Calibration device for machine

0001The invention relates to the calibration of coordinate positioning machines, such as machine tools or coordinate measuring machines.

0002Such machines typically comprise an arm (e.g. a toolholder) movable in three dimensions relative to a table or base on which, for example, a workpiece is supported. It is frequently desired to calibrate such machines to determine the accuracy to which for example a toolholder on a machine tool may describe a circular trajectory with respect to a fixed centre on the workholder of the machine.

0003A known device for performing such a calibration operation is described in US patent 4,435,905. The device comprises an elongate telescopic bar provided with a ball at each end. In use, each of the balls is retained in a socket provided on the toolholder and the workholder respectively, and the toolholder is then driven in a circular path about the centre of the ball retained in the socket on the workholder. A single axis transducer provided on the bar measures any variation in the centre-to-centre spacing of the balls, and thus determines the extent to which the tool holder path varies from a circular path. It is also known from GB 2,210,978 to provide a ball on each of the tool holder and workholder, and an interconnecting transducer between the two balls. The interconnector comprises a bar having a socket at one end for universally pivotally engaging one of the balls, and a plurality of forks at the other end of the bar for engaging the other of the balls to enable both universal pivotal motion, and linear motion of the other of the balls relative to the bar. A pair of single axis transducers are provided on the bar to determine the variation in spacing between the two balls as the tool holder is driven in a circular path.

0004In each of the above mentioned calibration devices, it is a requirement in order to accurately calibrate the machine to determine the centre of motion about which, for example, the tool holder is to be driven. In these known devices, this centre is defined by the centre of the ball which is secured to the workholder of the machine. Existing methods for determining the position of the centre of the ball retained on the workholder are based on trial and error. For example, with the toolholder and workholder engaged with the bar, the toolholder is moved to diametrically opposing points about a position which is assumed to be the centre of the ball retained on the workholder. The difference in the outputs between the single axis transducer readings at each of the two locations of the toolholder is used to determine a distance by which the centre of the ball retained on the workholder is to be moved in order to position it coincidentally with the position at which its centre is assumed to be. Such methods are time consuming.

0005Accordingly, the present invention provides apparatus for calibrating a coordinate positioning machine, the machine having a table, and an arm movable relative to the table in at least two dimensions, the apparatus comprising: a male and a female support member, each adapted to be retained on one of the movable arm and the table, transducer apparatus comprising an elongate telescopic bar having a male coupling member at one end for forming a universal pivotal connection with the female support member, and a female coupling member at the other end for forming a universal pivotal connection with the male support member, and means for measuring the distance between said two male members, characterised in that retaining means are provided for one of said support members, for permitting a range of 3-dimensional movement relative to the part of the machine on which said one support member is retained and for rigidly and releasably retaining said one support member in a fixed position within said range.

0006The apparatus according to the invention enables easy determination of the location of the point about which (for example) the movable arm of the machine is to be driven in a circular path, simply by engaging the male and female support members provided on the machine with each other, and measuring the position of the movable arm when the two support members are engaged.

0007Preferably, both the male members will be provided by spherical elements, and the female support member is provided by a magnetic socket. In one embodiment the female coupling member is also provided by a magnetic socket. In a further embodiment the female coupling member will preferably enable both universal pivotal motion and linear motion of the male support member relative to the bar.

0008Embodiments of the present invention will now be described, by way of example, and with reference to the accompanying drawings in which; <ul id="ul0001" list-style="none"><li>Fig 1 is an elevation of an apparatus according to a first embodiment of the present invention;</li><li>Figs 2 to 5 show details of the apparatus shown in Fig 1;</li><li>Fig 6 is an elevation of an apparatus according to a second embodiment of the present invention; and</li><li>Figs 7 to 10 show details of the apparatus of Fig 6.</li></ul>

0009Referring now to Fig 1 the movable arm 10 of a machine is movable (such movement being measured relative to a datum by transducers on the machine) relative to a table 12 in three dimensions, and the arm supports a female support member provided by a socket 14. The socket 14 is shown in more detail in Fig 2 and comprises a cylindrical cup 16 supporting three equi-spaced support elements provided by balls 18 in its inner rim. A magnet 20 is supported in the base of the cup 16, and the magnet 20 urges a male coupling member in the form of a spherical element 22 provided at one end of a telescopic bar 24 into engagement with the three balls 18. A foam pad 26 situated adjacent the balls 18 is impregnated with a suitable oil (e.g. a silicone based oil) to enable a continual supply of lubricant between the surfaces of the spherical element 22 and the balls 18.

0010The other end of the bar 24 is engaged with a male support member provided by a spherical element 28 for both universal pivotal motion and linear motion relative to the element 28. The mechanism enabling this movement is shown additionally in Fig 3 and comprises a pair of elongate parallel extending rods 30 and a leaf-spring 32 which urges the spherical element 28 into engagement with the rods 30. A pair of foam pads 34 are provided on the leaf-spring 32 which are impregnated with lubricant to provide continual lubrication between the surface of spherical element 28, the rods 30 and leaf-spring 32. A movable piston 36 is provided at the other end of the bar 24, and the piston 36 abuts the surface of the element 28. The piston 36 is part of a linear displacement transducer 38 provided inside the bar 24 which, when calibrated, determines the distance between the spherical elements 22 and 28. Linear displacement transducers are well known per se and will not be described further. The output of the transducer 38 is transmitted via a cable 60 to a processing device (not shown).

0011The spherical element 28 is supported on the table 12 of the machine by a retaining device 40 which is itself supported in a workholder 42. The retaining device 40 permits movement in 3-dimensions of the spherical element 28 relative to the table 12 and can rigidly clamp the element 28 relative to the table 12. The retaining device 40 is shown in more detail in Figs 4 and 5 and comprises a clamping device having a pair of jaws 44 which enclose a cylindrical bore 46. The jaws 44 may be opened and closed by screw threaded engagement of a clamping bolt 48 with each of the jaws 44. The retaining device also includes a structure for supporting element 28 in clamping device. As is best illustrated in Fig 4, the spherical element 28 is supported on a carrying bar or stem 50, itself supported by a spherical carrying member 52 which lies inside of the bore 46. A spring 54 is provided within the bore 46 to urge the carrying member 52 out of the bore 46.

0012During a calibration operation, it is desired to drive the male coupling member provided by spherical element 22 in a circular path about a point defined by the centre of the male support member, provided by spherical element 28. In order to instruct the machine to drive the arm 10 in such a circular path it is necessary to determine the position of the centre of the spherical element 28. Thus, prior to performing a calibration operation, with the bar 24 disengaged from the socket 14 and element 28, the arm of the machine 10 is moved so that the female support member, provided by socket 14 may engage the spherical element 28. Easy engagement of the socket 14 and element 28 is enabled by slackening the clamping bolt 48 to enable free movement of spherical carrying member 52 in the bore 46, and thus free movement of the element 28; the spring 54 provides a resilient vertical support for the carrying member 52 (and thus element 28) and further aids easy engagement of socket 14 and element 28. Once the socket 14 and the element 28 are engaged, the clamping bolt 48 is re-tightened thus fixing the position of the spherical element 28 relative to the table 12. As mentioned above the position of the arm 10, and thus the socket 14 is known relative to a datum from the transducers provided on the machine, and since the socket 14 is engagement with the element 28, the position of the centre of the spherical element 28 at this time is easily determinable.

0013The socket 14 is then disengaged from the spherical element 28 (the retaining device now preventing any movement of the element 28) and the bar 24 connected between the arm 10 and the table 12 as shown in Fig 1. The arm may thus then be driven in a circular path around the centre of the element 28 since the position of the centre has just been determined.

0014While in the illustrated embodiment a male support member (i.e. the spherical element 28) is provided on the table 12, and a female support member (e.g. socket 14) is provided on the arm 10, entirely equivalent arrangements are envisaged with e.g. socket 14 on the table 12 and element 28 on the arm 10. Additionally the retaining device 40 may be provided on the arm 10, with the male or female support member which is provided on the table 12 being rigidly supported thereto.

0015An alternative ball-bar apparatus according to the present invention will now be described with reference to Fig 6. In this embodiment the movable arm 110 (e.g. the spindle) of a machine tool carries a female coupling member provided by a magnetic socket 114, and is movable in 3-dimensions relative to a table 112. A male coupling member provided by a spherical reference element 128 is supported on the table 112 by a retaining device 140. The retaining device 140 is similar to the retaining device 40 described in the first embodiment, and has a clamping device operable by a clamping bolt 148 to retain a carrying member (not shown). The carrying member is connected to the carrying stem 150, which in turn supports spherical element 128. In this embodiment of the present invention the retaining device 140 has a magnetic base and is thus magnetically retained on the table 112.

0016The male and female support members 128,114 are interconnected by a transducer apparatus provided by a telescopic ball-bar 124. A male coupling member provided by spherical element 122, and situated at one end of the bar 124 is magnetically retained in the socket 114 to enable universal pivotal motion of the bar 124 relative to the arm 110. The other end of the ball-bar 124 has a female coupling member provided by a magnetic socket 130, in which the spherical element 128 is retained. A linear displacement transducer (not shown) provided within the ball-bar 124 determines (when calibrated) the spacing of the spherical elements 122,128. The output of the transducer is sent via a cable 160 to a computer in order to calibrate the machine. The ball-bar of this embodiment is set up and operated to calibrate the machine exactly as described with reference to the first embodiment of the present invention.

0017Figures 7 to 10 illustrate alternative forms of magnetic socket, which are used in conjunction with the ball-bar 124 of Fig 6. A socket 170 has a cup-shaped cylindrical body 172 having an axis A, in which a magnet 174 is retained. The body 172 supports three radially extending, and equi-spaced support elements, provided by ball-ended screws (made of a copper alloy such as phosphor-bronze) 176 in correspondingly screw-threaded bores. In use, the spherical elements 122;128 are urged into the socket 170 by the magnet 174 and rest against the three areas of surface provided by the ball-ends of the screws 176. The position in which the spherical elements 122;128 are retained in the socket 170 may be adjusted by adjusting the radial displacement of the screws 176.

0018Referring now to Fig 9, a socket 180 comprises a cup-shaped cylindrical body 182 having an axis B and a permanent magnet 184 supported in the cup. The body 182 supports three axially extending copper alloy ball-ended screws 186 which are equi-spaced about axis B. In use, the spherical elements 122;128 are urged into the socket 180 by the magnet 184 and are supported by three areas of surface provided by the ball-ends of the screws 186. As with the first embodiment of the present invention, the position of the spherical elements 122;128 within the socket 180 may be adjusted by adjusting the radial position of the screws 186.

0019Referring now to Fig 10, the screws 176;186 are retained in position by lock nuts 190 which lie in an enlarged portion 192 of the screw-threaded bore 194, and bear against a flat face 196 of the body 172;182 which surrounds the screws 176;186.

0020In tests of the arrangements described above (in which the spherical elements 122;128 are made of chrome plated steel) it has been found that no deterioration of the surface of the spherical elements 122;128 occurs after 10,000 revolutions of the elements within the socket. By providing three areas of surface whose positions are adjustable within the body 172;182 of the socket 170;180, any alteration in the position of the spherical elements 122;128 occurring over a large number of revolutions due to wear of the ball-ended surfaces of the screws 176;186 may be offset by adjusting the position of the screws.

0021It is not essential for the screws 176;186 to be of phosphor bronze; advantages in wear may be obtained where the material from which the screws 176;186 are made has a substantially lower hardness than the material of the spherical elements 122;128. Also, the screws 176;186 do not have to be adjustable within the body 172;182; their position may be fixed. Nor is it essential for the screws to be equi-spaced, or for them to extend radially or axially provided that they support the spherical elements 122;128 in a stable position. Further, it is not essential for the ends of the screws 176,186 to be ball-ended, provided that the ends are such that the spherical elements are stably supported.

0022In an alternative embodiment of socket, the areas of surface for supporting the spherical elements 122;128 may be provided by a ring situated inside the rim of the body 172;182 of the socket. According to this alternative the ring may be of e.g. phosphor bronze of PTFE.