Drill bit and method of manufacturing same

Abstract: A drill bit (10) and a manufacturing method thereof that ensures integral mounting of a drill (14) and a drill holder (12) with a simplified construction and in a simplified manner. The drill bit (10) has a drill (14) with a bit (14b) and a shank (14c), and a drill holder (12) formed with an axial insertion hole (11) for fitting the shank (14c). An engaging part (14e) is formed at a base end of the drill (14). A blind hole (13) that intersects the insertion hole (11) is formed in the drill holder (12) to form a restricting part (10e), or a stopper (40) equivalent to the restricting part (10e) is fitted in the bottom of the blind hole (13). Engagement of the top of the stopper (40) or the restricting part (10e) with the engaging part (14e) prevents relative rotation between the drill (14) and the drill holder (12).

Description: This invention relates to a drill bit comprising a drill and a drill holder for holding the drill, a drill tool provided with the drill bit, and a manufacturing method of the drill bit. A drill bit is known, for instance as disclosed in Japanese Examined Patent Publication No. 8-18213/1996, comprising a drill part formed with a spiral bit on a circumference thereof and a holding part with a larger diameter than the drill part. The drill bit is drivingly rotated in a state that the holding part is held by, for instance, an electric drive source. There has been a common practice, when manufacturing the drill bit, to shape the drill part and the holding part as one-piece unit by machining one metallic material with a lathe or its like. However, the above machining process is cumbersome and requires a lot of processing, leading to a remarkable production cost rise. As a means to solve the above problem, several drill bits have been proposed, as shown in FIGs. 16A to 16C. In any of these prior art drill bits, the drill bit comprises a drill corresponding to the drill part and a holder corresponding to the holding part and is constructed such that a shank formed at a base end of the drill is inserted into an insertion hole formed in the holder to joint the drill with the holder together. The specific construction of these drill bits is described one by one with reference to FIGs. 16A to 16C. A drill bit 100 shown in FIG. 16A has a drill 200 including a main part 201 formed at a lead end thereof and a cylindrical shank 202 formed at a base end thereof. The drill main part 201 and the shank 202 are integrally formed. A spiral bit 203 is formed on the drill main part 201, and an external thread 204 is formed at an end of the shank 202. The drill bit 100 has a holder 300 shaped into a cylinder with a larger diameter than the drill 200. A screw hole 301 is formed inside a lead end of the holder 300 concentrically with an axis of rotation of the drill 200. Screwing the external thread 204 of the shank 202 into the screw hole 301 enables mounting the drill 200 onto the holder 300. The drill bit 101 shown in FIG. 16B is constructed by inserting a shank 202 into an insertion hole 311 formed in a holder 310 and adhering the joint part of the shank 202 and the insertion hole 311 with a brazing filler metal 312 including silver and nickel alloy, in place of providing the external thread 204 and the screw hole 301. A drill bit 102 shown in FIG. 16C is constructed such that a flat part 204 is formed on an outer circumference of a shank 202, and a press fit hole (screw hole) 322 is formed extending from an outer circumference of a holder 320 toward a shank insertion hole 321 to communicate with the flat part 204. In this arrangement, inserting the shank 202 into the shank insertion hole 321 so as to set the flat part 204 in the press fit hole 322 and press fitting a metal piece 400 into the press fit hole 322 (or screwing an external thread 400 into the screw hole 322) enables preventing relative rotation of a drill 200 to the holder 320. The drill bit 100 shown in FIG. 16A is constructed by screw joint of the drill 200 with the holder 300. Accordingly, the joint state may be loosened due to vibration during working with a result that the drill 200 unnecessarily vibrates in spite of holding of the holder 300 and in a worst case, the drill 200 may come out from the holder 300. In the case of the drill bit 101 shown in FIG. 16B, if a large torque acts upon the drill bit 101 during working, adhesion of the joint part may be weakened, leading to breakage of the joint part. In such a case, the drill 200 may idly rotate relative to the holder 310 (namely, the torque is not transmitted to the drill 200), making the drill bit 101 unusable. Particularly, filling of the filler 312 between the shank 202 and the insertion hole 311 is insufficient (poor filling state), even application of a small torque may break the joint part. This is not desirable when seeking for reliability of the drill bit. The construction of the drill bit 102 shown in FIG. 16C also does not provide an effective solution to prevent relative rotation of the drill 200 to the holder 320 because there is a possibility that the metal piece (or external thread) 400 may be loosened and come out from the press fit hole (or the screw hole) 322 due to vibration and centrifugal force generated during working. Other fixation means such as close fitting the shank 202 in the insertion hole 311 by shrinkage fit or press fit can be applied. However, such fixation by close fit cannot sufficiently resist a large torque generated when start rotating the drill bit, and occurrence of a problem that the drill rotates relative to the holder cannot be avoided. GB 210513 and FR 2644089 each disclose a drill bit according to the preamble of claim 1. In view of the aforementioned problems residing in the prior art, it is an object of the invention to provide a drill bit, a drill tool provided with the drill bit, and a drill bit manufacturing method that can securely prevent rotation of a drill relative to a drill holder with a simplified construction and in a simplified manner. To solve the above problems, this invention adopts the following arrangements. According to one aspect of the invention there is provided a drill bit comprising a drill including a bit and a shank, and a drill holder formed with an axial insertion hole to receive the shank, the shank being formed with an engaging part, characterised in that axial insertion holes are formed in axially opposite ends of the drill holder each to receive the shank of an associated drill, the drill holder being formed with a blind hole extending from an outer circumference thereof to intersect each insertion hole, the blind hole terminating with an abutment surface which is engaged by the engaging part of each shank to prevent relative rotation between the drills and the drill holder. In this arrangement, fittingly inserting the shanks of the drills into the insertion holes of the drill holder to engage the engaging part of the shank with the abutment surface securely prevents a relative rotation of the drill to the drill holder. Accordingly, simply mounting the thus constructed drill holder onto a certain drive source for rotation secures integral rotation of the drill bit for drilling. In a first embodiment of the invention, a stopper is inserted into the blind hole whereby the top of the stopper defines the abutment surface, the top of the stopper engages the engaging part of the shank in a state that the stopper is inserted in the bottom of the hole. In this arrangement, even if an external force such as vibration is exerted to the drill bit, eliminated is a possibility that the stopper comes off from the hole, and thus secures prevention of a relative rotation of the drill and the drill holder for a long span. In a second embodiment of the invention, the abutment surface is formed integrally with the drill holder to define the bottom of the blind hole, an engagement of the abutment surface (restricting part) and the engaging part restricts a relative rotation of the drill to the drill holder. In this arrangement, fitting the shank of the drill into the insertion hole of the drill holder to engage the engaging part of the shank with the abutment surface formed at the bottom end of the insertion hole securely prevents a relative rotation of the drill to the drill holder. Accordingly, simply mounting the thus constructed drill holder onto a certain drive source for rotation secures integral rotation of the drill bit for drilling. Further, since the engaging part is integrally formed with the drill holder by forming the hole that intersects the bottom end of the insertion hole, the number of parts lessens, and a step of press fitting a stopper pin, as having been performed conventionally, is not required. According to a further aspect of the invention, there is provided a method of manufacturing the drill bit of the first embodiment defined above and comprising the steps of: a) forming an engaging part in each shank; b) forming a blind hole extending from an outer circumference of the drill holder in such a way as to intersect each insertion hole; c) inserting a stopper in the bottom of the hole; and d) fitting the shanks in the insertion holes to engage the engaging parts with a top of the stopper. In this method, performing the step of fitting the shanks into the insertion holes to such a degree as to engage the top of the stopper with the engaging parts after inserting the stopper in the hole securely prevents a relative rotation of the drills to the drill holder. According to a still further aspect of the invention, there is provided a method of manufacturing the drill bit of the second embodiment defined above and comprising the steps of: a) forming a blind hole in the drill holder such that the hole radially extends part way through each insertion hole and intersects a bottom end of each insertion hole to form an abutment surface at the bottom end of the insertion holes; b) forming an engaging part at an end of each shank to engage the abutment surface; and c) fitting the shanks into the insertion holes to such a degree as to engage the engaging parts with the abutment surface to restrict a relative rotation of the drills to the drill holder. In this method, performing the step of fitting the shanks into the insertion holes so as to engage the engaging parts with the abutment surface after forming the abutment surface at the bottom end of the insertion holes by combination of the insertion holes and the blind hole securely prevents a relative rotation of the drills to the drill holder without increasing the number of parts. These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings. Of the drawings: Fig. 1 is a partially cutaway exploded perspective view of a first embodiment of a drill bit according to this invention; Fig. 2 is a partially cutaway perspective view showing an assembled state of the drill bit; Fig. 3 is a perspective view showing the assembled state of the drill bit in upright posture; Fig. 4 is a cross section taken along the line A-A in Fig. 5; Fig. 5 is a cross sectional front view of the drill. bit; Fig. 6 is a cross section taken along the line C-C in Fig. 5; Fig. 7 is a cross sectional front view showing an entire construction of a drill tool provided with the drill bit and a drill bit holder; Figs. 8A and 8B are enlarged views showing essential parts of the drill tool, Fig. 8A being a cross sectional front view showing a state that mounting of the drill bit onto the drill bit holder is locked, Fig. 8B being a cross section taken along the line D-D in Fig. 8A; Figs. 9A and 9B are enlarged views showing essential parts of the drill tool, Fig. 9A being a cross sectional front view showing a state that the locked state of the drill bit onto the drill bit holder is released, Fig. 9B being a cross section taken along the line E-E in Fig. 9A; Figs. 11A and 11B are a plan view and a cross sectional front view respectively showing essential parts of a second embodiment of a drill bit according to this invention; Fig. 11C is a cross section taken along the line F-F in Fig. 11B; Fig. 12 is a perspective view of the second drill bit; Figs. 13A and 13B are respectively a plan view and a cross sectional front view of a drill holder in the second drill bit; Figs. 15A to 15C are cross sectional side views each showing a modification of a transverse cross section of a non-circular part in the aforementioned drill bit; and Figs. 16A to 16C are cross sectional diagrams each illustrating a drill bit of prior art. A first embodiment of this invention is described with reference to Figs. 1 to 9B. A drill bit 10 in this embodiment includes a drill holder 12 and a pair of drills 14 connected to opposite ends of the drill holder 12 respectively. An outer shape of the drill holder 12 is described. The drill holder 12 includes a pair of disk parts 18 formed at axially opposite ends thereof identical in shape with each other and having a diameter larger than the drill 14, and an intermediate part 16 formed between the disk parts 18. The intermediate part 16 has a non-circular cylindrical shape with a substantially square shape in cross section. The non-circular part 16 is formed by slicing off four parts, from a cylinder, over a circumference of the cylinder in such a manner as to leave four flat planes 15. In this embodiment, four corner ends of the non-circular part 16 in cross section has a circular arc shape and are continued to the disk parts 18. In other words, the drill holder 12 has a combined configuration of cylinder and prism such that a circular cylinder is sliced off to leave the plane 15 at plural positions over the circumference of the intermediate part while also leaving the axially opposite ends (disk parts 18) thereof. A cylindrical centering shaft 19 is formed between the drill holder 12 and the corresponding drill 14 with a diameter larger than the drill 14 and smaller than the disk part 18. An internal construction of the drill holder 12 is described. The drill holder 12 is formed with an insertion hole 11 extending along an axis of rotation thereof and passing therethrough, and a hole 13 with a closed bottom (hereinafter also referred to as "a blind hole") which extends from a center of one of the four flat planes 15 radially inward to intersect the insertion hole 11. Note that the term "blind hole" is meant to be a hole with a bottom closed as opposing to a through hole that has an open bottom. A cylindrical stopper 40, for instance, a metal piece is inserted (press fit in this embodiment) into the bottom of the blind hole 13. The outer diameter of the stopper 40 is the same or slightly larger than the inner diameter of the blind hole 13, as shown in FIGs. 4 and 5. The stopper 40 is press fitted by hammering down the stopper 40 with a tool such as hammer and punch. Fixing the stopper 40 on the bottom of the blind hole 13 in this way prevents coming off of the stopper 40 and facilitates fitting operation of the drills 14 which is described below. Insertion of the stopper 40 can be done by other way than press fit. A clearance fit of the stopper 40 such that the stopper 40 is fitted in the blind hole 13 with a clearance may also enable assembling of the drill 14. The diameter of the stopper 40 and the blind hole 13 may be smaller than the insertion hole 11. However, setting the diameter of the stopper 40 and the blind hole 13 larger than the insertion hole 11 (equivalent to the diameter of the insertion hole 11 in FIG. 4) prevents falling off of the stopper 40 from the insertion hole 11 during an assembly process and facilitates the assembling. The height of the stopper 40 (vertical dimension in FIG. 5) is set such that about half of a top thereof protrudes inside the insertion hole 11 when the stopper 40 is press fitted into the bottom of the blind hole 13. The drill 14 is integrally formed with a drill main part 14a formed with a spiral bit 14b and a cylindrical shank 14c. The outer diameter of the shank 14c is set the same or slightly larger than the inner diameter of the insertion hole 11 and the length thereof is set slightly longer than half of the length of the insertion hole 11. A tongue-like engaging part 14e protruding in the axial direction of the drill 14 is formed at a distal end 14d of a base end of the shank 14c. The engaging part 14e is formed by cutting out the base end of the shank 14c radially halfway. Fitting the shank 14c into the insertion hole 11 allows the distal end 14d of the shank 14c to abut against an outer circumference of the stopper 40 while positioning the engaging part 14e on the top of the stopper 40 (namely to cover the top of the stopper 40 from the opposite side of the bottom of the blind hole 13). In this state, the engaging part 14e eccentrically away from the center of axis of the drill 14 engages the top of the stopper 40. Thereby, a relative rotation of the drill 14 to the drill holder 12 about the axis thereof is restricted, and the drill 14 and the drill holder 12 are rotatable together about the axis thereof. Since the stopper 40 engages with both of the engaging parts 14e respectively provided at the drills 14 while pressingly fitted in the blind hole 13 with the closed bottom, the stopper 40 does not come out from the blind hole 13 even if vibration or centrifugal force is transmitted to the stopper 40 during rotation of the drill 14 . The drill bit 10 having the above construction can be manufactured according to, for instance, the following steps: I The engaging part 14e is formed by machining the base end of the shank 14c of the drill 14. After forming the insertion hole 11, the blind hole 13 is formed at a center of any one of the plural planes 15 of the non-circular part 16 such that the blind hole 13 intersects the insertion hole 11. II The stopper 40 is fitted (for instance, press fitted) in the bottom of the blind hole 13. The size of the stopper 40 is set such that the head thereof protrudes inside the insertion hole 11 when inserted. III The shank 14c is inserted into the insertion hole 11 such that the engaging part 14e engages with the stopper 40 to restrict a rotation of the drill 14 relative to the holder 12. In the step III, the shank 14c may be fitted into the insertion hole 11 with a clearance and then fixed thereat with a brazing or its equivalent. Alternatively, the shank 14c may be close fitted into the insertion hole 11. In the latter case, the drill bit can be assembled integrally without a specific fixing means. The close fit may be performed by press fit, or shrinkage fit (heating the internal threaded part to allow it to expand, inserting the external threaded part into the heated internal threaded part, and then allowing the expanded internal threaded part to shrink for close fit) or expand fit (cooling the external threaded part to allow it to shrink, inserting the external threaded part into the internal threaded part, and then allowing the external threaded part to expand for close fit). The shrinkage fit and the expand fit may be performed within an allowed temperature range. The entirety of the shank 14c may not necessarily be subject to close fit into the insertion hole 11. For instance, a small gap may be defined between the shank 14c and the insertion hole 11 to press fit the engaging part 14e in the gap between the head of the stopper 40 and an inner circumference of the insertion hole 11 for fixation. In this case, an assembling operation of the drill bit as a whole is further facilitated. Next, a drill bit holder 20 to which the drill bit 10 is mounted is described with reference to FIGs. 7 to 9B. As shown in these drawings, the drill bit holder 20 is integrally formed with a shank 22 that is connected to a drive source for rotation (not shown) and a drill bit mount 24 to which the drill bit 10 is mounted. The drill bit mount 24 is formed with a drill housing hole 26 of a circular shape in cross section. One of the drills 14 is housed in the housing hole 26 with the centering shaft 19 fitted therein almost in close fit state, thereby centering the drill bit 10. Namely, in this embodiment, the drill housing hole 26 functions as a fitting hole for centering. A fitting hole 27 with a circular shape in cross section is formed in the drill bit mount 24 near an entrance of the drill housing hole 26 with such a size as to accommodate the disk parts 18 of the drill holder 12 therein almost in a close fit state. Fitting of the disk parts 18 into the fitting hole 27 also enables centering of the drill bit 10. A lock mechanism 30 to lock mounting of the drill bit 10 onto the drill bit mount 24 is provided in a perimeter of the fitting hole 27. More specifically, as shown in FIGs. 8A to 9B, the lock mechanism 30 is constructed such that a plurality of through-holes (two in the drawings) 28 passing radially through a circumferential wall of the drill bit mount 24 are formed equidistantly (at 180 ° interval in the drawings) in the circumferential wall encasing the fitting hole 27. A ball (engaging member) 32 is housed in each of the through-holes 28. Each of the balls 32 has a diameter larger than the thickness of the circumferential wall in which the through-hole 28 is formed. The ball 32 is movable between an engaged position where the ball 28 bulges into the fitting hole 27, as shown in FIGs. 7, 8A, and 8B, and a release position where the ball 32 retracts from the engaged position radially outward, as shown in FIGs. 9A and 9B. The arranged position of the through-hole 28 and the ball 32 is determined such that the ball 32 engages with a stepped part 17 formed between the non-circular part 16 and the rear disk part 18 when the disk parts 18 are fitted in the fitting hole 27 as shown in FIGs. 7, 8A and 8B and the ball 32 is set at the engaged position. An outer circumference of the circumferential wall of the drill bit mount 24 is formed into a small diametrical part 25 with a diameter slightly smaller than the other outer circumference of the drill bit mount 24. A retaining sleeve (retainer) 34 is fitted over the small diametrical part 25 almost in close fit state to be axially slidable. A circumferential groove 34a opened toward a lead end of the drill bit mount 24 and a circumferential groove 34b formed in spaced relation to the circumferential groove 34a rearward (right-side in FIGs. 8A and 8B) are formed in an inner circumference of the retaining sleeve 34. A compression coil spring 36 is housed in the circumferential groove 34a, with a lead end of the retaining sleeve 34 (left end in FIGs. 8A and 9B) held by a stopper ring 38 fixed to the outer circumference of the drill bit mount 24. The retaining sleeve 34 is urged rearward by a compressive force of the coil spring 36. In this arrangement, when an external force is not exerted, a rear end of the retaining sleeve 34 is abutted against a step 25a formed at a rear end of the small diametrical part 25 and retained thereat. The configuration of the retaining sleeve 34 is set such that in the above retaining state, a part of the retaining sleeve 34 between the circumferential grooves 34a, 34b presses the ball 32 from outward to set the ball 32 at the engaged position shown in FIGs. 8A and 8B, and the ball 32 is shifted to the release position shown in FIGs. 9A and 9B where the ball 32 is retracted radially outward, namely into the circumferential groove 34b when an external force is applied to the retaining sleeve 34 to move the sleeve 34 forward. Next, an operation of the drill tool according to this invention is described. First, as shown in FIGs. 7, 8A, and 8B, when the drill bit 10 is mounted to the drill bit holder 20, the ball 32 set at the engaged position restricts the drill bit 10 from loosening out of the drill bit holder 20 due to abutment of the ball 32 against the step part 17 formed between the non-circular part 16 and the rear disk part 18. The ball 32 also restricts rotation of the drill holder 12 in the fitting hole 27. In this arrangement, when a torque is applied to the drill bit holder 20 from the drive source, the torque is securely transmitted to the drill bit 10, thus rotating the entirety of the drill tool. This arrangement enables a desirable drilling operation with use of the drill 14 set at the lead end of the drill bit 10. In the case where the drill 14 loses or weakens the drilling ability due to breakage or the like, the drill bit 10 is temporarily dismounted from the drill bit holder 20. Specifically, nipping the retaining sleeve 34 with fingers or the like and exerting an external force in such a way as to slide the retaining sleeve 34 axially forward against the compressive force of the coil spring 36 enables dismounting of the drill bit 10 from the drill bit holder 20. Then, the ball 32 is retracted into the circumferential groove 34b, and shifted from the engaged position to the release position. In this state, the drill bit 10 is ready to be dismounted from the drill bit holder 20. Next, the drill bit 10 is mounted in the opposite direction opposite the previously mounted state (namely, the unused drill 14 that has been housed in the housing hole 26 of the drill bit mount 24 is directed outward at the lead end of the drill bit holder 20) while housing the broken or damaged drill 14 in the drill housing hole 26, and the centering shaft 19 and the disk parts 18 are respectively fitted in the housing hole 26 and the fitting hole 27. Thereafter, when the holding force of the retaining sleeve 34 is released, the retaining sleeve 34 is retracted to such a position as to abut the rear end thereof against the step 25a due to the compressive force of the coil spring 36, thereby setting the ball 32 at the engaged position again. When the ball 32 is set at the engaged position, mounting of the drill bit 10 onto the drill bit holder 20 is locked to enable restart of drilling operation with use of the newly set drill 14. When observing the construction of the drill bit 10, forming the non-circular part 16 with a substantially square shape in cross section at the intermediate part of the drill holder 12 enables preventing a relative rotation of the drill 14 to the drill holder 12 without providing a circumferential groove for fittingly confining a ball as a stopper, as has been performed conventionally. Also, when observing the construction of the drill bit holder 20, the drill bit holder 20 is formed with the drill housing hole 26 and the fitting hole 27 both of which have a circular shape in cross section, unlike the conventional drill bit holder formed with a polygonal hole. Thus, the drill bit holder 20 can be manufactured according to a simplified manufacturing process, which remarkably reduces the production cost of the drill tool as a whole. Particularly, in this embodiment, the entire configuration of the drill holder 12 is formed such that the outer circumference of the intermediate part of the cylindrical member is sliced off at plural positions to leave the flat planes 15 while also leaving the axially opposite ends (disk parts 18). In this arrangement, the drill holder 12 is manufactured by simply removing the plural parts over the circumference of the cylindrical member by machining or the like to shape the flat planes 15. Further, the drill holder 12 having the above simplified construction prevents loosening off of the drill 14 from the drill holder 12 and relative rotation of the drill 14 to the drill holder 12 (namely, torque transmission from the drill bit holder 20 to the drill 14 is secured) at the same location. In addition, the disk parts 18 have the identical shape. Fitting the disk parts 18 in the fitting hole 27 having a circular shape in cross section enables centering of the drill bit 10. In other words, since the drill holder 12 functions as a shaft member for centering the drill bit 10, in addition to the function as a holder, an accurate centering is ensured with a compact construction. Further, forming the centering shaft 19 independently of the drill holder 12, as shown in this embodiment, further ensures centering. In this invention, the specific construction of the drill bit holder 20 and the specific mounting structure of the drill bit 10 are not limited to the ones shown in the drawings and can be designed desirably according to designer's choice. Next, a second embodiment of this invention is described with reference to Figs. 11 to 13B. Similar to the first embodiment, a drill bit 10 in this embodiment includes a drill holder 12 and a pair of drills 14 connected to the axially opposite ends thereof respectively. The shape of the drill 14 and the outer shape of the drill holder 12 are the same as the one shown in the first embodiment, and hence description thereof is omitted herein. The drill holder 12 is formed with a pair of insertion holes 11 extending, along an axis of rotation thereof, from a surface of the axially opposite ends toward an axially intermediate part except the intermediate part. A blind hole 13 with a closed bottom is formed in the drill holder 12 extending from one of corner ends of a non-circular part 16 radially inward toward the intermediate part. The blind hole 13 is formed into a substantially elliptical shape in plan view, as shown in Fig. 11A, and extends radially inward to such a degree as to intersect a bottom part of both of the insertion holes 11. The blind hole 13 extends up to a radially intermediate position (substantially depthwise center in the drawing of FIG. 11B) of the insertion holes 11. In other words, the depth of the blind hole 13 is limited to a certain level. Combination of the insertion holes 11 and the blind hole 13 defines a restricting part 10e protruding from an inner circumference of the insertion holes 11 radially inward at the bottom end of the insertion holes 11 (axially intermediate position of the drill holder 12 in FIG. 11B). Fitting a shank 14c of each of the drills 14 deep into the corresponding insertion hole 11 allows a distal end 14d of the shank 14c to abut against a side surface of the restricting part 10e, while rendering an engaging part 14e of each of the drills 14 in pressing contact with a top of the restricting part 10e. In this way, engaging the engaging part 14e eccentrically away from a center of axis of the drill 14 with the restricting part 10e prevents a relative rotation of the drill 14 to the drill holder 12 about an axis thereof, and ensures rotation of the drill 14 along with the drill holder 12 about the axis thereof. Further, since the restricting part 10e is integrally formed with the drill holder 12, there is no possibility of loosening off of the restricting part from the drill holder 12 that could have happened in the case of the prior art drill bit provided with a stopper pin. The drill bit 10 can be produced, for instance, according to the following steps. The engaging part 14e is formed by machining a base end of the shank 14c of the drill 14. After forming the insertion holes 11 by digging a hole in the opposite ends of the drill holder 12 except the axially intermediate part, the blind hole 13 with a closed bottom is formed such that the blind hole 13 extends from one of the corner ends of the non-circular part (boundary part between the adjacent planes 15) radially inward toward the insertion holes 11 to such a degree as to intersect the insertion holes 11 halfway through. Thereby, the restricting part 10e is bridgingly formed between the insertion holes 11. The insertion holes 11 may be formed after forming the blind hole 13. Next, fitting the shank 14c into the insertion hole 11 engages the engaging part 14e with the restricting part 10e. Thus, a relative rotation of the drill 10 to the drill holder 12 is easily prevented. In this embodiment, the shank 14c may be subject to clearance fit into the insertion hole 11 and fixed thereat by a brazing or the like. Alternatively, the shank 14c may be fitted into the insertion hole 11 by close fit such as press fit and shrinkage fit. As an altered arrangement, fixation can be accomplished by securing a small clearance between the shank 14c and the insertion hole 11 and press fitting the engaging part 14e in the clearance between a head of the restricting part 10e and an inner circumference of the insertion hole 11. The present invention can take the following modifications and alterations whithin the scope of the appended claims. (1) In the first embodiment, the specific construction and protruded amount of the head (top) of the stopper 40 are desirably set according to designer's choice. As a best mode, however, shaping the stopper 40 such that the top surface thereof is flat and passes or aligns in spaced relation to a center of axis of the insertion hole 11, as shown in Fig. 5, simplifies the shape of the stopper 40 and the engaging part 14e that engages the stopper 40. Also, this arrangement ensures a sufficient engagement. Note that the head of the stopper 40, which should be flat in the best mode, hardly deforms even after hammering down the stopper 40 into the blind hole 13, and hence, press fit operation of the stopper 40 by a hammer or the like is surely performed. (2) In this invention, it is possible to fit the shank 14c into the insertion hole 11 according to clearance fit and then fix the shank 14c thereat by a brazing or its like. In this case, solely depending on the brazing as performed conventionally may not withstand a large torque applied to the fixed part. In this invention, however, engagement of the stopper 40 (restricting part 10e) with the engaging part 14e assures prevention of relative rotation of the drill to the drill holder (namely, withstands the large torque). (3) In the first embodiment, shaping the distal end 14d of the shank 14c into a circular arc to allow a planar contact with the circumference of the stopper 40 increases a contact area of the distal end 14d of the shank 14c with the circumference of the stopper 40. Thereby, the mounted state of the drill 14 onto the drill holder 12 is more stabilized. (4) In the second embodiment, the specific construction and protruded amount of the restricting part 10e are desirably set according to designer's choice. As a best mode, however, shaping the restricting part 10e such that the top surface thereof is flat and passes or aligns in spaced relation to a center of axis of the insertion hole 11, as shown in Fig. 11B, simplifies the shape of the restricting part 10e and the engaging part 14e that engages the restricting part 10e. Also, this arrangement ensures a sufficient engagement. The diameter and shape of the blind hole 13 may be set desirable in accordance with the shape of the restricting part 10e. (5) In this invention, the shape of the drill holder 12 is not limited to the one in the above embodiments. For instance, the shape disclosed in Japanese Examined Patent Publication No. 8-18213/1996 may be used. (6) The shape of the drills 14 at opposite ends of the drill bit 10 may not necessarily be identical to each other. For instance, the diameter of the bit of one of the drills 14 may be varied from that of the other drill 14. (7) The centering shaft 19 of the drill bit 10 can be omitted as needed. In the case where the centering shaft 19 is provided, although, in the foregoing embodiment, the drill housing hole 26 of the drill bit holder 20 into which the drill 14 is accommodated is used as a fitting hole for centering as well as housing, a fitting hole for centering may be provided independently from the drill housing hole 26. (8) The shape of the drill bit holder 20 is not limited to the one shown in the foregoing embodiments. For instance, an inner end of the engaging member (end part that abuts against the drill bit 10) may be shaped into a spherical form, whereas the other part thereof that is to be housed in the through-hole 28 may be shaped into a column. Also, the shape of the retainer is not limited to the one shown in the aforementioned embodiments. For instance, the retainer may be of an annular spring that holds the engaging member at the engaged position from outward. (9) The cross sectional shape of the non-circular part 16 is not limited to a substantially square shape. For instance, substantially a hexagonal shape shown in FIG. 15A or other polygonal shape may be applicable. Further, although a shaping (removal) amount increases, the non-circular part 16 may be shaped into a complete polygon (e.g., square in FIG. 15B) whose size is obviously smaller than the diameter of the disk part 18. Alternatively, at least a part of the flat plane 15 may be curved into a recess 15', as shown in FIG. 15C. As a best mode, however, a square or a substantial square may be more preferable in the aspect of easy machining and considering the fact that the more the number of apexes of the polygon increases, the more a torque transmission weakens (namely, the smaller number of apexes of the polygon is better).

Claims: A drill bit (10) comprising a drill (14) including a bit (14b) and a shank (14c), and a drill holder (12) formed with an axial insertion hole (11) to receive the shank (14c), the shank (14c) being formed with an engaging part (14e), characterised in that axial insertion holes (11) are formed in axially opposite ends of the drill holder (12) each to receive the shank (14c) of an associated drill (14), the drill holder (12) being formed with a blind hole (13) extending from an outer circumference thereof to intersect each insertion hole (11), the blind hole (13) terminating with an abutment surface which is engaged by the engaging part (14e) of each shank (14c) to prevent relative rotation between the drills (14) and the drill holder (12). The drill bit (10) as recited in claim 1 in which a stopper (40) is inserted into the blind hole (13) whereby the top of the stopper defines the abutment surface. The drill bit (10) as recited in claim 1, wherein the abutment surface is formed integrally with the drill holder (12) to define the bottom of the blind hole (13). The drill bit (10) as recited in any one of claims 1 to 3, wherein the engaging part (14e) is formed by cutting away part of the shank (14c) into a shape engageable with the abutment surface. The drill bit (10) as recited in claim 3, wherein the abutment surface is flat and extends substantially along a center axis of the insertion holes (11). The drill bit (10) as recited in claim 1, wherein the engaging parts (14e) are press fitted on the abutment surface in the insertion holes (11). The drill bit (10) as recited in claim 1, wherein the shanks (14c) are each fitted into their insertion hole (11) to such a degree as to axially abut the engaging part (14e) against the abutment surface. The drill bit (10) as recited in claim 6, wherein a base end of each shank (14c) is so formed as to axially come into planar contact with the abutment surface. The drill bit (10) as recited in claim 2, wherein the diameter of the blind hole (13) and the stopper (40) is set equal to or larger than a.diameter of the insertion holes (11). The drill bit (10) as recited in claim 2, wherein the stopper (40) is press fitted in the bottom of the blind hole (11). The drill bit (10) as recited in any one of claims 1 to 10, wherein axially opposite ends of the drill holder (12) are each shaped into a disk part (18) identical to each other with a diameter larger than the drill (14), and an intermediate part (16) of the drill holder (12) between the disk parts (18) is formed into a non-circular part such that a cross section thereof is a polygon or a substantial polygon with a diameter of a circumscribed circle thereof equal to or smaller than the diameter of the disk part (18). The drill bit (10) as recited in claim 11, wherein the non-circular part (16) is formed by slicing off an intermediate part, from a cylinder, at plural positions over a circumference of the cylinder in such a way as to leave a flat plane (15) corresponding to the removed part while leaving the axially opposite ends corresponding to the disk parts (18) having a diameter larger than the bit (10) of the drill. The drill bit (10) as recited in claim 11, wherein the non-circular part (16) has a substantially square shape in cross section. The drill bit (10) as recited in claim 11, wherein a cylindrical centering shaft (19) with a diameter larger than the drill (14) and smaller than the disk part (18) is formed between each of the disk parts (18) and the corresponding drill (14). A drill tool comprising a drill bit (10) as claimed in any one of claims 1 to 14 together with a drill bit holder (20) formed with a housing hole (26) to house the drill bit (10) concentrically therein. A drill tool comprising a drill bit (10) as claimed in claim 11 together with a drill bit holder (20) formed with a circular drill housing hole (26) to house one of the drills (14) of the drill bit (10) and a fitting hole (27) of a circular shape in cross section to fit the disk parts (18) of the drill holder (12), the drill bit holder (20) provided, at a circumferential wall thereof encasing the fitting hole (27), with an engaging member (32) movable between an engaged position where the engaging member (32) engages a step part (17) formed between the rear disk part (18) and the non-circular part (16) to prevent loosening off of the drill bit (10) and a relative rotation of the drill (14) to the drill holder (12), and a release position where the engaging member (32) is retracted from the engaged position radially outward to allow a disengagement of the drill bit (10), and a retainer (34) operable to change the position of the engaging member between(32) the engaged position and the release position. The drill tool as recited in claim 16, wherein a cylindrical centering shaft (19) with a diameter larger than the drill (14) and smaller than the disk part (18) is provided between each of the drills (14) and the corresponding disk part (18), and the drill bit holder (20) is formed with the fitting hole for centering the drill bit (10) as the centering shaft (19) is fitted therein. A method of manufacturing a drill bit as claimed in claim 2 and claims dependent therefrom comprising the steps of: a) forming an engaging part (14e) in each shank (14c); b) forming a blind hole (13) extending from an outer circumference of the drill holder (20) in such a way as to intersect each insertion hole (11); c) inserting a stopper (40) in the bottom of the hole (13); and d) fitting the shanks (14c) in the insertion holes (11) to engage the engaging parts (14e) with a top of the stopper (40). The manufacturing method as recited in claim 18, further comprising the step of partially cutting way a certain part of each shank (14c) to form the engaging parts (14e). The manufacturing method as recited in claim 18, further comprising the step of press fitting the engaging parts (14e) in between the top of the stopper (40) and the insertion holes (11). The manufacturing method as recited in claim 18, further comprising the step of fitting the shanks (14c) into the insertion holes (11) to such a degree as to allow an end of each shank (14c) to abut axially against the top of the stopper (40). A method of manufacturing a drill bit as claimed in claim 3 and claims dependent therefrom comprising the steps of: a) forming a blind hole (13) in the drill holder (20) such that the hole (13) radially extends part way through each insertion hole (11) and intersects a bottom end of each insertion hole (11) to form an abutment surface at the bottom end of the insertion holes (11); b) forming an engaging part (14e) at an end of each shank (14c) to engage the abutment surface; and c) fitting the shanks (14c) into the insertion holes (11) to such a degree as to engage the engaging parts (14e) with the abutment surface to restrict a relative rotation of the drills (14) to the drill holder (20). The manufacturing method as recited in claim 22, further comprising the step of partially cutting away a certain part of each shank (14c) to form the engaging parts (14e). The manufacturing method as recited in claim 22, further comprising the step of fitting the shanks (14c) into the insertion holes (11) to such a degree as to allow an end of each shank (14c) to abut axially against the abutment surface.

Inventors: AJIMI KUNIO, AJIMI, KUNIO
Applicants: HOUSE BM CO LTD, HOUSE B.M. CO., LTD.
Application year: 1999
Main classification: B23B31
IPC codes: B23B51, B23B31