Method and apparatus for controlling the depth of cut in the radial direction of a rotary cutting tool in a machine tool

A method and an apparatus for controlling a radial depth of cut of a rotary cutting tool in a machine tool, in particular in a machining center is disclosed wherein an eccentric mechanism for controlling the radial depth of cut of the tip of the cutting tool is not contained in a main shaft as in a conventional machine tool. Instead, it is provided outside the main shaft, and the radial depth of cut of the tip of the cutter is controlled by a tool holder to be automatically changeably fit to the forward end of the main shaft such that a tool holding shaft mounted to the tool holder by means of an eccentric mechanism so as to have the eccentricity relative thereto be adjustable is adapted to be controlled through a depth of cut central shaft passed through the bore of a draw-bar, concentrically disposed within the bore of the main shaft, by means of a servo-motor through a differential gear mechanism and the eccentric mechanism.

BACKGROUND OF THE INVENTION 
The present invention relates to a machine tool and more particularly to a 
method for controlling the depth of cut in the radial direction of a 
rotary cutting tool in a numerically controlled machine tool such as a 
machining center and an apparatus for carrying out the same. 
Hitherto, in a numerically controlled machine tool such as a machining 
center, when the depth of cut in the radial direction (hereinafter 
referred to as the "radial depth of cut") of the tip of the cutting tool 
is to be controlled, it has been a common practice for an eccentric 
mechanism to be contained within the bore of the main spindle, and the 
control is carried out by radially shifting the tip of the tool by means 
of the eccentric mechanism. However, with such a system, since an 
eccentric bore having a large diameter must be formed substantially over 
the whole length of the main spindle, not only is its machining difficult, 
but also the rigidity of the main spindle is decreased. In order to 
maintain rigidity, the outer diameter of the main spindle must be 
increased. 
Furthermore, since it is impossible for the amount of eccentricity to be 
made large, there is the problem that the radial depth of cut of the 
rotary cutting tool is inevitably limited. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide a method for 
controlling the radial depth of cut of a rotary cutting tool in a 
numerically control machine tool such as a machining center as well as an 
apparatus for carrying out the method, the machine tool being provided 
with a rotary main spindle that is adapted to interchangeably fit a tool 
holder at its forward end and carrying out machining by the relative 
movement between the rotary cutting tool held by the tool holder and a 
workpiece, wherein an eccentric mechanism to control the radial depth of 
cut of the cutting tool is not contained in the main spindle, the 
machining is made easy, there is no decrease in the rigidity of the main 
spindle, the increase in the outer diameter of the main spindle is 
suppressed, and a larger radial depth of cut of the rotary cutting tool is 
possible than when the eccentric mechanism is contained in the main shaft 
as in a conventional machine tool. 
In accordance with the present invention a method and an apparatus for 
controlling the radial depth of cut of a rotary cutting tool in a machine 
tool are provided, in particular for a machining center which is provided 
with a rotary main spindle adapted to interchangeably fit a tool holder 
and which carries out the machining by the relative movement between the 
cutting tool and a workpiece. In the present invention, the tool holder is 
adapted to hold the cutting tool such that when the tool holder is 
disengaged from the main spindle, the radial depth of cut of the tip of 
the cutting tool can be adjusted by revolving an eccentric mechanism which 
has been locked at the fundamental angular position of the tool holder. 
The tool holder is adapted to be engaged with the forward end of the main 
spindle which has been stopped at a predetermined angular position, 
simultaneously the lock of the eccentric mechanism is released, the tool 
holder fit to the main spindle is fastened by a fastening mechanism 
mounted within the bore of the main shaft, a depth of cut control shaft 
axially moveably arranged within the bore of the fastening mechanism is 
adapted to be rotated by a servo-motor through a differential mechanism in 
synchronization with the main spindle, the depth of cut control shaft is 
adapted to engages with the eccentric mechanism at the fundamental angular 
position of the tool holder, and the serve-motor is adapted to control the 
radial direction of the tip of the cutting tool from the fundamental 
angular position by a numerical control value of the rotational angle 
through the depth of cut control shaft and the eccentric mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Prior to entering the description of the preferred embodiment, the 
principles behind the control of the radial depth cut of a rotary bit or 
cutting tool in a machine tool by the provision of an eccentric mechanism 
in a tool holder in accordance with the present invention will be 
explained with reference to all of the attached drawings, but with 
particular reference to FIG. 1. 
FIG. 1 is a front elevational view of a tool holder exchangeably mounted to 
the front end of a main spindle of a machine tool wherein the reference 
character S is the center of the main spindle, C is the center of a tool 
holding shaft of a tool holder, eccentric with respect to the center S, on 
which a rotary bit (200) is mounted, e is the amount of eccentricity of 
center C from center S, the reference numeral (200) is the rotary bit 
mounted on the tool holding shaft of the tool holder, .theta. is the 
rotational angle of the tip of bit (200) from the fundamental rotational 
position, P.sub.0 is the position of the top of the rotary bit (200) at 
the rotational fundamental angular position (i.e. the position when 
.theta.=0), and P is the position of the tip of the rotary bit (200) when 
the rotational angle of the tool holding shaft about center C is .theta.. 
Other various values are defined as follows: 
CP: the distance from the center C of the tool holding shaft to position P 
of the tip of the bit (200) (determined by the position of the tip of the 
rotary bit); 
SP: the distance-from the center S of the main spindle to position P of the 
tip of the bit (determining the radius of the bore to be formed in a 
workpiece by the tip of the bit); 
SC: the amount of eccentricity, e (determined by the position of the center 
C relative to the center S); 
From the above data, the following equation can be formulated: 
##EQU1## 
Since CP and SC are constant, by controlling the rotational angle .theta. 
of the tool holding shaft through the rotation of an eccentrically 
positioned tool holding shaft (112) by means of a servo-motor (40), the 
cutting rotational radius r=SP of the tip of the rotary bit which 
corresponds to the bore of the workpiece to be opened by the bit can be 
controlled as desired between a minimum radius, r min, and a maximum 
radius, r max. In this case, r min is the minimum boring radius of the tip 
P of the bit which the eccentrically positioned tool holding shaft (112) 
generates at the fundamental angular position P.sub.0, i.e. when 
.theta.=0, while r max is the maximum boring radius of the tip P of the 
bit which occurs at .theta.=180.degree.. The difference r max-r min =2e is 
the maximum depth of cut. Therefore, by suitably setting the reduction 
ratio between the servo-motor (40) and the eccentric tool holding shaft 
(112), it is possible to control the rotational angle of the tool holding 
shaft. For example, it is possible to control the rotational angle so as 
to obtain a rotation of 0.001.degree. pulse between .theta.=0 and 
.theta.=180.degree.. Therefore, it will be apparent that the depth of cut 
of the tip P of the rotary bit can be controlled so as to vary from zero 
to a maximum of 2e. 
Now, one embodiment of the present invention which is constituted so as to 
operate on the basis of the above-described principles will be fully 
explained. 
The present invention which is embodied in a numerically controlled machine 
tool and shown in FIGS. 2 to 6 principally comprises three units: (A) a 
headstock, (B) a depth of cut controlling gear box, and (C) a radial 
direction controlling rotary tool holder. 
The functions of the respective units are summarized below. 
(A) Headstock 
This unit is a driving mechanism for a main spindle to give cutting force 
to the rotary bit or cutting tool, and comprises an electric motor to be 
mounted on the body of the headstock, gear trains to change the speed of 
the main spindle, and a main spindle to which fastens a tool holder for 
controlling the radial depth of cut. 
(B) Depth of Cut Controlling Gear Box 
This unit comprises a differential gear mechanism adapted to effect 
synchronized rotation of the main spindle and a radial depth of cut 
controlling shaft as well as to cause relative rotation between the main 
spindle and the radial depth of cut controlling shaft by superimposing on 
the synchronized rotation the rotation of a servo-motor, and a connecting 
mechanism to transmit the relative rotation to the radial depth of cut 
controlling tool holder. 
(C) Radial Direction Controlling Rotary Tool Holder 
This unit comprises a mechanism to decrease the relative rotation between 
the main spindle and the depth of cut controlling shaft and to convert the 
relative rotation to a radial depth of cut of the tip of the rotary bit by 
the use of an eccentric mechanism, and a lock mechanism adapted to 
automatically lock and unlock the tool holding shaft at the time of 
clamping to or unclamping from the main spindle of the tool holder. 
Next, separate explanations will be given of the following mechanisms which 
consist of the combination of the above described units: (I) a rotation 
transmission mechanism, (II) an interlocking mechanism, and (III) a 
mechanism for controlling radial depth of cut. 
(I) Rotation Transmission Mechanism (FIG. 2a) 
A main spindle 2 which is rotatably supported within the body 1 of a 
headstock A and adapted to be rotatably driven by a drive mechanism (not 
shown) has on its rear end portion a gear 3 secured which transmits 
rotation of main spindle 2 to a gear 6 through an idle gear 5 that is 
freely rotatably supported by the body 4 of a gear box B for controlling 
depth of cut. Gear 6 is fixedly secured to a shaft 8 which is rotatably 
supported within a differential gear box 7 so that the rotation of gear 6 
is transmitted to a gear 9, also fixedly secured to shaft 8, within gear 
box 7. The body 4 of the depth of cut controlling gear box B is secured to 
the body 1 of the headstock A, and the differential gear box 7 is 
rotatably supported by the body 4 of the depth of cut controlling gear box 
B by bearing 10 and 11. Gear 9 meshes with a gear 13 secured on a shaft 12 
which is rotatably supported within differential gear box 7, and a gear 14 
secured to shaft 12 meshes with a gear 15 which is secured to a shaft 18 
which is in turn rotatably mounted on differential gear box 7 by bearings 
16 and 17, bearings 16, 17 being coaxial with the center axis of 
differential gear box 7. The rotation of gear 14 is transmitted through 
shaft 18 to a gear 19 which is secured to shaft 18 outside the 
differential gear box 7. The rotation of gear 19 is transmitted to gear 22 
through an idle gear 21 which is secured to a shaft 20 which is in turn 
rotatably supported by the body 4 of the depth of cut controlling gear box 
B. 
Gear 22 is supported on a depth of cut controlling shaft 24 so as to be 
axially slidably thereon, but is prevented from rotation relative thereto 
by means of a sliding key 25. The depth of cut controlling shaft 24 passes 
through a bore formed in a draw-bar 23 which is loosely axially mounted 
within the bore formed within main spindle 2. Gear 22 is rotatably 
supported by bearings 27, 28, and 29 to a nut-piece 26 which is fixedly 
secured to draw-bar 23. In addition, gear 22 is rotatably mounted on the 
body 4 of the depth of cut controlling gear box B by a bearing 30 and at 
the same time supported so as to be axially slidably by means of a key 31. 
A gear 43 which is connected through a key to the output shaft of a depth 
of cut controlling servo-motor 40 for the tip of the bit, mounted on the 
body 4 of depth of cut controlling gear box B, and also rotatably 
supported by the body 4 of the depth of cut controlling gear box B by 
bearings 41, 42 meshes with a gear 44 secured to differential gear box 7, 
whereby the rotation of the servo-motor 40 is adapted to rotate 
differential gear box 7 through gears 43 and 44. 
At this point it should be mentioned that the numbers of teeth of gears 3, 
6, 9, 13, 14, 15, 19 and 22 are to be appropriately selected such that a 
rotational speed ratio of 1:1 is obtained between the main spindle 2 and 
the depth of cut controlling shaft 24. That is, when the servo-motor 40 is 
not operated, the main spindle 2 and the depth of cut controlling shaft 24 
synchronously rotate, but when the servo-motor 40 comes into operation its 
rotation is transmitted to gear 44 through gear 43, causing the 
differential gear box 7 to be rotated. In this case, a planetary gear 
mechanism revolves shaft 12 around gear 9 so that the rotational angle of 
differential gear box 7 is superimposed on the rotational angle of shaft 
8, i.e. the angle of revolution of shaft 12 is superimposed on a 
rotational angle determined by gears 9, 13, 14, and 15 and the 
superimposed angle of revolution is given to shaft 18, and the output of 
shaft 18 is transmitted to gear 22 through gears 19 and 21. Thus, there is 
interposed a differential gear mechanism between main spindle 2 and the 
depth of cut controlling shaft 24, and a rotational angle which is 
controlled by the depth of cut servo-motor 40 is adapted to cause a 
desired relative rotation at a desired relative speed between main spindle 
2 and the depth of cut controlling shaft 24. 
(II) Interlocking Mechanism (FIG. 2a) 
The depth of cut controlling shaft 24 which loosely passes through the bore 
of draw-bar 23 so as to be rotatable and axially shiftable therein also 
passes through the through hole of the piston 51 of a cylinder 50 for 
clamping a bit and is rotatively supported by bearings 53 at its rear end 
portion, the bearings 53 being suitably supported by the body 1 of 
headstock A. 
The depth of cut controlling shaft 24 is adapted to be advanced towards the 
forward end of the main spindle 2 by the piston 54a of a cylinder 54 to 
drive the depth of cut controlling shaft 24 axially for fastening and 
releasing a connecting shaft 102, and, when piston 54a is advanced 
forward, slot 24a formed at the forward end portion of the depth of cut 
controlling shaft 24 is caused to be engaged with the engaging portion 
102a formed at the rear end portion of connecting shaft 102 for 
controlling the radial depth of cut of the bit, connecting shaft 102 being 
mounted in the through hole of a pull-stud 101 of the radial depth of cut 
controlling tool holder 100 which is possible to be mounted in a tapered 
hole 2a formed at the forward end of the main spindle 2 (see FIG. 2b). 
When piston 54a of drive cylinder 54 for depth of cut controlling shaft 24 
is moved rearward, the depth of cut controlling shaft 24 and connecting 
rod 102 become disconnected. Cylinder 54 is appropriately secured to the 
body 1 of the headstock A. 
In this case, similar to usual practice, the forward and rearward stroke 
ends of the piston 54a of the drive cylinder 54 for the depth of cut 
controlling shaft 24 are detected by proximity switches 55 and 56, 
respectively, provided at the rear part of cylinder 54 so that its 
positions to clamp and unclamp connecting shaft 102 are detected and 
confirmed. 
The depth of cut controlling shaft 24 can, at the time of the clamping of 
connecting shaft 102, be positioned at a predetermined angular position by 
the servo-motor 40, and the phase of the slot 24a for clamping connecting 
shaft 102 is adjusted so that it corresponds to the phase of the engaging 
projection 102a of the connecting shaft 102 which is positioned so that 
the radial depth of cut controlling tool holder 100 is engaged with the 
main spindle 2. In this case, the angular position of the depth of cut 
controlling shaft 24 is detected by a proximity switch 57 for detecting 
the rotary angular position. Clamping and unclamping of the radial depth 
of cut controlling tool holder 100 are carried out by clamping and 
unclamping pull-stud 101 of the radial depth of cut controlling tool 
holder 100 by a well-known bit fastening mechanism as follows. Upon 
advancing piston 51 of cylinder 50 for a bit clamping a bit the draw-bar 
23 is moved towards the forward end of the main spindle 2 through gear 22 
and nut-piece 26, whereby the radial depth of cut controlling tool holder 
100 is unfastened and urged forward, while, upon retraction of piston 51 
towards the forward end of the main spindle 2, gear 22 and nut-piece 26 
are freed, whereby draw-bar 23 is drawn towards the rear end of main 
spindle 2 by the action of belleville springs 58, causing the radial depth 
of cut controlling tool holder 100 to be clamped to the main spindle 2. 
(III) Radial Depth of Cut Controlling Mechanism (FIG. 2b) 
The radial depth of cut controlling tool holder 100, which is adapted to be 
automatically mounted to or dismounted from the forward end of the main 
spindle 2 by a well-known automatic tool changing device (not shown) has a 
connecting rod 102 which is coaxial with the rotating center line S of the 
main spindle 2 and passes through the bore of pull-stud 101, and is 
rotatably supported within the bore of a tapered shank 103 by bearings 104 
and 105. A gear 106 is fixedly secured to shaft 102 and meshes with a gear 
108 which is rotatably supported by tapered shank 103 and the body 107 of 
tool holder 100. Gear 108 meshed with a gear 115 which is secured to an 
eccentric depth of cut feed shaft 114. Shaft 114 is arranged in parallel 
with rotating center S of main spindle 2 and has its one end rotatably 
supported by the body 107 of the tool holder 100 by a bearing 109 and has 
its other end supported by a bearing 113 mounted in a tool holding shaft 
112 which is rotatably supported by bearings 111 in the eccentric bore of 
an eccentric casing 110 which is fixedly secured to the forward end of the 
body 107 of tool holder 100 so as to form a part thereof. The eccentric 
depth of cut feed shaft 114 is connected to the input shaft 118 of a 
reduction gear unit 100a by a key 116. Reduction gear unit 100a has an 
output portion 120 which is fastened to tool holding shaft 112, output 
portion 120 being given, through a stationary part 119 fastened to tapered 
shank part 103, a relative rotational speed difference between the 
rotating speed of main shaft 2 and the rotating speed of the eccentric 
depth of cut feed shaft 114 with the speed difference being reduced to a 
defined reduction ratio of the reduction gear unit 100a. This differential 
rotation is transmitted to the tool holding shaft 112 to which the output 
part 120 of the reduction gear unit 100a is connected, and further 
transmitted to a boring bar 122 through an eccentric bracket 121. Since 
the rotating center C of the boring bar 122 is deviated from the rotating 
center line S of the main spindle 2 by an eccentricity e, the tip position 
P of the cutting tool 200 (the boring tool) to be mounted on the boring 
bar 122 is rotated around the eccentrically positioned tool holding shaft 
112, whereby the radial distance relative to the center line S of main 
spindle 2, i.e. the boring center, is varied. As a result, the control of 
the radial dimensions of tne bored hole is made possible by the rotational 
control of servo-motor 40. 
At the time of clamping or unclamping of the radial depth of cut 
controlling tool holder 100 to or from the main spindle 2, an engaging 
driving key 130 provided at the forward end of main spindle 2 which has 
been previously indexed at a predetermined angular position by the 
detections of rotational angular position detectors 301 and 302 to be 
described later (see FIG. 2a) is made to automatically engage with a key 
groove 131 formed at the grip portion of tapered shank 103 of tool holder 
100 at the time of automatic tool interchange (see FIGS. 5 and 6). At this 
time, by abutting the tip portion 132a of a slide pin 132 for locking the 
connecting shaft 102 against the forward end surface of the key 130 of the 
main spindle 2, the slide pin 132 is caused to be shifted against the 
action of a spring 133, resulting in sliding of a pin 134 connected to the 
forward end of pin 132 to cooperate with a grooved cam formed in a lock 
pin 135. This causes lock pin 135, which is formed with the grooved cam 
and has been projected into the tooth space of gear 106 at a predetermined 
position, to be slid radially outwards, releasing connecting shaft 102 
from a lock condition in which radial direction depth of cut controlling 
tool holder 100 is released from main spindle 2 to a freely rotatable 
condition of connecting shaft 102. (See FIG. 2b) At the time of the 
release of the tool holder 100 from the main spindle 2, the tool holder 
100 is spaced apart from the forward end of the main spindle 2, freeing 
the slide pin 132, and pin 134 for the grooved cam is forced outwards by 
the action of the spring 133 so that the lock pin 135 projects into a 
tooth space of gear 106 at a predetermined position, resulting in a locked 
condition. 
The following is the operation and condition of the radial depth of cut 
controlling tool holder 100 at the time of its clamping. 
1. When a command for interchanging a bit is issued, main spindle 2 is 
indexed at a definite position by the detecting signal from detectors 301 
and 302, i.e. engaging driving key 130 is positioned at a definite 
position. 
2. Depth of cut controlling shaft 24 is positioned at the fundamental 
angular position, .theta.=0, by the operation of the servo-motor 40. (This 
position corresponds to the fundamental angular position of the tool 
holder 100 where the slot 24a of the depth of cut controlling shaft 24 is 
positioned at a predetermined angular position). 
3. The depth of cut controlling shaft 24 is at an unclamping position due 
to the action of cylinder 54. That is, piston 54a is retracted to the 
rightmost position as viewed in FIG. 2a. 
4. Draw-bar 23 is in an unclamped state. (Piston 51 of cylinder 50 for bit 
clamp is at the leftmost advanced position as viewed in FIG. 2a.) 
5. The radial depth of cut controlling tool holder 100 is locked at the 
fundamental angular position (.theta.=0) of connecting shaft 102 by the 
engagement of the projected lock pin 135 with a tooth space of gear 106. 
Under the conditions Nos. 1 to 5 described above, the tool holder 100 is 
automatically positioned by an automatic tool interchange device (not 
shown) in a position such that the groove 131 of the tool holder 100 can 
receive the engaging driving key 130 of the main spindle 2, and such that 
the tool holder 100 is clamped to the main spindle 2. Then, draw-bar 23 
becomes clamped by the retraction of piston 51 to the rightmost position 
as viewed in FIG. 2a as well as by the elasticity of the belleville 
springs 58, the bit being fastened, and the depth of cut controlling shaft 
24 comes to be in a state of engagement by the leftwards advancement of 
piston 54a of cylinder 54 as viewed in FIG. 2a, resulting in engagement of 
the engaging projection and engaging slot 102a and 24a, respectively, and 
the depth of cut controlling shaft 24 and connecting shaft 102 are 
connected together. On the other hand, the slide pin 132 of the connecting 
shaft 102 is urged towards the forward end of main spindle 2 and the 
connecting shaft 102 becomes unlocked from the tool holder 100. 
Further, the control and feed in the radial direction are carried out as 
follows. 
The position of the depth of cut controlling shaft 24 at the time of 
clamping of the tool holder 100 to the forward end tapered opening 2a of 
the main spindle 2 determines the fundamental position of the depth of cut 
in the radial direction of the tip of the bit or of the cutting tool 200. 
The release of the tool holder can be carried out substantially by 
reversing the above procedures Nos. 1 to 5, and the released tool holder 
100 is kept in a magazine by the automatic tool interchanging device with 
the eccentrically set tool holding shaft 112 being locked at the 
fundamental angular position (.theta.=0). 
The orientation of the main spindle 2 takes place using a magnetizing 
member as one of the rotational angular position detectors which is 
mounted at an appropriate position on the outer periphery of the main 
spindle 2 near its rear end portion, and a magnetic sensor 301 as the 
other of the rotational angular position detectors reacts to the magnetic 
field and directly detects a predetermined angular position at which the 
main spindle 2 is to be stopped so that the main electrical motor is 
electrically stopped in its operation. The magnetic sensor 301 is fixedly 
secured to the body 4 of the depth of cut controlling gear box B. 
Since the method in accordance with the present invention can be carried 
out as described above and the apparatus in accordance with the present 
invention has a constitution and operation as stated above, it will be 
appreciated that the present invention provides the following advantages: 
1. Since the eccentric mechanism is provided in the cutting tool portion, 
the main spindle can have an ordinary constitution, making the machining 
easy and not deteriorating the rigidity compared with a conventional 
apparatus in which the eccentric mechanism in contained within the main 
spindle; 
2. Since the reduction mechanism is installed within the tool holder and 
the reduction ratio is big, the effects of backlash, etc. of the 
differential gear mechanism are not large enough to damage the rigidity of 
the tip of the bit. 
3. Since the differential gear mechanism uses spur gears, it allows a 
higher rate of revolution, decreasing backlash and increasing the rigidity 
compared with a conventional differential gear mechanism wherein level 
gears are used. 
4. A larger amount of eccentricity can be realized. (This has a close 
relationship with effect No. 1 stated above, and as one example, an 
eccentricity of 5 mm, and a diameter of 20 mm can be given). 
5. Since a reduction mechanism having a larger reduction ratio (e.g. 120:1) 
is provided at the side of the tool holder, and the control system 
(servo-motor, differential gear mechanism, controlling shaft, etc.) 
necessiates little power, the mechanism can be made simple, reducing 
manufacturing costs. 
It is to be understood that although a single embodiment of the present 
invention has been illustrated and described above, it is not to be 
limited thereto except insofar as such limitation are included in the 
following claims.