GB2391498A

GB2391498A - Spindle unit with spindle supported internally and externally with gas bearings

Info

Publication number: GB2391498A
Application number: GB0318699A
Authority: GB
Inventors: Shinji Tanaka
Original assignee: Hitachi Via Mechanics Ltd
Current assignee: Hitachi Via Mechanics Ltd
Priority date: 2002-08-09
Filing date: 2003-08-08
Publication date: 2004-02-11
Anticipated expiration: 2023-08-08
Also published as: GB0318699D0; TWI301432B; GB2391498B; CN1485172A; TW200402344A; CN100482388C

Abstract

A blind hole (Fig 3, 7) is formed around a rotational axis O of a spindle 1 such that the hole is open at an end portion on a side opposite to a drill holding side. A center guide 30, having a plurality of air outlets 33 of a journal air bearing at its outer periphery, is fitted in the blind hole so as to be supported by a case 10. Compressed air is blown off from the air outlet so as to support the spindle in a diameter direction from a center side. Mass of such structure of the spindle can be made smaller, so that no unstable phenomenon occurs even if the spindle is rotated at high speed with high natural frequency. The centre guide may be fixed to the case. The push rod 20 is moved for attaching or detaching a tool. A further gas blowing off hole 13 may be formed at an inner peripheral face of said case.

Description

TITLE OF THE INVENTION

SPINDLE UNIT

BACKGROUND OF THE INVENTION

This invention relates to a spindle unit having a spindle, at one end of which tool holding means is provided, and gas bearing means for rotatably supporting the spindle.

Fig.5 is a cross-sectional view showing a spindle unit in a conventional printed board drilling machine. In Fig.5, a female type of tapered face It is formed at a top end (a lower end in the figure) of a spindle 1 which is stored in a case 10. The spindle 1 has a blind hole 7 which is formed along an axial direction, and a penetrating hole which penetrates in the axial direction is formed at a center portion of the bottom of the blind hole 7.

An inner face of this penetrating hole is the above-mentioned tapered face It which broads for a top end side (the lower side in the figure). And, a male type of a tapered face 2t, having the same angle as the tapered face It, is formed on an outer periphery of a collet 2. A hole 4 for engaging a drill (a tool) 3 is formed at a center portion of the collet 2 so as to elongate and to penetrate in the axial direction. The collet 2 has a plurality of slits (not shown) formed so as to reach the hole

4 from the tapered face 2t and formed at equal angular intervals in a circumference direction.

And, a rod 5 is supported on a rear side of the collet 2 (an upper side in the figure) on the inner peripheral side of the spindlel through a guide bush 6. The rod 5 is fixed and supported, being fitted in a center hole of the guide bush 6. A disc springs 8 are shrinkably provided between a bottom face 7a of the blind hole 7 which is formed at the inner face of the spindle 1 and the guide bush6so as to energize the collet 2 for the upper hand in the figure. The tapered face 2t and the tapered face It are closely contacted with each other, and then, the collet 2 is closed in the inner diameter direction owing to its elastic deformation, so that the drill 3 can be strongly held by frictional force between the inner face of the hole 4 and the surface of the drill 3.

The spindle 1 has a flange if uniting therewith so as to project in the outer peripheral direction from a neighborhood on the top end side (the lower side in the figure), and a rotor 9 which is connected with an outer peripheral face in the central portion in the axial direction and is comprised of a copper material in the shape of a ladder formed by connecting both ends thereof with each other so as to have an annular shape.

The case 10 is provided with a hole 11, in which

the spindle 1 fits so as to be rotatable. And, a housing portion llb is formed at the center portion I of the hole 11 in the axial direction so as to expand in the outer diameter direction, and a coil (a stator) 12 is located at a position in the housing portion lib, facing the rotor 9. The coil 12 is connected with an inverter power source (not shown).

A plurality of air outlets 13 (gas blowing off hole), which are included in a journal air bearing having air flow controlling orifices, are formed at a portion facing the hole 11 in the case 10 at a predetermined interval in an axis O direction, facing in the diameter direction. Gas (compressed air in this case) is blown off in a direction orthogonal to the axis O direction from the air outlet 13. As shown in the figure, two rows of air outlets 13 are respectively arranged in the upper and lower portions with respect to the coil 12. The journalair bearing having a group of the air outlets 13 arranged on the lower side with respect to the coil 12 is referred to as an air bearing A, and the journalair bearing haying a group of the air outlets 13 arranged on the upper side with respect to the coil 12 is referred to as an air bearing B. hereinafter. Each air outlet 13 is connected with a first supply port 14.

A top end side of the hole 11 (the lower side in

the figure) is provided with a spindle storing portion tic for rotatably storing the top end side I of the spindle 1, and an annular concave portion lid for storing the flange if in the spindle storing portion llc. On both upper and lower faces facing the flange if in the annular concave portion lid, a plurality of air outlets 15 are formed in the circumference direction. Air is blown off from each air outlet 15 in the axis O direction and each air outlet 15 is connected with a second supply port 16. One end of a push-rod 20 (an upper end of the figure) is fixed by a piston 22 having two flanges.

The piston 22 with two flanges is located in a double chamber lla comprising a cylinder portion, which is located on the upper portion of the case 10. A third and a fourth supply ports 23, 24 formed in the case 10 are respectively connected with the chamber lla.

A spring 25 prevents the piston 22 from dropping and contacting the pushrod20 with the rod 5 when supply of compressed air to the supply port 24 is stopped owing to some reason.

Subsequently, operations of the conventional spindle unit having the above-mentioned structure will now be explained. Compressed air is supplied through the supply port 23 so as to drop the piston 22. Following the drop of the piston, the push

rod 20 is moved on the lower hand in the figure so as to push the rod 5, and the collet 2 is moved on the lower hand in the figure against the energizing force of the disc springs 8.

When the tapered face 2t of the collet 2 departs from the tapered face it, the collet 2 is opened owing to no force acting in the direction of the inner diameter of the collet 2. In such a state, the drill 3 can be detached from the collet 2.

Subsequently, an another drill 3 is inserted into the collet 2, and thereafter supply of compressed air to the supply port 23 is stopped and the supply port24 is supplied with compressed air so as to lift the piston 22 up. Following these operations, the push-rod 20 is moved on the upper handin the figure.

Then, the tapered face 2t is contacted with the tapered face It with pressure by the energizing force of the disc springs 8 so as to close the collet 2,sothat the drill 3is strongly held by the spindle 1. Subsequently, the supply ports 14,16are supplied with compressed air. Then, the spindle 1 is supported in the diameter direction such that the axis thereof and the axis O of the hole 11 are corresponded with each other by the functions of the air bearings A and B. and is supported in the axis O direction by the function of the thrust air bearing

C. If the coil 12 is supplied with electric power in the above-mentioned state, rotational torque acts on the rotor 9 owing to the magnetic field

generated on the coil 12 so as to rotate the spindle Thereafter, the case 10 is moved in the axis O direction with means whichis not showninthe figurer and a workpiece is machined by the drill 3.

In recent years, demand for boring machining for boring holes having extremely thin diameter, such as a hole having a diameter of O.lmm or thinner, is increasing. Effective forming of a hole having a minimum diameter is to raise rotational speed of a spindle. In case ofbigh speed rotation of a spindle with an air bearing, it is necessary to avoid unstable phenomenon which is unique for the air bearing, such as excess run-out on a spindle, by setting the natural frequency of a spindle as high as possible.

It is effective to make a spindle lighter in order to raise the natural frequency of a spindle. In this case, two methods, that is, a method of making the outside diameter of a spindle smaller, and a method of making the whole length thereof shorter, are thinkable. If the outer diameter of a spindleis madesmaller, bearing stiffness (strength of bearing) at the time

of supporting a spindle with an air bearing lowers, so that thereis en anxiety of occurrence of unstable phenomenon when rotating a spindle at high speed.

In order to make the whole length of a spindle shorter, it is necessary to shorten the distance between the rows of the air outlets 13 comprising the air bearings A, B. or to provide one air bearing of both A, B. In any case, the bearing stiffness lowers owing to the air bearing, and then, there is an anxiety of occurrence of unstable phenomenon at the time of high speed rotation of a spindle.

The object of the present invention is to provide a spindle unit having a structure so as not to reduce bearing stiffness of an air bearing, improving work effectiveness by rotating a spindle at high speed, through which the above-mentioned problems in the prior art can be solved.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, the invention is a spindle unit having a spindle, at one end of which tool holding means is provided, and gas bearing means for rotatably supporting said spindle in a case member, comprising: a hollow portion formed in said spindle, extending from said one end to the other end along a rotation axis,;

a guide rod member inserted into said hollow portion along said axis, and supported by said case member in a coaxial manner with said axis; and said gas bearing means haying gas blowing off holes formed at an outer peripheral surface of said guide rod member for blowing gas off to a clearance between said outer peripheral face and an inner peripheral surface of said spindle, so that gas blown off from said gas blowing off holes supports said spindle just above its inner peripheral surface. According to this invention, the spindle can be certainly supported in the diameter direction by the journal air bearing having the gas blowing off hole, the air bearing structure can be simplified so as to make the whole length of the spindle shorter and make the spindle lighter, through which the natural frequency of the spindle can be raised. Therefore, the spindle can be rotated at high speed, being stably supported, without lowering the bearing stiffness of the air bearing, and the working efficiency can be improved, thereby. This effect leads to the small-sized unit.

In the above-mentioned case, it is further effective if said guide rod member is fixed by said case member so as to unite therewith.

Besides, it is further effective if said guide rod

member also functions as a push rod for operating said tool holding means by being supported by said case member so as to be freely moved in said direction of said axis of said spindle.

Besides, in the present invention, said gas bearing means further has a gas blowing off hole, formed at an inner peripheral face of said case member, for blowing gas off to a clearance between said inner peripheral face and an outer peripheral face of said spindle, so that gas blown off from said gas blowing off hole supports said spindle at its outer peripheral face.

An another aspect of the invention is the spindle unit, wherein an annular concave portion is formed at said case member, annularly broadening in said direction of said diameter of said spindle, an annular projecting portionis formed at saidspindle, annularly projecting in said direction of said diameter of said spindle so as to be stored in said annular concave portion, and said gas bearing means further has a gas blowing off hole formed at both side faces of said annular concave portion for blowinggas off to a clearance between said both side faces of said annular projecting portion, so that gas brown off from said gas blowing off holesupports said spindle in said axial direction.

According to this invention, the journal air

bearing having the gas blowing off hole (air outlet) located on the outer peripheral face of the guide rod member integrally supported by the case member in an at least direction of the diameter of the spindle, certainly supports the spindle in the diameter direction. Then, it is not necessary to provide the air bearing B (see Fig.5) owned by a conventional spindle unit. As the result, the length corresponding to the air bearing B in the whole length of a conventional spindle is not necessary so as to make it lighter. Therefore, the natural frequency of the spindle is raised. Then, the spindle can be rotated at high speed in the state of being stably supported without lowering the bearing stiffness when supporting the spindle with the air bearing. Besides, the spindle unit according to the invention may have no air bearing A (Fig.5) owned by the conventional spindle unit if the bearing stiffness of the journal air bearing is sufficiently high. In this case, the whole length of the spindle can be made shorter and the spindle can be made lighter in comparison with the spindle unit having only no air bearing B. BRIEF DESCRIPTION OF THE DRAWINGS

Fig.1 is a cross-sectional view of a spindle unit according to a first embodiment of the present

invention; Fig.2is across-sectional view of the spindle unit according to a second embodiment of the present invention; Fig.3is acrosssectionalview of the spindle unit according to a third embodiment of the present invention; Fig.4is across-sectionalviewof the spindle unit according to a fourth embodiment of the present invention; and Fig.5 is a cross-sectional view of a conventional spindle unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will now be explained hereinafter, referring to drawings.

(A first embodiment) Fig.1 is a cross-sectional view of a spindle unit according to a first embodiment of the present invention. In Fig.1, the same reference number is attached to an element having the same structure, an element (structure) having the same function as in Fig.5, and to an element (structure) having similar function to Fig.5, the shape of which is slightly different, and their explanation is omitted, thereby.

A spindle unit in this embodiment is provided with

the spindle l having the collet (tool holding means) 2 at its end (the lower side of Fig.l), and gas bearing means for rotatably supporting the spindle 1 with respect to the case 10, which are located in the case 10.

The case 10 has the hole 11 along the axial direction (up and down direction of Fig.l). The hole 11 has the double chamber lla for storing the piston with two flanges 22, the coil housing portion llb for housing the coil 12, the spindle storing portion tic for rotatably storing the top end portion of the spindle 1, and the annular concave portion lid, annularly broadening in the diameter direction of the spindle 1 so as to rotatably store the flange If of the spindle 1. A stepped portion lea for supporting a flange portion 30b of a center guide 30 (guide rod member) in a thrust direction is formed between the chamber lla and the coil storing portion llb in the case 10. The spindle storing portion tic is a clearance between the outer peripheral face of the spindle 1 and the inner peripheral face of the case 10.

The case 10 has both connection paths 37a, 38a respectively formed at different positions on the outer peripheral side of the coil storing portion lib. The connection path 37a connects with the first supply port 14 formed on the outer peripheral

face of the case 10 so as to penetrate, and connects with the air outlet 13 provided on the lower side with respect to the coil 12 through a connection path 37b. The connection path 38a connects with the second supply port 16 formed on the outer peripheral face of the case 10 so as to penetrate, and connects with the air outlet 15 through a connection path 38b.

The air bearing A which is a journal air bearing has a group of the air outlets13 located on the lower side with respect to the coil 12. That is, the air bearing A has the air outlet 13 formed at the inner peripheral face facing the spindle storing portion tic of the case 10, for blowing gas off to the clearance (the spindle storing portion llc) between the inner peripheral face and the outer peripheral face of the spindle 1, and comprises gas bearing means for supporting the spindle 1 by the gas blown off from the air outlet 13 at its outer peripheral face. Besides, the thrust air bearing C has a group of the air outlets 15. The spindle unit of this embodiment has no air bearing B. which is comprised of a group of the air outlets 13 located on the upper side with respect to the coil12, in the conventional spindle unit mentioned referring to Fig.5.

The spindle lhas the blind hole (a hollow portion) 7 formed extending from one end (the lower side of Fig.l) to the other end (the upper side of Fig.l)

along the axis (the axis O) direction. The spindle unit has the center guide 30 extending in the axial direction of the spindle 1, inserted into the blind hole 7. The gas bearing means for rotatably supporting the spindle 1 has the air outlet (gas blowing off hole) 33 formed at the outer peripheral face of the center guide 30, for blowing gas off to the clearance between the outer peripheral face of the center guide 30 and the inner peripheral face of the spindle 1, such that the gas blown off from the air outlet 33 can support the spindle 1 at its inner peripheral face.

The center guide 30 has en axial portion 30a having I a hollow chamber 32 described hereinafter in the axial direction, and the flange portion 30b formed on a rear end side of the axialportion 30a (the upper side in the figure) so as to unite therewith. The center guide 30 is integrally fixed by the case 10 in such a way that the top end of the axial portion 30a (the lower side of the figure) is fitted in the blind hole 7, having a clearance therebetween such I that its axis corresponds with the axis O. and the flange portion 30b is held by the stepped portion lea. That is, the center guide 30 is integrally supported by the case 10 in the diameter direction and the axial direction of the spindle 1 so as to secure a standard position of the spindle 1 to be

supported by a journal air bearing D mentioned hereinafter. Both holes 31 for slidably engaging the push-rod 20 are respectively formed at both end portions of the center guide 30 such that the center portion thereof penetrates in the axial direction.

The chamber32 having a diameter bigger then the hole 31 is positioned between the holes 31, 31 in the axial portion Boa.

A plurality of rows of air outlets 33 (three rows in the figure), which are provided at the journal air bearing (gas bearing means) D having air flow controlling orifices are located in the axis O direction on the outer peripheral face of the center guide 30 facing the inner peripheral face of the spindlelhaving the blind hole 7. And, a plurality of air outlets 33 are located in the circumference direction every row. That is, a plurality of the air outlets 33 are formed on the outer peripheral face of the axial portion 30a of the center guide 30 so es to penetrate and to connect with the chamber 32. A plurality of the air outlets 33 are respectively provided at almost equal intervals in the axial direction of the axial portion 30a so as to form a group by a plurality of the air outlets 33. And, the air outlets 33 are located at equal angularintervals in the circumference direction of the axial portion 30a in each group of the air

outlets. Bearing stiffness of the journal air bearing D is the same as or higher than one of the air bearing B as shown in Fig.5.

The push-rod 20 has a small diameter portion 20a, a big diameter portion 20b, a flange portion 20c formed at the lower portion of the big diameter portion 20b, a projecting portion 20d formed on the upper portion of the big diameter portion 20b, and a connection hole 34 formed on the upper side in the axial direction with respect to the intermediate portion of the push rod. The small diameter portion 20a is formed, being free to slide in the holes 31, 31 Of both ends of the center guide 30 end the chamber 32 such that the diameter thereof is almost the same as the hole 31 and is smaller than the chamber 32.

The big diameter portion 20b, the diameter of which is bigger than the small diameter portion 20a, is fitted in a center hole 22a of the piston 22, projecting on the upper hand from the flange portion 30b of the center guide 30. The projecting portion 20d, the diameter of which is smaller than the big diameter portion 20b, projects on the upper hand from the center hole 22a of the piston 22, and projects from a center hole lob of the uppermost portion of the case JO. An end of the connection hole 34 connects with the chamber 32 through a hole 35 which is formed at the intermediate portion of

the push-rod 20 so as to penetrate, and the other end thereof 36 connects with a source of compressed air (not shown). A reference mark 39 in the figure denotes a ring member fitted on the projecting portion 20d on the upper face portion of the piston 22. The push-rod 20 having the abovementioned structure receives energizing force acting on the upper hand by the spring 25 shrinkably provided between the flange portion 20c and the flange portion 30b in such a state that the small diameter portion 20a is inserted into the center of the axial portion 30a, the big diameter portion 20b is inserted into the center hole 22a of the piston 22, and the projecting portion 20d projects from the center hole 10b of the case 10.

Operations in this embodiment will now be explained. When the supply ports 14, 16 and the end portion 36 of the connection hole 34 of the push- rod 20 are respectively supplied with compressed air, the compressed air supplied from the end portion 36 blows off from the air outlet 33 of the center guide 30 fixed and supported by the case 10 through the flange portion 30b held by the stepped portion 10a so as to function as the journal air bearing D. The spindle 1 positioned on the outer peripheral side of the center guide 30 is thereby supported in the

diameter direction from the center side. And, the air bearing A supports the spindle 1 in the diameter direction from the outside, similar to the conventional case as shown in Fig.5. For this reason, the spindle 1 is supported in the diameter direction in a good condition such that the rotational axis of the spindle 1 corresponds with the axis O by the journal air bearing D and the air bearing A. And, the thrust air bearing C supports the spindle 1 in the axis O direction, similar to the conventional case.

In the above-mentioned state, the coil 12 is supplied with electric power so as to rotate the spindle 1. And, the case 10 is moved in the axis O direction by means (not shown) (up and down direction in the figure), and a workplace (not shown) is machined by the drill 3.

In this embodiment according to theinvention, the journal air bearing D, which is comprised of the air outlets 33 located on the outer peripheral face of the center guide 30, can certainly and stably support the spindle 1 in the diameter direction inside the spindle 1 as mentioned before, so that there is no necessity of providing the air bearing B (see Fig.5) in the conventional spindle unit.

Therefore, the length corresponding to the air bearing B can be cut from the whole length of the

spindle 1 so that the spindle 1 is made smaller and lighter (smallerin mass) than the conventionalone.

By doing so, natural frequency can be made higher.

For this reason, the spindle 1 can be rotated at high speed in the state of stably supported without lowering the bearing stiffness when supporting the spindle 1 by the air bearing. Besides' no unstable phenomenon occurs even if maximum rotational speed which is higher in comparison with a spindle having a conventional structure is set, so that machining efficiency can be improved.

The air bearing (gas bearing means) A provided in the above-mentioned embodiment has the air outlet (gas blowing off hole) 13 formed at the inner peripheral face of the case 10, for blowing gas off to the clearance between the inner peripheral face of the case 10 and the outer peripheral face of the spindle 1 so as to support the spindle 1 at its outer peripheral face by the gas blown off from the air outlet 13, so that the spindle 1 can be further stably supported by the air bearing A in addition to the above-mentioned journal air bearing D. Besides, in this embodiment, the case 10 is provided with the annular concave portion lid for storing the flange (annular projecting portion) If.

Furthermore, the spindle 1 is provided with the above-mentioned flange If formed, annularly

projecting in the diameter direction of the spindle 1, so as to be stored in the annular concave portion lid. And, the thrust air bearing (gas bearing means) C provided in this embodiment has the air outlet (gas blowing off hole) 15 formed at both side faces of the annular concave portion lid, for blowing gas off to the clearance between both side faces of the flange If, so that the gas blown off from the air outlet 15 supports the spindle 1 in the axial direction. Then, the spindle 1 stably supported in the diameter direction by the journal air bearing D and the air bearing A can be stably supported in the axial O direction by the thrust air bearing C. (A second embodiment) Subsequently, a second embodiment according to the invention will now be explained, referring to Fig.2. Fig.2 is a cross-sectional view of the spindle unit according to this embodiment. The spindle unit in this embodiment is different from one as shown in the former embodiment in Fig.1 in such an aspect that the center guide 30 is provided so as to unite with the push-rod 20, and the other portions are almost the same. So, in Fig.2, the same reference number is attached to an element having the same structure, an element (structure) having the same function as in Figs.1, 5, and to an element

(structure) having similar function, the shape of which is slightly different, to Figs.1, 5, end their explanation is omitted, thereby.

A small diameter portion led for fixing a cylindrical guide member 40 in the state of being inserted therein, and a stepped portion lOc formed on the lower portion side of the small diameter portion led are formed between the chamber lla and the coil storing portion llb in the case 10. The guide member 40 has an inner diameter almost the same as the outer diameter of the center guide 30 of this embodiment, and guides the center guide 30 slidably inserted in the axis O direction (up and down directions of Fig. 2), being held by the stepped portion lOc so as to be fixed in a small diameter portion led (that is, fixed by the case 10).

The center guide 30 in this embodiment has en axial portion 30c having the outer diameter almost the same as the axial portion 30a of the center guide 30 in Fig.1, an insertion portion 30d formed on the upper portion of the axial portion 30c, having the outer diameter almost the same as the big diameter portion 20b of the push-rod 20 as shown in Fig.l, a flange portion 30e provided between the axial portion 30c and the insertion portion 30d, and a projecting portion 30f having a diameter slightly smaller than the insertion portion 30d so as to

project from the center hole 22a of the piston 22 on the upper hand and to project from the center hole lOb of the case 10. The center guide 30 is substantially fixed by the piston 22, being held between the flange portion 30e and the ring member 39 so as to restrict a relative movement with respect to the piston 22.

Such a center guide 30 has the chamber 32 formed in the center portion extending along the axial direction, a plurality of rows (three rows in the figure) of the air outlets 33 comprising the journal air bearing D which are located in the axis O direction, and a pressing projection 30g projecting on the lower side for pushing down the rod 5 and the guide bush 6. A plurality of air outlets 33 are provided at equal angular intervals in the circumferencedirection of the center guide30 every each row located in the axis O direction.

As mentioned before, the center guide 30 in this embodiment has also the function of the push-rod 20 in the first embodiment. As mentioned before, the structure in the other portions and the operations in this embodiment are substantially the same as ones in the first embodiment, and overlapping explanation is omitted, thereby.

According to this embodiment, the center guide 30 also has the function of the push-rod 20, so that the structure of this unit can be further simplified in comparison with

the first embodiment in addition to the effects almost similar to the first embodiment. (A third embodiment) Fig.3is a cross-sectional view of the spindle unit according to a third embodiment of the invention.

In Fig.3, the same reference number is attached to an element having the same structure, an element (structure) having the same function as in Figs.1, 5, and to an element (structure) having similar function, the shape of which is slightly different, to Figs.l, 5, and their explanation is omitted, thereby. The spindle unit in this embodiment is different from one as shown in Fig.1 in such aspects that no air bearing A is provided, the flange If is located on the side opposite to the collet (tool holding portion) 2 of the top end of the spindle l (the upper side in Fig.3), and the piston 22A has one flange.

With the above-mentioned structure, the air outlet (gas blowing off hole) 15 provided on the lower hand in Fig.1 is located on the portion facing the flange If in the center portion of the case 10.

In case where the bearing stiffness of the journal air bearing Dis sufficient high, itis not necessary to provide the air bearing A formed in the spindle

unit in Fig.1, as shown in Fig.3. So, the whole length of the spindle 1 can be further made shorter and lighter in comparison with the case of Fig.1.

Besides, the flange If is located on the side opposite tothecollet 2 of the top end of the spindle 1, so that the center of gravity of the spindle 1 can be positioned near the center in the axis O direction. Then, an axial deviation of the spindle lcanbe further affectively prevented when rotating the spindle 1 at high speed. Furthermore, the piston in this embodiment is the piston 22A having one flange, so that the structure is simple and the spindle unit is lighter.

As mentioned before, the spindle unit in this embodiment has some aspects different from the spindle unit asshownin Fig.1. As mentioned before, but, the structure in the other portions and the operations in this embodiment are substantially the same as ones in the first embodiment, and overlapping explanation is omitted, thereby.

(A fourth embodiment) Fig.4isacross-sectional view of the spindle unit according to a fourth embodiment of the invention.

So, in Fig.4, the same reference number is attached to an element having the same structure, an element (structure) having the same function as in Figs.1, 5, and to an element (structure) having similar

function, the shape of which is rather difficult, to Figs.1, 5, and their explanation is omitted, thereby. The spindle unit in this embodiment is different from one as shown in Fig.1 in such aspects that no air bearing A is provided, the flange if is located on the side opposite to the collet (tool holding portion) 2 of the top end of the spindle 1 (on the upper side of Fig.4), and a chuck making use of centrifugal force (centrifugal chuck) is used as a tool holding unit.

In the above-mentioned chuck making use of centrifugal chuck (USP 5,997, 223, for instance), centrifugal force acting on a tool holding member by rotation of a spindle is used in place of the tool holding force of the holding member for holding a tool, and in order to attach or detach a tool, the rotational speed of the spindle is lowered so as to release the holding force. For this reason, the push rod 20 which is the means for attaching or detaching a tool in Fig.1, and the piston 22 which is the driving means of the push-rod 20 are not necessary. Therefore, the structural parts of the spindle 1 can be reduced, and the unit can be made lighter. The air bearing which is referred to in each embodiment is exemplary. So, a gas bearing making

use of the other gas in place of air is off course available in the present invention.

The present invention has been explained on the basis of the example embodiments discussed.

Although some variations have been mentioned, the embodiments which are described in the specification are illustrative and not limiting.

The scope of the invention is designated by the accompanying claims and is not restricted by the descriptions of the specific embodiments.

Accordingly, all the transformations and changes within the scope of the claims are to be construed as included in the scope of the present invention.

Claims

1. A spindle unit having a spindle, at one end of which tool holding means is provided, and gas bearing means for rotatably supporting said spindle in a case member, comprising: a hollow portion formed in said spindle, extending from said one end to the other end along a rotation axis; a guide rod member inserted into said hollow portion along said axis, and supported by said case member in a coaxial manner with said axis; and said gas bearing means having gas blowing off holes formed in an outer peripheral surface of said guide rod member for blowing gas off to a clearance between said outer peripheral face and an inner peripheral surface of said spindle, so that gas blown off from said gas blowing off holes supports said spindle just above its inner peripheral surface.

2. The spindle unit according to claim 1, wherein said guide rod member is fixed by said case member so as to unite therewith.

3. The spindle unit according to claim 1, wherein said guide rod member also functions as a push

rod for operating said tool holding means by being supported by said case member so as to be freely moved in said direction of said axis of said spindle.

4. The spindle unit according to claims 1 through 3, wherein said gas bearing means further has a gas blowing off hole, formed et en inner peripheral face of said case member, for blowing gas off to a clearance between said inner peripheral face and an outer peripheral face of said spindle, so that gas blown off from said gas blowing off hole supports said spindle at its outer peripheral face.

S.The spindle unit according to claims 1 through 4, wherein an annular concave portion is formed at said case member, annularly broadening in said direction of said diameter of said spindle, an annular projecting portion is formed at said i spindle, annularly projecting in said direction of said diameter of said spindle so es to tee stored in said annular concave portion, and said gas bearing means further has a gas blowing off hole -

formed at both side faces of said annular concave portion for blowing gas off to a clearance between said both side faces of said annular projecting portion, so that gas blown off from said gas

blowing of f hole supports said spindle in said axial direction.