WO2004007128A1

WO2004007128A1 - Workpiece mounting for precision machining

Info

Publication number: WO2004007128A1
Application number: PCT/AU2003/000917
Authority: WO
Inventors: Patrick John Mcclusky; Bruce Stanley Rodda
Original assignee: Anca Pty Ltd
Priority date: 2002-07-17
Filing date: 2003-07-17
Publication date: 2004-01-22
Also published as: WO2004007128A8; AU2002950263A0

Abstract

An apparatus and method are provided for mounting a workpiece (10) for a precision machining operation to be performed on the workpiece by a machine tool having a spindle rotatable about a rotational axis (12). An adaptor (20) mounted by the machine tool mounts the workpiece (10) so a rotational axis (13) of the workpiece is coaxial with the spindle axis (12). A selectively adjustable packing means (55) is interposed between the adaptor (20) and a base (45) fixed to the machine tool to determine the angle at which the adaptor (20) mounts the workpiece relative to the spindle axis (12) to enable, during a set up operation, reduction of the angularity error of the workpiece rotational axis (13). The packing means (55) includes two packing rings (56, 57) each having an axial thickness which varies around the circumference. The packing rings are adjustable in their rotational positions around the adaptor (20) to provide a selectively variable thickness in the axial direction at different angularly spaced positions enabling the inclination of the adaptor axis to the spindle axis (12) to be selectively varied.

Description

WORKPIECE MOUNTING FOR PRECISION MACHINING . This invention relates to mounting of workpieces for precision machining operations to be performed on the workpiece, particularly, although not exclusively, for mounting of a workpiece by a collet in a machine tool. The background to the present invention and the detailed description of the preferred embodiment of the invention will be particularly described in connection with mounting of a workpiece by a collet in a computer numerically controlled (CNC) machine, particularly one for tool cutting and grinding. However, the invention can be applied to other types of machine tools used for performing precision machining operations on a workpiece, such as cylindrical cutting tool holders, e.g. a lathe with a face mounted chuck. It is to be understood, therefore, that subsequent references to CNC tool cutting and grinding machines are not to be interpreted as limiting on the scope of applicability of the invention.

In CNC tool cutting and grinding machines there are several sources of error which can result in an inaccurately ground workpiece. This invention is principally concerned with reducing or compensating for possible run-out errors in a workpiece held by and extending from a split, draw-back collet in a CNC machine tool.

Conventionally, a split, draw-back collet in a CNC tool cutting and grinding machine has a cylindrical shank received in a bore in a collet adaptor or arbor mounted to a rotatable spindle, the collet having a split part at its front end with gripping jaws and a tapered external surface engageable with a complementary tapered surface of the mouth in the collet adaptor. The collet has a screw-threaded rear end engageable with a complementary screw-threaded front portion of a drawbar extending rearwardly relative to the collet adaptor. The engagement of the tapered surfaces causes the gripping jaws to tighten on the workpiece to clamp the workpiece securely for subsequent machining operations. This type of collet adaptor, however, can suffer from run-out errors of angularity and concentricity. Errors in angularity occur when the central longitudinal axis of the collet and clamped workpiece extends at an angle, albeit small, to the central axis of the rotatable spindle. Errors in concentricity occur when the central longitudinal axis of the collet and workpiece is offset from the central axis of the rotatable spindle. The present invention is primarily concerned with mounting of a workpiece with a reduced angularity run-out error. In published patent specification WO-00/13069 by the present applicant (which is not admitted to be disclosing information which is common general knowledge in the relevant industry), there is proposed a collet adaptor which provides adjustment pins which engage with a cylindrical portion of the collet at circumferentially spaced locations. The pins are selectively moveable radially so as to provide fine adjustment to align the longitudinal axis of the collet and workpiece held thereby accurately with the longitudinal axis of the spindle of the machine tool and thereby compensate for errors in angularity. The manufacture of the collet adaptor in this published patent specification would be relatively complex and the use of this apparatus would require significant skill to exercise the control to compensate for errors in angularity. The apparatus in this published patent specification would also require a specially constructed draw bar of thin walled tube which is used to draw the collet into the collet adaptor, the thin walled tubular construction being to allow some compliance in the rear mechanism and reduce resistance to front mechanism function.

It is an object of the present invention to provide an apparatus and a method for mounting of a workpiece for a precision machining operation which can provide considerable control for reducing run-out errors of angularity in setting up the apparatus for a machming operation.

It is a further and preferred object to provide an apparatus for mounting of a workpiece for a precision machining operation which can be relatively easy to use, but which nevertheless can provide great precision in setting up the workpiece. A related preferred object is to provide a method for mounting of a workpiece for a precision machining operation which can be relatively easy to perform, but which nevertheless enables great precision in setting up the workpiece.

It is a further preferred object of the present invention to provide an apparatus for mounting, a workpiece for a precision machining operation which can enable reduction of run-out errors in concentricity.

In accordance with a first aspect of the present invention, there is provided an apparatus for mounting a workpiece for a precision machining operation to be performed on the workpiece by a machine tool having a spindle rotatable about a rotational axis, the apparatus including an adaptor for receiving and mounting, either directly or indirectly, the workpiece, the adaptor itself being arranged to be mounted by the machine tool and secured thereto for rotation by the spindle of the machine tool, the adaptor in use being arranged to mount the workpiece (directly or indirectly) so that a rotational axis of the workpiece is coaxial with the rotational axis of the spindle of the machine tool, the apparatus further including selectively adjustable packing means interposed between the adaptor and a base fixed in use relative to the machine tool so as to adjustably determine the angle at which the adaptor mounts the workpiece relative to the rotational axis of the spindle and thereby enable, during a set up operation, reduction of the angularity error of the rotational axis of the workpiece in the machining operation.

By providing a packing means interposed between the adaptor and a base fixed relative to the machine tool and which can be varied so as to selectively adjust the axis of the adaptor (and any workpiece held directly or indirectly thereby) relative to the rotational axis of the spindle of the machine tool, fine control of the angularity is possible.

In the preferred embodiment, the adaptor is a collet adaptor or arbor for receiving and mounting a workpiece holder (namely a collet) which in use mounts the workpiece. In this embodiment, the collet adaptor is arranged to mount the collet which, in turn, mounts the workpiece coaxially with the rotational axis of the spindle of the machine tool. This particular type of mounting using a collet adaptor and associated collet will be primarily described throughout this specification.

The base which is fixed in use relative to the machine tool may be part of the machine tool and may be located at a front face of a headstock mounted by the spindle of the machine tool. Alternatively, the base which is fixed in use relative to the machine tool may comprise a component part of an assembly which includes the adaptor and the packing means, the base being arranged in use to be fixed to a headstock mounted by the spindle of the machine tool to thereby mount the assembly to the spindle.

In the preferred embodiment, the packing means has a selectively variable thickness in the axial direction at different angularly spaced positions around the axis of the adaptor whereby selectively varying the thickness of the packing means at different annularly spaced positions, the inclination of the adaptor axis relative to the rotational axis of the spindle can be selectively varied. The thickness of the packing means in the axial direction may be selectively variable in controlled increments so that the inclination of the adaptor axis to the rotation axis of the spindle can be progressively adjusted during the set up operation. Preferably the thickness of the packing means is selectively variable in substantially continuous increments. In a preferred embodiment the packing means includes at least one packing ring of a thiclcness in the axial direction which varies at different positions around the circumference of the packing ring, the packing ring being selectively adjustable in its rotational position around the adaptor. Preferably there are two packing rings, each having a thickness in the axial direction which varies around the circumference of the respective ring, the two packing rings being generally coaxial and rotationally adjustable at least relative to each other. Preferably the two packing rings are adjacent to and further preferably contact each other. In the preferred embodiment of the apparatus, the two packing rings are both independently rotationally adjustable relative to the adaptor and to the base. In a particular preferred embodiment, the or each packing ring comprises an annular ring of a thickness in the axial direction which varies around its circumference, the axial thickness varying continuously from a maximum thickness at one circumferential point, decreasing continuously to a minimum axial thickness at a point diametrically opposite the point of maximum thickness, and then continuously increasing again in thiclcness around the remainder of the circumference returning to the point of maximum thickness, the or each packing ring having one face which is planar, with the plane of that one face being orthogonal to an axis through the centre of the packing ring, the opposite face of the ring also being planar but with the plane of the opposite face being at an angle other than 90° to the axis through the centre of the annular ring. In the embodiment having two of the packing rings, each preferably has the same configuration as described above and the rings are arranged with their respective angled opposite faces facing and contacting each other whereby the two annular rings can be arranged so that the maximum axial thickness point on the circumference of one ring is directly facing the minimum axial thickness point on the circumference on the other ring and vice versa, whereby, the outwardly directed bearing faces of the two annular rings are parallel to each other and the two annular rings in combination form an annular packing ring of constant axial thickness throughout the circumference, the two annular rings being relatively rotatable so as to tilt the plane of the bearing face of at least one of the packing rings and hence tilt the axis through the centre of that packing ring relative to the axis through the other packing ring. The two packing rings may be located between a front face of the base and a rear face located around a cylindrical recessed part of the adaptor and facing the base. According to another aspect of the present invention there is provided a method of mounting a workpiece for a precision machining operation to be performed on the workpiece by a machine tool having a spindle rotatable about a rotational axis, the method including the steps of providing an adaptor for receiving and mounting, either directly or indirectly, the workpiece, mounting the adaptor and securing the adaptor so as to be rotatable by the spindle of the machine tool, mounting the workpiece (directly or indirectly) by the adaptor so that a rotational axis of the workpiece is coaxial with the rotational axis of the spindle of the machine tool, providing packing means interposed between the adaptor and a base which is fixed relative to the machine tool and selectively adjusting the packing means so as to thereby determine the angle at which the adaptor mounts the workpiece relative to the rotational axis of the spindle and thereby reduce the angularity error of the rotational axis of the workpiece in the machining operation.

Preferably the packing means has a selectively variable thickness in the axial direction at different angularly spaced positions around the axis of the adaptor, and the method includes selectively varying the thickness of the packing means at different annularly spaced positions so as to selectively vary the inclination of the adaptor axis relative to the rotational axis of the spindle. In this method, preferably the packing means includes at least one packing ring of a thickness in the axial direction which varies at different positions around the circumference of the packing ring, and the method includes selectively adjusting the packing ring in its rotational position around the adaptor. There are preferably two packing rings, each having a thiclcness in the axial direction which varies around the circumference of the respective ring, the two packing rings being generally coaxial and the method includes rotationally adjusting the packing rings relative to each other. The or each packing ring preferably comprises an annular ring of a thickness in the axial direction which varies around its circumference, the axial thickness varying continuously from a maximum thickness at one circumferential point, decreasing continuously to a minimum axial thickness at a point diametrically opposite the point of maximum thickness, and then continuously increasing again in thickness around the remainder of the circumference returning to the point of maximum thiclcness, each said at least one packing ring having one face which is planar, with the plane of that one face being orthogonal to an axis through the centre of the packing ring, the opposite face of the ring also being planar but with the plane of the opposite face being at an angle oilier than 90° to the axis through the centre of the annular ring. With this preferred embodiment there are preferably provided two of the packing rings each having the same configuration as described above and with their respective angled opposite faces facing and contacting each other, the method preferably including the step of arranging the two amiular rings so that the maximum axial thickness point on the circumference of one ring is directly facing the minimum axial thickness point on the circumference on the other ring and vice versa, whereby, the outwardly directed bearing faces of the two amiular rings are parallel to each other and the two annular rings in combination form an annular packing ring of constant axial thickness throughout the circumference, the method further including relatively rotating the two annular rings so as to tilt the plane of the bearing face of at least one of the packing rings and hence tilt the axis through the centre of that packing ring relative to the axis through the other packing ring.

Preparatory to a precision machining operation to be performed on the workpiece, the method preferably includes the step of approximately setting up the apparatus followed by measuring an angularity run-out error of the workpiece including measuring the direction of deviation of the axis of the workpiece from the spindle axis as well as the extent of that deviation, the method preferably further including the steps of: with the two rings in their initial position and the outside bearing faces parallel and orthogonal to the axis of the adaptor, simultaneously rotating the two rings in the same direction to substantially align a marker provided on the outside circumference indicating the maximum thickness of one ring and the minimum thickness of the other with the detected and measured direction of angularity run-out error, and then simultaneously rotating the two rings in opposite directions so as to increase the combined axial thickness of the two packing rings at one circumferential position, while diametrically opposite, reducing the axial thickness of the combination of the packing rings to bring the axis of the workpiece back towards and eventually to reach a direction collinear with or parallel to the axis of the spindle of the machine tool, thereby reducing or eliminating the angularity run-out error. The method preferably further includes a final step of securely fixing the adaptor to the base with the rings in their positions reducing or eliminating the angularity error. Possible and preferred features of the present invention will now be described with particular reference to the accompanying drawings. However it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the invention. In the drawings: Fig. 1 is a perspective view of a workpiece mounting apparatus according to preferred embodiment of the present invention,

Fig. 2 is a front view along the axis of rotation of the collet adaptor of the present invention,

Fig. 3 is a cross sectional view along the line A-A in Fig. 2, Fig. 4 is a cross sectional view along the line B-B in Fig. 2,

Fig. 5 is a cross sectional view along the line C-C in Fig. 2,

Fig. 6 is a cross sectional view along the line D-D in Fig. 2, and

Fig. 7 is a cross sectional view diagonally through a packing ring.

The apparatus illustrated in the drawings is for mounting a workpiece 10 for precision machining, e.g. such as a tool which is to be precision ground in a CNC tool cutting and grinding machine. The details of the CNC machine can be generally conventional and the only part of the machine tool which is illustrated is the front portion of the headstock 11 shown in Fig. 1. The headstock 11 is part of the spindle assembly of the machine tool which rotates about a main axis 12. For precision machining of the workpiece 10, the axis of rotation 13 of the workpiece 10 needs to be accurately coUinear with the axis of rotation 12 of the spindle of the machine tool. However, as discussed earlier, run-out errors of angularity and concentricity of the axis of rotation 13 of the workpiece 10 occur and, during set up, an attempt is made to reduce these errors.

The apparatus in the drawings includes a collet adaptor or arbor 20 which, has a main body 21 and a cylindrical rear body portion 22 with an axial bore 23 extending through the adaptor. At the forward end of the bore 23 is an outwardly tapered mouth 24.

In use, the adaptor 20 receives in the bore 23 a collet 30 which has a shank 32 closely received within the bore 23. The shank 32 as shown in Fig. 6 has an axial recess or groove 34 in its outer surface which is in registry with the pin assembly 35 which can be adjusted to project slightly into the bore 23 and into the recess 34 to prevent rotation of the collet 30 within the adaptor 20. This pin assembly is further described later in association with Fig. 6. The forward end of the collet 30 has a number of splits 37 defining jaws 38. The outside surfaces of the jaws 38 are frusto conical as shown at 39 so that when the collet 30 is drawn into the bore 23 of the adaptor 20, the frusto conical surfaces 39 are drawn into contact with the inside surfaces of the tapered mouth 24 causing the jaws 38 to securely clamp the workpiece 10. To draw the collet 30 into the adaptor 20, there can be a generally conventional draw bar 40 which is coupled to the rear end of the shank 32 of the collet 30 and which projects out the rear end of the bore 23 in the adaptor 20 for application of required axial force on the collet. The draw bar 40 is manufactured to close tolerances in size, shape and concentricity to meet clearance criteria when fitting within the rear part of the bore 23 in the adaptor. Referring to Fig. 6, the pin assembly 35 which cooperates with the axial recess or groove 34 along the surface of the collet shank 32 includes a pin 92 movable to an adjustable depth in the groove 34. The pin 92 has a body 94 surrounded by seal ring 96 and biased outwardly by spring 93 acting through washer 97. Height setting screw 95 is used to adjustably rotate the pin 92. A part-helical groove defining a cam track is provided on the outside surface of the body 94 and a cam engaging pin 98 laterally enters the cam defining groove in the surface of the body 94. The length of the helical groove in the axial direction of the body 94 limits the extent to which pin 92 can be advanced into the groove 34. Lock screw 99 enables the pin 98 to be tightly engaged against body 94 to lock the pin 92 in position. Four of such pin assemblies 35 are provided at 90° angular positions around the axis of the collet adaptor 20.

In the illustrated apparatus there is provided a base 45 illustrated as a mounting member or plate 46 which in use is fixed rigidly to the front of the headstock 11 by bolts 47. The mounting member 46 provides a mounting securely fixed to the machine tool and to which the main body 21 of the adaptor 20 is adjustably mounted to correct or compensate for run-out errors of angularity and concentricity as will be further described.

The main body 21 of the adaptor 20 has a cylindrical recessed part 53 and a generally peripheral ring 51 which provides a step or shoulder at the recessed part 53 having a rear face 52 facing towards the machine tool and, in particular, directly opposed to and spaced from the front face 48 of the mounting member 46. Interposed between the rear face 52 and the front face 48 is a packing means 55 which is selectively variable to determine the angle of the axis 13 of the collet adaptor 20 and workpiece 10 held thereby relative to the rotational axis 12 of the spindle. In particular, the packing means 55 is of selectively variable thickness at different angularly spaced positions around the axis of the cylindrical recess part 53 of the collet adaptor 20. To achieve the variations of thickness of the packing means 55, the illustrated embodiment provides two packing rings 56, 57 which abut each other and which determine the spacing between the rear face 52 and the front face 48. Each of the packing rings 56, 57 has one bearing face which is planar and is orthogonal to the axis through the centre of the respective ring and has an opposite face which is also planar but the plane of which is at an angle slightly less than 90° to the axis through the centre of the ring whereby the axial thiclcness of each ring varies continuously from a maximum thickness at one point on the circumference to a minimum thickness which is diametrically opposite. In Figs. 3 to 6 of the drawings, the rings 56, 57 may appear to have planar opposite faces which are both orthogonal to the axis but, at the scale of the drawings, the inclination of the angled face would not be discernible.

In Fig. 7 the packing ring 56 (of which the packing ring 57 can be a mirror image) is shown with the angle of the plane of face 60 to the axis 58 greatly exaggerated. Face 62 has its plane orthogonal to the axis 58 through the centre of the ring 56. Opposite bearing face 60 has its plane at an angle φ slightly less than 90° to the axis 58. The difference between the maximum axial thickness of each ring and the minimum axial thickness may be only in the order of a few microns. The angle of inclination of the angled face to the axis of the respective packing ring can be selected to achieve a maximum workpiece angular error compensation that is suitable for a typical machine tool construction, practicality of manufacture, and allows maximum rotational angular displacement between packing rings when making angular compensations down to 0.001mm. The proposed angle between the plane of the angled face and axis of the machine tool, i.e. angle φ in Fig. 7, may be between 89° 59min 52sec and 89° 59min 44sec. That is, the angle of the plane of the angled face to the true orthogonal plane (and to the opposite bearing face) i.e. angle θ in Fig. 7 may be between 8 and 16 seconds of arc.

The proposed difference in thickness between diametrical opposite points of maximum and minimum thickness on each of the rings is between 0.005mm and 0.010mm. However, this can be altered as required during manufacture of rings to increase maximum workpiece compensation or to increase rotational play necessary to achieve 1 micron adjustments.

In the embodiment illustrated in Fig. 1 the abutting faces 60, 61 of the rings 56, 57 respectively are the faces which are at an angle less than 90° to the axial direction and the outside faces 62, 63 which respectively abut the front face 48 of mounting member 46 and the rear face 52 of peripheral ring 51 are the bearing faces having their planes orthogonal to the axial direction.

With this arrangement, the rings 56, 57 can be initially located with the maximum thickness point of the circumference of one ring adjacent and abutting the minimum thiclcness portion of the other ring. In this position, the outside bearing faces 62, 63 will be parallel to each other and orthogonal to the axial direction 12. In this position, there nevertheless may be an error in angularity in which the axis 13 of the workpiece 10 held in the collet is at a slight angle to the true spindle axis 12 of the machine tool. However, by controlled incremental rotation of one or both of the rings, the combined axial thickness of the rings 56, 57 around part of the circumference will progressively start to increase while, at the diametrically opposite positions, the axial thickness of the two rings in combination will start to decrease. Where the packing rings are effectively in combination increasing in axial thiclcness, the rear face 52 of the ring 51 of the adaptor 20 will have an increasing outward force being applied to it while, diametrically opposite, the packing rings 56, 57 will be effectively reducing in axial thickness enabling face 52 to move relatively closer to fixed front face 48. It will be seen that this will progressively and incrementally slightly change the axis 12 of the adaptor 20 as a whole relative to the axis of the spindle of the machine tool.

During set up of the apparatus preparatory to a precision machining operation to be performed on the workpiece 10, the operator can approximately set up the apparatus and then measure the angularity run-out error of the workpiece 10. This includes measuring the direction of deviation of the axis 13 of the workpiece 10 from the spindle axis 12 as well as the extent of that deviation. With the two rings 56, 57 in their initial position and the outside bearing faces 62 and 63 parallel and orthogonal to the adaptor axis, the two rings can be simultaneously rotated in the same direction to substantially align a marker on the outside circumference (which indicates the maximum thickness of one ring and minimum thickness of the other) with the detected and measured direction of angularity run out error. From that point, the two annular packing rings 56, 57 can be then simultaneously rotated in opposite directions so as to apply the forces to the rear face 52 of the adaptor body 21 as described above so as to bring the axis 13 of the workpiece 10 back towards and eventually to reach a position coUinear or parallel with the spindle axis 12. With the angularity error thus eliminated or at least reduced, the collet adaptor 20 can then be securely fixed to the mounting member 46 by means of mounting bolts 70 (Fig. 5) (which up to this point in the set up operation would be loosened off).

However, to avoid the excessive movement of the collet adaptor 20 during the set up operation to reduce the angularity run out error and during the securing operation involving tightening of bolts 70, which might result in disruption of the setting achieved, the collet adaptor 20 throughout the set up operation may be mounted to the mounting base plate 46 by means of intermediate bolts 80. As best seen in Fig. 4, each intermediate bolt 80 is located in a bore 81 with belville washers or disk springs 82 applying tensile force to the underside of the head 83. The bolts 80 are of a shoulder screw type. The bolts 80 allow application of a predetermined tension to the mounting of the collet adaptor 20 to the mounting base plate 46 during the set up operation including during the positioning and rotation of the adjustable annular packing rings 56, 57 between faces 48 and 52. The tension, however, is sufficient to securely hold the setting achieved during the tightening of the bolts 70. To avoid tampering with the intermediate bolts 80, the bores 81 can be closed or plugged as shown at 85 after the desired tension setting has been calibrated.

Alternatively, to avoid tampering, the bolts 80 could be "reversed" so as to be adjustable from the rear of the base plate 46, i.e. by providing the bolts 80 with their heads within the base plate 46 and their shanks extending into the collet adaptor 20.

As referred to earlier, there is also commonly encountered a concentricity run-out error because the axis 13 of the workpiece 10 is not accurately coUinear with the spindle axis 12 but is parallel to and slightly displaced therefrom. The preferred apparatus illustrated in the drawings provides means for also reducing the concentricity error. In the illustrated embodiment of Fig. 6, this means comprises the provision of radial adjustment screws operating between the mounting base 45 and, firstly, the headstock 11 and, secondly, the body of the collet adaptor 20. In particular, the mounting base plate 46 is provided with at least three and preferably four radially extending bores 87 provided in cylindrical extension 86 of the base plate 46, each having therein a screw 88 to bear against the outside face of the headstock 11. This enables initial concentricity adjustment upon first assembling the base plate 46 and collet adaptor 20 with the headstock 11. There are also provided at least three and preferably four radially extending bores 90 each having therein a screw 91 which projects inwardly to bear against outside cylindrical face of the rear body portion 22 of the collet adaptor 20. By measuring the concentricity error and adjustment of the screws 91. the concentricity run out error of the workpiece 10 can be reduced or eliminated.

The annular packing rings 56, 57 can be movable angularly through an effectively infinite range of angular positions. The rings may include knurling 59 on the outer circumference to facilitate gripping for rotation. Also the rings may have radial bores 65, 66 at a number of locations around the perimeter to enable a suitable adjusting tool to engage with the rings and provide leverage for rotating the rings. The apparatus may also include stepped incremental guides to assist the operator set up the apparatus in known incremental steps of adjustment. One of the bores 66 as shown in Fig. 3 has a blind hole 67 axially intersecting it, the hole 67 opening through face 60 of the packing ring 56. A spring loaded ball 68 is installed in the hole 67 so as to project slightly beyond face 60 when in registry with a depression 69 in the face 61 of the packing ring 57. These depressions 69 can be provided at calibrated positions around the circumference of the face 61 of the packing ring 57. As the rings 56, 57 are rotated relative to each other during the set up operation, the ball 68 will, after predetermined angular movements, enter into a registering one of the depressions 69 with an audible click and/or a perceptible increase in resistance of the ring (5) to further angular movement, thus indicating to the operator that a predetermined incremental change in angularity had been reached. An operator may learn to determine the number of such incremental steps that are needed to correct an angularity error of a predetermined magnitude. It will be seen that the particular preferred embodiment described in detail with reference to the drawings can provide great control of the set up operation, particularly in reducing angularity error. The preferred apparatus can be relatively simple to use enabling less skilled operators to nevertheless achieve good results in setting up a workpiece for a precision machining operation, particularly in reducing run-out errors of angularity and concentricity. The degree of control and relative ease of use can also significantly reduce set up times. The method of mounting a workpiece according to the second aspect of the invention can be readily understood from the preceding description of the illustrated apparatus and its function and operation.

Although the apparatus is particularly designed and described for use with mounting of a workpiece by a collet which is received and mounted by a collet adaptor to the headstock of a CNC machine tool, the apparatus and method could also be adapted for use with cylindrical cutting tool holders such as lathes.

Claims

1. An apparatus for mounting a workpiece (10) for a precision machining operation to be performed on the workpiece by a machine tool having a spindle rotatable about a rotational axis (12), the apparatus including an adaptor (20) for receiving and mounting, either directly or indirectly, the workpiece (10), the adaptor (20) being arranged to be mounted by the machine tool and secured thereto for rotation by the spindle of the machine tool, the adaptor (20) in use being arranged to mount the workpiece (10) (directly or indirectly) so that a rotational axis (13) of the workpiece is coaxial with the rotational axis (12) of the spindle of the machine tool, the apparatus further including selectively adjustable packing means (55) interposed between the adaptor (20) and a base (45) fixed in use relative to the machine tool so as to adjustably determine the angle at which the adaptor (20) mounts the workpiece relative to the rotational axis (12) of the spindle and thereby enable, during a set up operation, reduction of the angularity error of the rotational axis (13) of the workpiece in the machining operation.

2. An apparatus as claimed in claim 1 wherein the packing means (55) has a selectively variable thickness in the axial direction at different angularly spaced positions around the axis of the adaptor (20) whereby selectively varying the thickness of the packing means (55) at different annularly spaced positions, the inclination of the adaptor axis relative to the rotational axis (12) of the spindle can be selectively varied.

3. Apparatus as claimed in claim 2 wherein the thiclcness of the packing means (55) in the axial direction is selectively variable controlled increments so that the inclination of the adaptor axis to the rotation axis (12) of the spindle can be progressively adjusted during the set up operation.

4. Apparatus as claimed in claim 2 or 3 wherein the thiclcness of the packing means (55) is selectively variable in substantially continuous increments.

5. Apparatus as claimed in any one of the preceding claims wherein the packing means (55) includes at least one packing ring (56, 57) of a thiclcness in the axial direction which varies at different positions around the circumference of the packing ring, the packing ring being selectively adjustable in its rotational position around the adaptor (20).

6. Apparatus as claimed in claim 5 wherein there are two packing rings (56, 57), each having a thickness in the axial direction which varies around the circumference of the respective ring (56, 57), the two packing rings being generally coaxial and rotationally adjustable at least relative to each other.

7. Apparatus as claimed in claim 6 wherein the two packing rings (56, 57) are adjacent to each other.

8. Apparatus as claimed in claim 6 or 7 wherein the two packing rings (56, 57) are both independently rotationally adjustable relative to the adaptor (20) and to the base (45) .

9. Apparatus as claimed in any one of claims 5 to 8 wherein each said at least one packing ring (56, 57) comprises an annular ring of a thickness in the axial direction which varies around its circumference, the axial thiclcness varying continuously from a maximum thickness at one circumferential point, decreasing continuously to a minimum axial thiclcness at a point diametrically opposite the point of maximum thickness, and then continuously increasing again in thickness around the remainder of the circumference returning to the point of maximum thickness, each said at least one packing ring (56, 57) having one face (62) which is planar, with the plane of that one face (62) being orthogonal to an axis (58) through the centre of the packing ring, the opposite face (60) of the ring also being planar but with the plane of the opposite face (60) being at an angle other than 90° to the axis (58) through the centre of the annular ring.

10. Apparatus as claimed in claim 9 wherein there are provided two of said packing rings (56, 57) each having the same configuration as claimed in claim 9 and with their respective angled opposite faces (60, 61) facing and contacting each other whereby the two annular rings (56, 57) can be arranged so that the maximum axial thickness point on the circumference of one ring is directly facing the minimum axial thiclcness point on the circumference on the other ring and vice versa, whereby, the outwardly directed bearing faces (62, 63) of the two annular rings are parallel to each other and the two annular rings (56, 57) in combination form an annular packing ring of constant axial thickness throughout the circumference, the two annular rings (56, 57) being relatively rotatable so as to tilt the plane of the bearing face (62, 63) of at least one of the packing rings and hence tilt the axis through the centre of that packing ring relative to the axis through the other packing ring.

11. Apparatus as claimed in claim 10 wherein the two packing rings (56, 57) are located between a front face (48) of the base (45) and a rear face (52) which is located around a cylindrical recessed part (53) of the adaptor (20) and faces the base (45).

12. Apparatus as claimed in any one of the preceding claims wherein the base (45) which is fixed in use relative to the machine tool is part of the machine tool and is located at a front face of a headstock (11) mounted by the spindle of the machine tool.

13. Apparatus as claimed in any one of claims 1 to 11 wherein the base (45) which is fixed in use relative to the machine tool comprises a component part of an assembly which includes the adaptor (20) and the packing means (55), the base (45) being arranged in use to be fixed to a headstock (11) mounted by the spindle of the machine tool to thereby mount the assembly to the spindle.

14. Apparatus as claimed in claim 13 and further including at least three concentricity adjustment screws (91) provided within respective radially extending bores (90) extending radially within the base (45), the screws (91) being selectively adjustable so as to project inwardly from the respective bores (90) to bear against an outside cylindrical face of a rear body portion (22) of the adaptor (20) so as to thereby enable selective adjustment of the concentricity of the axis of the adaptor (20) with the axis (12) of the rotation of the spindle of the machine tool thereby enabling reduction of the concentricity error of the workpiece (10).

15. Apparatus as claimed in claim 14 wherein the base (45) is provided with at least three radially extending bores (87) each of which has therein a respective screw (88) which projects inwardly to bear against an outside face of the headstock (11) of the machine tool, adjustment of the screws (88) thereby enabling initial concentricity adjustment upon first assembling the base (45) and adaptor (20) with the headstock (11) of the machine tool.

16. A method of mounting a workpiece (10) for a precision machining operation to be performed on the workpiece by a machine tool having a spindle rotatable about a rotational axis (12), the method including the steps of providing an adaptor (20) for receiving and mounting, either directly or indirectly, the workpiece (10), mounting the adaptor (20) and securing the adaptor (20) so as to be rotatable by the spindle of the machine tool, mounting the workpiece (10) (directly or indirectly) by the adaptor (20) so that a rotational axis (13) of the workpiece is coaxial with the rotational axis (12) of the spindle of the machine tool, providing packing means (55) interposed between the adaptor (20) and a base (45) which is fixed relative to the machine tool and selectively adjusting the packing means (55) so as to thereby determine the angle at which the adaptor (20) mounts the workpiece (10) relative to the rotational axis (12) of the spindle and thereby reduce the angularity error of the rotational axis (13) of the workpiece in the machining operation.

17. A method as claimed in claim 16 wherein the packing means (55) has a selectively variable thickness in the axial direction at different angularly spaced positions around the axis of the adaptor (20), the method including selectively varying the thickness of the packing means (55) at different annularly spaced positions so as to selectively vary the inclination of the adaptor axis relative to the rotational axis (12) of the spindle.

18. A method as claimed in claim 17 wherein the packing means (55) includes at least one packing ring (56, 57) of a thickness in the axial direction which varies at different positions around the circumference of the packing ring, the method including selectively adjusting the packing ring in its rotational position around the adaptor (20).

19. A method as claimed in claim 18 wherein there are two packing rings (56, 57), each having a thickness in the axial direction which varies around the circumference of the respective ring (56, 57), the two packing rings being generally coaxial and the method including rotationally adjusting the packing rings relative to each other.

20. A method as claimed in claim 18 or 19 wherein each said at least one packing ring (56, 57) comprises an annular ring of a thickness in the axial direction which varies around its circumference, the axial thickness varying continuously from a maximum thiclcness at one circumferential point, decreasing continuously to a minimum axial thickness at a point diametrically opposite the point of maximum thickness, and then continuously increasing again in thickness around the remainder of the circumference returning to the point of maximum thickness, each said at least one packing ring (56, 57) having one face (62) which is planar, with the plane of that one face (62) being orthogonal to an axis (58) through the centre of the packing ring, the opposite face (60) of the ring also being planar but with the plane of the opposite face (60) being at an angle other than 90° to the axis (58) tlirough the centre of the annular ring.

21. A method as claimed in claim 20 wherein there are provided two of said packing rings (56, 57) each having the same configuration as claimed in claim 20 and with their respective angled opposite faces (60, 61) facing and contacting each other, the method including arranging the two annular rings (56, 57) so that the maximum axial thickness point on the circumference of one ring is directly facing the minimum axial thickness point on the circumference on the other ring and vice versa, whereby, the outwardly directed bearing faces (62, 63) of the two annular rings are parallel to each other and the two annular rings (56, 57) in combination form an annular packing ring of constant axial thickness throughout the circumference, the method further including relatively rotating the two annular rings (56, 57) so as to tilt the plane of the bearing face (62, 63) of at least one of the packing rings and hence tilt the axis through the centre of that packing ring relative to the axis through the other packing ring.

22. A method as claimed in claim 21 wherein, preparatory to a precision machining operation to be performed on the workpiece (10), the method includes the step of approximately setting up the apparatus followed by measuring an angularity run-out error of the workpiece (10) including measuring the direction of deviation of the axis ( 13 ) of the workpiece (10) from the spindle axis (12) as well as the extent of that deviation, the method further including the steps of: with the two rings (56, 57) in their initial position and the outside bearing faces (62, 63) parallel and orthogonal to the axis of the adaptor (20), simultaneously rotating the two rings (56, 57) in the same direction to substantially align a marker provided on the outside circumference indicating the maximum thickness of one ring an the minimum thickness of the other with the detected and measured direction of angularity run-out error, and then simultaneously rotating the two rings (56, 57) in opposite directions so as to increase the combined axial thickness of the two packing rings (56, 57) at one circumferential position, while diametrically opposite, reducing the axial thickness of the combination of the packing rings (56, 57) to bring the axis (13) of the workpiece (10) back towards and eventually to reach a direction coUinear with or parallel to the axis (12) of the spindle of the machine tool, thereby reducing or eliminating the angularity run-out error.

23. A method as claimed in claim 22 and further including the final step of securely fixing the adaptor (20) to the base (45) with the rings (56, 57) in their positions reducing or eliminating the angularity error.