is worn far more rapidly than that of the rotary guide bush and hence it is more difficult to increase depth of cut and cutting speed when the stationary guide bush is employed.
Accordingly, it is an object of the present invention to provide a guide bush capable of solving such problems, having an inner surface to be in contact with a workpiece, having improved wear resistance, and capable of enabling an automatic lathe to machine a workpiece at an increased depth of cut and an increased cutting speed without damaging the workpiece or causing seizure between the guide bush and the workpiece. Another object of the present invention is to provide a method of efficiently manufacturing such a guide bush.
DISCLOSURE OF THE INVENTION
With the foregoing object in view, the present invention provides a guide bush of an approximate cylindrical form having a center bore formed along its center axis, which includes a taper outer surface formed on one longitudinal end portion and provided with slits to make the end portion elastic, a threaded portion formed on the other end portion to be mounted on the column of an automatic lathe, and an inner surface for holding a workpiece inside a portion on which the taper outer surface is formed. The guide bush is mounted on an automatic lathe so as to hold a workpiece inserted into the center bore with the inner surface for holding the workpiece rotatably and axially slidable at a position near a cutting tool, and is provided with a hard film coated on the inner surface for hold the workpiece.
Further, the present invention also provides having an inner surface to be in contact with a workpiece, coated with a hard carbon film, and a method of uniformly and firmly forming the hard carbon film in a short time over the inner surface of the guide bush.
The hard carbon film is formed of a hydrogenated amorphous carbon closely resembling diamond in properties. Therefore, hydrogenated amorphous carbon is also called diamondlike carbon (DLC).
The hard carbon film (DLC film) has a high hardness (not lower than Vickers 3000 Hv), is excellent in wear resistance and corrosion. resistance, and has a small coefficient of friction (about ⅛ that of a cemented carbide).
The guide bush of the present invention having an inner surface to be in sliding contact with a workpiece, coated with the hard carbon film, has wear resistance more excellent than the conventional guide bush having an inner surface attached with a cemented carbide or a ceramic material.
Accordingly, an automatic lathe employing the stationary guide bush of the present invention is able to achieve heavy machining, in which depth of cut is large and cutting speed is high, with high accuracy for an extended period of time without damaging the workpiece and causing seizure.
Preferably, the hard carbon film is formed on an intermediate layer formed over the inner surface of the guide bush to enhance the adhesion between the inner surface and the hard carbon film.
When the intermediate layer is formed of a two-layer film consisting of a lower layer of titanium, chromium or a compound containing titanium or chromium, and an upper layer of silicon, germanium or a compound containing silicon or germanium, the lower layer secures adhesion to the inner surface (carbon tool steel) of the guide bush, and the upper layer bonds firmly to the hard carbon film. Therefore, the hard carbon film adheres firmly to the inner surface of the guide bush with a high adhesion.
The hard carbon film may be formed on a hard lining member of a cemented carbide, such as tungsten carbide (WC), or a sintered ceramic material, such as silicon carbide (SiC), formed on the inner surface of the guide bush. An intermediate layer interposed between such a hard lining member and the hard carbon film will further enhance the adhesion of the hard carbon film.
A carburized layer may be formed in the inner surface of the guide bush at a portion of the substrate in the vicinity thereof and the hard carbon film may be formed over the carburized inner surface. When an intermediate layer similar to that mentioned above is formed between the carburized inner surface and the hard carbon film, the adhesion of the hard carbon film can be enhanced.
A method of forming a hard carbon film over the inner surface of a guide bush in accordance with the present invention comprises the following steps.
A guide bush for use on an automatic lathe is placed in a vacuum vessel provided with a gas inlet port and a evacuation port, and provided therein with an anode and a filament, and then an auxiliary electrode having the shape of a rod or a wire is inserted in the center bore of the guide bush defined by the inner surface, to be in sliding contact with a workpiece. The auxiliary electrode may be inserted in the center bore of the guide bush prior to placing the guide bush in the vacuum vessel.
A process gas containing carbon is supplied through the gas inlet port into the vacuum vessel after evacuating the vacuum vessel, and a hard carbon film is formed over the inner surface of the guide bush by applying a DC voltage to the guide bush, a DC voltage to the anode and an AC voltage to the filament for producing plasma.
The plasma may be produced in the vacuum vessel by applying a radio frequency voltage or a DC voltage to the guide bush without using the anode and the filament.
A hard carbon film of a uniform film thickness can be rapidly formed over the entire inner surface of the guide bush from the open end through the depth by supplyi
Kimura Sosaku
Koike Ryota
Kokubo Kunihiko
Miya Yukio
Sugiyama Osamu
Citizen Watch Co. Ltd.
Meeks Timothy
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