GB2262319A

GB2262319A - Linear bearing arrangement for a slide assembly

Info

Publication number: GB2262319A
Application number: GB9126473A
Authority: GB
Inventors: Ray Hudson
Original assignee: Eastman Machine Co Ltd; Eastman Machine Co
Current assignee: Eastman Machine Co Ltd; Eastman Machine Co
Priority date: 1991-12-13
Filing date: 1991-12-13
Publication date: 1993-06-16
Anticipated expiration: 2011-12-13
Also published as: GB2262319B; GB9126473D0

Abstract

A slide assembly comprises a first member (11, 17), a second member (13) movable relative to the first member, and guidance means for guiding such movement. The guidance means comprises at least one air bearing pair co-operable with a respective guide surface pair. The guide surface pair consists of a first surface of superaccurate planarity and a second surface of non-superaccurate planarity substantially parallel to the first surface, and the air bearing pair comprises a first air bearing (18) co-operating with the first surface and a second air bearing (19) co-operating with the second surface. The first bearing (18) is held by a fixed mounting, which may include an adjustment capability, and the second bearing (19) by a resilient mounting, such as a leaf spring, able to absorb irregularities in the planarity of the second surface and deviations in the parallelism of the surfaces. <IMAGE>

Description

SLIDE ASSEMBLY The present invention relates to a slide assembly, especially an assembly which provides precision guidance of measuring, machining or handling equipment for example in a co-ordinate measuring machine.

Highly accurate guidance of relatively moving parts is necessary in many different types of machine br instrument, of which a threedimensional co-ordinate measuring machine can be considered to be a representative example. Co-ordinate measuring requires movement of â measuring element relative to a stationary article, for example a sensor probe which contacts the article3 and recording the movement of the element in relation to a reference point so as to build up a tO-dimensional or three-dimensional measurement profile of the article. In a typical construction of such a measuring machine, a probe is carried by a spindle which is mounted in a carriage on a bridge movable relative to a support table for the article.Movement of the probe in the Z axis direction is by way of displacement of the spindle in a mount on the carriage, movement in the Y axis direction is by way of displacement of the carriage on the bridge and movement in the X axis direction is by way of displacement of the bridge over the table. The accuracy of the measurements obtained from signals generated through co-operation of the probe with the article is critically dependent on the accuracy of the bridge displacement, in particular its travel along a precisely rectilinear path. Any deviations from rectilinearity will ultimately be reflected in the measurement result.

In order to ensure precise guidance of the carriage in such a measuring machine it is established practice to define the bridge path by way of superaccurate guide surfaces, with two such guide surfaces being required for guidance in a horizontal plane and usually tt'.o for guidance in a vertical plane. Bearings on the bridge run along the guide surfaces to constrain the bridge, and thus the probe, to follow an exactly rectilinear path. The provision of guide surfaces to a superaccurate standard is both difficult and costly, with an additional complication being represented by the need to achieve exact parallelism of the two surfaces associated with each guidance plane.

Any departure of the surfaces from a precisely parallel relationship again adversely affects the accuracy of the measurement result. A consequence of these difficulties is the need to form the parallel guide surfaces on a more easily machinable component, such as a beam projecting upwardly from the support table. The beam must be produced with critical height and width accuracy and mounted free of distortion or misalignment.

It is therefore the principal object of the present invention to provide a slide assembly in which highly accurate guidance can be achieved by more economically producible measures, especially in the case of precision measuring machines or machine tools. A subsidiary object of the invention is to allow greater freedom of choice in the disposition of superaccurate guide surfaces in such a slide assembly, so as to remove the need for special constructions.

Other objects and advantages of the invention will be apparent from the following description.

According to the present invention there is provided a slide assembly comprising a first member, a second member slidingly movable relative to the first member, and guidance means for guiding movement of the second member relative to the first member and comprising an air bearing pair co-operable with a guide surface pair for guidance of said movement, wherein the guide surface pair comprises a first surface of superaccurate planarity and a second surface of nonsuperaccurate planarity substantially parallel to the first surface and the air bearing pair comprises a first air bearing held by fixed mounting means and co-operable with the first surface of the guide surface pair and a second air bearing held by resilient mounting means and co-operable with the second surface of the guide surface pair.

In a preferred embodiment the guidance means comprises a further air bearing pair co-operable with a further such guide surface pair for additional guidance of the movement of the second member, the surfaces of the further guide surface pair being substantially normal to the surfaces of the first-mentioned guide surface pair. The two guide surface pairs can thus provide guidance in, respectively, two mutually perpendicular planes.

The first and second surfaces can, for convenience, be provided on the same member, particularly the first member. The disposition of the surfaces is, however, a matter of choice depending on constructional requirements.

The resilient mounting means can comprise, for example a spring element, an elastomeric element or a pneumatic element. The fixed mounting means can comprise adjusting means for adjusting the position of the first air bearing, so that the spacing of the bearing relative to the associated guide surface can be set.

Depending on requirements, the or each first air bearing and/or the or each second air bearing can be divided into a plurality of spaced-apart air bearing elements, so that bearing action is provided at discrete and suitable spaced locations along the guide path defined by the associated guide surface pair.

In the case of an assembly with a plurality of air bearing pairs and guide surface pairs, for example as in a measuring machine, machine tool or precision instrument, the first member can define a slot and the second member can be movable in the slot and embrace the first member in the region of the slot, one air bearing pair being arranged in the slot and another in the slot region. For preference the air bearing pairs are arranged on the second member, with the surfaces of one guide surface pair being provided by the mutually opposed walls of the slot and the surfaces of the other guide surface pair by upper and lower faces of the first member.The first element can comprise a base element, such as a support table or a work base, and a rail mounted at a spacing from the base element to define the slot, the first surfaces of the two guide surface pairs then being provided by an uppermost face of the base element and by a side of the base element facing the rail.

If the assembly is to serve for measuring or machining purposes, one of the members, particularly the movable second member, can be provided with measuring or machining equipment.

An embodiment of the present invention will now be more particularly described with reference to the accompanying drawings, in which Fig. 1 is a partly sectioned front elevation, on the line I-I of Fig. 3, of a slide assembly embodying the invention, the assembly being present in a co-ordinate measuring machine; Fig. 2 is a detail view, to an enlarged scale, of the encircled area II in Fig. 1; and Fig. 3 is a side elevation of the assembly of Fig. 1.

Referring now to the drawings there is shown a slide assembly in the form of a co-ordinate measuring machine 10, which comprises a slab-like base element 11 supported on a stand 12, and a bridge 13 slidably mounted on the base element. Slideably mounted on a cross beam of the bridge 13 is a carriage 14, which carries a vertically movable spindle 15 with a sensor probe 16. For the purpose of coordinate measuring, the movement of the bridge 13 over the base element represents travel in an X axis direction, movement of the carriage 14 along the cross beam of the bridge 13 represents travel in a Y axis direction and movement of the spindle 15 in the carriage 13 represents travel in a Z axis direction.By means of displacement of the probe in these three directions the probe can be aligned with selected points on the contour of an article (not shown) fixed on the base element 11. A signal is induced, such as by deflection of the probe from a rest position in the case of a tactile probe, whenever the probe is activated. By evaluation of these signals, the travel in the X, Y and Z directions with respect to an origin or other point or points of reference is determined and from this travel the dimensions of the article can be calculated.

The base element 11, which consists of granite, aluminium or other suitable material, is of rectangular cross-section and has its upper face and righthand (in Fig. 1) side face finished to a superaccurate standard, for example with a maximum deviation of substantially + 3 microns from absolute planar)ty. Attached to the base element 11 or stand 12 is a guide rail 17, which is disposed at a spacing from the righthand side face of the element 11 so that a slot is present between the element 11 and the rail 17. The bridge 13 extends entirely around the base element 11 and is provided in its lower part with recessses receiving the side edge portions of the element 11, so that the bridge embraces these portions. In the region of the rail 17, the lower part of the bridge extends through the slot.

Guidance of the movement of the bridge 13 relative to the base element 11, and thus of the X axis movement of the probe 16 relative to an article to be measured, is provided by three air bearing pairs which are respectively co-operable with three guide surface pairs, each of which consists of a first, superaccurate surface and a second, non-superaccurate surface parallel to the first surface. Each air bearing pair consists of a first air bearing 18, which is held by a fixed, although adjustable, mounting and co-operates with the first surface of the associated guide surface pair, and a second air bearing 19, which is held by a resilient mounting and co-operates with the second surface of that pair.Two of the air bearing pairs are, as shown in the drawings, mounted on the bridge 13 and in particular in the recesses where the bridge embraces the side edge portions of the base element 11. The superaccurate surfaces for the first bearings 18 of these two bearing pairs are represented by respective portions of the superaccurately finished upper face of the base element, while the non-superaccurate surfaces for the associated second bearings 19 are represented by respective portions of the lower face of the base element. The lower face is planar and parallel to the upper face, but not finished to a superaccurate standard with respect to planarity or parallelism. The third air bearing pair is mounted on the bridge in the part thereof extending through the slot between the base element 11 and the rail 17.The superaccurate surface for the first bearing 18 of the third bearing pair is represented by the superaccurately finished side face of the base element, and the non-superaccurate surface for the second bearing 19 by the side of the rail 17 facing the base element. Because the rail does not require superaccurate finishing it can be of a material which is less expensive to incorporate into the machine.

Each of the first and second air bearings 18 and 19 is divided into two air bearing elements so that the bridge is stabilised against rocking. An individual air bearing can, however, be designed as a single element with a bearing area of sufficient size to ensure the requisite degree of stability. Each bearing element of each first air bearing 18 is mounted on the bridge 13 by a fixed mounting which incorporates a screw adjusting device 20 allowing precise setting of the spacing of that bearing element relative to the associated superaccurate guide surface. Each bearing element of each second air bearing 19 is mounted on the bridge by a resilient mounting in the form of a leaf spring which urges the bridge and the bearing element apart, thus the bearing element towards the associated non superaccurate guide surface.This resilient mounting can, if desired, include an adjustment to allow variation of the force acting on the respective bearing element in direction towards the associated guide surface. ' -' .

Each bearing element is pivotably connected by a spherical joint to the respective mounting so that the elements are self-aligning to accommodate for example, temperature-induced change in components of the machine, such as the rail 17 or the bridge itself.

In use of the measuring machine 10, compressed air is supplied at a pressure of, for example, 40-80 pounds per square inch through flexible pipes (not shown) to each of the air bearings where it escapes to provide a pressure air film between that bearing and the associated guide surface. The thickness of the air film will vary according to the selected tolerances and may be, for example, in the region of 5 microns. The first and second bearings of each air bearing pair act in mutually opposite directions to provide low friction guidance of the bridge 13 in vertical and horizontal planes relative to the base element 11 and rail 17, movement of the bridge being affected by a suitable drive (not shown).Rectilinearity of the travel path of the bridge to the required tolerance is ensured by the superaccurate guide surfaces provided exclusively on the base element 11 and co-operating with only one bearing - the first bearing 18 - of each bearing pair. Irregularities in the planarity of the non superaccurate guide surfaces, in particular tolerances greater than the standard of finish of the superaccurate surfaces, and deviations from strict parallelism of the opposing superaccurate and non superaccurate surfaces, are absorbed by the resilient mountings of the second bearings 19. Any deflections of the leaf springs caused by such irregularities and deviations are too small to change the spring force to a significant extent. The force thus remains substantially constant.To ensure that each first bearing maintains a constant relationship to the associated superaccurate surface and is not itself disturbed by irregularities in the opposing non-superaccurate surface, the force exerted by the leaf spring in the resilient mounting of the second bearing can be approximately equal to that of the air film at the first bearing; slight deflection of the resilient mounting causes virtually no change in the pressure of the air film at the first bearing 18 and thus no variation in the ride height of that bearing. The air film pressures at the first and second bearings 18 and 19 will normally be nominally equal when the bearings have similar areas under pressure. The desired pressure relationship can be secured by use of throttles (fixed or controllable3 in compressed air feeds, by spring adjustments and/or by other measures.

As previously explained, the movement of the bridge 13 relative to the base element 11 and rail 17 constitutes only one phase of the measuring process. The operation of such machines is well-known, however, and thus is not described in further detail.

The guidance system described above provides low-friction precision guidance of the bridge and permits substantially more economic construction of the machine in terms of manufacturing processes, materials, assembly and maintenance. Only the base element 11 need have superaccurately finished faces, and it is no longer necessary to apply superaccurate standards to the parallelism of the surfaces of each bearing surface pair. Moreover, because of the reduced number of superaccurate guide surfaces the guide system may retain its originally set tolerances for a longer period of time compared with known systems. The guidance system as applied to the described measuring machine is merely an example and changes and modifications can be made in accordance with requirements. For instance, the portion of the bridge 13 passing underneath the base element 11 can be eliminated, together with the associated second bearings 19. If these bearings are to be retained, they can be carried by, for example, cantilever brackets attached to the lower ends of the bridge columns.

Claims

1. A slide assembly comprising a first member, a second member slidingly movable relative to the first member, and guidance means for guiding movement of the second member relative to the first member and comprising an air bearing pair co-operable with a guide surface pair for guidance ot said movement, wherein the 'guide surface pair comprises a first surface of superaccurate planarity and a second surface of non-superaccurate planarity substantially parallel to the first surface and the air bearing pair comprises a first air bearing held by fixed mounting means and co-operable with the first surface of the guide surface pair and a second air bearing held by resilient mounting means and co-operable with the second surface of the guide surface pair.

2. An assembly as claimed in claim 1, wherein the guidance means comprises a further air bearing pair co-operable with a further such guide surface pair for additional guidance of said movement, the surfaces of the further guide surface pair being substantially normal to the surfaces of the first-mentioned guide surface pair.

3. An assembly as claimed in claim 1 or claim 2, wherein the first and second surfaces are provided on the same member.

4. An assembly as claimed in claim 3, wherein the first and second surfaces are provided on the first member.

5. An assembly as claimed in any one of the claims 1 to 4, wherein the resilient mounting means comprises a spring element.

6. An assembly as claimed in any one of claims 1 to 4, wherein the resilient mounting means comprises an elastomeric element.

7. An assembly as claimed in any of claims 1 to 4, wherein the resilient mounting means comprises a pneumatic element.

8. An assembly as claimed in any one of the preceding claims, wherein the fixed mounting means comprises adjusting means for adjusting the position of the first air bearing.

9. An assembly as claimed in any one of the preceding claims, wherein the or each first air bearing comprises a plurality of spacedapart air bearing elements.

10. An assembly as claimed in any one of the preceding claims, wherein the or each second air bearing comprises a plurality of spaced-apart air bearing elements.

11. An assembly as claimed in claim 2, wherein the first member defines a slot and the second member is movable in the slot and embraces the first member in a region adjacent to the slot, one air bearing pair being arranged in the slot and the other bearing pair in said region.

12. An assembly as claimed in claim 11, wherein the air bearing pairs are arranged on the second member, the surfaces of one guide surface pair being provided by mutually opposed walls of the slot and the surfaces of the other guide surface pair by upper and lower faces of the first member.

13. An assembly as claimed in claim 12, wherein the first member comprises a base element and a rail mounted at a spacing from the base element to define the slot, the first surfaces of the guide surface pairs being provided by an uppermost face of the base element and by a side of the base element facing the rail.

14. An assembly as claimed in any one of the preceding claims, wherein one of the members is provided with measuring equipment.

15. An assembly as claimed in any one of claims 1 to 13, wherein one of the members is provided with machining equipment.

16. An assembly substantially as hereinbefore described with reference to the accompanying drawings.