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Patent Searching and Data


Title:
DIAMOND TOOL WITH GROOVE
Document Type and Number:
WIPO Patent Application WO/2005/099950
Kind Code:
A1
Abstract:
The present invention relates to a cutting tip and a diamond tool using the cutting tip. An object of the invention is to provide a cutting tip and a diamond tool, wherein a good initial cutting ability can be maintained while using the tool for an extended period of time, and a complicated dressing work therefor can be omitted, along with smooth discharge of the cutting chips and cooling water. In the cutting tip and the diamond tool of the invention for achieving the object, a concave groove is formed along a cutting direction at a leading end thereof that initially comes into contact with a workpiece when cutting the workpiece.

Inventors:
KIM SUG GOO (KR)
KIM SHIN KYUNG (KR)
KIM SEONG HOON (KR)
JEONG JAE HYUN (KR)
Application Number:
PCT/KR2005/001073
Publication Date:
October 27, 2005
Filing Date:
April 13, 2005
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SHINHAN DIAMOND IND CO LTD (KR)
KIM SUG GOO (KR)
KIM SHIN KYUNG (KR)
KIM SEONG HOON (KR)
JEONG JAE HYUN (KR)
International Classes:
B23B27/20; B23B51/04; B23D61/02; B24D5/00; B24D5/12; B28D1/12; (IPC1-7): B23D61/02; B24D5/00
Foreign References:
JPH03161278A1991-07-11
JP2001334469A2001-12-04
KR20030027434A2003-04-07
US4993194A1991-02-19
JP2001071270A2001-03-21
Attorney, Agent or Firm:
Nam, Seung-hee (1330-9 Seocho-Don, Seocho-Gu Seoul 137-858, KR)
Download PDF:
Description:
Description

DIAMOND TOOL WITH GROOVE Technical Field [1] The present invention relates to a diamond tool. More particularly, the invention relates to a diamond tool having a concave groove formed at a leading end thereof such that a good initial cutting ability is maintained for an extended period of time, a front- face dressing process is not required, and cutting chips and powder and cooling water are smoothly discharged. [2] The present invention is also applied to tools such as a saw, a core drill, a cutter, a saw blade, a wire saw and the like. Background Art [3] In general, a diamond tool comprises a diamond grinding stone portion (abrasive-impregnated section) attached to a shank to cut and grind a workpiece, and a shank through which the grinding stone portion is mounted on a cutting or grinding machine. Here, the abrasive-impregnated section comprises a plurality of diamond particles and a metallic bonding material. The term 'diamond,' 'abrasives' or 'diamond particles' generally means a natural and synthetic diamond, cubic boron nitride, and ad¬ ditionally a super abrasive such as silicone carbide and alumina, and also a mixture of two or more thereof. Furthermore, the shank is commonly formed of a metallic material such as stainless steel and carbon steel. [4] Methods of bonding abrasives or a diamond-impregnated section to a shank include a sintered-tip welding method (hereinafter, referred to as a 'sintering method'), an electroplating method, a brazing method, and the like. In the sintering method, a metallic bonding material and abrasives are mixed, press-compacted, and sintered to form a cutting tip, and then the sintered cutting tip is bonded to a shank through silver brazing, laser welding, resistance welding, or the like. In the electroplating method, abrasives are attached to a shank through a wet electroplating process using a bonding material such as nickel. In the brazing method, a liquid paste of a metallic bonding material and a binder is coated on the shank, abrasives are dispersed therein, and the dispersed abrasives are bonded to the shank at an elevated temperature. Fig. 1 is a sectional view of abrasives 130 bonded to a shank 110 via a bonding material 120 re¬ spectively through a sintering method (Fig. 1 (a)), an electroplating method (Fig. 1 (b)), and a brazing method (Fig. 1 (c)). [5] Fig. 2 is a partial front view of a saw blade where abrasives 130 are attached to a shank 110 through a sintering method. Fig. 3 shows a section taken along line II- II in FIG. 2. A typical saw blade has the shape of a circular plate, and a plurality of cutting tips are formed at regular intervals along the circumference thereof to protrude in radial directions. Figs. 2 and 3 (also, Figs. 4 and 5) show only a part of the cutting tip. As previously described, in the sintering method, a metallic bonding material 120 and abrasives 130 are mixed, press-formed, and sintered. Thus, as shown in Fig. 3, the cutting tip has a structure where a plurality of abrasives 130 are non-uniformly dispersed in the metallic bonding material 120. This cutting tip is bonded to the shank 110 through a weldment 115 formed by laser welding, silver brazing, or resistance welding. Here, the cutting tip is provided with a blank 125 formed at a bonding area with the shank 110, as shown in Fig. 3. The blank 125 has no abrasive 130 so that subsequent laser welding with the shank 110 can be easily performed. [6] As another method of manufacturing a diamond tool, there is a method of simul¬ taneously compacting and sintering together with a shank, which is one type of sintering method. In this method, a shank is positioned at the center of a mold, and a mixture powder of metallic bonding material and diamond particles is filled in the mold. Then, the metallic bonding material and the abrasives are pressurized and sintered, along with the shank, thereby fabricating a diamond tool. This method is used most in manufacturing general cutter products. Hereinafter, therefore, a sintering method includes the method where a cutting tip is separately formed and bonded to a shank through a laser welding, a silver brazing, and a resistance welding, and the si¬ multaneous pressurizing and sintering method. [7] On the other hand, it should be noted that the size and shape of diamond particles in Figs. 2 and 3 (including other drawings) are exaggerated, relatively to those of a shank or the like, for the purpose of clear illustration therefor. Also, the number of il¬ lustrated diamond particles may be more or less than the actual number thereof. [8] Fig. 4 is a front view of a saw blade where abrasives 130 are bonded to a shank 110 through a brazing method or an electroplating method. Fig. 5 is a sectional view taken along line III-III in Fig. 4. As described above, in the brazing or electroplating method, the abrasives are directly attached to the shank 110, and thus the abrasives 130 is bonded to the surface of the shank 110 in a mono-layer, as shown in Fig. 5. Dissimilar to the above method where abrasives are attached directly to the shank 110, only the tip portion of a diamond tool may be formed separately using a brazing or elec¬ troplating method so as to have the same shape as the sintered tip, and then bonded to a shank through a laser welding, a silver brazing, a resistance welding, or the like, thereby enabling to obtain a diamond tool. (Hereafter, such a tip portion of a diamond tool including the sintered tip are referred to as a 'cutting tip') [9] Among the previously mentioned methods, in case where a typical saw blade man¬ ufactured by a sintering method is used for cutting a workpiece, its cutting tip is abraded as illustrated in Fig. 6. Fig. 6(a) is a partial cross-section of the sintered tip or cutting tip 140 (comprising a metallic bonding material and abrasives) and the shank with the tip 140 attached thereto in a saw blade shown in Fig. 3, which is unused after fabricated. When this saw blade is used for cutting a workpiece, initially the cutting face of the saw blade, i.e., the leading end portion 140a of the cutting tip 140 is in contact with the workpiece to thereby participate in the cutting work, and thereafter the lateral face 140b too is involved in the working job, along with the leading end portion 140a. In the above conventional saw blade, the leading end portion 140a of the cutting tip 140 is flat and thus its initial cutting ability is lowered due to an increased load in the initial cutting process. As this saw blade is used over time, the cutting tip 140 is worn gradually. At this time, the height and width of the cutting tip 140 are reduced and also the leading end portion 140a thereof becomes rounded, as shown in FIG. 6(b). This rounded shape of the leading end portion 140a results from the fact that the cutting work is performed most actively at the area where the leading edge and the lateral face of the cutting tip are met with each other. In this way, during the cutting process, the leading end portion 140a is worn to a rounded shape, and thus the friction area thereof is increased to thereby deteriorate the cutting ability thereof. In particular, as the saw blade is used repeatedly, the abrasives in the upper layer become released and the abrasives in the under layer does not become exposed, thereby further degrading the cutting performance thereof. In addition, this conventional saw blade fails to provide a good cutting performance partly because an appropriate passageway for the cutting chips and cooling water is not available when cutting a workpiece. Similarly, in case of diamond tools manufactured through a brazing or electroplating method, mostly both side edges of the leading end portion are abraded during a cutting work and consequently the leading end portion thereof comes to have a rounded shape. [10] Fig. 7(a) is a cross-section of a saw blade modified in order to improve the initial cutting ability where the leading end portion of the cutting tip 140 is protruded so as to have a sharp tip 140c at the central portion thereof. In this case, similarly to the saw blade of Fig. 6, as it is used repeatedly over time, the cutting tip 140 is abraded such that the height and the width of the cutting tip 140 is reduced and simultaneously the sharp tip 140c is changed into a rounded shape, as shown in Fig. 7(b). That is, although it may improve the initial cutting ability, the cutting chips and the cooling water cannot be smoothly discharged and the sharp tip 140c is abraded into a round shape to thereby increase the friction area, thus deteriorating the cutting ability thereof. Furthermore, in case where this shape of cutting tip 140 is fabricated through a sintering method, when press-compacting a preform, a boundary area 14Od between the leading end portion 140a and the lateral face 140b comes to have a different compaction density and strength. Therefore, when the compacted preform is sintered, cracks are created in this boundary area due to a different shrinkage rate from the surrounding portions, thus consequently leading to a defective product. Disclosure of Invention Technical Problem [11] The present invention is conceived to solve the aforementioned problems in the prior art. An object of the invention is to provide a cutting tip and a diamond tool using the same, wherein a good initial cutting ability can be maintained while using the tool for an extended period of time. [12] Another object of the invention is to provide a cutting tip and a diamond tool having the same, wherein a difficult front-face dressing process can be omitted, t hereby reducing the manufacturing cost thereof and discharging the cutting chips and the cooling water in a smooth manner. [13] A further object of the invention is to provide a cutting tip and a diamond tool, wherein creation of a crack can be suppressed during the manufacturing of the cutting tip through a sintering method, thereby enhancing the quality thereof. Technical Solution [14] According to an aspect of the present invention for achieving the above objects, there is provided a cutting tip being attached to a shank of a diamond tool, wherein a concave groove is formed along a cutting direction at a leading end thereof that initially comes into contact with a workpiece when cutting the workpiece. [15] The cutting tip may be manufactured through a sintering method. The cutting tip includes a base material and a plurality of abrasives attached to at least part of the surface of the base metal excepting an area to be bonded to the shank. In this case, a plurality of depressed portions are formed in the surface of the base material and a plurality of abrasives are attached to the inner space of the depressed portion. More preferably, the depressed portion may include a dimple type depressed portion, a groove type depressed portion, or a through-hole type depressed portion. The depressed portion may be formed under a projection at both sides of the concave groove. A plurality of second abrasives may be attached to the surface of the shank and the upper side of the depressed potion, to which the abrasives and bonding material are attached. The abrasive may include a synthetic or natural diamond, cubic boron nitride, silicone carbide, alumina, or a mixture of two or more thereof. The abrasives are attached to the shank through a bonding material using a brazing or electroplating method. The projection at both sides of the concave groove may be partially removed along the cutting direction so as to have the form of a gear. The cross-section per¬ pendicular to the cutting direction of the concave groove may have a V-shape, a U- shape, a W-shape, or a rectangular shape. [16] According to another aspect of the invention, there is provided a diamond tool comprising a shank and one or more cutting tips having the above-described features, wherein the cutting top is bonded to the shank. Alternatively, a diamond tool of the invention may have a concave groove formed along a cutting direction at a leading end thereof that initially comes into contact with a workpiece when cutting the workpiece. A projection at both sides of the concave groove may be partially removed along the cutting direction so as to have the form of a gear. The cross-section perpendicular to the cutting direction of the concave groove may have a V-shape, a U-shape, a W-shap e, or a rectangular shape. The diamond tool may include a saw, a core drill, a cutter, a saw blade, and a wire saw. [17] As described above, the diamond tool of the invention has a concave groove formed along the cutting direction at the leading end portion of the cutting tip. Thus, a good initial cutting ability can be maintained while using the tool for an extended period of time, and a dressing work therefor can be omitted, along with a smooth discharge of the cutting chips and cooling water. Here, the 'cutting direction' means a rotating direction of the tool when it rotates for a cutting work, and in case where the tool performs a linear (reciprocating) motion to cut a workpiece, it means the direction of linear motion. Description of Drawings [18] Fig. 1 is a sectional view of abrasives bonded to a shank respectively through a sintering method (Fig. 1 (a)), an electroplating method (Fig. 1 (b)) and a brazing method (Fig. 1 (c). [19] Figs. 2 and 3 are a front view and a sectional view of a saw blade where abrasives are bonded to a shank through a sintering method. [20] Figs. 4 and 5 are a front view and a sectional view of a saw blade where abrasives are bonded to a shank through a brazing or sintering method. [21] Fig. 6 is a sectional view of a conventional saw blade formed by a sintering method, showing a worn state of the cutting tip thereof. [22] Fig. 7 is a sectional view of another conventional saw blade formed by a sintering method, showing a worn state of the cutting tip thereof. [23] Fig. 8 is a perspective view, a front view and a sectional view of a saw blade according to the invention. [24] Fig. 9 shows a process of forming the cutting tip of a saw blade according to the invention. [25] Fig. 10 shows a worn state of the cutting tip of a saw blade according to the invention. [26] Fig. 11 shows a process of manufacturing a saw blade through a brazing method according to the invention. [27] Fig. 12 shows a process of manufacturing a saw blade through an electroplating method according to the invention. [28] Fig. 13 shows a process of manufacturing a saw blade having multiple abrasive layers through a brazing method according to the invention. [29] Fig. 14 shows a process of manufacturing a saw blade having multiple abrasive layers through an electroplating method according to the invention. [30] Figs. 15 and 16 are perspective views showing examples of a depressed portion formed in the surface of a shank. [31] Figs. 17 to 19 are a perspective view, a font view and a sectional view showing other examples of a depressed portion formed in the surface of a shank. [32] Figs. 20 to 22 are a perspective view, a font view and a sectional view showing other examples of a depressed portion formed in the surface of a shank. [33] Figs. 23 to 25 are a perspective view, a font view and a sectional view showing other examples of a depressed portion formed in the surface of a shank. [34] Figs. 26 and 27 are sectional views showing other examples of a depressed portion formed in the surface of a shank. [35] Fig. 28 is perspective and front views showing a modified saw blade according to the invention. [36] Fig. 29 is a sectional view showing another example of a depressed portion formed at the leading end of a saw blade according to the invention. [37] Fig. 30 shows a cord drill to which the invention is applied. [38] Fig. 31 shows a wire saw bead to which the invention is applied. Best Mode [39] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. As one example of a diamond tool or its cutting top according to the invention, a saw blade will be illustrated. [40] Fig. 8 is a perspective view (Fig. 8(a)), a front view (Fig. 8(b)) and a sectional view (Fig. 8(c)) illustrating a saw blade as an example of the diamond tool according to the invention, where the saw blade is denoted at 200. Fig. 8(c) is a sectional view taken along line VI-VI in Fig. 8(b). As shown in Fig. 8, the saw blade 200 is provided with a concave groove 240a formed at the leading end of a cutting tip 240 along the circum¬ ferential direction so as to form a sharp projection 240b at both sides (right and left in the Fig. 8(c)) of the concave groove 240a. The saw blade 200 of Fig. 8 is manufactured through a sintering method, i.e., the cutting tip 240 or sintered tip is formed by mixing and press-compacting a metallic bonding material and abrasives and then sintering the compacted mixture, and the sintered cutting tip 240 is bonded to a shank 210 through a weldment 215 formed by a laser welding, a silver brazing, a resistance welding, or the like. [41] The process for manufacturing the cutting tip 240 will be explained in greater detail. First, metal powder to be used as a bonding material is uniformly mixed ap- propriately according to the characteristics of the bonding material, and then coated with a resin material such as wax to thereby be made into a bulk form having a desired size, such as abrasives. Thereafter, a de-waxing treatment is carried out to remove the wax. Next, diamond particles are mixed with the above pre-treated metal powder to form a mixture of diamond particles and the metal powder. The mixture is press- formed with a desired pressure to thereby provide a pre-form having a certain desired shape. This press-forming, as shown in Fig. 9, needs an upper punch 310 and a lower punch 320 corresponding to the shapes of the upper and lower faces respectively of the cutting tip, and a mold 300 having an open top and bottom for the upper and lower punch 310 and 320 to be inserted thereto and having an inner wall face corresponding to the lateral shape of the cutting tip. After inserting the lower punch 320 into the open bottom of the mold 300, the pre-treated mixture of metal powder and diamond particles is injected into the inside of the mold 300. Thereafter, the upper punch 310 is inserted into the open top of the mold 300 and a pressure is exerted in the direction indicated by the arrow in Fig. 9 to thereby provide a preform for a cutting tip according to the invention. The cutting tip preform is sintered (heat-treated) so as to have desired mechanical and physical properties and the sintered cutting tip is bonded to a shank using a silver brazing, a laser welding, a resistance welding, or the like to thereby obtain a diamond tool. As illustrated, if the preform is formed so as to have sharp tip projections at both sides of the concave groove, generation of cracks is suppressed during the sintering, due to flowability of powder inside the mold. [42] Fig. 10 shows a worn state of the leading end portion in the cutting tip 240 of the saw blade 200. That is, Fig. 10(a) shows the cutting top 240 and part of the shank 210 bonded thereto of a saw blade of Fig. 8(a), which is unused after manufactured. When this saw blade is used for cutting a workpiece for a relatively short period of time, the sharp tip projections 240 formed at both sides of the concave groove 240a, which is formed at the leading end of the cutting top 120 along the cutting direction (i.e., cir¬ cumferential direction), is abraded at its tip. As the tool is used repeatedly, the cutting tip 240 is abraded as shown in Fig. 10(c). That is, the height and width of the cutting tip 240 becomes decreased, but the concavity of the concave groove 240a is maintained. It is because the abrasives near the concave groove 240a does not participate in the cutting and grinding work much, but besides the cutting job, the concave groove 240a serves as a discharge passageway for the cutting chips of a workpiece and the cooling water and thus keeps being abraded while maintaining the shape thereof, due to continuous friction with the cutting chips . In addition, in case where the cutting tip 240 is manufactured through a sintering method as in this embodiment, when the cutting tip is press-formed as shown in Fig. 9, the concentration of the preform becomes low at the central portion thereof and becomes high at the peripheral area thereof. Thus, the abrasives near the concave groove 240a do not make a great contribution to the cutting work, as compared with the sharp projections 240 at both sides of the concave groove 240a. However, the abrasives near the concave groove 240a are bonded by a bonding material less strongly than those in the projections 240b, i.e., it means that the abrasives near the concave groove 240a can be easily released therefrom. Accordingly, the sharp projections 240b at both sides of the concave groove 240a greatly contributes to a cutting job, and the concave groove 240a maintains its concavity, due to continuous friction with the cutting chips and the easy release of abrasives. Consequently, even in case where the saw blade 200 is used for a long period of time, the concave groove 240a is remained at the leading end of the cutting tip 240 so that, while cutting a workpiece, the initial cutting ability remains by the projections 240a at both sides of the concave groove 240a. Furthermore, the concave groove 240a serves as a discharge passageway for the cutting chips and the cooling water, thereby improving the cutting and grinding performance thereof. [43] In the above-described embodiment, a cutting tip is manufactured through a sintering method and bonded to a shank using a silver brazing, a laser welding, a resistance welding, or the like, to thereby obtain a diamond tool. However, the present invention is applied to the manufacturing of a diamond tool using an electroplating method or a brazing method. [44] Fig. 11 shows a process of manufacturing a diamond tool (saw blade) through a brazing method according to the invention. First, as shown in Fig. 1 l(a), a shank 410 having a concave groove 410a is provided. The concave groove 410a is formed at a position corresponding to the cutting face of a saw blade, i.e., the leading end portion of the cutting tip. A bonding material of paste form containing a brazing metal powder is coated on the shank 410 (Fig. 1 l(b)) and then plural abrasives 430 are dispersed in the coated paste (Fig. 1 l(c)). Here, the bonding material 420 of paste form used to bond the abrasives 430 to the shank 410 contains commonly a metal powder and a bond to provide a flowability to the metal powder, or the like. For the purpose of convenient description of the invention, hereinafter, the bonding material of paste form is referred to as a 'bonding material,' 'bonding paste' or 'paste.' In addition, a drying process may be added between the coating of bonding material and the dispersion of abrasive. The bonding paste 420 with plural abrasives 430 dispersed therein is dried at a certain desired temperature (Fig. 1 l(d)), and thereafter, is held in a vacuum furnace or a reduction/inert gas atmosphere furnace at a certain elevated temperature, where the metal power in the bonding material can flow in a liquid phase and react chemically, such that the brazing metallic bonding material is melted and solidified in the shank 410 and the abrasives 430 (Fig. 11 (e)). At this time, the holding temperature depends on the type of the commercialized pastes, for example, about 600~1300°C. The vacuum furnace is mostly a batch type furnace so that a good productivity cannot be expected, but in the reduction/inert gas atmospheric furnace, a continuous brazing process using a conveyor can be utilized to thereby enhance the production efficiency significantly. [45] In addition, Fig. 12 shows a process of manufacturing a diamond tool through an electroplating method according to the invention. First, as shown in Fig. 12(a), a shank 410 having a concave groove 410a is provided. Then, as shown in Fig. 12(b), a non- conductive film 415 is coated on those portions in the shank 410 that abrasives are not be bonded thereto, in order to prevent from being electroplated. Abrasives 430 are uniformly disposed on the surface of the shank 410 that the non-conductive film 415 is not coated thereon (Fig. 12(c)) and the resultant product is immersed in an elec¬ troplating bath to carry out a wet electroplating. At this time, the shank coated with the non-conductive film 415 and the non-conductive diamond particles are not elec¬ troplated and only those portions in the shank that the abrasives are disposed thereon are electroplated so that the abrasives 430 is bonded to the shank through a bonding material 420 formed by the electroplating (Fig. 12(d)). Upon completion of elec¬ troplating, it is preferred that the non-conductive film 415 be removed for subsequent processes such as coloring, etc. (Fig. 12(e)). [46] As described above, in case where the invention is applied to a diamond tool man¬ ufactured through a brazing or electroplating method, during the cutting process, the sharp tip projections at both sides of the concave groove improves the initial cutting ability and the concave groove serves as a discharge passageway for the cutting chips and the cooling water to thereby enhance the cutting ability thereof. In addition, the concave groove does not contribute to the cutting job as much as the sharp tip projections at both sides thereof. However, while the cutting chips are discharged through the concave groove, the concave groove is abraded continuously due to friction with the cutting chips . Thus, the abrasives at the front face can be con¬ tinuously exposed effectively and the leading end portion can be uniformly worn, as compared with the conventional diamond tools, in which the front face and both side faces of the leading end portion are mostly abraded, as shown in Fig. 6(b). [47] On the other hand, while the diamond tool of the invention is used for an extended period of time, it is abraded while retaining the shape of the concave groove, to thereby maintain its initial cutting ability . This can be considered to be the same case as in where a diamond tool has multiple abrasive layers formed by a sintering method. This is, during the cutting and grinding process, right after the abrasives of top layer are released or fallen apart, the very next abrasive layer comes to participate in the cutting work. As described above, therefore, in case where the present invention is applied to a diamond tool having a single abrasive layer formed through a common brazing or elec- troplating method, the sharp tip projections at both sides the concave groove enhance the initial cutting ability thereof and the concave groove serves as a discharge passageway for the cutting chips and the cooling water to thereby improve the cutting performance thereof. However, if the abrasives are fallen apart to some extent, the diamond tool cannot be used anymore. Thus, even though the life span of the tool is prolonged, the tool is worn while generally maintaining the shape of the concave groove so that it is meaningless to continuously keep a superior initial cutting ability. Therefore, in case where the present invention is applied to diamond tools man¬ ufactured by a brazing or electroplating method, if the abrasive layer is formed in a multi-layered form, the tools can maintain its initial cutting ability , as described above, while extending the service lifer thereof. [48] In order to form a multi-layered abrasive using the brazing or electroplating method, various methods have been proposed. Hereafter, a method of forming multiple layers by providing a depressed portion to the shank will be explained. It should be noted that the 'concave groove' used above is distinguished from a 'depressed portion', which will be hereinafter introduced. [49] Fig. 13 shows a process of forming multiple abrasive layers through a brazing method. In Fig. 13, a cutting tip having a depressed portion formed thereon is partially illustrated. As shown in Fig. 13(a), first, a plurality of depressed portion 520 are formed a shank 510. The depth d, the width w and the spacing s of the depressed portion are determined based on the size of abrasives. That is, considering the maximum diameter a of the abrasive, the depth d and the width w of the depressed portion 520 are predetermined, and these depressed portions are formed so as to be spaced apart from one another with a certain desired spacing s provided between neighboring depressed portions. Between neighboring depressed portions are formed a partition wall 521. In Fig. 13 (a), the cross-section of the depressed portion 520 is il¬ lustrated as a rectangular shape, but not limited thereto. Other shapes of the depressed portion 520 will be hereinafter explained. In order to form a lower abrasive layer, as shown in Fig. 13 (b), a bonding material 530a of paste form containing a brazing metal powder and a binder is coated inside the depressed portions 520 of the shank in Fig. 13 (a). Then, a plurality of abrasives 540a are filled in the bonding paste coated in the depressed portion 520 and thereafter the bonding material 530a with the abrasives contained therein are primarily dried at a desired temperature (Fig. 13 (c)). The bonding paste 530a may be pre-dried before the abrasives 540a is dispersed in the coated paste. Next, in order to form an upper abrasive layer, a bonding paste 530b is coated on the top of the dried mixture of the bonding paste 530a and the abrasives 540a and on the surface of the shank 510 (i.e., the top surface of the wall 521) (Fig. 13 (d)). Then, a plurality of abrasives 540b are dispersed in the above-coated bonding paste 53Ob and the mixture of the bonding paste 530b and the abrasives 540b is secondarily dried at a desired temperature (Fig. 13 (e)). Upon completion of drying, the resultant product is heat-treated in a vacuum or reduction/inert gas atmosphere brazing furnace at a certain desired temperature, such that the brazing metal powder can be melted and adhered to the abrasives 540a and 540b and the shank 510, as shown in Fig. 13 (e). [50] On the other hand, as described above, the bonding paste 530a or 530b is first coated in the depressed portions and then the abrasives 540a or 540b is dispersed in the coated bonding paste 530a or 530b. Alternatively, the bonding paste 530a and the abrasives 540a may be mixed first and then the mixture may be coated in the depressed portion 520. Similarly, the bonding paste 530b and the abrasive 540b may be mixed first to form a mixture, which may be then coated above the previous coated mixture and the top surface of the wall 521. In addition, the bonding material 530a filled in the depressed portion 520 and the bonding material 530b coated on the surface of the shank 510 may be the same or different from each other. The upper abrasive layer comprises the bonding material 530b and the abrasive 540b, but the lower abrasive layer comprises the bonding material 530a, the abrasive 540a and the wall 521 (part of the shank 510) between the depressed portions. Therefore, the composition of the bonding paste 530a and the bonding paste 530b may be made to become different from each other, and also the metal powder contained in each bonding paste 530a, 530b may be made different. In this way, the lower abrasive layer, which is to be exposed after the upper abrasive layer is released or fallen apart, can have a uniform and consistent cutting or grinding characteristic as in the upper abrasive layer. [51] Here, the metal powders contained in the bonding materials of the upper and lower abrasive layers are melted at the same time. However, since the metal powder of the bonding material 530a is filled in the depressed portion 520, the melted metal powder is held in place, along with the abrasives 540a, due to the surface tension thereof, and thus cannot easily flow with the melted metal powder of the bonding material of the upper abrasive layer. Accordingly, the abrasives dispersed in the upper and lower abrasive layers are retained in their places without being scattered or disturbed, thereby enabling to avoid a deviation with the thickness thereof. Consequently, the abrasives 540a filled in the plural depressed portions 520, which are formed in the surface of the shank 510, constitutes a lower abrasive layer, and the abrasives 540b disposed on the above abrasive 540a and the surface of the shank 510 constitutes an upper abrasive layer, i.e., multiple abrasive layers are formed on the shank. [52] With the diamond tool having the above construction, when in use, the upper abrasives 540b of the upper abrasive layer are fallen apart therefrom, the lower abrasive layer is subsequently exposed and the lower abrasives 540a contained therein comes to participate in the grinding and cutting work, thereby extending the service life of the tool. That is, although the lower abrasives 540a of the lower abrasive layer is retained inside the depressed portion 520, the wall 521 between the depressed portions 520 (in Fig. 13(a)) is easily abraded during a cutting or grinding process such that the abrasives 540a inside the depressed portion 520 come to protrude above the surface of the tool and participate in the cutting or grinding work. Therefore, the width w, the depth d, and the spacing s of the depressed portions 520 can be optimized, preferably, such that the wall 521 can be appropriately abraded. [53] On the other hand, considering the abrasive size a, the minimum width and depth of the depressed portion are preferred to be designed so as to be larger than the abrasive size, so that part of the abrasives can be held, in its entirety, inside the depressed portion. At this time, the spacing s between the neighboring depressed portions is designed preferably such that the upper and lower abrasive layers can have a same abrasive concentration and thus exhibit a uniform and consistent cutting or grinding speed or performance. [54] The diamond tool having multiple abrasive layers of the invention may be man¬ ufactured through an electroplating method, instead of the above brazing method. Fig. 14 illustrates a process for manufacturing a diamond tool having multiple abrasive layers using an electroplating method according to the invention. Similar to the brazing method, as shown in Fig. 14(a), first, a plurality of depressed portions 520 are formed in a shank 510. As shown in Fig. 14 (b), the top surface of the wall 521 is coated with a non-conductive film 526 in order to prevent from being electroplated. Abrasives 540a are filled inside the depressed portion 520 (Fig. 14 (c)) and then a wet electroplating process is performed in an electroplating bath. Thus, the electroplating is not processed on the non-conductive abrasives, but processed from the shank and gradually forms a bonding material 530a simultaneously while embedding the abrasives into the bonding material 530a being formed by the electroplating (Fig. 14 (d)). When the bonding material 530a is formed adequately inside the depressed portion 520, the shank 510 is removed from the electroplating bath, thereby completing a primary electroplating to form a lower abrasive layer. Thereafter, the non-conductive film 526 coated on top of the wall 521 is removed (Fig. 14 (e)), and then abrasives 540b are uniformly disposed on the lower abrasive layer and on the top of the wall 521 (Fig. 14 (f)). In this case, the depressed portion 520 and the wall 521 both become conductive and thus a secondary electroplating can be performed thereon to thereby form an upper abrasive layer (Fig. 14 (g)). In this way, a diamond tool having two or more abrasive layers can be man¬ ufactured. In the brazing method, preferably a bonding paste is filled in the depressed portion 520 first and then the abrasives are dispersed. In contrast, in the electroplating method, the abrasives 540a are filled inside the depressed portion 520 first and then a bonding material is electroplated inside the depressed portion 520, and at the same time the abrasives 540a are fixed into the bonding material and the shank. [55] On the other hand, the upper and lower abrasive layers may be formed through a combination of the brazing method and the electroplating method. That is, the upper and lower abrasive layers may be formed using either one of the brazing method and the electroplating method. Alternatively, the lower abrasive layer may be formed by the brazing method and the upper abrasive layer may be formed through the elec¬ troplating method, and vice versa. In a case where the lower and upper abrasive layers are formed respectively through a brazing method and an electroplating method, right after the step of Fig. 13 (c), the shank is to be heat-treated to bond the abrasives to the shank before performing next steps for electroplating. [56] The diamond tools manufactured through the above brazing and electroplating methods according to the invention can be modified in various ways. For example, in case of a brazing method, right after the step of Fig. 13 (c), the shank can be heat- treated to perform a fusion-bonding (brazing), thereby providing a diamond tool to be used without an upper abrasive layer. Similarly, in case of an electroplating method, after finishing the step of Fig. 14 (d) or Fig. 14 (e), the resultant product without an upper abrasive layer, in which the abrasive 540a and the bonding material 530a are provided in the depressed portion 520, can be used as a diamond tool. In another mod¬ ification, if the width w of and spacing s between the depressed portions 520 are not changed and the depth d of the depressed portion is made to be almost the same as the abrasive size a, two-layered abrasive can be formed, thereby providing a same cutting and grinding characteristic to the upper and lower abrasive layers when in use. In addition, dissimilar to Figs. 13 and 14, the size of the depressed portion 520 can made to be smaller than that of abrasives such that the abrasive is partially inserted into the depressed portion 520 and thus only the inserted portion of the abrasive is bonded to the shank through a brazing or an electroplating. In a case where an upper abrasive layer is formed on top of this lower abrasive layer, right after the abrasives of the upper layer is released therefrom, the abrasives of the lower layer can be exposed to thereby achieve a continuity of cutting or grinding work between the upper and lower abrasive layers. [57] As described above, in order for the abrasives to be continually exposed and protruded, the ratio d/a of the depth of depressed portion to the abrasive size is to be at least 1/4, and also the ratio w/a of the width of depressed portion to the abrasive size is preferred to be at least 1/4. In addition, in order to obtain uniform cutting and grinding characteristics, the ratio s/w of the spacing to the width of the depressed portion is preferred to be within a range of 0.2 to 0.8, in case where the abrasive concentration is substantially the same in the upper and lower layers. [58] In Figs. 13 and 14, the shape of the depressed portion 520 is illustrated to have a rectangular cross-section, but not limited thereto. As illustrated in Figs. 15 and 16, the depressed portion may have a semi-elliptic cross section 520a or a V-shaped cross section 520b. Besides, the depressed portion may have a semi-circular cross-section, a U-shaped cross-section, a wavy cross-section, or the like. On the other hand, in Figs. 13 and 14, the upper end portion of the wall 521 is illustrated to have a right angle edge, but it may be formed so as to have a round shape 52 IR, as shown in Figs. 15 and 16, thereby facilitating the flowability of paste and improving the adhesiveness of abrasives near the edge. This round shape 52 IR can be applied to the rectangular cross-section shown in Figs. 13 and 14. [59] In the above, the present invention has been described, referring to the cross- section of the depressed portion 520. Hereafter, the configuration of the whole depressed portions formed in the shank will be explained. In the description, the term 'depressed portion' includes all the shapes, which are sunken under the surface of the shank. For example, the shape of the depressed portion includes a dimple type such as a semi-sphere, a semi-ellipsoid, an inverse cone, a rectangular block, a cylinder or the like. In addition, it may includes an elongated groove type having a cross-section such as a semi-circle, a semi-oval, a U-shape, a V-shape, or a rectangular shape, and furthermore a through-hole type passing through the opposing sides of a shank and having various shape of cross-sections. Also, this depressed portion includes a space between projected portions, which may be formed on the surface of the shank through a coating process, an electroplating process, a bonding process or the like. Figs. 17, 20 and 23 are perspective views showing the tip member of a saw blade, where in the surface of the tip member is formed respectively a dimple type depressed portion 520c, an elongated groove type depressed portion 52Od, and a through-hole type depressed portion 52Oe. Figs. 18, 21 and 24 are front views of the tip members shown in Figs. 17, 20 and 23, respectively. Figs. 19, 22 and 25 are sectional views taken along line M-M, N-N, and 0-0 in Figs. 18, 21 and 24, respectively. [60] On the other hand, in Figs. 17 to 25, the size of the depressed portions 520c to 52Oe is exaggerated, relative to the tip member of the saw blade, for the purpose of clear il¬ lustration of the shape thereof. The number of the depressed portions is illustrated more or less than the actual number thereof. The shape, the size and the number of depressed portions are to be designed appropriately, depending on the strength and ductility of a workpiece, etc. [61] The above-described various shapes of the depressed portions may be, needless to say, combined with each other in various ways. In particular, the through-hole shown in Fig. 15 is formed in the thickness direction of the cutting tip, but in case of a thicker cutting tip, may be formed perpendicular to the thickness direction thereof, as il¬ lustrated in Figs. 26 and 27. [62] In case of the above diamond tools having multiple abrasive layers, as previously described, a concave groove may be formed at the leading end of the cutting tip along the circumferential direction (i.e., along the cutting direction) such that the shape of the concave groove can be maintained during the continuous cutting work. That is, as shown in Figs. 20 to 22, 26 and 27, a shank having a concave groove formed therein may be used for manufacturing a diamond tool having multiple abrasive layers. [63] If a depressed portion 52Od, 52Of, or 52Og is formed under the sharp tip projections at both sides of the concave groove 510d, 510f, or 510g as shown in Figs. 20 to 22, 26 and 27, mostly the abrasives disposed inside the depressed portion 52Od, 52Of, or 52Og participate in the cutting work to thereby improve the initial cutting ability thereof. The shank under the concave groove 510d, 510f, or 510g is abraded during the cutting of a workpiece and the concave groove 510d, 510f, or 510g serves as a discharge passageway for the cutting chips , thereby more easily wearing the concave grooves. Therefore, the diamond tools, which are manufactured by bonding abrasives to the shank having a concave groove of Figs. 20 to 22, 26 and 27 using a brazing or elec¬ troplating method, is abraded, but maintains the shape of the concave groove 510d, 510f, or 510g. In this way, the invention may be applied to a diamond tool having multiple abrasive layers through a brazing or electroplating method, in which a similar effect can be achieved, as in the case where the invention is applied to a diamond tool manufactured by a sintering method. [64] In the brazing and electroplating methods as described above, abrasives are directly bonded to a shank to thereby fabricate a diamond tool. As mentioned in the background art section, a cutting tip having the same shape as the sintered one may be manufactured separately through a brazing or electroplating method and then boned to a shank using a laser welding, a silver brazing, a resistance welding, or the like, thereby manufacturing a diamond tool. In this case, when abrasives are attached to a base material through a brazing or electroplating, it is preferable that the abrasives are not attached to a portion to be boned to a shank. [65] According to the invention, the leading end portion of a cutting tip may be modified in a way as illustrated in Fig. 28, thereby further improve the initial cutting ability and facilitating the discharge of cutting chips and cooling water. That is, the sharp tip projection 610b at both sides of the concave groove 610a is partially removed so as to have the shape of a gear as shown in the front view of Fig. 28(b). This cutting tip provides a good initial cutting ability due to a smaller initial contact area when cutting a workpiece. In addition, the cutting chips and the cooling water can be more smoothly discharged through the removed portions of the projection 610a, thereby further improving the cutting ability thereof. [66] In the previous embodiment, the concave groove formed in a cutting tip has a V- shape, but not limited thereto. The concave groove may have various other shapes, for example, a W-shape or a U-shape as illustrated in Fig. 29. [67] Although the present invention has been explained, illustrating a saw blade, it may be applied to other tools such as a saw, a core drill, a cutter, a wire saw, or the like, as previously mentioned. By way of an example, Figs. 30 and 31 show a core drill 700 and a wire saw bead 800 to which the invention is applied. In particular, the wire saw bead 800 of Fig. 31 is used to cut a workpiece in such a way that a plurality of such beads are inserted into a wire (not shown) through a through-hole 815 and performs a linear motion along the longitudinal direction of the wire. Thus, the wire saw bead 800 of a cylindrical shape is provided with plural concave grooves formed along the lon¬ gitudinal direction thereof (horizontal direction in the figure) and at regular intervals along the circumferential direction of the cylindrical bead. At this time, in case where a wire saw formed of plural beads inserted into a wire is operated (a linear motion) in a tilted state, the concave groove may be formed inclined relative to the length of the bead so as to compensate for the tilting of the bead, dissimilar to the concave groove 810a shown in Fig. 31. That is, the concave groove in the cylindrical bead can be formed in a helical form. The wire saw bead 800 depicted in Fig. 31 comprises of a cutting tip 810, i.e., a portion where abrasives are attached, and a shank, i.e., a portion where a wire is inserted. In this embodiment, according to the invention, a concave groove is formed along the cutting direction of a tool at the leading end portion of the cutting tip, i.e., at the front face of a diamond tool, although the shapes of the shank and the cutting tip are different from the previous embodiments. Industrial Applicability [68] As described above, a diamond tool manufactured according to the present invention can maintain a good initial cutting ability for an extended period of time. Thus, during use of the tool, a difficult front-face dressing process can be omitted, thereby reducing a manufacturing cost. In addition, a discharge passageway for the cutting chips and cooling water is spontaneously formed to thereby enhance the cutting performance thereof. Furthermore, the manufacturing of the cutting tip of the invention using a sintering method can suppress creation of a crack, which otherwise is likely to lead to a defective product.