Centering process in chucking work and apparatus therefor

A process and apparatus for centering accurately and speedy a workpiece on a magnet chuck mounted on a work spindle. A pair of action pads spaced away from one another comes into engagement with the workpiece attracted to the work spindle, thereby performing the centering operation. The action pads are mounted on a support plate that can freely turn on a fulcrum. As the fulcrum makes head towards a rotational center of the magnet chuck, the action pads comes into abutment in a rocking manner against the workpiece that is held at off-center relation in the chuck, thereby compensating the off-center relation to keep a center of the workpiece in alignment with the rotational center of the chuck.

FIELD OF THE INVENTION

The present invention relates generally to a machine tool including a lathe and so on having a work spindle equipped thereon with a magnetic chuck and, more particularly, to a process for centering a workpiece in a chucking phase on a chuck mounted on the machine spindle and an apparatus therefor.

BACKGROUND OF THE INVENTION

There are conventionally known processors that can perform accurate facing operations with no need of centering work on a hollow cylindrical workpiece. One example of the processors of the sort as stated just earlier is disclosed in Japanese Patent Laid-Open No. 2002-337012, which is envisaged turning a workpiece of material heat-treated to be highly hardened. The prior processor is composed of a machine spindle supported for rotation on a headstock fastened on a machine bed, a magnet chuck mounted on a lengthwise end of the machine spindle in a way revolving coaxially with the machine spindle, a loose headstock placed on the same rotational axis with headstock, and a three-jaw chuck-mounted on a lengthwise end of the loose headstock in a way revolving coaxially with the loose headstock. The three-jaw chuck or self-centering chuck is to transfer the workpiece held in the chuck to the magnet chuck with keeping the rotational axis of the work in alignment with the rotational axis of the magnet chuck, thereby performing the centering operation.

The processor constructed as stated earlier, nevertheless, has need of a carriage to carry the loose headstock thereon, and therefore could not get out of becoming bulky in construction. Moreover, the workpiece held in the three-jaw chuck, as deformed easily as shown inFIG. 9, is unfair disadvantageous to the centering operation and therefore poses an issue of causing variations in centering from one work to another.

As shown in, for example, Japanese Patent Laid-Open No. H10-43985, an automatic centering process for machining more massive parts has been developed which helps a circular workpiece make automatic centering with accuracy as well as with a very short time on a machine tool. The level of accuracy in centering to be varied depending on the property of workpiece can be selectively determined using an inspection means. With the prior automatic centering process recited earlier, the centering operation is performed with the combination of radially retractable driver units and an inspection unit to detect off-center deflection. The radially retractable driver units are each arranged at a point of three o'clock and another point of from six o'clock to eight o'clock around a circular surface of a workpiece held in a magnet chuck. At a centering mode, the magnet chuck switches to a weaker level in magnetic force. Simultaneously with this, the workpiece starts revolving while the driver units move center-ward and the inspection unit shifts towards a reference surface for measuring the accuracy of centering. After the accuracy of centering comes into a prescribed range that can be tolerated by the inspection unit, a signal to cease the centering work is issued to retract radially the driver units into their home positions where the magnet chuck switches automatically to a stronger level and the machine is turns into a cutting mode at the same time.

The automatic centering process constructed as stated earlier, however, involves for centering operation activating the driver units set at preselected two points separately from one another while detecting the accuracy of centering by the inspection unit. With the prior automatic centering process, thus, there are practical issues that the machine tool has to be made bulky in construction and also lacks in universality for a variety of workpiece size.

As disclosed in, for example, Japanese Patent Laid-Open No. 2002-260293, there is known a centering process to bring the center of a circular plate into coincident relation with the center of a turntable. With this prior centering process, a turntable onboard a circular plate is moved straight by a linear guide mechanism until the outward circular edge of the circular plate comes into abutment against stoppers, thereby getting the center of the circular plate matching the rotational center of the turntable. With the centering device to carry out the centering process as stated earlier, the stoppers are placed in a relation spaced away from one another with an interval less than the diameter of the turntable, so that the circular plate at the first place comes into engagement at the outward circular edge thereof with any one of the stoppers, followed by coming into abutment against the other stopper to make the off-center correction, thereby performing the accurate centering.

The prior centering device constructed as stated earlier needs to set the stoppers with high accuracy in their locations and also lacks in universality for wide variations in circular plate size.

A centering apparatus for a circular plate with no need of rotating a turntable is known conventionally as disclosed in, for example, Japanese Patent Laid-Open No. 2003-157589. The prior centering apparatus for the circular plate as recited earlier is comprised of a turntable to carry a circular plate thereon, and arms with pads at their opposite distal ends laid in diametric opposition with respect to the turntable in a way pivoted at their centers for rocking motion about their fulcrums. As the arms shift towards the center of the turntable, the pads at the opposite ends of the arms come into contact with the outward circular edge of the turntable to follow the contour of the turntable.

However, the centering apparatus for the circular plate in which the four pads are arranged to embrace the circular plate would have a margin of error because the four pads doing their works on the circular plate could vary from one pad to another pad in working position. Moreover, the machine tool has to be made bulky in construction.

SUMMARY OF THE INVENTION

The present invention, therefore, has as its primary object to solve the major challenges as described just above and to provide a process and an apparatus for centering a workpiece in a machine tool including a lathe and so on, which has a work spindle with a chuck thereon used for processing, turning, facing and inspecting operations applied on a workpiece having a circular part. More particularly, the present invention provides a process and an apparatus for centering a workpiece having a circular part, in which the centering operation to set the workpiece on the chuck by virtue of magnetic or vacuum attraction with the center of the workpiece lying in alignment with the rotational axis of the chuck can be achieved by just installation of a centering tool that is simple in construction. Thus, the process and apparatus for centering a workpiece constructed as stated earlier according to the present invention makes it easier to perform the centering operation in chucking work more accurately and rapidly in diverse working fields including processing, measuring and so on of the workpiece, thereby improving the working efficiency in the processing, turning, facing, measuring operations.

The present invention is concerned with a process for centering a workpiece attracted on a chuck mounted on a work spindle, using a centering tool comprising a support block and a pair of action pads lying on the support block with spaced away from one another in a way coming into engagement with a circular part of the workpiece; wherein the support block of the centering tool is supported to turn on a fulcrum; and wherein as the fulcrum of the support block moves head towards a rotational center of the chuck, the support block makes a rocking motion on the fulcrum in a way the action pads come into abutment from any one side against the workpiece that is held at off-center relation in the chuck, thereby compensating the off-center relation to keep a center of the workpiece in alignment with the rotational center of the chuck.

In an aspect of the present invention, a process for centering a workpiece is disclosed in which the chuck is a magnet chuck equipped on a machine tool to attract a cylindrical workpiece thereon and wherein the action pads are made to rise above the support block to come into either an outside or an inside circular surfaces of the workpiece, performing a centering operation of the workpiece on the chuck.

The present invention is further concerned with an apparatus for centering a workpiece; comprising a pair of action pads adapted to come into engagement with a circular part of a workpiece attracted to a chuck, a support block for the centering tool having the action pads thereon in a fashion the action pads are spaced away from one another, and a moving part to bear the support block for free turning on a fulcrum of the support block; and wherein as the moving part gets the fulcrum of the support block moving head towards a rotational center of the chuck, the action pads alternately come into engagement from any one side against the workpiece that is held at off-center relation in the chuck, thereby compensating the off-center of the workpiece to bring a center of the workpiece in alignment with the rotational center of the chuck.

In an aspect of the present invention, an apparatus for centering a workpiece is disclosed in which the chuck is a magnet chuck equipped on a machine tool to attract a cylindrical workpiece thereon and wherein the action pads are made to rise above the support plate of the support block to come into either an outside or an inside circular surfaces of the workpiece, perfoi ining a centering operation of the workpiece on the chuck. In another aspect of the present invention, the centering tool includes a support plate for the support block, a pair of action pads made in a fashion raised above the support plate, a shank fastened to the moving holder and mounted to the support plate to provide a fulcrum on which the support plate is allowed to turn freely, and a stopper fastened to the shank so as to come into abutment against recessed edges that are in opposition to one another on both sides of the stopper. In a further another aspect of the present invention, the moving part fastened to the shank of the support block is a holder mounted on a turret equipped on a machine tool.

The centering process and apparatus constructed as recited earlier can be well applied to a wide variety of machines. Just mounting the centering tool of simple construction to any moving part of a turret of machine tools is sufficient to perform quickly the centering operation to set the center of the workpiece in alignment with the rotational center of the chuck, such as magnet chuck and vacuum chuck, on which the workpiece having a circular part is attracted for cuts, such as turn and face, and measurements. High-speed centering of the workpiece on the chuck is desirable to complete the cuts including turning of inside and outside circular surfaces, facing flat surfaces on the ends of the cylindrical workpiece with high efficiency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a process and an apparatus for centering a workpiece according to the present invention will be explained hereinafter in detail with reference to the accompanying drawings. The present invention is preferably adapted to a centering process and apparatus for a workpiece, which is used in a diversity of machines including machine tools, inspection instruments, measuring instruments, semiconductor fabricating equipment, various robots, and so on.

The embodiment of the centering apparatus according to the present invention is shown inFIGS. 1 to 3in a fashion adapted for a machine tool1or lathe of the type equipped thereon with a magnet chuck5of circular contour. The magnet chuck5is composed of many permanent magnet pieces21whose flat front surfaces serve as attractive surfaces22. The magnet pieces21are positioned circularly around a work spindle6in a fashion extending radially outward of the work spindle6and revolving together with the work spindle6. The magnet chuck5utilizes the permanent-electro magnetic system that allows the control of the magnetic holding force. With the embodiment explained here, a centering tool10is clamped on a turret4, or a rotary indexing tool rest, among a plurality of cutting tools that are positioned circularly in a fashion each extending radially outwards. The turret4is mounted on an X-axis slider3. Moreover, the turret4is allowed to move in either of an X-axis or infeed direction and a Z-axis direction or laterally reciprocating direction inFIG. 1with respect to a sloping machine bed31. On the slant machine bed31, a cross slide or a Z-axis slider2is mounted for movement in the Z-axis direction while the X-axis slider3with the turret4thereon is supported on the Z-axis slider2for movement in the X-axis direction.

In preparation for centering operation on a workpiece9with using the centering tool10constructed according to the version illustrated, the magnet chuck5is first deactivated or made less in magnetic holding force and then the workpiece9is attracted tentatively along any one circular edge23thereof with less force to the magnet chuck5. After the completion of the centering operation for the workpiece9with respect to the magnet chuck5, the activation of the magnet chuck5to strengthen the magnetic holding force starts to perform the turning operation to cut the workpiece9. The hollow cylindrical workpiece9recited in the present embodiment is envisaged outer rings, inner rings and retainers in rolling-contact bearings. Cuts, such as turn and face, on the workpiece9attracted magnetically to the chuck5are performed on a circular external surface24, a circular internal surface25and any one exposed circular end23, excepting other circular end23coming into engagement with the magnet chuck5. With the embodiment thought of turning a circular internal surface25inside the workpiece9, the centering operation is carried out in a way getting action pads13of the centering tool10coming into engagement with the circular external surface24of the workpiece9.

Referring mainly toFIGS. 4 to 7, there is shown a preferred version of the centering tool10according to the present invention. The centering tool10, although constructed as shown hereinafter to make it easier to use it, but is not limited to such a version. A support frame constituting the centering tool10, for example, may be made in diverse configurations other than a support block11. With the version shown here, the centering tool10is comprised of the support block11, a pair of action pads13installed on the support block11in a way raised above the support block11, a journal26to bear the support block11so as to serve as a fulcrum (O), a shank12extending from the journal26to fit into a holder20, and a stopper14extending out of the shank12into a recess17to come into engagement and disengagement from recessed edges18on the support block11. The holder20for the centering tool10is installed in the turret4on the X-axis slider3in return for any one of the cutting tools that are positioned circularly at regular intervals. The turret4on the lathe provides the tool rest, or tool block, for mounting the cutting tools thereon. The shank12is adapted to fit into a matching hole made in the holder20in such a relation that fulcrum (O) lying on the rotational axis of the shank12makes head for a rotational center (OG) of the magnet chuck5in the X-axis direction at the indexed location.

With the centering tool10in the version illustrated, the action pads13coming into contact with the workpiece9to exert the force on the workpiece9are placed in opposition to the fulcrum (O) with respect to the workpiece9. Alternatively, the action pads13and the fulcrum (O) may be reversed each other with respect to the workpiece9. The cylindrical workpiece9is magnetically secured at any one circular end23thereof to a front surface22of the magnet chuck5.

Moreover, the magnet chuck5, although may employ any one of electromagnetic, permanent magnet and electro-permanent magnetic systems, but selects the electro-permanent magnetic system in the version illustrated. The electro-permanent magnetic system features increased holding or attraction power with less heat loss compared to only the permanent magnets.

With the centering tool10constructed as in the version illustrated, there are installed two action pads13in a fashion raised above any one surface of the support block11to provide points (PA) of action, where come into contact with the workpiece9. The action pads13are made of cylindrical bodies serving likewise a kind of cam follower, and supported on shafts30for rotation through bearings16. The action pads13mounted for rotation on the support block11are more universal or versatile for contact-movement with the workpiece9. The action pads13employed in the version illustrated is a tracking roller of stud type, or a cam follower, in which needle rollers are installed for the bearings16. Thus, the centering tool10can be made by only installation of the cam followers to the support block11. Two cam followers or action pads13are spaced apart from one another by a preselected distance (S) and mounted on the support block11that is pivoted for rocking movement at the fulcrum (O). Moreover, the shank12whose rotational axis lies on the fulcrum (O) is mounted on the support block11in a way extending above the side opposite to the action pads13.

The shank12adapted to fit into the holder20on the turret4is lengthwise cut on the circular surface thereof to make an axial flat surface19to make it easier to lock the shank12in the holder20at a desired angular orientation. A bolt extending through the holder20into abutment against the flat surface19of the shank12helps keep the shank12at the desired angular position. The shank12is integral at any one axial end thereof with the journal26, which fits into the support block11for rotation through a cross-roller bearing15that is comprised of an outer ring27, an inner ring28and rollers29interposed between the outer and inner rings27and28.

As the support block11is not necessarily allowed to turn over the circle in full, the stopper14is provided to limit the tolerated revolving range within a preselected revolving angle. A distal end of a bent arm32whose another end is fastened to the shank12extends into the recessed area17in the support block11, thereby providing the stopper14that defines the tolerated revolving range of the support block11when coming into collision against any one of the opposite edges18bordering the recess17made in the support block11. Thus, the action pads13are allowed to rock around the fulcrum (O) of the support block11. All the two action pads13are needed in their geometric relation is lying on both sides, one to each side, of the line of action on which the fulcrum (O) of the support block11gets closer to the rotational center (OG) of the magnet chuck5, while being separated apart from one another by a preselected distance (S) that allows the action pads13to keep the rolling-contact with the workpiece9irrespective of how extent the support block rocks. There is no point in positioning the action pads13with accuracy. With the version illustrated, the support block11and two action pads13are made in such a geometric relation that straight lines joining the fulcrum (O) of the support block11with each the centers (O1) and (O2) of the action pads13form roughly an isosceles triangle. Any configuration may be selected for the support block11as long as it allows the support block11to rock about the fulcrum (O) and also the action pads13to rest on the support block11. Other constructional demands, such as size and so on, of the centering tool10are free to choose depending on the scale of the workpiece9, the reforming ability of any deviation in location of the workpiece9which might be caused by magnetic attraction, and the like. Further, the centering tool10of the present invention is versatile for a wide range in size of the workpiece9.

Next, how the workpiece9is subjected to centering operation on the magnet chuck5will be described later with reference toFIG. 7. With the process for centering the workpiece according to the present invention, the centering operation for the workpiece9is completed while the chuck5rotates together with the workpiece9magnetically attracted to the chuck5. It will be certainty that the centering operation is equally applicable regardless of whether the magnet chuck5having the workpiece9attracted to the chuck5is rotating or ceases rotating. The action pads13are arranged to come into rolling-contact with the circular outside surface of the workpiece9to exert on the workpiece9. The centering tool10of the version illustrated is better adapted to turn a plurality of workpiece9in succession. After the workpiece9is held in the magnet chuck5, the centering operation will finish when the teetering action pads13come to a stop. Automation of the centering operations can be carried out by installation of any detector, not shown in the turret4to sense the rocking behavior of the action pads13and/or the support block11.

With the version illustrated, the following settings are needed in preparation for the centering operation of the workpiece9held in the chuck5.

At an early stage of the centering operation as shown inFIG. 7(C), a position to teach the completion of the centering operation is predicted depending on the size of the first workpiece9. The position to teach the completion of the centering operation is found by a point (OS) on which the fulcrum (O) of the support block11converges when the center (OW) of the workpiece9comes into coincidence with the rotational center (OG) of the magnet chuck5.

Second, as considering the size of the workpiece9can vary widely from one to the other, a targeted amount of centering is preset. The real amount of centering is determined short of the convergent point (OS) by any measuring instrument. Then, the support block11is moved to get closer to the convergent point (OS) depending on how far the measured amount is away from the targeted amount whereby the centering is completed. With the version illustrated, for example, the targeted amount of centering is set within the range of 0.02 mm.

Third, the measuring instrument is mounted on the turret4at any one of the locations reserved for the cutting tools.

Thus, the action pads13are once detached from the workpiece9short of the convergent point (OS). Instead, the measuring instrument comes into engagement with or gets closer to the workpiece9to confirm the degree of off-center of the workpiece9. When the off-center is still more than the targeted amount of centering, the action pads13again come into abutment against the workpiece9to resume compensating the off-center to complete the centering operation.

The centering process and the centering apparatus of the present invention can perform automatically a sequence of phases as stated earlier.

In a phase in which the center (OW) of the workpiece9lies above (in the figure) the center line (O)-(OG) for movement as shown inFIG. 7(A), the two action pads13are allowed to come into contact at their points (PA) of action with the circular outside surface24of the workpiece9so as to embrace the workpiece9. When the workpiece9is further rotated in eccentric relation with respect to the rotational center (OG), the workpiece9varies to be reformed in its position towards the rotational axis as the fulcrum (O) moves towards the rotational center (OG).

In another phase in which the center (OW) of the workpiece9lies below (in the figure) the center line (O)-(OG) for movement as shown inFIG. 7(B), the two action pads13are allowed to come into contact at their points (PA) of action with the circular outside surface24of the workpiece9so as to embrace the workpiece9. When the workpiece9is further rotated in eccentric relation with respect to the rotational center (OG), the workpiece9varies to be reformed in its position towards the rotational axis as the fulcrum (O) moves towards the rotational center (OG).

In a final phase in which the center (OW) of the workpiece9comes into coincidence with the rotational center (OG) after the fulcrum (O) has reached the convergent point (OS) for movement as shown inFIG. 7(C), the two action pads13are allowed to come into contact at their points (PA) of action with the circular outside surface24of the workpiece9so as to embrace the workpiece9, while the support block11comes to stop the rocking motion. This phase teaches the completion of the centering operation. After the completion of the centering operation for the workpiece9with respect to the magnet chuck5, the magnet chuck5is activated to strengthen the magnetic holding force exerted on the workpiece9to perform the turning operation on the lathe.

The centering process and apparatus working as stated earlier make it possible to machine a plurality of workpiece9continuously and automatically throughout from the centering operation to the turning operation in one chucking. With the centering apparatus constructed according to the present invention, the action pads13made to teeter are allowed to embrace the workpiece9so as continue keeping engagement with the workpiece9that rotates in highly eccentric orbits with respect to the magnet chuck5, thereby serving the smooth centripetal function. On the contrary, with the conventional centering tool kept against teetering, the action pads could not follow successfully every posture of the workpiece. Thus, the workpiece falls off from the magnet chuck and, therefore, such conventional centering tool is apt to fail in smooth centering of the workpiece.

Referring toFIG. 8, there are shown circularity33and coaxiality34found really on the workpiece9that was turned after centering operation according to the present invention, together with theoretical or ideal coaxiality35.

The results illustrated were achieved in internal cylindrical turning operations of the workpiece9that was a cylindrical material of inside diameter: φ110 mm, outside diameter: φ126 mm and axial length: 6 mm.

The action pads13of the centering apparatus came into contact with the circular outside surface24of the cylindrical material to center the cylindrical material. Then, the centered cylindrical material was held in the magnet chuck5and turned to cut the circular inside surface25. The measured result (2000 times in measuring magnification) after turning operation showed the centered material could be finished by turning work to the coaxiality34of 0.0042 mm and the circularity33of 0.0037 mm.

The result shown inFIG. 9was found on the workpiece9that was cut by internal turning work with held in the conventional three-jaw chuck and, then untethered from the chuck to be inspected after the completion of internal turning. As apparent from the measured result (500 times in measuring magnification), though the coaxiality34was achieved to 0.008 mm, the circularity33on the inside diameter was found only 0.066 mm and the workpiece9finished by turning work remained depressed at three locations where the three-jaw chuck grasped the workpiece9in chucking work.