Patent Publication Number: US-2022234120-A1

Title: Spindle and cutting apparatus including the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-012081 filed on Jan. 28, 2021. The entire contents of this application are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to spindles and cutting apparatuses including the spindles. 
     2. Description of the Related Art 
     Cutting apparatuses known in the related art machine workpieces with rotatable machining tools. Such a cutting apparatus brings a machining tool into contact with a workpiece at a predetermined angle while three-dimensionally changing the positions of the workpiece and the machining tool relative to each other so as to machine the workpiece into a desired shape. 
     JP 2020-28935 A, for example, discloses a cutting apparatus including a spindle provided with a main shaft rotatably supported by a bearing, and a collet chuck that is inserted into a through hole defined in the main shaft and is able to grip a machining tool. The machining tool is detachably gripped by the collet chuck of the spindle. A workpiece is secured to a holder. For example, assuming that a Cartesian coordinate system for a machining space is defined by X, Y, and Z axes perpendicular to each other, the spindle is movable freely along the Y axis and the Z axis and able to rotate the machining tool around the Z axis, and the holder is movable freely along the X axis in the machining space and able to rotate the workpiece around the X axis and the Y axis. 
     During machining of the workpiece, high-speed rotation of the main shaft creates negative pressure around the bearing. Such negative pressure may cause chips produced during machining of the workpiece to penetrate into the spindle and adhere to the bearing. The adhesion of the chips to the bearing interferes with rotation of the main shaft, making it difficult to machine the workpiece with the machining tool. To solve such problems, JP 2019-188502 A discloses a spindle including an air purger to which air for air purging is to be supplied. Depending on the location of a bearing, supplying air for air purging may unfortunately fail to create sufficient positive pressure around the bearing, resulting in penetration of chips into the spindle and adhesion of the chips to the bearing. 
     SUMMARY OF THE INVENTION 
     Accordingly, preferred embodiments of the present invention provide spindles that are each able to prevent or reduce adhesion of chips to bearings. 
     A spindle according to a preferred embodiment of the present invention includes a housing, a bearing, a main shaft, a collet chuck, and a draw bar. The bearing is housed in the housing. The main shaft is rotatably supported by the bearing. The main shaft is provided with a through hole passing through the main shaft in an axial direction of the main shaft. The main shaft includes a first end located on a first side in the axial direction and a second end located on a second side in the axial direction. The collet chuck extends into the through hole. The collet chuck is movable in the axial direction. The collet chuck includes a first collet end located adjacent to the first end of the main shaft and a second collet end located adjacent to the second end of the main shaft. The first collet end of the collet chuck is able to grip a machining tool to machine a workpiece. The draw bar extends into the through hole. The draw bar is movable in the axial direction. The draw bar includes a first bar end located adjacent to the first end of the main shaft and a second bar end located adjacent to the second end of the main shaft. The first bar end of the draw bar is connected to the second collet end of the collet chuck. The housing is provided with an air inlet, an air passage, and an air outlet. The air inlet is located above the bearing. Air is introduced into the housing through the air inlet. The air passage is located below the bearing such that the air introduced into the housing through the air inlet flows below the bearing. The air outlet is located below the bearing. The air flowing below the bearing is discharged out of the housing through the air outlet. 
     The spindle includes the air passage defined in the housing such that the air introduced into the housing through the air inlet flows below the bearing. The air flowing below the bearing is discharged out of the housing through the air outlet. An inner portion of the housing located below the bearing is thus maintained at a positive pressure. Because air outside the housing is kept from flowing into the air passage through the air outlet, chips produced during machining of the workpiece with the machining tool are prevented from penetrating into the air passage through the air outlet accordingly. Consequently, the bearing is kept in a clean condition and is thus able to effectively support the main shaft such that the main shaft is rotatable. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a cutting apparatus according to a preferred embodiment of the present invention. 
         FIG. 2  is a front view of a cutting apparatus according to a preferred embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of a cutting apparatus according to a preferred embodiment of the present invention. 
         FIG. 4  is a perspective view of a workpiece to which an adapter according to a preferred embodiment of the present invention is attached. 
         FIG. 5  is a perspective view of a tool magazine and a clamp according to a preferred embodiment of the present invention. 
         FIG. 6  is a front view of the tool magazine and a clamp according to a preferred embodiment of the present invention. 
         FIG. 7  is a cross-sectional view of a spindle according to a preferred embodiment of the present invention. 
         FIG. 8  is a partially enlarged cross-sectional view of a spindle according to the preferred embodiment of the present invention. 
         FIG. 9  is a bottom view of a housing according to a preferred embodiment of the present invention. 
         FIG. 10  is a perspective view of a second securer according to a preferred embodiment of the present invention. 
         FIG. 11  is a block diagram of a cutting apparatus according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Spindle units and cutting apparatuses according to preferred embodiments of the present invention will be described below with reference to the drawings. The preferred embodiments described below are not intended to limit the invention in any way. Components and elements having the same functions are identified by the same reference signs, and description thereof will be simplified or omitted when deemed redundant. 
       FIG. 1  is a perspective view of a cutting apparatus  10  according to the present preferred embodiment.  FIG. 2  is a front view of the cutting apparatus  10 .  FIG. 3  is a cross-sectional view of the cutting apparatus  10 . The following description is based on the assumption that when an operator is facing the front of the cutting apparatus  10 , a direction away from the rear of the cutting apparatus  10  and toward the operator is a forward direction, and a direction away from the operator and toward the rear of the cutting apparatus  10  is a rearward direction. The terms “left”, “right”, “up”, and “down” respectively refer to left, right, up, and down with respect to the operator facing the front of the cutting apparatus  10 . The reference signs F, Rr, L, R, U, and D in the drawings respectively represent front, rear, left, right, up, and down. In the present preferred embodiment, the cutting apparatus  10  is located in a Cartesian coordinate system defined by X, Y, and Z axes perpendicular to each other. The X axis in the present preferred embodiment extends substantially in a front-rear direction. As illustrated in  FIG. 3 , the X axis in the present preferred embodiment is inclined relative to a horizontal direction by an angle θ. Alternatively, the X axis may extend in the same direction as the horizontal direction. The Y axis extends in a right-left direction. The Z axis extends substantially in an up-down direction. As illustrated in  FIG. 3 , the Z axis in the present preferred embodiment is inclined relative to a vertical direction by the angle θ. Alternatively, the Z axis may extend in the same direction as the vertical direction. The reference sign θ x  represents a rotational direction around the X axis. The reference sign θ y  represents a rotational direction around the Y axis. The reference sign θ z  represents a rotational direction around the Z axis. These directions are defined merely for the sake of convenience of description and do not limit in any way how the cutting apparatus  10  may be installed or how preferred embodiments of the present invention may be practiced or implemented. 
     The cutting apparatus  10  machines a workpiece  5  (see  FIG. 4 ). Examples of machining processes to be performed by the cutting apparatus  10  include cutting. The cutting apparatus  10  grinds the workpiece  5  when necessary. The cutting apparatus  10  machines the workpiece  5  into a dental article. Examples of such a dental article include a dental prosthetic (such as a crown, an inlay, an onlay, or a veneer), an artificial tooth, and a denture base. The workpiece  5  has, for example, a block shape (such as a prismatic shape). Alternatively, the workpiece  5  may have a disk shape. Examples of material(s) for the workpiece  5  include: resin materials, such as polymethyl methacrylate (PMMA) resin, polyether ether ketone (PEEK) resin, and hybrid resin; ceramic materials, such as glass ceramic and zirconia; wax; and gypsum. When zirconia is selected as a material for the workpiece  5 , semi-sintered zirconia, for example, is used. The workpiece  5  is not limited to any particular shape or material. 
     As illustrated in  FIG. 4 , the present preferred embodiment involves attaching an adapter  8  (which may also be referred to as a “holder”) to the workpiece  5 . The workpiece  5  having the adapter  8  attached thereto is housed in the cutting apparatus  10  and subjected to cutting. In the present preferred embodiment, the adapter  8  includes a plate  8 A and a connecting pin  8 B. The plate  8 A is connected to the workpiece  5 . The connecting pin  8 B protrudes from the plate  8 A. The connecting pin  8 B is inserted into an insertion hole  50 A (see  FIG. 3 ) defined in a clamp  50  (which will be described below). The adapter  8  holds the workpiece  5 . 
     As illustrated in  FIG. 1 , the cutting apparatus  10  has a box shape. The cutting apparatus  10  includes a case body  12 , a front cover  25 , and a controller  48 . The case body  12  includes a lower wall  13 , a left wall  14  (see also  FIG. 2 ), a right wall  15 , a rear wall  16  (see also  FIG. 3 ), an upper wall  17 , a front wall  18 , a bottom partition  19  (see  FIG. 3 ), a rear partition  20  (see  FIGS. 2 and 3 ), an upper partition  21  (see  FIG. 3 ), and a side partition  23  (see  FIG. 3 ). The left wall  14  extends upward from the left end of the lower wall  13 . The right wall  15  extends upward from the right end of the lower wall  13 . The rear wall  16  extends upward from the rear end of the lower wall  13 . The left end of the rear wall  16  is connected to the rear end of the left wall  14 . The right end of the rear wall  16  is connected to the rear end of the right wall  15 . The front wall  18  extends upward from the front end of the lower wall  13 . The left end of the front wall  18  is connected to the front end of the left wall  14 . The right end of the front wall  18  is connected to the front end of the right wall  15 . The front wall  18  is provided with an opening  18 A (see  FIG. 2 ). The upper wall  17  is connected to the upper ends of the left wall  14 , the right wall  15 , the rear wall  16 , and the front wall  18 . As illustrated in  FIG. 3 , the bottom partition  19  is disposed above the lower wall  13 . The upper partition  21  is disposed above the bottom partition  19  and below the upper wall  17 . The rear partition  20  is disposed forward of the rear wall  16  and rearward of the front wall  18 . The side partition  23  is disposed rightward of the left wall  14  and leftward of the right wall  15 . The side partition  23  extends upward from the bottom partition  19 . The side partition  23  is connected to the bottom partition  19 , the upper partition  21 , and the rear partition  20 . 
     As illustrated in  FIG. 3 , an internal space  26  is defined in the cutting apparatus  10 . The internal space  26  is surrounded by the bottom partition  19 , the left wall  14  (see  FIG. 2 ), the rear partition  20 , the upper partition  21 , the side partition  23 , and the front wall  18 . The internal space  26  serves as a machining area where the workpiece  5  is to be machined. A first housing space  27  (see  FIG. 2 ) is defined in the cutting apparatus  10 . The first housing space  27  is surrounded by the bottom partition  19 , the right wall  15  (see  FIG. 2 ), the rear partition  20 , the upper partition  21 , the side partition  23 , and the front wall  18 . A conveyor  58  (which will be described below) is housed in the first housing space  27 . 
     As illustrated in  FIG. 2 , the front cover  25  is provided on the front ends of the left wall  14  and the right wall  15  such that the front cover  25  is movable substantially in the up-down direction. Opening the front cover  25  by moving the front cover  25  upward brings the internal space  26  into communication with an external space. Closing the front cover  25  by moving the front cover  25  downward separates the internal space  26  from the external space.  FIG. 2  illustrates the cutting apparatus  10 , with the front cover  25  moved upward such that the internal space  26  is in communication with the external space. The front cover  25  is provided with a window  26 A. The window  26 A is made of, for example, a transparent acrylic plate. The operator is thus able to visually check the internal space  26  through the window  26 A. The window  26 A is smaller than the opening  18 A defined in the front wall  18 . 
     As illustrated in  FIG. 2 , the cutting apparatus  10  includes a spindle unit  60 , a carriage  38 , a tool magazine  40  (see also  FIG. 5 ), the clamp  50 , and the conveyor  58 . A second housing space  28  (see  FIG. 3 ) is defined in the cutting apparatus  10 . The second housing space  28  is surrounded by the upper partition  21 , the left wall  14 , the right wall  15 , the rear partition  20 , the upper wall  17 , and the front wall  18 . The carriage  38  and a portion of the spindle unit  60  are disposed in the second housing space  28 . Another portion of the spindle unit  60  is disposed in the internal space  26 . The spindle unit  60  is inserted through an opening  21 H (see  FIG. 3 ) defined in the upper partition  21 . The tool magazine  40  and the clamp  50  are disposed in the internal space  26 . The carriage  38  is an example of a unit conveyor. The carriage  38  is equipped with the spindle unit  60 . The carriage  38  is movable in a Z-axis direction and a Y-axis direction. As used herein, the term “Z-axis direction” refers to a direction along the Z axis, and the term “Y-axis direction” refers to a direction along the Y axis. The carriage  38  moves the spindle unit  60  in the Z-axis direction and the Y-axis direction. The carriage  38  includes a first carriage  38 A and a second carriage  38 B. The first carriage  38 A is supported by a pair of first guide shafts  39 A extending in the Y-axis direction. The first carriage  38 A is movable in the Y-axis direction along the first guide shafts  39 A by a first driver  38 C (see  FIG. 11 ). The first guide shafts  39 A are provided in the second housing space  28  (see  FIG. 3 ). The left ends of the first guide shafts  39 A are connected to the left wall  14 . The right ends of the first guide shafts  39 A are connected to the right wall  15 . The second carriage  38 B is supported by a pair of second guide shafts  39 B extending in the Z-axis direction. The second carriage  38 B is movable in the Z-axis direction along the second guide shafts  39 B by a second driver  38 D (see  FIG. 11 ). The second guide shafts  39 B are provided on the first carriage  38 A. Movement of the first carriage  38 A in the Y-axis direction thus moves the second carriage  38 B in the Y-axis direction accordingly. The first driver  38 C and the second driver  38 D are controlled by the controller  48 . 
     As illustrated in  FIG. 2 , the conveyor  58  is disposed in the first housing space  27 . The conveyor  58  is disposed rightward of the tool magazine  40 . The conveyor  58  includes a shaft  58 A extending in the Y-axis direction. A portion of the shaft  58 A (e.g., the right end portion of the shaft  58 A) is disposed in the first housing space  27 . Another portion of the shaft  58 A (e.g., the left end portion of the shaft  58 A) is disposed in the internal space  26 . The tool magazine  40  is provided on the left end portion of the shaft  58 A. The conveyor  58  is movable in an X-axis direction by a third driver  58 B (see  FIG. 11 ). As used herein, the term “X-axis direction” refers to a direction along the X axis. The conveyor  58  moves the tool magazine  40  in the X-axis direction. The third driver  58 B is controlled by the controller  48 . 
     As illustrated in  FIG. 5 , the tool magazine  40  is able to hold a plurality of machining tools  6 A and a detecting tool  6 B. The tool magazine  40  is provided between the clamp  50  and the conveyor  58 . Movement of the conveyor  58  in the X-axis direction moves the tool magazine  40  in the X-axis direction. The tool magazine  40  includes a first portion  40 A to hold the machining tools  6 A and the detecting tool  6 B, a second portion  40 B located rearward of the first portion  40 A and connected to the shaft  58 A, and a third portion  40 C located rearward of the second portion  40 B. The first portion  40 A of the tool magazine  40  is provided with a plurality of through holes  42 A in which the machining tools  6 A and the detecting tool  6 B are to be held. In the present preferred embodiment, the number of through holes  42 A is six, for example. The through holes  42 A pass through the tool magazine  40  in the up-down direction. The machining tools  6 A and the detecting tool  6 B are each inserted into an associated one of the through holes  42 A such that the upper portion of each of the machining tools  6 A and the detecting tool  6 B is exposed. Replacement of the machining tool  6 A or the detecting tool  6 B involves returning the machining tool  6 A or the detecting tool  6 B gripped by a collet chuck  67  of a spindle  62  (which will be described below) to the associated through hole  42 A, moving the spindle unit  60  to a position over the machining tool  6 A or the detecting tool  6 B to be used next, and causing the collet chuck  67  to grip the upper end of the machining tool  6 A or the detecting tool  6 B located under the collet chuck  67 . 
     The machining tools  6 A each have a rod shape. The cutting apparatus  10  uses the machining tool(s)  6 A in cutting the workpiece  5 . Each machining tool  6 A gradually wears out in the course of cutting the workpiece  5 . Each machining tool  6 A is made of a conductive material, such as metal. The detecting tool  6 B has a rod shape. The cutting apparatus  10  uses the detecting tool  6 B in making automatic corrections involving suitably correcting the relative positions of the workpiece  5  and the spindle unit  60 , the relative positions of the tool magazine  40  and the spindle unit  60 , and/or the relative positions of the clamp  50  and the spindle unit  60 . The detecting tool  6 B is made of a conductive material, such as metal. 
     As illustrated in  FIG. 2 , a rotary shaft  44  is provided inside the shaft  58 A. The rotary shaft  44  supports the clamp  50  such that the clamp  50  is rotatable. The rotary shaft  44  extends in the right-left direction. The rotary shaft  44  is coupled to the clamp  50  and the conveyor  58 . A drive motor  44 A (see also  FIG. 11 ) is provided on the conveyor  58 . The drive motor  44 A is controlled by the controller  48 . The rotary shaft  44  is rotatable in the rotational direction θ y  around the Y axis by the drive motor  44 A. The rotation of the rotary shaft  44  in the rotational direction θ y  around the Y axis causes the clamp  50  to rotate in the rotational direction θ y  around the Y axis. The rotary shaft  44  is rotatable independently of the shaft  58 A. The rotation of the rotary shaft  44  in the rotational direction θ y  around the Y axis thus does not cause the shaft  58 A to rotate in the rotational direction θ y  around the Y axis. 
     As illustrated in  FIG. 6 , the clamp  50  is provided on the left end of the rotary shaft  44 . The clamp  50  is disposed leftward of the tool magazine  40 . The clamp  50  holds the adapter  8  such that the adapter  8  is detachable from the clamp  50 . As illustrated in  FIG. 5 , the number of adapters  8  the clamp  50  is able to hold is three in the present preferred embodiment, for example. The clamp  50  holds the workpiece(s)  5  through the adapter(s)  8 . The clamp  50  is an example of a holder. As illustrated in  FIG. 3 , the clamp  50  is provided with more than one insertion hole  50 A. In the present preferred embodiment, the number of insertion holes  50 A is three, and the three insertion holes  50 A are aligned in the front-rear direction of the clamp  50 , for example. The connecting pins  8 B (see  FIG. 4 ) of the adapters  8  are each inserted into an associated one of the insertion holes  50 A. The connecting pins  8 B inserted into the insertion holes  50 A are secured to the clamp  50  with screws  50 B (see  FIG. 5 ). The clamp  50  is movable together with the tool magazine  40 . Specifically, the tool magazine  40  and the clamp  50  are movable in the X-axis direction by the conveyor  58 . The rotation of the clamp  50  in the rotational direction Oy around the Y axis does not cause the tool magazine  40  to rotate in the rotational direction Oy around the Y axis. 
     As illustrated in  FIG. 7 , the spindle unit  60  includes the spindle  62  and an actuator  61  (see  FIG. 3 ). As illustrated in  FIG. 3 , the actuator  61  is detachably provided on the upper portion of the spindle  62 . The actuator  61  controls movement of a draw bar  68  (see  FIG. 7 ) of the spindle  62  in the up-down direction. The draw bar  68  will be described below. The actuator  61  controls attaching and detaching of the machining tool  6 A or the detecting tool  6 B to and from the collet chuck  67  (which will be described below) of the spindle  62 . 
     As illustrated in  FIG. 7 , the spindle  62  includes a housing  63 , a first bearing  64 A, a second bearing  64 B, a third bearing  64 C, a main shaft  65 , a spindle motor  66 , the collet chuck  67 , the draw bar  68 , an urger  70 , a stopper  71 , a cover  72 , a first securer  74 , a second securer  76 , a third securer  78 , a coolant container  80 , air inlets  90 , an air passage  94  (see  FIG. 8 ), and an air outlet  96  (see  FIG. 8 ). The terms “above” and “below” as used herein in connection with the spindle  62  respectively refer to, for example, being located above and below in an axial direction K of the main shaft  65 . The axial direction K in the present preferred embodiment is parallel or substantially parallel to the Z axis. Specifically, the axial direction K is inclined relative to the vertical direction by the angle θ. 
     As illustrated in  FIG. 7 , the housing  63  has a cylindrical or substantially cylindrical shape. The housing  63  is provided with a first opening  63 A located in the lower portion of the housing  63 , and a second opening  63 B located in the upper portion of the housing  63 . The housing  63  includes a first bearing holder  63 C located adjacent to the first opening  63 A, and a second bearing holder  63 D located adjacent to the second opening  63 B. The first bearing holder  63 C holds the first bearing  64 A and the second bearing  64 B. The second bearing holder  63 D holds the third bearing  64 C. The second bearing holder  63 D is provided with through holes  63 DH. Compressed air supplied from a compressor  98  (see  FIG. 11 ) passes through the through holes  63 DH. The compressor  98  will be described below. The through holes  63 DH pass through the second bearing holder  63 D in the up-down direction. The first bearing holder  63 C is provided with a plurality of through holes  63 CH (see also  FIG. 9 ). In the present preferred embodiment, the number of through holes  63 CH is four, and the four through holes  63 CH are disposed at regular intervals (e.g., intervals of 90 degrees) in a circumferential direction of the first bearing holder  63 C, for example. The first bearing holder  63 C may be provided with any other suitable number of through holes  63 CH. The through holes  63 CH pass through the first bearing holder  63 C in the up-down direction. The through holes  63 CH are located outward of the first bearing  64 A and the second bearing  64 B. 
     As illustrated in  FIG. 7 , the first bearing  64 A, the second bearing  64 B, and the third bearing  64 C are housed in the housing  63 . The first bearing  64 A and the second bearing  64 B are located adjacent to the first opening  63 A of the housing  63 . The second bearing  64 B is disposed on the first bearing  64 A. The first bearing  64 A and the second bearing  64 B are held by the first securer  74  and the second securer  76 . More specifically, the first bearing  64 A is held directly by the first securer  74  and the second securer  76 , and the second bearing  64 B is held indirectly by the first securer  74  and the second securer  76  through the first bearing  64 A. The third bearing  64 C is located adjacent to the second opening  63 B of the housing  63 . The third bearing  64 C is held by the second bearing holder  63 D. The third bearing  64 C is disposed above the second bearing  64 B. The first bearing  64 A, the second bearing  64 B, and the third bearing  64 C support the main shaft  65  such that the main shaft  65  is rotatable. The first bearing  64 A is an example of a bearing. 
     As illustrated in  FIG. 7 , the main shaft  65  extends in the up-down direction. The main shaft  65  is housed in the housing  63 . The main shaft  65  is rotatably supported by the first bearing  64 A, the second bearing  64 B, and the third bearing  64 C. The main shaft  65  is provided with a through hole  65 H passing through the main shaft  65  in the axial direction K (which corresponds to the up-down direction in the present preferred embodiment). The main shaft  65  includes a first end  65 A and a second end  65 B. The first end  65 A is located on a first side in the axial direction K (which corresponds to the lower side in the axial direction K in the present preferred embodiment). The first end  65 A is located adjacent to the first opening  63 A. The first end  65 A is located outside the housing  63 . The second end  65 B is located on a second side in the axial direction K (which corresponds to the upper side in the axial direction K in the present preferred embodiment). The second end  65 B is located adjacent to the second opening  63 B. A portion of the through hole  65 H adjacent to the first end  65 A increases in diameter as the portion extends downward (i.e., as the portion extends away from the second end  65 B). 
     As illustrated in  FIG. 7 , the spindle motor  66  is housed in the housing  63 . The spindle motor  66  is disposed between the second bearing  64 B and the third bearing  64 C. The spindle motor includes a rotor  66 A and a stator  66 B. The rotor  66 A is integral with the main shaft  65 . The stator  66 B is disposed to face the rotor  66 A. Passage of a current through the stator  66 B rotates the main shaft  65  at high speed. The spindle motor  66  is controlled by the controller  48 . 
     As illustrated in  FIG. 7 , the collet chuck  67  extends in the up-down direction. The collet chuck  67  is inserted into the through hole  65 H of the main shaft  65 . The collet chuck  67  is movable in the axial direction K. The collet chuck  67  includes a first collet end  67 A and a second collet end  67 B. The first collet end  67 A is located adjacent to the first end  65 A of the main shaft  65 . In the present preferred embodiment, the first collet end  67 A is the lower end of the collet chuck  67 . The first collet end  67 A is able to grip either one of the machining tool  6 A and the detecting tool  6 B. A portion of the first collet end  67 A protrudes out of the through hole  65 H of the main shaft  65 . The second collet end  67 B is located adjacent to the second end  65 B of the main shaft  65 . In the present preferred embodiment, the second collet end  67 B is the upper end of the collet chuck  67 . The second collet end  67 B is located within the through hole  65 H of the main shaft  65 . 
     As illustrated in  FIG. 7 , the draw bar  68  extends in the up-down direction. The draw bar  68  is inserted into the through hole  65 H of the main shaft  65 . The draw bar  68  is movable in the axial direction K. The draw bar  68  is disposed above the collet chuck  67 . The draw bar  68  includes a first bar end  68 A and a second bar end  68 B. The first bar end  68 A is located adjacent to the first end  65 A of the main shaft  65 . In the present preferred embodiment, the first bar end  68 A is the lower end of the draw bar  68 . The first bar end  68 A is connected to the second collet end  67 B. The first bar end  68 A is located within the through hole  65 H of the main shaft  65 . The second bar end  68 B is located adjacent to the second end  65 B of the main shaft  65 . In the present preferred embodiment, the second bar end  68 B is the upper end of the draw bar  68 . The second bar end  68 B protrudes out of the through hole  65 H of the main shaft  65 . The second bar end  68 B is to be pressed by a push rod (not illustrated) of the actuator  61  (see  FIG. 3 ). The draw bar  68  includes a flange  69 . Contact of the flange  69  with a retainer  69 B, such as a nut, stops movement of the draw bar  68  in a first direction K 1 . As used herein, the term “first direction K 1 ” refers to a direction from the first end  65 A of the main shaft  65  to the second end  65 B of the main shaft  65 . 
     As illustrated in  FIG. 7 , the urger  70  is disposed around the draw bar  68 . The urger  70  is supported by the third securer  78  attached to the main shaft  65 . The urger  70  includes, for example, a plurality of disc springs. The draw bar  68  is inserted through the urger  70 . The urger  70  is in contact with the draw bar  68  so as to urge the draw bar  68  in the first direction K 1 . In the present preferred embodiment, the urger  70  is in contact with the flange  69  of the draw bar  68  so as to urge the draw bar  68  in the first direction K 1 . 
     As illustrated in  FIG. 7 , the stopper  71  is disposed laterally of the urger  70 . The stopper  71  is disposed around the urger  70 . The stopper  71  is able to come into contact with the flange  69  of the draw bar  68 . The stopper  71  comes into contact with the flange  69  of the draw bar  68  so as to prevent the draw bar  68  from moving in a second direction K 2  by a distance longer than a predetermined distance. As used herein, the term “second direction K 2 ” refers to a direction from the second end  65 B of the main shaft  65  to the first end  65 A of the main shaft  65 . 
     As illustrated in  FIG. 8 , the first securer  74  is secured to the main shaft  65 . The first securer  74  is housed in the housing  63 . The first securer  74  holds the first bearing  64 A and the second bearing  64 B from below. More specifically, the first securer  74  holds an inner ring  64 AA of the first bearing  64 A from below. The first securer  74  restricts movement of the first bearing  64 A and the second bearing  64 B in the axial direction K. The first securer  74  has a ring shape. The first securer  74  is, for example, a nut. 
     As illustrated in  FIG. 8 , the second securer  76  is secured to the housing  63 . The second securer  76  is housed in the housing  63 . The second securer  76  is located outward of the first securer  74 . The first securer  74  and the second securer  76  have a gap therebetween. The second securer  76  holds the first bearing  64 A and the second bearing  64 B from below. More specifically, the second securer  76  holds an outer ring  64 AB of the first bearing  64 A from below. The second securer  76  restricts movement of the first bearing  64 A and the second bearing  64 B in the axial direction K. As illustrated in  FIG. 10 , the second securer  76  has a ring shape. The second securer  76  includes a first portion  76 A pressing the first bearing  64 A from below, a second portion  76 B secured to the housing  63 , and an opening  76 C. The first portion  76 A includes an upper surface  76 AA in contact with the outer ring  64 AB of the first bearing  64 A, and an inclined surface  76 AB extending obliquely downward from the upper surface  76 AA. The first portion  76 A is provided with recesses  76 AH recessed downward from the upper surface  76 AA and the inclined surface  76 AB. In the present preferred embodiment, the number of recesses  76 AH is four, for example. The recesses  76 AH are each in communication with an associated one of the through holes  63 CH (see  FIG. 8 ) of the housing  63 . 
     As illustrated in  FIG. 7 , the third securer  78  is secured to the main shaft  65 . The third securer  78  is secured to the second end  65 B of the main shaft  65 . The third securer  78  restricts movement of the third bearing  64 C in the axial direction K. The third securer  78  holds the urger  70  and the stopper  71 . The third securer  78  is, for example, a nut. 
     As illustrated in  FIG. 7 , the cover  72  covers the first opening  63 A of the housing  63 . The cover  72  is a component of the housing  63 . As illustrated in  FIG. 8 , the cover  72  is provided with a through hole  72 H through which the main shaft  65  passes. The cover  72  is disposed at a distance from the main shaft  65 . Rotation of the main shaft  65  thus does not cause the cover  72  to rotate. 
     As illustrated in  FIG. 8 , the air outlet  96  is defined in the housing  63 . More specifically, the air outlet  96  is defined between the cover  72  and the main shaft  65 . The air outlet  96  opens downward in the axial direction K of the main shaft  65 . The air outlet  96  is located below the first bearing  64 A. As will be described below, air flowing below the first bearing  64 A is discharged out of the housing  63  through the air outlet  96 . 
     As illustrated in  FIG. 7 , the coolant container  80  is attached to the housing  63 . The coolant container  80  has a tubular shape. The coolant container  80  includes a supply port  81  to which a coolant is to be supplied, flow passages  82  through which the coolant flows, and nozzles  83  from which the coolant is to be discharged onto the machining tool  6 A. The supply port  81  is connected through a liquid supply passage (not illustrated) to a storage tank  37  (see  FIG. 3 ) provided in the case body  12 . Examples of the liquid supply passage include an easily deformable tube made of resin. The coolant stored in the storage tank  37  is supplied to the supply port  81  by actuating a pump  85  (see  FIG. 11 ) provided in the case body  12 . As indicated by the arrows L 1  in  FIG. 7 , actuating the pump  85  discharges the coolant onto the machining tool  6 A from the nozzles  83 . The pump  85  is controlled by the controller  48 . The supply port  81  is in communication with the nozzles  83  through the flow passages  82 . The nozzles  83  are defined in the coolant container  80  attached to the housing  63 . The nozzles  83  are located outward of the air outlet  96 . The nozzles  83  open toward the machining tool  6 A. The coolant, which has been discharged onto the machining tool  6 A from the nozzles  83 , returns to the storage tank  37  through the internal space  26 .  FIG. 8  illustrates the spindle  62 , with the coolant container  80  detached from the housing  63 . 
     As illustrated in  FIG. 7 , the air inlets  90  are defined in the housing  63 . Air is introduced into the housing  63  through the air inlets  90 . The air inlets  90  are disposed above the first bearing  64 A. The air inlets  90  are disposed above the third bearing  64 C. The air inlets  90  are connected through a gas supply passage (not illustrated) to the compressor  98  (see  FIG. 11 ) provided outside the case body  12 . Examples of the gas supply passage include an easily deformable tube made of resin. The compressor  98  is controlled by the controller  48 . The compressor  98  supplies compressed air into the housing  63  through the air inlets  90 . The air compressed by the compressor  98  may have a pressure of about 0.1 MPa or higher (e.g., about 0.2 MPa±0.05 MPa), for example. 
     As illustrated in  FIG. 8 , the air passage  94  is defined in the housing  63  such that the air introduced into the housing  63  through the air inlets  90  (see  FIG. 7 ) flows below the first bearing  64 A. The air passage  94  includes a first portion  94 A, a second portion  94 B, and a third portion  94 C. The first portion  94 A is located below the first bearing  64 A. The first portion  94 A is defined by the first bearing  64 A and the recesses  76 AH of the second securer  76 . The air flowing through the first portion  94 A passes along a surface of the first bearing  64 A. The first portion  94 A is in communication with the through holes  63 CH of the first bearing holder  63 C. The second portion  94 B is located laterally of the first securer  74 . In the present preferred embodiment, the second portion  94 B is located outward of the first securer  74 . The second portion  94 B is located above the air outlet  96 . The second portion  94 B is defined by the first securer  74  and the second securer  76 . Specifically, the second portion  94 B is a gap created between the first securer  74  and the second securer  76 . The second portion  94 B is in communication with the first portion  94 A. The third portion  94 C is located under the first securer  74 . The third portion  94 C is defined by the first securer  74  and the cover  72 . Specifically, the third portion  94 C is a gap created between the first securer  74  and the cover  72 . The third portion  94 C is in communication with the second portion  94 B and the air outlet  96 . 
     The following description discusses how air flows inside the housing  63 . As indicated by the arrow F 1  in  FIG. 7 , actuating the compressor  98  (see  FIG. 11 ) supplies compressed air into the housing  63  through the air inlets  90 . The air, which has been supplied into the housing  63  through the air inlets  90 , passes through the through holes  63 DH of the second bearing holder  63 D and flows to the spindle motor  66 . The compressed air passes between the rotor  66 A and the stator  66 B and flows into the through holes  63 CH of the first bearing holder  63 C. As indicated by the arrow F 2  in  FIG. 8 , the compressed air, which has flowed into the through holes  63 CH, flows through the first portion  94 A of the air passage  94  and then through the second portion  94 B of the air passage  94 . Because the compressed air passes below the first bearing  64 A (typically under the first bearing  64 A), the pressure around the first bearing  64 A is positive pressure. The compressed air, which has flowed through the second portion  94 B, is discharged out of the housing  63  through the third portion  94 C and the air outlet  96 . 
     As described above, the spindle  62  according to the present preferred embodiment includes the air passage  94  defined in the housing  63  such that air introduced into the housing  63  through the air inlets  90  flows below the first bearing  64 A. The air flowing below the first bearing  64 A is discharged out of the housing  63  through the air outlet  96 . An inner portion of the housing  63  located below the first bearing  64 A is thus maintained at a positive pressure. Because air outside the housing  63  is kept from flowing into the air passage  94  through the air outlet  96 , chips produced during machining of the workpiece  5  with the machining tool  6 A are prevented from penetrating into the air passage  94  through the air outlet  96  accordingly. Consequently, the first bearing  64 A is kept in a clean condition and is thus able to effectively support the main shaft  65  such that the main shaft  65  is rotatable. 
     The spindle  62  according to the present preferred embodiment includes the first securer  74  secured to the main shaft  65  and holding the first bearing  64 A from below. The air passage  94  includes the first portion  94 A located below the first bearing  64 A, and the second portion  94 B located laterally of the first securer  74 . In the present preferred embodiment, the second portion  94 B is located outward of the first securer  74 . The air outlet  96  is located below the second portion  94 B. Air thus flows more smoothly through the air passage  94 . Consequently, the inner portion of the housing  63  located below the first bearing  64 A is maintained at a positive pressure, and the first bearing  64 A is reliably held by the first securer  74 . 
     The spindle  62  according to the present preferred embodiment includes the second securer  76  secured to the housing  63 , holding the first bearing  64 A from below, and located outward of the first securer  74 . The first portion  94 A of the air passage  94  is defined by the first bearing  64 A and the recesses  76 AH of the second securer  76 . The second portion  94 B of the air passage  94  is defined by the first securer  74  and the second securer  76 . Air thus flows more smoothly through the air passage  94 . Consequently, the inner portion of the housing  63  located below the first bearing  64 A is maintained at a positive pressure, and the first bearing  64 A is reliably held by the second securer  76 . 
     The spindle  62  according to the present preferred embodiment includes the air outlet  96  that opens downward in the axial direction K of the main shaft  65 . When the air outlet  96  opens downward in the axial direction K of the main shaft  65 , chips may easily penetrate into the air passage  94  through the air outlet  96 . In the present preferred embodiment, however, the inner space of the housing  63  located below the first bearing  64 A is maintained at a positive pressure. Air is thus reliably discharged from the air outlet  96  so as to prevent chips from scattering to the air outlet  96 . 
     The spindle  62  according to the present preferred embodiment includes the coolant container  80  attached to the housing  63 . The coolant container  80  is provided with the nozzles  83  which are located outward of the air outlet  96  and from which a coolant is to be discharged onto the machining tool  6 A. The use of a coolant may cause chips of the workpiece  5  to mix with the coolant and scatter to the air outlet  96  together with the coolant. In the present preferred embodiment, however, the inner space of the housing  63  located below the first bearing  64 A is maintained at a positive pressure. Air is thus reliably discharged from the air outlet  96  so as to prevent the chips and the coolant from scattering to the air outlet  96 . 
     Preferred embodiments of the present invention have been described thus far. The preferred embodiments described above, however, are only illustrative. The present invention may be embodied in various other forms. 
     In the foregoing preferred embodiments, the through holes  63 CH are disposed outward of the first bearing  64 A and the second bearing  64 B. Alternatively, the through holes  63 CH may be disposed at any other suitable locations. The through holes  63 CH may be disposed inward of the first bearing  64 A and the second bearing  64 B. In this case, the through holes  63 CH are defined, for example, in the main shaft  65 . The first securer  74  may be provided with recesses similar to the recesses  76 AH of the second securer  76  so as to allow air introduced into the housing  63  through the air inlets  90  to flow below the first bearing  64 A. 
     In the foregoing preferred embodiments, the air outlet  96  opens downward in the axial direction K of the main shaft  65 . Alternatively, the air outlet  96  may open in any other suitable direction. The air outlet  96  may open in a direction intersecting the main shaft  65  (e.g., a direction perpendicular or substantially perpendicular to the main shaft  65 ). 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.