One aspect of the invention relates to rotary drive couplings, or universal couplings, having high vibration damping isolation and desensitising properties, combined with misalignment insensitivity/tolerance and, when desired, thermal decoupling. It finds application, for example, for coupling a remotely mounted motor to the driven part of a device, machine or instrument where the vibration, misalignment (of motor axis, or separation, relative to device axis) and/or thermal consequences of a direct mounting of a motor on the device, machine or instrument would be unacceptable.

Precision mechanical stationary interfaces, such as annular spherical interfaces on the tetrahedral machine mounting structures described in our patent application referred to above also normally need to interface consistently with large, reproducible contact area to exhibit high compressive stiffness, stability, and vibration damping properties. Interfacing of this kind may also be improved by replication methods, that is, replicating one surface directly on to the matching surface, by use of the method of the invention.

According to the present invention there is provided a method of forming a structure which includes two components having surfaces intended to mate with one another, comprising the steps of coating one of the surfaces with a release agent, positioning the two components in their intended relationship with one another with their said surfaces slightly spaced from one another, filling the space between the surfaces with a hardenable

interface material, and hardening the interface material in adherent relationship with that other of the said surfaces which is not coated with the release agent.

The hardenable interface material may be a viscous but curable polymeric material, or it may be a coating or sheet of deformable solid material which is applied to one or other of the surfaces before positioning the two components in their intended relationship and thereby deforming the said coating or sheet into close conformity with the surfaces. Or, again, discrete bearing-pad segments may be adhered to the said other surface, prior to positioning the two components in their intended relationship with the said one surface engaging the bearing pad segments, and thereafter filling the remaining space between the surfaces with the hardenable interface material. The method may comprise the further steps of separating the components, removing the release agent from the said one of the surfaces, and bringing the components once again into their intended relationship with the said one surface mated with the interface material which is adherent to the other surface; and, with or without those further steps, the method may include the step of securing the two components fixedly together if they are intended to be in a fixed relationship.

The invention also provides a structure formed by the foregoing method, which includes two components having surfaces mating with one another with an interface material disposed between the surfaces and non-adherent to one but adherent to the other of them.

In such a structure, the two components may be movable relative to one another, with movement of the interface material, adherent to the said other surface, over the said one surface; or the two components may be fixedly secured together.

The interface material may be a cured polymeric material, which may be loaded with particles of a filler material. The interface material may be one which possesses vibration-damping properties. Also, the release agent may be a material which

possesses vibration-damping properties, and it may be retained on the said one surface; or the release-agent coating may be removed from the said one surface and replaced by another material, which possesses vibration-damping properties. The invention will be more fully understood from the following description of embodiments thereof with reference to the accompanying drawings, in which:-

Figure 1 is a front elevational view of a tetrahedral mounting device for a machine tool, a modification of an embodiment of the tool described in U.K. Patent Application No.2194182A; Figure 2 is a lateral sectional view, on the line II-II of Figure 1, of the mounting device and machine tool shown in Figure 1 ;

Figure 3 is a section, on a larger scale, through a workpiece mounting table for the machine tool; Figure 4 is a section, on a larger scale, through a toolhoider shown in Figures 1 and 2; and Figures 5 and 6 are sectional representations showing diagrammatically the "replication" of spherical surfaces in carrying out the present invention.

The tetrahedral mounting structure shown in Figures 1 and 2, which is as described and claimed in UK Patent Application No ^' . 2194182A, includes six rod-like members la - If. Each member comprises a tubular outer strut 2a - 2f enclosing a tensile rod

3a - 3f. At each apex of the tetrahedral structure is a hollow sphere 4a - 4d. Each sphere comprises a pair of mating hemispheres 5a - 5d, 6a - 6d. The halves of the hemispheres are held together by threaded bolts 7. Each sphere abuts the outer strut of three of the rod-like members and is clamped thereto by means of bolts 8 attached to the ends of the corresponding enclosed tensile rod. Mounted on the rod-like members la, lb and lc which are remote from the base constituted by members Id, le and If is a machine tool holder 9. The machine tool holder is

clamped to rod-like members la, lb and lc by means of threaded bolts 10 and space blocks 11, and preferably the mounting interfaces are "replicated", by application of the method according to the invention, as described below. The machine tool holder 9 and the hollow sphere 4d have respective bores 12 and 21, coaxial with the machine principal axis 13, through which passes a drive shaft 14 coupled (through a spherical joint with "replicated" interface in accordance with the invention)- to a rotor 15A rotatably mounted in an air bearing stator 15B and carrying a grinding tool 16, as shown in Figure 2 and (on a larger scale) in Figure 4. The bearing stator 15B has an extension 15C to provide an extended viscous damping workface 15D. Mounted on the base rod-like members Id, le and If preferably also with "replicated" mounting interfaces in accordance with the invention, is a vee slideway 17 bearing a workpiece holder 18 to impart linear motion to a workpiece 19 beneath the grinding tool 16. The workpiece holder is provided with a dividing strip 18A. Interfaces 18B are provided with a viscous damping medium. The slideway has a hollowed-out portion 20 so that the wall thickness is similar to that of the other structural components. The hollow sphere remote from the base contains a bore 2T through which the drive shaft passes.

On the areas between the dividing strip of the sliding workpiece holder 18 and the matching slot in the vee slideway 17 and between the stator of the air bearing rotary spindle 15 and the bore of the machine tool holder 9, there is relative motion, the viscous drag of the damping medium being arranged to be insignificant for the rate of motion required. The geometry of the damping interface can be matched to the modes of the vibration expected or most needing attenuation, for instance the plane vertical damping interfaces between the driving strip of the sliding workpiece holder 18 and the matching slot in the vee slideway 17 will respond efficiently to horizontal mode vibrations from the drive to the sliding workpiece holder and the principal axis (vertical) vibrations; whereas the cylindrical

damping interface between the stator of the air bearing rotary spindle 15 and the bore of the machine tool holder 9 will respond efficiently to torsional vibrations from the rotary spindle and to principal axis (vertical) vibrations. Similarly, appropriate vibrational damping may be provided in the spherical coupling between the drive shaft 14 and the rotor 15A.

It has been found that the thickness of the viscous damping layer (i.e. the gap between the solid surfaces thus comprising the damping interface) can be 50 micrometres or greater with suitably high-viscosity damping fluids, so that no tribological (fretting, pick up, wear) problems will arise with dimensional tolerances within normal engineering practice. Close proximity of the two surfaces, as on some existing machines, is not needed; this again avoids tribological problems, and the inconsistent vibration damping which frequently results from stick-slip when the two surfaces are in close proximity or intermittent contact.

As referred to above, the spherical coupling between the drive shaft 14 and the rotor 15A is formed in accordance with the invention with surface replication at its mating surfaces, as will now be described with reference to Figure 5.

As shown in Figure 5, the lower end surface of the shaft 14 is formed with a hemispherical recess in which a spherically-formed projection 15E on the upper end of the rotor 15A is accommodated concentrically with the surface of the recess but with a clearance since the projection 15E is of smaller radius than the recess on the shaft 14. In applying the method of the invention to this coupling the spherical surface of the projection 15E is first coated with a layer of a release agent 22, then the shaft 14 and rotor 15A are positioned in their intended relationship with one another, with the spherical projection 15E located concentric with the surface of the recess in the shaft 14. The space between the two parts is then filled with a suitable hardenable material, such as a curable polymeric material such as a polyester material or epoxy resin or the like, which is then cured or otherwise hardened to form a hardened

solid coating or layer 23 which adheres to the surface of the recess on the shaft 14 but which is prevented by the release agent 22 from adhering to the projection 15E to the shape of which, however, it closely conforms. The layer 23, which replicates the surface shape of the projection 15E, has to be of sufficient thickness to accommodate the geometrical and dimensional mismatch of the two surfaces, including any irregularity of the recess surface to which it adheres. Interface compressive stiffness can be enhanced by minimising the thickness of the replicating layer and by maximi-sing its elastic modulus (in the case of curable polymeric materials, this can be enhanced by loading/filling the material with high modulus solids).

The release agent layer should preferably be as thin as possible (so as not to produce a significant final radius mismatch between the convex and concave surfaces). Where vibration damping properties are required of the interface, the release agent layer can often be made of material which itself provides friction and/or viscous damping so that it need not be removed after the layer 23 has hardened. If this is not possible for some reason, for example the unsuitability of some material combinations, the parts can be separated after the layer 23 has hardened and the release agent may then be removed and replaced with damping material of similar thickness before reassembling the parts. The material of the surface replicating layer 23 can often be loaded/filled with vibration damping additives so that there is no need to add an extra damping material layer to the interface.

A variant of the construction, and the method of formation, illustrated in Figure 5 is illustrated in Figure 6. As before, the spherical surface of the projection 15E is coated with a layer 22 of a suitable release agent, but in this case the concave spherical surface of the recess in the shaft 14 is coated with an adhesi-ve layer 24 into which discrete bearing-pad segments 25 of a solid but preferably deformable material are

pressed so as to adhere them to the concave surface. Only then are the parts 14 and 15E positioned in their intended relationship with one another, with the pads 25 bearing against the surface of the projection 15E and, preferably, deformed into close conformity therewith. The interface material 23 is then introduced to fill the remaining space between the convex and concave surfaces and hardened, as in the case described with reference to Figure 5.

The use of discrete bearing-pad segments, as in Figure 6, is particularly suitable where the mating surfaces between which they are disposed are spherical and the positioning between them of a continuous sheet of solid material, even if deformable, would be difficult or impossible. However, in the case of mating cylindrical surfaces the desired layer of interface material may conveniently be provided in the form of a continuous sheet of suitable deformable but hardenable material which is then, when the surfaces are brought into their intended relationship, made to adhere to the one surface but prevented by the release agent from adhering to the other, to which, however, it is deformed into conformity by the pressure between the surfaces (this pressure being then maintained while the sheet material is hardened).

It will be apparent that, although it is the convex spherical surface in Figures 5 and 6 to which the release agent is applied and the concave surface to which the layer of interface material is adhered, the converse arrangement is equally applicable.

Although the invention has been described with reference to Figures 5 and 6 as applied to a joint between two components which are to be relatively movable in use, it may also be applied in cases where the two components will be secured together in fixed relationship in the structure which contains them. Thus a surface-replicating interface layer may be provided, in accordance with the invention, between the space blocks 11 of the structure shown in Figures 1 and 2 and the tubular struts 2a, 2b and 2c on which they are secured in ating-surface relationship.

Equally, the upper ends of the tubular struts 2a, 2b and 2c abut the hemisphere 6d at spherical zones of the hemispherical surface of the latter; and the imperfections of the inter-surface fit, and consequent non-uniformity of stress at the joint, may be minimised at this position also, by use of the invention to provide interface material between the tubular strut and the hemisphere 6d (and likewise at other similar points of the structure).