WO2008069373A1

WO2008069373A1 - Deep hole drilling tool, apparatus having the same, and method for drilling deep hole

Info

Publication number: WO2008069373A1
Application number: PCT/KR2007/002166
Authority: WO
Inventors: Dong Ki Baek; Cheong Woon Park; Seung Ho Kim; Sung Yeoul Kim; Hung Suk Song
Original assignee: Doosan Engine Co., Ltd
Priority date: 2006-12-05
Filing date: 2007-05-03
Publication date: 2008-06-12
Also published as: KR20080052174A; KR100887855B1

Abstract

A drilling tool for forming a second hole which is arranged in parallel with a first hole formed along a longitudinal direction of a workpiece and overlaps at least partially with the first hole includes a guide bar unit for being inserted into the first hole to move along the first hole in an event of drilling the second hole; and a drill arranged at the guide bar unit to be rotatable around a central axis of the second hole, the drill having a cutter for drilling the second hole, arranged at one end thereof, wherein the guide bar unit guides a movement of the drill in the longitudinal direction of the workpiece.

Description

DEEP HOLE DRILLING TOOL, APPARATUS HAVING THE SAME, AND METHOD FOR DRILLING DEEP HOLE

Technical Field The present invention relates to a drilling tool for forming a deep hole in a workpiece such as a fuel rail mounted on a diesel engine for a ship, a drilling apparatus having the same drilling tool and a drilling method. More particularly, the drilling tool of the present invention is configured to form a plurality of deep holes in the workpiece so that they overlap each other.

Background Art

In a conventional diesel engine for ships, fuel injection and valve opening/closing timings are adjusted by controlling a cam driving unit connected to a crank shaft of the engine. However, in a new electronically controlled engine, timings for fuel injection and valve opening/closing are adjusted by a mechanical hydraulic control technique in terms of whole cylinders or an individual cylinder. This technique shows advantageous effects such as reduced fuel consumption, enhanced combustion efficiency, low noise and vibration and reduction in contaminated exhaust. In particular, in the conventional engine for a ship adopting an injection technique based on cam driving, engine performance is optimized in high load and inj ection pressure inevitably has a relatively lower value. On the other hand, in the electromagnetically controlled engine, injection pressure generation and an injection process are independent from engine rotation rate so that fuel pressure and injection timing can be adjusted according to operating conditions of the engine. Thus, high pressure injection is enabled even at a low rotation rate of the engine. Accordingly, the electronically controlled engine can overcome drawbacks of a low speed engine such as low efficiency at low load actuation so as to run stably in a low rate range while reducing pollution caused by exhaust as well as fuel consumption in the low load actuation.

In the electromagnetically controlled engine, fuel is fed into a combustion chamber of the engine through a fuel rail that is also referred to as "common rail." The fuel rail has a longitudinal hole penetrating the fuel rail in a longitudinal direction and a plurality of small holes extending radially from the longitudinal hole to feed fuel from the longitudinal hole into the combustion chamber of the engine. The fuel rail should be maintained at high pressure (e.g., about 900bar) to feed fuel to the engine and thus is designed to withstand high pressure. In order for the fuel rail to withstand high pressure while stably feeding fuel to respective engines, the longitudinal hole of the fuel rail has a unique configuration of two holes (first and second holes) overlapping with each other, which are also referred to as a peanut-shaped hole or peanut hole.

The peanut-shaped hole is formed by sequentially drilling first and second holes. However, since the first and second holes overlap with each other, a drill head forming the second hole tends to slip toward the first hole, which obstructs a drilling process and degrades manufacturing precision.

Describing the process of forming the peanut-shaped hole in more detail, a drill tube is mounted on a motor spindle and then a first hole is drilled in a workpiece fixed to an eccentric jig using the drill tube fixed to the motor spindle. When the first hole is completed, a second hole is formed, in which the workpiece is subjected to torsional force caused by the rotational force of the drill. In order to prevent the torsional force, a piling piece is inserted into the first hole and both ends of the piling piece are welded with the workpiece. Then, the second hole is drilled in the workpiece about the center of the second hole. Once the second hole is completed, the piling piece is removed from the workpiece.

In this case, however, drilling time is prolonged due to the piling piece being welded to the first hole for the purpose of drilling the second hole and then being removed from the first hole. In particular, since the center of the second hole is different from the axial center of the workpiece, a centering operation for locating the central point of the second hole needs a large amount of time. In addition, during the forming of the first hole, the first hole having a long dimension causes sagging in the drill tube. Such sagging in the drill tube causes vibration in the drill tube, degrades drilling efficiency and manufacturing precision of the first hole such as linearity owing to vibration, and forms scratches on the inner periphery of the drilled hole.

Furthermore, since the eccentric jig is fixed to a curved surface of the outer periphery, the position of the workpiece can be changed during the drilling process, which may degrade manufacturing precision such as linearity. When the position of the workpiece is changed, time loss takes place to set the workpiece in position. Then, the process of drilling the second hole is carried out intermittently, consuming a large amount of time.

Disclosure of Invention Technical Problem

The present invention has been made to solve the foregoing problems of the prior art and therefore an object of the present invention is to provide a drilling tool, and a drilling apparatus and a drilling method using the same, which can drill deep holes more rapidly and precisely in a workpiece such as a fuel rail so that the deep holes overlap at least partially with each other.

Technical Solution

According to an aspect of the invention for realizing the object, the invention provides a drilling tool for forming a second hole which is arranged in parallel with a first hole formed along a longitudinal direction of a workpiece and overlaps at least partially with the first hole. The drilling tool includes a guide bar unit for being inserted into the first hole to move along the first hole in an event of drilling the second hole; and a drill arranged at the guide bar unit to be rotatable around a central axis of the second hole, the drill having a cutter for drilling the second hole, arranged at one end thereof, wherein the guide bar unit guides a movement of the drill in the longitudinal direction of the workpiece.

According to another aspect of the invention for realizing the object, the invention provides a drilling apparatus for forming first and second holes along a longitudinal direction of a workpiece, in which the second hole is arranged in parallel and overlaps at least partially with the first hole. The drilling tool includes a motor spindle; a first hole drilling tool comprising a drill tube selectively coupling at one end with the motor spindle, a drill head arranged at the other end of the drill tube for drilling the first hole, and at least one pad arranged on an outer peripheral surface of the drill tube in order to support the drill tube in an inner peripheral surface, thereby preventing the drill tube from sagging; and a second hole drilling tool connected at one end to the motor spindle and provided at the other end with a cutter for drilling the second hole. According to an embodiment of the invention, the first hole drilling tool includes a tube adaptor interposed between the drill head and the drill tube, wherein the tube adaptor is provided at an outer peripheral surface with at least one pad contacting an inner peripheral surface of the first hole.

In addition, the drilling apparatus may further include an eccentric jig having an annular configuration for supporting the workpiece inserted thereinto, inner and outer peripheries of the eccentric jig having different central axes; and a block interposed between at least one jig reference surface formed on the workpiece and the eccentric jig to fix the workpiece to the eccentric jig.

The guide bar unit may include an insert for being inserted into the first hole to move along the first hole in the event of drilling the second hole; and a support fixedly provided in the insert, the drill being inserted into and rotatably supported by the support.

The drilling apparatus may further include an inner tube for transmitting a rotational force of the motor spindle to the drill, wherein the inner tube is connected at one end to the other end of the drill and at the other end to the motor spindle, and an outer tube rotatably receiving the inner tube therein, and connected at one end to the support and at the other end to a tool attachment unit, which is provided with the motor spindle and movable in the longitudinal direction of the workpiece, so as to move the guide bar unit along the longitudinal direction of the workpiece . In the drilling apparatus, a pad may be provided on at least one of an outer peripheral surface of the guide bar unit and an outer peripheral surface of the drill, the pad configured to reduce friction with the workpiece, and the drill may have a hollow area for discharging chips cut by the cutter.

According to further another aspect of the invention for realizing the object, the invention provides a drilling method including a) forming a jig reference surface on an outer peripheral surface of a workpiece; b) mounting at least one eccentric jig on the jig reference surface of the workpiece by using a block; c) drilling a first hole in the workpiece where the eccentric jig is mounted, by using a first hole drilling tool; d) detaching and disassembling the eccentric jig from the workpiece, overturning the eccentric jig 1QO\E, and fixedly inserting the eccentric jig into the workpiece; and e) drilling a second hole in the workpiece by using a second drilling tool so that the second hole overlaps at least partially with the first hole.

Advantageous Effects

According to the present invention as mentioned above, in the case of forming first and second holes which extend along a longitudinal direction of a workpiece and overlap at least partially with each other, it is possible to improve manufacturing precision and reduce machining time.

Description of Drawings FIG. 1 is a schematic perspective view illustrating a workpiece drilled by a drilling apparatus according to an embodiment of the invention;

FIG. 2 is a schematic sectional view illustrating the drilling apparatus equipped with a first hole drilling tool according to an embodiment of the invention; ^■

FIG. 3 is a schematic sectional view illustrating the workpiece fixed to the eccentric jig shown in FIG. 2 ; FIG. 4 is a schematic sectional view illustrating a procedure of drilling a first hole in the workpiece with the first hole drilling tool shown in FIG. 2;

FIG. 5 is a schematic perspective view illustrating a second hole drilling tool according to an embodiment of the invention;

FIG. 6 is a side elevational view illustrating the second hole drilling tool shown in FIG. 5;

FIG. 7 is a schematic sectional view illustrating the second hole drilling tool shown in FIG. 5; FIG. 8 is a front elevational view illustrating the second hole drilling tool shown in FIG. 5; and

FIGS. 9 and 10 are sectional views illustrating a procedure of drilling a second hole in a workpiece with the drilling apparatus according to the embodiment of the invention.

Best Mode

Hereinafter a drilling apparatus and a drilling method according to an embodiment of the invention will be described in detail.

Referring to FIGS. 1 to 8, the drilling apparatus according to this embodiment of the invention is configured to form first and second holes 2 and 3 in a workpiece 1 so that the first and second holes 2 and 3 overlap with each other at least partially, and includes a support 10, a tool attachment unit 11, tube-side vibration absorbers 12, a pressure head 13, a workpiece attachment unit 14, eccentric jigs 15, workpiece-side vibration absorbers 16, a first hole drilling tool 20 and a second hole drilling tool 30.

The tool attachment unit 11 is configured to support the first hole and second hole drilling tools 20 and 30 as well as to rotate the first hole and second hole drilling tools 20 and 30 so as to drilling the workpiece 1. The tool attachment unit 11 is arranged on the support 10 so as to be linearly movable along an arrow A. A motor spindle 11a is arranged inside the tool attachment unit 11, and the first hole and second hole drilling tools 20 and 30 are connected to the motor spindle 11a so as to be rotated by the same.

The tube-side vibration absorbers 12 are arranged on the support 10 to support a drill tube 21 of the first hole drilling tool 20 and an outer tube 35 of the second hole drilling tool 30, thereby absorbing vibration occurring in the drill tube 21 and the outer tube 35.

The pressure head 13 supports the drill tube 21 or the outer tube 35 so as not to sag while guiding the same to move in the direction of the arrow A.

The workpiece attachment unit 14 is configured to fix the workpiece 1, and has a chuck for fixing one end of the workpiece 1.

Each of the eccentric jigs 15 is devised to prevent that a centering procedure takes a long time owing to the eccentricity between the center of the workpiece 1 and those of the first and-second holes 2 and 3. That is, the eccentric jig 15 has an annular shape with the inner periphery being positioned eccentric with respect to the outer periphery. In detail, the inner periphery of the eccentric jig 15 is eccentric with respect to the outer periphery in proportion to the distance between the central axes of the first and second holes 2 and 3 separated from each other. Therefore, after the first hole 2 is drilled, the eccentric jigs 15 are turned over 180°, inserted into the workpiece 1 and then supported on the workpiece-side vibration absorbers 16. Then, a second hole 3 can be drilled without having to perform a centering procedure. As problems exist, the jigs are fixed to the circular outer periphery of the workpiece 1 and thus the workpiece 1 is not easily fixed but tends to change its position. According to this embodiment, j ig reference surfaces 4 are cut on the workpiece 1 and blocks 5 are inserted into between the eccentric jig 15 and the jig reference surfaces 4 so that the eccentric j ig 15 can be mounted on the workpiece 1. With the workpiece

1 fixed to the eccentric jig 15 by inserting the blocks 5 into between the plane jig reference surfaces 4 and the eccentric jig 15, it is possible to prevent any position shift of the workpiece 1 while saving centering time. In the meantime, the workpiece 1 is supported by the workpiece-side vibration absorbers 16 which are arranged periodically. The workpiece-side vibration absorbers 16 act similar to the aforementioned tube-side vibration absorbers 12 to support the workpiece 1 while absorbing vibrations of the workpiece 1.

The first hole drilling tool 20 is devised to drill the first hole 2 in the workpiece 1, and includes a drill tube 21, a drill head 22 arranged on one end of the drill tube 21 and a tube adaptor 23 connecting the drill tube 21 with the drill head 22. Both of the tube adaptor 23 and the drill tube 21 are provided with pads 24 protruding radially . The pads 24 serve to reduce frictions between the drill tube 21 and the inner periphery of the first hole 2 or the tube adaptor 23 and the inner periphery of the first hole

2 while supporting the drill tube 21 so as not to sag. With the drill tube 21 supported so as not to sag, it is possible to improve drilling precision such as the linearity of the first hole 2 as well as to restrict vibrations owing to the sagging of the drill tube 21 thereby preventing drilling conditions from changing. With the drilling conditions maintained constant, it is possible to prevent frequent replacement of insert tips.

The drill tube 21 is connected at one end to the motor spindle 11a of the tool attachment unit 11 and at the other end to the drill head 22 via the tube adaptor 23 in order to transmit rotational force from the motor spindle 11a to the drill head 22 via the drill tube 21 and the tube adaptor 23. The drill head 22 is mounted with an insert tip in use for a drilling operation and thus, when rotating, can drill the first hole 2.

As shown in FIGS. 5 to 8, the second hole drilling tool 30 includes a guide bar unit 31, a drill 36 and pads 41. The guide bar unit 31 is devised to guide the movement of the drill 36 while the drill 36 perforates the second hole 3, and includes an insert 32 to be inserted into the first hole 2 in the event of drilling the second hole 3 and a support 33 fixed to the insert 32 and rotatably supporting the drill 36.

The insert 32 is inserted into the first hole 2 and moves along the first hole 2 without rotational motion while the second hole 3 is being drilled. The insert 32 is fabricated with the same configuration as that of the first hole 3 and substantially the same radius as that of the first hole 3 so that the insert 32, when inserted into the second hole 3, can minimize the movement of the workpiece 1 except for in the longitudinal direction., More specifically, the insert 32 preferably has a crescent cross section. The support 33 is fixed to the insert 32 and has a hollow area 33a therein so that the drill 36 can be inserted into and rotatably supported by the support 33. An outer tube connector 34 is formed in the outer periphery of one end portion of the support 33, and configured to couple with the outer tube 35 which is fixed to the tool attachment unit 11. The outer tube connector 34 is shaped as threads, and the outer tube 35 also has threads in the inner periphery of one end thereof, corresponding to the threads of the outer tube connector 34. That is, the outer tube 35 is connected to the support 33, threaded into the outer tube connector 34. The other end of the outer tube 35 is fixed to the tool attachment unit 11, which is movable in the direction of the arrow A (see FIG. 2), but not connected to the motor spindle 11a. Therefore, the guide bar unit 31 moves linearly in the direction of the arrow A but does not rotate. Accordingly, even in a case where the drill 36 supported by the support 33 rotates to drill theworkpiece 1, it is possible to prevent the workpiece 1 from rotating around the central axis of the second hole 3 by using the guide bar unit 31 and thereby more precisely drill the second hole 3 in the workpiece 1.

The drill 36 is provided at one end thereof with a cutter 37 and the other end with an inner tube connector 38 connected with the inner tube 39. The inner tube connector 38 is provided with threads in the outer periphery of one end thereof like the outer tube connector 34, and the inner tube 39 is also provided with threads in the inner periphery of one end thereof, corresponding to the threads of the inner tube connector 38. The inner tube 39, in a state inserted into the outer tube 35, is connected to the motor spindle 11a of the tool attachment unit 11. With this configuration, the rotation of the motor spindle 11a causes the inner tube 39 to rotate inside the outer tube

35 and the drill 36 connected to the inner tube 39 to rotate inside the support 33. As the drill 36 rotates, the cutter

37 provided at one end of the drill 36 cuts the workpiece 1 to perforate the second hole 3. The drill 36 also has an inner hollow area 40, so that chips, which are created in the cutting of the second hole 3, can be discharged through the hollow area 40 of the drill 36 and the inner tube 39.

The pads 41 protruding radially from the outer peripheries of the guide bar unit 31 and the drill 36 are configured to reduce frictions between the guide bar unit 31 and the inside periphery of the first hole 2 and between the drill 36 and the second hole 3. Preferably, the pads 41 are made of a material having a hardness higher than that of the workpiece 1 in order to efficiently decrease the frictions between the guide bar unit 31 and the inside periphery of the first hole 2 and between the drill 36 and the second hole 3.

Now a method will be described according to an embodiment of the invention, for drilling a peanut-shaped deep hole. In general, the method is carried out to sequentially form the first hole 2 and then the second hole 3 in the workpiece 1.

Describing procedures of forming the first hole 2, at least one planar jig reference surface 4 is first cut on the workpiece 1. Then, the eccentric jigs 15 are mounted on the workpiece 1. The eccentric jigs 15 are fixed to the workpiece 1 with the blocks 5 interposed between the jig reference surface 4 and the eccentric jigs 15. Then, the workpiece 1 with the eccentric jigs 15 attached thereto is mounted on the workpiece-side vibration absorbers 16, centered, and fixed to the workpiece attachment unit 14 via a chuck. In sequence, the first hole drilling tool 20 is mounted on the motor spindle 11a of the tool attachment unit 11 and supported on the tube-side vibration absorbers 12 and the pressure head 13. Then, the first hole drilling tool 20 is centered. Next, the motor spindle 11a is driven to rotate the first hole drilling tool 20. As the first hole drilling tool 20 rotates, the drill head 22 perforates the first hole 2 in the workpiece 1.

Alternatively, a balance weight (not shown) can be attached to the workpiece 1 so that the drill head 22 rotates at the same time as the workpiece 1 rotates around the center of the first hole 2 in the drilling of the first hole 2 in order to perforate a deep hole in the workpiece 1.

As the first hole 2 is drilled, the sagging of the drill tube 21 may cause vibration, thereby changing cutting conditions or creating scratches on the inner periphery of the first hole 2. However, according to this embodiment , the pads 24 arranged on the drill tube 21 and the tube adaptor 23 support the drill tube 21 and the tube adaptor 23 on the inner periphery of the first hole 2, thereby preventing the drill tube 21 from sagging.

When the drilling of the first hole 2 is completed, the drilling of the second hole 3 is started. In order to drill the second hole 3, the eccentric jigs 15 are disassembled from the position fixed for the drilling of the first hole 2 and detached from the workpiece 1. Then, the eccentric jigs 15 are turned over 180° and mounted on the workpiece 1 again. Next, the eccentric jigs 15 are mounted on the workpiece-side vibration absorbers 16, respectively, and the workpiece 1 is fixed to the workpiece attachment unit 30. Then the workpiece 1 is set in a position for the drilling of the second hole 3. This is because the inner periphery of the eccentric jigs 15 is not formed concentric with the outer periphery of the eccentric jigs 15. That is, the inner periphery of the eccentric jigs 15 is eccentric to the outer periphery thereof.

Then, the inner tube 39 and the outer tube 35 are mounted on the tool attachment unit 11 and connected to the second hole drilling tool 30. In more detail, the inner tube 39 is inserted into the outer tube 35, the inner tube 39 is connected at one end to the inner tube connector 38, and the outer tube 35 is connected at one end to the outer tube connector 34. Then, the inner tube 39 is connected at the other end to the motor spindle 11a and the outer tube 35 is connected at the other end to a fixed part (not shown) of the tool attachment unit 11 which does not rotate.

When the workpiece 1 and the second hole drilling tool 30 are mounted completely, the motor spindle 11a is rotated. Then, the inner tube 39 rotates within the outer tube 35 so that the drill 36 connected to the inner tube 39 via the inner tube connector 38 rotates inside the support 33 in a supported state. In response to the rotation of the drill 36, the cutter 37 arranged at one end of the drill 36 starts the drilling of the second hole 3 by cutting the workpiece 1.

As the drilling of the second hole 3 starts, the tool attachment unit 11 moves in the direction of an arrow B as shown in FIG. 9. Then, the second hole drilling tool 30 drills the second hole 3 while moving in the direction of the arrow B. The insert 32 of the guide bar unit 31 is inserted into the first hole 2, and performs only a linear movement along the longitudinal direction of the workpiece 1 without performing a rotational movement. With this configuration, while the cutter 37 rotating to cut the workpiece may transmit a torsional force to the workpiece 1, the insert 32 can prevent torsion or vibration of the workpiece 1, thereby improving the manufacturing precision such as linearity of the second hole 3 formed in the workpiece 1. It is also possible to prevent frequent replacement of the cutter 37 or changes in machining conditions caused by the torsion of the workpiece 1. That is, the second hole 3 can be drilledmore quickly . However , as the drilling depth of the second hole 3 increases, sagging of the inner and outer tubes 39 and 35 is caused by the weight of the second hole drilling tool 30 and the inner and outer tubes 39 and 35 and the axial load of the second hole 3 acting on the workpiece 1 , thereby possibly decreasing the rotation rate and feed rate thus creating vibration. In view of these matters, the second hole 3 is not penetrated by a single procedure but, when the total length of the second hole 3 exceeds a predetermined dimension of for example 11.4m, the total length of the second hole 3 is drilled by halves .

Referring to FIG. 9, a first half 3a of the second hole 3 is formed in the workpiece 1 where the first hole 2 is formed from the right of the drawing to a half point of the total axial length of the second hole 3 by using the second drilling tool 30. Then, as shown in FIG. 10, a secondhalf 3bof the secondhole 3 is formed in the workpiece 1 from the left of the drawing to the half point of the total axial length of the second hole 3. Although it has been illustrated that the second hole 3 is drilled from the right and the left, respectively, so that the present invention can be more clearly understood, when the second hole 3 is drilled to a substantially middle point of the length of the workpiece 1, the workpiece 1 is changed in position or turned over and then the second hole 3 is drilled again in the direction of an arrow C from a point opposite to a starting point of the first drilling as shown in FIG. 10. The drilling in the direction of the arrow C is carried out until the second half 3b communicates with the first half 3a. Chips created during the drilling of the second hole 3 are discharged on lubricant through the hollow area 40 of the drill 36 and the inner tube 39. In the drilling of the second hole 3, the pads 41 are arranged to protrude radially from the outer peiripheral surfaces of the insert 32, the support 33 and the drill 36 so as to contact the inner peripheral surface of the second hole 3. This can reduce frictions between the second drilling tool 30 and the inner peripheral surface of the second hole 3 of the workpiece 1, thereby facilitating the movement of the second hole drilling tool 30.

Industrial Applicability The present invention can be applied not only to a process of drilling a peanut-shaped hole in a fuel pipe of a diesel engine, particularly for a ship, but also to any process of drilling a plurality of deep holes in a workpiece where the deep holes overlap at least partially with each other.

According to the present invention as set forth above, in the drilling of the second hole, the guide bar unit is inserted into andmoves along the first hole without rotation in order to guide the second drilling tool along the longitudinal direction of the workpiece, thereby preventing any torsion of the workpiece or vibration of the second hole drilling tool. This as a result can improve manufacturing precision of the second hole such as linearity as well as enhance machining rate. Furthermore, with the support pads arranged to protrude radially from the outer peripheral surface of the drill tube of the first hole drilling tool, it is possible to decrease frictions in the inner peripheral surfaces of the first hole drilling tool and the first hole of the workpiece, thereby facilitating the movement of the first hole drilling tool. In particular, the movement of the first hole drilling tool can be further facilitated by connecting the drill head with the drill tube via the tube adaptor and arranging the pads on the tube adaptor. In addition, with the pads supporting the first hole drilling tool on the inner peripheral surface of the first hole, it is possible to prevent sagging of the first hole drilling tool, thereby preventing vibration of the first drilling tool. This as a result can improve manufacturing precision of the first hole such as linearity.

Moreover, with the jig reference surface formed on the workpiece and the eccentric jig fixed to the workpiece using the block interposed between the jig reference surface and the eccentric jig, it is possible to more stabilize the mounting of the workpiece as well as further facilitate the centering of the workpiece.

In a case where the length or depth of the second hole exceeds a predetermined dimension, the second hole can be drilled by halves of the total depth in order to prevent sagging of the drilling tool and associated vibration, thereby further enhancing the precision of the second hole.

Claims

1. A drilling tool for forming a second hole which is arranged in parallel with a first hole formed along a longitudinal direction of a workpiece and overlaps at least partially with the first hole, comprising: a guide bar unit for being inserted into the first hole to move along the first hole in an event of drilling the second hole; and a drill arranged at the guide bar unit to be rotatable around a central axis of the second hole, the drill having a cutter for drilling the second hole at one end thereof, wherein the guide bar unit guides a movement of the drill in the longitudinal direction of the workpiece in an event of drilling the second hole.

2. The drilling tool according to claim 1, wherein the guide bar unit includes: an insert for being inserted into the first hole to move along the first hole in the event of drilling the second hole; and a support fixedly provided in the insert, the drill being inserted into and rotatably supported by the support .

3. The drilling tool according to claim 2, wherein thedrillhas aninner tube connector at the other end thereof, the inner tube connector connected to an inner tube which is rotatable by a motor spindle.

4. The drilling tool according to claim 3, wherein the support is provided with an outer tube connector for being connected with an outer tube into which the inner tube is rotatably inserted.

5. The drilling tool according to claim 1, comprising a pad provided on at least one of an outer peripheral surface of the guide bar unit and an outer peripheral surface of the drill, the pad configured to reduce friction with the workpiece.

6. The drilling tool according to claim 1, wherein the drill has a hollow area for discharging chips cut by the cutter.

7. The drilling tool according to claim 1, wherein the workpiece is a fuel rail for a ship.

8. A drilling apparatus for forming first and second holes along a longitudinal direction of a workpiece, in which the second hole is arranged in parallel and overlaps at least partially with the first hole, comprising: a motor spindle; a first hole drilling tool comprising a drill tube with one end for being selectively connected to the motor spindle, a drill head arranged at the other end of the drill tube for drilling the first hole, and at least one pad arranged on an outer peripheral surface of the drill tube in order to support the drill tube in an inner peripheral surface, thereby preventing the drill tube from sagging; and a second hole drilling tool connected at one end to the motor spindle and provided at the other end with a cutter for drilling the second hole.

9. The drilling apparatus according to claim 8, wherein the first hole drilling tool includes a tube adaptor interposed between the drill head and the drill tube, wherein the tube adaptor is provided at an outer peripheral surface with at least one pad contacting an inner peripheral surface of the first hole.

10. The drilling apparatus according to claim 8, comprising : an eccentric jig having an annular configuration for supporting the workpiece inserted thereinto, inner and outer peripheries of the eccentric jig having different central axes; and a block interposed between at least one jig reference surface formed on the workpiece and the eccentric jig to fix the workpiece to the eccentric jig.

11. The drilling apparatus according to claim 8, wherein the second hole drilling tool comprises: a guide bar unit for being inserted into the first hole to move along the first hole in an event of drilling the second hole; and a drill arranged at the guide bar unit to be rotatable around a central axis of the second hole, the drill having a cutter for drilling the second hole at one end thereof, wherein the guide bar unit guides a movement of the drill in the longitudinal direction of the workpiece.

12. The drilling apparatus according to claim 11, wherein the guide bar unit includes: an insert for being inserted into the first hole to move along the first hole in the event of drilling the second hole; and a support fixedly provided in the insert, the drill being inserted into and rotatably supported by the support .

13. The drilling apparatus according to claim 12, comprising an inner tube connected at one end to the other end of the drill and at the other end to the motor spindle so that the inner tube transmits a rotational force of the motor spindle to the drill.

14. The drilling apparatus according to claim 13, comprising an outer tube rotatably receiving the inner tube therein, and connected at one end to the support and at the other end to a tool attachment unit, which is provided with the motor spindle and movable in the longitudinal direction of the workpiece, so as to move the guide bar unit along the longitudinal direction of the workpiece.

15. The drilling apparatus according to claim 11, wherein a pad is provided on at least one of an outer peripheral surface of the guide bar unit and an outer peripheral surface of the drill, the pad configured to reduce friction with the workpiece.

16. The drilling apparatus according to claim 11, wherein the drill has a hollow area for discharging chips cut by the cutter.

17. The drilling apparatus according to claim 11, wherein the workpiece is a fuel rail for a ship.

18. A drilling method comprising: a) forming a jig reference surface on an outer peripheral surface of a workpiece; b) mounting at least one eccentric jig on the jig reference surface of the workpiece by using a block; c) drilling a first hole in the workpiece where the eccentric jig is mounted, by using a first hole drilling tool; d) detaching and disassembling the eccentric jig from the workpiece, overturning the eccentric jig 180°, and fixedly inserting the eccentric jig into the workpiece; and e) drilling a second hole in the workpiece by using a second drilling tool so that the second hole overlaps at least partially with the first hole.

19. The drilling method according to claim 18, wherein the step e) comprises: drilling a first half of the second hole from one end of the workpiece into a middle point of the workpiece where the first hole is formed; and drilling a second half of the second hole from the other end of the workpiece so that the second half of the second hole communicates with the first half of the second hole .