Improvements in bearings

Abstract: A machine is disclosed having novel air bearings for supporting one machine part for translational movement on another part in a first direction X. Each bearing takes the form of a track (5A,14,16) on one of the parts providing concave part-cylindrical bearing surfaces (5A,14A,16A) and pads 25 on the other part providing confronting convex part-cylindrical bearing surfaces 25A. The arc lengths of the part-cylindrical bearing surfaces are such as to provide restraint against relative movements between the machine parts in at least one sense of both of the two directions (Y,Z) orthogonal to the direction (X) of the translational movement. The advantages are relative cheapness of manufacture compared to vee-bearings and a capability of the bearings to self-align under load.

Description: The present invention relates to a machine tool or coordinate measuring machine having bearings for translational slideway systems. In one embodiment the invention relates to gas bearings for such systems. In mounting machine parts to slide on gas bearings it is often required to provide bearing surfaces between the parts which will restrain movement of the machine parts in two orthogonal directions. In the past it has been common to provide two air bearing surfaces at right angles on one part with confronting air bearing surfaces on the other part to form a vee guide. It is important for the relative alignment of the two orthogonal surfaces to be extremely accurate in order that the very small clearances required for proper operation of the air bearing are not adversely affected by relative misalignment of the surfaces. This is difficult to achieve in practice and results in costly machining operations, and/or the provision of some form of articulation to enable self-alignment to take place. Also it is known to provide an air bearing in the form of a hollow cylinder mounted for movement along a cylindrical guideway. Such bearings, however, are not practical in a machine, for example a measuring machine, having long guideways since the guideway can only be supported at its ends, and it is difficult under such circumstances to ensure adequate stiffness of the guideway to maintain measurement accuracy. Another known bearing arrangement is shown in French patent specification no. 466,849. In this arrangement a machine element is supported and guided for axial translation along a spaced pair of cylindrical bearing rods which are mounted on a support structure. German Patent Specification No. 31 03 093 A1 describes a machine in which a movable carriage is mounted on two transversely spaced guide cylinders. The guide cylinders are in turn supported on fixed bearing surfaces formed on fixed axially spaced pillars. The bearing surfaces are part-cylindrical, and the axial spacing of the pillars is such that each cylinder is supported on at least two of the bearing surfaces. The present invention has for one of its objects the provision of a machine tool or coordinate measuring machine in which the disadvantages of the above-mentioned prior art bearings are obviated or at least reduced. According to the present invention there is provided a machine tool or coordinate measuring machine according to claim 1, the preamble of which is known from DE-A-31 03 093, having an axis, a first part movable relative to a second, support part in a first direction parallel to the axis, and bearing means for supporting the movable part on the support part, said bearing means comprising: bearing elements carried by the machine parts and spaced apart in a second direction orthogonal to the axis so as to lie on opposite sides of the axis, said elements defining respective convex and concave confronting part-cylindrical surfaces on respective ones of the machine parts, the confronting part-cylindrical surfaces extending over arcs the lengths of which are sufficient to provide restraint against relative movements between the surfaces in both senses of a third direction orthogonal to both of the first and second directions but which subtend angles not greater than 180°, whereby the surfaces of said bearing elements carried by one of the machine parts face in opposite senses of the second direction and comprise continuous elongate surfaces, the confronting surfaces of the bearing elements carried by the other one of the machine parts are formed on three relatively short pads, two of which are positioned on one side of the axis and are spaced in the first direction, the third of which is positioned on the opposite side of the axis, and resilient means are provided for allowing limited relative movement to take place between the oppositely facing surfaces on one of the machine parts, and for providing a pre-load on said surfaces. Such a bearing provides constraint for the structure against pitch roll and yaw rotations and against linear movements in the second and third directions while allowing sliding movements of the moving part along the support part in the first direction. In a preferred embodiment the bearing is an air bearing and means are provided for supplying air under pressure between the confronting surfaces of the two machine parts. The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings in which: Fig 1 illustrates the overall construction of a scanning machine including cylindrical air bearings made in accordance with the present invention. Fig 2 is a perspective view of the carriage of the machine of Fig 1 showing the air bearings in more detail. Fig 3 is an enlarged end view of one of the convex parts of a cylindrical air bearing of Fig 1 in a jig during an alignment operation. Fig 4 is an enlarged view of a modified bearing element of the present invention. Referring now to the drawings in Fig 1, there is shown a machine having a base 1, a pillar 2 upstanding from one end of the base and which is elongate in a direction defined as the X co-ordinate direction of the machine. Two beams 3 and 4 are positioned on top of the pillar, one at each end thereof, and which extend in a direction orthogonal to the X direction, defined as the Y co-ordinate direction, to overlie the base. Each of the beams 3 and 4 has at least part of its top surface formed as an air bearing surface 5A and 5B, for supporting a bridge member 6. The air bearing surface 5A of beam 3 is formed as a part-cylindrical air bearing surface in accordance with the present invention, and the air bearing surface 5B of beam 4 is a flat surface. The bridge member 6, the beams 3 and 4 and the top surface of the base 1 surround the working volume of the machine. The bridge member 6 supports a carriage 7 which comprises two vertical pillars 9 and 10 separated by a cross-member. The carriage 7 is supported on bridge member 6 for movement in the X direction on air bearings on each side of the bridge member 6. Each air bearing consists of a concave bearing element 14 or 16 on the bridge member 6, and a convex bearing element 25 on the carriage 7 (to be described with reference to Fig 2). As can be seen from Fig 1, the concave bearing elements 14,16 are formed with part cylindrical bearing surfaces 14A,16A. The pillar 9 supports a spindle 11 which in turn carries a probe 8 which extends downwardly into the working volume of the machine for taking measurements on a workpiece (not shown). The probe may be of any desired design, but in the particular example of the machine described herein, the probe is of the type described in our co-pending British Patent Application No. 9111364.7 to perform a scanning measurement on a workpiece. Movement of the carriage in the X direction is produced by a sledge 18 running in channels 20 and 21 formed on a track 22 fixed to the bridge member. The sledge is connected to the carriage preferably near to the mean centroid (in the Y and Z directions) of the relevant moving mass, by a pair of flexible rods (only one of which is shown at 23) attached to a bracket on the bridge member 6. Movement of the probe in Y direction is provided by a similar sledge (not shown) mounted for movement in the Y direction on a track mounted between the beams 3 and 4, and connected to the bridge member 6 by a pair of flexible drive rods. The sledge is driven in similar manner from a rack mounted on the track by a pinion driven by a sledge-mounted motor (none of which is illustrated in the drawings). Movement of the probe in the Z direction is provided by a motor 65 (Fig 1) mounted on the exterior of the pillar 9. The motor has a shaft passing through the wall of the pillar and which carries at its end a pinion engaging a rack (not shown) extending in the Z direction and mounted on a shaft inside the pillar to which the probe is connected. The movement of the probe shaft is guided by a further air bearing (not shown) within the pillar. All of the above-described movements of the probe are measured by scale and scale reader assemblies which may be of any known type suitably disposed on all the respective axes. Signals from the scale readers are passed in known manner to the machine control system for providing feedback to a closed loop control system (not shown) which controls the overall operation of the machine. For a detailed description of the air bearings of the present invention reference will be made to the bearings supporting the carriage 7 for movement on the bridge member 6, but it is to be understood that the same design principles can be applied to the other bearings of the machine. It will also be understood that the positions of the convex and concave surfaces described below on the moving and fixed parts of the machine may be reversed. The air bearing surfaces 14A and 16A of the bearing elements 14,16 on the bridge member 6 are continuous, concave surfaces of part-cylindrical form extending along each side of the bridge member. These may be formed in any known manner, e.g. by grinding or turning, die-forming or by a replication technique. For example, in one method where the bridge member is made from a lightweight metal such as Aluminium, the bridge member is mounted on a surface or slideway of a grinding machine and a high speed diamond fly cutter is passed down each side. This operation is preferably carried out without re-positioning the workpiece so that accurate relative alignment is maintained between the two surfaces. Another method of manufacture is to cut the concave surfaces to a reasonably accurate standard, line them with a curable resin material with low shrinkage characteristics, and impress into the resin, during curing, a cylindrical mandrel made with very precise form. By this means accurate replication of the form of the mandrel along the whole length of the concave surface can be maintained. The other bearing elements making up the air bearings are formed as convex part-cylindrical pads (Fig 2), of which two are mounted on a hollow platform 26 connected to pillar 9 of carriage 7 to form the front bearing, and one is connected to pillar 10 to form the rear bearing, thus providing part-cylindrical bearing surfaces 25A at three locations for supporting the carriage 7 on the bridge member 6. Each of the pads may be formed in any convenient manner, but the preferred manner is to form a complete cylinder, which is relatively easy to manufacture to an accurate size with high quality surface finish, and then to cut it longitudinally into two halves. This has the benefit that the two convex, part-cylindrical pads so formed will not be completely semi-cylindrical, so that when they are connected to the platform 26, the centre of the circle from which the curved surface is formed will be in the material of the platform 26. Since all of the forces on the curved surface will be radial, they will all be directed towards said centre in the material of the platform and any lateral component will be reacted by the material of the platform with no additional bending component. If the shrinkage of the resin, which is used in the replication technique, can be accurately controlled, the mandrel used for forming the concave surface of the bearing may be made out of appropriate bearing material so that it can be subsequently cut up to form the convex bearing pads. As can be seen from Fig 3 the part-cylindrical pads, once cut to length, are drilled to form two air passages 28 therethrough which terminate in orifices 28A to enable pressurised air to be directed between the bearing surfaces. The figure shows part of a jig 30 which is used to align the bearing pads as they are assembled onto the bridge member 12. The jig has a first elongate vee-groove on one side for receiving two of the pads 25, and which serves to align them onto a common axis. The jig has a second vee-groove on its opposite side, which is very accurately aligned with the first vee-groove, for receiving the third bearing pad. Only one of the pads 25 to be aligned in the first vee groove is shown in the jig, the position of the pad in the second vee-groove only being indicated in dotted lines on the opposite side of the jig. The bearing pads are attached to the bridge member 12 while in position in the jig as follows: Air supply nipples 32 each with an integral spacer 34 are passed through the wall of the platform 26. The bearing pads are screwed onto the other ends of the nipples, the spacers ensuring that gaps 36 remain between the bearing pads and the wall 26. At this stage the pads are free to tilt in both planes to ensure correct alignment of their axes with the vee-groove in both planes. Glue is added into the gaps 36 between the pads and the wall to stabilise the assembly. To ensure that the alignment of the pad is not disturbed in use, a nut 38 is screwed onto the nipple once the glue has set. A similar procedure is used for attaching the other bearing pads. The three part-cylindrical bearing pads 25 provide constraint for the structure against pitch, roll, and yaw rotations and against linear movements in the orthogonal Z and Y directions while allowing sliding movements of the carriage along the bridge member in the X direction. In order to provide the necessary pre-load on the bearings in the Y direction to maintain the correct relative positions of the bearing surfaces in that direction, a slit 40 is made in the top of pillar 10 (Fig 2), and a strong spring indicated diagrammatically at 42 is placed in the pillar under compressive stress. The result is that the force of the spring urging the outer wall of the pillar away from the end of the cross-member 12 in the Y direction is reacted at the bottom of the pillar producing a force in the Y direction urging the bearing pad into contact with the concave surface of the bearing on the bridge member 6. This construction not only provides the pre-load in the Y direction required for proper operation of the air bearings, but it also provides freedom for the rear bearing pad 25 to translate in the Y direction to avoid overconstraint of the bearing system. The fact that the bearing elements 25 are capable of self-aligning about the axis of the part-cylindrical bearing surfaces is a significant advantage in allowing the above-described bearing pre-load arrangement to be used. This is because when the spring load is applied to the bearing system there will inevitably be pivoting of part of the pillar 10 which can be accommodated in the bearing. Thus any distortion in the Y-Z plane applied to the bearing by the pre-load forces, or any other external forces will not cause bearing mis-alignment. Another novel feature of the present invention is the way in which the pre-load in the Y direction and the angular orientation of the air orifices are combined to improve the stiffness of the bearing. Referring again to Fig 3 it can be seen that the air orifices are directed so that the included angle between them α is greater than 90°, e.g. it may be 100°. Thus the inclination α/2 of the direction of the gas jets to the direction Y of the pre-load is greater than 45°. The effect of this is to ensure that the forces produced by the air orifices in operation have a greater component in the Z direction than in the Y direction thus providing for greater stiffness in the Z direction than in the Y direction, the latter being provided by the spring 42. Thus a more even stiffness all round can be provided. Although the bearing has been described with reference to an air bearing embodiment, other gaseous fluids may be used. However, the advantages of the part-cylindrical form are equally applicable to a journal bearing. For example, if the resin used in the replication technique for forming the concave surfaces is a low friction material it is not necessary to provide the air film to lubricate the bearing. Thus the invention can also provide a part-cylindrical journal bearing having the self-aligning feature and of relatively simple inexpensive construction. Another enhancement which may be used is to truncate the convex bearing elements 25 as shown in Fig 4 reducing the part-cylindrical bearing surfaces 25A to two separate portions on opposite sides of the bearing element 25. This enables the bearing to behave like a vee-bearing while retaining the self-aligning capability of half cylinder.

Claims: A machine tool or coordinate measuring machine having an axis (X), a first part (7) movable relative to a second, support part (6) in a first direction parallel to the axis (X), and bearing means (14,16,25) for supporting the movable part on the support part, said bearing means comprising: bearing elements (14,16;25) carried by the machine parts (6,7) and spaced apart in a second direction (Y) orthogonal to the axis (X) so as to lie on opposite sides of the axis (X), said elements defining respective concave and convex confronting part-cylindrical surfaces (14A,16A,25A) on respective ones of the machine parts, the confronting part-cylindrical surfaces extending over arcs the lengths of which are sufficient to provide restraint against relative movements between the surfaces in both senses of a third direction (Z) orthogonal to both of the first (X) and second (Y) directions but which subtend angles not greater than 180°, characterised in that: the surfaces (14A,16A) of said bearing elements carried by one of the machine parts (6) face in opposite senses of the second direction and comprise continuous elongate surfaces, the confronting surfaces of the bearing elements (25A) carried by the other one of the machine parts (7) are formed on three relatively short pads, two of which are positioned on one side of the axis (X) and are spaced in the first direction, the third of which is positioned on the opposite side of the axis, and resilient means (40,42) are provided for allowing limited relative movement to take place between the oppositely facing surfaces on one of the machine parts, and for providing a pre-load on said surfaces. A machine tool or coordinate measuring machine as claimed in claim 1 characterised in that the movable part comprises two pillars (9,10) positioned one on each side of the support part (6) the bearing elements on the movable member (7) being fixed to said pillars at one end thereof, the pillars being interconnected at the other end thereof by a cross-member (12) and wherein the resilient means (40,42) are provided for urging at least one of the pillars towards the other one to produce said pre-load on the bearing surfaces of the bearing elements. A machine tool or coordinate measuring machine as claimed in claim 2 characterised in that one of said pillars (10) is hollow at least at its said other end and has a slit (40) formed therein which extends from its said other end part way along the length of the pilar, and further characterised in that the resilient means comprises a compression spring (42) provided inside the hollow part of the pillar which reacts against the cross-member (12) to produce said pre-load. A machine tool or coordinate measuring machine as claimed in any preceding claim characterised in that gaseous fluid supply means (32) are provided for pressurising the bearings. A machine tool or coordinate measuring machine as claimed in claim 4, characterised in that the gaseous fluid supply means (32) includes jets (28A) for directing gaseous fluid between the confronting bearing surfaces in a direction lying in the plane of said two orthogonal directions (Y,Z) at an angle of inclination α/2 to the direction (Y) of the pre-load which is greater than 45°. A machine tool or coordinate measuring machine as claimed in claim 1, characterised in that each of the relatively short bearing elements (25) is attached to its respective structure and has a convex, part-cylindrical bearing surface (25A) extending outwardly therefrom, and the angle subtended by the convex bearing surface is less than 180° so that the centre of the arc of the bearing surface lies within the structure.

Inventors: MCMURTRY DAVID ROBERTS, WRIGHT DAVID ALLAN, MCMURTRY, DAVID ROBERTS, WRIGHT, DAVID ALLAN
Applicants: RENISHAW METROLOGY LTD, RENISHAW METROLOGY LIMITED
Application year: 1992
Main classification: B23Q1
IPC codes: F16C29, F16C32, B23Q1