This invention relates to a metrological apparatus for measuring surface characteristics, such as roundness, straightness and form.

EP-A-0240151 describes a metrological apparatus for measuring the roundness of a workpiece mounted on a turntable. A stylus gauge is supported relative to the turntable and generates a signal representative of the deflection of the stylus along a measurement direction as the stylus follows a surface of the workpiece on rotation of the turntable about a rotation axis. Measurement data is sent to a processor which fits the measurement points with a best fit circle and compares the deviation of the measurement points from the best fit circle. In order to fit and analyse the data, it is generally necessary to have centred data. Centring can be achieved through software correction or by mechanical means. Examples of mechanical centring devices are iris devices and scroll chucks.

An iris device comprises first and second annular supports arranged coaxially for relative rotation about their axis and a plurality of arms coupled between the annular supports. One end of each arm is pivotally coupled to a point of the first annular support and the other end of each arm is pivotally coupled to a point on the second annular support. The edges of the arms are curved, each having a major and a minor circumference with the minor circumference arranged to face towards the axis. A workpiece to be centred with the axis is positioned between the arms. Relative rotation of the annular supports causes the minor circumferences of the arms to move towards the axis to move the workpiece towards the axis. It is however difficult to manufacture the edges of the arms with sufficient accuracy and deviations from true circularity of the edges may prevent the arms from contacting the workpiece surface symmetrically, which may reduce the accuracy of subsequent centring. Furthermore, the points of contact with the workpiece may not be the same for different centring operations. The separation between contact points is particularly marked when the size of the workpiece is changed. Consequently, iris devices require further workpiece adjustment for every measurement.

A scroll chuck comprises a scroll plate arranged for rotation relative to a chuck body which supports a workpiece. The scroll plate has a spiral guide path and jaws arranged to move on the guide path within radial constraint paths of the chuck body. Each jaw has a plurality of teeth for engaging the spiral path. In most designs, the jaws are cut to closely reflect the spiral geometry. When relative rotation is effected, the jaws track the spiral within the constraint paths with the effect that the jaws move in directions defined by the radial constraint paths. In use, the jaws are scrolled inwardly towards a workpiece to tighten around the workpiece. The spiral geometry imposes limitations on the tolerances of the components, and imperfections in the spiral path and/or jaws give rise to asymmetry in the device, resulting in inaccurate centring. Another problem is that the device typically has a high gear ratio, with many rotations of the scroll plate being required to translate the jaws through their full range of movement. For a scroll plate with a spiral path which crosses a radius of the scroll plate n times, n rotations of the scroll plate are generally required to translate the jaws across the radius. This means that centring operations are slow and cumbersome.

According to a first aspect, there is provided metrological apparatus comprising a sensing assembly for carrying a sensor to sense a surface of a workpiece, means for effecting relative rotation of the workpiece and the sensing assembly about a rotation axis, a workpiece centring assembly comprising a workpiece support surface and a sleeve, the workpiece support surface and sleeve being arranged for relative rotation, and jaws arranged for movement along guide paths of the workpiece support surface to centre the workpiece on the workpiece support surface, wherein connectors are pivotally coupled between the jaws and the sleeve, and wherein the relative rotation causes the connectors to pivot to cause movement of the jaws along the guide paths.

The connectors enable the conversion of rotational movement of the sleeve to movement of the jaws. The connectors may be linear, for example, or have another geometry that can easily be manufactured to a high degree of precision. The guide paths may also be linear, for example, with the advantage that they may also be manufactured to a high degree of precision, thus helping optimise the accuracy of jaw movement and thus the accuracy of a centring operation.

According to a second aspect, there is provided metrological apparatus comprising a sensing assembly for carrying a sensor to sense a surface of a workpiece, means for effecting relative rotation of the workpiece and the sensing assembly about a rotation axis, a workpiece centring assembly comprising a workpiece support surface and sleeve, the workpiece support surface and sleeve being arranged for relative rotation, and jaws arranged for movement with respect to the workpiece support surface to centre the workpiece on the workpiece support surface , further comprising elongate bearings arranged for rotation with the workpiece support surface, slidable members supporting the jaws and arranged for movement along the elongate bearings, and connectors having pivotal couplings to respective slidable members and the sleeve, wherein the relative rotation causes the connectors to pivot about their pivotal couplings to allow movement of the slidable members along the elongate bearings and consequential movement of the jaws with respect to the workpiece support surface.

Repeated use of the apparatus can result in wear to tips of the jaws through repeated 4ngagement with surfaces of a workpiece or workpieces. When the jaw tips are initially pointed or tapered to provide point contacts to a workpiece, such wear can reduce the definition of the contact which can lead to inaccuracy. Each slidable member may support a respective jaw by a detachable coupling between the slidable member and jaw, providing the advantage that the jaws can be removed so that they can be reshaped or replaced to address the effects of wear. Additionally or alternatively they can be replaced with new jaws suitable for engaging a differently sized or shaped workpiece. The elongate bearings may be linear bearings that can easily be manufactured to a high degree of precision to enable the movement of jaws on highly defined paths provided by the linear bearings.

According to a third aspect, there is provided metrological apparatus comprising a sensing assembly including means to sense a surface of a workpiece, means for rotating the workpiece and the sensing assembly relative to one another about a rotation axis, a workpiece centring assembly comprising a workpiece support surface and a sleeve, the workpiece support surface and sleeve being arranged for relative rotation, and jaws arranged for movement along guide paths of the workpiece support surface to centre the workpiece on the workpiece support surface, further comprising a base plate supporting the sleeve and coupled to the workpiece support surface for rotation with the workpiece support surface, and a guidance mechanism coupling the jaws to the sleeve such that the relative rotation causes movement of the jaws along the guide paths, wherein the workpiece support surface is releasably coupled to the base pate 80 and the guidance mechanism is coupled to the base plate to allow removal and replacement of the workpiece support surface.

Repeated use of the apparatus can result in wear to the top plate due to abrasion by the movement of jaws and/or workpieces across its surface. Over time, such wear could affect the accuracy or symmetry of jaw movement and/or of workpiece movement and therefore reduce the accuracy of a centring operation. The possibility of removing the workpiece support surface is advantageous in mitigating the effects of wear by enabling the workpiece support surface to be reshaped, for example by having its surface flattened or re-ground, or to be replaced with a fresh workpiece support surface. Additionally or alternatively, the workpiece support surface could be replaced with a new workpiece support surface configured to support a differently sized or shaped workpiece. According to a fourth aspect, there is provided metrological apparatus comprising a sensing assembly arranged to carrying a sensor configured to sense a surface of a workpiece, a rotation mechanism arranged to effect relative rotation of the workpiece and the sensing assembly about a rotation axis, a workpiece centring assembly comprising a workpiece support surface and a sleeve, the workpiece support surface and sleeve arranged to be rotatable with respect to one another, and jaws arranged to move along guide paths of the workpiece support surface to centre the workpiece on the workpiece support surface, wherein connectors are pivotally coupled between the jaws and the sleeve, and wherein the relative rotation causes the connectors to pivot to allow movement of the jaws along the guide paths. According to a fifth aspect, there is provided metrological apparatus comprising a sensing assembly arranged to carrying a sensor configured to sense a surface of a workpiece, a rotation mechanism arranged to effect relative rotation of the workpiece and the sensing assembly about a rotation axis, a workpiece centring assembly comprising a workpiece support surface and sleeve, the workpiece support surface and sleeve arranged to be rotatable with respect to one another, and jaws arranged to move with respect to the workpiece support surface to centre the workpiece on the workpiece support surface , further comprising elongate bearings arranged to be rotated with the workpiece support surface, slidable members supporting the jaws and arranged to move along the elongate bearings, and connectors having pivotal couplings to respective slidable members and the sleeve, wherein the relative rotation causes the connectors to pivot about their pivotal couplings to allow movement of the slidable members along the elongate bearings and consequential movement of the jaws with respect to the workpiece support surface.

According to a sixth aspect, there is provided metrological apparatus comprising a sensing assembly arranged to carrying a sensor configured to sense a surface of a workpiece, a rotation mechanism arranged to effect relative rotation of the workpiece and the sensing assembly about a rotation axis a workpiece centring assembly comprising a workpiece support surface and a sleeve, the workpiece support surface and sleeve to be rotatable with respect to one another, and jaws arranged to move along guide paths of the workpiece support surface to centre the workpiece on the workpiece support surface, further comprising a base plate supporting the sleeve and coupled to the workpiece support surface to be rotatable with the workpiece support surface, and a guidance mechanism coupling the jaws to the sleeve such that the relative rotation causes movement of the jaws along the guide paths, wherein the workpiece support surface is releasably coupled to the base pate 80 and the guidance mechanism is coupled to the base plate to allow removal and replacement of the workpiece support surface.

According to one embodiment, there is provided metrological apparatus comprising a sensing assembly including means to sense a surface of a workpiece, means for rotating the workpiece and the sensing assembly relative to one another about a rotation axis, a workpiece centring assembly comprising a top plate for supporting the workpiece and a plurality of jaws arranged for movement in a radially symmetric configuration along radial paths of the top plate to engage a surface of the workpiece to move the workpiece towards a central axis of the centring assembly, and alignment means for aligning the central axis with the rotation axis, wherein the centring assembly comprises a circumferential sleeve arranged for rotation relative to the top plate and a guidance mechanism coupling the jaws to the sleeve such that the relative rotation causes reciprocating radial movement of the jaws, wherein the full range of movement of each jaw is provided by a portion of one full rotation of the sleeve relative to the top plate.

In one possibility the guidance mechanism comprises a plurality of radially arranged elongate bearings arranged not to rotate relative to the top plate, a plurality of slidable members coupling the jaws to respective elongate bearings through radial slots in the top plate, the slidable members being arranged for radial translation on guide paths defined by the elongate bearings, the guide paths being aligned with the radial paths and a plurality of connectors, an inner end of each connector being coupled to a respective slidable member and an outer end of each connector being coupled to the sleeve so that rotation of the sleeve relative to the top plate causes circumferential translation of the outer ends and consequential radial translation of the inner ends, respective slidable members and respective jaws.

According to another embodiment, there is provided metrological apparatus comprising a sensing assembly including means to sense a surface of a workpiece means for rotating the workpiece and the sensing assembly relative to one another about a rotation axis a workpiece centring assembly comprising a top plate for supporting the workpiece and a plurality of jaws arranged for movement in a radially symmetric configuration along radial paths of the top plate to engage a surface of the workpiece to move the workpiece towards a central axis of the centring assembly and alignment means for aligning the central axis with the rotation axis, wherein the centring assembly comprises a circumferential sleeve arranged for rotation relative to the top plate and a guidance mechanism coupling the jaws to the sleeve, the guidance mechanism elongate bearings arranged not to rotate relative to the top plate, orientations of the elongate bearings corresponding to the radial paths, slidable members for coupling the jaws to respective elongate bearings, the slidable members being arranged for radial translation on guide paths defined by the elongate bearings, and connectors coupling respective slidable members to the sleeve such that relative rotation of the sleeve and the top plate causes reciprocating radial movement of the slidable members and their respective jaws.

According to yet another embodiment, there is provided metrological apparatus comprising a sensing assembly including means to sense a surface of a workpiece means for rotating the workpiece and the sensing assembly relative to one another about a rotation axis a workpiece centring assembly comprising a top plate for supporting the workpiece and a plurality of jaws arranged for movement in a radially symmetric configuration on radial paths of the top plate to engage a surface of the workpiece to move the workpiece towards a central axis of the centring assembly, further comprising a circumferential sleeve supported on a base plate, the sleeve being arranged for rotation relative to the top and base plates, and a guidance mechanism coupling the jaws to the sleeve such that the rotation of the sleeve relative to the top and base plates causes reciprocating radial movement of the jaws on their radial paths, and alignment means for aligning the central axis with the rotation axis, wherein the guidance mechanism is coupled to the base plate and wherein the top plate is removably coupled to the base plate to allow replacement of the top plate to support a different sized work piece.

According to yet another embodiment, there is provided a workpiece centring assembly comprising a top plate for supporting a workpiece a plurality of jaws arranged for movement in a radially symmetric configuration along radial paths of the top plate to engage a surface of the workpiece to move the workpiece towards a central axis of the assembly, a circumferential sleeve arranged for rotation relative to the top plate, and a guidance mechanism coupling the jaws to the sleeve such that the relative rotation causes reciprocating radial movement of the jaws, wherein the full range of movement of each jaw is provided by a portion of one full rotation of the sleeve relative to the top plate.

According to yet another embodiment, the centring assembly comprises a base plate having a circular bearing for supporting the sleeve for rotation, wherein the top plate is coupled to the base plate to prevent relative rotation therebetween. The guidance mechanism may be coupled to the base plate and the top plate may be removably coupled to the base plate enabling the top plate to be replaced. Optionally the guidance mechanism is affixed to the base plate, optionally by affixing the elongate bearings to the base plate. In an alternative embodiment, the guidance mechanism is coupled to the top plate, and may be affixed to the top plate.

In a possibility, the slidable members are arranged to run on respective elongate bearings and the length of the guide paths is equal to the full range of movement of the slidable member on the 5 elongate bearings. The length of the guide path is preferably less than the radius of the sleeve.

In a possibility, the jaws are detachably coupled to the respective slidable members enabling the jaws to be replaced. The detachable coupling may comprise protrusions and recesses provided on opposing faces of the jaws and the respective slidable members, wherein the recesses are configured to receive the protrusions. Optionally, the protrusions are pegs and the recessed are correspondingly 10 shaped hole. The jaws may be configured to centre a substantially workpiece having rotational symmetry about an axis normal to the top plate of the centring assembly. Preferably the jaws are configured to centre a workpiece comprising a hollow cylinder. Preferably the jaws are configured to move together to contact an external surface of the workpiece and/or to move apart to contact an inner surface of the workpiece.

15 Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of metrological apparatus;

Figure 2 is a perspective view of an embodiment of a workpiece centring assembly and alignment device; and

20 Figure 3 is an exploded view of an embodiment of a workpiece centring assembly.

Figure 1 shows metrological apparatus 1. Apparatus 1 has a turntable 4 for effecting relative rotation of a workpiece 6 and a sensing assembly 22 about a rotation axis A. The turntable 4 supports a centring assembly 100 which supports the workpiece 6. In the illustrated example, an alignment device 30 is provided between the turntable 4 and the centring assembly 100. The sensing assembly

25 22 comprises a stylus 14 having a stylus tip 14a, a stylus gauge 16 carrying the stylus 14 and a

transducer (not shown). The transducer is coupled to the stylus 14 for monitoring movement of the stylus tip 14a. A support structure 2 of the apparatus 1 comprises a base 3, a column 8 supporting a carriage 10 for movement along the column in a Z direction, a mounting arm 24 carried by the carriage 10 and movable in the X direction relative to the carriage 10, and an attitude arm 20

30 mounted to the mounting arm 24 and rotatable relative to the mounting arm to adjust the attitude of the stylus gauge 16 which is mounted to the mounting arm 24. The base 3 supports the column 8 and the turntable 4.

In use, as the turntable 4 rotates, the stylus tip 14a follows the surface of a workpiece 6. Any deflection of the stylus tip 14a is converted by the transducer into an electrical signal which is processed by a remote processing unit (not shown) to determine at least one of a number of surface characteristics of the workpiece, for example roundness. In the present example, the stylus 14 is biased towards the workpiece surface and is deflectable in a measurement direction against that biasing.

Figure 2 shows the centring assembly 100 and alignment device 30. The centring assembly 100 has a workpiece support surface 40 for supporting the workpiece 6, a plurality of jaws 42 and a sleeve 60. The workpiece support surface 40 and the sleeve 60 are arranged for relative rotation. The jaws 42 are arranged for movement with respect to the workpiece support surface 40 to centre the workpiece 6 on the workpiece support surface 40. In the illustrated example, each jaw 42 is moveable along a respective guide path 50 of the workpiece support surface 40. In the present example the jaws 42 have pointed tips or tapered ends for providing point contacts with the workpiece 6 to enable precise centring. The jaws 42 are shown as being movable radially inwardly from positions towards the edge of the workpiece support surface 40 to contact an exterior surface of a workpiece 6. Additionally or alternatively the jaws 42 can be moved apart from positions towards the centre of the workpiece support surface 40 to contact an interior surface of a workpiece 6.

It will be understood that the described apparatus is particularly appropriate for measuring the roundness of hollow workpieces because the centring assembly 100 can access the interior as well as exterior surfaces of the workpiece to centre the workpiece. In particular hollow cylindrical components including cylindrical bearings and annular ring bearings may be centred. Such bearings have a broad range of applications, for example larger bearings with diameters ranging from 200mm to 300mm may be used in cars and washing machines, while smaller bearings with diameters in the range ΙΟμηι to 2mm may be used in electric fans.

In Figure 2, the centring assembly 100 is shown supported by the alignment device 30. The alignment device 30 has a platform 30a which supports the centring assembly 100 and screw mechanisms 32 for adjusting the position of the platform to align a centre of the workpiece support surface 40 with the rotation axis A. In the present example, two screw mechanisms in the form of manually rotatable thumbscrews 32 are provided for translating the platform in a plane normal to the rotation axis A. In the present example, the screw mechanisms are spaced apart by 90 degrees to translate the platform in orthogonal directions (X, Y) in the plane. Figure 3 is an exploded view showing the interior of the centring assembly 100. The centring assembly 100 comprises the jaws 42 and workpiece support surface 40 having guide paths 50, and sleeve 60 as described above, a guidance mechanism coupling the jaws 42 to the sleeve 60 and a base plate 80. The guidance mechanism is contained within the sleeve 60.

The guidance mechanism comprises elongate bearings or guide rails 70, members 62 slidable along the elongate bearings and connectors 68 coupling the slidable members to the sleeve 60. Each slidable member 62 is coupled to a respective jaw 42 through the jaw's respective guide path 50. The coupling between a jaw 42 and the corresponding slidable member 66 is provided by protrusions 46 and recesses 66 provided on opposing faces of the jaw 42 and slidable member 62. In the illustrated example, the protrusions 46 are provided on the jaws 42 and the recesses 62 are provided on the slidable members 62. Each jaw 42 may additionally be coupled to its respective slidable member 62 with a screw 44. The protrusions 46 may be pegs and the recesses 66 correspondingly shaped holes. Also in the illustrated example, the bearings are linear bearings

Each slidable member 62 is arranged to move along a respective elongate bearing 70. Each connector 68 has an inner end pivotally coupled to a respective slidable member 62 and an outer end pivotally coupled to the sleeve 60. In the present example, the guide paths 50 extend radially from the centre of the workpiece support surface in a radially symmetric configuration, the elongate bearings 70 have a radially symmetric configuration and the couplings of the connectors 68 to the sleeve 60 are equally spaced around the sleeve 60 such that the connectors 68 have a radially symmetrical configuration. The base plate 80 is irrotably coupled to the workpiece support surface 40 and the base plate 80 and workpiece support surface 40 are arranged for relative rotation with respect to the sleeve. The sleeve 60 is coupled to a large circular bearing 94 which is supported against a shoulder 88 of the base plate to enable precise rotation. In the illustrated example, a plurality of blocks 92 is provided on the base plate 80 to which the top plate 40 is affixed with screws 72. Each of the base plate 80 and the sleeve 60 has a handle 86, 84 for enabling relative rotation of the base plate 80 and the sleeve 60. The base plate 80 also has recesses 90 to which, in the present example, the bearings 70 are coupled. In the present example, the bearings 70 are coupled to the recesses 90 with screws. In the illustrated example, the recesses 90 extend radially from a centre of the base plate 80 and have a radially symmetric configuration which matches the configuration of the bearings 70 and is aligned with the configuration of bearings 70 by the couplings therebetween. The recesses 90 are coplanar so that the bearings 70 have a coplanar configuration when coupled to the base plate 80. The workpiece support surface 40 is coupled to the base plate 80 such that the guide paths 50 are aligned with the elongate bearings 70.

In use, relative rotation of the sleeve 60 and the workpiece support surface 40 and base plate 80 is effected by moving apart the base plate handle 86 and sleeve handle 84. The base plate handle 86 can be used to hold the base plate 80 steady to mitigate the effect of any turntable rotation on the magnitude of relative rotation. The relative rotation causes the connectors 68 to pivot about both pivotal couplings. The pivoting allows the outer end of the connectors 68 to be rotated with the sleeve 60. The outer ends are translated circumferentially and angularly relative to the inner ends. The pivoting of the inner ends and the relative movement of the inner and outer ends allows the inner ends to move in directions defined by their respective bearings 70. The movement of the inner ends causes their respective slidable members 62 to move along their respective bearings 70. The movement of each slidable member 62 provides movement of its respective jaw 42 along its guide path 50. In operation, jaws 42 move along their respective guide paths 50 and into contact with a surface of the workpiece 6 to urge to urge the workpiece 6 toward the centre of the workpiece support surface 40 to centre the workpiece 6 on the workpiece support surface 40.

The couplings of the jaws 42 to the slidable members 62 may allow the jaws to be removed. The jaws 42 may be replaced with differently sized jaws for centring a different sized workpiece, for example, or for performing some other customised centring operation, or because the jaws 42 have become worn through use. In the present example, the workpiece support surface 40 can be uncoupled from the base pate 80 by being unscrewed form the blocks of the base plate 80. This allows the workpiece support surface 40 to be removed so that it can be treated, for example by re-flattening or regrinding, to address the effects of wear through use. Additionally or alternatively, removal of the workpiece support surface 40 enables its replacement with a different workpiece support surface for accommodating a differently sized workpiece In the present example, the shortest one of the guide path 50, the length of the elongate bearings 70 and the length of the connectors 68 defines the maximum angle through which the sleeve 60 can be rotated relative to the workpiece support surface 40 and base plate 80. The ratio of circumferential translation of the sleeve 60 to radial translation of the jaws 42 is generally approximately unity. Generally, the maximum range of radial movement of the jaws 42 is provided by rotating the sleeve through one radian. Therefore, in the present example, approximately one third of a full relative rotation provides the full range of movement of the jaws 42. This feature provides time saving benefits over a standard scroll chuck which typically requires many full rotations of its scroll plate relative to its chuck body to translate its jaws across a full radius of its body. Furthermore, the approximately unity ratio of circumferential to radial translation in the described apparatus renders the centring mechanism 100 more intuitive to a user, enabling a user better to judge the degree of rotation required to bring the jaws 42 into contact with the workpiece 6. This may help the user avoid damaging the workpiece 6 by over tightening the jaws 42.

A calibration procedure may be performed prior to a first measurement operation to ensure correct alignment with respect to the rotation axis by adjustment using the alignment device 30. In an example calibration procedure, an ideal workpiece, for example a perfectly cylindrical workpiece, is centred on the workpiece support surface by the jaws 42. A measurement operation is then performed on a surface of the ideal workpiece as it is rotated about the rotation axis A on the turntable. Data analysis is performed on the measurement data to compute the distance of the centre of the workpiece 6 from the rotation axis A. The data analysis may be performed using any appropriate centring software including that disclosed in US 5,926,781 or any other appropriate analytical tool. The alignment device 30 may then be used to align a centre of the workpiece support surface 100 with the rotation axis A. Repeated calibration steps may be performed as appropriate until the workpiece support surface 40 is centred. Once centred, any of the workpiece 6, top plate 40 or jaws 42 can be replaced without the need for further calibration using the alignment device.

ALTERNATIVES AND EMBODIMENTS In the above discussion of the metrological apparatus 1, a turntable 4 is disclosed for effecting relative rotation of the workpiece 6 and sensing assembly 22. As another possibility, a different mechanism could be provided for effecting the relative rotation, for example, a mechanism could be provided for rotating the sensing assembly 22, and in particular the stylus tip 14a, about the workpiece 6. As described above, the stylus gauge is mounted on a carriage which is movable in the Z direction along the column 8. As another possibility, the carriage may be fixed on the column or the carriage may be omitted and the mounting arm fixed to the column. Also as described above, the mounting arm is slidable in the extraction relative to the carriage. As another possibility, the mounting arm may be fixed relative to the carriage. As described above, the attitude arm 20 enables the orientation of the stylus gauge to be changed. As another possibility, the attitude arm may be omitted all fixed and so not movable. The stylus gauge 16 may be movably mounted to the attitude arm 20 to allow rotation of the stylus 14 between different measurement directions. This is advantageous when measurement of an exterior and interior surface of a workpiece 6 is required. Where the attitude arm is movable, then it may be moveably mounted to the mounting arm 24 so that its attitude can be changed between an attitude parallel to the rotation axis A to measure an edge of a workpiece 6, and an attitude transverse to the rotation axis A to measure an upper or lower surface of the workpiece 6. The moveable mounting 24 may be provided by a kinematic mounting, for example. Any combination of these features may be provided.

The described alignment device 30 two screw mechanisms 32 spaced apart by 90 degrees. In other examples, a different number of screw mechanisms 32 are provided, and the screws may not be equally spaced to allow translation of the platform in non-orthogonal directions. The screws 32 may be standard thumb screws. Any other appropriate mechanical mechanism for translating the platform may be provided. In an example, the alignment device 30 may includes a centring motor and be configured to perform a conventional automated centring operation using the centring motor. It may be possible to omit the alignment device 30.

In the described embodiment, the jaws 42 have pointed tips. In another example, the jaws 42 have tapered tips. In another example, the jaws 42 have a tip geometry configured to cooperate with a surface of the workpiece 6. If the jaws are required only to measure internal or only to measure external surfaces, then only the corresponding ends of the jaws may be pointed or tapered. Couplings of the jaws 42 to the slidable members are described above as comprising protrusions 46 provided on the jaws 42, wherein the protrusions are pegs, and recesses 66 provided on the slidable members 62, wherein the recesses 66 are holes with shapes corresponding to the pegs. Any other appropriately shaped protrusions and recesses could be provided. Any other suitable form of coupling could be provided, for example, screws or other detachable fixing mechanisms. In embodiments where removal of the jaws 42 is not required, the jaws 42 could be undetachable from the slidable members 62, for example, by being fixed with adhesive. In another example, the jaws 42 could be continuous with the slidable members 62.

In the described embodiment, the guidance mechanism is coupled to the base plate 80 by couplings of the bearings 70 to the base plate 80. In another embodiment, the guidance mechanism is coupled to the workpiece support surface 40. In this example, the coupling may be provided by couplings of the elongate bearings 70 to the workpiece support surface 40. This arrangement may facilitate achieving corresponding configurations for the elongate bearings 70 and guide paths and may increases the ease of assembling the centring assembly 100 so that their configurations are aligned.

As another possibility, the connectors 68 could be coupled directly between the jaws 42 and the sleeve to move the jaws 42 along the guide paths. In this example, an end of each connector 68 would be coupled to a surface of a respective jaw 42 protruding through the jaw's guide path 50.

Three jaws 42 are disclosed, but a different number of jaws 42, being at least two jaws 42, could be provided. In some embodiments, at least some of the guide paths 50 may not extend radially from the centre of the workpiece support surface 40 and the guide paths 50 may not have a radially symmetric configuration. The elongate bearings 70 may not have a radially symmetric configuration. The couplings of the connectors 68 to the sleeve 60 may be unequally spaced around the sleeve 60 such that the connectors 68 do not have a radially symmetrical configuration. The guide paths 50, elongate bearings 70 and connectors 68 could be arranged such that the jaws 42 are arranged to move in a symmetric configuration, including a square configuration, or an asymmetric configuration, such as a T-shaped configuration. An asymmetric configuration may be advantageous to centring an asymmetrical workpiece, such as cam having an odd number of edges, a gear having an odd number of teeth or any other workpiece having radial asymmetry. The connectors 68 are illustrated as linear but are preferably S-shaped, such that each inner end of each connector 68 curves away from its slidable member 62 to help the connector 68 to stay clear of the slidable member 62 as it moves along its elongate bearing 70. In some embodiments, the connectors 68 are substantially inflexible but in other embodiments they could have a degree of elasticity, for example to help protect a delicate workpiece 6 from damage by softening the impact of the jaws 42 on the workpiece 6. The connectors 68 could comprise spirals, lazy arms or be jointed.

By adjusting one or more of the features of the centring assembly 100 as indicated, a centring assembly could be configured to centre a radially symmetric workpiece or a radially asymmetric workpiece. ln an embodiment, a biasing mechanism is provided to bias the sleeve against the direction of its rotation relative to the workpiece support surface 40 and base plate 80. This has the effect of biasing the jaws against moving towards the workpiece which can help prevent damage to the workpiece. In one possibility, the biasing mechanism is a spring-loaded biasing mechanism. In an embodiment, a biasing lever is provided through the sleeve and a spring is coupled between an inner end of the lever and the nearest one of the blocks. The lever is moveable along a slot between first and second positions which are spaced apart around the sleeve. The lever may be retained in one of the first and second positions by a groove running transverse to the slot. When the sleeve is to be rotated in a clockwise direction relative to the workpiece support surface and base plate, the lever is moved to the first position to bias the sleeve against clockwise rotation. When the sleeve is to be rotated anticlockwise, the lever is moved to the second position to bias the sleeve against anticlockwise rotation. The two biasing arrangements mean that the jaws can be biased away from the workpiece surface whether they are moving together to engage an exterior workpiece surface or apart to engage an exterior workpiece surface.