Machine tool with movable positioning device

Abstract: A machine tool for machining a workpiece (11), the workpiece (11) being fastened to a support (9) which has its bearings in a frame (1) of the machine tool and is rotatable about an axis of rotation (13), while a tool (37) is movable relative to the workpiece (11) by means of a positioning device (35) provided on a carrier (17) of the machine tool. The carrier (17) is movably guided relative to the support (9) along a straight guide (23, 25) of the frame (1) in a direction parallel to the axis of rotation (13) by means of static fluid bearings (19, 21), so that a space easily accessible to an operator is created between the carrier (17) and the support (9) through displacement of the carrier (17). The carrier (17) can be coupled to the frame (1) by means of coupling members (51) provided with bolts (75), a head of each bolt (75) forming a piston (73) which can be moved in a cylinder (59), which is hinged to the carrier (17). In the operational condition of the machine tool, in which the workpiece (11) is machined with the tool (37), each bolt (75) is subjected to a tensile stress which is achieved through the supply of pressurized fluid to the cylinders (59), so that a particularly rigid coupling of the carrier (17) to the frame (1) is obtained. In particular, so-called masters for projection television screens in the form of circular Fresnel lenses of a comparatively large diameter are manufactured with these machine tools.

Description: The invention relates to a machine tool for machining a workpiece which is fastened to a rotatable support which has its bearings in a frame of the machine tool, which machine tool is provided with a positioning device for locating a tool fastened on the positioning device relative to the workpiece, the positioning device being provided on a carrier which is couplable to the frame by means of coupling members, wherein the tool is movable in at least an x-direction parallel to the support by means of the positioning device and the carrier is movable along a flat guide of the frame in a y-direction perpendicular to the x-direction and away from the support, and wherein the coupling members for coupling the carrier to the frame are provided with bolts which are couplable to fastening members. On conventional machine tools, and particularly on turning lathes, a tool such as a chisel is fastened to a transverse carriage which is movably guided along a straight guide of a longitudinal carriage in a direction transverse to a direction of a horizontal axis of rotation of the rotatable support of the workpiece. The longitudinal carriage of such a machine is movably guided along a straight guide of a frame of the machine. Such machine tools are in general only suitable for machining workpieces of a comparatively small diameter. For machining of workpieces having a comparatively large diameter, use is mostly made of machine tools fitted with a plate which is rotatable about a vertical axis of rotation for supporting the workpiece, a tool necessary for machining the workpiece being fastened to a positioning device provided at a portal-shaped construction positioned above the rotatable plate. Such machine tools provided with a horizontal plate for supporting the workpiece, however, have the disadvantage that chips formed during machining of the workpiece remain on the workpiece, which may adversely affect the quality of the workpiece. In addition, such a workpiece clamped on a horizontal plate can become deformed during clamping or removing under the influence of its own weight. Especially in the case of comparatively thin plate-shaped workpieces having a large diameter, such as, for example, masters for projection television screens in the form of so-called circular Fresnel lenses, these disadvantages can be avoided through the use of a machine tool having a plate rotatable about a horizontal axis of rotation for supporting the workpiece, the tool being movable relative to the workpiece by means of a positioning device attached to a frame of the relevant machine. Moreover, the machine operator's view of the workpiece in such a machine is generally better than in the case of a machine tool having a horizontal plate for supporting the workpiece. A machine tool of the kind mentioned in the opening paragraph and provided with a plate-shaped support for a workpiece rotatable about a horizontal axis of rotation is known from US 2,167,755. In this known machine tool the carrier for the positioning device is a bed which extends in a direction perpendicular to the support and which is slidable in said direction away from the support along elongated ways, which are provided on a base mounted centrally with respect to the support. This bed may be clamped in adjustable positions upon the base by means of clamping bolts having heads on their lower ends engaging within a T-slot extending longitudinally along the base. A disadvantage of this known machine tool is that the rigidity and positioning accuracy provided by said clamping bolts, which constitute the coupling members by means of which the carrier or bed is couplable to the frame or base, is limited, so that the accuracy of the known machine tool is limited. The invention has for its object to provide a machine tool of the kind mentioned in the opening paragraph in which the coupling members for coupling the carrier to the frame have an improved rigidity and positioning accuracy. According to the invention, the machine tool is for this purpose characterized in that each bolt is subjected to a tensile stress during operation after coupling of the carrier to the frame, which tensile stress is achieved by means of a piston which is movable by a fluid in a cylinder and is connected to the relevant bolt. By subjecting each bolt of the coupling members to a tensile stress achieved by means of a piston which is movable by a fluid in a cylinder and is connected to the relevant bolt, a particularly sturdy, effective and easily realisable coupling between the carrier and the frame of the machine tool is obtained, so that the positioning accuracy of the coupling and the machining accuracy of the machine tool are relatively high. A particular embodiment of a machine tool according to the invention which provides a simple construction of the coupling members for coupling the carrier to the frame is characterized in that each of the cylinders is attached to the carrier, while a head of each bolt constitutes the piston by means of which the tensile stress is applied to the relevant bolt during operation. A further embodiment of a machine tool according to the invention is characterized in that the cylinders are pivotably coupled to the carrier. The use of a pivotable coupling of the cylinders to the carrier prevents undesirable stresses which may occur in the coupling members by which the carrier can be coupled to the frame. A still further embodiment of a machine tool according to the invention is characterized in that the carrier and the frame are provided with mutually fitting curved support surfaces which are in pressure contact with one another after coupling of the carrier to the frame and which extend around the bolts. It is achieved in this way that the carrier with the positioning device provided on it can be brought into an accurate position relative to the frame and the workpiece support having its bearings therein while being coupled to the frame. In addition, the use of the said support surfaces achieves that the transmission to the frame of the machining forces exerted on the positioning device can take place over a comparatively large surface. Undesirable pressure concentrations are avoided in this way. A special embodiment of a machine tool according to the invention is characterized in that the carrier comprises a girder which extends parallel to the x-direction and is movably guided near its two ends along a flat guide of a first frame block and a flat guide of a second frame block, the frame blocks being situated at a distance from one another and extending substantially parallel to the y-direction. In this embodiment, a space between the support and the carrier, which is accessible to a machine operator after movement of the carrier away from the supportr, is provided by the spacing between said mutually spaced frame blocks. Since the space between the support and the carrier is accessible to the operator, the operator is able to enter said space so as to stand in front of the support, by which the mounting or removal of a workpiece having a relatively large diameter or a high weight is considerably facilitated. In addition, the two frame blocks provide a rigid and stable support for the carrier. A further embodiment of a machine tool according to the invention, which provides a substantially frictionless guiding of the carrier of the positioning device along the frame, is characterized in that the carrier is guided along the flat guide of the frame by means of a static fluid bearing. The invention will now be explained in more detail with reference to the drawing in which: Fig. 1 diagrammatically shows an embodiment of a machine tool according to the invention, Fig. 2 shows a cross-section of one of the coupling members by which the carrier of the machine tool shown in Fig. 1 can be coupled to the frame, the components being represented at a distance from one another, and Fig. 3 shows the components shown in Fig. 2 in a position in which the carrier is coupled to the frame. The embodiment of the machine tool shown in Fig. 1 comprises a frame 1 which is provided with two frame blocks 3 and 5 which are situated at a distance from one another and which extend substantially parallel to one another in a horizontal direction represented as the y-direction in Fig. 1. The two frame blocks 3 and 5 are interconnected at one end of each of these frame blocks by means of a cross beam 7 which extends in a horizontal x-direction perpendicular to the y-direction indicated in Fig. 1. A vertical, disc-shaped support 9 of a plate-shaped workpiece 11 has its bearings in the cross beam 7 of the frame 1. The support 9 is rotatable about an axis of rotation 13, which is substantially parallel to the y-direction, by driving means which are not shown. The workpiece 11 in that case is fastened to a side 15 of the support 9 facing away from the cross beam 7. A girder 17 (carrier) is provided at the side 15 of the support 9, which girder extends in the x-direction parallel to the cross beam 7. Near its two ends, the girder 17 is movably guided along a flat guide 23 of the frame block 3 and a flat guide 25 of the frame block 5 by means of static fluid bearings 19 and 21, respectively, the guides 23 and 25 being formed by upper surfaces of the frame blocks 3 and 5, which two upper surfaces are situated in a horizontal plane parallel to the x- and y-directions. The static fluid bearings 19, 21 are of a type which is known per se and are not indicated further in Fig. 1. The girder 17, as is shown diagrammatically in Fig. 1, is also provided with roller members 27 and 29 which bear on a straight guide rail 31 of the frame block 3 and a straight guide rail 33 of the frame block 5, respectively, the guide rails 31 and 33 extending parallel to the y-direction on the mutually facing lateral surfaces of the frame blocks 3 and 5. When the fluid bearings 19, 21 are operational, the girder 17 can be moved by hand by an operator of the machine tool with little force in the y-direction along the frame blocks 3 and 5, undesirable displacements in the x-direction being prevented by the guide rails 31 and 33 and the roller members 27 and 29. The girder 17 constitutes a carrier for a positioning device 35 by which a tool 37 fastened to the positioning device 35, such as, for example, a chisel, is movable relative to the workpiece 11. A horizontal upper surface 39 of the girder 17 parallel to the x- and y-directions in that case forms a base surface along which a longitudinal carriage 41 of the positioning device 35 is movable in the x-direction and along which a transverse carriage 43 of the positioning device 35 is movable relative to the longitudinal carriage 41 in the y-direction. The tool 37 is fastened to a ϕ-carriage 45 of the positioning device 35, which ϕ carriage is rotatably guided about an axis of rotation 47 indicated in Fig. 1 along the upper surface 39 of the girder 17 and along a circular cylindrical guide 49 of the transverse carriage 43. The means by which the longitudinal carriage 41, the transverse carriage 43, and the ϕ carriage 45 are supported, displaced and/or rotated are not important for the essence of the invention and are accordingly shown only diagrammatically in Fig. 1. The girder 17 is coupled to the cross beam 7 of the frame 1 by means of coupling members 51 in the operational position of the machine tool represented in Fig. 1. In this operational position, the workpiece 11 fastened to the support 9 can be machined by the tool 37 fastened to the positioning device 35 through a rotational movement of the workpiece 11 about the axis of rotation 13, a feed motion of the transverse carriage 43 in the y-direction, and a displacement of the longitudinal carriage 41 in the x-direction. As is shown in more detail in Figs. 2 and 3, the coupling members 51 each comprise a first block 53 attached to the girder 17 and comprising a circular cylindrical chamber 55 with a centreline 57 which is parallel to the y-direction. Inside the chamber 55 there is a cylinder 59 provided with a circular cylindrical shell 61 whose centreline 63 is situated in a plane transverse to the x-direction and intersects the centreline 57 of the chamber 55 in point of intersection 65, with a ring-shaped bottom 67 transverse to the centreline 63 of the shell 61, and with a head 69 which is hinged on the first block 53 by means of a pivot pin 71 which goes through the point of intersection 65 and extends in the x-direction. The chamber 55 surrounds the shell 61 with some clearance, so that the cylinder 59 in the situation represented in Fig. 2 can pivot through small angles about an axis which goes through the intersection point 65 and is parallel to the x-direction. A piston 73 is present in the cylinder 59. The piston 73 is formed by a head of a bolt 75, while a rod 77 of the piston 73 is formed by a shaft of the bolt 75, whose centreline coincides with the centreline 63 of the cylinder shell 61. The rod 77 is provided with screw thread 79 at its free end. At the side of the first block 53 facing the cross beam 7, where the rod 77 projects from the chamber 55 of the block 53, this block 53 is bounded by a first curved support surface 81 of the block 53, which support surface 81 forms a boundary of a spherical surface whose centre is situated on the centreline 63 of the cylinder shell 61. As is shown in Figs. 2 and 3, furthermore, the coupling members 51 each comprise a second block 83 attached to the cross beam 7 of the frame 1. A stepped bore is provided in the second block 83, whose centreline coincides with the centreline 57 of the chamber 55 of the first block 53. The said stepped bore comprises two bore portions 85 and 87 which are interconnected by a bore portion 89 whose diameter is smaller than the diameters of the bore portions 85 and 87. Inside the bore portion 87, which is situated near a side of the second block 83 facing the girder 17, a support piece 91 is attached. The boundary surface of the support piece 91 facing the girder 17 forms a second curved support surface 93 whose shape is complementary to that of the first support surface 81 of the first block 53. The support piece 91 is further provided with a bore 95 which is situated in the extended direction of the bore portion 89 of the second block 83 and whose diameter equals the diameter of the bore portion 89. In the operational position of the machine tool mentioned above, in which the girder 17 is coupled to the cross beam 7 by means of the coupling members 51, the various parts of the coupling members 51 occupy positions as represented in Fig. 3, a nut 97 being tightened by hand on the end of the rod 77 provided with screw thread 79 and present in the bore portion 85 of the second block 83. In the operational position, a pressurized fluid will be supplied to the cylinder 59 between the piston 73 and the bottom 67 in order to maintain a considerable tensile stress in the rod 77, so that a particularly rigid and strong coupling of the girder 17 to the cross beam 7 of the frame 1 is achieved. Undesirable stresses in the parts of the coupling members 51 are avoided through the use of the said hinged connection between the cylinder 59 and the girder 17. The two mutually fitting curved support surfaces 81 and 93 are in pressure contact with one another in the operational position. The support surfaces 81, 93 are manufactured accurately and are provided in an accurate position relative to the girder 17 and the cross beam 7, respectively. Thus, the girder 17 with the positioning device 35 provided on it is arranged in an accurate position relative to the cross beam 7 of the frame 1 with the support 9 of the workpiece 11 journalled in it, which renders possible an accurate positioning of the tool 37 relative to the workpiece 11 by means of the positioning device 35. If an operator of the machine tool wants to fasten a workpiece to the support 9 or wants to remove a workpiece from the support 9, the fluid present between the pistons 73 and the bottoms 67 in the cylinders 59 of the coupling members 51 is first depressurized, upon which the nuts 97 are unscrewed from the rods 77. Then the static fluid bearings 19 and 21 are activated, after which the girder 17 with the positioning device 35 is moved along the flat guides 23 and 25 of the frame blocks 3 and 5 by hand in a direction away from the support 9 over such a distance that the space thus created between the girder 17 and the support 9 is accessible to the operator for fastening the workpiece to or removing it from the support 9. When the girder 17 is at a certain distance from the support 9, the static fluid bearings 19 and 21 can be de-activated, so that then the girder 17 will rest squarely on the frame blocks 3 and 5. In this position of the girder 17 relative to the support 9, the operator of the machine tool is also capable of fastening a tool to or removing it from the positioning device 35 in a simple manner. To make the machine tool operational again after a workpiece has been fastened to the support 9 or a tool to the positioning device 35, the operator of the machine tool will again activate the static fluid bearings 19 and 21, and move the girder 17 towards the support 9 in such a way that the rods 77 of the coupling members 51 return to the position shown in Fig. 3. The nuts 77 are then screwed on the ends of the rods 77 again by hand, after which pressurized fluid is supplied to the cylinders 59 of the coupling members 51 between the pistons 73 and the bottoms 67. The rods 77 are tensioned again in this way, so that a rigid and accurate coupling of the girder 17 to the cross beam 7 of the frame 1 is realised. The machine tool described above comprises a vertically arranged disc-shaped support on which a plate-shaped workpiece is clamped. More particularly, so-called masters for projection television screens in the form of circular Fresnel lenses are manufactured with these machine tools by means of a cutting operation. It should be noted that other embodiments of machine tools are also possible, provided with a carrier on which a positioning device for a tool is present and which is movable relative to the workpiece, while a space accessible to an operator of the machine tool is created through displacement of the carrier relative to the workpiece. Thus, the machine tool may be provided with, for instance, a drum-shaped support rotatable about a horizontal axis of rotation with a flexible, plate-shaped workpiece clamped around this support, the carrier of the positioning device being displaceable along a straight guide of the frame in a direction transverse to the said axis of rotation. With such a machine tool, for example, masters for linear Fresnel lenses may be manufactured through a cutting operation of the tool. It should further be noted that the carrier of the positioning device may also be guided along the frame in a conventional manner such as, for example, by means of roller members attached to the carrier and guide rails attached to the frame blocks. If these types of guidance are used, an operator of the machine tool will have to exert a greater force on the carrier when displacing the latter than in the case of static fluid bearings. It should be noted that the static fluid bearings by which the carrier of the positioning device is guided along the frame blocks may also be operational in the operational condition of the machine tool described above. Coupling of the carrier to the frame in this condition takes place exclusively through the said coupling members, so that an optimal positioning accuracy is achieved.

Claims: A machine tool for machining a workpiece (11) which is fastened to a rotatable support (9) which has its bearings in a frame (1) of the machine tool, which machine tool is provided with a positioning device (35) for locating a tool (37) fastened on the positioning device (35) relative to the workpiece (11), the positioning device (35) being provided on a carrier (17) which is couplable to the frame (1) by means of coupling members (51), wherein the tool (37) is movable in at least an x-direction parallel to the support (9) by means of the positioning device (35) and the cater (17) is movable along a flat guide (23, 25) of the frame (1) in a y-direction perpendicular to the x-direction and away from the support (9), and wherein the coupling members (51) for coupling the carrier (17) to the frame (1) are provided wit bolts (75) which are couplable to fastening members (83, 91), characterized in that each bolt (75) is subjected to a tensile stress during operation after coupling of the carrier (17) to the frame (1), which tensile stress is achieved by means of a piston (73) which is movable by a fluid in a cylinder (59) and is connected to the relevant bolt (75). A machine tool as claimed in Claim 1, characterized in that each of the cylinders (59) is attached to the carrier (17), while a head of each bolt (75) constitutes the piston (73) by means of which the tensile stress is applied to the relevant bolt (75) during operation. A machine tool as claimed in Claim 1 or 2, characterized in that the cylinders (59) are pivotably coupled to the carrier (17). A machine tool as claimed in Claim 1, 2 or 3, characterized in that the carrier (17) and the frame (1) are provided with mutually fitting curved support surfaces (81, 93) which are in pressure contact with one another after coupling of the carrier (17) to the frame (1) and which extend around the bolts (75). A machine tool as claimed in Claim 1, 2, 3 or 4, characterized in that the carrier comprises a girder (17) which extends parallel to the x-direction and is movably guided near its two ends along a flat guide (23) of a first frame block (3) and a flat guide (25) of a second frame block (5), the frame blocks (3, 5) being situated at a distance from one another and extending substantially parallel to the y-direction. A machine tool as claimed in Claim 1, 2, 3, 4 or 5, characterized in that the carrier (17) is guided along the flat guide (23, 25) of the frame (1) by means of a static fluid bearing (19, 21).

Inventors: DONA MARINUS JOSEPHUS JAKOBUS, HAGEN JOHANNES LUDOVICUS MARIA, DONA, MARINUS JOSEPHUS JAKOBUS, HAGEN, JOHANNES LUDOVICUS MARIA
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: B23Q1
IPC codes: B23B21, B23Q1, B23B3