JPH09136286A - Positioning device and machine tool having three legs - Google Patents

Abstract

(57) [Abstract] [Problem] When a spherical bearing is used in the joint, the positioning accuracy is low, and the joint is easily disengaged when operating at high speed. Further, since the allowable swing angle is small, the moving area of the movable member is extremely limited. SOLUTION: Three legs are constituted by three pairs of link mechanisms 23 forming a quadrangle with two arms 20 and connecting members 21 and 22 arranged at one end and the other end of the arm 20, respectively. This is a machine tool that moves the connecting member 21 of 1 through the drive side member 25 to move the end plate 24 attached to the connecting member 22 at the other end in parallel in space. The connecting members 21 and 22 are a housing attached to the driving side member 25 or the end plate 24, a shaft rotatably supported by the housing, and an arm 20 that is orthogonal to the rotation center axis of the shaft. It is provided with a pair of connecting portions that respectively connect both end portions of the shaft and the end portion of the arm 20 so as to freely swing around the axis.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positioning device for moving a movable member in space by means of three legs. For example, a work (workpiece) is machined by rotating a spindle provided on the movable member. Related to machine tools.

[0002]

2. Description of the Related Art Industrial robots in which a plurality of robot arms are connected in series via joints are widely used. However, since this robot arm has a cantilever structure, it is necessary to move a moving part having a large mass. There is also a possibility that the error or play of the joint portion is enlarged and the positioning accuracy is lowered. On the other hand, in a machine tool such as a machining center, in order to relatively move a spindle and a workpiece, it is necessary to move a spindle head having a large mass, a column, a table and the like by a large driving force.

On the other hand, for example, Japanese Patent Publication No. 4-4531
Japanese Unexamined Patent Publication No. 0 discloses a positioning device which moves a movable member in a space by three sets of link mechanisms which form three legs and move three-dimensionally. The structure of this positioning device is called "parallel mechanism". Compared with conventional industrial robots and machine tools, the moving part has a light weight, so the driving force is small and it can move at high speed. There is. The technology related to this parallel mechanism is
JP-A-5-237732 and JP-A-5-50033.
No. 7, JP-A-6-190667 and US Pat. No. 4,976,582.

[0004]

If a spherical bearing is used for the joint when the three legs for moving the movable member in the space are constituted by three sets of quadrilateral link mechanisms, the error of the joint will occur. There is a problem that the joint is easily disengaged if the positioning accuracy is low and the robot is moved at high speed due to play.
Further, the load capacity in the thrust direction is only a fraction of the load capacity in the radial direction, and the clearance in the bearing portion is several times as large as that in the radial direction, and the value reaches 100 μm. Therefore, it was difficult to use a positioning device using a spherical bearing as a machine tool or an industrial robot that requires high rigidity and high accuracy (NIKKEI MECHANICAL (March 1995).
Issued on the 20th of a month)). Since the spherical bearing has a small allowable swing angle, the moving area of the movable member is significantly limited. In addition, since each link mechanism of the parallel mechanism moves three-dimensionally, it is difficult to control the parallel mechanism, and the creation of a machining program is sophisticated and complicated.

The present invention has been made to solve the above problems, has a joint structure with high positioning accuracy and high rigidity, can move a movable member in a wide moving area, and has three sets. An object of the present invention is to provide a positioning device and a machine tool in which control of a link mechanism is simplified.

[0006]

In order to achieve the above-mentioned object, a positioning device having three legs according to the present invention is arranged between two arms and one end and the other end of the arms, respectively. The three linking mechanisms that form a quadrangle with the connected connecting member form three legs, and the connecting member at the one end is attached to the connecting member at the other end by moving the connecting member via the drive side member. A device for parallelizing and positioning a movable member in a space, wherein the connecting member includes a housing attached to the drive side member or the movable member,
A shaft rotatably supported by the housing, and both ends of the shaft and an end of the arm so that the arm freely swings around an orthogonal axis substantially orthogonal to the rotation center axis of the shaft. And a pair of connecting portions for respectively connecting and.

In a preferred embodiment, a radial bearing for restricting radial movement of the shaft and pre-loading the shaft between the shaft and the housing, and a radial bearing for supporting the shaft. A thrust bearing that restricts the movement in the direction of the rotation center axis and applies a preload to axially support the shaft is mounted, and at the connecting portion, a pin is attached to the shaft end portion in the orthogonal axial direction. The spherical member on the outer peripheral surface fitted to the pin and the spherical inner peripheral surface of the engaging member attached to the end of the arm are brought into sliding contact with each other so that the engaging member can slide.

Further, a base end portion of the drive side member is swingably supported on the frame side, and a tip end portion is swingably connected to the connecting member at the one end, and meshes with a pinion rotated by a drive motor. A reciprocating rack is swingably connected to the tip of the driving side member, and the driving side member is pulled upward by an urging member with substantially the same urging force as gravity acting at the mounting position of the urging member. Is preferably provided. For example, the positioning device is a machine tool, and a spindle head having a spindle that is rotationally driven is provided on the movable member.

A preferred specific positioning device is a three-legged three-legged link mechanism having three arms of a quadrilateral structure with two arms and connecting members respectively arranged between one end and the other end of the arms. And moving the connecting member at the one end through the drive-side member, the movable member attached to the connecting member at the other end and provided with the spindle head is translated in space, A machine tool that rotates a spindle. The connecting member includes a housing attached to the drive-side member or the movable member, a shaft rotatably supported by the housing, and the arm orthogonal to the rotation center axis of the shaft. A pair of connecting portions that respectively connect both end portions of the shaft and the end portion of the arm are provided so as to freely swing around an axis. Between the shaft and the housing, a radial bearing that regulates the radial movement of the shaft and applies a preload to axially support the shaft, and a movement of the shaft in the rotation center axis direction are provided. A thrust bearing that regulates and applies a preload to axially support the shaft is mounted, and a pin is attached to the shaft end portion in the orthogonal axial direction at the connecting portion, and an outer peripheral surface spherical surface fitted to the pin is attached. -Shaped member and the spherical inner peripheral surface of the engagement member attached to the end of the arm are slidably contacted to each other so that the engagement member is slidable, and the base end of the drive-side member faces the frame. A rack that is swingably supported and has a tip end swingably connected to the connecting member at the one end, and a rack that reciprocates by meshing with a pinion that is rotated by a drive motor is swingable along the tip end of the drive side member. In a row Is, by the driving-side member is a tension spring, it is pulled upward at substantially the same biasing force of gravity acting on the attachment position of the tension spring.

[0010]

DETAILED DESCRIPTION OF THE INVENTION An example of an embodiment of the present invention will be described below with reference to FIGS. The positioning device having three legs according to the present invention can be applied to an industrial robot that performs an assembly operation, a transfer operation, or the like, or a machine tool that performs machining such as cutting of a work (workpiece),
In this embodiment, the case of a machine tool is shown. It should be noted that the positioning device only needs to have at least three legs composed of three sets of link mechanisms, and may have other legs separately.

FIG. 1 is a front structural view of a machine tool according to the present invention, and FIG. 2 is a schematic view for explaining the machine tool of FIG. As shown in FIG. 1, a vertical machine tool 1 as a positioning device includes four vertically installed pillars 2, a ceiling member 3 horizontally attached to an upper portion of the pillar 2, and a ceiling member 3. It has a table 5 which is located below and is horizontally attached to the column 2 for detachably mounting a work 4 or a pallet (not shown).

A space between the ceiling member 3 and the table 5 becomes a processing area 6, and most of the parallel mechanism mechanism 7 is arranged in the processing area 6. The processing area 6 is an enclosed space surrounded by a splash cover 8 to prevent chips and coolant from scattering. The upper part of the ceiling member 3 is a drive chamber 10 surrounded by a cover 9, and here, a drive motor 11 of the parallel mechanism mechanism 7 is provided.
The driving part such as is arranged.

As shown in FIGS. 1 and 2, in the parallel mechanism mechanism 7, two arms 20 and a connecting member 21 and the other end that are arranged between one end (for example, an upper end) of the arms 20 and are connected to each other. (For example, a parallelogram is preferable) and three sets of link mechanisms 23 that are arranged between (for example, lower ends) and are connected to each other.
The three legs that move the end plate 24 as a movable member three-dimensionally are configured. The connecting members 21 and 22 of this embodiment have substantially the same structure and dimensions, and form a portion corresponding to the short side of the parallelogram, but the connecting members 21 and 22 have different structures and dimensions. Thus, the link mechanism may have a trapezoidal or other quadrilateral shape.

Then, by moving the connecting member 21 at one end through the driving side member 25, the connecting member 2 at the other end is moved.
The end plate 24 attached to the No. 2 is moved in parallel and positioned in space while always maintaining a horizontal posture. The end plate 24 is formed in a hexagonal plate shape, and a spindle head 27 having a spindle 26 that is rotationally driven is attached to the center of the end plate 24. End plate 2
A tool 28 is attached to the main shaft 26 arranged downward from No. 4.
Is detachably mounted. The spindle head 27 has a spindle 2
A spindle motor 36 for rotating 6 is attached.

The pillar 2 and the ceiling member 3 constitute a frame of the machine tool 1, and a support portion 29 for projecting downward and supporting the parallel mechanism mechanism 7 is formed in the central portion of the ceiling member 3.
Are integrally formed. The support portion 29 integrally has three support members 30 facing downward, and these support members 30 are uniformly arranged in the circumferential direction. The base end portion 31 of the drive side member 25 is swingably supported by the frame side, that is, the support member 30, and the drive side member 2 is provided.
A tip portion 32 of 5 is swingably connected to the connecting member 21 at one end.

The rack 34 meshes with the pinion 33 that is rotated by the drive motor 11, reciprocates as shown by the arrow H, and is swingably connected to the drive-side member tip portion 32. The pinion 33 and the rack 34 constitute a booster mechanism. The driving-side member 25 is pulled upward by a tension spring 35 as a biasing member with a biasing force substantially the same as the gravity acting on the attachment position B of the tension spring 35. The link mechanism 23, the drive side member 25, and the rack 3 having the above configuration
4 and a pinion 3 that meshes with the rack 34
Three sets of unit mechanisms 38 configured by the drive motor 11 for rotating 3 are provided, and the end plates 24 supported by the lower end of the link mechanism 23 are horizontally moved in space by these three sets of unit mechanisms 38. Position.

The spindle motor 36 and the three drive motors 11
Are controlled by the control device 37.
The control device 37 controls the locus of the tool 28 and drives the spindle motor 36 by controlling each drive motor 11 according to a predetermined machining program to move and deform each link mechanism 23 of each motion part. The spindle 26 and the tool 28 are rotated at high speed.

FIG. 3 is a vertical cross-sectional view showing the lower portion of the link mechanism 23, and FIG. 4 is a plan view of the lower portion of the link mechanism 23, showing a part of it in section. FIG. 5 is a partial cross-sectional structural view showing the upper part of the unit mechanism 38, and FIG. 6 is a bottom view of the upper part of the unit mechanism 38 seen from below, and a part thereof is shown in cross section. 3, 4 and 6
As shown in FIG. 3, the connecting members 21 and 22 having substantially the same size and structure are the driving side member 25 or the end plate 2.
4, a housing 40 mounted on the housing 4, a shaft 41 rotatably supported by the housing 40, and an arm 23 so as to freely swing about an orthogonal axis D that is substantially orthogonal to the rotation center axis C of the shaft 41. , A pair of connecting portions 42 respectively connecting the both ends of the shaft 41 and the end 20a of the arm 20.

As shown in FIG. 3, between the shaft 41 and the housing 40, a needle bearing which is a radial bearing for restricting the radial movement of the shaft 41 and preloading the shaft 41 for axial support. 43 and a pair of ball bearings (or roller bearings) 44, which are thrust bearings that regulate the movement of the shaft 41 in the direction of the rotation center axis C and preload the shaft 41 to support the shaft 41. . The needle bearing 43 and the ball bearing 44 can be replaced with a single angular bearing. Since the needle bearing 43 itself is preloaded in advance, the bearing portion 43 has high accuracy with almost no error or play.

The shaft 41 includes a head portion 45 having a large diameter,
A cylindrical portion 46 extending from the head portion 45 in the direction of the central axis of rotation C to have a circular cross section, and a male screw 47 formed at the tip end portion;
It has a protrusion 48 that protrudes outward in the direction of the rotation center axis C from the head portion 45, and is integrally formed as a whole.
A forked portion 49 that opens outward in the direction of the rotation center axis C is formed at the tip end of the cylindrical portion 46 and the protruding portion 48, and the arm end portion 20a is connected to the forked portion 49. . It should be noted that the shaft 41 may have an assembly structure that can be divided into the left and right parts in FIG. 3 in order to facilitate the incorporation of the bearings 43 and 44 into the shaft 41, but the entire structure is integrated as in the present embodiment. It is preferable that the strength and length accuracy of the shaft 41 are improved.

A pair of ball bearings 44 for holding the housing 40 from both sides are strongly fixed by being sandwiched from both sides by a head portion 45 and a nut 50 screwed into a male screw 47. As a result, each ball bearing 44
Is preloaded from the outside in the direction of the rotation center axis C to eliminate the play, and the shaft 41 is accurately supported so as not to move in the direction of the rotation center axis C with respect to the housing 40 via the ball bearing 44. To be done. Since the needle 43a is mounted between the inner peripheral surface 51 of the needle bearing 43 and the outer peripheral surface 41a of the shaft 41 in a preloaded state, the shaft 4 is not attached to the housing 40.
1 is rotatably and accurately supported so as not to move in the radial direction.

A spherical inner peripheral surface 5 is provided in the arm end 20a.
The engaging member 54 having the number 5 is inserted and locked by the pin 58. The pins 52 are attached to the bifurcated portions 49 formed at both ends of the shaft 41 so as to extend in the orthogonal axis D direction. The outer peripheral surface spherical member 53 fitted to the pin 52 and the spherical inner peripheral surface 55 of the engaging member 54 of the arm end portion 20a are slidably in spherical contact with each other to make the engaging member 54 slidable. There is. That is, the forked portion 49 and the spherical member 53 are integrated via the pin 52, and the spherical movement of the engaging member 54 with respect to the spherical member 53 becomes possible, so that the engaging member 54 and the arm 20 are separated from each other. As shown by the arrow E in FIG. 4, it can relatively swing slightly. Therefore, even if there is a positional deviation between the shaft 41 and the arm 20, this is absorbed by the sliding movement of the arm 20 integrally with the engaging member 54 at the connecting portion 42, so that the link mechanism 23 deforms. Is done smoothly.

The spherical member 53 and the inner peripheral surface 5 of the engaging member
5 may be brought into direct contact with each other as described above, but a bush having lubricity formed of synthetic resin or the like may be used for both members 5
You may interpose between 3 and 54. Further, the connecting portion 42 is
If a ball bearing or the like that has a high rigidity and can be preloaded is used instead of the sliding contact, the resistance can be further reduced and the arm 20 can be swung smoothly.

The arm 20 connected to the connecting members 21 and 22 in this way swings about the central axis C of rotation and about the orthogonal axis D by the joint portion of the connecting members 21 and 22, so that two movements are performed. By combining them, they freely swing about the intersection P of both axes C and D, and the same three-dimensional movement as the spherical bearing can be realized. Moreover, since the rotation angle of the shaft 41 is unlimited and the swing angle with respect to the orthogonal axis D is large, the overall swing angle is large and the arm 20 can move in a wide area.

As shown in FIGS. 3 and 4, the connecting members 22 at the lower end of each link mechanism 23 are evenly arranged in the circumferential direction, and are positioned at predetermined positions on the end plate 24 by the bolts 57 and the pins 57a. It is fastened and fixed.
Note that, in FIG. 4, in order to clarify the structure of the connecting member 22, a case where the arm 20 takes a different posture for each connecting member 22 and a case where the cutting surface is different are shown.

As shown in FIGS. 5 and 6, the shaft 59
Is hung between the two support materials 30, and the support material 3
It is fixed to the bottom surface of 0 with a bolt 59a. The base end portion 31 of the T-shaped drive side member 25 is rotatably supported by the shaft 59. The arm portion 60 integrally extending outward from the base end portion 31 swings up and down around the base end portion 31 as shown by an arrow F. Tip 3 of arm 60
The housing 40 of the connecting member 21 at the upper end is fastened and fixed to the second member 2 by bolts 61. A rack 34 is swingably connected to the upper end of the arm tip 32 as shown by an arrow G. The rack 34 is swingable via a bearing 63 attached to the arm tip 32 and a pin 62 inserted in the bearing 63 and attached to the rack tip 34b.

A locking portion 67 for locking the lower end of the tension spring 35 is provided on the upper surface of the attachment position B in the middle of the arm 60, and the upper end of the tension spring 35 is provided on the ceiling member 3. It is locked to the locking portion 3a. The arm portion 60 is always pulled upward by the tension spring 35, and the tension spring 3
The spring force (biasing force) of 5 is approximately the same as the gravity acting on the locking portion 67. Accordingly, the load due to the gravity is not applied to the pinion 33, and the moving portion including the rack 34, the driving side member 25, and the link mechanism 23 can move at high speed with a small driving force.

A pinion 33 for moving the rack 34 back and forth.
A pressing mechanism 65 for constantly pressing the rack 34 to the pinion 33 side is disposed in the drive chamber 10 at the height position of. The pressing mechanism 65 contacts the flat surface 34a formed on the rear surface side of the rack 34 and freely rotates with the roller 6.
4, a shaft 69 that fits in the sleeve 68 so as to be movable back and forth in the axial direction to support the roller 64, and a compression spring 66 that biases the shaft 69 in the rack 34 direction as shown by an arrow J. There is. Since the roller 64 is constantly pressed to the rack 34 side via the shaft 69 by the urging force of the compression spring 66,
The rack 34, while being sandwiched between the pinion 33 and the roller 64, reliably moves back and forth as shown by an arrow H without being separated.

Next, the operation of this embodiment will be described. When a drive signal is output from the control device 37 to the three drive motors 11 and the spindle motor 36, each drive motor 11 rotates the pinion 33, and the spindle motor 36 rotates the spindle 26 at a high speed. Due to the rotational movement of the pinion 33, the rack 34 swings around the pin 62 as indicated by the arrow G and moves as indicated by the arrow H.

The arm 60 is moved by the arrow F by the operation of the rack 34.
Since it swings up and down around the base end portion 31 as described above, the link mechanism 23 moves three-dimensionally via the connecting member 21 at the upper end connected to the arm end portion 32. As a result, the end plate 24 is moved in parallel with high positioning accuracy while maintaining the horizontal posture, and the tool 28 rotated by the main shaft 26 accurately performs machining such as tapping or hole machining on the work 4.

In the case of the prior art in which the spherical bearing is used for the joint portion of the link mechanism, the positioning accuracy is poor and there is a possibility that the joint portion may come off when moving at high speed. On the other hand, in the present invention, since the shaft 41 is rotatably supported by the housing 40 via the bearings 43 and 44 and the arm 20 is connected via the connecting portion 42, the rigidity and motion of the link mechanism 23 are increased. The speed can be significantly improved. Also,
The movement of the shaft 41 in the radial direction and the direction of the rotation center axis C is regulated and preloaded in both directions to support the shaft, and the arm 20 is swingably supported around the orthogonal axis D via the connecting portion 42. There is. Therefore, there is no error or play in the joint portions of the connecting members 21 and 22, and the rigidity and accuracy of the joint structure of the link mechanism 23 is improved, so that the joint portions do not move even when the link mechanism 23 moves three-dimensionally at high speed. It does not come off, and the end plate 24 can be positioned with high accuracy.

Since the shaft 41 can freely rotate with respect to the housing 40 and the arm 20 can swing with respect to the shaft 41 at a large swing angle via the connecting portion 42, the movable area of the end plate 24 is You can move to any place in a wide moving area with almost no restrictions. Also, arm 2
Since 0 swings about the rotation center axis C of the shaft 41 and the orthogonal axis D that is substantially orthogonal to the rotation center axis C, the swing center of the arm 20 is always located at the intersection P of both axes C and D. Therefore, the control for moving the three sets of link mechanisms 23 is simplified, and the machining program is easily created.

Further, at the connecting portion 42, the arm 20 is slidably in contact with the shaft 41 on the spherical surface. Therefore, even if there is a manufacturing error in each component of the link mechanism 23, this error is caused by the spherical sliding contact portion. Will be absorbed and the link mechanism 23
Deforms smoothly while always maintaining a parallelogram with high accuracy. Further, since the spherical connection structure is adopted, the entire structure of the connecting portion 42 becomes compact.

As described above, since the rigidity and accuracy of the joint structure of the link mechanism 23 are improved, the present invention can be applied to a machine tool. However, assembling work, painting work, carrying work, measuring work, working work, etc. It can also be applied to industrial robots used in. For example, in the case of a transfer robot, a vacuum suction cup may be attached to the lower surface of the end plate 24 to suck and handle an object to be transferred. Further, if a touch sensor or a laser oscillator is attached to the end plate 24, it becomes a measuring instrument or laser processing machine for three-dimensional measurement, respectively. The same reference numerals in the drawings indicate the same or corresponding parts.

[0035]

Since the present invention is constructed as described above, the rigidity and positioning accuracy of the link mechanism can be improved. Further, the movable member can be moved in a wide moving area, and further, the control of the three sets of link mechanisms becomes simple.

[Brief description of the drawings]

1 to 6 are views showing an example of an embodiment of the present invention, and FIG. 1 is a front structural view of a machine tool.

FIG. 2 is a schematic diagram for explaining the machine tool of FIG.

FIG. 3 is a vertical cross-sectional view showing a lower portion of a link mechanism.

FIG. 4 is a plan view of a lower portion of the link mechanism.

FIG. 5 is a partial cross-sectional structural view showing the upper part of the unit mechanism.

FIG. 6 is a bottom view of the upper part of the unit mechanism as viewed from below.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Machine tool (positioning device) 3 Ceiling member (frame) 11 Drive motor 20 Arm 20a End part of arm 21 Connecting member at upper end (connecting member at one end) 22 Connecting member at lower end (connecting member at other end) 23 Link mechanism 24 End plate (movable member) 25 Drive side member 26 Spindle 27 Spindle head 31 Base end part 32 Tip part 33 Pinion 34 Rack 35 Extension spring (biasing member) 40 Housing 41 Shaft 42 Connecting part 43 Needle bearing (radial bearing) 44 Ball bearing (thrust bearing) 52 pin 53 outer peripheral surface spherical member 54 engaging member 55 spherical inner peripheral surface B mounting position of biasing member C rotation center axis D orthogonal axis

Claims (5)

[Claims] 1. Three legs are formed by three sets of quadrilateral link mechanisms, each of which is composed of two arms and a connecting member disposed between one end and the other end of the arm. In which the movable member attached to the connecting member at the other end is translated and positioned in space by moving the connecting member of the driving side member, wherein the connecting member is the driving side member. Alternatively, a housing attached to the movable member, a shaft rotatably supported by the housing, and the arm so as to freely swing around an orthogonal axis substantially orthogonal to the rotation center axis of the shaft, A positioning device having three legs, comprising a pair of connecting portions respectively connecting both ends of the shaft and an end of the arm. 2. A radial bearing for restricting radial movement of the shaft and pre-loading the shaft between the shaft and the housing, and a rotation center axis of the shaft. A thrust bearing that regulates the movement in the direction and applies a preload to axially support the shaft, and in the connecting portion, a pin is attached to the shaft end portion in the orthogonal axial direction and is fitted to the pin. 7. The engaging member is slidable by slidably contacting the formed spherical member on the outer peripheral surface with the spherical inner surface of the engaging member attached to the end of the arm. 1. A positioning device having three legs according to 1. 3. A base end portion of the drive side member is swingably supported on the frame side, and a tip end portion is swingably connected to the connecting member at the one end, and meshes with a pinion rotated by a drive motor. A rack that reciprocates is swingably connected to the tip of the drive-side member, and the drive-side member is pulled upward by a biasing member with a biasing force that is substantially the same as the gravity acting at the mounting position of the biasing member. A positioning device having three legs according to claim 1 or 2, wherein the positioning device has three legs. 4. The three legs according to claim 1, wherein the positioning device is a machine tool, and a spindle head having a spindle driven to rotate is provided on the movable member. Positioning device having. 5. Three legs are constituted by three sets of quadrilateral link mechanisms, each of which is composed of two arms and a connecting member disposed between one end and the other end of the arms, respectively. Machine tool for rotating the main shaft of the spindle head by moving the connecting member of the second embodiment through the drive side member to move in parallel the movable member attached to the connecting member at the other end and provided with the spindle head. The connecting member includes a housing attached to the drive-side member or the movable member, a shaft rotatably supported by the housing, and the arm substantially orthogonal to a rotation center axis of the shaft. A pair of connecting portions that respectively connect both ends of the shaft and an end of the arm so as to freely swing about the orthogonal axis, and the shuffling member is provided between the shaft and the housing. A radial bearing that regulates the radial movement of the shaft and preloads the shaft to support the shaft, and a radial bearing that regulates the movement of the shaft in the rotation center axis direction and preloads the shaft. A thrust bearing is mounted, and at the connecting portion, a pin is attached to the shaft end portion in the orthogonal axial direction, and an outer peripheral surface spherical member fitted to the pin and an end portion of the arm are attached. The spherical inner peripheral surface of the engaging member is slidably contacted with the engaging member so that the engaging member is slidable, the base end of the driving-side member is swingably supported on the frame side, and the distal end is A rack that is swingably connected to a connecting member at one end and that reciprocates by meshing with a pinion that is rotated by a drive motor is swingably connected to the leading end of the drive-side member. , This tension A machine tool having three legs, which is pulled upward by a biasing force that is substantially the same as the force of gravity acting on a mounting position of a spring.