A multi-spindle unit for holding and rotating a plurality of rotary cutting tools has a unit body fixed to a first spindle of a machine tool, a rotatable shaft concentric with the unit body, and a plurality of secondary spindles each having a tool mounting portion at one end thereof. The rotatable shaft is coupled at one end portion to the unit body for receiving a torque from the holder body, the rotatable shaft being displaceable relative to the holder body. A rotation transmitting mechanism is provided to transmit a rotary motion of the rotatable shaft to the secondary spindles. A positioning member is fixedly disposed radially outwardly of the first spindle, and a cylindrical casing is disposed radially outwardly of the rotatable shaft such that the casing and the shaft are rotatable relative to each other. The casing is engageable with the positioning member for positioning thereof by the positioning member, and thereby positioning the secondary spindles.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a multi-spindle unit attached to 
a spindle of a machine tool for supporting a plurality of cutting tools 
rotated by the machine spindle. More particularly, the invention is 
concerned with such a unit for rotary cutting tools, which allows the 
rotary cutting tools to perform high-precision machining, unaffected by 
positioning error of the unit with respect to the machine spindle. 
Various tool holders have been used for mounting drills, milling cutters, 
reamers, boring bars and other rotary cutting tools on spindles of machine 
tools such as drilling, milling and boring machines, and machining centers 
which are capable of automatically performing multiple kinds of cutting 
operations. Such a tool holder supports a tool at its one end portion, and 
is adapted to be removably fixed to the machine spindle at the other end 
portion. 
However, surfaces of the tool holder for fitting, positioning and other 
purposes with respect to the machine spindle are liable to wear due to 
mounting and dismounting of the tool holder to and from the machine 
spindle. Further, these surfaces are subject to cutting chips, dust and 
dirt, and other foreign matter. As a result, the tool holder does not 
always make a sufficiently snug fit in the spindle. Thus, it has been 
difficult or impossible to avoid a positioning error of the tool holder 
due to misalignment or inclination of its centerline with respect to the 
centerline of the spindle. This positioning error or misalignment of the 
tool holder results in an increase in run-out of the boring bar at the 
free end of the holder, and consequently leads to machining errors such as 
an oversize bore diameters bored by the boring bar. That is, the 
positioning error of the tool holder lowers the accuracy of machining with 
the tool. Such positioning error of a tool holder will give rise to 
serious trouble, particularly in fine-boring, reaming and other machining 
operations which require relatively high accuracy. 
In view of the above problem, one of the inventors of the present 
application and his co-workers proposed, in the pending patent application 
Ser. No. 625,960 filed June 29, 1984 (assigned to the assignees of the 
present application), a holder for a rotary cutting tool which is capable 
of preventing a decrease in accuracy of machining by the tool due to 
positioning error of the holder with respect to the machine spindle. This 
tool holder has a holder body fixed to a spindle of a machine tool and a 
rotatable shaft disposed concentrically with the holder body and having a 
tool mounting portion at its one end. The rotatable shaft is coupled at 
its other end portion to the holder body for receiving a torque from the 
holder body and is axially and radially displaceable relative to the 
holder body. Further, the tool holder has a positioning member fixedly 
disposed radially outwardly of the spindle and a casing disposed radially 
outwardly of the rotatable shaft such that the casing and the shaft are 
rotatably relative to each other. The casing is engageable with the 
positioning member for accurate positioning thereof by the positioning 
member, and thereby positioning the rotatable shaft, while the holder body 
is fixed to the spindle. 
In such a tool holder, a rotary cutting tool is fixed to one end portion of 
the rotatable shaft. The rotatable shaft is positioned by the casing, 
which is accurately positioned by the positioning member disposed fixedly 
around the outer circumference of the free end of the spindle. 
Accordingly, a possible misalignment of the holder body with respect to 
the spindle will not affect a machining accuracy of the rotaty cutting 
tool carried by the rotatable shaft. In other words, the accurate 
positioning of the casing permits a high-precision machining. 
Thereafter, however, there have arisen requirements for improvements in not 
only machining accuracy but also machining efficiency, by accomplishing 
simultaneous machining of plural bores. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a 
multi-spindle unit, which is a rotary cutting tool holder attached to a 
first spindle of a machine tool and equipped with a plurality of secondary 
spindles which are simultaneously driven by the first spindle, and 
positioned with high accuracy, irrespective of a positioning error of the 
multi-spindle unit with respect to the first spindle of the machine. 
According to the invention, there is provided a multi-spindle unit for 
holding a plurality of rotary cutting tools, which is attachable to a 
first spindle of a machine tool for rotary cutting movements of the 
cutting tools by the first spindle, comprising: 
(1) a unit body removably fixed to the first spindle for rotation thereof 
about an axis of the first spindle; 
(2) a rotatable shaft axially and radially displaceable relative to the 
unit body, the rotatable shaft being coupled at one end portion thereof to 
the unit body for receiving a torque from the unit body; 
(3) a positioning member fixedly disposed radially outwardly of the first 
spindle; 
(4) a casing disposed radially outwardly of the rotatable shaft such that 
the casing and the shaft are rotatable relative to each other, the casing 
being engageable with the positioning member for accurate alignment of its 
centerline with the first spindle and for accurate circumferential 
positioning thereof about its centerline with the unit body is fixed to 
the first spindle; 
(5) a plurality of secondary spindles disposed in the casing at a plurality 
of locations radially spaced from the axis of the rotatable shaft and 
rotatable about axes thereof parallel to the axis of the rotatable shaft, 
each secondary spindle having a tool mounting portion at its respective 
free end portion which protrudes from the casing to rotatably support a 
cutting tool; and 
(6) a rotation transmission mechanism disposed within the casing, for 
transmitting rotary movement of the rotatable shaft to the secondary 
spindles. 
In the above multi-spindle unit, the casing is accurately positioned by the 
positioning member and the secondary spindles supporting the individual 
rotary cutting tools are positioned with high accuracy by the casing, 
unaffected by a positioning error of the unit body with respect to the 
first spindle. In other words, since the alignment of the centerline of 
the casing and the circumferential angular position thereof about the 
centerline are accurately established with respect to the first spindle, 
each of the secondary spindles held by the casing is accurately positioned 
in the radial and circumferential directions, thereby permitting drilling, 
reaming, boring, and other machining operations with high accuracy. 
Further, cutting reaction forces applied from the plurality of rotary 
cutting tools to the plurality of secondary spindles are received by the 
machine tool such as a boring machine, a drilling machine and a machining 
center, with high rigidity via the casing and the positioning member. This 
effectively prevents displacement of each rotary cutting tool caused by 
the cutting reaction force and allows considerable heavy-duty cutting, 
thereby ensuring simultaneous machining of a plurality of bores with 
remarkably improved machining efficiency. 
According to one embodiment of the invention, the rotation transmission 
mechanism comprises a first eccentric shaft portion provided at one end of 
the rotatable shaft remote from the unit body. The first eccentric shaft 
portion is eccentric with respect to the rotatable shaft. The transmission 
mechanism further comprises a plurality of secondary eccentric shaft 
portions corresponding to the secondary spindles. Each of the secondary 
eccentric shaft portions is provided on one end of the corresponding 
secondary spindle opposite to the end thereof at which the cutting tool is 
mounted. The secondary eccentric shaft portion is eccentric with respect 
to the corresponding secondary spindle, such that the amount of 
eccentricity thereof to the secondary spindle is equal to that of the 
first eccentric shaft portion to the rotatable shaft. The transmission 
mechanism further comprises an oscillating plate which has a plurality of 
holes engaging the first eccentric shaft portion and the secondary 
eccentric shaft portions to transmit the rotary movement of the rotatable 
shaft to the secondary spindles. 
According to another embodiment of the invention, the rotation transmission 
mechanism comprises a train of gears connecting the rotatable shaft and 
the secondary spindles. 
According to a further embodiment of the invention, the casing comprises a 
first end plate engageable with the positioning member, a second end plate 
and an annular spacer connecting the first and second end plates with a 
distance therebetween axially of the rotatable shaft. The secondary 
spindles are rotatably supported by the second end plate. 
In one form of the above further embodiment of the invention, the second 
end plate is provided with a plurality of boss or bushing members 
corresponding to the secondary spindles and secured to the second end 
plate, and further provided with a plurality of bearings corresponding to 
the plurality of boss members. The secondary spindles are rotatably 
supported by the boss members via the bearings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
To further clarify the present invention, a preferred embodiment of a 
multi-spindle unit as applied to rotary cutting tools for a machine tool 
will be described in greater detail, referring to FIGS. 1 and 2. 
In FIG. 1, the reference character 2 designates a cylindrical spindle 2 of 
a boring machine, which is rotated about its axis and acts as a first 
spindle. A multi-spindle unit 6 for holding a plurality of rotary cutting 
tools 4 is attached to a free end of the first spindle 2. The 
multi-spindle unit 6 simultaneously imparts rotary movements of the first 
spindle 2 to the rotary cutting tools 4. 
The unit 6 comprises a unit body 8 removably fixed to the first spindle 2, 
a rotatable shaft 10 which is axially and radially displaceable relative 
to the unit body 8 and is coupled at one end portion thereof to the unit 
body 8 for receiving torque from the unit body 8, a casing 12 which is 
rotatable relative to the rotatable shaft 10, a plurality of secondary 
spindles 14 which are rotatably supported parallel to the rotatable shaft 
10 by the casing 12, and an oscillating plate 16 which transmits rotations 
of the rotatable shaft 10 to the secondary spindles 14. 
The unit body 8 consists of a shank member 15 and a torque member 34 which 
is of ring shape and is screwed into shank member 15. The shank member 15 
has a stepped axial hole 20 and an outer circumferential tapered surface 
22. The axial hole 20 is formed through a radially central portion of the 
shank member 15. The outer circumferential tapered surface 22 gradually 
decreases in outside diameter toward a rear end portion, i.e. a 
small-diameter end portion, of the shank member 15. A pull stud 24 is 
fixed to the rear end portion of the shank member 15, i.e., of the unit 
body 8, such that the stud 24 extends from the unit body 8 in the axial 
direction. The unit body 8 includes an annular flange portion 26 which 
protrudes radially outwardly from the outer circumference at its 
large-diameter end, and perpendicularly to the axis of the unit body 8. 
The annular flange portion 26 has a cutout 28 which receives or engages a 
boss 32 provided on the end face of the first spindle 2 at a location out 
of the center thereof, in order to prevent a relative rotation between the 
first spindle 2 and the unit body 8 when the unit body 8 has been 
sufficiently pulled into the first spindle 2, that is, when the outer 
circumferential tapered surface 22 of the unit body 8 has come into close 
contact with an inner tapered surface 30 formed in the free end portion of 
the first spindle 2. 
The rotatable shaft 10 is a stepped diameter shaft having a circular cross 
section. A small-diameter portion 10a of the rotatable shaft 10, with 
which the unit body 8 is associated, loosely fits in a through-hole formed 
in the torque member 34. An inner circumferential surface of the torque 
member 34 has a key slot 36 parallel to the centerline thereof. A key 38 
fixed to the rotatable shaft 10 is fitted to the key slot 36 of the 
rotatable shaft 10 such that the rotatable shaft 10 is able to move 
relative to the unit body 8 in the axial direction and cannot rotate 
relative to the unit body 8. A bolt 40 is threaded axially in the 
small-diameter portion 10a and a medium-diameter portion 10b of the 
rotatable shaft 10. Belleville or coned disc springs 42 are disposed, 
acting as elastic members, between the head of the bolt 40 and the torque 
member 34, to bias the rotatable shaft 10 toward the unit body 8. In other 
words, the rotatable shaft 10 and the unit body 8 are coupled to each 
other so that they are movable axially relative to each other against a 
resilient force of the coned disc springs 42. Further, they are coupled to 
allow a small degree of inclination and radial displacement of axes of the 
unit body 8 and the rotatable shaft 10 relative to each other. 
The casing 12 is mounted on the rotatable shaft 10 via a radial bearing 44 
and a thrust ball bearing 46 such that the casing 12 is rotatable relative 
to the rotatable shaft 10. The casing 12 has an end face 50 which is 
abuttable on the flange portion 26 of the unit body 8. When the unit 6 is 
not mounted on the first spindle 2, the end face 50 is pressed against the 
end face of the flange portion 26 by the biasing force of coned disc 
springs 42 which is transmitted via the bolt 40, the rotatable shaft 10, 
and the thrust ball bearing 46. When the unit 6 is mounted on the first 
spindle 2, the end face 50 is slightly spaced from the flange portion 26 
as shown in FIG. 1. The end face 50 has a recess 52 and the flange portion 
26 has a protrusion 54 which engages the recess 52 for preventing the unit 
body 8 and the casing 12 from rotating relative to each other. 
Further, the casing 12 has an outer circumferential tapered surface 56 and 
an end face 58. The outer circumferential tapered surface 56 is tapered 
such that its diameter is gradually decreased toward the unit body 8. The 
end face 58 extends outwardly in the radial direction from the large 
diameter end of the outer circumferential tapered surface 56. The casing 
12 is held in close contact with a positioning member 60 at the outer 
circumferential tapered surface 56 and the end face 58. The positioning 
member 60 is a generally annular member which is accurately pre-positioned 
and secured to a body 62 of the boring machine so as to surround the free 
end portion of the first spindle 2. The positioning member 60 has an inner 
circumferential tapered surface 64 which can be tight-fitted to the 
aforementioned outer circumferential tapered surface 56, and an end face 
66 which is abuttable on the aforementioned end face 58. That is, the end 
faces 58 and 66 act as abutment faces. The positioning member 60 is 
provided with a positioning protrusion 68 at a location spaced as far as 
possible from the centerline thereof. The postioning protrusion 68 
accurately engages a positioning cutout 70 which is formed in the casing 
12 to fit the protrusion 68, so that the circumferential angular position 
of the casing 12 with respect to the positioning member 60 is exactly 
established. 
In addition to a coned disc-shaped first member 72 having the 
aforementioned end face 50, the outer circumferential tapered surface 56, 
the end face 58 and other portions, the casing 12 comprises a short 
cylindrical second member 74 and a disc-shaped third member 76. The second 
member 74 is secured to a radially outward portion of the first member 72, 
and the third member 76 covers an open end of the second member 74. 
Therefore, the casing 12 forms a closed cylindrical container which 
consists of a first end plate in the form of the first member 72, a second 
end plate in the form of the third member 76, and an annular spacer in the 
form of the second member 74 connecting the two end plates with a distance 
therebetween axially of the rotatable shaft 10. The third member 76 has a 
plurality of fitting holes 78 at positions located radially away from the 
centerline of the aforementioned rotatable shaft 10, i.e., the centerline 
of the casing 12 as clearly illustrated in FIG. 2. Boss members 80 are 
fixedly fitted in the holes 78. Each of the secondary spindles 14 is 
rotatably supported by the corresponding boss member 80 via a bearing 
housing 82, a radial bearing 84, and thrust bearings 86 and 88. Each 
secondary spindle 14 consists of several members for the convenience of an 
assembly. After the assembly, each secondary spindle 14 functions as a 
unitized spindle. The secondary spindle 14 has a tool mounting hole 90 for 
accomodating a rotary cutting tool 4 at its one end portion. At its other 
end portion, the secondary spindle 14 has an eccentric shaft portion 92, 
the axis of which is offset a certain distance from the axis of rotation 
of the spindle 14. 
The eccentric shaft portion 92 of each secondary spindle 14 is engaged with 
the oscillating plate 16 via a radial bearing 94. The oscillating plate 16 
is generally a disc-shaped member. A radially outward portion of the 
oscillating plate 16 has a plurality of holes into which the eccentric 
shaft portions 92 are fitted. An axially central portion of the 
oscillating plate 16 has a fitting hole in which is rotatably received, 
via a radial bearing 98 and thrust bearings 100 and 102, an eccentric 
shaft portion 96 which is projectingly provided at the free end portion of 
the rotatable shaft 10. The rotatable shaft 10, its eccentric shaft 
portion 96, the secondary spindles 14 and their eccentric shaft portions 
92 are all disposed parallel to each other, and an eccentricity of the 
eccentric shaft portion 96 to the rotation axis of the rotatable shaft 10 
is equal to that of the eccentric shaft portion 92 to the secondary 
spindle 14. Therefore, when the rotatable shaft 10 rotates, the 
osscillating plate 16 makes oscillating motions in a circular locus around 
the rotation axis of the rotatable shaft 10, whereby rotations of the 
rotatable shaft 10 are transmitted to the secondary spindles 14. As seen 
from the foreoing, a rotation transmission mechanism of the present 
embodiment comprises the oscillating plate 16, and the eccentric shaft 
portions 92 and 96. 
After the rotary cutting tools 4, such as reamers, drills, and boring 
tools, are set in the secondary spindles 14, the unit 6 constructed as 
described hitherto is mounted to the boring machine. Before the unit 6 is 
mounted on the boring machine, the unit body 8 and the casing 12 are 
biased into close contact with each other at the flange portion 26 and the 
end face 50 by the resilient force of the coned disc springs 42. Also, the 
unit body 8 and the casing 12 are not allowed to rotate relative to each 
other by the engagement of the protrusion 54 and the recess 52. 
When the unit 6 is in such a state, the unit body 8 is inserted into the 
bore (30) of the first spindle 2 while the positioning protrusion 68 of 
the positioning member 60 is aligned with the positioning cutout 70 of the 
casing 12. More specifically, the positioning protrusion 68 and the 
positioning cutout 70 are first engaged to position the casing 12 
circumferentially. Then, the outer circumferential tapered surface 56 of 
the casing 12 is brought into contact with the inner circumferential 
tapered surfaces 64 of the positioning member 60. The pull stud 24 is then 
pulled by a drawbar (not shown) and the outer circumferential tapered 
surface 22 of the unit body 8 is tight-fitted on the inner circumferential 
tapered surface 30 of the first spindle 2. As this tight-fitting action 
occurs, the coned disc springs 42 are compressed by the torque member 34 
and a resultant resilient force of the coned disc springs 42 is 
transmitted to the casing 12 via the bolt 40, the rotatable shaft 10 and 
the thrust ball bearing 46, forcing the casing 12 against the positioning 
member 60. Therefore, the outer circumferential tapered surface 56 of the 
casing 12 is tight-fitted on the inner circumferential tapered surface 64 
of the positioning member 60. Also, the end face 58 of the casing 12 is 
brought into abutment on the end face 66 of the positioning member 60, 
thereby eliminating misalignment and inclination of the centerline of the 
casing 12. In this case, since the rotatable shaft 10 and the secondary 
spindles 14 have been accurately positioned to the casing 12, and the 
rotatable shaft 10 is axially and radially displaceable relative to the 
unit body 8, the casing 12 is accurately positioned by the positioning 
member 60, unaffected by a positioning error of the unit body 8 with 
respect to the first spindle 2. Therefore, the secondary spindles 14 are 
accurately positioned because they are accurately supported by the casing 
12. 
When the unit 6 is mounted on the boring machine, rotations of the spindle 
2 are transmitted to the unit body 8 by the engagement of the protrusion 
32 and the cutout 28, and are then transmitted to the rotatable shaft 10 
by the engagement of the key slot 36 of the torque member 34 and the key 
38. The rotations of the rotatable shaft 10 are further transmitted to the 
secondary spindles 14 through the eccentric shaft portion 96, the 
oscillating plate 16 and the eccentric shaft portions 92, thereby rotating 
the rotary cutting tools mounted on the second spindles 14. Accordingly, 
if a workpiece is positioned at a certain location with respect to the 
unit 6, and the unit 6 or the workpiece is advanced so that the workpiece 
and the unit 6 move toward each other, a plurality of holes can be 
machined at the same time. In this case, the casing is in close contact 
with the end face 66 of the positioning member 60 at a location 
sufficiently spaced from the centerline. Also, the positioning protrusion 
68 and the positioning cutout 70 are accurately engaged at a location 
sufficiently spaced from the centerline. Therefore, the casing 12 is not 
inclined or rotated by cutting reaction forces which are applied from the 
rotary cutting tools 4 to the secondary spindles 14, thus permitting the 
unit 6 to support a plurality of rotary cutting tools with high rigidity. 
When it is required to dismount the unit 6 from the first spindle 2, a 
pulling force applied to the pull stud 24 is removed to permit the unit 
body 8 to be disengaged from the first spindle 2 with the biasing force of 
the springs 42, and the flange portion 26 of the unit body 8 is brought 
into contact with the end face 50 of the casing 12. Then, by further 
applying a pushing force to the unit body 8 via the pull stud 24, the 
pushing force is transmitted from the flange portion 26 to the casing 12, 
and the engagement of the casing 12 with the positioning member 60 is 
released, whereby the unit 6 is ready to be dismounted. 
While the present invention has been described in its preferred embodiment 
suitable for holding rotary cutting tools for a boring machine, it is to 
be understood that the invention may be otherwise embodied. 
For example, it is not necessarily a requirement to use a rotation 
transmission mechanism comprising the oscillating plate 16 and the 
eccentric shaft portions 92 and 96 for transmitting rotations of the 
rotatable shaft 10 to the plural second spindles 14. Instead, it is 
possible to utilize another rotation transmission mechanism, such as a 
gear train, for transmitting rotations of the rotatable shaft 10 to the 
second spindles 14. Also, a means for positioning the casing 12 to the 
positioning member 60 is not limited to the one used in the embodiment. As 
described in the specification and shown in the accompanying drawing of 
the patent application identified in the introductory part of the present 
application, it is possible to employ another means, e.g., to provide 
plural positioning pins on the casing parallel to the centerline thereof 
and accurately fit the positioning pins into positioning holes formed in 
the positioning member. 
It will be obvious from the foregoing detailed description that many 
changes and modifications can be made to the embodiment described in 
detail without departing from the spirit or scope of the invention.