Spindle head for a machine tool

A spindle head for a machine tool rotatably carries a tool spindle, which is drivingly connected with an output shaft of a drive motor provided in axial alignment therewith. A drawing bar received in an axial through hole of the tool spindle is urged by means of springs for clamping on the tool spindle a tool being inserted into a front tapered bore of the tool spindle. In order to move the drawing bar against the springs and thereby to unclamp the tool from the tool spindle, an annular cylinder device is provided around the tool spindle. This cylinder device, when supplied with pressurized fluid, moves first and second operating members toward and away from each other in the axial direction of the tool spindle. The first operating member is abuttable with a first abutting member fixed on the tool spindle, while the second operating member is abuttable with a second abutting member, which is secured to the drawing bar, with opposite end portions thereof extending externally of the tool spindle through elongate holes formed in the tool spindle. Accordingly, movement of the first and second operating members toward each other causes the second abutting member and the drawing bar to move against the springs without applying substantial thrust force to the tool spindle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to a spindle head for a machine 
tool and more particularly, to a spindle head capable of axially moving a 
drawing bar in a tool spindle against springs for unclamping from the tool 
spindle a tool being inserted in a receiving bore formed at a front end of 
the tool spindle. 
2. Description of the Prior Art 
Generally, in one method of rotationally driving a tool spindle, a drive 
motor may be directly connected to the tool spindle. This method is 
advantageous in that it is possible to diminish the size of a spindle head 
rotatably carrying a tool spindle, because the method does not need to use 
certain accessories such as a gear train. However, spindle heads for 
machining centers must be provided with a hydraulic cylinder device for 
axially moving against the springs of a drawing bar received in a tool 
spindle. Such cylinder device is conventionally disposed at the rear side 
of the tool spindle. Accordingly, the space for installation of a drive 
motor is not available at the rear side of the tool spindle, and this 
makes it very difficult to apply the above-noted drive method to the 
spindle heads for machining centers. 
Further, there has been proposed a machine tool spindle head in which a 
cylinder device for axially moving a drawing bar is disposed at a lateral 
side of a tool spindle so as to secure the space for installation of a 
drive motor directly connected to the tool spindle. However, such 
arrangement may unavoidably result in enlarging the size of the spindle 
head if an attempt were made to incorporate the cylinder device in the 
spindle head. Such arrangement also makes the spindle head complicated in 
construction due to the necessity of a mechanism for transmitting power 
from the cylinder device to a drawing bar operating mechanism provided in 
the tool spindle. 
In addition, the drawing bar operating mechanism exerts a pushing force 
only upon the drawing bar when operated for unclamping a tool from the 
tool spindle. This results in overloading of the bearings rotatably 
supporting the tool spindle, whereby the rotational accuracy of the 
bearings is deteriorated. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide an 
improved spindle head for a machine tool which permits not only the direct 
drive of a tool spindle by a drive motor, but also incorporation of a 
drawing bar actuation cylinder device in a frame of the spindle head 
without substantial enlargement of the frame in dimension. 
Another object of the present invention is to provide an improved spindle 
head for a machine tool of the character set forth above, wherein the 
thrust force applied to a drawing bar for unclamping a tool from a tool 
spindle is prevented from being exerted on bearings rotatably supporting 
the tool spindle. 
Briefly, in a spindle head for a machine tool according to the present 
invention, a drive motor is mounted on a head frame, with an output shaft 
thereof being in axial alignment with a tool spindle and being connected 
with a rear end portion of the tool spindle. The tool spindle has received 
therein a drawing bar movable in an axial direction of the tool spindle 
and incorporates therein a spring which urges the drawing bar to clamp on 
the tool spindle a tool being inserted into a receiving bore formed at a 
front end portion of the tool spindle. An annular cylinder device is 
provided around the tool spindle for permitting the tool spindle to pass 
therethrough. The annular cylinder device, when supplied with pressurized 
fluid, moves first and second operating members toward and away from each 
other in the axial direction of the tool spindle. The first operating 
member is abuttable with a first abutting member fixed on the tool 
spindle, while the second operating member is abuttable with a second 
abutting member secured to the drawing bar and extending from opposite end 
portions thereof externally of the tool spindle through elongate holes 
formed on the tool spindle. Accordingly, the second abutting member is 
moved by the second operating member relative to the tool spindle, whereby 
the drawing bar is moved against the spring. 
With this configuration, since the cylinder device is formed at its central 
portion with a through bore, it is possible to directly connect the tool 
spindle with the drive motor through the through bore. Further, the 
cylinder device is annular so as to occupy only a relatively small space, 
and this makes it possible to incorporate the cylinder device in the frame 
of the spindle head without substantially enlarging the dimension of the 
frame. In addition, power from the cylinder device can be transmitted to 
the drawing bar through a simple machanism. 
Moreover, the thrust force applied from the second operating member to the 
drawing bar through the second abutting member is effectively cancelled 
within the tool spindle because the abutting engagement simultaneously 
occurs between the first operating member and the first abutting member. 
This prevents such thrust force from being exerted upon bearings rotatably 
carrying the tool spindle, whereby a desired rotational accuracy of the 
bearings can be maintained for a long period of time.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to the drawings and particularly to FIG. 1 thereof, there is 
shown a spindle head 10 for a machining center having a frame 10a to which 
a bearing sleeve 13 is secured. The sleeve 13 rotatably carries a tool 
spindle 11 therein through bearings 12. The spindle 11 is integrally 
connected at its rear end with an output shaft 21 of a drive motor 20 
through a coupling 22 and is formed at its front end with a tapered bore 
14 for receiving a tool T therein. 
The tool spindle 11 is formed with an axial through hole 15, in which 
provided are a drawing bar 16 and a set of washer springs 25 for 
rearwardly urging the drawing bar 16. The drawing bar 16 is provided at 
its front end with a tool arresting head 18 which includes, for example, 
an annular member 19 and a number of steel balls 17 held in the annular 
member 19 for radial movement, as is well known in the art. When the 
drawing bar 16 is rearwardly moved against the springs 25, the steel balls 
17 are radially inwardly moved along a cam surface (not shown) to thereby 
arrest a pulling stud Tc of the tool T. 
The spindle head frame 10a fixedly mounts a guide cylinder 28 at the rear 
side of and is in axial alignment with the tool spindle 11. The sliding 
piston 29 is received in the cylinder 28 for sliding movement in an axial 
direction of the tool spindle 11. This sliding piston 29 is cylindrical 
and has secured to its front end a tool unclamping cylinder device 30 
which comprises a cylinder 31, a piston 32 and an end cap 33. The cylinder 
31, piston 32 and end cap 33 are cylindrical or annular to define at their 
central portions a through bore 34, through which the rear end portion of 
the tool spindle 11 passes. The piston 32 has secured to its front end a 
first annular operating member 35 which faces a first abutting member 38 
threadedly fixed on the tool spindle 11. The end cap 33 has secured to its 
rear end a second annular operating member 36 which faces a second 
abutting member 39. This second abutting member 39 is secured to opposite 
ends of a radial extension 41, as best shown in FIG. 2. The radial 
extension 41 is secured to the drawing bar 16, with opposite ends thereof 
radially protruding beyond the external surface of the tool spindle 11 
through elongate holes 40 formed in the tool spindle 11. 
The first and second operating members 35 and 36 support needle bearings 45 
and 46 at inner end surfaces thereof, respectively. These needle bearings 
permit the first and second abutting members 38 and 39 to rotate while the 
same are in contact, respectively, with the first and second operating 
members 35 and 36. Accordingly, it is possible to clamp the tool T onto, 
and to unclamp it from the tool spindle 11 even when the same is rotating. 
Further, a pin 49 rearwardly extends from the rear end surface of the 
guide cylinder 28. This pin 49 is engaged with a hole 48a of a plate 48 
secured to the sliding piston 29 to work against rotation of the sliding 
piston 29. 
The operation of the apparatus as described above will be described 
hereinbelow. FIG. 1 shows a state wherein the drawing bar 16 has been 
rearwardly drawn by the force of the springs 25, with the arresting head 
18 clamping the tool T being received in the tapered bore 14 of the tool 
spindle 11. In this state, the sliding piston 29 and the tool clamping 
cylinder 30 integrally connected thereto are at a rearward end position 
because low-pressurized fluid is continuously supplied from a 
low-pressurized fluid supply P2 into a front chamber 28a of the guide 
cylinder 28. Consequently, clearances in the axial direction are 
maintained between the first operation member 35 and the first abutting 
member 38 and between the second operating member 36 and the second 
abutting member 39, so that no contact takes place therebetween during 
rotation of the tool spindle 11. 
ln order to unclamp the tool T being received in the tapered bore 14, first 
a magnetic change-over valve 50 is switched to supply pressurized fluid 
from a high-pressurized fluid supply P1 into a front chamber 30a of the 
tool unclamping cylinder 30. The piston 32 is thus retracted to bring the 
first operating member 35 into abutting engagement with the first abutting 
member 38. This abutting engagement discontinues the retraction movement 
of the piston 32 relative to the tool spindle 11, which, however, 
counteracts so as to advance the cylinder 31, the end cap 33 and the 
second operating member 36 relative to the tool spindle 11. Because the 
advance movement of the cylinder 31 relative to the piston 32 is further 
continued, the second operating member 36 is brought into abutting 
engagement with the second abutting member 39 and advances the drawing bar 
16 against the force of the springs 25. Consequently, the pulling stud Tc 
of the tool T is no longer arrested by the steel balls 17 on, in other 
words, the tool T is unclamped from the tool spindle 11 so as to make 
itself ready for tool exchange operation. 
During unclamping movement, although the thrust force from the second 
operating member 36 acts on the tool spindle 11 through the second 
abutting member 39, the radial extension 41, the drawing bar 16 and the 
washer springs 25, such thrust force is cancelled by the first abutting 
member 38 abutting with the first operating member 35. Accordingly, no 
thrust overload is applied to the bearings 12 and the bearing sleeve 13 
which supports the tool spindle 11. This avoids damage to the bearings 12 
and the sleeve 13, whereby high rotational accuracy of the tool spindle 11 
can be maintained for a long period of time. 
After the used tool T is removed from the tapered bore 14 and a new tool T 
is inserted therein, the change-over valve 50 is switched back to supply 
pressurized fluid into a rear chamber 30b of the tool unclamping cylinder 
device 30. The piston 32 is thus forwardly moved to disengage the first 
operating member 35 from the first abutting member 38, during which time 
the sliding piston 29 and the cylinder 31 integral therewith are 
rearwardly moved to disengage the second operating member 36 from the 
second abutting member 39, because low-pressurized fluid is continuously 
supplied into the front chamber 28a of the guide cylinder 28. 
Such movements of the piston 32 and the cylinder 31 permit the drawing bar 
16 to be rearwardly moved by the force of the springs 25. The tool 
arresting head 18 thus arrests the new tool T by engaging the steel balls 
17 with the pulling stud Tc of the new tool T, whereby the clamping and 
unclamping operation of the tools T are completed. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the foregoing teachings. lt is therefore to be 
understood that within the scope of the appended claims, the present 
invention may be practiced otherwise than as specifically described 
herein.