Calibration device and method

Abstract

A calibration apparatus for a coordinate positioning machine, such as a machine tool, is described. The apparatus (200) includes a base (30; 130; 202), a calibration artifact (34; 134), and a biasing mechanism (32) that attaches the calibration artifact (34; 134) to the base (30; 130; 202), such that the calibration artifact (34; 134) can be moved relative to the base (30; 130; 202) by application of an external force. The device further includes a releasable lock that, when locked, secures the calibration artifact (34; 134) relative to the base (30; 130; 202).

Description

[Technical field]

[0001] The present invention relates to a calibration device for use with a coordinate positioning apparatus such as a machine tool, and in particular to a reduced complexity calibration device and methods associated with the use of such a device. [Background technology]

[0002] Various coordinate positioning devices are known. For example, numerically controlled machine tools are widely used in manufacturing to cut components such as metal parts for vehicles, aircraft, etc. In order to cut features with high accuracy (e.g. within a few microns), the measurement probes of such machine tools need to be calibrated. This is particularly true when using machine tools that have one or more rotational axes (e.g. mill-turn or 5-axis machine tools) where the orientation of the part relative to the cutting tool changes during the cutting procedure.

[0003] An important part of a typical calibration procedure, especially for a machine tool that has at least one rotary axis, is to establish the location of the center of a ring gauge, calibration sphere, etc. relative to a reference position on the machine tool. Once established, the center can serve as a machine datum point in the work coordinate system upon which all subsequent measurement and calibration procedures are based.

[0004] It is known to establish the location of the sphere centre in a plane parallel to the machine tool bed (typically referred to as the XY plane) using a dial test indicator or touch probe mounted on the machine tool spindle. Measuring the sphere centre location along an axis perpendicular to the machine tool bed (typically referred to as the Z axis) is more complex and has heretofore typically been performed using a variety of manual procedures. For example, it is known to move a reference tool of known length along the Z axis towards the calibration sphere under manual control. The reference tool is advanced towards the sphere until a gage block of known thickness is "sandwiched" between the reference tool and the sphere. It has been found that such manual procedures can be inaccurate and calibration results vary from operator to operator.

[0005] International Patent Publication No. WO2017 / 121990 describes a calibration device that enables the establishment of the center of a calibration sphere relative to the reference position of a machine tool. The device includes a calibration artifact that is biased to a known and repeatable rest position relative to the base of the device, but can be displaced from the rest position when a sufficiently high force is applied thereto (e.g., by a bar of calibrated length). A sensor is provided within the device to measure the magnitude of the deflection away from the rest position (e.g., along the Z-axis of the machine), thereby enabling the position of the calibration device (e.g., within the Z-axis) to be determined with a high level of accuracy using an extrapolation technique. The device of WO2017 / 121990 enables very accurate and fully automated measurement of the sphere position, but the inventors have found that the calibration device is relatively expensive to manufacture and use.

SUMMARY OF THE INVENTION

[0006] According to a first aspect of the present invention, there is provided a calibration device for a coordinate positioning machine, comprising: a base; a calibration artifact; a deflection mechanism for attaching the calibration artifact to the base and enabling the calibration artifact to move relative to the base upon application of an external force; The calibration device is further provided with a releasable lock for fixing the calibration artifact relative to the base when the device is locked.

[0007] There is thus provided a calibration device suitable for use in calibrating a coordinate positioning machine, such as a machine tool, a coordinate measuring machine (CMM), a robot, etc. The calibration device preferably comprises a base releasably attachable to a part of the coordinate positioning machine. For example, the base may be magnetically attachable to the bed of the machine tool. The calibration device also comprises a calibration artefact, such as a calibration sphere (e.g. a sphere having a radius precisely measured to a traceable metric), a partial sphere, a dome surface, a ring gauge, or a flat surface attached to the base via a deflection mechanism.

[0008] The deflection mechanism allows the calibration artifact to move relative to the base, but the device is also characterized by a lock that allows the device to prevent such relative movement if necessary. In other words, the deflection mechanism can be "unlocked," thereby allowing the calibration artifact to move relative to the base, or "locked," thereby preventing the calibration artifact from moving relative to the base. This configuration allows the calibration artifact to be moved to a deflected position relative to the base by applying an external force when unlocked. For example, a bar or reference tool of known length carried on the spindle of the machine may engage and move the calibration artifact relative to the base. Once moved to such a deflected position, the lock can be actuated (i.e., locked) such that the calibration artifact remains in that deflected position even when the length bar or reference tool is disengaged. Thus, the calibration artifact is fixed relative to the base, and the position of the calibration artifact is known when the deflected position is set using a reference tool (e.g., a length bar, a tool of known length, etc.) that has a known position in the coordinate system of the coordinate positioning machine. The calibration artifact can then remain locked in place and can be used for further calibration tasks.

[0009] Thus, the calibration device of the present invention has the advantage that it can provide a calibration artifact having a known position (e.g., one, two or three dimensions) in the machine coordinate system, but without the need to sense when it comes into contact with the calibration artifact and without the need to measure the amount of deflection of the calibration artifact. Instead, the inventors recognize that the calibration artifact can be moved to any position, as long as that position can be determined in the machine tool coordinate system. Thus, the calibration device of the present invention is less complex and therefore less expensive to manufacture than prior art devices, such as those described in prior patent publication WO2017 / 121990, while also eliminating the subjective element of sandwiching a gauge block of known thickness between a reference. A step of locking the calibration device is required, but even if this is done manually, it can only be done after the machine motion has stopped. That is, there is no need to access the calibration device by overriding a safety interlock or the like when the machine is moving.

[0010] The deflection mechanism, when unlocked, may limit the movement of the calibration artifact to some extent (e.g., the movement may be limited to translation along one or more axes). In one embodiment, the deflection mechanism may conveniently include a guide that provides linear translation of the calibration artifact when the releasable lock is unlocked. In other words, the deflection mechanism may include a guide that guides the calibration artifact to move linearly (i.e., along a linear axis) back and forth when the releasable lock is unlocked. The deflection mechanism may prevent rotation of the calibration artifact. In one example, linear movement of the calibration artifact is allowed along an axis parallel to the movable shaft of the deflection mechanism. This linear movement axis of the calibration artifact may be aligned, in use, to approximately coincide with the longitudinal axis of a reference tool. Such a reference tool may then be moved to engage the calibration artifact along the longitudinal axis of the calibration artifact such that the calibration artifact is deflected along the linear movement axis. The length of the reference tool is, by definition, known and therefore the position of the calibration artefact along its axis is known.

[0011] If the deflection mechanism restricts the calibration artifact to move only along a particular axis (e.g., a linear axis of translation), the associated reference point defined by the artifact may be positioned to be located on or near the axis. In other words, the calibration artifact may be conveniently positioned on a linear axis. For example, the center of a calibration sphere may be positioned to be substantially on the linear translation axis and thus translate back and forth along the linear translation axis. This prevents or reduces any external forces applied to the deflection mechanism (i.e., when the calibration artifact moves) from causing off-axis movement or deflection (deformation) of the device. This, in turn, helps to ensure that when the releasable lock is locked, the calibration artifact does not substantially change position when the deflection force is removed (e.g., when the reference tool is disengaged).

[0012] Advantageously, the biasing mechanism comprises a bias. The bias may bias the calibration artifact towards a rest position. The bias may be provided by one or more springs (e.g., coil springs). The force exerted by the bias is preferably insufficient to overcome the locking force exerted by the lock. Thus, the bias results in movement of the calibration artifact only when the lock is in an unlocked state. In other words, when the lock is locked, the bias is insufficient to overcome the locking force and thus the calibration artifact is fixed relative to the base. The rest position to which the calibration artifact is biased does not need to be repeatedly defined (i.e., the rest position adopted by the calibration artifact does not affect the accuracy of the calibration).

[0013] The releasable lock may be implemented in various ways. The releasable lock may be a remotely actuable lock. For example, the releasable lock may be an electrically or pneumatically actuated lock that can be automatically (non-manually) locked / unlocked by sending an appropriate command to the device (e.g., via a wire or pneumatic line). Preferably, the releasable lock is manually actuated (i.e., manually actuated by an operator / user). Thus, the releasable lock conveniently comprises a manually actuated locking member. The locking member is preferably a locking lever. Alternatively, the locking member may be a twist lock member.

[0014] The calibration device may comprise any suitable calibration artifact. Thus, the calibration artifact may be any item having known dimensions (e.g., an artifact of known size). The calibration artifact may comprise a flat surface. The calibration artifact may comprise a dome-shaped surface (e.g., a surface with the curvature of a large diameter sphere). One or more dimensions of the artifact may be known by prior measurement against a calibrated (traceable) standard. For example, one or more dimensions of the artifact may be previously measured with a coordinate measuring machine (CMM) that is calibrated against a relevant (national or international) calibration standard. The calibration artifact may comprise a ring gauge, a cube, a disk (e.g., having a known radius), etc. Conveniently, the calibration artifact comprises a sphere. The sphere may have a known radius. For simplicity and accuracy, a sphere of known radius is preferred, since it is possible to determine the location of the sphere center (e.g., in x, y, z coordinates) by measuring the positions of multiple points on the sphere. The calibration sphere may be a full sphere or a partial sphere (eg, it may comprise only a segment of a sphere, or it may include a flat area or recess for mounting to a stem).

[0015] A calibration artifact may provide a datum point in the work coordinate system in one, two, or all three dimensions. For example, a calibration sphere may provide a three-dimensional datum point (e.g., a sphere center). A ring gauge may provide a two-dimensional datum point (e.g., a center point of a ring in a particular plane). A flat surface may provide a one-dimensional datum point (e.g., a position of a plane that comprises the surface). A calibration device may include both a (movable / lockable) calibration artifact and an additional calibration artifact. For example, a device may include a (movable / lockable) flat or dome-shaped surface and an additional artifact that is not movable (e.g., a ring gauge or a sphere that is fixedly attached to the base of the device). In this way, features of both the (movable / lockable) calibration artifact and the additional calibration artifact may be used to define a datum point in the work coordinate system in three dimensions.

[0016] The calibration device may include a housing attached to or formed as part of the base. The deflection mechanism may be partially or completely contained within the housing. The deflection mechanism may comprise an elongated stem that attaches the calibration artifact to the base. The calibration artifact may be attached directly to the stem, or an additional component (e.g., an angled wedge, an extension piece, etc.) may attach the calibration artifact to the stem. Thus, the orientation of the calibration artifact and the attached stem may be selected as needed (e.g., to allow for vertical and / or horizontal mounting of the calibration device).

[0017] The base may be attached to the machine in several ways. For example, it may be bolted to the machine bed. Advantageously, the base may be equipped with a magnetic mounting device for releasably fixing the base to a metal surface of the coordinate positioning machine (e.g. the bed of a machine tool). In other words, a magnetic base may be used that can releasably attach the device to a metal surface. This holds the device securely but allows it to be easily removed.

[0018] The calibration device may be configured to be mounted on any coordinate positioning machine. Advantageously, the calibration device is configured to be mounted on a coordinate positioning machine comprising a machine tool. The machine tool may have at least three, at least four, or at least five axes of motion. The machine tool may have at least one axis of rotation. The machine tool may have at least two axes of rotation. The machine tool may be a turning machine, lathe, or mill-turn machine. The calibration device may be mountable to a machine tool bed that is movable relative to a tool-holding spindle. The invention also extends to a coordinate positioning machine comprising the calibration device. For example, a machine tool comprising the calibration device may be provided.

[0019] According to a second aspect of the invention, there is also provided a method of calibrating a coordinate positioning machine (e.g. a machine tool), the method comprising the steps of (i) mounting a calibrated tool (e.g. a bar or tool of known length) and a calibration artefact (e.g. a calibration sphere) on the coordinate positioning machine, (ii) moving the calibrated tool using the coordinate positioning machine to engage the calibration artefact such that the calibration artefact is moved from an initial position to a deflected position, and (iii) locking the calibration artefact in the deflected position such that the calibration artefact is maintained in the deflected position after disengagement from the calibrated tool.

[0020] A further step may then be performed: (iv) using a coordinate positioning machine to move a measurement probe to measure the position of a number of points on the surface of the calibration artefact. The measurement probe may be a tactile measurement probe with a stylus for contacting the artefact, e.g. a touch-trigger measurement probe or an analogue (scanning) probe. Alternatively, the measurement probe may be a non-contact measurement probe. The measurement probe may be carried on the spindle of the machine. Thus, the measurement probe may be used to measure the position of the calibration artefact. For example, the xy coordinates of a calibration sphere may be found using the measurement probe and used in combination with the Z coordinate of the sphere centre, known by placing the calibration artefact in a stationary position using a calibration tool.

[0021] Advantageously, the calibration artefact comprises a sphere. The position of the centre of the sphere can then be used to define a reference position in the local coordinate system of the coordinate positioning machine. This sphere centre (reference) position can be determined in the machine coordinate system and used for further calibration procedures.

[0022] In a preferred embodiment, a calibration device according to the first aspect of the invention is used in the method of the second aspect of the invention. The calibration device may then provide a calibration artefact and step (iii) of the method may comprise locking a releasable lock of the device.

[0023] A further aspect of the invention provides a calibration device for a coordinate positioning machine comprising a base, a calibration artefact, and a lockable mechanism for mounting the calibration artefact to the base, the lockable mechanism being capable of adopting an unlocked state in which the calibration artefact is movable relative to the base by application of an external force, and a locked state in which the position of the calibration artefact is locked relative to the base. The calibration artefact may be biased towards a rest position when in the unlocked state. The calibration artefact may comprise a calibration sphere having a known radius. The lockable mechanism may comprise a manually actuated member for changing between the locked and unlocked states. The base may comprise a magnet enabling mounting to the bed of the machine tool.

[0024] Also described herein is a calibration device. The calibration device may be for a coordinate positioning machine. The calibration device may comprise a base. The calibration device may comprise a calibration artefact. The calibration device may comprise a deflection mechanism. The deflection mechanism may attach the calibration artefact to the base. The deflection mechanism may allow the calibration artefact to be moved relative to the base by application of an external force. The device may further comprise a (releasable) lock. When locked, the lock may secure the calibration artefact relative to the base. The device may include any of the features described above and / or be used in any of the methods described above. [Brief description of the drawings]

[0025] The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1

Figure 2

Figure 3a

Figure 3b

Figure 4

Figure 5

Figure 6

Figure 7a

Figure 7b

Figure 7c

Figure 8

[0026] FIG. 1 shows a multi-axis machine tool. The machine tool comprises a spindle 2 which can be rotated at high speed around its centre, typically around what is called the S-axis. The spindle 2 comprises a tapered mount 6 for receiving a tapered shank of a cutting tool 4 or other accessories, such as a measuring probe, so that tools and accessories can be loaded into the spindle 2, if required. The spindle 2 can be moved in space by the machine tool along three linear axes. These machine tool axes are typically called the X-axis, the Y-axis and the Z-axis. A table 10 is provided on which the workpiece 12 is mounted. The table 10 can also be tilted around the A-axis and rotated around the B-axis.

[0027] During cutting, the cutting tool 4 is rotated at high speed about the S axis and the machine tool controller follows a list of instructions defined in the cutting program to move the tool 4 along the desired cutting path relative to the workpiece 12. The cutting path may include translational movements of the spindle along the X, Y and Z axes, as well as rotational movements about the A and B axes. In order to remove the required material from the workpiece, the position of the tool tip relative to the workpiece needs to be known precisely, even when rotational movements about the A and B axes occur. Therefore, various techniques and devices have been developed over the years to perform such calibrations.

[0028] As will be known to those skilled in the art, many automated machine tool calibration techniques involve using a spindle-mounted measurement probe to make various measurements of a calibration sphere (i.e., a sphere of known radius) mounted on the machine tool bed. These measurements typically depend on knowing with high accuracy the location of the calibration sphere's center relative to a reference location of the machine tool (e.g., the origin of the x, y, z machine tool coordinate system). Accordingly, various techniques have been developed over the years to accurately measure the sphere center location. Once measured, subsequent on-machine measurements (e.g., for calibration, workpiece measurement, or tool measurement purposes, etc.) can be tied to the sphere center location.

[0029] 2, a prior art technique is described that allows the position of the center of a calibration sphere 20 that is fixedly (immovably) attached to the bed of a machine tool to be determined. The sphere may be fixed to a base or another structure within the machine tool, typically on which a workpiece is placed. Thus, the sphere may be attached to a work surface of the machine tool, which may comprise one or more rotational axes of the machine tool.

[0030] First, it is noted that several techniques are known that allow the XY position of the calibration sphere to be established with high precision. For example, a dial test indicator (DTI) may be attached to the spindle of a machine tool and used to "clock" the diameter of the calibration sphere 20 near the equator of the sphere. This is typically done by jogging (i.e. moving under manual control) the X and Y position until the dial indicator no longer deflects when the spindle is rotated. When this is achieved, the XY position of the spindle is the center position of the sphere.

[0031] It is also known to measure the XY centre of a sphere using a touch-trigger probe mounted on the spindle of a machine tool: the rotational position of the spindle (i.e. the angle of rotation about the S axis) is oriented to zero degrees, the sphere centre is measured and the XY position of the sphere centre recorded.

[0032] The spindle is then rotated 180° and the XY position of the sphere centre is re-measured. The average between the two measured XY sphere centre positions is used as the centre position, i.e. any error due to eccentric mounting of the touch probe spherical stylus relative to the spindle centre of rotation is eliminated.

[0033] Although the techniques mentioned above make it possible to find the XY sphere center position, they usually cannot establish the Z position of the sphere with sufficient accuracy for machine tools that have one or more rotational axes. Therefore, several separate techniques are known for determining the Z position of the sphere.

[0034] The most commonly used technique is illustrated in Figure 2 and involves the use of a so-called feeler gauge block 24. This can be a block or sheet of material of known (e.g. calibrated) thickness. A reference tool 22 of known, calibrated length is loaded into the spindle of the machine tool. The spindle is moved so that the reference tool 22 is located above the topdeadcentre of the sphere 20. A feeler gauge or gauge block 24 is placed between the end of the reference tool 22 and the sphere 20. Using the manual "jog" function of the machine tool, the tool is manually moved down until the gauge block 24 is just "squeezed" between the reference tool 22 and the sphere 20. This manual process requires the engineer to "feel" when the gauge is free to move, but has no play or clearance gap. Once this is achieved at the current machine position, the length of the tool and the calibrated radius of the sphere are used to calculate the centre Z position of the sphere.

[0035] It is also known to use a bespoke length setting device to set the Z position. The device, called the Base-Master, is manufactured by Big Daishowa in Japan and supplied by Metrology Software Products Limited, Alnwick, UK, and is fitted with a repeatable unidirectional indicator. The indicator is attached to the machine tool spindle and positioned over the XY centre of a fixed sphere. The jog control of the machine tool is used to move the spindle down until the Base-Master just touches the top of the sphere. Such contact is indicated by an LED attached to the Base-Master device. The (known) length of the Base-Master, the current Z position and the radius of the sphere are then used to calculate the centre of the sphere in the Z axis.

[0036] As mentioned above, WO2017 / 121990 describes a calibration device that allows for establishing the center of a calibration sphere relative to a reference position of a machine tool in an automated manner. The device includes a calibration artifact (e.g., a calibration sphere) that is biased by a relatively high spring force to a known, repeatable, rest position relative to its base. Upon a sufficiently high force applied by a calibrated length bar (or other reference tool) carried on the spindle of the machine tool, the calibration sphere moves away from the rest position. An analog sensor is provided within the calibration device to measure the magnitude of the deflection away from the rest position. A measurement of the position of the spindle (i.e., holding the length bar) as the length bar moves to deflect the calibration sphere is combined with a measurement of the calibration sphere measured by the analog sensor. Extrapolation techniques are then used to accurately determine the position of the calibration sphere when it is in its rest position. This process is described in further detail in WO2017 / 121990.

[0037] Although the device of WO2017 / 121990 provides highly accurate position information in a fully automated manner, the device itself is complex and can be expensive to manufacture. For example, the rest position adopted by the calibration artefact must be highly repeatable and the analogue measurement sensors must provide accurate position measurements. Also, a separate computer is required to combine data from the machine tool and the calibration device to establish the rest position of the calibration artefact. This makes the set-up and operation of the apparatus relatively expensive and complex.

[0038] The present invention arises from the inventors' realization that it is not necessary to measure the position of a calibration artifact (e.g., a calibration sphere) only after it has been fixed in place in the machine tool. Instead, the calibration sphere can be moved to any (but known) position by being moved along the Z-axis by a reference tool (e.g., a calibrated length bar). As long as the calibration sphere is locked in place (e.g., fixed relative to the machine tool bed to which it is attached) before it is disengaged from the calibrated length bar (i.e., such that the length bar is retracted), the calibration sphere can provide a known reference position in the machine tool coordinate system. This eliminates the need to sense contact between the length bar and the calibration sphere and to measure the amount of deflection of the calibration sphere. Thus, a lower cost and simpler device is provided. We now describe the apparatus of the present invention and how it can be used for calibration purposes.

[0039] 3 a and 3 b show a first embodiment of a calibration device of the present invention comprising a magnetic base 30 , a deflection mechanism 32 and a calibration sphere 34 .

[0040] The magnetic base 30 is a commercially available magnetic base manufactured by Misumi Group of Japan that can be attached to a metal machine tool bed with a holding force of approximately 800 N. The magnetic base 30 can be securely secured to the machine tool bed, but is releasable, by rotation of knob 36 from an "off" position to an "on" position. As described below, other designs of magnetic bases or different types of mounting mechanisms (e.g., bolts, screws, etc.) can be used to securely secure the device to the machine tool.

[0041] The deflection mechanism 32 comprises a housing 38 fixed to the top of the magnetic base 30 and a movable shaft 40 to which the calibration sphere 34 is indirectly attached. The calibration sphere 34 is attached to a further shaft 42 which is in turn attached to the movable shaft 40 via a 45 degree wedge element 44. This configuration allows the calibration sphere to be measured from different directions, thereby allowing the calibration device to be mounted on horizontal, vertical or inclined surfaces. It would be possible to attach the calibration sphere 34 to the movable shaft 40 in a variety of different ways. In this example, the movable shaft 40 can be translated back and forth in the direction indicated by the arrow 41 (i.e. parallel to the longitudinal axis of the movable shaft 40).

[0042] The calibration device further includes a lock contained within the housing 38, the structure and operation of which will be described in more detail below with reference to FIG. 4. The lock, which can be manually actuated by an operator by moving a lever 46, locks the position of the movable shaft 40 relative to the housing 38. When the lock is in a locked state, it thus fixes the movable shaft 40 relative to the housing 38. The locked state is shown in FIG. 3b. However, the movable shaft 40 can still translate back and forth (i.e., along the indicated direction 41) when the lock is in an unlocked state. The unlocked state is shown in FIG. 3a. As will be described below, a spring (not shown) is also provided within the housing to bias the movable shaft 40 away from the housing 38 when in the unlocked state (i.e., the shaft is biased upwards by the spring when the device is in the orientation shown in FIG. 3a).

[0043] Thus, the calibration device can adopt a locked state (i.e., locked configuration) in which the calibration sphere has a fixed position relative to the magnetic base 30, and an unlocked state (i.e., unlocked configuration) in which the calibration artifact cannot move relative to the housing (i.e., is fixed). It is important to note that the lock can be actuated (i.e., locked) when the moveable shaft is moved to any position relative to the housing (i.e., such movement is possible in the unlocked state). As explained below, this allows the calibration sphere to be moved to a predetermined position and locked in that position for subsequent measurements to be made. When the device is locked, movement of the calibration sphere relative to the base is prevented.

[0044] Figure 4 shows further details of the internal deflection mechanism and lock of the calibration device shown in Figures 3a and 3b, and therefore like features are assigned like reference numbers.

[0045] As mentioned above, the calibration device comprises a movable shaft 40, which is assembled into the housing 32 and is shown in FIG. 4 as a separate item. The movable shaft 40 is generally cylindrical but includes a V-shaped recess 60 running along a central portion of its length. A first end 62 of the movable shaft 40 includes a threaded hole in which is mounted a wedge element 44 that holds the calibration sphere 34. A second end 64 of the movable shaft 40 includes an enlarged portion having an opening for receiving a coil spring 66. When assembled into the device, the second end 64 of the movable shaft 40 engages the coil spring 66, thereby biasing the movable shaft 40 upward (i.e., upward when in the orientation shown in FIG. 4).

[0046] Locking is provided by a locking mechanism comprising a ball 70 which engages a first end of a slidable insert 72 held in a slot in the housing. In particular, the ball 70 is urged into contact with the V-shaped recess 60 of the movable shaft 40 by a number of disc springs 74 held in the insert 72. The second end of the slidable insert 72 engages a camshaft 76. The camshaft 76 is generally an elongated cylinder, but has a flat area 78 (i.e., a small diameter) for engaging the second end of the slidable insert 72. A threaded hole 80 is also provided in the camshaft 76 to allow the radially projecting locking lever 46 to be attached to the camshaft 76. FIG. 4 shows the camshaft 76 when installed in the device as an individual component.

[0047] Movement of the lever 46 (e.g., between the positions shown above in Figs. 3a and 3b) rotates the camshaft 76, thereby radially moving the slidable insert relative to the movable shaft 40. Movement of the lever 46 thus rotates the camshaft 76, thereby increasing or decreasing the retention force applied to the movable shaft 40 via the ball 70. The arrangement is configured to allow the lever 46 to move between a locked state and an unlocked state. In the unlocked state, the ball 70 is only pressed into the movable shaft 40 with a light force, thereby allowing the movable shaft 40 to translate back and forth within the channel of the housing. In the unlocked state and in the absence of an applied external force, the movable shaft 40 is pushed (upwards) by the coil spring 66, thereby fully extending from the housing 32. It should be noted that in the unlocked state, the ball 70 is still located within the V-shaped recess 60 of the movable shaft 40, thereby limiting the axial range of movement of the movable shaft (i.e., preventing the spring from pushing the movable shaft 40 out of the housing). The spring force of the coil spring 66 is selected to allow axial deflection of the movable shaft 40 when engaged, for example, by a reference tool (e.g., a calibrated length bar) held in a machine tool spindle. In the locked state, the ball 70 is urged into engagement with the movable shaft 40 with sufficient force to secure that shaft 40 relative to the housing 32. Moving from the unlocked state to the locked state locks (fixes) the calibration sphere in its current position (which may be a position to which it has been deflected by, for example, the application of an external force).

[0048] 5 and 6, a variation of the above calibration device is described that includes a different locking mechanism.

[0049] FIG. 5 shows a calibration device comprising a magnetic base 130 attached to a housing 132, similar to the devices described above. A calibration sphere 134 is also attached to a movable shaft 140. A lock comprising a locking ring 146 is also provided. The movable shaft 140 can move axially relative to the housing 132 when the locking ring 146 is rotated to an unlocked state. However, when the locking ring 146 is rotated to a locked configuration, the movable shaft 140 (and thus the calibration sphere 134) is fixed relative to the magnetic base 134. Markings on the housing 132 and the locking ring 146 may indicate whether the lock is locked or unlocked. Thus, the device of FIG. 5 provides the same functionality as the devices described above with reference to FIGS. 3a, 3b and 4, but includes a different mechanism for implementing the lock.

[0050] With reference to FIG. 6, further details of the locking mechanism of the calibration device described with reference to FIG. 5 are shown. In common with the arrangements of FIGS. 3a, 3b and 4, the movable shaft 140 includes a recess into which the ball 170 is pressed by the insert 172. However, the insert 172 is pushed towards the movable shaft 140 by the rotatable disk 180 which is in rolling engagement with an inner surface recess 182 formed in the locking ring 146. The recess 182 in the locking ring 146 varies in radius so that rotation of the ring causes the rotatable disk 180 to move the insert towards or away from the movable shaft 140, thereby exerting different amounts of force via the ball 170. In particular, this arrangement is configured to provide a locked state in which the movable shaft 140 is fixed relative to the housing 132, and an unlocked state in which the movable shaft 140 can move back and forth. A spring (not shown) is also provided to urge the distal end of the movable shaft 140 away from the housing when in the unlocked state.

[0051] 7a-7c, an example of how the calibration device of the present invention can be used to calibrate a machine tool will now be described.

[0052] FIG. 7a shows a schematic of a calibration device 200 having a base 202 that is rigidly fixed (e.g., by a magnet or bolt, etc.) to a machine tool bed 204. The calibration device 200 comprises a calibration sphere 206 attached to the base 202 by an elongated shaft 208. The device includes a locking mechanism that can be placed in an unlocked state that allows the shaft to move in the Z direction relative to the base 202, and a locked state in which the calibration sphere 206 is fixed relative to the case 202. In the unlocked state, the elongated shaft 208 is biased in the positive Z direction to the rest position shown. A manual actuator 210 allows the user to change the device between a locked (L) state and an unlocked (U) state as desired.

[0053] In Figure 7a, a calibrated length bar 212 held in the spindle of a machine tool (not shown) is also shown being moved in the negative Z direction towards the calibration sphere 206. The position of the end of the length bar 212 is precisely known in the machine coordinate system (i.e., because by definition it has a known length relative to a reference point on the spindle).

[0054] FIG. 7b shows the same arrangement after the calibrated length bar 212 has been brought into contact with the calibration sphere 206. The method of contact is performed manually by controlling the machine tool (e.g., using a jog wheel) to engage the end of the length bar 212 with the calibration sphere 206. After engagement between the length bar 212 and the calibration sphere 206, the relative motion is continued a short distance, thereby deflecting (moving) the calibration sphere 206 to the deflected position shown in FIG. 7b. After the machine motion has stopped, the user may open the machine tool housing and manually switch the calibration device to the locked state using the actuator 210. The position of the center of the calibration sphere in the Z-axis (with a known radius) is then known in the machine coordinate system (i.e., because the corresponding positions of the ends of the calibration length bar are known).

[0055] As shown in FIG. 7c, the calibrated length bar 212 can be pulled out of the calibration sphere 206, but the position of that sphere 206 does not move any significant distance (i.e., because the calibration device is now locked). The spindle probe 218 can then be loaded onto the spindle of the machine tool probe and used in known manner to accurately establish the location of the sphere center in the xy plane. The calibration device then provides a calibration sphere with a precisely known position in the machine tool (x, y, z) coordinate system. The sphere center can then serve as the machine datum point on which all subsequent probe measurements are based (e.g., for calibration procedures, etc.).

[0056] Referring to FIG. 8, an alternative calibration device 300 of the present invention is shown. The device includes a base 302 mountable to the bed of a machine tool. A calibration flat 306 (i.e., a calibration artifact having a flat surface) is attached to the base 302 via a lockable deflection mechanism. Following the example above, a calibrated length bar 312 may deflect the calibration flat 306, which may then be locked into position using a manual actuator 310. Thus, the position of the calibration flat 306 is known in the Z axis (i.e., defines a Z axis datum position). An additional calibration sphere 320 fixedly mounted to the base (note that this may alternatively be fixed elsewhere on the machine tool) may be used to provide a reference position in the x and y axes. In this way, the datum position may be defined in three dimensions in a coordinate system.

[0057] The calibration flat 306 shown in FIG. 8 may be replaced by a calibration artifact or a disk with a domed contact surface rather than a flat. For example, the calibration disk may have a top surface with a radius of curvature that lies on a large (virtual) sphere. This reduces length errors that may occur if there is an angular misalignment of the calibrated length bar 312 and the calibration disk when they are brought into contact (e.g., if either is not aligned with the Z axis as shown in FIG. 8). This domed disk configuration ensures that the flat-ended calibrated length bar 312 will contact the highest part of the domed surface even with slight angular misalignment. Such a configuration is particularly suitable for measuring flat-ended tools.

[0058] The above-described embodiment is merely one example of the invention. Alternative locking mechanisms, calibration artifacts, housings, bases, etc. could be used to practice the invention. The multi-axis machine tool described above is a multi-axis milling machine, but the technique could be used on any type of machine tool (e.g., lathe, mill-turn machine, etc.). Similarly, the device could be used on coordinate positioning machines other than machine tools (e.g., dedicated coordinate measuring machines or robots) to implement a wide range of measurement and / or calibration procedures.

Claims

1. 1. A calibration device for a coordinate positioning machine, comprising: With the base, A calibration artifact; a biasing mechanism for mounting the calibration artifact to the base and for allowing the calibration artifact to move relative to the base upon application of an external force; 11. A calibration device, comprising: a releasable lock that, when locked, secures the calibration artifact to the base.

2. The calibration device of claim 1 , wherein the deflection mechanism includes a guide that guides the calibration artifact to move linearly back and forth along a linear axis when the releasable lock is released.

3. The calibration device of claim 2 , wherein the calibration artifact lies on the linear axis.

4. A calibration device according to claim 1 , wherein the deflection mechanism comprises a bias for urging the calibration artefact towards a rest position.

5. A calibration device according to any preceding claim, wherein the releasable lock comprises a manually actuated locking member.

6. The calibration device of claim 5 , wherein the manually actuated locking member comprises a locking lever or a twist lock member.

7. A calibration device according to any preceding claim, wherein the lock comprises a remotely operable lock.

8. A calibration device according to claim 1 , wherein the calibration artefact comprises a calibration sphere.

9. A calibration device according to claim 1 , wherein the deflection mechanism comprises an elongated stem that attaches the calibration artefact to the base.

10. A calibration device according to any preceding claim, wherein the base is provided with a magnetic mounting device for releasably securing the base to a metal surface of the coordinate positioning machine.

11. A calibration device according to any preceding claim, configured for mounting on the coordinate positioning machine comprising a machine tool.

12. A machine tool comprising a calibration device according to any one of claims 1 to 11.

13. 1. A method of calibrating a coordinate positioning machine, comprising the steps of: (i) mounting a calibrated tool and a calibration artefact on said coordinate positioning machine; (ii) moving the calibrated tool into engagement with a calibration artefact using the coordinate positioning machine such that the calibration artefact moves from an initial position to a deflected position; (iii) locking a calibration artifact in the deflected position after disengaging from the calibrated tool such that the calibration artifact is maintained in the deflected position.

14. 14. The method of claim 13, comprising: (iv) using the coordinate positioning machine to move a measurement probe to measure the positions of a plurality of points on a surface of the calibration artefact.

15. 15. A method according to claim 13 or 14, wherein the calibration artefact comprises a sphere, the position of the centre of the sphere being used to define a reference position in a local coordinate system of the coordinate positioning machine.

16. A method according to any of claims 13 to 15, wherein a calibration device according to any of claims 1 to 11 provides the calibration artefact, and step (iii) comprises actuating a releasable lock of the calibration device.

17. 1. A calibration device for a coordinate positioning machine, comprising: With the base, Calibration artifacts, and a lockable mechanism for attaching the calibration artifact to the base; A calibration device, wherein the lockable mechanism can adopt an unlocked state in which the calibration artifact is movable relative to the base upon application of an external force, and a locked state in which the position of the calibration artifact is locked relative to the base.

18. The calibration device of claim 17 , wherein the calibration artifact is biased towards a rest position when in the unlocked state.

19. The calibration device of claim 17 , wherein the calibration artifact is a calibration sphere having a known radius.

20. The calibration device of claim 17 , wherein the lockable mechanism comprises a manually actuated member for changing between the locked and unlocked states.

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