Patent Publication Number: US-6220931-B1

Title: Feeding a grinding wheel in grinding method

Description:
This is a divisional application No. 09/048,273 filed Mar. 26, 1998, the disclosure of which is incorporated herein by reference, now U.S. Pat. No. 6,036,585. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a grinder and a grinding method for significantly precisely grinding at least one sides of a hard and thin work, for example a wafer utilized for a semiconductor device. 
     In general, a conventional grinder has spindles rotatively supported by spindle heads thereof in such a manner that a grinding wheel is secured to the leading end of each spindle. Moreover, a feeding unit comprising a motor and a ball screw is connected to the spindle head. When the spindle heads are fed and moved in the axial direction by the feeding units while the grinding wheels are rotated by the rotating motors, the outer surfaces of the work are ground. 
     The conventional grinder has the structure that each unit for feeding the grinding wheel comprises the motor and the ball screw. When the ball screw is rotated by the motor, the operation for feeding the spindle head is performed. However, the grinding wheels cannot precisely be fed because of insufficient rigidity of the machine and frictional resistance of sliding portions when the feeding operation must precisely be performed in order of microns or sub-microns. Thus, there arises a problem in that a precise grinding operation cannot be performed. 
     As a grinder capable of grinding both side surfaces of the work, Japanese Utility-Model Examined Publication No. Hei. 1-8282 teaches a conventional twin-head grinder, for example. The grinder disclosed therein has a structure that a C-shape column frame allowed to project over the frame of the grinder by a cantilever method forms a grinding head for supporting the upper grinding wheel. A bending moment acts on the column by a reaction of the grinding operation which acts on the grinding wheel because of a grinding resistance generated during the machining operation. Moreover, a thermal displacement takes place, causing a precise grinding operation to be inhibited. Therefore, a frame having an upper bed disposed above a lower bed has been disclosed. Moreover, the upper and lower grinding wheels provided for the foregoing frame are mounted to upper and lower spindles. The spindles are rotated by motors and belts disposed on the sides of the spindles. The upper spindle is vertically moved by a feeding means comprising a rack which is operated by hydraulic pressure or air pressure. 
     As a means for vertically moving the upper grinding wheel, a structure has been disclosed in, for example, Japanese Patent Unexamined Publication No. Sho. 61-270043. The means comprises a feeding means for vertically moving spindle heads by motors, ball screws and nuts provided for the spindle heads. The main spindle is, in the spindle head, supported by a hydrostatic bearing. Moreover, each grinding wheel is rotated by a built-in type motor. 
     The conventional grinder as disclosed in the above-mentioned Japanese Patent Unexamined Publication No. Sho. 61-270043 has realized a frame which is free from a thermal displacement and a rotatively supporting means with which the main spindles are not thermally expanded. On the other hand, this conventional grinder has the feeding mechanism comprising the rack or the ball screw which is operated by the motor serving as the drive source and having the structure that the grinding heads are vertically guided along the sliding surfaces, so that there is a problem that the conventional grinder cannot substantially perform precise feeding because of a sliding resistance or the like. 
     Further, in the conventional grinder, the motors which are drive sources for vertically moving the grinding heads and the ball screws are disposed on the sides of the spindles for rotating the grinding wheels, so that a lateral load is applied to the spindle. Accordingly, there is a great possibility that smooth and precise feeding cannot be performed and intercepted. 
     SUMMARY OF THE INVENTION 
     The present invention has been found to overcome the problems experienced with the conventional techniques. 
     It is an object of the present invention to provide a grinder and grinding method which is capable of precisely feeding and moving grinding wheels to predetermined grinding positions to grind a hard material, such as a semiconductor wafer. 
     In addition, it is also object of the present invention to provide a grinder and grinding method which is capable of exhibiting sufficient rigidity and performing a precise grinding operation. 
     Further, it is an object of the present invention to provide a grinder of twin head type capable of adjusting the levelness (horizontal inclination) of the grinding wheel so as to grind the both side surfaces of work precisely and to make the same parallel. 
     The object of the present invention can be achieved by a grinder that includes the following: 
     a first grinding wheel drive unit stood erect in a vertical direction, the first grinding wheel drive unit including a first spindle rotatable and a first housing for rotatably supporting the first spindle; 
     a first grinding wheel held at an end of the first spindle; 
     a first feeding means for moving the first housing in the vertical direction, the first feeding means having a mechanism for converting rotational movement into linear movement; and 
     a first hydrostatic radial bearing for movably supporting the first housing. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the first feeding means comprises a motor, a ball screw and a nut portion which are coupled with one another, and the ball screw is coaxially disposed with the first housing. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the first grinding wheel drive unit includes: 
     a second hydrostatic radial bearing provided within the first housing for directly and rotatably supporting the first spindle; and 
     a first hydrostatic thrust bearing for rotatably supporting the first spindle. 
     The above-mentioned construction of the grinder according to the present invention, more advantageously, further includes: 
     a first supplemental feeding means for moving the first grinding wheel in a vertical direction, the first supplemental feeding means having a first pressure regulating mechanism capable of regulating the pressure of a fluid in the first hydrostatic thrust bearing which rotatively supports the first spindle. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the first hydrostatic radial bearing includes a plurality hydrostatic radial bearing portions which are separated from each other in the vertical direction. 
     The above-mentioned construction of the grinder according to the present invention, advantageously, further includes: 
     a second grinding wheel drive unit disposed opposite to the first grinding wheel drive unit and stood erect in a vertical direction, the second grinding wheel drive unit including a second spindle rotatable and a second housing for rotatably supporting the first spindle; 
     a second grinding wheel held at an end of the second spindle in a state in which the second grinding wheel is held in parallel substantially with and opposite to the first grinding wheel; and 
     a second feeding means for moving the second housing in the vertical direction, the second feeding means having a mechanism for converting rotational movement into linear movement; and 
     a third hydrostatic radial bearing for movably supporting the second housing. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the second feeding means has a motor, a ball screw and a nut portion which are coupled with one another, and the second housing is coaxially disposed with the ball screw. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the second housing includes: 
     a fourth hydrostatic radial bearing for rotatably supporting the second spindle; and 
     a second hydrostatic thrust bearing for rotatably supporting the second spindle. 
     The above-mentioned construction of the grinder according to the present invention, advantageously, further includes: 
     a second supplemental feeding means for moving the second spindle in the vertical direction, the second supplemental feeding means having a second pressure regulating mechanism capable of regulating the pressure of a fluid in the second hydrostatic thrust bearing which rotatively supports the second spindle. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the third hydrostatic radial bearing comprises a plurality of hydrostatic radial bearing portions which are separated from each other in the vertical direction. 
     Note that the above-mentioned object can be achieved by a grinder, according to the present invention, includes: 
     a first grinding wheel drive unit stood erect in a vertical direction, and having a first spindle rotatable and a first supporting member for rotatably supporting the first spindle; 
     a first grinding wheel held at an end of the first spindle; 
     a first feeding means for moving the supporting member in the vertical direction, the first feeding means having a mechanism for converting rotational movement into linear movement; 
     a first hydrostatic thrust bearing for rotatively supporting the first spindle; and 
     a second feeding means for moving the first grinding wheel in a vertical direction, the second feeding means having a first pressure regulating mechanism capable of regulating the pressure of a fluid in the first hydrostatic thrust bearing. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the regulation of the pressure of the fluid in the first hydrostatic thrust bearing is performed by changing the back pressure of the hydrostatic thrust bearing. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the first pressure regulating mechanism includes: 
     a fluid pressure generating device; 
     two fluid supply pipe passages for allowing the fluid pressure generating device to communicate with the first hydrostatic thrust bearing to supply the fluid therein; 
     a supply pressure regulating mechanism provided at least one of the two fluid supply pipe passages. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the supply pressure regulating mechanism has an apparatus for adjusting a restriction of the fluid. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, the first spindle has a flange portion projecting radially, the first hydrostatic thrust bearing rotatably supports the flange portion, and an opening of one of the two fluid supply pipe passages faces the upper surface of the flange portion, and an opening of the other fluid supply pipe passage faces the lower surface of the flange portion. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, each of the fluid supply pipe passages is respectively provided with an apparatus for adjusting a restriction of the fluid. 
     The above-mentioned construction of the grinder according to the present invention, advantageously, further includes: 
     a second grinding wheel drive unit disposed opposite to the first grinding wheel drive unit and stood erect in the vertical direction, the second grinding wheel drive unit including a second spindle rotatable and a second supporting member for rotatably supporting the first spindle; 
     a second grinding wheel held at an end of the second spindle in state in which the second grinding wheel is held in parallel substantially with and opposite to the first grinding wheel; and 
     a third feeding means for moving the second supporting member in the vertical direction, the third feeding means having a mechanism for converting rotational movement into linear movement. 
     The above-mentioned construction of the grinder according to the present invention, advantageously, further includes: 
     a second hydrostatic thrust bearing for rotatively supporting the second spindle; and 
     a fourth feeding means for moving the second spindle in the vertical direction, the fourth feeding means having a second pressure regulating mechanism which is capable of regulating the pressure of the fluid in the second hydrostatic thrust bearing. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, 
     the first grinding wheel drive unit has a drive motor for rotating the first spindle, 
     the first feeding means has a motor, a ball screw and a nut portion which are coupled with one another, and 
     the axial line of the ball screw, a rotational axis of the drive motor and the axial line of the first spindle are in line with one another. 
     In the above-mentioned construction of the grinder according to the present invention, more advantageously, 
     the first grinding wheel drive unit has a first drive motor for rotating the first spindle, 
     the second grinding wheel drive unit has a second drive motor for rotating the second spindle, 
     the first feeding means has a first motor, a first ball screw and a first nut portion which are coupled with one another, 
     the third feeding means has a second motor, a second ball screw and a second nut portion which are coupled with one another, and 
     the rotational axis of the first drive motor, the rotational axis of the second drive motor, the axial line of the first ball screw, the axial line of the first spindle, the axial line of the second ball screw and the axial line of the second spindle are in line with one another. 
     In the above-mentioned construction of the grinder according to the present invention, advantageously, 
     regulation of the pressure of the fluid in the first hydrostatic thrust bearing of the second feeding means is performed by changing the back pressure of the first hydrostatic thrust bearing, and 
     regulation of the pressure of the fluid in the second hydrostatic thrust bearing of the fourth feeding means is performed by changing the back pressure of the second hydrostatic thrust bearing. 
     In the above-mentioned construction of the grinder according to the present invention, more advantageously, 
     the first feeding means has a first motor, a first ball screw and a first nut portion, 
     the third feeding means has a second motor, a second ball screw and a second nut portion, and 
     the axial line of the first ball screw, the axial line of the first spindle, the axial line of the second ball screw, the axial line of the second spindle, the axial line of the first hydrostatic thrust bearing and the axial line of the second hydrostatic thrust bearing are in line with one another. 
     In the above-mentioned construction of the grinder according to the present invention, more advantageously, 
     the first pressure regulating mechanism comprises a first fluid pressure generating device, two fluid supply pipe passages for allowing the first fluid pressure generating device to communicate with the first hydrostatic thrust bearing, and a first supply pressure regulating mechanism provided at least one of the two fluid supply passages, and 
     the second pressure regulating mechanism comprises a second fluid pressure generating device, two fluid supply passages for allowing the second fluid pressure generating device to communicate with the second hydrostatic thrust bearing, and a second supply pressure regulating mechanism provided for at least one of the two fluid supply passages. 
     In the above-mentioned construction of the grinder according to the present invention, more advantageously, 
     the first spindle has a first flange portion projecting radially at an end thereof, 
     the first hydrostatic thrust bearing rotatively supports the first flange portion, 
     an opening of one of the two fluid supply pipe passages faces the upper surface of the first flange portion, and an opening of the other fluid supply pipe passage adjacent to the first hydrostatic thrust bearing faces the lower surface of the first flange portion, 
     the second spindle has a second flange portion projecting radially at an end thereof, 
     the second hydrostatic thrust bearing rotatively supports the second flange portion, and 
     an opening of one of the two fluid supply passages faces the upper surface of the second flange portion, and an opening of the other fluid supply passage faces the lower surface of the second flange portion. 
     Further note that the above-mentioned object can be attained by a grinding method according to the present invention including: 
     a first feeding step of feeding a grinding wheel mounted on a spindle in a vertical direction while converting the motion from rotational movement into linear movement; and 
     a second feeding step of feeding the grinding wheel in the vertical direction while regulating the pressure of the fluid in a hydrostatic thrust bearing which rotatively supports the spindle. 
     In the above-mentioned grinding method according to the present invention, advantageously, the first feeding step has 
     a high-speed feeding step for feeding the grinding wheel at high speed to a position near a work to be ground by the grinding wheel, and 
     a low-speed feeding step for feeding the grinding wheel at low speed to bring the grinding wheel into contact with the work after high-speed feeding has been performed. 
     In the above-mentioned grinding method according to the present invention, advantageously, the second feeding step comprises a precise feeding step in which more precise feeding interval as compared with the low-speed feeding step can be performed. 
     In the above-mentioned grinding method according to the present invention, advantageously, the amount of feeding in the second feeding step can continuously be changed. 
     In the above-mentioned grinding method according to the present invention, advantageously, the second feeding step comprises: 
     a precise feeding step in which more precise feeding interval as compared with the low-speed feeding step can be performed; and 
     a finishing step of feeding the work in which more precise feeding interval as compared with the precise feeding step can be performed. 
     Further note that the above-mentioned object can be achieved by a grinding method, according to the present invention, comprising the steps of: 
     feeding in a vertical direction a housing which rotatably supports a spindle while said housing is being held by a hydrostatic radial bearing. 
     Furthermore note that the above-mentioned object can also be attained by a grinder, according to the present invention, includes: 
     an upper grinding wheel drive unit stood erect in the vertical direction; 
     an upper spindle rotatively disposed in the upper grinding wheel drive unit; 
     an upper motor for rotating the upper spindle; 
     an upper grinding wheel held at an end of the upper spindle; 
     an upper feeding means having a mechanism for converting rotational movement into linear movement and arranged to move the upper grinding wheel in the vertical direction, wherein 
     the rotational axis of the upper motor, the rotational axis of the upper spindle, the rotational axis of the lower spindle and the axial line of the upper feeding means are in line with one another. 
     In the above-mentioned structure of the grinder according to the present invention, advantageously, further including 
     precise feeding means having a first pressure regulating mechanism which is capable of regulating the pressure of a fluid in a first hydrostatic thrust bearing for rotatively supporting the first spindle and arranged to move the first grinding wheel in the vertical direction, wherein 
     the axial line of the precise feeding means is in line with the axial line of the upper feeding means. 
     Moreover, note that the above-mentioned object can also be attained by a twin-head grinder, according to the present invention, includes: 
     an upper grinding wheel drive unit stood erect in the vertical direction; 
     an upper spindle rotatively disposed in the upper grinding wheel drive unit; 
     an upper motor for rotating the upper spindle; 
     an upper grinding wheel held at an end of the upper spindle; 
     upper feeding means having a mechanism for converting rotational movement into linear movement and arranged to move the first grinding wheel in the vertical direction; 
     upper precise feeding means having an upper pressure regulating mechanism which is capable of regulating the pressure of a fluid in the upper hydrostatic thrust bearing which rotatively support the upper spindle and arranged to move the upper grinding wheel in the vertical direction; 
     a lower grinding wheel drive unit disposed opposite to the upper grinding wheel drive unit and stood erect in the vertical direction; 
     a lower spindle rotatively dispose in the lower grinding wheel drive unit; 
     a lower motor for rotating the lower spindle; 
     a lower grinding wheel held at an end of the lower spindle in such a manner that the lower grinding wheel is held in parallel substantially with and opposite to the upper grinding wheel; 
     lower feeding means having a mechanism for converting rotational movement into linear movement and arranged to move the lower grinding wheel in the vertical direction; and 
     lower precise feeding means having a lower pressure regulating means which is capable of regulating the pressure of the fluid in the lower hydrostatic thrust bearing which rotatively supports the lower spindle and arranged to move the lower grinding wheel in the vertical direction, wherein 
     the rotational axis of the upper motor, the rotational axis of the lower motor, the rotational axis of the upper spindle, the rotational axis of the lower spindle, the axial line of the upper feeding means, the axial line of the lower feeding means, the axial line of the upper precise feeding means and the axial line of the lower precise feeding means are in line with one another. 
     In addition, note that the above-mentioned object can also be attained by a grinder, according to the present invention, including: 
     a grinding wheel drive unit stood erect in the vertical direction; 
     a spindle rotatively disposed in the grinding wheel drive unit; 
     a grinding wheel holder disposed at an end of the spindle; and 
     a levelness adjustment apparatus for compensating the levelness of the grinding wheel holder by regulating the pressure of the fluid. 
     Furthermore, to achieve the object, according to one aspect of the present invention, there is provided a twin-head grinder having two grinding wheels which are moved in the axial direction while the two grinding wheels are rotated so that a work is ground, the twin-head grinder including: spindles for the grinding wheels arranged in such a manner that at least either of the spindles is rotatively borne by benches for the grinding wheels through a hydrostatic thrust bearing; and pressure regulating means for regulating the pressure of a fluid which is supplied to at least either of supply ports formed in the hydrostatic thrust bearing and allowed to communicate with each other in a direction opposite to each other in a direction of the axial line of the spindles. 
     When a work is ground by the above-mentioned twin-head grinder according to the present invention, the benches for the grinding wheels are quickly fed to positions adjacent to the work by the motors and the ball screws while the grinding wheels are rotated. Then, the feeding mode is switched to feeding for a grinding operation so that the work is ground in a quantity near a predetermined quantity. Finally, the pressure regulating means regulates the pressure of the fluid which is supplied to at least either of the supply ports of the hydrostatic thrust bearing. As a result, the bearing balance of the spindles in the axial direction realized by the hydrostatic thrust bearing is changed so that the grinding wheels are precisely fed and moved to predetermined grinding positions. 
     In the above-mentioned twin-head grinder according to the present invention, advantageously, the pressure regulating means has regulating means for changing the pressure by adjusting a restriction of the fluid. 
     Furthermore, in the above-mentioned twin-head grinder, advantageously, has the structure that the pressure regulating means comprises a regulating means for adjusting a restriction of the pressure. When the pressure is changed by the regulating means, the pressure of the fluid which must be supplied to the supply port of the hydrostatic thrust bearing can easily be regulated. 
     Moreover, To achieve the above-mentioned objects, according to one aspect of the present invention, there is provided a twin-head grinder having grinding wheel drive units which are stood erect and which include upper and lower spindles each of which can be rotated by a motor and arranged in such a manner that upper and lower grinding wheels are held substantially in parallel with and opposite to each other by the upper and lower spindles and a work is inserted into a space between the two grinding wheels so that the work is rotated and ground, the twin-head grinder including: first feeding means provided for each of the upper- and lower-grinding wheel drive units and each having a mechanism for converting each of the upper and lower grinding wheels from rotational movement into reciprocative straight movement so as to vertically move the upper and lower grinding wheels toward the work; and second feeding means provided for at least either of the upper- and lower-grinding wheel drive units, the second feeding means feeding the grinding wheel for a short distance by fluid pressure so as to precisely finish the work. 
     Each of the upper- and lower-grinding wheel drive units has the housing in the guide which is stood erect. Moreover, the spindle is rotatively disposed in the housing. To vertically move the grinding wheel by the first feeding means, the housing is vertically moved directly by the first feeding means. Since the mechanism for converting the rotational movement into the linear reciprocative movement is employed, the housing can quickly be fed so as to be allowed to approach the grinding wheel. Furthermore, the grinding wheel can be fed as it is so that the work is ground. 
     The second feeding means directly moves the spindle in the vertical direction. The second feeding means is provided for each of the upper- and lower-grinding wheel drive units, only the upper-grinding wheel drive unit or only the lower-grinding wheel drive unit. Moreover, a function is realized, with which feeding in sub-micron units which cannot easily be performed by the first feeding means, can be performed. 
     In the above-mentioned twin-head grinder according to the present invention, advantageously, the first feeding means incorporates a motor, a ball screw and a nut portion, and the axial line of the ball screw is made to be in line with the axial lines of the upper and lower spindles. 
     In the above-mentioned twin-head grinder according to the present invention, advantageously, the second feeding means supports the spindle by a hydrostatic thrust bearing and a hydrostatic radial bearing thereof and changes the back pressure of the hydrostatic thrust bearing so as to enable the spindle to move vertically. 
     The second feeding means is composed of the first hydrostatic radial bearing and a hydrostatic thrust bearing. The difference between the upper back pressure and the lower back pressure of the thrust bearings is used so that the spindles are moved precisely. 
     In the above-mentioned twin-head grinder according to the present invention, advantageously, the axial line of the upper spindle and the axial line of the lower spindle are made to be in line with each other, and axial lines of the upper and lower motors for rotating the upper and lower spindles, the first feeding means and the second feeding means are made to be in line with the axial lines of the upper and lower spindles. 
     The structure that the axial lines of the foregoing six units are made to be in line with the axial lines of the spindles means a structure that the axial lines of drive sources are made to be in line with those of the spindles in place of the structure in which motors for rotating the spindles are disposed on the side of the spindles so as to rotate the spindles by belts. Specifically, the above-mentioned structure can be realized by built-in type motors. Moreover, the drive source for generating the force for rotating the first feeding means and the linear reciprocative movement conversion mechanism are disposed to be in line with one another. In addition, the second feeding means is structured in such a manner that the force for the feeding operation is generated on the same axial line. Thus, the upper and lower spindles, the upper and lower first and seventh motors and the first and second feeding means are disposed in line with one another. As a result of the above-mentioned structure, the overall structure of the grinder has a symmetrical structure with respect to the axial lines of the spindles. Therefore, significantly stable mechanical rigidity can be realized and an extremely precise grinding operation can be performed. 
     The nature, utility and principle of the invention will be more clearly understood from the following detailed description and the appended claims when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is cross sectional view showing an essential portion of an embodiment of a twin-head grinder according to the present invention; 
     FIG. 2 is an enlarged cross sectional view showing a pressure regulator of a grinding wheel feeding unit; 
     FIG. 3 is a cross sectional view showing a state of an operation of the unit shown in FIG. 2; 
     FIG. 4 is a perspective view showing an essential portion of an apparatus for adjusting the levelness of a lower grinding wheel; 
     FIG. 5 is an enlarged cross sectional view showing a pressure regulator of a levelness adjusting apparatus; 
     FIG. 6 is a cross sectional view taken along line VI—VI shown in FIG. 5; 
     FIG. 7 is a cross sectional view showing a state of an operation of the unit shown in FIG. 6; 
     FIG. 8 is a graph showing the operation of the pressure regulator of the grinding wheel feeding unit; 
     FIG. 9 is a graph showing an operation for feeding a bench for a grinding wheel which is performed by the pressure regulator; 
     FIG. 10 is a graph showing an operation for a pressure regulator for the lower grinding wheel holder; 
     FIG. 11 is a graph showing an operation of the pressure regulator for feeding the grinding wheel holder; 
     FIG. 12 is front view schematically showing a twin-head grinder as a second embodiment of the present invention; 
     FIG. 13 is a side view showing the twin-head grinder; 
     FIG. 14 is a cross sectional view showing an upper-grinding wheel drive unit of the twin-head grinder according to the present invention; 
     FIG. 15 is a lateral cross sectional view showing a hydrostatic radial bearing of the upper-grinding wheel drive unit; 
     FIG. 16 is a cross sectional view showing a lower-grinding wheel drive unit; 
     FIG. 17 is a plane view showing a work support unit; and 
     FIG. 18 is a cross sectional view showing an essential portion of the work support unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the twin-head grinder according to the present invention will now be described with reference to the drawings. 
     As shown in FIG. 1, a lower spindle head  11  is mounted on a lower frame (not shown). A lower spindle  12  is rotatively supported in the central portion of the lower spindle head  11 . A lower grinding wheel  13  is mounted through a grinding wheel holder  14  formed integrally with the top end portion of the lower spindle  12 . A modifying motor  15  which is rotated when the grinding wheel is modified/corrected by a dressing operation is mounted on the side surface of the lower spindle head  11 . When the modifying motor  15  is rotated, the lower grinding wheel  13  is rotated at low speed through a pulley  16 , a belt  17 , a pulley  18  and a lower spindle  12  for the purpose of modifying/correcting the grinding wheel  13 . A lower machining motor  91  for rotating the grinding wheel holder  14  is included in the lower spindle head  11  so that the lower grinding wheel  13  is rotated at high speed when the work is machined. 
     A work holder  19  is disposed on a lower frame in such a manner that the work holder  19  is positioned adjacent to a position above the lower grinding wheel  13 . A work holding through hole  20  is formed in the central portion of the work holder  19 . A work  21  is inserted into the work holding through hole  20  of the work holder  19  in such a manner that a projection formed on the work holder  19  (not shown) and a groove formed on the work  21  are engaged to each other. Thus, the work  21  and the work holder  19  are rotated by a motor (not shown). When a machining operation is performed, the work  21  is rotated at low speed. Moreover, the lower surface of the work  21  is mounted on the lower grinding wheel  13 . 
     The upper spindle head  22  is mounted on an upper frame (not shown) in such a manner that the upper spindle head  22  can be moved vertically so that the upper spindle head  22  is disposed above the lower spindle head  11 . To cause the upper spindle  23  to extend in line with the lower spindle  12  when the work  21  is machined, the upper spindle  23  is rotatively supported in the central portion of the upper spindle head  22  through pairs of hydrostatic radial bearings  24  and  25  and hydrostatic thrust bearings  26  and  27 . An upper grinding wheel  28  is mounted on a grinding wheel holder  29  formed integrally with the lower end of the upper spindle  23 . 
     The hydrostatic radial bearings  24  and  25  have supply ports  24   a  and  25   a  for supplying oil serving as a pressure fluid to the outer surface of the upper spindle  23 . The hydrostatic thrust bearings  26  and  27  have supply ports  26   a  and  27   a  for supplying oil to the two opposed end surfaces of a flange portion  23   a  of the upper spindle  23 . 
     A grinding wheel modifying motor  30  is mounted on the side surface of the upper spindle head  22 . When the motor  30  is rotated, the upper grinding wheel  28  is rotated at low speed through a pulley  31 , a belt  32 , a pulley  33  and the upper spindle  23 . 
     An upper machining motor  90  is included in the upper spindle head  22  so as to rotate the upper grinding wheel  28  at high speed when the work is machined. A grinding wheel feeding motor  34  is mounted on an upper frame (not shown). When the motor  34  is rotated, the upper spindle head  22  is, through a ball screw  35 , quickly fed to a position near the work  21 , and then fed at low speed to a predetermined machining position through a guide (not shown). 
     A hydrostatic pump  36 , which is a fluid supply source, is connected to the supply ports  24   a  and  25   a  of the hydrostatic radial bearings  24  and  25  and the supply port  26   a  of the hydrostatic thrust bearing  26  through a supply pipe passage  37  and to the supply port  27   a  through a supply pipe passage  39 . The fluid under predetermined pressure is supplied from the hydrostatic pump  36  to the supply ports  24   a ,  25   a ,  26   a  and  27   a  through the supply pipe passages  37  and  39 . 
     A pressure regulator  38 , which acts as a pressure regulating means, is connected to the supply pipe passage  39  extending from the hydrostatic pump  36  to the supply port  27   a  of the hydrostatic thrust bearing  27 . After the upper spindle head  22  has quickly been fed to a position near the work  21  by the motor  34  and the ball screw  35 , a grinding operation is started in such a manner that the upper spindle head  22  is precisely fed. Then, the pressure regulator  38  regulates the pressure of the fluid which is supplied from the hydrostatic pump  36  to the supply port  27   a  of the hydrostatic thrust bearing  27 . Thus, the upper spindle  23  is furthermore precisely fed so that the upper grinding wheel  28  is moved downwards for a small distance to the predetermined grinding position. 
     As shown in FIG. 2, a fluid inlet port  41  and a fluid outlet port  42  are, apart from each other for a predetermined distance, formed in the outer surface of a housing  40  of the pressure regulator  38 . An adjustment rod  43  is movably inserted into the housing  40 . A small-diameter portion  43   a  and a large-diameter portion  43   b  are provided on the outer surface of the adjustment rod  43 . When the adjustment rod  43  has been moved to the left, the large-diameter portion  43   b  is moved to the left within a region between the fluid inlet port  41  and the fluid outlet port  42 , as shown in FIG.  3 . Thus, a choking passage  44  is formed among the fluid inlet port  41 , the fluid outlet port  42  and the small-diameter portion  43   a.    
     A regulating motor  45  constituting an adjustment member, as an adjustment means, is mounted on the outer surface of the housing  40 . A ball screw  46  is rotatively supported by the outer surface of the housing  40  through a bearing member  47  so as to be connected to a motor shaft of a regulating motor  45  through a coupling  48 . A nut  49  is attached to an outer end of the adjustment rod  43  through a joining plate  50 , and then threadedly engaged with the ball screw  46 . An encoder  51  is attached to the regulating motor  45  so as to detect an amount of movement of the adjustment rod  43  in accordance with the number of revolutions of the regulating motor  45 . A cover  52  is mounted on the housing  40  to cover the regulating motor  45 , the encoder  51 , the ball screw  46  and the nut  49 . 
     When the regulating motor  45  is rotated in a state in which the small-diameter portion  43   a  of the adjustment rod  43  is disposed between the fluid inlet port  41  and the fluid outlet port  42  as shown in FIG. 2, the adjustment rod  43  is moved to the left in FIG.  2  through the ball screw  46  and the nut  49 . As a result, as shown in FIG. 3, the large-diameter portion  43   b  of the adjustment rod  43  is moved to a position between the fluid inlet port  41  and the fluid outlet port  42 . Thus, the choking passage  44  is formed among the fluid inlet port  41 , the fluid outlet port  42  and the small-diameter portion  43   a . The length of the choking passage  44  is changed in accordance with the distance for which the adjustment rod  43  has been moved. Therefore, as shown in FIG. 1, the pressure of the fluid which is supplied to the supply port  27   a  of the hydrostatic thrust bearing  27  is lowered in accordance with the length of the choking passage  44  thus formed. As a result, the bearing balance of the upper spindle  23  in the axial direction realized by the two hydrostatic thrust bearings  26  and  27  is changed so that the upper grinding wheel  28  is precisely downwards fed. 
     As shown in FIGS. 1 and 4 to  7 , a levelness (horizontal inclination) adjustment unit  53  is disposed to correspond to the grinding wheel holder  14  of the lower grinding wheel  13 , the levelness adjustment unit  53  having a plurality of (for example, eight in this embodiment) pressure regulators  54  disposed on the lower spindle head  11  apart from one another by predetermined intervals. The pressurized fluid is supplied from the hydraulic pump  36  to the lower surface of the grinding wheel holder  14  of the lower grinding wheel  13  through the supply pipe passage  37 , valves  70  and each of the pressure regulators  54 . The pressure of the fluid, that is, each of the pressure regulators  54  is adjusted so that the horizontal inclination of the lower grinding wheel  13  is adjusted. The plurality of the pressure regulators  54  correspond to the plurality of the valves  70 . 
     That is, a fluid inlet port  56 , which is connected to the supply pipe passage  37  extended from the hydraulic pump  36 , is formed at an end of a housing  55  of the pressure regulators  54 . Moreover, a fluid outlet port  57  allowed to communicate with the lower surface of the grinding wheel holder  14  is formed in the outer surface of the housing  55 . An adjustment rod  58  is rotatively inserted into the housing  55 . A fluid passage  59  allowed to communicate with the fluid inlet port  56  is formed in the central portion of the adjustment rod  58 . Moreover, a choking passage  60  allowed to communicate with the fluid passage  59  and the fluid outlet port  57  is formed on the outer surface of the adjustment rod  58 . The use of the levelness adjustment unit  53  is very effective and useful to prevent the work  21 , the grinding wheel and the elements around them from being heated, because the adjustment of the levelness (horizonal inclination) of the grinding wheel in order of micron can be made with the levelness adjustment unit  53 . Note that this heat generation which is occurred between the work and the grinding wheel being rotated in inclined state could not be avoided by using a prior art, particularly, a prior art utilizing the mechanical manner (such as a jack device). 
     An adjustment motor  61  is mounted on an end of the housing  55  through a bracket  62 . A motor shaft of the adjustment motor  61  is connected to the adjustment rod  58  through a coupling  63 . An encoder  64  is attached to the adjustment motor  61  so as to detect the amount of rotations of the adjustment rod  58  in accordance with the number of revolutions of the adjustment motor  61 . 
     As shown in FIGS. 1,  6  and  7 , the adjustment rod  58  is rotated by the adjustment motor  61  of a pressure regulator  54  previously selected by a control means (not shown). Thus, the length of the choking passage  60  interposed between the fluid passage  59  and the fluid outlet port  57  is changed. The pressure of the fluid which is supplied from the fluid outlet port  57  of the selected pressure regulator  54  to the lower surface of the grinding wheel holder  14  is changed so that the horizontal inclination of the lower grinding wheel  13  is precisely adjusted. 
     The operation of the twin-head grinder having the above-mentioned structure will now be described. 
     When the twin-head grinder is operated to perform the grinding operation, the work  21  is brought to a position at which the work  21  is brought into contact with the lower grinding wheel  13  in a state in which the work  21  is rotatively held by the work holder  19 , as shown in FIG.  1 . In the above-mentioned state, the lower grinding wheel  13  is rotated by the lower machining motor  91 . Moreover, the upper grinding wheel  28  is rotated by the upper machining motor  90 . The upper spindle head  22  is moved downwards by the grinding wheel feeding motor  34  through the ball screw  35  so that the upper grinding wheel  28  is quickly moved to the position near the work  21 . Then, the feeding mode is switched to the low-speed mode for the machining operation so that the upper grinding wheel  28  is fed to the predetermined machining position. 
     When the feeding operation is performed by the grinding wheel feeding motor  34  and the ball screw  35 , the small-diameter portion  43   a  of the adjustment rod  43  of the pressure regulator  38  is positioned between the fluid inlet port  41  and the fluid outlet port  42 . Thus, the choking passage  44  does not exist among the fluid inlet port  41 , the fluid outlet port  42  and the small-diameter portion  43   a . Therefore, oil under predetermined pressure is supplied from the hydraulic pump  36  to the upper hydrostatic thrust bearing  26  through the supply pipe passage  37 . Moreover, the fluid under the above-mentioned pressure is supplied to the lower hydrostatic thrust bearing  27  through the supply pipe passage  39  and the pressure regulator  38 . Therefore, the upper spindle  23  is rotatively borne by the two hydrostatic thrust bearings  26  and  27  in such a manner that predetermined balance is maintained in the axial direction. 
     Then, the adjustment rod  43  is moved to the left by the regulating motor  45  of the pressure regulator  38 , as shown in FIG.  3 . Thus, the large-diameter portion  43   b  is moved so that the choking passage  44  is formed in the housing  40  at a position between the fluid inlet port  41  and the fluid outlet port  42 . The length of the choking passage  44  is changed in accordance with the amount of movement of the adjustment rod  43 . 
     In accordance with the change in the length of the choking passage  44  of the pressure regulator  38 , the pressure of the fluid which is supplied to the lower hydrostatic thrust bearing  27  is lowered, as shown in FIG.  8 . Since the pressure of the fluid is lowered as described above, the bearing balance of the upper spindle  23  realized by the two hydrostatic thrust bearings  26  and  27  is changed. As a result, the upper grinding wheel  28  is precisely moved in units of sub-microns, as shown in FIG.  9 . Therefore, the upper grinding wheel  28  is accurately moved to the predetermined grinding position. 
     In accordance with the relationship between the length of the choking passage  44  and the amount of the movement of the spindle obtainable from FIGS. 8 and 9, the length of the choking passage, that is, the relationship between the amount of rotations of the regulating motor  45  and the amount of the movement of the upper spindle can be obtained. As a result, the upper spindle  23  can precisely be set. 
     As shown in FIG. 9, an influence of a pressure curve shown in FIG. 8 causes relatively great feeding to be realized in region A in the precise feeding operation. In region B, relatively small feeding can be realized in the precise feeding operation. Therefore, the region B is used to perform a final stage for feeding the grinding wheel. 
     The relationship between the length of the choking passage  60  of the plurality of the pressure regulators  54  and the pressure of the fluid at the fluid outlet port  57  of the pressure regulators  54  as shown in FIG. 10 can be obtained. In accordance with the obtained relationship, the relationship between length and the amount of downward deviation of the corresponding portion of the grinding wheel holder  14  which is brought into contact with the pressure regulators  54  as shown in FIG. 11 can be obtained. 
     As a result, the relationship between the length of the choking passage  60 , that is, the adjustment motor  61  and the amount of the downward deviation of the contact portion of the grinding wheel holder  14  can be obtained. When a predetermined pressure regulator  54  is selected from the plurality of the pressure regulators  54 , the precise angle of inclination of the grinding wheel holder  14  and the lower grinding wheel  13  can be adjusted. 
     An effect obtainable from the above-mentioned embodiment will now be described. 
     The twin-head grinder according to this embodiment has the structure that the upper spindle  23  of the upper grinding wheel  28  is rotatively borne by the upper spindle head  22  through the hydrostatic thrust bearings  26  and  27 . The pressure regulator  38  serving as a pressure regulating means is provided which regulates the pressure of the fluid which must be supplied to the supply port  27   a  of the supply ports  26   a  and  27   a  of the hydrostatic thrust bearings  26  and  27 . 
     Since the pressure of the fluid which must by supplied to the supply port  27   a  of the hydrostatic thrust bearing  27  is regulated by the pressure regulator  38 , the upper grinding wheel  28  can precisely be fed to the predetermined grinding position. Moreover, a rigid and precise grinding operation can be performed. 
     The twin-head grinder according to this embodiment has the structure that the pressure regulator  38  is provided with the regulating motor  45  for changing the choking passage  44  and the length of the choking passage  44 . When the length of the choking passage  44  is changed by the rotations of the regulating motor  45 , the pressure of the fluid which must be supplied to the supply port  27   a  of the hydrostatic thrust bearing  27  can easily be adjusted. As a result, the upper grinding wheel  28  can precisely be fed. 
     The twin-head grinder according to this embodiment has the levelness adjustment unit  53  to correspond to the grinding wheel holder  14  of the lower grinding wheel  13 . Thus, the pressurized fluid is supplied to the lower surface of the grinding wheel holder  14  of the lower grinding wheel  13  through each of the pressure regulators  54  of the levelness adjustment unit  53 . When the pressure of the fluid discharged from the selected pressure regulator  54  is changed, the angle of inclination of the lower grinding wheel  13  can be adjusted. Therefore, the lower grinding wheel  13  can be adjusted to be in parallel substantially with the upper grinding wheel  28  so that the upper and lower grinding wheels  28  and  13  are able to precisely and in parallel machine the upper and lower surfaces of the work  21 . 
     The following modification of the embodiment of the present invention will now be described. 
     The foregoing embodiment may be arranged in such a manner that the supply pipe passage  37  is connected to the lower hydrostatic thrust bearing  27  to supply the fluid under the predetermined pressure. Moreover, the pressure regulator  38  may be connected to the upper hydrostatic thrust bearing  26  so as to regulate the pressure of the fluid which must be supplied to the supply port  26   a.    
     Another modification of the above-mentioned embodiment may be employed in which the pressure regulator  38  is connected to each of the upper and lower hydrostatic thrust bearings  26  and  27  so as to individually regulate the pressure of the fluid which must be supplied to each of the supply ports  26   a  and  27   a.    
     A modification of the above-mentioned embodiment may be employed in which the means for regulating the pressure of the fluid which must be supplied to the hydrostatic thrust bearings  26  and  27  is a pressure regulator  54  having a rotative adjustment rod  58  as shown in FIGS. 5 to  7  in place of the pressure regulating valve  38  having the slidable adjustment rod  43  as shown in FIGS. 2 and 3. 
     A modification of the above-mentioned embodiment may be employed in which an operation handle or an operation button is connected to the ball screw  46  in place of the regulating motor  45  of the pressure regulator  38  for the grinding wheel feeding unit so as to move the adjustment rod  43  by manually operating the operation handle or the operation button. 
     The above-mentioned embodiment may be modified in such a manner that an operation handle or an operation button is connected to the adjustment rod  58  in place of the adjustment motor  61  of the pressure regulators  54  of the levelness adjustment unit  53  so as to rotate the adjustment rod  58  by manually operating the operation handle or the operation button. 
     Since the one aspect of the present invention has the above-mentioned structure, the following effect can be obtained. 
     The grinder according to the present invention enables the grinding wheels to precisely be fed to predetermined grinding positions. Thus, a precise grinding operation can be performed. 
     The one aspect of the present invention has the structure that the length of the choking passage of the pressure regulating means is changed. Thus, the pressure of the fluid which must be supplied to the supply portion of the hydrostatic thrust bearing can easily be regulated. As a result, the grinding wheel can precisely be fed. 
     A second aspect of the present invention has a essential structure that comprising: 
     a first grinding wheel drive unit stood erect in a vertical direction, said first grinding wheel drive unit including a first spindle rotatable and a first housing for rotatably supporting said first spindle; 
     a first grinding wheel held at an end of said first spindle; 
     a first feeding means for moving said first housing in the vertical direction, said first feeding means having a mechanism for converting rotational movement into linear movement; and 
     a first hydrostatic radial bearing for movably supporting said first housing. 
     In addition to this structure, in the second embodiment described hereinafter, the second feeding means, which is capable of precisely feeding the grinding wheel, is optionally provided so as to grind the work more precisely. 
     Further to this structure in the second aspect of the present invention, the axial lines of the first and second feeding means are preferably made to be in line with each other in order to grind the work more precisely. 
     Note that the basic idea of the above mentioned structure according to the second aspect of the present invention applicable to both of a single head type grinder and a twin-head type grinder. 
     A second embodiment according to the second aspect of the present invention having the structure that the second feeding means is optionally provided for the upper-grinding wheel drive unit will now be described. As shown in FIGS. 12 and 13, a frame  101  is formed by securing a table  103  to the upper surface of a bed  102 . An opening portion is formed in the central portion of each of the upper plates  104  and  105  of the bed  102  and the table  103 . The upper plate  105  of the table  103  is supported by a holder  106  composed of a plurality of walls or poles. Moreover, the upper plate  104  of the bed  102  and the upper plate  105  of the table  103  are made to be substantially in parallel with each other. 
     An upper-grinding wheel drive unit  107  is provided for the upper plate  105  of the table  103 . Moreover, a lower-grinding wheel drive unit  108  is provided for the upper plate  104  of the bed  102 . The spindles  109  and  110  disposed in the corresponding upper- and lower-grinding wheel drive units  107  and  108  are coaxially disposed in line with each other. An upper grinding wheel U is mounted to the lower surface of an upper grinding wheel holder  111  disposed at the lower end of the upper spindle  109 , while a lower grinding wheel L is mounted to the upper surface of a lower grinding wheel holder  112  disposed at the upper end of the lower spindle  110 . Moreover, a work support unit  113  is disposed between the upper and lower grinding wheels U and L. 
     As shown in FIG. 14, the upper-grinding wheel drive unit  107  provided for the table  103  has a structure that a cylindrical upper housing  115 , which can vertically be moved by a first feeding means  116 , is disposed in a cylindrical upper guide  114  secured to the table  103 . The upper spindle  109 , which is rotated by a first motor  117  and which can vertically be moved by a second feeding means  118 , is disposed in the upper housing  115 . Moreover, the upper housing  115  is supported by a first hydrostatic radial bearing  119  with respect to the upper guide  114 , as shown in FIG.  15 . 
     The first motor  117  for rotating the upper spindle  109  is a built-in type motor disposed in the upper housing  115 . A stator of the first motor  117  is secured to the inner surface of the upper housing  115 , while a rotor of the same is secured to the outer surface of the upper spindle  109 . Since each of the upper housing  115  and the upper spindle  109  has a circular cross sectional shape, the axial line of the first motor  117  and that of the upper spindle  109  are in line with each other. 
     The first feeding means  116  is a mechanism for converting rotations of the motor into a linear reciprocative movement in the vertical direction. As shown in FIG. 14, a second motor  121  is secured to a head cap  120  which covers an upper end opening of the upper guide  114  in such a manner that the axial line of the second motor  121  is made to be in line with that of the upper spindle  109 . A first ball screw  123  is, by a coupling  124 , connected to an output shaft  122  of the second motor  121 . On the other hand, a first nut portion  126  is provided for a top cap  125  secured to cover the top opening of the upper housing  115 . The first ball screw  123  is threadedly engaged with and mounted to the first nut portion  126 . When the second motor  121  is rotated, the upper housing  115  is vertically moved in the upper guide  114  through the first hydrostatic radial bearing  119   a  and  119   b  while the upper housing  115  is being supported by the first hydrostatic radial bearings  119   a  and  119   b.    
     As shown in FIG. 14, the first hydrostatic radial bearings  119   a  and  119   b  are separated from each other in the vertical direction. 
     With a structure in which the upper housing  115  is rotatably supported by the first hydrostatic radial bearings  119   a  and  119   b , it is possible to support the upper housing  115  through a liquid in a non-contact manner, so that there is no friction resistance between the upper housing  115  and the cylindrical upper guide  114 . In addition to this, a rigidity of elements supporting the upper spindle  109  can be increased, so that the upper spindle can be fed precisely in order to sub-micron. 
     In addition to this, the upper housing  115 , the second motor  121 , the first ball screw  123 , a coupling  124  and the first nut portion  126  are coaxially provided with one another, the rigidity of elements supporting the upper spindle  109  is further increased, so that the upper spindle can be fed more precisely. 
     As shown in FIG. 14, the second feeding means  118  has a structure that the second hydrostatic radial bearing  127  disposed above and below the first motor  117  in the upper housing  115  rotatively supports the upper spindle  109 . Moreover, a flange  128  is provided at the lower portion of the upper spindle  109  at a position upper than the upper grinding wheel holder  111 . The outer portion of the flange  128  is supported by hydrostatic thrust bearings  129   a  and  129   b  which hold the foregoing outer portion from upper and lower positions in the vertical direction. A fluid pump  130 , which is specifically a hydraulic pump for supplying a pressurized fluid to the hydrostatic thrust bearings  129   a  and  129   b  and the second hydrostatic radial bearing  127 , supplies pressurized fluid through the pressure regulators  131   a  and  131   b . When the pressure regulators  131   a  and  131   b  have performed adjustment operations, their back pressures are changed. The difference in the pressure is used to precisely move the upper spindle  109  in the vertical direction. The second hydrostatic radial bearing  127  and the hydrostatic thrust bearings  129   a  and  129   b  are disposed on the outside of the upper spindle  109 . Their axes are in line with the axial line of the upper spindle  109 . 
     Third and fourth motors  132  and  133  are mounted to the table  103  provided with the upper-grinding wheel drive unit  107  in such a manner that the output shafts of the third and fourth motors  132  and  133  face downwards. Moreover, the third and fourth motors  132  and  133  are disposed on the line of the diameter of the upper guide  114  so as to be positioned opposite to each other. The third motor  132  rotates an arm  134  having a dresser D which is used when dressing of the grinding wheel is performed. The fourth motor  133  rotates an arm  135  having a sensor S for detecting abrasion of the grinding wheel. 
     As shown in FIG. 16, the lower-grinding wheel drive unit  108  has a saddle  137  which is capable of slidably moving along rails  136  provided for the upper plate  104  of the bed  102 . The saddle  137  has a lower guide  138  extending downwards. The lower housing  139  is disposed within the lower guide  138  in such a manner that the lower housing  139  can be moved vertically. Moreover, the lower spindle  110  is rotatively disposed within the lower housing  139 . The lower grinding wheel holder  112  is disposed at an upper end projecting over the lower housing  139  of the lower spindle  110 . Moreover, the lower grinding wheel L is secured to the upper surface of the lower grinding wheel holder  112 . When the position of the saddle  137  is adjusted, the axial line of the lower spindle  110  is made to coincide with the axial line of the upper spindle  109 , and then a grinding operation is performed. 
     The saddle  137  is slidably moved by a structure formed by engaging, to a second nut portion  142  provided for the saddle  137 , a second ball screw  141  which is rotated by a fifth motor  140  secured to the upper plate  104  of the bed  102 . When the fifth motor  140  is rotated, the saddle  137  is slidably moved along the rails  136 . The rails  136  have a guiding structure (not shown) in such a manner that one of the rails  136  is formed into a V-groove and the other one of the same has a flat shape. The saddle  137  is slidably moved so as to usually be positioned in the central portion of the bed  102  when the grinding operation is performed. When, for example, dressing of the upper grinding wheel U mounted on the upper-grinding wheel drive unit  107  is performed, the saddle  137  is retracted from the central portions so as to allow the dressing operation to be performed. 
     The lower housing  139  is engaged in the lower guide  138  in such a manner that the lower housing  139  is able to vertically be moved by a means arranged similarly to the upper-grinding wheel drive unit  107 . As shown in FIGS. 15 and 16, the lower housing  139  is supported by a third hydrostatic radial bearings  143   a  and  143   b  with respect to the lower guide  138 . 
     As shown in FIG. 16, the third hydrostatic radial bearings  143   a  and  143   b  are separated with other in the vertical direction. 
     Further, as shown in FIG. 14, the second upper feeding means  118  has a structure that the second hydrostatic radial bearing  127  disposed above and below the first motor  117  in the upper housing  115  rotatively supports the upper spindle  109 . Moreover, a flange  128  is provided at the lower portion of the upper spindle  109  at a position upper than the upper grinding wheel holder  111 . The outer portion of the flange  128  is supported by hydrostatic thrust bearings  129   a  and  129   b  which hold the foregoing outer portion from upper and lower positions in the vertical direction. A fluid pump  30 , which is specifically a hydraulic pump for supplying a pressurized fluid to the hydrostatic thrust bearings  129   a  and  129   b  and the second hydrostatic radial bearing  127 , supplies pressurized fluid through the pressure regulators  131   a  and  131   b . When the pressure regulators  131   a  and  131   b  have performed adjustment operations, their back pressures are changed. The difference in the pressure is used to precisely move the upper spindle  109  in the vertical direction. The second hydrostatic radial bearing  127  and the hydrostatic thrust bearings  129   a  and  129   b  are disposed on the outside of the upper spindle  109 . Their axes are in line with the axial line of the upper spindle  109 . 
     The lower housing  139  is vertically moved with respect to the lower guide  138  by a first lower feeding means  144 . In a usual case, the first lower feeding means  144  supports a work W mounted on the work support unit  113  at a position at which the work W must be supported. When the reverse side of the work W is ground, the lower grinding wheel L is moved upwards so as to be ground. When dressing is performed, the lower grinding wheel L is moved downwards to a position lower than the usual height. The first lower feeding means  144  has a short stroke and a structure not to vertically move the lower grinding wheel L during the machining operation. 
     As shown in FIG. 16, the structure of the first lower feeding means  144  is arranged in such a manner that a third nut portion  145  is formed to project over the outer surface of the lower housing  139 . Moreover, a third ball screw  147  which is rotated by a sixth motor  146  secured to the side portion of the lower guide  138  is threadedly engaged with the third nut portion  145 . When the sixth motor  146  is rotated, the lower housing  139  can vertically be moved. 
     The above-mentioned structure is arranged in such a manner that the axial line of the first lower feeding means  144  is made to be substantially in parallel to the lower spindle  110 . The structure according to the present invention is not limited to this. The structure of the first feeding means  116  of the upper-grinding wheel drive unit  107  may have an inverted structure (not shown) so that the axial line of the first lower feeding means  144  coincides with the axial line of the lower spindle  110 . 
     Further, as shown in FIG. 16, a lower second feeding means  300  which is substantially the same as that of the upper-grinding wheel drive unit  107  is provided. That is, a flange radially expanded from the lower spindle  110  is provided at the upper portion of the lower spindle  110  at a position lower than the upper grinding wheel holder  111 . The outer portion of the flange  700  is supported by hydrostatic thrust bearings  300   a  and  300   b  which hold the foregoing outer portion from upper and lower positions in the vertical direction. A fluid pump  500 , which is specifically a hydraulic pump for supplying a pressurized fluid to the hydrostatic thrust bearings  300   a  and the second hydrostatic thrust bearing  300   b , supplies pressurized fluid through the pressure regulators  400   a  and  400   b . When the pressure regulators  400   a  and  400   b  have performed adjustment operations, their back pressures are changed. The difference in the pressure is used to precisely move the lower spindle  110  in the vertical direction. The second hydrostatic thrust bearings  300   a  and  300   b  and the hydrostatic radial bearings  600   a  and  600   b  are disposed on the outside of the lower spindle  110  to support the same. Their axes are in line with the axial line of the lower spindle  110 . 
     The lower spindle  110  engaged in the inside portion of the lower housing  139  is rotated at high speed by the seventh motor  148  in the form of the built-in shape when the grinding operation is performed. Similarly to the upper-grinding wheel drive unit  107 , the built-in type seventh motor  148  has a stator secured to the inner surface of the lower housing  139  and a rotor secured to the outer surface of the lower spindle  110 . Thus, the axial line of the seventh motor  148  is made to be in line with the axial line of the lower spindle  110 , that is the seventh motor  148  is coaxially disposed with the lower spindle  110 . 
     The work support unit  113  is disposed above the upper plate  104  of the bed  102 , and the work support unit  113  is positioned upper than the lower grinding wheel L installed on the lower spindle  110 . As shown in FIGS. 16 to  18 , the work support unit  113  has a structure that a stationary table  149  is disposed on the upper plate  104  of the bed  102 . Moreover, a horizontal guide  150  is secured above the stationary table  149  so as to be substantially in parallel to the stationary table  149  while being apart from the same for a predetermined distance. A sliding table  151 , which is slidably moved on the upper surface of the horizontal guide  150  in the same direction as a direction in which the saddle  137  for supporting the lower-grinding wheel drive unit  108  is slidably moved, is moved by a fourth ball screw  153  which is rotated by an eighth motor  152  secured to the stationary table  149  as shown in FIG.  17 . Note that the sliding table  151  is formed into a frame shape having a circular opening formed in a rectangular plate. 
     A plurality of guide rollers  154  each having a V-groove are disposed around the edge of the circular opening in the sliding table  151  at the same intervals in the circumferential direction of the circular opening. The guide rollers  154  rotatively support a rotational frame  155  in the form of an annular shape. As shown in FIG. 18, a follower gear  156  is formed on the outer surface of the rotational frame  155 . A main drive gear  158  provided for a shaft of a ninth motor  157  provided for the sliding table  151  is engaged to the follower gear  156  so as to be rotated when the rotations of the ninth motor  157  have been started. Moreover, a support plate  159  is secured to the lower surface of the rotational frame  155  in such a manner that the internal space of the rotational frame  155  is covered. The support plate  159  is formed by a plate having a thickness smaller than that of the work W. The support plate  159  is arranged under a tension in the horizontally outward direction so that deflection and deformation of the support plate  159  because of the dead weight are prevented. Moreover, a setting hole  160  for detachably setting the work W to the central portion of the support plate  159  is formed. An engagement portion  161  in the form of a projection is formed at the inner end of the setting hole  160  so as to transmit the rotational force to the work W. On the other hand, the work W has an engagement portion H in the form of a recess which is so-called a “notch” arranged to be engaged to the engagement portion  161 . The work W is received in the setting hole  160  from an upper position so as to be placed on the lower grinding wheel L mounted to the lower spindle  110 . The work W sometimes has a cut portion (not shown) so-called an orientation flange. Also in this case, a means for transmitting the rotational force to the work W is similar to that employed when the notch is provided. 
     When the work W is set and the ninth motor  157  is rotated, the rotational frame  155  is therefore rotated by the main drive and follower gears  158  and  156 . Thus, the work W set in the setting hole  160  of the support plate  159  provided for the rotational frame  155  is rotated in synchronization with the rotations of the rotational frame  155 . 
     The structure is formed as described above. The work W is set to the lower grinding wheel B, and then the second motor  121  is quickly moved rotated so that the upper housing  115  is downwards. When the upper grinding wheel U has approached the upper surface of the work W, the first and seventh motors  117  and  148  which are built-in motors are rotated to rotate the upper and lower spindles  109  and  110 . Thus, the downward moving speed of the upper housing  115  is considerably decelerated so that a suitable grinding operation is performed. 
     With this operation so far, since the upper housing  115  is rotatably supported by the first hydrostatic radial bearings  119   a  and  119   b  while the upper housing  115  is being disengaged with the upper guide  114 , it is possible to grind the work precisely in order of sub-micron. 
     However, if it is required to grind the work more precisely, the rotations of the second motor  121  of the first feeding means  116  are interrupted. Moreover, the back pressure applied to the flange  128  of the lower hydrostatic thrust bearing  129   a  of the hydrostatic thrust bearings  129   a  and  129   b  is moderately reduced. Thus, the upper grinding wheel U is, in sub-micron units, moved downwards. As a result of the above-mentioned downward movement, significantly precise finishing is performed. 
     The structure in which the second lower feeding means is provided for the lower-grinding wheel drive unit  108  is arranged into a shape (not shown) similarly to the second upper feeding means  118 . However, the structure is turned upside down. In this case, the work W is set to the work support unit  113 , and then the upper grinding wheel U is moved downwards by the first feeding means  116  of the upper-grinding wheel drive unit  107 . Thus, the work W is held between the upper and lower grinding wheels U and B, and then the upper and lower grinding wheels U and L and the work W are rotated. The lower grinding wheel L is slowly and upwards moved by the first lower feeding means  144  so that the grinding is performed precisely. Then, if required, the second lower feeding means is operated so that the precise grinding operation is performed. Thus, finishing or more precise grinding to realize a predetermined state is performed. 
     When the first and second feeding means  116 ,  118 ,  144  are provided for the upper- and lower-grinding wheel drive units  107  and  108 , the two sides of the work W can simultaneously and precisely be finished by the upper and lower second feeding means. 
     In the above-mentioned description of the second embodiment according to the present invention, the twin-head grinder is explained. However, without saying that, the embodiment is applicable into a single-head grinder. 
     With the grinder according to the present invention comprising: a first grinding wheel drive unit stood erect in a vertical direction, said first grinding wheel drive unit including a first spindle rotatable and a first housing for rotatably supporting said first spindle; a first grinding wheel held at an end of said first spindle; a first feeding means for moving said first housing in the vertical direction, said first feeding means having a mechanism for converting rotational movement into linear movement; and a first hydrostatic radial bearing for movably supporting said first housing, it is possible to make a rigidity of the grinder increase and to grind the work precisely in order of sub-micron. 
     In addition, the grinder according to the present invention has the structure that switching is performed between the first feeding means which is capable of performing a quick feeding operation and the second feeding means which is capable of performing a precise feeding operation to grind a work. Since the feeding means suitable to fast feeding, feeding for the rough machining and feeding for the precise finishing can be selected, the work can significantly precisely be finished in shortest machining time. 
     The grinder according to the present invention has the structure that the axial line of the ball screw of the first feeding means is in line with the axial lines of the upper and lower spindles. Therefore, the grinding wheels can stably be fed. 
     The grinder according to the present invention comprises the second feeding means having the hydrostatic thrust bearings which support the spindles. The back pressure of the hydrostatic thrust bearing is changed so that the spindles are vertically moved. Therefore, the spindles can smoothly and precisely be moved in the vertical direction. As a result, significantly precise machining can be performed. 
     The grinder according to the present invention has the structure that all of the axial lines of the upper and lower spindles, the motors for rotating the upper and lower spindles and the first and second feeding means are made to be in line with one another. Therefore, a mechanically and thermally rigid structure can be realized. Moreover, a required amount of feeding can precisely be realized by the first and second feeding means. As a result, a work can reliably be finished to have a required thickness. 
     Further, the grinding method according to the present invention, it is possible to grind a work precisely by feeding a grinding wheel by a fine amount. 
     While there has been described in connection with the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the invention.