Abstract:
The main spindle device for a machine tool is provided with: a bearing for rotatably supporting the main spindle; a bearing case for holding the bearing; a housing inside which the bearing case is disposed; and a stiffness-switching device, which supports the bearing case with respect to the housing and is able to switch the stiffness of the bearing case support between multiple levels. When the stiffness-switching device is supporting the bearing case at a low stiffness, the main spindle device is formed so that the bearing case slides with respect to the housing. A friction member is disposed on the area where the bearing case slides with respect to the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase patent application of International Patent Application No. PCT/JP2013/067916, filed Jun. 28, 2013, which is hereby incorporated by reference in the present disclosure in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a spindle device of a machine tool, and the machine tool. 
       BACKGROUND ART 
       [0003]    There has hitherto been known a machine tool which carries out processing such as cutting by moving a tool relative to a workpiece. The machine tool comprises a spindle device including a spindle to which the tool is fitted and a motor or the like for rotating the spindle. The spindle device is generally formed so that the tool can be replaced, and a plurality of processes can be carried out by tool replacement. 
         [0004]    Patent Literature 1 discloses a peripheral velocity switching device which can change a preload of a bearing of the spindle, and switches a bearing set of the spindle according to a rotational speed of the spindle. As disclosed, this switching device applies a preload to a large bearing set disposed in front of the spindle during low-speed rotation. When a switch piston is moved by hydraulic pressure, the preload is applied to the large bearing set. On the other hand, during high-speed rotation, the large bearing set is in a state of no preload application. As disclosed, a tapered part is formed at an end of the switch piston, and a preload is absorbed when the preload of a pressure fluid is excessively large. 
         [0005]    Patent Literature 2 discloses a spindle device which includes a diaphragm attached to an outer peripheral surface of a rear bearing case for supporting a spindle and an inner wall of a spindle head. The diaphragm supports the rear bearing case on an inner wall of a housing of the spindle head. As disclosed, the diaphragm supports the spindle while being elastically deformed in an axial direction but supports the spindle with high rigidity in a radial direction. 
       PATENT LITERATURE 
       [0006]    Patent Literature 1: Japanese Utility Model Registration No. 3076394 
         [0007]    Patent Literature 2: Japanese Examined Patent Publication (Kokoku) No. H07-37811 
       SUMMARY OF THE INVENTION 
       [0008]    In general, processing accuracy of a machine tool can be improved by increasing the support rigidity for supporting a spindle. For example, when a workpiece is cut, the cutting can be carried out according to a desired amount. Alternatively, deeper cutting which increases the amount to be cut at once can be carried out. The spindle device is manufactured by setting the support rigidity so as to mainly satisfy the requirement of the deeper cutting. For example, the spindle device is designed to satisfy support rigidity required when a tool having a large diameter is fitted. 
         [0009]    Depending on a type of tool, the tool may vibrate during processing. In other words, a reaction force applied to the tool which is currently processing a workpiece may cause the tool to vibrate. When the tool vibrates, processing accuracy of a processed surface of the workpiece is reduced, and processing efficiency is also reduced. In particular, when processing such as cutting is carried out by using a tool having a small diameter or a tool having a long length, vibration occurs more easily. For example, the processed surface may be finished by the tool having the small diameter. When the tool having the small diameter is fitted to the spindle device to carry out processing, rigidity of the tool is less than the support rigidity of the spindle of the spindle device, and thus the tool may vibrate. For example, chattering vibration may occur. As result, processing accuracy (processed surface quality) or processing efficiency is reduced in some cases, for example, chattering marks may be formed on the processed surface. 
         [0010]    Thus, in general, it is necessary to increase the support rigidity of the spindle in order to improve the processing accuracy of the deeper cutting. However, when the support rigidity of the spindle is enlarged, damping performance of the vibration of the spindle is lowered. The vibration sometimes occurs when the tool has small diameter or the like. To suppress the vibration generated during the processing carried out by the tool having the small diameter, a damping function for damping the vibration may be provided to the tool. However, in this case, a special tool is required to be used, resulting in a normal tool that may not be used. 
         [0011]    A spindle device of a machine tool according to the present invention comprises a spindle to which a tool is fitted, a bearing for rotatably supporting the spindle, a bearing case for holding the bearing, a housing in which the spindle, the bearing, and the bearing case are arranged, a rigidity switching device supporting the bearing case on the housing and capable of switching support rigidity for supporting the bearing case at a plurality of stages of rigidity including low rigidity and high rigidity higher than the low rigidity, and a vibration damping device disposed between the housing and the bearing case and actuated when the rigidity switching device supports the bearing case with the low rigidity. 
         [0012]    According to the invention described above, the vibration damping device can include a friction member disposed in a contact part of the housing and the bearing case. 
         [0013]    According to the invention described above, the rigidity switching device can be formed to move the bearing case along a rotational axis of the spindle in the housing, the bearing case includes a tapered part in which a diameter of an outer surface is gradually reduced, and the rigidity switching device can support the bearing case with the high rigidity by moving the bearing case to press the tapered part to the housing, and support the bearing case with the low rigidity by providing the tapered part with a gap. 
         [0014]    According to the invention described above, the spindle device can further comprise a plate-like member supporting the bearing case on the housing and having a thickness direction set nearly parallel to the rotational axis of the spindle, and the bearing case can have an end of a side on which the tool is disposed and which is supported by the rigidity switching device, and an end of a side opposite to the side on which the tool is disposed and which is supported by the plate-like member. 
         [0015]    According to the invention described above, the spindle device can further comprise a displacement sensor for detecting displacement of the spindle with respect to the housing. 
         [0016]    According to the invention described above, the rigidity switching device normally supports the bearing case with the high rigidity, and can be switched to support the bearing case with the low rigidity when a predesignated tool is fitted to the spindle. 
         [0017]    According to the invention described above, the rigidity switching device can support the bearing case with the low rigidity when a diameter of the tool fitted to the spindle is smaller than a preset value, and support the bearing case with the high rigidity when the diameter of the tool fitted to the spindle is equal to or larger than the preset value. 
         [0018]    A machine tool according to the present invention comprises the aforementioned spindle device, and when the rigidity switching device switches the support rigidity of the bearing case to the low rigidity, a tool length offset value of a NC device is rewritten only by a value equivalent to a moving amount of the bearing case along the rotational axis of the spindle. 
         [0019]    The present invention can provide a spindle device of a machine tool which can prevent vibration of a small-diameter tool or the like and improve processing accuracy (processed surface quality) for a workpiece and processing efficiency, and the machine tool. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a schematic partial sectional view of a spindle device according to an embodiment. 
           [0021]      FIG. 2  is a schematic enlarged sectional view of a bearing case portion of the spindle device according to the embodiment. 
           [0022]      FIG. 3  is another schematic enlarged sectional view of the bearing case portion of the spindle device according to the embodiment. 
           [0023]      FIG. 4  is a diagram of a stability limit line of a spindle device according to comparative example. 
           [0024]      FIG. 5  is a diagram of a stability limit line of the spindle device according to the embodiment. 
           [0025]      FIG. 6  is a schematic diagram illustrating a machine tool, a CAD device, and a CAM device according to the embodiment. 
           [0026]      FIG. 7  is a schematic enlarged sectional view of a bearing case portion of another spindle device according to the embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring to  FIGS. 1 to 7 , a spindle device of a machine tool according to an embodiment of the present invention will be described. The spindle device which rotates a spindle having a tool fitted thereto can be disposed in any machine tool. For example, the spindle device can be disposed in a machine tool such as a horizontal or vertical machining center, a milling machine, or a boring machine. 
         [0028]      FIG. 1  is a schematic partial sectional view of the spindle device according to the embodiment. The spindle device includes a spindle  1  for supporting a tool  2 , and a housing in which the spindle  1  is disposed. The housing according to the embodiment includes a spindle head housing  10 , a front housing  11 , and a front end housing  12 . 
         [0029]    The spindle device according to the embodiment includes a built-in type rotary machine. A stator  7  is disposed on an inner surface of the spindle head housing  10 . A rotor  6  is disposed on an outer surface of the spindle  1 . The stator  7  and the rotor  6  constitute the rotary machine. The spindle  1  is formed into a cylindrical shape, and rotated in the spindle head housing  10 . 
         [0030]    In the embodiment, in an extending direction of a rotational axis  101  of the spindle  1 , a side on which the tool  2  is fixed is referred to as a front side, and a side which is opposite to the fixed side of the tool  2  is referred to as a rear side. The extending direction of the rotational axis  101  of the spindle  1  is referred to as a Z-axis direction. An end of the front side of the spindle  1  is supported by the front housing  11  via a bearing, namely a front bearing, and bearing cases  14  and  15  for holding the front bearing. 
         [0031]    The front bearing supports the spindle  1  which is rotatable around the rotational axis  101 . The front bearing according to the embodiment includes a pair of angular bearings  21  arranged back to back, and a roller bearing  22 . The roller bearing  22  according to the embodiment includes a cylindrical roller. The angular bearing  21  provides, even when a force is applied to the spindle  1  in the extending direction of the rotational axis  101 , a force to counter this force. The angular bearing  21  has a function of suppressing movement of the spindle  1  in the extending direction of the rotational axis  101 . 
         [0032]    The bearing cases  14  and  15  are formed to cover the angular bearings  21  and the roller bearing  22 . The bearing case  14  is disposed between the front bearing and the front housing  11 . The bearing case  14  and the bearing case  15  according to the embodiment are fixed to each other. The bearing case  14  and the bearing case  15  are formed by separate members. However, it is not limited to the above-described embodiment, and the bearing case  14  and the bearing case  15  may be integrally formed by one member. The front bearing and the bearing cases  14  and  15  are arranged in the housing. 
         [0033]    An end of the rear side of the spindle  1  is supported by the spindle head housing  10  via a rear bearing and a bearing case  17  supporting the rear bearing. The rear bearing supports the spindle  1  to be rotatable around the rotational axis  101 . The rear bearing according to the embodiment includes a roller bearing  23 . The roller bearing  23  according to the embodiment includes a cylindrical roller. An outer ring of the roller bearing  23  is pressed toward the bearing case  17  via a bearing pressing member  18 . This roller bearing  23  is supported by the spindle head housing  10  via the bearing case  17 . 
         [0034]    A nut  30  is fixed to the end of the rear side of the spindle  1 . By fastening the nut  30 , an inner ring of the roller bearing  23  is fixed to the spindle  1  via a sleeve  25  and collars  26  and  27 . In addition, by fastening the nut  30 , the inner ring of the roller bearing  23  bites into a tapered part of the spindle  1  to apply a preload to the roller bearing  23 . 
         [0035]    In the spindle  1 , a draw bar  42  is disposed to extend along the rotational axis  101 . The draw bar  42  is formed into a bar shape. A disc spring  43  is disposed around the draw bar  42 . A nut  44  is disposed at an end of a rear side of the disc spring  43 . The nut  44  is fixed to the draw bar  42 . 
         [0036]    The spindle  1  includes a collet  45  engaged with the draw bar  42 . The draw bar  42  is urged to a rear side indicated by an arrow  111  by the disc spring  43  to be engaged with the collet  45 . At this time, the collet  45  is set in a state where an end of a front side is open. The collet  45  is engaged with a tool holder  3  to hold the tool holder  3 . On the other hand, the collet  45  is formed to be disengaged from the tool holder  3  when the draw bar  42  is pressed toward the front side. 
         [0037]    The spindle device includes a hydraulic cylinder  5  for pressing the draw bar  42  to the front side. When the hydraulic cylinder  5  is driven to press the draw bar  42 , fixing of the tool holder  3  is released. The tool  2  is fixed to the tool holder  3 . The tool  2  can be replaced together with the tool holder  3 . 
         [0038]    When a workpiece is processed, the pressing of the draw bar  42  by the hydraulic cylinder  5  is released. The tool holder  3  is fixed by an urging force of the disc spring  43  to a tapered hole of the front side of the spindle  1 . Thus, during the processing of the workpiece, a state where the tool  2  is fixed to the spindle  1  is set. The tool  2  rotates or moves integrally with the spindle  1 . 
         [0039]      FIG. 2  is a schematic enlarged sectional view of the front bearing and the bearing case portion. Referring to  FIGS. 1 and 2 , the outer rings of the angular bearing  21  and the roller bearing  22  are supported by the bearing cases  14  and  15 . A collar  36  is disposed between the angular bearing  21  and the roller bearing  22 . 
         [0040]    The bearing case  14  includes a cylindrical part  14   b  formed to be nearly constant in thickness along the rotational axis  101 . In the embodiment, a gap part  35  having a slight gap between a surface of an outer side of the cylindrical part  14   b  and an inner surface of the front housing  11  is formed. Height of the gap part  35  is preferably, for example, about several tens of μm in a diameter direction of the spindle  1 . In other words, the surface of the outer side of the cylindrical part  14   b  and the inner surface of the front housing  11  are preferably separated from each other by several tens of μm. 
         [0041]    The bearing case  14  includes a piston part  14   c  at the end of the front side. The bearing case  14  includes a tapered part  14   a  formed between the piston part  14   c  and the cylindrical part  14   b . The tapered part  14   a  has its outer diameter gradually reduced toward the rear side. The front housing  11  includes a tapered hole part  11   a  formed in a portion facing the tapered part  14   a . The tapered hole part  11   a  has its inner diameter gradually reduced toward the rear side. The tapered hole part  11   a  according to the embodiment is formed to come into surface-contact with the tapered part  14   a  when the tapered part  14   a  is pressed, and simultaneously the piston part  14   c  comes into surface-contact with the front end of the front housing  11 . Thus, the bearing case  14  is bound two sides. 
         [0042]    The ends of the rear sides of the bearing cases  14  and  15  are supported by a diaphragm  31 . The diaphragm  31  is a plate-like member formed into a plate shape by a metal. The diaphragm  31  has its thickness direction which is set nearly parallel to a direction of the rotational axis  101  of the spindle  1 . The diaphragm  31  has its end of an outer side in a diameter direction fixed to the front housing  11 , and its end of an inner side in the diameter direction fixed to the bearing case  15 . 
         [0043]    A nut  29  is disposed in front of the roller bearing  22  via a collar  37 . A sleeve  24  is disposed behind the angular bearing  21 . By fastening the nut  29  to the spindle  1 , the roller bearing  22  is pressed via the collar  37 . An inner ring of the roller bearing  22  bites into the tapered part of the spindle  1  to apply a preload to the roller bearing  22 . Further, by fastening the nut  29 , an inner ring of the angular bearing  21  of the front side is pressed to the rear side via the collar  36 . This pressing force is transmitted to an outer ring of the angular bearing  21  of the front side, an outer ring of the angular bearing  21  of the rear side, and an inner ring of the angular bearing  21  of the rear side in this order. Accordingly, a preload is also applied to the angular bearing  21 . 
         [0044]    A seal member  16  is disposed in front of the nut  29 . The seal member  16  suppresses flowing-out of lubricating oil supplied to the front bearing, and prevents entry of chips into the spindle device. The seal member  16  constitutes, for example, a labyrinth seal having concave and convex patterns formed on its surface. 
         [0045]    The spindle device according to the embodiment includes a moving device for moving the bearing cases  14  and  15  in the direction of the rotational axis  101  of the spindle  1  in the housing. An oil chamber  63  is formed in front of the piston part  14   c  of the bearing case  14 . An oil path  61  is formed in the front end housing  12 . The oil path  61  communicates with the oil chamber  63 . Eight equally distributed oil chambers  64  are formed on a front end surface of the front housing  11  of the rear side of the piston part  14   c . An oil path  62  is formed in the front housing  11 . The oil path  62  communicates with the oil chamber  64 . 
         [0046]    The moving device according to the embodiment includes a hydraulic device  8  for supplying high-pressure control oil to the oil chambers  63  and  64 . By supplying the high-pressure control oil to one of the oil chambers  63  and  64  from the hydraulic device  8 , the piston part  14   c  can be moved along the extending direction of the rotational axis  101 . In other words, the moving device can move the beating cases  14  and  15  along the direction of the rotational axis  101  of the spindle  1 . 
         [0047]    The spindle device according to the embodiment includes a vibration damping device for accelerating damping of vibrations generated in the bearing case  14  and the spindle  1 . The vibration damping device is provided between the housing and the bearing case  14 . The vibration damping device according to the embodiment includes a friction member  65  fixed to the front end housing  12 . The friction member  65  is provided in a contact part of the housing and the bearing case  14 . The friction member  65  is disposed to face an end surface of the front side of the bearing case  14 . The friction member  65  according to the embodiment is formed into a plate shape. The friction member  65  has its thickness direction set nearly parallel to the direction of the rotational axis  101 . 
         [0048]    The spindle device according to the embodiment includes a rigidity switching device capable of switching, at a plurality of stages, support rigidity for supporting the bearing cases  14  and  15  on the housing. The rigidity switching device according to the embodiment supports the bearing cases  14  and  15 , at two stages, low rigidity and high rigidity higher than the low rigidity. The rigidity switching device includes a moving device, and switches rigidity by moving the bearing cases  14  and  15 . 
         [0049]    In the state illustrated in  FIG. 2 , the spindle  1  is supported with high rigidity. The hydraulic device  8  supplies high-pressure control oil to the oil chamber  63  as indicated by an arrow  112 . From the oil chamber  64 , the stored control oil flows out as indicated by an arrow  113 . The piston part  14   c  of the bearing case  14  is pressed by hydraulic pressure of the control oil supplied to the oil chamber  63 . The bearing cases  14  and  15  are pressed to the rear side as indicated by an arrow  114 . 
         [0050]    The tapered part  14   a  of the bearing case  14  closely contacts to the tapered hole part  11   a  of the front housing  11  and, simultaneously, the rear side of the piston part  14   c  of the bearing case  14  closely contacts to the front end surface of the front housing  11 . Accordingly, the ends of the front sides of the bearing cases  14  and  15  are bound in a diameter direction indicated by an arrow  116  and an axial direction indicated by the arrow  114 . The ends of the rear sides of the bearing cases  14  and  15  are bound by the diaphragm  31  in the diameter direction. The spindle  1  has the ends of the front sides bound in the diameter direction and in the axial direction in addition to the ends of the rear sides of the bearing cases  14  and  15 , and thus the bearing cases  14  and  15  can be supported on the housing with high rigidity. 
         [0051]      FIG. 3  is another schematic enlarged sectional view of the portions of the front bearing and the bearing case of the spindle device according to the embodiment. In a state illustrated in  FIG. 3 , the spindle  1  is supported with low rigidity. The hydraulic device  8  supplies high-pressure control oil to the oil chamber  64  as indicated by an arrow  112 . The control oil stored in the oil chamber  63  flows out as indicated by an arrow  113 . As a result, the bearing cases  14  and  15  move along the rotational axis  101  in a direction indicated by an arrow  115 . In other words, the bearing cases  14  and  15  move to the front side. The tapered part  14   a  of the bearing case  14  moves away from the tapered hole part  11   a  of the front housing  11 . Moving amounts of the bearing cases  14  and  15  in this case are preferably small, about several hundreds of μm. The front end surface of the piston part  14   c  of the bearing case  14  comes into contact with the friction member  65 . By the movement of the bearing case  14  to the front side, slight gaps are formed between the tapered part  14   a  and the tapered hole part  11   a  and between the piston part  14   c  and the front surface of the front housing  11 . The gap between the tapered part  14   a  and the tapered hole part  11   a  is, for example, several tens of μm. 
         [0052]    The bearing cases  14  and  15  support the angular bearing  21  and the roller bearing  22 , and the angular bearing  21  and the roller bearing  22  support the spindle  1 . Accordingly, the movements of the bearing cases  14  and  15  cause the movement of the spindle  1  in the direction indicated by the arrow  115 . The seal member  16  protrudes from a surface of the front end housing  12 . This moving amount pz of the spindle  1  is, for example, about several hundreds of μm as described above. 
         [0053]    When the bearing cases  14  and  15  move to the front side, the diaphragm  31  is slightly deformed. The ends of the rear sides of the bearing cases  14  and  15  are bound by the diaphragm  31  in the diameter direction. However, the end of the front side of the bearing case  14  is set in a state of not receiving strong binding. Thus, the bearing cases  14  and  15  can be supported on the housing with low rigidity. In other words, the spindle  1  can be supported on the housing with low rigidity. 
         [0054]    Therefore, by moving the bearing cases  14  and  15 , the support rigidity of the spindle  1  can be changed. In the rear bearing for supporting the end of the rear side of the spindle  1 , referring to  FIG. 1 , the inner ring slides with respect to the roller of the roller bearing  23  to move the spindle  1 . 
         [0055]    A normal state of the spindle device according to the embodiment is the state of the high rigidity illustrated in  FIG. 2 , and employed for many processing operations. In particular, since the spindle  1  is supported with the high support rigidity, the state illustrated in  FIG. 2  is suitable when the tool  2  of the large diameter is used, when deeper cutting having a large cutting-in amount is carried out, or the like. The high support rigidity can be achieved by pressing the tapered part  14   a  and the piston part  14   c  of the bearing case  14  into contact with the front housing  11 . As a result, processing can be carried out with high accuracy. 
         [0056]    On the other hand, when processing such as cutting is carried out by using the tool  2  of the small diameter, contact between the tool  2  and a processed surface may generate vibration in the tool  2 . In such a case, the spindle device is set in the state of low rigidity illustrated in  FIG. 3 . In this state, the ends of the rear sides of the bearing cases  14  and  15  are strongly bound by the diaphragm  31  in the diameter direction. However, the ends of the front sides of the bearing cases  14  and  15  can vibrate with slight amplitude in the diameter direction indicated by the arrow  116  since the ends are not bound strongly. 
         [0057]    Small vibration generated in the tool  2  during the processing is transmitted to the spindle  1 . The small vibration of the spindle  1  causes sliding of the end surface of the piston part  14   c  of the bearing case  14  with the friction member  65 . Accordingly, the vibration of the spindle  1  can be damped by friction. As a result, the vibration generated in the tool  2  is suppressed to enable processing with high accuracy. In particular, when the tool  2  of the small diameter is used, the processing can be carried out while suppressing chattering vibration. This processing is suitable when finish processing is carried out by using the tool  2  of the small diameter or the like. Thus, the vibration damping device according to the embodiment is actuated when the rigidity switching device supports the bearing case  14  with low rigidity. 
         [0058]    The fiction member  65  for suppressing the vibration of the spindle  1  preferably employs a member which smoothly slides with the bearing case  14  even with micro vibration. For example, the friction member  65  is preferably made of a material having a small difference between a static friction coefficient and a dynamic friction coefficient. Alternatively, the friction member  65  is preferably a low friction sliding member. 
         [0059]    The friction member  65  according to the embodiment is made of a fluorine resin. The fluorine resin is a material having a static friction coefficient set larger by about 1.0 to 1.1 times than a dynamic friction coefficient. Accordingly, even with vibration of very small amplitude, the friction member  65  and the bearing case  14  smoothly slide with each other. Vibration energy can be effectively converted into thermal energy. Even when chattering vibration occurs, the vibration can be effectively damped by friction. Further, in the spindle device according to the embodiment, the oil chamber  63  is formed in front of the piston part  14   c . Accordingly, the control oil also moves to a surface of the friction member  65 , and a viscous damping effect of the control oil can be provided. 
         [0060]    The fiction member  65  according to the embodiment is fixed to the front end housing  12 . However, it is not limited to the above-described embodiment, and the friction member  65  can be disposed in a portion where the bearing case slides with respect to the housing. For example, the fiction member may be disposed in the bearing case. 
         [0061]    The diaphragm  31  is preferably formed so that rigidity in the diameter direction can be sufficiently large. In addition, preferably, an elastic force of the diaphragm  31  in the extending direction of the rotational axis  101  is appropriately large. Such a diaphragm  31  can be made of metal. 
         [0062]      FIG. 4  is a diagram illustrating a stability limit line when a tool of a small diameter is fitted to a spindle device according to comparative example to carry out processing simulation. The spindle device according to comparative example does not include the rigidity switching device of the embodiment, but always supports a spindle with high support rigidity.  FIG. 5  is a diagram illustrating a stability limit line when a tool of a small diameter is fitted to the spindle device according to the embodiment to carry out processing simulation.  FIG. 5  illustrates a state where the rigidity switching device supports the spindle with low support rigidity. A horizontal axis indicates a rotational speed of the spindle, while a vertical axis indicates a cutting-in amount of the tool. A result when an end mill of a small diameter is used as the tool is illustrated. 
         [0063]    It can be understood from  FIG. 4  that when the tool of the small diameter is used in the state of supporting the spindle with the high rigidity, chattering vibration occurs, and a cutting-in amount indicating a lower limit of the stability limit line is very small. On the other hand, it can be understood from  FIG. 5  that by supporting the spindle with low rigidity and using the friction member to reduce vibration via friction damping, a cutting-in amount indicating a lower limit of the stability limit line is larger than that of the comparative example. 
         [0064]    In the spindle device according to the embodiment, even in a state where tool vibration easily occurs, a cutting-in amount can be increased, and stable processing can be carried out. In other words, the spindle device according to the embodiment can improve processing accuracy (processed surface quality) and processing performance. A reason for this may be that the reduction of the support rigidity of the spindle enables vibration of the spindle in the same vibration mode as that of the tool vibration, and the sliding of the friction member enables damping of the vibration. Thus, the spindle device according to the embodiment can suppress vibration generated in the tool itself. In particular, vibration generated when a tool having a small diameter, a tool having a long tool-length, a tool in which vibration easily occurs, or the like is used can be effectively damped. In addition, by switching the support rigidity to the high rigidity, even when the tool of the large diameter is used or the like, processing can be carried out with high processing accuracy and processing performance. 
         [0065]    An example of the tool may be a tool which carries out processing while rotating, specifically an end mill. An example of a tool diameter in which vibration such as chattering vibration occurs is 10 mm or less. For example, when an end mill having a tool diameter of 10 mm or less is used, the spindle  1  is preferably supported with low support rigidity. When an end mill having a tool diameter larger than 10 mm is used, the spindle  1  is preferably supported with high support rigidity. 
         [0066]    Referring to  FIGS. 2 and 3 , a gap is formed between the seal member  16  and the front end housing  12 . In a region where this gap is formed, a displacement sensor  66  is disposed in the front end housing  12 . The displacement sensor  66  detects displacement of the spindle  1  with respect to the housing in a Z-axis direction. A gap is formed between the front end housing  12  and the nut  29 . In a region where this gap is formed, a displacement sensor  67  is disposed in the front end housing  12 . The displacement sensor  67  detects displacement of the spindle  1  with respect to the housing in a Y-axis direction. 
         [0067]    Further, in an X-axis direction (direction vertical to paper surface) orthogonal to the Y axis and the Z axis, a displacement sensor for detecting displacement of the spindle  1  with respect to the housing is disposed. The displacement sensor for the X-axis direction is disposed at a position rotated by 90° around the rotational axis  101  with respect to the displacement sensor  67  for the Y-axis direction. Thus, the spindle device according to the embodiment includes a plurality of sensors for detecting displacement when the spindle  1  moves with respect to the housing. 
         [0068]    The displacement sensor  66  for detecting displacement in the Z-axis direction can detect, when the bearing cases  14  and  15  have been moved by the hydraulic device  8 , whether the bearing cases have moved by desired amounts. In addition, the moving amounts pz of the bearing cases  14  and  15  can be corrected based on an output from the displacement sensor  66 . When the output from the displacement sensor  66  deviates from a predetermined range during processing of the workpiece, it is judged that abnormality of the support rigidity of the spindle  1  occurs. In this case, control for stopping the processing of the workpiece can be carried out. Further, for example, control for displaying an alarm on an operation panel can be carried out. 
         [0069]    In addition, on the basis of an output from the displacement sensor  67  for detecting displacement in the Y-axis direction and an output from the displacement sensor for detecting displacement in the X-axis direction, displacement or a vibration state of the spindle  1  during the processing of the workpiece can be detected. For example, when amplitude of vibration deviated from a permissible range in the X-axis direction or the Y-axis direction is detected during the processing of the workpiece, control for stopping the processing can be carried out. In addition, an alarm for notifying abnormality can be displayed on the operation panel. 
         [0070]    By providing the displacement sensor for detecting the displacement of the spindle with respect to the housing in this manner, abnormality of the moving device can be quickly detected, and continuance of the processing in the lowered state of the processing accuracy can be avoided. 
         [0071]    In the bearing case  14  according to the embodiment, the end of the side on which the tool is disposed is supported by the rigidity switching device. In the bearing case  14 , the end of the side opposite to the side on which the tool is disposed is supported by the diaphragm  31 . By employing this configuration, when the support rigidity is low, the spindle  1  can be effectively vibrated. In addition, by a support using the diaphragm  31 , slight movement can be permitted in the direction of the rotational axis  101  of the bearing case  14  while supporting the bearing case  14  with high support rigidity in the diameter direction. 
         [0072]    The diaphragm  31  is preferably disposed with a predetermined distance from the tapered part  14   a . This predetermined distance enables the spindle  1  to be supported at two positions separate from each other in the axial direction. The spindle  1  can be supported more reliably with high support rigidity. In order to effectively vibrate the spindle, the diaphragm  31  is preferably disposed at a position separate from the friction member  65 . 
         [0073]    Referring to  FIG. 3 , when the tool having the small diameter is used or the like, the spindle  1  is moved by a moving amount pz in the Z-axis direction to carry out processing. Accordingly, a processing error of the moving amount pz may occur on the processed surface of the workpiece. Next, control carried out by the machine tool for correcting the processing error of the moving amount pz during the processing of the workpiece will be described. 
         [0074]      FIG. 6  is a schematic diagram illustrating a workpiece processing system according to the embodiment. The processing system according to the embodiment includes a machine tool  85  of a numerical control type, and a device for generating input numerical data  84  which is input to the machine tool  85 . A shape of the workpiece is designed by a CAD (computer aided design) device  81 . The CAD device  81  supplies workpiece shape data  82  to a CAM (computer aided manufacturing) device  83 . 
         [0075]    The machine tool  85  includes a control device for controlling processing, movement of the workpiece, or the like. The control device according to the embodiment includes a NC (Numerical Control) device  86 . The CAM device  83  generates input numerical data  84  which is input to the NC device  86  based on the shape data  82 . The input numerical data  84  includes, for example, data indicating a path of a tool tip point when a predetermined tool  2  is used. The input numerical data  84  includes, for example, coordinate values of X, Y and Z axes. 
         [0076]    The NC device  86  of the machine tool  85  drives each axis servo motor  87  based on the input numerical data  84 . Each axis servo motor  87  includes an X-axis servo motor, a Y-axis servo motor, a Z-axis servo motor or the like. Accordingly, the tool  2  can be moved relatively to the workpiece. 
         [0077]    The input numerical data  84  which is input to the NC device  86  includes a tool diameter. The NC device  86  according to the embodiment determines whether the tool diameter of the tool  2  is less than a predetermined value. When the tool diameter of the used tool  2  is less than the predetermined value, the NC device  86  performs control for supporting the bearing cases  14  and  15  with low rigidity. In other words, when the diameter of the tool  2  fixed to the spindle  1  is less than the predetermined value, the rigidity switching device performs control for moving the bearing cases  14  and  15  to the front side. 
         [0078]    On the other hand, when the diameter of the used tool is not small, the NC device  86  performs control for supporting the bearing cases  14  and  15  with high rigidity. In other words, when the diameter of the tool  2  fixed to the spindle  1  is equal to or more than the predetermined value, the rigidity switching device performs control for moving the bearing cases  14  and  15  to the rear side. 
         [0079]    Thus, the machine tool according to the embodiment can improve processing accuracy by changing the support rigidity for supporting the spindle according to the tool to be fitted. 
         [0080]    When the machine tool includes an automatic tool exchanging device for automatically replacing a tool, fitting of a tool having a tool diameter smaller than the predetermined value is detected, and the rigidity switching device can be controlled so as to support the spindle with low support rigidity. Fitting of a tool having a tool diameter equal to or larger than the predetermined value is detected, and the rigidity switching device can be controlled so as to support the spindle with high support rigidity. When the tool is automatically replaced, as illustrated in  FIG. 1 , driving of the hydraulic cylinder  5  by the control device enables the tool holder  3  to which the tool  2  is fixed to be detached from or attached to the spindle  1 . 
         [0081]    When the spindle  1  is supported with low support rigidity, the NC device  86  can perform control for offsetting a Z-axis tool-length of the tool  2  by a value corresponding to the moving amount pz of the bearing case  14 . This control enables correction of a processing error generated in the Z-axis direction when the spindle is supported with low support rigidity. The NC device  86  can drive each axis servo motor  87  on the corrected tool path. 
         [0082]    The example of offsetting the tool in the NC device  86  of the machine tool  85  has been described in the above description. However, it is not limited to the above-described embodiment. For example, when a tool path is generated by the CAM device  83 , in the case of a tool having a small diameter, a tool length may be corrected in advance or a tool path may be offset. 
         [0083]    The aforementioned rigidity switching device makes the tapered part  14   a  of the bearing case  14  to closely attach to the tapered hole part  11   a  of the front housing  11 . However, the rigidity switching device is not limited to this as long as the bearing case is supported by moving the bearing case to press the tapered part to the housing. Further, the rigidity switching device is not limited to the manner of contacting or separating the tapered part of the bearing case with or from the front housing, but can switch the support rigidity by any configuration. 
         [0084]      FIG. 7  is a schematic enlarged sectional view of portions of a front bearing and a bearing case of another spindle device according to the embodiment.  FIG. 7  illustrates a state where a spindle  1  is supported with low rigidity by using a tool having a small diameter. A rigidity switching device of the other spindle device includes a hydraulic chamber member  71 . The hydraulic chamber member  71  is hollow inside, forming a hydraulic chamber. The hydraulic chamber member  71  is disposed in a region of an inner surface of a front housing  11  opposite to a bearing case  14 . The hydraulic chamber member  71  is formed, for example, so that the hydraulic chamber can extend along a rotational axis  101 . 
         [0085]    A slight gap is formed between an outer surface of the bearing case  14  and the inner surface of the front housing  11 . Height of the gap part  35  is, for example, about several tens of μm. An end surface of a front side of the bearing case  14  is in contact with a friction member  65  by an elastic force of a diaphragm  31 . When processing is carried out by using a tool having a small diameter, as in the case of the aforementioned spindle device, the spindle  1  is vibrated. In this case, the end surface of the front side of the bearing case  14  slides with the friction member  65  to enable vibration to be damped by friction. 
         [0086]    The hydraulic chamber member  71  includes a thin part  71   a  opposite to the bearing case  14 . The thin part  71   a  is formed to be elastically deformed toward the bearing case  14  as indicated by an arrow  123  when high hydraulic pressure is supplied from a hydraulic device  8 . The thin part  71   a  can be made of metal such as spring steel. 
         [0087]    When a usual tool or a tool having a large diameter is used, the hydraulic device  8  supplies high-pressure control oil into the hydraulic chamber member  71  through oil paths  73  and  74  as indicated by arrows  121  and  122 . Swelling of the thin part  71   a  enables the bearing case  14  to be pressed to the inside in a radial direction. As a result, the bearing case  14  can be supported with high rigidity. Accordingly, the spindle  1  can be supported with high support rigidity. 
         [0088]    On the other hand, when a tool having a small diameter is used or the like, hydraulic pressure in the hydraulic chamber member  71  is reduced. Through this control, the thin part  71   a  is separated from the bearing case  14  to enable unbinding of the bearing case  14 . Actions and effects similar to those described above can be provided by such a rigidity switching device. 
         [0089]    In addition, in the other spindle device illustrated in  FIG. 7 , since the spindle  1  does not move in the direction of the rotational axis  101 , it is not necessary to perform control for offsetting the tool length of the tool  2  by the predetermined moving amount, as the aforementioned case. Thus, control of each axis servo motor can be simplified. 
         [0090]    In the spindle device according to the embodiment, the front bearing includes two angular bearings  21  and one roller bearing  22 . However, it is not limited to the above-described embodiment, and the front bearing can employ any type of a bearing. Any number of bearings can also be employed. For example, the front bearing may include a roller bearing without using any angular bearing. For a rear bearing, similarly, the spindle can be supported by any configuration. 
         [0091]    The rigidity switching means according to the embodiment is formed to switch the support rigidity for supporting the spindle at two stages. However, it is not limited to the above-described embodiment, and the rigidity switching means may be formed to switch the support rigidity at three stages or more. For example, in the embodiment illustrated in  FIG. 7 , the hydraulic pressure supplied to the hydraulic chamber member  71  from the hydraulic device  8  may be switched at three stages or more. 
         [0092]    In the aforementioned embodiment, it is determined whether the used tool having the diameter smaller than the predetermined value or the tool having the diameter equal to or larger than the predetermined value so as to switch the rigidity. However, an operator of the machine tool may designate a number of an easily vibrated tool in advance without determination based on a tool diameter, and when the NC device  86  determines that the tool with the number is fitted to the spindle by a tool replacing operation, high rigidity may be switched to low rigidity. 
         [0093]    The vibration damping device according to the embodiment is formed so that the friction member  65  and the end surface of the front side of the bearing case  14  can slide with each other. However, it is not limited to the above-described embodiment, and the vibration damping device may be formed to accelerate damping of the vibration generated in the spindle  1 . For example, the vibration damping device may include, when the bearing cases  14  and  15  are supported with low rigidity, an elastic member which protrudes from the inside of the housing toward the bearing cases  14  and  15 . This elastic member can be made of an elastic material such as rubber. The elastic member is housed in the housing in the case of high rigidity, and protrudes from the housing in the diameter direction in the case of low rigidity to be able to press the bearing case. Pressing the bearing case by the elastic member enables the vibration of the bearing case and the spindle to be damped. 
         [0094]    The aforementioned embodiments can be combined in any appropriate way. Throughout the drawings described above, the same or equivalent portions are demoted by the same reference signs. The aforementioned embodiments are only exemplary, not limitative of the present invention. The embodiments include changes which are within the scope of claims. 
       REFERENCE SIGNS LIST 
       [0095]      1  spindle 
         [0096]      2  tool 
         [0097]      8  hydraulic device 
         [0098]      10  spindle head housing 
         [0099]      11  front housing 
         [0100]      12  front end housing 
         [0101]      14 ,  15  bearing case 
         [0102]      17  bearing case 
         [0103]      21  angular bearing 
         [0104]      22  roller bearing 
         [0105]      31  diaphragm 
         [0106]      35  gap part 
         [0107]      63 ,  64  oil chamber 
         [0108]      65  friction member 
         [0109]      66 ,  67  displacement sensor 
         [0110]      71  hydraulic chamber member 
         [0111]      71   a  thin part 
         [0112]      84  input numerical data 
         [0113]      85  machine tool 
         [0114]      86  NC device 
         [0115]      87  each axis servo motor