Clamping device for machine tools

Abstract: The invention provides a clamping device for machine tools capable of preventing the occurrence of impressions. In a clamping device for machine tools, a tailstock (movable carriage) is provided on a top face of a fixed bed and the tailstock is positioned and fixed with respect to the fixed bed. The tailstock is supported by linear-motion bearings (guides) which are disposed between the tailstock and the fixed bed. Blocks are slidably engaged with rails extending in the travel direction with rollers interposed between them. Further, a recess portion extending in the travel direction is formed in the top surface of the fixed bed. A cylinder mechanism is arranged so that left and right inner walls in the recess portion are pressed by a pair of pistons provided on the undersurface of a headstock.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clamping device for machine tools in which a movable carriage, provided linearly movable on a fixed bed, is positioned and fixed. 2. Discussion of the Related Art In a lathe, for example, that has a headstock secured onto a fixed bed and a movable tailstock, a workpiece is held by a spindle of the tailstock and a chuck of the headstock. Thus, the machining process is carried out by rotating the workpiece. In this case, to prevent undesirable movement of the workpiece, the tailstock must be securely positioned and fixed to the fixed bed with a clamping device. A conventional clamping device is shown in Fig. 1. In this clamping device, a tailstock 50 is set on a fixed bed 51 with their sliding surfaces 51a, 50a in sliding contact with each other. A piston member 53b is disposed in a hydraulic chamber 53a formed in a pinching plate 53 of the tailstock 50. The piston member 53b is elevated with oil pressure. A clamping force F is supplied to the hydraulic chamber 53a so that a ceiling portion 51b of the fixed bed 51 is pinched between the piston member 53b and the tailstock 50. Japanese Utility Model Laid-Open Publication No. 2570872 discloses a clamping device in which a protrusion formed at the undersurface of a movable table is pinched by a pair of piezoelectric elements provided on a fixed base. A structure could be envisioned in which the tailstock 50 is movably supported by linear-motion bearings 55, as shown in Fig. 2, instead of the structure in which the tailstock 50 slides on the fixed bed 51. These linear-motion bearings 55 are generally structured, so that rails 56 fixed to the fixed bed 51 and blocks 57 fixed to the tailstock 50 are engaged with each other with rollers (not shown) interposed in between. In this way, frictional resistance can be reduced. However, if this type of linear-motion bearing is used in the aforementioned conventional clamping device, there is a possibility that the rails 56 and the blocks 57 may suffer impressions. This occurs as a result of the rollers causing a reaction force f equivalent to the clamping force F acting directly on the linear-motion bearings 55. In addition, when using linear-motion bearings, the clamping force is inferior in proportion to a decrement of friction coefficient, which causes further problems. In the structure of the aforementioned publication, the table is pinched between piezoelectric elements provided on the base, with a longer travel distance of the table. Therefore it is necessary to set multiple piezoelectric elements in a number corresponding to the travel distance, which disadvantageously increases the number of component parts. From the document DE-A-3347221 there is disclosed a clamping device having: A moveable carriage on a fixed bed; a guide; a (strip shaped)-plate disposed on the fixed bed and extending in a travel direction of said moveable carriage; and two pistons of the cylinder mechanism oriented opposed to each other and acting on the plate interposed there between. From the Document US-A-4681506 there is disclosed a clamping device for machine tools comprising: A moveable carriage on a top face of a fixed bed so that said moveable carriage can be positioned and fixed to said fixed bed; a guide disposed between said moveable carriage and said fixed bed, said guide extending in a direction in which said moveable carriage travels so that said moveable carriage is supported by said guide; a recess portion formed in the top surface of said fixed bed and extending in a travel direction of said moveable carriage; and a cylinder mechanism disposed on said moveable carriage, said cylinder mechanism having at least two pistons oriented in opposite directions so as to be respectively apposed to left and right inner walls of said recess portion so that said inner walls are pressed by said pistons when said pistons are extended. In the clamping device of the last cited document there is no equilibrium of the forced acting on the carriage; further at the same time it is space consuming since the pistons do not act on opposite walls of the recess and the bed. SUMMARY OF THE INVENTION It is an object of the present invention to provide a clamping device as in the last cited document without these shortcomings. This problem is solved in that the cylinder mechanism is disposed at an undersurface of said moveable carriage and by following features: a moveable rack fixed to at least one of the pistons opposed to one of the left and right inner walls; and a fixed rack attached to one of said inner walls, wherein said fixed rack is opposed to and engageable with said moveable rack. Preferably the clamping device is further comprising a plurality of cylinder mechanisms disposed on the undersurface of the moveable carriage. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partial cross sectional view showing a problem that is overcome by the present invention; Fig. 2 is a partial cross sectional view showing a conventional clamping device; Fig. 3 is a right side view of a clamping device of a slant bed type lathe according to an embodiment of the present invention; Fig. 4 is a sectional side view showing an unclamped state of the clamping device according to the present invention; Fig. 5 is a sectional plan view showing a clamped state of the clamping device according to the present invention; Fig. 6 is a sectional view of linear-motion bearings in the embodiment according to the present invention; Fig. 7 is a schematic perspective view of the lathe; and Fig. 8 is a right side view showing a clamping device according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are described in further detail with reference to the accompanying drawings. Figs. 3 to 7 are views for explaining a clamping device for a slant bed type lathe (machine tool) according to an embodiment of the invention. In these figures, identical or corresponding elements are designated by like reference numerals. Referring to Figs. 3 to 7, a slant bed type lathe generally has a structure such that a tailstock 3 is right-and-left movably set on the right side of the slant type bed 2, a headstock 4 is fixed on the left side of bed 2, and a tool post 5 is back-and-forth, right-and-left movably set behind the center line between the headstock 4 and the tailstock 3. A chuck 6 for grasping the workpiece is fitted to the headstock 4, and a tailstock spindle 7 for holding the workpiece between itself and the chuck 6 is fitted to the tailstock 3. In this tailstock 3, a tailstock body 3b is mounted on a base 3a, and the tailstock spindle 7 is back-and-forth movably fitted to the tailstock body 3b. As shown in Fig. 7, a turret head 8 for holding a plurality of tools K is rotatably fitted to the tool post 5. A turret indexing unit (not shown) for indexing the turret head 8 to make a required tool selectively usable is provided inside the turret head 8 and tool post 5. The tailstock 3 is guided and supported by two linear-motion bearings 10 so as to be movable along the direction of the spindle. These linear-motion bearings 10, which are disposed on the top face 2a of the fixed bed 2, each comprises a rail 11 extending in parallel to the travel direction and two blocks 12 disposed on the bottom face of the tailstock 3 and slidably fitted to the rail 11. As shown in Fig. 6, each rail 11 has a rod formed in a rectangular shape in cross section and is tightly secured to the fixed bed 2 with a bolt 15 inserted through the central part of the rail 11. Also, each block 12 is generally an inverted U-shape in cross section and is in rolling contact with the rail 11 with a large number of rollers 13 interposed between them. Each block 12 is tightly secured to four corners of the undersurface of the base 3a of the tailstock 3 with bolts 16. In addition, balls may be interposed in place of the rollers 13. As shown in Figs. 3 and 5, a clamping device 20 is provided between the tailstock 3 and the fixed bed 2. This clamping device 20 comprises one or more cylinder mechanisms 24 set within a recess portion 21 recessed between the two rails 11 of the fixed bed 2. In this example, two cylinder mechanisms 24 are present. The clamping device 20 also comprises a rack mechanism 45 which is driven into engagement and disengagement by the cylinder mechanisms 24. The recess portion 21 comprises left and right inner walls 21a, 21b and a bottom wall 21c, as viewed from the right side of the lathe 1, and is formed over the right-and-left overall length of the fixed bed 2. The cylinder mechanisms 24 have a structure such that first and second pistons 22, 23 are provided in cylinder holes 27b, 27b formed in a cylinder member 27 so that the first and second pistons 22, 23 are advanceable and retractable alternately in opposite directions. Rod portions 28a of the first and second pistons 22, 23 are projected outward from the cylinder holes 27b, respectively. In each of the cylinder members 27, the cylinder hole 27b is bored through a rectangular parallelepiped cylinder body 27a. Lid plates 29, 30 are tightened to both end faces of the cylinder body 27a with a plurality of bolts 31, respectively. The rod portion 28a is projected from an opening 29a in one side of lid plate 29. The cylinder member 27 is tightly fixed to the undersurface of the base 3a of the tailstock 3 with a plurality of bolts 25. Further, oil seals 32 are provided at the inner circumferential surface and both end faces of the cylinder hold 27b, and at the inner circumferential surface of the opening 29a of the lid plate 29, respectively. The cylinder hole 27b is internally divided into first and second hydraulic chambers 34, 35 by the first and second pistons 22, 23, respectively. The first and second pistons 22, 23 are advanced by oil pressure supplied into the first hydraulic chamber 34 and retracted by oil pressure supplied into the second hydraulic chamber 35. The first and second pistons 22, 23 are set so as to be movable back-and-forth along a direction parallel to the top face 2a of the fixed bed 2 and perpendicular to the travel direction of the tailstock 3. The first piston 22 is opposed to the right inner wall 21b within the recess portion 21, and the second piston 23 is opposed to the left inner wall 21a on the opposite side. Thus, the first and second pistons 22, 23 are in pressure contact with the left and right inner walls 21a, 21b in a mutually pressing fashion. In this way, the cylinder member 27 and the tailstock 3 are positioned and fixed at a specified position. A movable rack 36 is provided so as to be stretched to the rod portion 28a of the first piston 22 and tightly fixed with bolts 37. Also, a fixed rack 38 having a length corresponding to the travel distance of the tailstock 3 is disposed on the right inner wall 21b of the recess portion 21. This fixed rack 38 is securely tightened with bolts 39 to a bracket 38a securely bolted to the right inner wall 21b. Further, the movable rack 36 is opposed to and engageable with the fixed rack 38. Next, the operation and effects of this embodiment are explained below. In the lathe 1 of this embodiment, the tailstock 3 is positioned and fixed. The workpiece is held by the chuck 6 of the headstock 4 and the tailstock spindle 7 of the tailstock 3. In this manner, specific machining processes are carried out with the tool while the workpiece is being rotated. In order to position and fix the tailstock 3, the fixing position of the tailstock 3 is determined, then the first and second pistons 22, 23 are advanced. The movable rack 36 fixed to the first piston 22 is engaged with the fixed rack 38 while the second piston 23 presses the left inner wall 21a of the recess portion 21. Thus, the cylinder member 27 and the tailstock 3 are positioned and fixed. According to the present embodiment, the cylinder member 27 is fixed to the undersurface of the tailstock 3, and then the tailstock 3 is positioned and fixed by pressing the left and right inner walls 21a, 21b of the recess portion 21 of the fixed bed 2 by means of the first and second pistons 22, 23 provided within the cylinder member 27. The resulting clamping force acts only on the left and right inner walls 21a, 21b of the fixed bed 2, and the clamping force never acts on the linear-motion bearings 10. As a result, the occurrence of any impressions on the rails 11, 11 and the blocks 12 is prevented, and there is no lowering of the clamping force. This makes the linear-motion bearings 10 particularly practical, and frictional resistance can be reduced. Further, since the cylinder member 27 for holding the first and second pistons 22, 23 is fixed to the tailstock 3, it is only necessary to dispose a small number of pistons regardless of the travel distance of the tailstock 3. Therefore, the number of pistons can be reduced, and thus the cost for component parts can also be reduced, as compared with the use of piezoelectric elements disposed on the base side. In this embodiment, since the movable rack 36 is fixed to the first piston 22, and the fixed rack 38 is fixed to the right inner wall 21b of the recess portion 21, the clamping force of the tailstock 3 can be enhanced with a simple structure and wearing out of the workpiece can be prevented with higher reliability. Further, since the first and second pistons 22, 23 are placed in parallel to each other and alternately in opposite directions, the pressing force to the fixed bed 2 can be exerted with good balance. Moreover, the placement space within the recess portion 21 can be ensured with ease. Fig. 8 is a view for explaining a clamping device according to another embodiment of the invention. In the figure, the same reference numerals as in Fig. 3 designate like or equivalent component parts. In the clamping device 40 of this embodiment, a strip shaped plate 41 having a length corresponding to the travel length of the tailstock 3 is provided in the recess portion 21 of the fixed bed 2 and secured with a bolt 41a. Further, a housing 43 is fixed to the undersurface of the tailstock 3. A pair of pads 42, 42 are provided opposite to each other within the housing 43 with the plate 41 interposed between them. Further, pistons 44 for pressing the individual pads 42 using oil pressure are provided within the housing 43. The pads 42, when released from being pressed, are returned by return springs (not shown). According to this embodiment, since the plate 41 fixed to the fixed bed 2 is pinched by a pair of pistons 44 set on the undersurface of the tailstock 3, the clamping force acts only on the plate 41 and never acts on the linear-motion bearings 10. Thus, in this case also, effects generally similar to those of the foregoing embodiment can be obtained. For the clamping device 40, it is also possible to use, for example, commercially available disc brake parts for vehicles, in order to lower costs. Although the above embodiments have been described with respect to the case where the tailstock of the lathe is clamped, the clamping device of the present invention is not limited to this. The invention is also applicable for clamping steady rest, movable tables, pallets and the like. Also, although the tailstock is guided and supported by linear-motion bearings in the above embodiments, the present invention is not limited to this arrangement. For example, the present invention is also applicable to slide guides implemented by bringing into sliding contact the sliding surfaces of the fixed bed and the tailstock into sliding contact. In this case also, the clamping force does not act on the slide guide surface and thus does not result in undesirable indentations on the guide.

Claims: A clamping device for machine tools comprising: a moveable carriage provided on a top face of a fixed bed (2) so that said movable carriage can be positioned and fixed to said fixed bed (2); a guide disposed between said moveable carriage and said fixed bed (2), said guide extending in a direction in which said moveable carriage travels so that said moveable carriage is supported by said guide; a recess portion (21) formed in the top face of said fixed bed and extending in a travel direction of said moveable carriage; and a cylinder mechanism (24) disposed at said moveable carriage, said cylinder mechanism (24) having at least two pistons (22, 23) oriented in opposite directions so as to be respectively opposed to left and right inner walls (21a, 21b) of said recess portion (21) so that said inner walls are pressed by said pistons (22, 23) when said pistons (22, 23) are extended, characterized in that the cylinder mechanism (24) is disposed at an undersurface of said movable carriage and by following features: a moveable rack (36) fixed to at least one of the pistons (22, 23) opposed to one of the left and right inner walls (21a, 21b); and a fixed rack (38) attached to one of said inner walls (21a, 21b), wherein said fixed rack (38) is opposed to and engageable with said moveable rack (36). A clamping device for machine tools as claimed in claim 1, further comprising a plurality of cylinder mechanisms (24) disposed on the undersurface of the moveable carriage.

Inventors: NAKAMINAMI MASAMITSU, SAKASHITA YOSHINORI, NAKAMINAMI, MASAMITSU, SAKASHITA, YOSHINORI, Nakaminami, Masamitsu, c/o Mori Seiki Co., Ltd., Sakashita, Yoshinori, c/o Mori Seiki Co., Ltd.
Applicants: MORI SEIKI SEISAKUSHO KK, MORI SEIKI CO., LTD.
Application year: 2000
Main classification: B23Q1
IPC codes: B23Q1