MECHANICAL DEVICES AND STRUCTURES

Abstract: A very stiff mounting structure comprises at least three pyramidally disposed rod-like members (1a, 1b, 1c) rigidly coupled together at one end (4d) and with their other ends rigidly coupled to a base (22) either to stiffen the base and/or to mount a device such as a tool or instrument with respect to it. Three pyramidally disposed members (1a, 1b, 1c) may be mounted on a tetrahedral base (22), or four such members on respective corners of a square pyramid, to form therewith a framework in which to mount, for example, a machine tool or measuring instrument, the base preferably being formed of similar rod-like members; or the base may be a plate or other structure of a separately existing machine or other entity.

Description: MECHANICAL DEVICES AND STRUCTURES This Invention relates to mechanical devices and structures. In particular, It relates to structures having an affinity with those described In our pending British Patent Application No.2194182A. Published British Patent Application No. 2194182A describes and claims a mounting device comprising six rod-like members the ends ~~of which are rigidly coupled together to form a substantially tetrahedral structure, three of the members which are not co-planar having fixedly coupled to them mounting means for, for example, a machine tool, and the other three members constituting a triangular base and, suitably, having fixedly coupled to them means for locating a workpiece on which the machine tool is to work. The six members may all be of equal length, yielding a regular tetrahedral structure, or the three members having the mounting means coupled to them may be of different length from the remaining three members constituting the base. The published Application describes the advantageous physical and mechanical properties (stiffness, vibration response, and thermal response characteristics) which may be attained by the use of such a structure. It is found that these advantages may be retained virtually uncompromised in a mounting structure which comprises a rigid base and a first set of rod-like members disposed pyramidally with one end of each rigidly coupled to corresponding ends of the others at a common vertex and the other end of each rigidly coupled to the base, even when the rod-like members of the said,., first set are more than three in number and/or the base is not ' itself formed simply of a further set of three rod-like members. According to the present invention, therefore, there is provided a mounting structure which comprises a rigid base and a first set of at least three rod-like members disposed pyramidally with one end of each rigidly coupled to corresponding ends of the others at a common vertex and the other end of each rigidly coupled to the base, wherein the base is other than simpTy a triangular structure formed of a further set of three rod-like members. Such a mounting structure may also include mounting means comprising a rigid member fixedly coupled to the pyra idally- disposed rod-like members and in spaced relationship with the base. The base of such a mounting structure according to the invention may be a more complicated three-dimensional triangulated structure, or may be, for example, a solid rigid metal plate or other structure to which the said other ends of the pyramidally disposed rod-like members are rigidly coupled at spaced-apart locations; and the plate or other structure may itself be an integral part of a separately existing machine or other article. The rod-like members of the said first set may be three or four in number, and the base may have the form of a tetrahedron or a square pyramid, with the said other ends of the members of the first set secured to respecitive vertices of the base forming a triangle or a square respectively. As contemplated in the published UK Application already referred to, the coupling together of the said one ends of the rod-like members at their common vertex may be such that their pyramidal disposition is, in effect, that of a somewhat truncated pyramid. The invention will be more fully explained and understood from the following description of embodiments thereof with reference to the accompanying drawings, in which:- Figure 1 1s a front elevational view of a tetrahedral machine tool mounting device as 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 shown in Figure 1; Figure 3 is a section, on a larger scale, through a workpiece mounting table for a machine tool shown in Figure 1 and 2; Figure 4 is a section, on a larger scale, through a toolholder shown in Figures 1 and 2; Figure 5 represents the geometrical structure of the mounting device shown in Figures 1 and 2; Figures 6, 7 and 8 represent geometrical structures according to the invention, with tetrahedral stiffening components; and Figures 9, 10, 11 and 12 are practical realisations of the structures of Figures 5, 6, 7 and 8. The tetrahedral mounting structure shown in Figures 1 and 2 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. The mounting interface is preferably replicated, as described and claimed in our co-pending Application No. (Reference No. 132386) having the same filing date as the present Application. 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 carrying an air bearing spindle having a rotor 15A and a stator 15B and a grinding tool 16, as shown in Figure 2 and (on a larger scale) in Figure 4. The stator has an extension 15C to provide an extended viscous damping workface 15D. Mounted on the base rod-like members Id, la and If is a vee slideway 17 bearing a workpiece holder 18 to impart linear motion to a workpiece 19 beneath the grinding tool 16. As shown in Figure 3, 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. On the areas between the dividing strip 18A 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. This vibration damping principle using viscous means can also, as described in published British Application No. 2194182A, be applied to the rod-like members la - If, which may contain viscous damping fluid; and the damping affect may be increased if damping interfaces are machined or built into or on, the components, which can be done without compromising structural static stiffness, i.e. the damping is essentially in parallel, rather than series, with the structural loop so the net result of adding the viscous damping can only be an increase of structure dynamic stiffness. 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 the chosen 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, 1s not needed; again avoiding tribological problems, and the inconsistent vibration damping which frequently results from stick-slip when the two surfaces are in close proximity or intermittent contact. The geometrical form of the known mounting device shown in Figures 1 and 2 is represented diagrammatically in Figure 5 as a regular tetrahedron whose edges represent the rod-like members la to If as shown in the corresponding Figure 9 which is a simplified perspective view of the mounting structure shown in Figures 1 and 2. The machine principal axis 13 is represented in both Figures 5 and 9 by a broken line referenced PA. In accordance with a first embodiment (not illustrated) of the present invention, the base of the structure, instead of being constituted by the three rod-like members Id, le, and If and the spheres 4a, 4b and 4c by which they are joined, is a solid metal plate or other structure to which the lower ends of the members la, lb and lc are rigidly coupled at spaced-apart locations. The plate or other structure may be free-standing, or may itself be an integral part of a separately existing article such as a machine on which, for example, ancillary equipment is mounted by means of the pyramidally disposed members la, lb and lc and mounting means secured (like the tool holder 9 shown Figures 1 and 2) to the members la, lb and lc in spaced relationship with the said plate or other structure. # Again, the pyramidally disposed members la, lb and lc, with their common spherical joint 4d, may be mounted on the plate or other structure not (or not solely) for the purpose of mounting ancillary equipment upon 1t but rather (or additionally) for imparting added stiffness or rigidity to the plate or structure. In a second (likewise non-illustrated) embodiment of the invention, the set of pyramidally-disposed rod-like members having their mutually remote ends rigidly coupled to the base are more than three in number, for example four or five, usually but not necessarily of equal length, arranged in the form of a four-edged or five-edged pyramid; and, as in the above-described first embodiment, the base to which the mutually remote ends of those members are secured may be either a plate or panel which may be free-standing or a component part of a separately- existing entity, or some other structure which may in this case be formed of a set of four or five rod-Hke members coupled end-to-end in a preferably planar four- or five-sided polygon (with or without the addition of additional rod-like members to provide a fully triangulated structure) and a further set of four or five rod-like members each having one end rigidly coupled at a common vertex to the corresponding ends of the others and having its other end (like the said other end of each of the said first set of rod-like members) rigidly coupled to a respective one of the corners of the said four- or five-sided polygon. A further embodiment of a mounting structure in accordance with the invention is represented diagra matically and in perspective outline in Figures 6 and 10 respectively, in which the pyramidally disposed members la, lb and lc are rigidly coupled to a base 22 which is itself in the form of an inverted regular tetrahedron and is constituted by co-planar rod-like members Id', le', and If coupled together by spherical joints 4a', 4b' and 4c', together with further rod-like members la 1 , lb' and lc' coupled together at their lower ends by a spherical joint 4d' and each coupled at Its upper end to a respective one, of the joints 4a', 4b' and 4c 1 . The base 22 is, in fact, a mirror Image of the structure shown in Figure 9 r and the details of Its construction may be identical with those shown in Figure 2. Since all the rod-like members shown in Figure 10 are of equal length, the base 22 provides a collinear extension PA' of the principal axis PA and increased working space within the structure in which mounting means (for Instance, like the tool holder 9 " and workpiece holder 17, 18 shown in Figures 1 and 2 ) may be secured in place on the members la, lb and lc or la', lb' and lc' or Id', le' and If for mounting tools and workpieces or co-operating components of other functional apparatus in a desired spaced relationship to one another along the extended principal axis. Axial symmetry, high resonant frequencies and other desired qualities remain uncompromised, as compared with the structure shown in Figures 1 and 2. Further lengthening of the extended principal axis can be provided, if required, by lengthening the members la, lb and lc and/or la', lb', and lc', within the limitation imposed by the need to maintain high resonant frequencies for these members. Another embodiment, not shown in the drawings but closely analogous to that represented, in Figures 6 and 10, has the overall form not of two tetrahedra back-to-back as in Figures 6 and 10 but of two square pyramids back-to-back, in that it comprises a first set of four rod-like members disposed pyramidally with one end of each rigidly coupled to the corresponding ends of each of the other three at a common vertex and the other end of each rigidly coupled to a base in the form of a square pyramid comprising eight rod-like members - four rigidly coupled end-to-end, to form a square at whose corners respective ones of the first set are mounted, and four more each having one end mounted at a respective one of those corners and having their other ends coupled together in a pyramidal structure which mirrors that of the first set of members. Such a structure is not fully triangulated (unless it also comprises a further member secured as a diagonal of the square) but nevertheless is a very favourable structure, with a large available working space for mounting tools or the like within its bounds, and provides all the advantages of a fully triangulated structure, virtually uncompromised. Structural symmetry may be almost entirely maintained, and other advantageous properties remain again largely uncompromised if, as in a further embodiment represented in Figures 7 and 11, a base 22' is provided which creates an auxilliary principal axis PA" intersecting the axis PA at an angle of 90° (as shown ) or at some other selected angle. The base 22' comprises, to that end, the members Id', le' and If and spherical joints 4a', 4b' and 4c' as shown in Figure 10, and also the members la 1 , lb' and lc' having one end of each coupled together by spherical joint 4d'; but in this embodiment, although the members la' and lc' are secured as before to the joints 4a' and 4c' respectively, they and the member lb' are swung together through 90° about the line of member If and the end of member lb' which is remote from the joint 4d' is rigidly located by a further spherical joint 23 and further rod-like members 24, 25 and 26 (similarly constructed) which are secured between the joint 23 and, respectively, the joints 4a 1 , 4b' and 4c'. This arrangement allows for mounting means to be secured in place, along the axis PA, on the members la, lb and lc and/or the members Id', le' and If and also, along the auxilliary axis PA", on the members la', lb' and lc' and/or the members If, 24 and 26. It will be seen that if the angle between the axes PA and PA" is to be 90° the member 25 will be of greater length than that of all the other rod-like members, and care is therefore needed to keep its resonant frequency near or equal to that of those other members. It will be seen that the base 22' comprises a non-regular first tetrahedron (having edges constituted by the members 24, 25 and 26 and vertices 4a', 4b' 4c' and 23 to three of which the members la, lb and lc are rigidly coupled) and, mounted over one of the lower triangular sides of this first tetrahedron, the pyramidal structure constituted by the members la', lb' and lc' and their common coupling joint 4d'. Other mounting structures in accordance with the invention may be designed for particular purposes; for example, each of the lower triangular sides of the tetrahedral base 22 shown in Figure 10 may have fitted over it a pyramidal structure like that constituted by the members la, lb, lc and 4d, and each of these will then enable mounting means to be fitted along respective ones of three auxilliary axes each intersecting the axis PA at an angle. Nor need those pyramidal structures be isosceles; if they are of unequal lengths, as represented in Figures 8 and 12, their common vertex may be offset, if that is required for some special purpose, from the principal axis PA as defined by the symmetrical case. It will be understood that it is not essential to employ mounting spheres for coupling together adjacent ends of the various rod-like members of a structure. A solid "hinge" comprising a weld contact or pin joint just sufficient to give static stiffness may be used instead. Advantageously, vibration damping may be introduced at mounting decoupling points, in accordance with our co-pending Application No. (Reference 132386). The use of mounting structures according to the invention is not limited to machine tools. They may be used in any application, such as surface metrology where a high degree of stiffness and rigidity is required. As mentioned earlier, a small degree of truncation of an apex formed where ends of rod-like members are coupled together is contemplated in some applications, for example, to facilitiate apical mounting of subsidiary structures. Referring again to the (non-illustrated) first embodiment of the invention described above as including a base in the form of a plate, it will be understood that such an embodiment may be the result of adding a pyramidal structure of the kind represented by rod-like members la, lb and lc and their coupling joint 4d as shown in Figures 1 and 2, for example, as an additive feature to another entity/structure to provide positional accuracy, dynamic stiffness, etc. for it or for a critical component mounted on the pyramidal addition, for example to isolate such component environmentally (vibration, distortion due to thermal effects, etc.) relative to the other entity/structure whilst retaining its position relative thereto. Such an additive feature might be provided, for instance, for the critical mounting of a cutting tool, sensor, or measuring device on a structurally or environmentally flawed existing entity or structure, or as a cost-effective (that is, avoiding the need to discard the whole flawed entity/structure) improvement to an existent entity/structure. An example of such an additive fixture would be the addition of three hemispherical mounting interfaces to an existent entity/structure, each of the three hemispherical interfaces then having secured to it an end of one of the three rod-like members of the pyramidal additive fixture. Such a three-point mounting enables a tool or other device to be mounted with minimum self-stressing at the mounting. The mounting points in reality have to be extended to finite areas, but the minimum departure from perfect interfacing will result if these surfaces are spherical, preferably pin-jointed as in the spherical joints shown in Figures 1 and 2 in order to minimise self-stressing. CLAIMS 1. A mounting structure which comprises a rigid base and a first set of at least three rod-like members disposed pyramidally with one end of each rigidly coupled to corresponding ends of the others at a common vertex and the other end of each rigidly coupled to the base, wherein the base is other than simply a triangular structure formed of a further set of three rod-like members. 2. A mounting structure as claimed in Claim 1 and Including mounting means comprising a rigid member fixedly coupled to the pyramidally-disposed rod-like members and in spaced relationship with the base. 3. A mounting structure as claimed in Claim 1 or Claim 2 wherein the said rod-like members are of substantially equal dimensions to one another. 4. A mounting structure as claimed in any of Claims 1 to 3, wherein the said rod-like members are coupled at their said one ends by means of a substantially spherical hollow member. 5. A mounting structure as claimed in Claim 4, wherein the hollow member comprises a pair of mating hemispheres. 6. A mounting structure as claimed in any of Claims 1 to 3, wherein the said rod-like members are coupled together by means of a welded contact. 7. A mounting structure as claimed in any of Claims 1 to 3, wherein the said rod-like members are coupled together by means of a pin joint. 8. A mounting structure as claimed in any one of the preceding claims wherein decoupling means is provided between the said rod-like members to inhibit the transmission of vibrations. 9. A mounting structure as claimed In any one of the preceding claims wherein susceptibility to vibration is decreased by maximising the fundamental frequencies of the components of the device by using low aspect ratio components. 10. A mounting structure as claimed in any one of the preceding claims wherein the said rod-like members are of two-part construction, each comprising a strut member and a tensile member. 11. A mounting structure as claimed 1n Claim 10 wherein each rod-Hke member includes a tubular member having a tensile rod therethrough. 12. A "mounting structure as claimed in Claim 11, wherein at least one of the tensile rods is provided with an adjusting screw system to tune the fundamental frequency of vibration of the corresponding tubular member. 13. A mounting structure as claimed in Claim 11, wherein at least one of said tubular members is provided with viscous damping material between the tubular member and the tensile rod. 14. A mounting structure as claimed in any of the preceding claims, wherein the base is a metal plate or other structure to which the said other ends of the pyramidally disposed rod-like members are rigidly coupled at spaced-apart locations.. 15. A mounting structure as claimed in Claim 14, wherein the said plate or other structure is part of a separately existing machine. 16. A mounting structure as claimed in any of Claims 1 to 13, wherein the base is a three-dimensional triangulated structure and each of the said rod-like members has its said other end rigidly coupled to a respective vertex thereof. 17. A mounting structure as claimed in any of Claims 1 to 15, wherein the rod-like members of the said first set are three 1n number. 18. A mounting structure as claimed In Claim 17, wherein the base 1s of tetrahedral form. 19. A mounting structure as claimed in Claim 18, wherein the base and the said first set of rod-like members together form a double tetrahedron.

Claims: 20. A mounting structure as claimed in Claim 17, wherein the base comprises a first tetrahedron, having one triangular side defined by three vertices to which the said other ends of the first set of three rod-Hke members are respectively coupled, and at least one further set of. three rod-Hke members pyramidally disposed and mounted over another triangular face of the tetrahedron. 21. A mounting structure as claimed in any of Claims 1 to 15, wherein the rod-like members of the said first set are four or five in number. 22. A mounting structure as claimed in Claim 21, wherein the base has the form of a square pyramid comprising eight rod-like members of which four are rigidly coupled end-to-end to form a square, at whose corners respective ones of the said first set of rod-like members are mounted, and four more rod-like members each having one end mounted at a respective one of those corners and having their other ends coupled together at a common pyramidal vertex. 23. A mounting structure as claimed in any of Claims 16 to 22, wherein the base is formed of rod-like members rigidly coupled together at their ends. 24. A mounting structure as claimed in Claim 23, wherein the rod-like members of the base and the means coupling their ends together are of similar construction to the said first set of rod-like members and the end-coupling means thereof. 25. A machine tool incorporating a mounting structure as claimed in any of the preceding claims. 26. A measuring instrument incorporating a mounting structure as claimed in any of Claims 1 to 25. 1/t SUBSTITUTE SHEET

Inventors: LINDSEY KEVIN, LINDSEY, KEVIN
Applicants: NAT RES DEV, NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1989
Main classification: B23Q1
IPC codes: B23Q1, B23Q3, F16M13