Calibration apparatus and methods

Abstract

A calibration apparatus for coordinate positioning machines, such as machine tools, is described. The apparatus (200) includes a base (30; 130; 202), a calibration article (34; 134), and a deflection mechanism (32) that attaches the calibration article (34; 134) to the base (30; 130; 202) and allows the calibration article (34; 134) to be moved relative to the base (30; 130; 202) by applying an external force. The apparatus further includes a releasable lock that, when locked, fixes the calibration article (34; 134) relative to the base (30; 130; 202).

Description

[0001] This invention relates to a calibration apparatus for use with coordinate positioning devices such as machine tools. In particular, this invention relates to a calibration apparatus with reduced complexity and an association method using such an apparatus.

[0002] Various coordinate positioning devices are known. For example, CNC machine tools are widely used in manufacturing to cut parts, such as metal parts for vehicles, aircraft, etc. To cut features with high accuracy (e.g., within a few micrometers), it is necessary to calibrate the measuring probes on such machine tools. This is especially true when using machine tools with one or more rotary axes (e.g., mill-turning centers or five-axis machine tools), in which the orientation of the part relative to the cutting tool is changed during the cutting process.

[0003] Especially for machine tools with at least one axis of rotation, a key part of a typical calibration procedure is establishing the position of the center of the ring gauge, calibration ball, etc., relative to the reference position of the machine tool. Once established, this center can then serve as the machine reference point in the working coordinate system, and all subsequent measurement and calibration procedures are based on this machine reference point.

[0004] It is known to establish the center position of a sphere in a plane parallel to the machine tool's bed (typically referred to as the XY plane) using a dial indicator or touch probe mounted on the machine tool's spindle. Measuring the center position of the sphere along an axis perpendicular to the machine tool's bed (typically referred to as the Z-axis) is more complex and has so far typically been performed using various manual procedures. For example, it is known to manually move a reference tool of known length along the Z-axis toward the calibration sphere. The reference tool is advanced toward the sphere until a gauge block of known thickness is precisely "clamped" between the reference tool and the sphere. Such manual procedures can be inaccurate, and calibration results have been found to vary depending on the operator.

[0005] International Patent Publication WO 2017 / 121990 describes a calibration device that allows the center of a calibration sphere to be established relative to a reference position on a machine tool. The device includes a calibration article biased relative to its base to a known and repeatable resting position, but the calibration article can be removed from this resting position when a sufficiently large force is applied to it (e.g., by a calibrated length bar). A sensor is provided within the device to measure (e.g., along the z-axis of the machine) the magnitude of deflection from the resting position, thereby allowing the use of extrapolation techniques to determine the position of the calibration device (e.g., on the z-axis) with high accuracy. While the device of WO 2017 / 121990 enables highly accurate and fully automated measurement of the sphere's position, the inventors have found that the manufacture and use of the calibration device are relatively expensive.

[0006] According to a first aspect of the present invention, a calibration apparatus for a coordinate positioning machine is provided, the calibration apparatus comprising:

[0007] Base;

[0008] Calibration products, and

[0009] A deflection mechanism that attaches the calibration article to the base and allows the calibration article to be moved relative to the base by applying an external force.

[0010] The device is characterized in that it further includes a releasable lock that, when locked, fixes the calibration article relative to the base.

[0011] Therefore, a calibration device is provided suitable for calibrating coordinate positioning machines, such as machine tools, coordinate measuring machines (CMMs), robots, etc. The calibration device includes a base, which is preferably releasably attached to a portion of the coordinate positioning machine. For example, the base may be magnetically attached to the bed of a machine tool. The calibration device also includes calibration articles, such as a calibration sphere (e.g., a sphere having a radius accurately measured according to traceable measurement standards), a partial sphere, a domed surface, a ring gauge, or a platform attached to the base via a deflection mechanism.

[0012] The deflection mechanism allows the calibration article to move relative to the base, but the device is characterized by also including a lock that allows and prevents such relative movement as needed. In other words, the deflection mechanism can be "unlocked" to allow the calibration article to move relative to the base, or "locked" to prevent the calibration article from moving relative to the base. This arrangement allows the calibration article to be moved relative to the base to a deflected position by applying an external force when unlocked. For example, a known-length bar or reference tool carried in the machine's spindle can engage the calibration article and move it relative to the base. Once moved to such a deflected position, the lock can be engaged (i.e., locked) so that the calibration article remains in that deflected position even when the bar or reference tool is disengaged. Therefore, if the deflection position is set using a reference tool (e.g., a bar, a tool of known length, etc.) with a known position in the coordinate system of the coordinate positioning machine, the calibration article becomes fixed relative to the base, and the position of the calibration article is known. The calibration article can then remain locked (fixed) in place and can be used for further calibration tasks.

[0013] Therefore, the calibration device of the present invention has the following advantages: it can provide a calibration article with a known position (e.g., in one, two, or three dimensions) in the machine coordinate system, but this can be done without sensing when the calibration article comes into contact and without measuring the amount of deflection of the calibration article. Instead, the inventors have recognized that the calibration article can be moved to any arbitrary position, as long as that position can be determined in the machine coordinate system. Therefore, the calibration device of the present invention has lower complexity and thus lower manufacturing cost than prior art devices (such as those described in prior art patent publication WO 2017 / 121990), while also eliminating the subjective factor of clamping a gauge block of known thickness between the reference tool and the calibration sphere. Although a step of locking the calibration device is required, even if done manually, it can be done only after the machine has stopped moving; that is, when the machine is moving, it is not necessary to touch the calibration device, for example, through an over-control safety interlock.

[0014] The deflection mechanism may, when unlocked, restrict the movement of the calibration article to some extent (e.g., 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 article when the releasable lock is unlocked. In other words, the deflection mechanism may include a guide that guides the calibration article to move back and forth linearly (i.e., along a linear axis) when the releasable lock is unlocked. The deflection mechanism can prevent rotation of the calibration article. In one example, linear movement of the calibration article is allowed along an axis parallel to the movable axis of the deflection mechanism. This linear axis of motion of the calibration article can be approximately aligned in use to coincide with the longitudinal axis of a reference tool. Such a reference tool can then be moved to engage the calibration article along its longitudinal axis, causing the calibration article to deflect along its linear axis of motion. By definition, the length of the reference tool is known, and therefore the position of the calibration article along this axis is known.

[0015] If the deflection mechanism restricts the movement of the calibration article to a single axis (e.g., a linear translation axis), the relevant reference point defined by the article can be arranged on or near that axis. In other words, the calibration article can conveniently be positioned on a linear axis. For example, the center of the calibration sphere can be arranged substantially on the linear translation axis and thus translate back and forth along it. This prevents or reduces any external forces applied to the deflection mechanism (i.e., when the calibration article is moved) that could cause off-axis movement or deflection (deformation) of the device. This, in turn, helps ensure that once the releasable lock is engaged, the calibration article does not significantly change position when the deflection force is removed (e.g., when the reference tool disengages).

[0016] Advantageously, the deflection mechanism includes a biasing element. The biasing element can bias the calibration article toward a stopping position. The biasing element can be provided by one or more springs (e.g., coil springs). The force applied by the biasing element is preferably insufficient to overcome the locking force applied by the lock. Therefore, the biasing element causes movement of the calibration article only when the lock is unlocked. In other words, when the lock is locked, the biasing element is insufficient to overcome the locking force, and thus the calibration article remains stationary relative to the base. The stopping position to which the calibration article is biased does not need to be repeatedly defined (i.e., the stopping position adopted by the calibration article has no effect on the calibration accuracy).

[0017] Releasable locks can be implemented in various ways. A releasable lock can be a remotely actuated lock. For example, a releasable lock can 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 an electrical or pneumatic line). Preferably, a releasable lock is manually actuated (i.e., it is manually activated by the operator / user). Therefore, a releasable lock conveniently includes a manually actuated locking member. The locking member is preferably a locking lever. Alternatively, the locking member can be a torsion lock member.

[0018] The calibration apparatus can include any suitable calibration article. Therefore, a calibration article can be any item with one or more known dimensions (e.g., an article of known size). A calibration article can include a flat surface. A calibration article can include a domed surface (e.g., a surface with the curvature of a large-diameter sphere). One or more dimensions of the article can be known through prior measurements to a calibrated (traceable) standard. For example, one or more dimensions of the article can have been previously measured on a coordinate measuring machine (CMM) calibrated according to an applicable (national or international) calibration standard. A calibration article can include a ring gauge, a cube, a disk (e.g., with a known radius), etc. Conveniently, the calibration article includes a sphere. The sphere can have a known radius. For simplicity and accuracy, a sphere with a known radius is preferred because the location of the center of the sphere (e.g., x, y, z coordinates) can be determined by measuring the positions of multiple points on the surface of the sphere. The calibration sphere can be a complete sphere or a partial sphere (e.g., it can consist only of a section of the sphere or include a flat area or recess for attachment to a rod).

[0019] Calibration artifacts can provide reference points in the working coordinate system in one, two, or all three dimensions. For example, a calibration sphere can provide a three-dimensional reference point (e.g., the center of the sphere). A ring gauge can provide a two-dimensional reference point (e.g., the center point of the ring in a plane). A flat surface can provide a one-dimensional data point (e.g., the location of the plane containing the surface). Calibration devices can include (movable / lockable) calibration artifacts and additional calibration artifacts. For example, the device can include a (movable / lockable) flat or dome-shaped surface and non-movable additional artifacts (e.g., a ring gauge or sphere non-movably attached to the base of the device). In this way, the characteristics of both the (movable / lockable) calibration artifact and the additional calibration artifacts can be used to define reference points in the working coordinate system in three dimensions.

[0020] The calibration device may include a housing attached to or formed as part of a base. A deflection mechanism may be partially or completely housed within the housing. The deflection mechanism may include an elongated rod for attaching a calibration article to the base. The calibration article may be directly attached to the rod, or other components (e.g., angled wedges, extensions, etc.) may be used to attach the calibration article to the rod. Therefore, the orientation of the calibration article and the rod to which it is attached can be suitably selected (e.g., to allow for vertical and / or horizontal mounting of the calibration device).

[0021] The base can be mounted to the machine in a variety of ways. For example, the base can be bolted to the machine bed. Advantageously, the base can include a magnetic attachment device for releasably securing the base to a metal surface of a coordinate positioning machine (e.g., to the bed of a machine tool). In other words, a magnetic base that allows the device to be releasably attached to a metal surface can be used. This allows the device to be securely held but easily removed.

[0022] The calibration device can be configured for attachment to any coordinate positioning machine. Advantageously, the calibration device is configured for attachment to a coordinate positioning machine including 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 rotary axis. The machine tool may have at least two rotary axes. The machine tool may be a turning machine, a lathe, or a mill-turn machine. The calibration device may be attachable to a bed of a machine tool capable of moving the spindle relative to the tool holder. The invention also extends to coordinate positioning machines including a calibration device. For example, a machine tool including a calibration device may be provided.

[0023] According to a second aspect of the invention, a method for calibrating a coordinate positioning machine (e.g., a machine tool) is also provided. The method includes the steps of: (i) mounting a calibrated tool (e.g., a length bar or a tool of known length) and a calibration article (e.g., a calibration sphere) on the coordinate positioning machine; (ii) using the coordinate positioning machine to move the calibrated tool into engagement with the calibration article, such that the calibration article moves from an initial position to a deflected position; and (iii) locking the calibration article in the deflected position, such that the calibration article remains in the deflected position after disengagement from the calibrated tool.

[0024] Further step (iv) can then be performed: using a coordinate positioning machine to move a measuring probe to measure the position of multiple points on the surface of the calibration article. The measuring probe can be a tactile measuring probe with styluses for contact with the article; for example, a contact-triggered measuring probe or an analog (scanning) probe. Alternatively, the measuring probe can be a non-contact measuring probe. The measuring probe can be carried in the machine's spindle. Therefore, the measuring probe can be used to measure the position of the calibration article. For example, the measuring probe can be used to find the xy coordinates of the calibration sphere and combined with the z coordinate of the sphere's center, known through the use of a calibration tool, to place the calibration article in a stationary position.

[0025] Advantageously, the calibration article includes a sphere. The center position of the sphere can then be used to define a reference position in the local coordinate system of the coordinate positioning machine. This center (reference) position of the sphere can be determined in the machine coordinate system and used for further calibration procedures.

[0026] In a preferred embodiment, the 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 can then provide a calibration article, and step (iii) of the method may include locking the device with a releasable lock.

[0027] Another aspect of the invention provides a calibration apparatus for a coordinate positioning machine, the apparatus comprising a base, a calibration article, and a lockable mechanism for attaching the calibration article to the base, wherein the lockable mechanism can be in the following states: an unlocked state in which the calibration article is movable relative to the base by applying an external force; and a locked state in which the position of the calibration article is locked relative to the base. When in the unlocked state, the calibration article can be biased toward a rest position. The calibration article may include a calibration sphere having a known radius. The lockable mechanism may include a manually actuating member for changing between the locked and unlocked states. The base may include a magnet to allow attachment to the bed of a machine tool.

[0028] This document also describes a calibration device. The calibration device can be used in a coordinate positioning machine. The calibration device may include a base. The calibration device may include a calibration article. The calibration device may include a deflection mechanism. The deflection mechanism can attach the calibration article to the base. The deflection mechanism can allow the calibration article to be moved relative to the base by applying an external force. The device may further include a (releasable) lock. When locked, the lock can fix the calibration article 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.

[0029] The invention will now be described by way of example only, with reference to the accompanying drawings, in which;

[0030] Figure 1 A multi-axis machine tool is shown.

[0031] Figure 2 This illustrates a calibration technique using existing technology that employs a calibration sphere.

[0032] Figure 3a and Figure 3b The calibration device of the present invention is shown, which includes a locking lever in an unlocked state and a locked state, respectively.

[0033] Figure 4 Showing more details Figure 3a and Figure 3b The various internal parts of the device,

[0034] Figure 5 Another calibration device of the present invention is shown, which includes a torsion lock mechanism.

[0035] Figure 6 Showing more details Figure 5 The various internal parts of the device,

[0036] Figures 7a to 7c It demonstrates how calibration devices can be used to help calibrate machine tools, and

[0037] Figure 8 An alternative embodiment of the calibration device is shown.

[0038] Figure 1A multi-axis machine tool is demonstrated. The machine tool includes a spindle 2 that can rotate at high speed about an axis typically referred to as the S-axis. The spindle 2 includes a wedge-shaped mount 6 for receiving a wedge-shaped shank for cutting tools 4 or other accessories (such as measuring probes); this allows tools and accessories to be loaded into the spindle 2 as needed. The spindle 2 can move in space along three linear axes of the machine tool; these machine tool axes are typically referred to as the X-axis, Y-axis, and Z-axis. A worktable 10 on which a workpiece 12 is mounted is provided. The worktable 10 can tilt about the A-axis and also rotate about the B-axis.

[0039] During cutting, the cutting tool 4 rotates at high speed around the S-axis, and the machine tool controller moves the tool 4 relative to the workpiece 12 along the desired cutting path according to a list of instructions defined in the cutting program. The cutting path may include translational movements of the spindle along the X, Y, and Z axes, as well as rotational movements around the A and B axes. To remove the desired material from the workpiece, even during rotational movements around the A and B axes, precise knowledge of the tool tip's position relative to the workpiece is required. Therefore, various techniques and devices have been developed over the years to perform such calibrations.

[0040] As is well known to those skilled in the art, many automated machine tool calibration techniques involve using spindle-mounted measuring probes to perform various measurements on a calibration sphere (i.e., a sphere of known radius) mounted to the machine tool. These measurements typically rely on a highly accurate understanding of the position of the center of the calibration sphere relative to a reference position on the machine tool (e.g., the origin of the x, y, z machine coordinate system). Therefore, a wide variety of techniques have been developed over the years to accurately measure the center position of the sphere. Once measured, subsequent machine-on-the-machine measurements (e.g., for calibration, workpiece measurement, or tooling purposes) can be correlated with that sphere center position.

[0041] Reference Figure 2 This section describes prior art techniques that allow for determining the position of the center of a calibration sphere 20, which is fixedly (immovably) mounted to a machine tool. The sphere can be fixed to a base or another structure within the machine tool where a workpiece is typically placed. Thus, the sphere can be mounted to a working surface of the machine tool, which may include one or more axes of rotation of the machine tool.

[0042] First, it should be noted that several techniques are known that allow for the accurate establishment of the XY position of a calibration sphere. For example, a dial test indicator (DTI) can be mounted in the spindle of a machine tool and used to "record" the diameter of the calibration sphere 20 near its mid-latitude. This is typically accomplished by gently pushing (i.e., moving under manual control) the X and Y positions until the dial indicator no longer deflects as the spindle rotates. When this is achieved, the XY position of the spindle is the center position of the sphere.

[0043] It is also known to measure the XY ball center position using a contact-triggered probe mounted in the spindle of a machine tool. The spindle rotation position (i.e., the rotation angle around the S-axis) is oriented to zero degrees, and the ball center is measured and its XY position is recorded. The spindle is then rotated 180°, and the ball center XY position is measured again. The average of these two measured XY ball center positions is used as the center position; that is, any error caused by the eccentric mounting of the contact probe's spherical stylus relative to the spindle's rotation center is eliminated.

[0044] While the techniques described above allow for finding the center position of an XY sphere, they are typically not accurate enough to establish the Z-position of a sphere for machine tools with one or more axes of rotation. Therefore, several separate techniques for determining the Z-position of a sphere are known.

[0045] The most commonly used technology is Figure 2 The diagram illustrates and relates to the use of a so-called feeler gauge block 24; this block can be a material block or sheet of known (e.g., calibrated) thickness. A reference tool 22 of known, calibrated length is loaded into the machine tool spindle. The spindle is moved such that the reference tool 22 is positioned above the top dead center of the sphere 20. The feeler gauge or gauge block 24 is positioned between one end of the reference tool 22 and the sphere 20. Using the machine tool's manual "push" function, the tool is manually moved downwards until the gauge block 24 is precisely "clamped" between the reference tool 22 and the sphere 20. This manual process requires the engineer to "feel" when the gauge moves freely but without any play or clearance. Once this is achieved, the center Z position of the sphere is calculated using the current machine position, tool length, and calibrated sphere radius.

[0046] It is also known to use a custom-designed length setting device to set the Z-position. The device, called the Base-Master, is manufactured by Big Daishowa Corporation of Japan and supplied by Metrology Software Products Limited of Alnico, UK. This device includes a repeatable unidirectional indicator. This indicator is mounted in the machine tool spindle and positioned above the XY center of the fixed sphere. The spindle is moved downwards using a push control of the machine tool until the Base-Master just contacts the top of the sphere; this contact is indicated by an LED mounted on the Base-Master device. The center of the sphere on the Z-axis is then calculated using the (known) length of the Base-Master, the current Z-position, and the sphere radius.

[0047] As described above, WO 2017 / 121990 describes a calibration apparatus that allows for the automated establishment of the center of a calibration sphere relative to a reference position on a machine tool. The apparatus includes a calibration article (e.g., a calibration sphere) that is biased relative to its base to a known and repeatable stopping position by a relatively large spring force. The calibration sphere is moved away from this stopping position when a calibrated length bar (or other reference tool) carried in the machine tool spindle applies a sufficiently large force to it. An analog sensor is provided within the calibration apparatus to measure the deflection from the stopping position. The measurement of the spindle position (i.e., the position of the length bar it holds) is combined with the measurement of the calibration sphere acquired by the analog sensor as the length bar is moved to deflect the calibration sphere. Extrapolation techniques are then used to accurately determine the position of the calibration sphere when it is in its stopping position. This process is described in more detail in WO 2017 / 121990.

[0048] While the apparatus described in WO 2017 / 121990 provides highly accurate positional information in a fully automated manner, the apparatus itself is complex and can be costly to manufacture. For example, the stopping position of the calibration article must be highly repeatable, and the analog measurement sensors must provide accurate position measurements. A separate computer is also required to combine data from the machine tool and the calibration apparatus to determine the stopping position of the calibration article. This makes the equipment relatively expensive and complex to set up and operate.

[0049] This invention stems from the inventors' recognition that it is unnecessary to measure the position of a calibration article (e.g., a calibration sphere) only after it has been fixed in a predetermined position within a machine tool. Instead, the calibration sphere can be moved to any (but known) position; for example, by means of a reference tool (e.g., a calibrated length bar) along the z-axis. As long as the calibration sphere is locked in place (e.g., fixed relative to the bed of the machine tool on which the calibration sphere is mounted) before disengaging from the calibrated length bar (i.e., when 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 the contact between the length bar and the calibration sphere or measure the amount of deflection of the calibration sphere. Thus, a less costly and less complex device is provided. The device of the present invention and how it can be used for calibration purposes will now be described.

[0050] Figure 3a and Figure 3b A first embodiment of the calibration device of the present invention is shown, which includes a magnetic base 30, a deflection mechanism 32 and a calibration sphere 34.

[0051] The magnetic base 30 is a commercially available magnetic base manufactured by the Misumi Group of Japan. This base allows attachment to the bed of a metal machine tool with a holding force of approximately 800 N. By rotating the knob 36 from the "off" position to the "on" position, the magnetic base 30 can be securely but releasably fixed to the machine tool bed. As described below, other designs of the magnetic base or different types of attachment mechanisms (e.g., bolts, screws, etc.) can be used to securely fix the device to the machine tool.

[0052] The deflection mechanism 32 includes: a housing 38 fixed to the top of the magnetic base 30; and a movable shaft 40 to which the calibration ball 34 is indirectly attached. The calibration ball 34 is attached to another shaft 42, which is in turn attached to the movable shaft 40 via a 45-degree wedge element 44. This arrangement allows the calibration ball to be measured from different directions, thus allowing the calibration device to be mounted on horizontal, vertical, or inclined surfaces. The calibration ball 34 can be attached to the movable shaft 40 in various different ways. In this example, the movable shaft 40 can be translated back and forth in the direction indicated by arrow 41 (i.e., parallel to the longitudinal axis of the movable shaft 40).

[0053] The calibration device further includes a lock housed within the housing 38; reference will be made below. Figure 4 To describe the structure and operation of this lock in more detail. The operator can manually activate the lock using lever 46, which locks the position of the movable shaft 40 relative to the housing 38. Therefore, when locked, the lock fixes the movable shaft 40 relative to the housing 38. The locked state is... Figure 3b As shown in the diagram. However, when the lock is in the unlocked state, the movable axis 40 can still translate back and forth (i.e., along the indicated direction 41). The unlocked state is... Figure 3a As shown in the diagram. As 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., when the device is in the unlocked state). Figure 3a In the orientation shown, the shaft is biased upward by the spring.

[0054] Therefore, the calibration device can be configured in two states: a locked state (i.e., a locked configuration), in which the calibration ball has a fixed position relative to the magnetic base 30; and an unlocked state (i.e., an unlocked configuration), in which the calibration ball cannot move relative to the housing (i.e., it is stationary). It is important to note that the lock can be engaged (i.e., locked) when the movable axis has been 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 ball to be moved to a certain position and then locked in that position for subsequent measurements. When the device is locked, movement of the calibration ball relative to the base is prevented.

[0055] Figure 4 It shows Figure 3a and Figure 3b Further details of the internal deflection mechanism and lock of the calibration device shown. Therefore, the same features are assigned the same reference numerals.

[0056] As explained above, the calibration device includes a movable shaft 40; this movable shaft is... Figure 4 The movable shaft 40 is shown as being integrated into the housing 32 and also as a separate item. The movable shaft 40 is generally cylindrical but includes a V-shaped recess 60 extending along its central portion. A first end 62 of the movable shaft 40 includes a threaded hole to which a wedge-shaped element 44 holding the calibration ball 34 is attached. A second end 64 of the movable shaft 40 includes a widened portion having an aperture for receiving a coil spring 66. When assembled within the device, the second end 64 of the movable shaft 40 engages the coil spring 66, thereby moving upwards (i.e., when in...). Figure 4 The orientation shown is upward) biased movable axis 40.

[0057] The lock is provided by a locking mechanism including a ball 70, which engages with a first end of a slidable insert 72 fixed in a groove within the housing. Specifically, the ball 70 is pressed by a plurality of disc springs 74 fixed within the insert 72 into contact with a V-shaped recess 60 of a movable shaft 40. The second end of the slidable insert 72 engages with a camshaft 76. The camshaft 76 is typically an elongated cylinder but has a flat region 78 (i.e., a reduced diameter) for engaging the second end of the slidable insert 72. A threaded hole 80 is also provided in the camshaft 76 to allow a radially protruding locking lever 46 to be attached thereto. Figure 4 The camshaft 76 is shown as a separate component installed within the device.

[0058] Lever 46 (for example, in the above) Figure 3a and Figure 3bThe movement of the lever 46 (between the positions shown) rotates the camshaft 76, thereby causing the slidable insert to move radially relative to the movable shaft 40. Therefore, the movement of the lever 46 rotates the camshaft 76, thereby increasing or decreasing the holding force applied to the movable shaft 40 via the ball 70. This arrangement is configured such that the lever 46 can move between a locked and unlocked state. In the unlocked state, the ball 70 is pushed into the movable shaft 40 with only a small force, allowing the movable shaft 40 to translate back and forth within the channel of the housing. In the unlocked state and without the application of external force, the movable shaft 40 is pushed (upward) by the coil spring 66, thus fully extending it from the housing 32. It should be noted that in the unlocked state, the ball 70 remains within the V-shaped recess 60 of the movable shaft 40, thereby limiting the axial range of motion 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 with, for example, a reference tool (e.g., a calibrated length bar) held in the machine tool spindle. In the locked state, sufficient force is applied to force ball 70 to engage with movable shaft 40, thereby fixing shaft 40 relative to housing 32. Thus, moving from the unlocked state to the locked state locks the calibration ball (fixes it in place) in its current position (e.g., a position that the calibration ball may have been deflected to by the application of an external force).

[0059] refer to Figure 5 and Figure 6 The following will describe variations of the above-mentioned calibration device, including different locking mechanisms.

[0060] Figure 5 A calibration device, similar to the one described above, is shown, including a magnetic base 130 to which a housing 132 is attached. A calibration ball 134 is also attached to a movable shaft 140. A lock, including a locking ring 146, is also provided. When the locking ring 146 is rotated to the unlocked state, the movable shaft 140 can move axially relative to the housing 132. However, when the locking ring 146 is rotated to the locked configuration, the movable shaft 140 (and therefore the calibration ball 134) is fixed relative to the magnetic base 134. Markings on the housing 132 and the locking ring 146 indicate whether the lock is locked or unlocked. Therefore, Figure 5 The device provides the above reference Figure 3a , Figure 3b and Figure 4 The described devices have the same function, but include different mechanisms for implementing the lock.

[0061] refer to Figure 6 References are shown Figure 5 Further details regarding the locking mechanism of the described calibration device. Figure 3a , Figure 3b and Figure 4The arrangement is identical, with the movable shaft 140 including a recess into which the ball 170 is pressed by the insert 172. However, the insert 172 is pressed against the movable shaft 140 by a rotatable disk 180, which rolls into engagement with an inner surface recess 182 formed in a locking ring 146. Variations in the radius of the recess 182 in the locking ring 146 cause rotation of the ring to move the insert toward or away from the movable shaft 140, thereby applying varying amounts of force via the ball 170. Specifically, 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 force the distal end of the movable shaft 140 away from the housing in the unlocked state.

[0062] Next reference Figures 7a to 7c The following will describe examples of how the calibration apparatus of the present invention can be used for machine tool calibration.

[0063] Figure 7a A calibration device 200 with a base 202 is schematically shown, which is securely fixed to a machine tool bed 204 (e.g., by a magnet or bolts). The calibration device 200 includes a calibration ball 206 attached to the base 202 via an elongated shaft 208. The device includes a locking mechanism that can be placed in: an unlocked state, in which the shaft is allowed to move relative to the base 202 in the z-direction; and a locked state, in which the calibration ball 206 is fixed relative to the base 202. In the unlocked state, the elongated shaft 208 is biased in the positive z-direction to the shown stop position. A manual actuator 210 allows the user to switch the device between the locked (L) and unlocked (U) states as needed.

[0064] exist Figure 7a The diagram also shows a calibrated length bar 212 held in the spindle (not shown) of the machine tool as it is moved toward the calibration ball 206 in the negative z direction. 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 of the spindle).

[0065] Figure 7b The same arrangement is shown after the calibrated length bar 212 has made contact with the calibration ball 206. The contact is made manually by controlling a machine tool (e.g., using a starter wheel) to engage the end of the length bar 212 with the calibration ball 206. After engagement between the length bar 212 and the calibration ball 206, a short relative movement continues, causing the calibration ball 206 to deflect (move) to... Figure 7bThe deflection position is shown in the diagram. After the machine stops moving, the user can open the machine cover and manually switch the calibration device to the locked state using actuator 210. The position of the center of the calibration sphere (which has a known radius) on the z-axis is thus known in the machine coordinate system (i.e., because the corresponding position of the end of the calibrated length bar is known).

[0066] like Figure 7c As shown, the calibrated length bar 212 can be withdrawn from the calibration sphere 206, but the position of the 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 into the spindle of the machine tool probe and used in a known manner to accurately establish the position of the sphere's center in the xy plane. The calibration device then provides a calibration sphere with a known high-accuracy position within the machine tool's (x, y, z) coordinate system. The center of the sphere can then serve as a machine reference point, upon which all subsequent probe measurements (e.g., for calibration procedures, etc.) are based.

[0067] Reference Figure 8 An alternative calibration apparatus 300 of the present invention is illustrated. This apparatus includes a base 302 for attaching to a machine tool. A calibration platform 306 (i.e., a calibration article having a flat surface) is attached to the base 302 via a lockable deflection mechanism. According to the example above, a calibrated length bar 312 can deflect the calibration platform 306, which can then be locked in place using a manual actuator 310. Therefore, the position of the calibration platform 306 is known on the z-axis (i.e., it defines a z-axis reference position). An additional calibration sphere 320, immovably attached to the base (note that this can alternatively be fixed to other locations on the machine tool), can be used to provide reference positions on the x and y axes. In this way, reference positions can be defined in three dimensions of the coordinate system.

[0068] Figure 8 The calibration platform 306 shown can be replaced by a calibration article or disk with a domed rather than flat contact surface. For example, the calibration disk could have an uppermost surface with a radius of curvature located on a large (virtual) sphere. This reduces length errors should any angular misalignment exist between 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, such as...). Figure 8 As shown, length errors may occur if the flat-ended, calibrated length bar 312 contacts the highest part of the dome surface, even with slight angular misalignment. This arrangement is particularly suitable for measuring tools with flat ends.

[0069] The above embodiments are merely examples of the present invention. Alternative locking mechanisms, calibration articles, housings, bases, etc., can also be used to implement the present invention. The multi-axis machine tool described above is a multi-axis milling machine, but this technology can be used with any type of machine tool (e.g., lathes, mill-turn machines, etc.). Similarly, the device can be used with 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. A method of calibrating a coordinate positioning machine using a calibration device, the calibration device comprising: A base, a calibration artefact comprising a sphere, a partial sphere or a domed surface, a deflection mechanism attaching the calibration artefact to the base and allowing the calibration artefact to be moved relative to the base by the application of an external force, and a releasable lock which when locked immobilises the calibration artefact relative to the base, the method comprising the steps of: (i) installing a calibrated tool and the calibration artefact on the coordinate positioning machine, (ii) using the coordinate positioning machine to move the calibrated tool into engagement with the calibration artefact so that the calibration artefact is moved from an initial position to a deflected position, (iii) locking the calibration artefact in the deflected position so that it remains in the deflected position after disengagement from the calibrated tool, and (iv) using the coordinate positioning machine to move a measurement probe to measure the positions of a plurality of points on the surface of the calibration artefact.

2. The method of claim 1, wherein, The calibration artefact comprises a sphere and the centre position of the sphere is used to define a reference position in the local coordinate system of the coordinate positioning machine.

3. The method of claim 1, wherein, The deflection mechanism comprises a guide which guides the calibration artefact to move back and forth linearly along a linear axis when the releasable lock is unlocked.

4. The method of claim 3, wherein, The calibration artefact is located on the linear axis.

5. The method of claim 1, wherein, The deflection mechanism comprises a biasing member for biasing the calibration artefact towards a rest position.

6. The method of claim 1, wherein, The releasable lock comprises a manually activated locking member and step (iii) comprises manually actuating the releasable lock.

7. The method of claim 6, wherein, The manually activated locking member comprises a locking lever or a twist lock member.

8. The method of claim 1, wherein, The releasable lock comprises a remotely activated lock.

9. The method of claim 1, wherein, The calibration artefact comprises a calibration sphere.

10. The method of claim 1, wherein, The calibration artefact comprises a partial sphere or a domed surface.

11. The method of claim 1, wherein, The deflection mechanism comprises an elongate rod attaching the calibration artefact to the base.

12. The method of claim 1, wherein, The base comprises magnetic attachment means for releasably securing the base to a metal surface of a coordinate positioning machine.

13. The method of claim 1, wherein, The coordinate positioning machine is a machine tool.

14. A calibration device for a coordinate positioning machine, the calibration device comprising: a base, a calibration artefact comprising a sphere, a partial sphere or a domed surface, and a deflection mechanism attaching the calibration artefact to the base and allowing the calibration artefact to be moved relative to the base by the application of an external force, characterised in that the device further comprises a releasable lock which when locked immobilises the calibration artefact relative to the base; and the calibration device is adapted to be calibrated using a measurement probe for the calibration coordinate positioning machine using the method of claim 1.

15. The calibration device of claim 14, wherein, The deflection mechanism comprises a guide which guides the calibration artefact to move back and forth linearly along a linear axis when the releasable lock is unlocked.

16. The calibration device of claim 15, wherein, The calibration artefact is located on the linear axis.

17. The calibration device of claim 14, wherein, The deflection mechanism comprises a biasing member for biasing the calibration artefact towards a rest position.

18. The calibration device of claim 14, wherein, The releasable lock comprises a manually activated locking member.

19. The calibration device of claim 18, wherein, The manually activated locking member comprises a locking lever or twist lock member.

20. The calibration device of claim 14, wherein, The releasable lock comprises a remotely activated lock.

21. The calibration device of claim 14, wherein, The deflection mechanism comprises an elongate rod attaching the calibration artefact to the base.

22. The calibration device of claim 14, wherein, The base comprises magnetic attachment means for releasably securing the base to a metallic surface of a coordinate positioning machine.

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