Boring tool

A boring tool includes a concave groove formed on the outer peripheral surface of a tool body on the side opposite from a cutting blade with respect to an axis of the tool body. The concave, groove 10 is formed so as to extend from a position x, which is separated rearwardly from a front end face of a head portion, toward the rear side, and to be open downward from above a rake face. Furthermore, a sub-groove is formed between the concave groove and a chip pocket. Accordingly, the natural frequency is increased by reducing the weight of the head portion while limiting rigidity reduction, thereby restricting chattering.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a boring tool for use in internal 
machining or the like for a prepared hole formed in a workpiece. 
2. Description of the Related Art 
As a tool for machining the inner periphery of a prepared hole of a 
rotating workpiece, for example, a boring tool is known in which a tip 
made of a cemented carbide alloy is detachably screwed to a front end 
portion of an approximately cylindrical tool body. 
This boring tool is inserted in a prepared hole of a workpiece, which is 
supported by a main shaft of a machine tool and rotates at high speed, so 
as to cut the inner peripheral surface of the prepared hole with a cutting 
blade formed in the tip. 
Since machining with the boring tool is performed in a state in which the 
tool body shaped like a shaft is extendedly projected, chattering is prone 
to arise. 
For this reason, chattering has been hitherto prevented by making the tool 
body thick as long as it does not interfere with discharging of chips, by 
making the entire tool body of a high-speed steel, or by other means, in 
order to increase the rigidity of the tool body. 
As disclosed in Japanese Unexamined Utility Model No. 4-2505, a boring tool 
that restricts chattering by making a tool body thicker toward the end is 
known to include a second chip pocket open on the outer peripheral surface 
of the tool body opposite from the side where a first chip pocket is open, 
in order to enhance chip discharging efficiency. In these cases, however, 
it is impossible to prevent the amplitude of chattering from increasing 
due to the resonance that arises when the frequency of external force 
(cutting resistance) applied from the workpiece to the tool and the 
natural frequency of a machine system including the tool coincide with 
each other. This is a problem in a case in which the inner peripheral 
surface of a deep prepared hole must be precisely finished, a case in 
which high dimensional accuracy is necessary, and in other cases. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above circumstances, and 
an object of the present invention is to prevent chattering by reducing 
the weight of a boring tool without reducing rigidity so as to increase 
the natural frequency of the boring tool itself. 
According to the present invention, a boring tool includes a concave groove 
formed on the outer peripheral surface of a tool body on the side opposite 
from a cutting blade with respect to the axis of the tool body so as to be 
open downward from above a rake face, as viewed from the direction of the 
axis. 
In this boring tool, since the weight of an end portion of the tool body is 
reduced and the natural frequency of the tool-itself is increased by 
forming the concave groove in the end portion, it is possible to increase 
the difference between the natural frequency of a machine system including 
the tool and the frequency of external force, and to thereby restrict 
chattering. Moreover, by forming the concave groove in such a position, a 
satisfactory thickness is secured along the direction in which the 
principal force of the cutting force acts, and a part is left to function 
as a rib for the thrust force of the cutting force. Therefore, rigidity 
can be maintained. 
Preferably, the boring tool further includes a sub-groove formed between a 
chip pocket and the concave groove. 
In this boring tool, the sub-groove is added between the concave groove and 
the chip pocket in addition to the concave groove formed in the 
aforementioned position, which makes it possible to further reduce the 
weight of the end portion of the tool body, and to thereby further 
restrict chattering. 
Preferably, the concave groove does not have an opening at the front end of 
the tool body. 
Since the concave groove is formed so as not to be open at the front end of 
the tool body in this boring tool, it is possible to further restrict 
rigidity from decreasing due to the formation of the concave groove, 
compared with a case in which the concave groove is open at the front end 
of the tool body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A boring tool according to an embodiment of the present invention will be 
described below with reference to FIGS. 1 to 6. 
FIG. 1 is a plan view of a forward end portion (hereinafter referred to as 
a "head portion") of a boring tool of this embodiment, as viewed from the 
side of a rake face of a tip. In these figures, numerals 1, 2, 3, and 4 
denote a tool body, a head portion, a tip, and a chip pocket, 
respectively, and the letter O denotes the axis of the tool body 1. 
The tool body 1 has an approximately cylindrical outline, and the chip 
pocket 4 is formed in the head portion 2 that represents the end portion 
of the tool body 1. 
The chip pocket 4 is indented inward in the radial direction around 
approximately one third of the circumference of the head portion 2 that is 
circular in cross section, and extends from a front end face 2a of the 
head-portion 2 toward the rear end. A rear end portion of the chip pocket 
4 forms an inclined face that gently links to the outer peripheral surface 
of the tool body 1. 
At the front end of a face 4a of the chip pocket 4 that points back in the 
workpiece rotating direction, a tip mount 5 is formed to be indented a 
step lower than the face 4a, as shown in FIG. 4. On the tip mount 5, the 
tip 3 having a cutting blade made of a cemented carbide alloy or the like 
is seated, and is detachably mounted thereon by a clamp screw 6 in a state 
in which a rake face 3a of the tip 3 and face 4a chip pocket 4 are 
substantially coplanar. 
In addition to the chip pocket 4, the head portion 2 is provided with a 
cutout portion 7 that is formed by cutting out the outer peripheral 
surface of the head portion 2 opposite from the cutting blade with respect 
to the axis O of the tool body 1. 
This cutout portion 7 is formed to be gradually indent inwardly in the 
radial direction from the rear end of the outer peripheral surface of the 
head portion 2 toward the front end, as shown in FIG. 1. 
Furthermore, the cutout portion 7 is provided with a concave groove 10 for 
the purpose of reducing the weight of the head portion 2. The concave 
groove 10 is formed to extend by a predetermined length from a position x, 
which is separated rearwardly from the front end face 2a of the head 
portion 2, toward the rear side. That is, a front end 10a of the concave 
groove 10 is not open on the front end face 2a of the head portion 2, as 
distinguished from the chip pocket 4 that is open on the front end face 2a 
of the head portion 2. 
The concave groove 10 is open downward from above the rake face 3a, as seen 
from the axial direction of the tool body 1, that is, when the tool body 1 
is seen from the front side toward the rear side along the axis O, as 
shown in FIG. 2. 
In the head portion 2 having the concave groove 10 structured as described 
above, a satisfactory thickness is secured in the direction in which a 
principal force F1 of the cutting force acts, and a rib portion 2a, for 
supporting a thrust force F2 is also secured, as shown in FIG. 5a. 
A sub-groove 11 is formed between the chip pocket 4 and the concave groove 
10 in the head portion 2. The sub-groove 11 is provided to further reduce 
the weight of the head portion 2, and links to the front end of the chip 
pocket 4 via a cross ridge L. Furthermore, the sub-groove 11 is indented 
from the outer peripheral surface of the head portion 2 in approximately 
the shape of a fan in plan view (see FIG. 1), in which the rake face 3a of 
the tip 3 is seen from directly above, so that sub-groove 11 gradually 
widens from the front side to the rear side of the head portion 2. 
On the other hand, the tool body 1 is provided, on its base-end side, with 
two parallel flat faces 21 (one of which is not shown) that are formed by 
planing the outer periphery of the tool body 1, as shown in FIG. 6. These 
flat faces 21 are provided to prevent the tool body 1, supported on a tool 
post of a machine tool, from rotating so that the tool body 1 is not 
rotated by cutting resistance applied from a workpiece that rotates at 
high speed on the axis O together with the main shaft of the machine tool. 
As described above, in the boring tool of this embodiment, the weight of 
the head portion 2 is reduced by forming the cutout portion 7 so as to 
substantially surround the periphery of the concave groove, as shown in 
FIGS. 2 and 3, and the concave groove 10 in the head portion 2, separate 
from the chip pocket 4, thereby increasing the natural frequency. 
Furthermore, the weight of the head portion 2 is further reduced by 
forming the sub-groove 11 in addition to the cutout portion 7 and the 
concave groove 10, thereby further increasing the natural frequency. This 
makes it possible to increase the difference between the natural frequency 
of the machine system including the boring tool and the frequency of 
external force, and to thereby restrict chattering due to resonance. As 
shown in FIG. 3, the length of groove 10 is such as to be substantially 
the equivalent of the length of head portion 2. 
By forming the cutout portion 7 and the concave groove 10 in such 
positions, a satisfactory thickness is secured along the direction in 
which the principal force F1 of the cutting force acts, as shown in FIGS. 
5a to 5d, thereby limiting rigidity reduction. 
In other words, chattering is restricted with rigidity reduction limited by 
removing extra thickness while maintaining the second moment of area with 
respect to the axis Y perpendicular to the principal force F1. 
Since the rib portion 2a is also secured to support the thrust force F2, 
rigidity is increased with respect to the stress that acts in the 
direction along the thrust force F2, so that chattering is not apt to 
arise in this direction. 
Furthermore, since the front end of the concave groove 10 is not open on 
the front end face 2a of the head portion 2, reduction in rigidity is 
further limited. 
From the above, according to the boring tool of this embodiment, since 
chattering is restricted by preventing resonance during machining while 
maintaining tool rigidity as before, it is possible to enhance working 
accuracy and to prevent fracture at the tip. In addition, it is possible 
to increase the amount of projection of the tool body, and to thereby 
precisely finish the inner peripheral surface of a prepared hole having a 
large depth. 
While the sub-groove 11 is formed between the chip pocket 4 and the concave 
groove 10 in the description of this embodiment, this is not always 
necessary. That is, even when the boring tool has only the concave groove 
10 having the above structure in the head portion 2, it can reduce 
chattering compared with the conventional boring tool while-maintaining 
tool rigidity. 
As is apparent from the above description, the present invention can offer 
the following advantages: 
(a) In the boring tool according to the present invention, chattering can 
be restricted by forming the concave groove in the head portion so as to 
reduce the weight of the head portion and to thereby increase the natural 
frequency of the tool itself. Moreover, since rigidity is prevented from 
being reduced due to the formation of the concave groove by forming the 
concave groove in the outer peripheral surface on the side opposite from 
the cutting blade with respect to the axis of the tool body, it is 
possible to enhance working accuracy and to prevent fracture at the tip, 
and furthermore, it is possible to increase the amount of projection of 
the tool body, and to thereby precisely finish the inner peripheral 
surface of a prepared hole having a large depth. 
(b) The weight of the head portion is further reduced by forming the 
concave groove in the above-described position, and forming the sub-groove 
as well between the concave groove and the chip pocket, thereby increasing 
the natural frequency of the tool itself. Therefore, it is possible to 
further restrict chattering. 
(c) When the concave groove is formed so as not to be open at the front end 
of the head portion in order to further restrain rigidity from being 
reduced due to the formation of the concave groove, compared with a case 
in which the concave groove is open at the front end of the head portion, 
it is possible to further restrict chattering.