Cutting tool

A cutting tool comprises a bit-holder fed at a prescribed feed amount f per rotation of the workpiece, and a throw-away tip which is formed of superhard metal, which has an outer peripheral surface of prescribed shape, an inner peripheral surface provided concentrically with the outer peripheral surface, similar to the prescribed shape of the outer peripheral surface and defining a hollow part and a predetermined thickness t and which is detachably mounted to the bit-holder. A distance .omega. between the inner peripheral surface of the throw-away tip and the outer peripheral surface thereof is defined within a range satisfying the following formula represented by; f.sub.max .ltoreq..omega..ltoreq.t.

BACKGROUND OF THE INVENTION 
The present invention relates to a cutting tool having a throw-away tip 
and, more particularly, to a resource-saving type cutting tool. 
A throw-away tip to function as a cutting edge of a bit tool generally has 
a variety of merits, e.g., a lower cost per a cutting edge than a 
resharpening type brazed tip, less handling time for replacing a worn tip 
with a new tip, reproducibility in the positional relationship between a 
bit-holder after replacement and a new tip to be similarly retained to the 
state before the replacement, and has hence a wide application in an 
industrial field. Since the throw-away tip is, however, formed of 
expensive materials such as, tungsten, cobalt, etc., it has a disadvantage 
of expensive cost. 
FIG. 1A is a perspective view showing a conventional throw-away tip 
integral with a bit-holder. As shown in FIG. 1A, a throw-away tip 1 has a 
through hole 5 to be engaged with a tip locking pin 4 tiltably mounted 
with a shim 3 of a bit-holder 2. This pin 4 is, as shown in FIG. 2, 
inclined toward the wall surface 7 at the tip seat side of the bit-holder 
2 by rotating and implanting a clamping screw 6. When this tip 1 is 
attached to the bit-holder 2, the tip locking pin 4 is inserted into the 
mounting hole 5 to arrange the tip 1 on the shim 3, and the clamping screw 
6 is turned and implanted. The locking pin 4 is inclined by this turning 
toward the wall surface 7, the tip 1 is urged onto the wall surface 7, and 
is locked onto the bit-holder 2. The throw-away tip 1 thus mounted at the 
bit-holder 2 is contacted with a workpiece to cut the same. In ordinary 
cutting work, a relief face wear 8a and a rake face wear 8b will 
respectively occur on a relief face 1a and a rake face 1b both forming a 
cutting edge of the tip 1, as shown in FIGS. 1A and 1B. The width V.sub.B 
of the relief face wear 8a which relates directly to the dimensional 
accuracy of the workpiece and the roughness of the finished surface of the 
workpiece is normally less than 0.5 mm, and a relatively small amount of 
wear of the cutting edge takes place at the extremely restricted parts. 
In cutting theory, in case, for example, of an ordinary three-dimensional 
cutting, as shown in FIG. 3, of a cylindrical workpiece 9, a cutting 
resistance force P0 can be divided into a main component P1 of force, a 
feeding component P2 of force and a back component P3 of force. The 
magnitudes of the respective components of force normally depend on the 
material of the workpiece 9, the shapes of the tip 1 and the bit-holder 2, 
the cutting conditions, etc. and the main component P1 of force is 
commonly prominent. It is appreciated from this that the size and hence 
the width of the tip 1 along a direction of the feeding component P2 of 
force is not so necessarily required as the size and hence the thickness 
of the tip 1 along a direction of the main component P1. However, in the 
conventional tip 1, the width of the tip 1 is normally larger than the 
thickness of the tip due to the feasibility of locking to the bit-holder 2 
and to the interchangeability of the tip. As described above, an effort to 
positively reduce the capacity of the tip rationally as much as possible 
to save resources is not carried out in the conventional throw-away tip by 
considering the wearing state and the cutting resistance. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a cutting tool which is 
capable of eliminating the aforementioned drawbacks and disadvantages of 
the conventional cutting tool and which comprises a resource-saving type 
throw-away tip which is inexpensive and interchangeable with the 
conventional tip without reducing the strength and the cutting performance 
by considering the wear occurring positions, wearing extent, the magnitude 
and the direction of the cutting resistance. 
According to an aspect of the present invention, there is provided a 
cutting tool which comprises a bit-holder fed at a prescribed feed amount 
f per revolution of the object to be ground; and a throw-away tip which is 
formed of a superhard metal, which has an outer peripheral surface of 
prescribed shape, an inner peripheral surface provided concentrically with 
the outer peripheral surface, similar to the prescribed shape of the outer 
peripheral surface and defining a hollow part and a predetermined 
thickness t and which is detachably mounted to the bit-holder, wherein a 
distance .omega. between the inner peripheral surface of the throw-away 
tip and the outer peripheral surface thereof is defined within a range 
satisfying the relation formula represented by: 
EQU f.sub.max .ltoreq..omega..ltoreq.t.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first preferred embodiment of a cutting tool according to the present 
invention will be described in detail with respect to the case of the 
application to a lathe with reference to FIGS. 4 through 10 of 
accompanying drawings. 
In the figures, reference numeral 12 designates a throw-away tip for 
turning, schematically formed in a hollow triangular pillar shape and 
formed of superhard metallic material, e.g., tungsten, cobalt, etc. This 
throw-away tip 12 has a cavity 13 passing through a thicknesswise 
direction. The outer periphery of the cavity 13 is defined by the three 
inner lateral surfaces of the throw-away tip 12, and is set similarly to 
the three outer lateral surfaces for defining the outer periphery of the 
throw-away tip 12. Distances or tip widths (.omega.) between the outer 
lateral surfaces and the corresponding inner lateral surfaces of the tip 
12 are respectively equally set. The respective outer lateral surfaces of 
the throw-away tip 12 are formed parallel to the respective corresponding 
inner lateral surfaces. 
An insert 14 is intimately inserted into the cavity 13. The outer periphery 
of the insert 14 is set correspondingly to the inner periphery of the 
cavity 13 with integral surface. The insert 14 has a mounting hole 15, to 
which a tip locking pin 4 mounted on a shim 3 of a bit-holder 2 is 
inserted, and perforated in a thicknesswise direction. Necessary portions 
of the cavity 13 of the tip 12, the insert 14, the mounting hole 15 and 
the tip locking pin 4 are chamfered so as to effectively and readily lock 
to the bit-holder 2 of the tip 12 and to improve the cutting and working 
properties of the tip 12. These portions may also be, in addition to the 
chamfer, rounded or tapered. 
The width (.omega.) of the tip to become the most advantageous feature of 
the present invention will be described in detail. It is first important 
to know in the bit tool on which a throw-away tip is mounted which part of 
the cutting edge 11 receives a stress at cutting time, what magnitude of 
the stress is applied up to which position of a rake face 1b cutting chips 
is contacted when the cutting is executed at a prescribed feed amount f of 
the bit tool. Accordingly, the cutting chip contacting length l of a 
workpiece to be cut in case that the workpiece is cut by a bit tool on 
which a throw-away tip is generally mounted will initially be described 
with reference to FIGS. 6 and 7. 
FIG. 6 is a view of a similar two-dimensional cutting state of a cutting 
tool. In FIG. 6, the state that a cylindrical workpiece 9 is, for example, 
being cut on the outer periphery thereof by a bit tool on which a 
triangular pillar-shaped throw-away tip 1 is mounted is shown. A 
side-cutting edge angle (not shown) is supposed to be zero, and a nose 10 
is also, for the purpose of approximating to the two-dimensional cutting, 
set similar to zero. Reference numeral 11 illustrates a cutting edge. FIG. 
7 is a sectional view taken along the line with arrows VII and VII in FIG. 
6. The length of the fretted scar of the cutting chip H presented on the 
rake face 1b of the throw-away tip 1 after cutting is measured by a tool 
microscope under such cutting conditions. When the cutting chip contacting 
length l is obtained from the length of the fretted scar of the cutting 
chip H and a ratio l/d of the cutting chip contacting length l to the 
cutting length d is observed, it is identified to be approximately 3 to 5. 
Particularly when cutting oil is employed, the ratio l/d is resulted in 
2.7. In FIGS. 7 through 10, the same reference numerals are designated to 
the components and parts equal in the components and parts and their 
functions to those shown in FIG. 6, and the description will be omitted. 
The stress on the rake face 1b of the throw-away tip will be described with 
reference to FIG. 8. FIG. 8 is a characteristic diagram in which the 
strengths of the stresses on the respective positions on the rake surface 
of the throw-away tip are measured, the stresses of the rake face 1b are 
plotted in an ordinate axis, and the distances from the cutting edge 11 
are plotted in an abscissa axis. According to the characteristic diagram 
in FIG. 8, the magnitude of vertical stress (.sigma..sub.t) acting 
vertically to the rake face 1b exponentially increases as it approaches 
the cutting edge 11, and disappears at the position (cutting chip 
contacting region boundary) isolated by the cutting chip contacting length 
l from the cutting edge 11. On the other hand, frictional stress 
(.tau..sub.t) acting horizontally to the rake face 1b exhibits a 
triangular distribution (.tau..sub.t(a)) or a trapezoidal distribution 
(.tau..sub.t(b)), and disappears at the position (cutting chip contacting 
region boundary) isolated by the cutting chip contacting length l from the 
cutting edge 11. 
The cutting tool of three-dimensional cutting will be described with 
reference to FIG. 9. FIG. 9 is a characteristic diagram of vertical stress 
(.sigma..sub.t) with respect to the rake face in three-dimensional cutting 
state. In FIG. 9, the vertical stresses (.sigma..sub.t) acting to the 
various position on the rake face 1b in the three-dimensional cutting are 
actually measured, and are exhibited with equivalent stress lines, thereby 
depicting the distribution state. According to the characteristic diagram 
in FIG. 9, the vertical stress (.sigma..sub.t) abruptly increases as it 
approaches from the position (cutting chip contacting region boundary) 
isolated by the cutting chip contacting length l from the cutting edge 11 
of the rake face 1b to the cutting edge 11, and increases as it approaches 
from the cutting end I of the cutting edge 11 to the nose 10. The wear of 
the rake face 1b of the throw-away tip occurs due to the friction of the 
cutting chip H on the rake face 1b, and the region where the friction 
occurs is in the range of the cutting chip contacting length l from the 
cutting edge 11. 
A cutting work with a cutting tool for limiting the cutting chip contacting 
length l will be then described with reference to FIG. 10. FIG. 10 is a 
schematic sectional view of a concept of cutting work with a cutting chip 
contacting area constraint tool for limiting the cutting chip contacting 
length l to l' (l'&lt;l). In FIG. 10, the edge part of the cutting chip 
contacting area constraint tool J is formed thicker than the other part to 
the position isolated by the distance l' from the cutting edge 11, and has 
such features that the cutting resistance F of this type is decreased as 
compared with that of an ordinary bit tool with the cutting chip 
contacting length l, the wear of the tool and particularly of the rake 
face 1b is reduced, and the lifetime of the edge part is prolonged. 
The foregoing description can be summarized as below. 
(1) The cutting chip contacting length l on the rake face 1b is 
substantially with five times of the cutting thickness d. 
(2) The vertical stress (.sigma..sub.t) acting to the various positions on 
the rake face 1b exponentially increases as it approaches to the cutting 
edge 11, and increases from the cutting end I to the nose 10 in the 
edgewise direction of the cutting edge 11. 
(3) The wear of the rake face of the tip normally occurs within the range 
of the cutting chip contacting length l. 
(4) That main component of the cutting resistance which is acting in the 
thicknesswise direction of the tip is generally prominent. 
(5) The cutting resistance of the contacting area constraint tool for 
limiting the cutting chip contacting length artificially decreases, and 
the wear of the rake face of the tool is less and the lifetime of the tool 
results in extension. 
The following results are obtained according to the experiments executed on 
the basis of the aforementioned facts. That is, it is identified that, 
when the width (.omega.) of the tip is set longer than the maximum value 
(fmax) of the optimum or most frequently used feed amount for the tip, the 
tip will less break in an ordinary light cutting, and the thickness of the 
tip is more important. As the width (.omega.) of the tip is increased, the 
tip approaches the conventional tip, and hence consumes expensive and rare 
mineral resources. In the cutting tool of the present invention, the 
feature resides in the positive omission of the part not directly 
contributing the actual cutting as a hollow part, and the upper limit of 
the value of the width (.omega.) of the tip is 5fmax (where fmax 
represents the maximum value of the extent of the feed amount of the tip 
per one revolution of a workpiece in case of three-dimensional cutting), 
as the length of the limit in which the cutting chip actually contacts 
with the rake face of the tip, or the thickness (t) of the tip, as 
selected either larger one. Since the thickness (t) of the tip is 
generally larger than the 5fmax, the width (.omega.) of the tip is 
selected in the range of fmax&lt;.omega..ltoreq.t as a whole. The following 
conditions are adapted for the using conditions of the tip. 
That is, in an ordinary light cutting, the width (.omega.) of the tip is 
less than the five times of the maximum value (fmax) of the feed amount, 
therefore the cutting chip contacting length (l) is in this case 
artificially limited to (l'). Thus, the cutting chip contacting area 
constraint tool for limiting the cutting chip contacting length to (l') is 
used to perform the cutting work. In a heavy cutting or in a hard cutting 
work, the width (.omega.) of the tip is less than the thickness (t) of the 
tip. In case that the width (.omega.) of the tip is set to the same as or 
to the vicinity of the thickness (t) of the tip, the tip is lightly saved 
in the resources as compared with the conventional tip, but the part which 
does not contribute to the actual cutting exists in the amount 
corresponding to the difference (t-5fmax) between the tip of the present 
invention and the conventional tip. In addition, in case of heavy or hard 
cutting work, the strength of the tip and particularly the feeding 
component of force and the back component of force are improved in 
strength, thereby prolonging the lifetime of the tip according to the 
present invention. 
The sequence of determining the width (.omega.) of the throw-away tip of 
the present invention will be concretely described. For instance, in case 
of a tip of square shape having 19.05 mm of one side, 6.35 mm of thickness 
(t), 1.6 mm of nose R, 5 mm of cutting depth, and material of a workpiece 
to be cut made of medium carbon steel (having 60 kgf/mm.sup.2 of tensile 
strength), its optimum feed is 0.4 to 0.8 (mm/revolution). In this manner, 
the value of the width (.omega.) of the tip can be determined in an 
ordinary case by fmax.ltoreq..omega..ltoreq.5fmax and accordingly 
0.8.ltoreq..omega..ltoreq.4.0, and can be determined in worse cutting 
condition, e.g., intermittent cutting or the like by 
5fmax.ltoreq..omega..ltoreq.t and accordingly 
4.0.ltoreq..omega..ltoreq.6.35. Examples of numerals determined in this 
manner will be listed in the following Tables 1 and 2. 
TABLE 1 
______________________________________ 
(Dimension: mm) 
For light cutting 
Range of Range of 
Size of tip feed f width .omega. 
Determined 
Thick- 
Overall Nose (mm/ fmax .ltoreq. .omega. .ltoreq. 
value of 
ness (t) 
length R rotation) 
5 fmax width .omega. 
______________________________________ 
3.18 9.525 0.8 0.2-0.5 
0.5 .ltoreq. .omega. .ltoreq. 2.5 
2.0 
4.76 12.70 1.2 0.3-0.6 
0.6 .ltoreq. .omega. .ltoreq. 3.0 
3.0 
6.35 19.05 1.6 0.4-0.8 
0.8 .ltoreq. .omega. .ltoreq. 4.0 
4.0 
7.94 25.4 2.4 0.6-1.2 
1.2 .ltoreq. .omega. .ltoreq. 6.0 
5.0 
______________________________________ 
TABLE 2 
______________________________________ 
(Dimension: mm) 
Range of 
For heavy cutting 
Size of tip feed f Range of Determined 
Thick- 
Overall Nose (mm/ width .omega. 
value of 
ness (t) 
length R rotation) 
5 fmax .ltoreq. .omega. .ltoreq. t 
width .omega. 
______________________________________ 
3.18 9.525 0.8 0.2-0.5 
2.5 .ltoreq. .omega. .ltoreq. 3.18 
3.0 
4.76 12.70 1.2 0.3-0.6 
3.0 .ltoreq. .omega. .ltoreq. 4.76 
4.0 
6.35 19.05 1.6 0.4-0.8 
4.0 .ltoreq. .omega. .ltoreq. 6.35 
5.0 
7.94 25.4 2.4 0.6-1.2 
6.0 .ltoreq. .omega. .ltoreq. 7.94 
7.0 
______________________________________ 
When the throw-away tip 12 thus constructed is mounted on the bit-holder 2, 
the insert 14 is first inserted into the cavity 13 of the tip 12 
integrally. Thereafter, the tip locking pin 4 is then inserted into the 
mounting hole 15 of the insert 14. In this state, the clamping screw 6 is 
turned in the pushing direction, the pin 4 is thus inclined in the 
direction toward the wall surface 7 of the tip seat side, the insert 14 
and the tip 12 thus engaged with the insert 14 are urged under pressure to 
the wall surface 7 of the tip seat side, and the tip 12 is thus locked to 
the bit-holder 2. 
In the tip thus constructed according to the first embodiment of the 
present invention as described above, the cavity 13 is so formed as to be 
approximate or similar to the shape of the outer periphery of the tip 12 
and respectively equal to the distances from the outer lateral surfaces to 
the corresponding inner lateral surfaces by considering the wearing shape 
and the cutting resistance of the tip. Therefore, the size of the cavity 
13 becomes much larger than the conventional mounting hole 5, and the tip 
12 can be formed with less material than the conventional tip without 
deteriorating the strength and the cutting performance. Thus, the quantity 
of material used, e.g., expensive tungsten, cobalt or the like is 
decreased in the tip 12, thereby contriving the resource-saving, and 
providing inexpensive cutting tool less than the conventional tool. 
Furthermore, the insert 14 is formed in the state that the contour is 
approximate or similar to the shape of the cavity 13 of the throw-away tip 
12, has the mounting hole 15 to which the tip locking pin 4 is inserted, 
and is intimately inserted into the cavity 13, and both tip 12 and insert 
14 are locked to the bit-holder 2. Therefore, the following advantages and 
effects can be obtained: 
(a) The tip 12 can be mounted at the conventional bit-holder 2 by employing 
the insert 14, and an interchangeability between the tip 12 of the present 
invention and the conventional tip can be provided. 
(b) Since the insert 14 does not contribute directly to the cutting action 
nor wear, it can be formed of inexpensive material, and can also be used 
repetitively. 
The present invention is not limited to the particular first embodiment 
described above. Various other changes and modifications may be made 
within the spirit and scope of the present invention. Other preferred 
embodiments of the present invention will be described in detail. In the 
following description, the components and parts similar or equivalent to 
those in the first embodiment will be denoted by the like reference 
numerals and will be omitted for the convenience of the description. 
A second preferred embodiment will now be described with reference to FIG. 
11. 
When the throw-away tip 12 as described above is locked to the bit-holder 
2, the insert 14 to be engaged with the cavity 13 of the tip 12 may be 
employed as in the first embodiment. On the other hand, as shown as the 
second embodiment in FIG. 11, an insert 22 for pressing and locking the 
tip 12 to the shim 3 of the bit-holder 2 may be used. This insert 22 has 
the first part 20a intimately inserted into the cavity 13 and the second 
flanged part 20b formed integrally with the first part 20a, projected 
radially outwardly from the first part 20a and having a lower surface 
engaged with the upper surface of the tip 12. A chip breaker plate 21 is 
fixedly provided on the outer periphery of the second part 20b. A through 
hole to which a screw 23 is inserted is formed at the center of the insert 
22. The outer periphery of the chip breaker plate 21 is reduced inwardly 
at a prescribed distance from the outer periphery of the tip 12 to define 
the chip breaker. The insert 22 and the tip 12 are locked via the screw 23 
to the bit-holder 2. 
And, as shown as a first modification in FIG. 12, it may also be possible 
to fix the tip 12 on the bit-holder 2 without using the insert 14. That 
is, the shim 3 is attached to the bit-holder 2 by means of a first screw 
16a. The tip 12 is set on the one end of a pressure member 17 by means of 
a second screw 16b. In the other end of the pressure member 17 is provided 
a first serration part 18a. A second serration part 18b corresponding to 
the first serration part 18a is provided on the upper surface of the 
bit-holder 2. The pressure member 17 has a loose through hole 17a. The tip 
12 is fixed on the shim 3 by the pressure member 17 which is fixed to the 
bit-holder 2 by means of a third screw 16c through the loose through hole 
17a under the condition of pressuring the tip 12 onto the shim 3. The 
position of the chip breaker plate 21 is defined by the engaging the first 
serration part 18a with the second serration part 18b. 
A variety of modifications may be prepared as will be described below 
according to the present invention. 
(a) As shown by first through fifth modifications in FIGS. 13A through 13E, 
respectively, the planar shape of the tips 24a through 24e may be formed 
in rhombic, parallelogrammic, ship-like, elliptical, or circular shape 
respectively having cavities 25a through 25e of the similar shape. Inserts 
26a through 26e are respectively inserted correspondingly to the cavities 
25a through 25e. 
(b) The foregoing description is directed in the first embodiment to the 
preferred embodiment of the throw-away tip for a lathe, but the throw-away 
tip used for cutting tools of all types and shapes e.g., milling, balling, 
drilling, etc. may be applied with the throw-away tip and the insert 
according to the present invention. 
The foregoing description is also directed in the second embodiment to 
another preferred embodiment of the throw-away tip in which the chip 
breaker is defined by the chip breaker plate fixedly provided on the outer 
periphery of the insert. However, the above throw-away tip is not limited 
to the particular construction described above, but may be constructed as 
a third preferred embodiment shown in FIGS. 14 and 15 within the spirit 
and scope of the present invention. 
In FIGS. 14 and 15, a tap bolt 28 provided with a flange 27 has a hexagonal 
hole 29 at its head. This bolt 28 has a male threaded part 30 consecutive 
to the head. The bolt 28 has a hollow part 31 to which a pin 4 of the 
bit-holder 2 is inserted is formed at the center. A thin chip breaker 
plate 32 is separately provided on the upper surface of the insert 36. A 
through hole 33 to which the male threaded part 30 is loosely inserted is 
formed at the center of the chip breaker plate 32. The chip breaker plate 
32 has an outer diameter necessary for the tip 24e to have a prescribed 
chip breaker. The bolt 28 is so threaded at the male threaded part 30 with 
the female threaded part 35 formed on the inner periphery of an insert 34 
that the chip breaker plate 32 is interposed between the head of the bolt 
28 and the insert 34 and they are integrally fixed. The insert 34 thus 
integrally provided with the chip breaker plate 32 is intimately inserted 
into the cavity 25e of the tip 24e. In this state, a predetermined chip 
breaker is formed via the chip breaker plate 32 on the upper surface of 
the tip 24e. When the clamping screw 6 is turned to insert, the pin 4 is 
inclined toward the wall surface 7 at the tip seat side, the tip 24e is 
urged under pressure to the wall surface 7, and is locked to the 
bit-holder 2. The thickness of the insert 34 is so formed as not to be 
larger than the thickness of the tip 24e. Since the chip breaker plate 32 
is, therefore, fixed onto the upper surface of the insert 34 and is also 
brought into contact with the upper surface of the tip 24e, the chip 
breaker plate can perform the function for breaking the cutting chip in 
the same manner as the chip breaker formed integrally with the 
conventional tip shown in FIG. 2. 
According to the third preferred embodiment embodying the cutting tool of 
the present invention, the chip breaker is composed of the tip 24e and the 
chip breaker plate 32 formed integrally with the insert 34 via the bolt 
28. Therefore, the cavity 25e may be formed larger than the conventional 
cavity. 
Since the insert 34 and the flanged tap bolt 28 do not necessitate wear 
resistance and hardness as the tip 24e and the chip breaker plate 32, they 
may be formed of relatively inexpensive material. Further, the shape and 
the size of the chip breaker may be selected to be adapted for desired 
cutting conditions and the material of a workpiece to be cut. Since the 
conventional tip is provided integrally with the chip breaker, the tip 
should be thrown away when the tip itself can be used but its chip breaker 
is damaged not to be durable for use. However, only the chip breaker plate 
may be replaced in the third embodiment of the tip, which is thus 
economic. Furthermore, in case of cutting discontinuously a workpiece or 
cutting a workpiece having small ductility with preferable cutting chip 
treatment, the tip may be used without the chip breaker by removing the 
chip breaker plate from the tip. 
When a larger cavity is formed in a tip which does not relate directly to 
cutting for the purpose of resource-saving and the like, it is difficult 
to so increase the cavity as to be due to the space in which the chip 
breaker is heretofore provided. However, in the third embodiment embodying 
the cutting tool of the present invention, since the chip breaker plate 
may function as a chip breaker when the end of the chip breaker plate 
reaches the upper surface of the tip, a cavity larger than the 
conventional cavity of the tip with the chip breaker may be formed within 
the tip, and greater resource-saving and less expensive tip may be 
provided. In addition, the conventional bit-holder to which a tip with a 
hole is applied may be used without any modification for the conventional 
bit-holder with a pin, an eccentric pin type bit-holder, etc. (not shown). 
According to the aforementioned first through third embodiments embodying 
the cutting tool of the present invention, the outer peripheral surface of 
the insert is, as shown, consecutively formed. The present invention is 
not limited to these particular embodiments described above, but may also 
be composed as the fourth preferred embodiment shown in FIGS. 16 through 
19. 
An insert 36 of the tip of the fourth embodiment has a single slot 37 
radially inwardly extending on the outer peripheral surface thereof. That 
is, the outer peripheral surface of the insert 36 is discontinuous at the 
slot 37. This slot 37 extends, in the fourth embodiment, to the mounting 
hole 15 and communicates with the mounting hole 15. However, the slot 37 
may not extend to the mounting hole 15. 
The operation of the insert 36 thus composed will be described in detail. 
FIGS. 17 and 18 show the state that the circular insert 36 with the slot 
37 shown in FIG. 16 is inserted into the cavity 25e of the cylindrical 
hollow throw-away tip 24e. In FIG. 16, the insert 36 slightly opened due 
to the slot 37 is restricted by the inner wall of the cavity 25e of the 
throw-away tip 24e, and is engaged with the tip 24e in thus compressed 
state. Therefore, the insert 36 will produce a recoiling force tending to 
return to the original shape, with the result that the outer peripheral 
surface of the insert except the slot 37 is intimately contacted with the 
inner wall of the cavity 25e of the throw-away tip 24e. 
When the pin 4 of the bit-holder is inclined upon turning of the clamping 
screw 7 in the state that the insert 36 is thus intimately contacted with 
the tip 24e, the pin 4 tends to expand the slot 37 of the insert 36 in the 
tip 24e. Therefore, the insert 36 is strongly contacted with the inner 
wall of the throw-away tip 24e. Accordingly, the tip of this embodiment 
can be used in stable state even against the fluctuated vibration caused 
by the variation of the cutting resistance or the like, thereby improving 
the lifetime of the cutting tool. 
On the other hand, since cutting heat generally occurs during the cutting 
work to cause the tip to become extremely high temperature, the tip is 
thermally damaged, is resultantly fastened in wear and is thus shortened 
in its lifetime. When the insert in the fourth embodiment of the present 
invention is used, since the slot is formed on the outer peripheral 
surface of the insert, the surface area exposed and contacted with the 
atmospheric air is increased as compared with the insert in which such a 
slot is not formed. In this manner, the dissipating and cooling effect of 
the cutting heat of the tip can be improved, the thermal damage of the tip 
can be thus suppressed, and the lifetime of the throw-away tip can be 
accordingly improved. Since the part of the increased volume of the 
inserted due to the thermal expansion is further absorbed and alleviated 
by the slot space formed at the insert, the damage of the throw-away tip 
due to the thermal expansion of the insert can be eliminated. Furthermore, 
inasmuch as the insert itself does not contribute directly to cutting 
work, it may be repetitively used and is consequently economic. 
The foregoing description is directed to the example of the cylindrical 
hollow throw-away tip 24e in which the insert is inserted, but this 
embodiment may also employ the throw-away tip of the shape other than the 
cylindrical hollow shape within the spirit and scope of the present 
invention. 
FIG. 20 shows a sixth modification of the fourth embodiment of the insert 
of the throw-away tip in the state that a triangular pillar-shaped insert 
38 provided with a slot 40 is inserted into the cavity 13 of the hollow 
triangular pillar-shaped throw-away tip 12. 
FIG. 21 shows a seventh modification of the fourth embodiment of the insert 
of the throw-away tip in the state that a square pillar-shaped insert 43 
provided with a slot 42 is inserted into the cavity 41 of a hollow square 
pillar-shaped throw-away tip 40. In these two modifications, the slots 40 
and 42 are opened at the respective outer lateral surfaces of the 
pillar-shaped inserts. However, as shown as an eighth modification in FIG. 
22, the slot 40 of the insert 38 may be opened at the corner of the 
pillar-shaped insert. 
In a ninth modification of the insert in FIG. 23, the slot 40 is formed on 
the outer periphery of a tapered insert 45 having a tapered outer 
periphery 44. 
FIG. 24 shows the state that the tapered insert 45 shown in FIG. 23 is 
inserted into a cylindrical hollow throw-away tip 47 having a tapered 
cavity 46. Since the outer periphery of the insert 45 and the inner 
surface of the cavity 46 are coupled in tapered state and the slot 40 
formed at the insert 45 is restricted and compressed by the inner surface 
of the cavity 46 of the throw-away tip 47, both are further intimately 
engaged with one another. 
In the sixth through ninth modifications, the single slot is formed at each 
insert and is formed in the state communicated with the mounting hole 15. 
However, the present invention may not be limited to these particular 
structures described above, but as shown as a tenth modification in FIG. 
25, a plurality of slots 40, three in this particular example, may be 
formed, and may not always be communicated with the mounting hole 15. FIG. 
26 shows the state that the insert 39 shown in FIG. 25 is inserted into 
the throw-away tip 12 of hollow triangular pillar-shape. 
FIG. 27 shows as an eleventh modification of the insert in the state that a 
square pillar-shaped insert 50 having a plurality of slots 49 is inserted 
into the cavity 41 of the hollow square pillar-shaped tip 40. 
FIG. 28 shows as a twelfth modification of the insert in the state that a 
circular insert 52 having a plurality of slots 51, 52 on the inner and the 
outer peripheral surfaces respectively is inserted into the cavity 25e of 
a hollow circular pillar-shaped throw-away tip 24e. A second insert 54 is 
inserted into the mounting hole 15 of the insert 53. In this case, the 
material of the second insert 54 may be different from the insert 53 with 
slots. For instahce, only the second insert 54 making direct contact with 
the pin 4 of the bit-holder 2 may be formed of a material having high wear 
resistance. When the slots are formed also at the inner peripheral surface 
of the insert, the effect of the slots formed at the outer peripheral 
surface thereof can be enhanced. 
FIG. 29 shows as a thirteenth modification of the insert in the state that 
a generally triangular pillar-shaped insert 39 formed with a slot 40 
having a wavy shape is inserted into the cavity 13 of the hollow 
triangular pillar-shaped throw-away tip 12. 
The foregoing description is directed to the modifications of the insert in 
which the shape of the mounting hole is circular, but may also be 
polygonal or tapered. 
According to the fourth embodiment and the sixth through thirteenth 
modifications thereof, at least one slot is formed at the insert of the 
throw-away tip and hence have the following advantages and effects. 
(a) Since the slot is formed at the insert and are incorporated with 
relatively elastic property, the insert is compressed by the inner wall of 
the cavity of the throw-away tip to thus cause a recoiling force. Inasmuch 
as the insert is thus intimately engaged with the throw-away tip, the 
throw-away tip can be rigidly mounted at the bit-holder via the insert, 
and can accordingly be used stably against the vibration of the tool 
during cutting work, the lifetime of the cutting tool can be prolonged, 
and the cutting tool can be thus used under excellent cutting conditions. 
(b) Since the pin tends to push and expand the width of the slot formed at 
the insert when the throw-away tip is mounted via the pin at the 
bit-holder, the throw-away tip can be more rigidly engaged with the 
insert, thereby further improving the stability of the cutting tool during 
cutting work. 
(c) Since the surface area of the insert is increased due to the formation 
of the slot at the insert, its heat dissipating and cooling effects with 
the atmospheric air are improved, and the decrease in the lifetime of the 
throw-away tip due to the thermal damage can be effective prevented. 
(d) Since the part of the increased volume of the insert due to the thermal 
expansion of the cutting heat can be partly absorbed by the slot formed at 
the insert, the damage of the throw-away tip can be effectively 
eliminated. 
(e) Since the insert itself does not contribute directly to the cutting 
work, it is not damaged, but can be repetitively used extremely 
economically. 
The foregoing description as to the first embodiment of the throw-away tip 
according to the present invention is directed to that the outer lateral 
surface of the throw-away tip 12 is formed parallel to the inner lateral 
surface thereof. However, as exemplified by a fifth embodiment and a 
fourteenth modification thereof respectively in FIGS. 30A and 30B, each 
outer lateral surface of the throw-away tip 55a and 55b may be inclined at 
the prescribed angle .theta. with respect to each inner lateral surfaces 
thereof, respectively. In this embodiment, the distance (.omega.) between 
the outer lateral surface and the corresponding inner lateral surface is 
defined by the shortest distance therebetween. The width of the part which 
actually contributes to the cutting can be increased by providing the 
inclination, and thereby remedying against the variation in the cutting 
margin thereof. Furthermore, the strength of the cutting edge of the 
cutting tool can be improved on the basis of the distribution of the 
actual vertical stress. 
The foregoing description as to the first embodiment of the throw-away tip 
according to the present invention is directed to that the outer periphery 
of the insert 14 is set correspondingly to the inner periphery of the 
throw-away tip 12 for defining the cavity 13. However, the outer periphery 
of the insert 56 may be, as shown as a sixth embodiment in FIG. 31, 
composed of a first part facing to one part of the inner periphery of the 
throw-away tip 12 and a second part not facing to the other part of the 
inner periphery. In other words, the outer periphery or the insert 56 may 
be intimately contacted with the inner periphery of the throw-away tip 12 
at the first part and may be isolated from the inner periphery at the 
second part. Because the insert 56 is actually functioned when the 
throw-away tip 12 is pushed to lock to the wall surface 7 at the tip seat 
side via the insert 56 by the inclination of the pin 4 inserted into the 
mounting hole 15. Therefore, the first part making contact with the inner 
peripheral surface of the throw-away tip 12 is necessary for the part 
confronting the wall surface 7 at the tip seat side, but the residual part 
is not necessary to be contacted with the inner peripheral surface of the 
throw-away tip 12. The mounting hole may not always be formed from the 
through hole, but be tormed from the slot 57 extending along the 
thicknesswise direction from the side face defining the second part as 
shown by a fifteenth modification in FIG. 32A. 
FIGS. 32B through 32J show sixteenth through twenty-fourth modifications 
according to the present invention.