Drill

Abstract: A drill capable of drilling a deep hole while keeping high chip discharging ability and having a helical flute (6) whose width and helix angle (α) will not change even after the cutting edges have been regenerated by regrinding. The drill has a tip portion (1) formed with a helical flute having a constant width and helix angle (α1), a rear portion (3) formed with a helical flute which are greater in width and smaller in helix angle (α3) than the helical flute formed in the tip portion, and a middle portion (2) provided between the tip portion and rear portion and formed with a helical flute smoothly connecting with the helical flutes of the tip portion and rear portion and having a gradually increasing width and a gradually decreasing helix angle (α2) from the tip portion toward the rear portion.

Description: BACKGROUND OF THE INVENTION This invention relates to a drill which shows high drilling performance in forming a deep hole and which maintains stable performance even after its edge has been regenerated by regrinding, according to the preamble of claim 1. As shown in Fig. 6, a conventional drill typically has a helical chip discharge flute 6 having a constant sectional area and extending at a helix angle of α. Since the flute is helical, chip has to move a long distance along such a flute 6 until they are discharged from the drilled hole. Therefore, the flute is frequently clogged with chip. To discharge chip smoothly even in drilling a deep hole, Examined Japanese Patent Publication JP 6088168 B discloses a drill having a chip discharge flute 6 that extends straight, i.e. at a zero helix angle at its portion 6' near the drill tip (see Fig. 5). Unexamined Japanese Patent Publication JP 7164227 A proposes to gradually increase the sectional area of the flute and gradually reduce its helix angle from the drill tip to discharge chip more smoothly. One problem with the drill disclosed in Examined Japanese Patent Publication JP 6088168 B is that it is difficult to effectively use screw effects to obtain a chip pushing force while chip is flowing along the straight portion 6'. Rather, chip in the straight portion is merely pushed up by chip nearer to the drill tip in the axial direction of the drill, so that they will not be discharged with a strong force. Thus, because a hole is drilled deeper, it will become increasingly difficult to smoothly discharge chip. The drill disclosed in Unexamined Japanese Patent Publication JP 7164227 A has a problem in that if the drill tip is reground, the width and helix angle of the flute will change, so that the shape of chip produced also will change, making it difficult to discharge chip in a stable manner. The closest prior art is a drill with all the features of the preamble of claim 1 (FR 130 2191 A). This document shows a drill comprising a tip portion formed with a helical flute having a constant width and helix angle, a rear portion formed with a helical flute which has a constant width and helix angle but is greater in width and smaller in helix angle than the helical flute formed in said tip portion, and a middle portion provided between said tip portion and said rear portion. Further, it is known to design the transition portion with a gradually increasing width and a gradually decreasing helix angle from one portion having a constant width and helix angle to another portion having a constant width and helix angle (EP 126 409 B). The object of this invention is to improve the ability of the drill to discharge chip in drilling a deep hole, and to maintain its high performance after regrinding. SUMMARY OF THE INVENTION According to this invention, there is provided a drill according to claim 1. Further, the drill tip portion preferably has a length of 1D-2D (D is the diameter of the drill) and the drill middle portion preferably has a length of 1D-3D. The flute width ratios are the ratio of the central angles a1, a2 and a3 of the helical flutes 6 shown in Figs. 2-4 to the central angles c1, c2 and c3 of the lands 5. If the heels 7 shown in Figs. 2-4 are chamfered, the chamfered portions are excluded in calculating the flute width ratios. A drill having a helical flute formed therein, the helical flute having a constant width and helix angle at the tip portion of the drill, and the helical flute at the remaining portion of the drill having a gradually increasing width and a gradually decreasing helix angle toward the terminal end of the helical flute. Since the helical flute has a helix angle gradually decreasing toward the rear end of the drill, chip flow distance chip is short compared with a conventional drill having a constant helix angle. The helical flute in the drill rear portion has also a helix angle, so that chip produced near the drill tip can be pushed out smoothly by screw effects. This also contributes to suppressing increase in the machining power. In the drill tip portion, the helical flute has a constant width and helix angle, so that the shape of the tip of the drill will never change even after the drill has been repeatedly reground, so that its performance will not change, either. Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of an embodiment of the drill of this invention; Fig. 2 is a sectional view taken along line I-I of Fig. 1; Fig. 3 is a sectional view taken along line II-II of Fig. 1; Fig. 4 is a sectional view taken along line III-III of Fig. 1; Fig. 5 is a side view of a conventional drill; Fig. 6 is a side view of another conventional drill; and Fig. 7 is a graph showing machining power needed for each drill specimen. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a drill embodying this invention. Figs. 2-4 show sections taken along lines I-I, II-II and III-III of Fig. 1, respectively. The illustrated drill has a shank 4 integrally formed at its rear end. In the figure, the letter C indicates the central axis of the drill. The drill has a helical flute 6 having a constant width and a helix angle α1 in a tip portion 1 of the drill near its tip. In a middle portion 2 of the drill, the flute 6 has a width that gradually increases toward the rear end of the drill and a helix angle α 2 that gradually decreases toward the drill rear end. In a rear portion 3 of the drill, the flute 6 has a large, constant width and has a helix angle α 3 that is smaller than α 1. The drill according to the present invention satisfies the following conditions: α1 > α3, a1/c1 < a3/c3, α3 ≦ α2 ≦ α1, a1/c1 ≦ a2/c2 ≦ a3 /c3 This drill is suited for use in drilling deep holes. Of the helix angles at the respective portions, α1 is constant within the range of 15° -30 ° , α 3 is constant within the range of 3-8 ° and α2 is equal to α1 at the end near the drill tip and changes gradually until it becomes equal to α3 at the rear end. The ratios of the width of the flute to the width of the remaining portion in the drill tip portion 1 and that in the drill rear portion 2, i.e. a1 : c1 and a3 : c3, should be constant within the range of 0.5 - 1 : 1 and 0.8 - 1.3 : 1, respectively. The length l 1 of the drill tip portion 1 should not be too long to keep sufficiently high chip discharging properties and should not be too short to prevent undue reduction in the number of times the cutting edges can be reground. The length l 1 should be preferably equal to or greater than the drill diameter D and equal to or smaller than twice the drill diameter D. The length l 2 of the drill middle portion 2 should be long enough to prevent sharp change in the width of the helical flute which is undue to keep high chip discharging properties. The length l 2 should preferably be between 1D and 3D. The length l 3 of the drill rear portion 3 is determined to a suitable value, taking into consideration the depth of the hole to be drilled. the drill rear portion of the drill shown in Fig. 1. In the embodiment of Fig. 1, an insert 8 made from cemented carbide and having cutting edges 9 and margins 10 is brazed to the tip of the drill. But this invention is also applicable to a drill made from a single material or a drill in which margins extend to near the terminal end of the helical flute. We will now briefly describe how the helical flute according to this invention is formed. If a cemented carbide insert is to be attached to the tip of the drill, the flute has to be worked before the insert is brazed. For this purpose, a ball end mill of the size matching the size of an intended flute is moved longitudinally of the drill from its tip. To change the flute width, the ball end mill is shifted to one side of the flute while moving it longitudinally. In this case, the flute is formed by moving the end mill only once. But from the portion of the flute where its width is changing to the flute end, the flute is left unmachined along its other side. Thus, the ball end mill is moved along the flute again to machine the other side. At the portion of the drill where the ball end mill has moved twice, a rib is formed on its bottom. Such a rib is removed by buffing or cutting. The helical groove is thus finished. [Examples] For a performance test, drills having a diameter D = 15 mm and a flute length of 110 mm with a cemented carbide insert brazed thereto and having an oil hole were prepared. They include a drill of the type shown in Fig. 5 (Specimen No. 13), a drill of the type shown in Fig. 6 (No. 11), a drill basically of the same type as the drill of Fig. 1 with the angle α1 equal to α3 (No. 12), and drills of the type shown in Fig. 1 (Nos.1-10). Specimen Nos. 11 to 13 are conventional drills, while Specimen Nos. 1-10 are drills according to the present invention. The insert attached to each of these drills is made from K30 under ISO standard or its equivalent. Table 1 shows specifications of these drills and test results. In the performance test, the test drills were used to drill holes in a workpiece made from alloy steel SCM440 (Brinell hardness HB = 300) under the following conditions, and increases in machining power due to clogging with chip was measured. Machining conditions: Machining speed V = 50 m/min Feed f = 0.25 mm/rev Depth of hole d = 107 mm (7D) Other conditions: wet machining using machining oil Chip produced while drilling a hole tends to sharply increase the machining power. Thus, by measuring the machining power, it is possible to determine how efficiently chip is discharged. From the table, it is apparent that the drills according to the present invention can discharge chip more efficiently than the conventional drills. Specimens 1-3, having helix angles and flute width ratios within the preferred range, were especially high in the ability to discharge chip, as evidenced by the fact that the maximum machining power was equal to or under 4.1 kw. Specimens No. 1 to 3 were also high in their ability to drill straight holes. That is, none of the holes drilled by these drills were bent by more than 0.15 mm. Specimen No. 4 was low in the maximum machining power but the hole drilled by this drill was bent by 0.56 mm. This is presumably because the flute was too wide. Fig. 7 shows fluctuations in the machining power for Specimens Nos. 1, 3-5 and 11-13. For the No. 4 drill, the cutting power fluctuated at a small amplitude, which shows that this drill is low in rigidity. For Specimens Nos. 11 and 12, the machining power increased sharply when the hole depth increased to about 4D. For the drills according to the present invention, the cutting power did not increase sharply until the hole depth increased to about 5D. Compared with No. 3, Nos. 5, 6 and 9 are not so marked in their ability to lower the machining power. But in the case of the drills according to this invention, it is possible to improve the ability to discharge chip by changing helix angles and flute width ratios at various portions. For Specimen No. 13, such improvement in performance is practically impossible. (For exmaple, if the helix angle at the tip portion were increased, chip would have to be moved an unduly long distance until discharged, and if it were reduced, the chip flowing force due to screw effects will drop.) For the above reason, the drills of the invention are advantageous over Specimen No. 13. As described above, the drill according to this invention has a helical flute which is wider in the drill rear portion than in the drill tip portion. Its helix angle is smaller in the rear portion than in the tip portion. This drill can form a deep hole while discharging chip smoothly without the possibility of getting clogged with chip. Since the width and helix angle of the flute are constant in the drill tip portion, the drill will show unchanged performance after its cutting edges have been regenerated. That is, even after regeneration, the shape of chip will not change at all. This assures a stable drilling.

Claims: A drill comprising a tip portion (1) formed with a helical flute (6) having a constant width and helix angle (α1) and a rear portion (3) formed with a helical flute (6) which has a constant width and helix angle (α3) but is greater in width and smaller in helix angle than the helical flute (6) formed in said tip portion (1), and a middle portion (2) which is provided between said tip portion (1) and said rear portion (2), characterized in that the middle portion (2) is formed with a helical flute smoothly connecting with the helical flutes of said tip portion (1) and said rear portion (3) and having a gradually increasing width and a gradually decreasing helix angle (α2) from said tip portion (1) toward said rear portion (3) and that a groove width ratio a1 : c1 in said tip portion (1) is 0.5-1 : 1, wherein a groove width ratio a3 : c3 in said rear portion (3) is 0.8-1.3 : 1, wherein the helix angle of the helical flute formed in said tip portion (1) is 15° - 30°, and wherein the helix angle of the helical flute (6) formed in said rear portion (3) is 3° - 8°. Drill as claimed in claim 1, characterized in that the length of said tip portion (1) is 1D to 2D wherein D is the diameter of the drill and the length of said middle portion (2) is 1D to 3D.

Inventors: JINDAI MASAAKI, OKAMOTO JUN, JINDAI, MASAAKI, OKAMOTO, JUN, Jindai, Masaaki, c/o Itami Works, Okamoto, Jun, c/o Itami Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1997
Main classification: B23B51
IPC codes: B23B51