Boring device

A boring device includes a casing and an upwardly biased tool spindle supported in the casing so as to move in a feed direction and a return direction opposite thereto. A first thrust ring with a track surface is disposed coaxial with the tool spindle and is fixed thereto. A second thrust ring with a track surface is also disposed coaxial with the tool spindle and fixed to the casing such that the track surfaces of the first and second thrust rings face each other. A rolling element retaining ring is provided coaxially with the tool spindle between the first and second thrust rings and has bores circumferentially arranged at predetermined angular intervals. Rolling elements are fitted in the bores of the rolling element retaining ring such that the rolling elements roll on the track surfaces of the first and second thrust rings. Receptacles complementary to the bores are formed in the track surface of the first thrust member. A coupling mechanism is provided to allow the rolling element retaining member to directly or operatively engage with the second thrust ring and disengage therefrom.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a boring device, and more particularly to a 
drilling machine with an improved efficiency for removing chips formed by 
a boring tool. 
2. Description of Related Art 
A boring tool such as an annular cutter or a twist drill which has cutting 
edges at its tip is formed with flutes in its outer peripheral wall which 
are continuous with the cutting edges. Chips formed during the boring 
operation are taken out of the hole along the flutes. Chips are broken at 
such a length as is determined by the material of a workpiece, a cutting 
speed and a feed speed of the tool, and other factors. 
When a thrust is applied to the boring tool at a substantially constant 
feed speed, the chips are relatively long. As the chips are continuously 
formed, their weight and removal resistance increase, lowering the chip 
removal efficiency of the tool. Thus, chips are clogged in the spaces 
between the flutes and a hole being bored. The increased cutting 
resistance impairs the free cutting performance, reducing the boring 
efficiency, and the cutting edges are undesirably worn out. Further, 
friction heat makes the cutting edges soft. 
In order to solve the above problems, there has been proposed a chip 
breaking member fixed to a drill casing disposed above the cutting tool 
such that the breaking member hits against a continuous chip to forcibly 
break the same due to impact or resistance given by the breaking member. 
When such a chip breaking member is disposed adjacent to the boring tool, 
however, it hinders the boring operation. In addition, the continuous chip 
may take a helical shape or meander. Thus, it is not assured that the 
breaking member hits against the chip. The chips may be continuously 
removed without being broken. 
Further, the usage of a gun drill has been proposed. The gun drill supplies 
cutting oil under a high pressure through the boring tool to chips being 
removed so that the cutting oil causes the chips to flow out of the spaces 
between the flutes and the hole. However, this requires a special tool or 
a tool head. Therefore, this proposal is not suitable for a boring device 
such as a portable drill press or a relatively small drilling machine. 
U.S. Pat. Nos. 2,458,929, 2,514,758, 2,514,759 and 2,474,720 disclose drill 
presses each provided with a chip breaker which performs chip breaking 
during the return movement of the spindle. However, each of these chip 
breakers breaks the chip into pieces having one set length, and thus lack 
the flexibility to break the chip into a plurality of lengths, a given 
length being chosen in accordance with the prevailing boring conditions. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a boring device for 
selecting an appropriate cutting length for the chip in accordance with 
boring conditions, and smoothly removing the thus broken chip pieces from 
a hole being formed in a workpiece. 
In order to achieve the above object of the present invention, there is 
provided a boring device comprising a casing, a tool spindle supported in 
the casing so as to be axially moved in an axial feed direction and in an 
axial return direction opposite thereto and biased by a spring in the 
axial return direction. Between the casing and the tool spindle, a first 
thrust member is provided coaxial with the tool spindle and fixed thereto. 
The first thrust member has a circumferentially extending track surface. 
Between the casing and the tool spindle a second thrust member is also 
provided coaxially with the tool spindle and fixed to the casing. The 
second thrust member has a circumferentially extending track surface 
facing the track surface of the first thrust member. A rolling element 
retaining member is disposed between the first and second thrust members 
and has at least three rolling element receiving bores which are arranged 
to be opposed to the track surfaces of the first and second thrust members 
and to be spaced apart from each other in the circumferential direction. 
Rolling elements are held in the respective rolling element receiving 
bores such that the rolling elements project from the rolling element 
receiving bores. Receptacles are formed in the track surface of either the 
first thrust member or the second thrust member in the same arrangement as 
the rolling element receiving bores such that the receptacles 
simultaneously receive the rolling elements. A coupling mechanism for 
allowing the rolling element retaining member to be engaged with the 
thrust member which is not formed with the receptacles and disengaged 
therefrom. 
The mechanism has such a structure that it causes the rolling element 
retaining member to be engaged with the thrust member having the 
receptacles and disengaged therefrom. 
In the boring device according to a preferred embodiment of the present 
invention, a tool spindle having a lower end receiving the shank of the 
boring tool is provided on the casing so as to be axially reciprocated. 
Two axially opposed thrust rings are fixed to the tool spindle and the 
casing, respectively. At least three rolling elements are disposed between 
the thrust rings in rolling contact therewith and are separated 
circumferentially of the thrust members through predetermined angles by a 
rolling element retaining ring. The rolling element retaining ring is 
supported on the tool spindle such that the retaining ring is selectively 
connected to one of the thrust rings and free from both thrust rings. A 
plurality of receptacles are formed in the track surface on the other 
thrust ring. The receptacles are arranged in the same angular relation as 
that of the rolling elements and are simultaneously engaged by the rolling 
elements every time the tool spindle rotates through a predetermined 
angle. 
According to another preferred embodiment, the rolling element retaining 
ring can also be connected to the other thrust ring such that the tool 
spindle is not intermittently moved in the return direction, while a hole 
is being formed in a workpiece. 
According to still another preferred embodiment, the thrust rings and the 
rolling element retaining ring are concentrically arranged. 
According to the present invention, chips can be broken into pieces having 
two lengths, a given length being chosen in accordance with the prevailing 
boring conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a drilling machine includes electromagnetic base 2 for 
electromagnetically attracting ferromagnetic workpiece 1 in a desired 
position thereof. Base 2 is located under frame 3. Boring device 5 is 
supported on the front wall of frame 3 to vertically reciprocate with 
respect to the workpiece 1 by handle 4 or an automatic feed unit (not 
shown). 
FIGS. 2 to 4 show an embodiment of the present invention. 
Casing 6 of boring device 5 houses an electric motor (not shown). Referring 
to FIG. 2, the rotation of motor shaft 7 reduced by two sets of helical 
gears 8A to 8D is transmitted to tool spindle 10. Boring tool 9 such as an 
annular cutter and a drill is detachably mounted in the lower end of tool 
spindle 10. 
The lower end of tool spindle 10 projects from housing block 11 threadably 
engaged with the lower end of casing 6 and fixed thereto (FIG. 2). An 
intermediate portion of tool spindle 10 is supported by radial needle 
bearing 12 mounted in housing block 11. The upper end portion of tool 
spindle 10 is supported by another radial needle bearing 14 in partition 
plate 13 of casing 6. The portions of spindle 10 which contact and are 
supported by the bearings 12 and 14 are respectively slightly longer than 
the thickness of bearings 12 and 14. Therefore, tool spindle 10 can be 
slightly reciprocated in the axial direction thereof. 
Flange-like thrust ring or first thrust ring 15 in the first embodiment is 
fixed onto the outer circumferential surface of the portion of spindle 10 
between radial needle bearings 12, 14 so as to rotate together with 
spindle 10. Disk-like thrust ring or second thrust ring 16 is located 
right above first thrust ring 15 so as to face ring 15 and is separated 
therefrom by a predetermined distance. The outer circumferential surface 
of ring 16 is fixed to the inner wall of casing 6, while tool spindle 10 
is loosely fitted in the central hole of ring 16. 
Ball retaining ring or rolling element retaining ring 20 has ball receiving 
bores or rolling element receiving bores 22 which receive respective 
thrust balls 21 as three rolling elements and defines the relative 
positions of balls 21. Retaining ring 20 is located between thrust rings 
15 and 16 and is loosely and rotatably fitted onto the outer surface of 
tool spindle 10. 
As shown in FIG. 4, distances D1, D2, and D3 between center C2 of ring 16 
and the centers of bores 22 differ from each other (D1&gt;D2&gt;D3). Center C2 
coincides with the axis of tool spindle 10. Central angles .theta.1, 
.theta.2, .theta.3 are defined by center C2 and the centers of adjacent 
ball receiving bores 22. Balls 21 are received in ball receiving bores 22 
and project from the upper and lower surfaces of retaining ring 20 so as 
to be in rolling contact with the opposed track surfaces of first and 
second thrust rings 15 and 16. 
As shown in FIG. 2, compression coil spring 26 disposed between bearing 12 
and first thrust ring 15 urges ring 15 upwardly via upper spacer 25. The 
upper end surface of first thrust ring 15 is elastically urged against 
thrust balls 21. Accordingly, spindle 10 is supported by ring 15 and 
biased in the thrust direction. 
Each of first and second thrust rings 15 and 16 is elastically urged 
against three thrust balls 21 rolling on different tracks L1, L2 and L3 so 
as to prevent excessive wear caused by the rolling contact of all thrust 
balls 21 on a single track. Each of thrust rings 15 and 16 is supported by 
thrust balls 21 at three points. Central angles .theta.1, .theta.2, and 
.theta.3 and distances D1, D2, and D3 are adjusted so that total load is 
applied to the central axis of tool spindle 10. With the above 
countermeasure, tool spindle 10 can be stably supported and urged in the 
thrust direction without applying an inclination force to thrust rings 15 
and 16, ball retaining ring 20, and thrust balls 21 and without causing 
eccentric rotation of thrust balls 21 regardless of the spindle speed. 
Localized wear of individual thrust balls 21 and track surfaces can also 
be prevented. 
As shown in FIG. 4, spherical receptacles or recesses 27 are formed in the 
regions of corresponding tracks L1, L2 and L3 on which balls 21 roll. 
Recesses 27 have the same depth and are arranged in the same positional 
relation as that of thrust balls 21 in retaining ring 20. Namely, central 
angles extended to the centers of adjacent thrust balls 21 from center C1 
are .theta.1, .theta.2 and .theta.3, respectively, and tracks L1, L2 and 
L3 are separated from center C1 by D1, D2 and D3, respectively. When three 
thrust balls 21 are simultaneously fitted in recesses 27, first thrust 
ring 15 and tool spindle 10, which are elastically biased by compression 
coil spring 26 in the upward direction, i.e., in the return direction, are 
retracted by a distance equal to the depth of recess 27. The depth of 
recesses 27 is equal to substantially a maximum allowable thickness of 
chips formed by boring tool 9. The maximum allowable thickness of chips is 
determined by the maximum speed of spindle 10 and a maximum allowable feed 
speed of boring device 5, both of which are defined by a maximum boring 
capacity of the boring device, as well as the material of work piece 1. 
Radial projections 30 and 31 are formed on outer portions of upper end 
surface or track surface 15a of first thrust ring 15 and lower end surface 
or track surface 16a of second thrust ring 16. Recesses or receptacles 32 
and 33 complementary to projections 30 and 31 are formed in the outer 
portions of upper and lower end surfaces of ball retaining ring 20. 
U-shaped guide piece 34 radially holds the outer peripheral portion of ball 
retaining ring 20 so that the portion of ring 20 slides on piece 34 when 
ring 21 rotates. Bolt 37a passes through a vertical elongated hole 36 
formed in the skirt of the casing 6 and is threaded in piece 34. 
Belleville spring 35 is disposed between the outer wall of casing 6 and 
the inner face of knob 37 integral with bolt 37a. By tightening knob 37, 
bolt 37a is fixed to casing 6. After knob 37 is loosened, bolt 37a is 
moved together with knob 37 along hole 6a. Ring 20 is also moved by piece 
34 in the same direction as bolt 37a, thereby adjusting the position of 
ring 20. When ring 20 is lifted, projections 31 of second thrust ring 16 
engage recesses 33 of retaining ring 20. On the contrary, lowered ring 20 
enables recesses 32 of ring 20 to receive projections 30 of first thrust 
ring 15. In an intermediate position of retaining ring 20, no engagement 
occurs between projections 31 and recesses 33 or between projections 30 
and recesses 30. 
When projections 31 engage respective recesses 33, ball retaining ring 20 
is coupled with second thrust ring 16, as shown in FIG. 5. As first thrust 
ring 15 is rotated by tool spindle 10, thrust balls 21 in ring 20 revolves 
about their axes without changing their position with respect to second 
thrust ring 16 and are simultaneously fitted in respective recesses 27 
once, every time tool spindle 10 rotates first thrust ring 15 once, as 
shown in FIG. 6. Then, spindle 10 is axially retracted by compression coil 
spring 26 in the upward direction, i.e., in the return direction. 
Therefore, tool spindle 10 is moved in the upward direction, i.e., the 
return direction by a distance equal to the depth of recess 27 once per 
one revolution of spindle 10 and instantaneously separates the cutting 
edge from the surface being cut of workpiece 1, thereby instantaneously 
interrupting cutting. The chip length corresponds to substantially one 
revolution of boring tool 9. 
When projections 31 are disengaged from recesses 33 by lowering ring 20 to 
its intermediate position as shown in FIG. 2, ball retaining ring 20 is 
released from thrust rings 15 and 16. As first thrust ring 15 is rotated 
together with tool spindle 10, thrust balls 21 roll on the track surfaces 
of first and second thrust rings 15, 16. The distance by which each ball 
21 travels on the track surface on first thrust ring 15 is equal to the 
distance by which the ball 21 rolls on the track surface on second thrust 
ring 16. As first thrust ring 15 is rotated, therefore, balls 21 rotate 
about the axis of retaining ring 20 together therewith through half the 
angle of the rotational angle of first thrust ring 15. As a result, thrust 
balls 21 instantaneously engage corresponding recesses 27 once, every time 
first thrust ring 15 rotates twice together with tool spindle 10, and tool 
spindle 10 slides in the return direction by a distance equal to the depth 
of recess 27 at the same moment, when the engagement between balls 21 and 
recesses 27 takes place. The cutting edges of boring tool 9 are separated 
from the surface in the hole being formed in the workpiece 1 and the 
cutting operation is instantaneously interrupted. Therefore, the chip is 
cut into pieces each having a length corresponding to two revolutions of 
boring tool 9. 
When projections 30 engage respective recesses 32 and thrust balls 21 are 
also received in respective recesses 27 as shown in FIG. 7, ball retaining 
ring 20 is coupled to first thrust ring 15. Balls 21 in retaining ring 20 
are kept fitted in corresponding recesses 27 and revolve about their own 
axes without changing their positions with respect to first thrust ring 15 
and are in rolling contact with the track surface on second thrust ring 
16. Therefore, tool spindle 10 fixed by first thrust ring 15 is not 
axially displaced during its own rotation. In this case, the chip length 
is not controlled. 
An annular cutter can be used as a boring tool 9 and is detachably fixed to 
cutter arbor 39 of tool spindle 10 by set screw 40. Pilot pin 41 passes 
through boring tool 9 in alignment with the axis of tool 9 such that pin 
41 can extend from or be retracted into the lower end of tool 9. When 
boring tool 9 is attached to tool spindle 10, the tip 41a of pilot pin 41 
is urged by press piece 43 which is urged downward by compression coil 
spring 42 in cutter arbor 39 of spindle 10. Tip 41a of pilot pin 41 
extends from the lower end face of tool 9 and serves as the center 
thereof. During the boring operation, tip 41a of pin 41 contacts the 
surface of the workpiece, and pin 41 runs idle. When boring machine 5 is 
moved to the uppermost position upon completion of boring, pilot pin 41 
pushes chips from boring tool 9 by the biasing force of compression coil 
spring 42. 
The operation of the first embodiment will be described below. 
When the depth of a hole to be cut by boring tool 9 is relative large, 
loosened operation knob 37 is moved to the center of elongated hole 36, as 
shown in FIG. 2, so that ball retaining ring 20 is separated from thrust 
rings 15 and 16. And then knob 37 is retightened to fix guide piece 34 to 
inner wall surface 6a of casing 6 at the center of hole 36. 
When tool spindle 10 is rotated, thrust balls 21 roll on the opposed track 
surfaces of first and second thrust rings 15 and 16, and balls 21 are 
rotated together with ring 20 once, every time tool spindle 10 rotates 
twice. Balls 21 are simultaneously fitted in respective recesses 27 formed 
in the regions of tracks L1, L2, and L3 once per every two revolutions of 
spindle 10. Spindle 10 and ring 15 urged upward by compression coil spring 
26 are moved upward by a distance equal to the depth of recess 27 in the 
return direction (upward in FIG. 2). The spindle 10 moves from the state 
in FIG. 2 to the state in FIG. 3. 
During the boring operation, handle 4 or an automatic feeding unit (not 
shown) feeds boring tool 9 through spindle 10. Upon simultaneous 
engagement of thrust balls 1 with recesses 27, spindle 10 is moved upward 
by the above-mentioned distance. This exhibits the same effect that the 
feeding of boring tool 9 is stopped. Since the axial displacement of 
spindle 10 is the same as substantially the maximum allowable thickness of 
the chip, tool 9 temporarily stops cutting workpiece 1. Then, the chip is 
broken into pieces each having a length corresponding to two revolutions 
of tool 9, and the thus broken chip pieces are intermittently removed from 
a hole being formed. 
If the cutting diameter of boring tool 9 is large and the boring depth is 
smaller than those in the first case, operation knob 37 is moved to the 
upper end of elongated hole 36 to engage projections 31 with recesses 33. 
Ball retaining ring 20 is coupled to second thrust ring 16. As tool 
spindle 10 is rotated, thrust balls 21 are rotated about their axes 
without changing their positions with respect to second thrust ring 16 and 
are in rolling contact with thrust ring 15. Balls 21 are simultaneously 
fitted in corresponding recesses 27 once per one revolution of tool 
spindle 10. First thrust ring 15 fixed to tool spindle 10 are moved by a 
distance equal to the depth of recess 27 in the return direction, as shown 
in FIG. 6. 
The chip is cut into pieces each having a length corresponding to one 
revolution of boring tool 9, and the broken chip pieces are intermittently 
removed from the hole being formed. 
There are such cases where chips are not required to be broken, that a work 
piece is thin, that a work piece is made of such material as cast iron 
from which continuous chips are not formed, or that a cutting tool has a 
chip-breaking function although it is poor. In such cases, operation knob 
37 is set to the lower end of elongated hole 36 to engage projections 30 
with recesses 32, as shown in FIG. 7. 
Thrust balls 21 remaining fitted in recesses 27 are rotated together with 
first thrust ring 15 and roll on the track surface of second thrust ring 
16. Tool spindle 10 does not retract during its rotation. In this case, 
chips are not broken. 
When an annular cutter having a relatively large cutting diameter is used 
in place of the annular cutter in the first embodiment, two recesses 27 
are formed diametrically opposed to each other in the region of each of 
tracks L1, L2, and L3. Spindle 10 can be selectively retracted once or 
twice per one revolution thereof. 
FIG. 8 shows a modification of the first embodiment which is usable when 
first thrust ring 15 is not exerted by a heavy thrust. Three thrust balls 
21 roll on a common track L. In the region of track L, three recesses 27 
are formed to simultaneously receive respective balls 21 with central 
angles .theta. equal to each other. Spindle 10 is selectively retracted 
six or three times per every two revolutions of tool spindle 10. 
The number of thrust balls, the number of tracks and the number of recesses 
are properly selected to make the number of retraction of tool spindle 10 
per one revolution thereof be 1/2 or more. 
Recesses 27 may be formed in second thrust ring 16 in place of first thrust 
ring 15. Ball retaining ring 20 is connected to first thrust ring 15. 
A second embodiment is substantially the same as the first embodiment shown 
in FIGS. 2 to 8, except that the upper end face of flange-like first 
thrust ring 15 does not have projections 30 and the lower end face of ball 
or rolling element retaining ring 20 does not have recesses 32. Other 
arrangements of the second embodiment are the same as those of the first 
embodiment, this embodiment being not shown in the drawings. 
With the second embodiment, tool spindle 10 is also retracted to break the 
chips every time spindle 10 rotates once or twice. The only exception is 
that spindle 10 cannot be prevented from retracting when it rotates. 
FIGS. 9 to 13 show a third embodiment. The same numerals as in the first 
embodiment denote the same parts in the third embodiment, and a detailed 
description thereof will be omitted. 
First thrust ring 151 is fixed to tool spindle 10 located above radial 
needle bearing 12. Upward facing and arcuated track surface 51 having the 
same radius of curvature of thrust ball 21 is formed on outer 
circumferential surface 151a of ring 151 to receive a thrust load from 
balls 21. Second thrust ring 161 is fixed to the inner wall of casing 6. 
The inner circumferential surface of ring 161 faces track surface 51 and 
is spaced therefrom by a predetermined distance. Downward facing and 
arcuated track surface 51 having the same radius of curvature as that of 
thrust ball 21 is formed on inner circumferential surface 161a of ring 161 
to receive the thrust load from balls 21. 
Ball or rolling element retaining ring 201 is made of a metal plate and has 
ball receiving bores 221 at an equal circumferentially angular interval of 
120.degree. with respect to the axis of ball retaining ring 20. In other 
words, the central angles of ball bores 221 are equal to each other. Bores 
221 receive three thrust balls 21 at the same circumferential angular 
interval as bores 221. Balls 21 are partially exposed from the inner and 
outer circumferential surfaces of ball retaining ring 201 and are in 
rolling contact with track surfaces 51 and 52. 
First thrust ring 151 is upwardly urged through upper spacer 25 by 
compression coil spring 26 disposed between radial needle bearing 12 and 
first thrust ring 151. In other words, ring 151 is biased together with 
tool spindle 10 in a direction away from the work piece 1, i.e., in the 
return direction. First thrust ring 151 elastically urges thrust balls 21 
against second thrust ring 161. 
Three spherical recesses or receptacles 53 having the same depth are formed 
on track surface 51 of first thrust ring 151 at an equal circumferential 
angular intervals of 120.degree.. When balls 21 are simultaneously fitted 
in respective recesses 53 during the rotation of spindle 10, spindle 10 
fixed to first thrust ring 151 is retracted by a distance equal to the 
depth of recess 53 (upward in FIG. 9). The depth of recess 53 is the same 
as that of the first embodiment. 
Lower edge portion of ball retaining ring 201 extends downward from the 
space between first and second thrust rings 151 and 161 and is formed with 
a plurality of semispherical notches 54. Bolt 37a with knob 37 on its one 
end is inserted in vertically elongated hole 36 and threaded into guide 
piece or nut 34 which can slide on the inner wall of casing 6 along 
elongated hole 36. 
Belleville spring 35 is provided between the inner face of knob 37 and the 
outer wall of casing 6. By loosening knob 37, bolt 37a can be moved to a 
required position in hole 36. After tightening knob 37, bolt 37a is fixed 
to casing 6 through nut 34 and spring 35. 
When knob 37 is moved upward, bolt 37a engages one of notches 54, thereby 
coupling retaining ring 201 with casing 6. Thus, ring 201 is stationary 
with respect to second thrust ring 161. 
As first thrust ring 151 rotates together with tool spindle 10, thrust 
balls 21 revolve about their own axes in the stationary ball retaining 
ring 201. Balls 21 are fitted in recesses 53 three times per one 
revolution of first thrust ring 151. Spindle 10 is displaced in the return 
direction under the biasing force of compression coil spring 26 three 
times per one revolution of spindle 10 by a distance equal to the depth of 
recess 53. The cutting edges of boring tool 9 is separated from the face 
being cut of the hole in the workpiece 1 to interrupt cutting of the 
workpiece. The chip is therefore cut into pieces each having a length 
corresponding to 1/3 revolution of boring tool 9. 
When bolt 37a is disengaged from notch 54 by lowering knob 37, retaining 
ring 201 rotates freely with respect to first and second thrust rings 151, 
161. When first thrust ring 151 is rotated by tool spindle 10, thrust 
balls 21 roll on track surfaces 51, 52 of first and second thrust rings 
151, 152. 
The displacement of first thrust ring 151 relative to thrust balls 21 is 
equal to the displacement of thrust balls 21 with respect to second thrust 
ring 161. As first thrust ring 151 is rotated, therefore, retaining ring 
201 as well as balls 21 rotates at half the rotational speed of first 
thrust ring 151. Thrust balls 21 are fitted in corresponding recesses 53 
three times every time first thrust ring 151 as well as spindle 10 rotates 
twice, thereby retracting tool spindle 10. Tool spindle 10 retracts by a 
distance equal to the depth of recess 53 three times per every two 
revolutions thereof. The boring operation is thus interrupted and the chip 
is cut into pieces each having a length corresponding to 2/3 revolution of 
boring tool 9. 
The operation of the third embodiment will be described below. 
If the cutting diameter of boring tool 9 is small and the boring depth is 
relatively large, knob 37 is moved to the lower end of elongated hole 36 
to separate bolt 37a of knob 37 from notch 54 of ball retaining ring 20. 
As tool spindle 10 is rotated and thrust balls 21 in ball retaining ring 
201 roll on track surfaces 51, 52 of first and second thrust rings 151, 
161, thrust balls 21 are simultaneously fitted in respective recesses 53 
three times per every two revolutions of drill spindle 10. As a result, 
tool spindle 10 and first thrust ring 151 are moved in the return 
direction (upward in FIG. 9) by a distance equal to the depth of recess 53 
which is a substantial maximum allowable thickness of the chip. Tool 
spindle 10 is moved from the state in FIG. 9 to the state in FIG. 10. 
The boring operation is interrupted which is performed by a cutting tool 9 
fed to the workpiece 1 by means of handle 4 or an automatic feeding device 
(not shown). Therefore, the chip is broken into pieces each having a 
length corresponding to 1/3 revolution of tool 9, and the cut pieces are 
intermittently removed from a hole being formed. 
If the cutting diameter of boring tool 9 is larger and the cutting depth is 
smaller than those in the case described above, knob 37 is set to the 
upper end of elongated hole 36 to engage bolt 37a with one of recesses 54 
of ball retaining ring 201, as shown in FIG. 11. Retaining ring 201 is 
stationary with respect to second thrust ring 161. 
As tool spindle 10 is rotated, thrust balls 21 revolve about their own axes 
without rotating about the axis of retaining ring 201. When balls 21 are 
simultaneously fitted in respective recesses 53 formed in track surface 51 
of first thrust ring 151 three times per one revolution of tool spindle 
10, tool spindle 10 as well as first thrust ring 151 are moved by a 
distance equal to the depth of recess 53 in the return direction. 
Therefore, tool spindle 10 moves from the state in FIG. 11 to the state in 
FIG. 12. The chip is broken into pieces each having a length corresponding 
to 1/3 revolution of boring tool 9, and the cut chip pieces are 
intermittently removed from the hole being formed. 
In the third embodiment, boring tool 9 is moved in the return direction 3 
or 6 times per every two revolutions of tool spindle 10. However, if an 
annular cutter having a relatively large cutting diameter is used, six 
recesses 53 may be formed in track surface 51 at an equal circumferential 
angular interval of 60.degree.. Spindle 10 can be selectively retracted 
three or six times per revolution thereof. 
The number of thrust balls and the number of recesses are properly selected 
to make the number of retraction of the tool spindle per one revolution 
thereof set to 3/2 or more. 
In the third embodiment, the recesses are formed in first thrust ring 151, 
but may be formed in second thrust ring 161. 
In addition, the boring tool attached to the lower end of the tool spindle 
is not limited to the annular cutter. A twist drill may be used therefor. 
With the above embodiments, the present invention is applied to a drilling 
machine with a portable electromagnetic base. However, it is also 
applicable to a compact boring machine or an electric drill.