Drill

Abstract

This drill (1) comprises a shaft-shaped tool body (10) that is rotated about a central axis, the drill having a chip discharge groove (14) formed on the outer periphery of the tool body (10), and a cutting blade (15) formed at an intersecting ridge portion between the chip discharge groove (14) and a tip flank (13) of the tool body (10)). The tool body (10) has a base material and a hard coating (17) formed in a region of a prescribed length from the tip of the base material. The chip discharge groove (14) has, in the central axis direction, in order from the tip of the tool body (10), a first section (S1) having a first surface roughness and a second section (S2) positioned closer to the rear end side than the first section (S1) and having a second surface roughness less than the first surface roughness. The first section (S1) has a length of at least 0.5 times a tool diameter (D). The hard coating (17) is formed in the first section (S1), and a hard coating (S17) is not formed in the second section (S2).

Description

drill

[0001] This application claims priority from Japanese Patent Application No. 2023-150250, filed on September 15, 2023, the contents of which are incorporated herein by reference.

[0002] Conventionally, there has been known a drill in which the surface of a hard coating provided in a chip discharge flute is smoothed to improve chip discharge performance (see, for example, Patent Document 1).

[0003] Japanese Patent No. 6828824 (B)

[0004] Smoothing the inner surface of the chip discharge flute can prevent chip clogging. However, polishing the surface of the hard coating on the rake face connected to the cutting edge reduces the wear resistance of the hard coating. Furthermore, because the surface of the hard coating is inherently rough, even if the surface of the hard coating is smoothed, it is difficult to achieve good chip discharge when the chip discharge flute is long.

[0005] An object of the present invention is to provide a drill that has improved chip removal performance while ensuring the wear resistance of the hard coating near the cutting edge.

[0006] According to one aspect of the present invention, there is provided a drill including a shaft-shaped tool body that rotates about a central axis, a chip flute formed on the outer periphery of the tool body, and a cutting edge formed at the intersection ridge between the tip flank of the tool body and the chip flute. The tool body includes a substrate and a hard coating formed in a region of a predetermined length from the tip of the substrate. The chip flute has, in the central axial direction, in order from the tip of the tool body, a first section having a first surface roughness, and a second section located rearward of the first section and having a second surface roughness smaller than the first surface roughness. The first section has a length of at least 0.5 times the tool diameter D. The hard coating is formed in the first section, while the hard coating is not formed in the second section.

[0007] According to the above configuration, the hard coating is formed only on the tip of the drill, and the substrate surface with a small surface roughness is exposed in the chip discharge flute in the rear second section. This allows chips moving in the chip discharge flute in the second section to be smoothly moved toward the rear end. Compared to a configuration in which a hard coating is formed over the entire chip discharge flute or a configuration in which a normally ground substrate surface is exposed in the rear chip discharge flute, chip discharge performance can be improved. Furthermore, a hard coating is formed in the first section of the tip, resulting in a large surface roughness. Since the smoothed hard coating is partially removed or thinned, wear of the hard coating is likely to occur on areas such as the rake face, which are subject to heavy loads. In this configuration, the surface of the hard coating in the first section has a large surface roughness, making it less likely to wear around the cutting edge.

[0008] The surface roughness of the substrate in the second section may be smaller than the surface roughness of the substrate in the first section. The surface roughness between the sections may be made different depending on the surface roughness of the substrate. Since the surface roughness in the second section can be further reduced, chip removal performance is improved.

[0009] The hard coating may have a surface having directional striations exposed in the first section, and a smooth surface of the mirror-ground substrate exposed in the second section. Because the hard coating near the cutting edge has a streaked surface, it becomes easier for the lubricating liquid to penetrate to the cutting edge, thereby suppressing damage to the cutting edge.

[0010] The hard coating may be formed in a third section between the first section and the second section, the third section having a third surface roughness that is smaller than the first surface roughness and larger than the second surface roughness.

[0011] The hard coating may be formed in a third section between the first section and the second section, the third section having a third surface roughness that is smaller than the first surface roughness and larger than the second surface roughness, and the hard coating may not be formed in the third section.

[0012] The surface roughness of the substrate in the second section and the third section may be smaller than the surface roughness of the substrate in the first section. A surface of the hard coating having directional striations may be exposed in the first section, and a smooth surface of the mirror-ground substrate may be exposed in the second section.

[0013] According to one aspect of the present invention, a drill is provided that ensures the wear resistance of a hard coating near the cutting edge while improving chip removal performance.

[0014] Fig. 1 is a side view of a drill according to an embodiment. Fig. 2 is a partial side view showing an enlarged view of a tip portion of the drill according to an embodiment. Fig. 3 is a front view of the drill according to an embodiment. Fig. 4 is a diagram showing a first example of the drill. Fig. 5 is a diagram showing a second example of the drill.

[0015] Fig. 1 is a side view of the drill of this embodiment, Fig. 2 is a partial side view showing an enlarged tip portion of the drill of this embodiment, and Fig. 3 is a front view of the drill of this embodiment.

[0016] The drill 1 of this embodiment is a drill for deep hole drilling. The drill 1 has a generally cylindrical shape centered on a central axis (axial line) O. The drill 1 has an axial body (tool main body) 10 and a shank 11, each of which extends along the central axis O. The drill 1 performs machining on the workpiece by rotating the tip surface of the body 10 facing the workpiece and rotating around the central axis O. The drill 1 of this embodiment is a solid drill having a substrate made of cemented carbide or the like. The drill 1 has a hard coating made of a metal nitride or the like coating the surface of the substrate.

[0017] The direction along the central axis O of the drill 1 (the direction in which the central axis O extends) is called the axial direction. Within the axial direction, the direction from the shank 11 toward the body 10 is called the tip side, and the direction from the body 10 toward the shank 11 is called the rear side. The direction perpendicular to the central axis O is called the radial direction. Within the radial direction, the direction approaching the central axis O is called the radially inner side, and the direction away from the central axis O is called the radially outer side. The direction going around the central axis O is called the circumferential direction. Additionally, within the circumferential direction, the direction in which the body 10 rotates during drilling is called the forward rotation side, and the opposite side is called the rear rotation side.

[0018] The body 10 has a pair of cutting edges 15, 15 located at the tip of the body 10, and a pair of chip discharge grooves 14, 14 extending spirally around the outer periphery of the body 10 from the cutting edges 15, 15 toward the rear end side.

[0019] The drill 1 has two coolant holes 9 formed symmetrically about the central axis O from the rear end face of the shank 11 toward the tip end, twisting with the same lead as the chip discharge flutes 14. These coolant holes 9 pass between the chip discharge flutes 14 in the body 10 and open to the tip flank 13. When drilling using the drill 1, coolant such as cutting fluid or compressed air is ejected from the two coolant holes 9. The tip flank 13 is formed by a two-stage flank. Of the two-stage tip flank 13, the flank located on the opposite side to the tool rotation direction T has a larger clearance angle. The coolant hole 9 opens to the flank located on the opposite side to the tool rotation direction T of the two-stage flank.

[0020] The chip discharge flutes 14 twist rearward in the tool rotation direction T at a constant twist angle on the outer periphery of the body 10 as they extend from the cutting edge 15 toward the rear end in the direction of the central axis O. The pair of chip discharge flutes 14, 14 are arranged symmetrically with respect to the central axis O. The drill 1 of this embodiment is used for deep hole drilling, and the length of the chip discharge flutes 14 along the direction of the central axis O is, for example, in the range of 8.0D to 30.0D relative to the outer diameter of the cutting edge 15 of the drill 1. The outer diameter of the cutting edge 15 is the outer diameter of a circle formed by a rotation locus around the central axis O at the outer peripheral end of the cutting edge 15, i.e., the tool diameter D.

[0021] The cutting edges 15, 15 are formed at the tip of the body 10 at the intersection ridges of the inner wall surfaces of the pair of chip discharge grooves 14, 14 facing forward in the tool rotation direction T and the tip flank 13. The pair of cutting edges 15, 15 are arranged symmetrically with respect to the central axis O. The cutting edges 15 extend toward the rear end as they approach the outer periphery (radially outward) of the body 10, and are given a point angle.

[0022] The body 10 has lands 16, 16 between the two chip discharge grooves 14, 14. A first margin 16A is provided at the end of the land 16 on the front side in the tool rotation direction T. A second margin 16B is provided at the end of the land 16 on the rear side in the tool rotation direction T. The first margin 16A and the second margin 16B extend spirally along the edge of the chip discharge groove 14. The body 10 has a relief surface 16C recessed radially inward between the first margin 16A and the second margin 16B.

[0023] In the drill 1 of this embodiment, a hard coating is formed only on the tip portion of the body 10, and the chip flutes 14, excluding the tip portion, are smoothed by mirror grinding. In other words, the hard coating is formed in a region of a predetermined length from the tip of the substrate. With this configuration, the chip flutes 14 of the drill 1 are divided into multiple sections with different surface roughness along the axial direction. The drill 1 of this embodiment will be described in more detail below using two examples.

[0024] 4 is a diagram showing a first example of the drill 1 according to the present embodiment. As shown in FIG. 4, the hard coating 17 is formed only on the tip end portion of the base material of the drill 1, and is not formed on the rear end portion of the base material. In the first example, the hard coating 17 is formed in a region extending approximately one-third of the way from the tip of the body 10.

[0025] As shown in the lower part of Figure 4, the wall surface of the chip discharge flute 14 of the base material of the drill 1 is partially mirror-finished. More specifically, the tip portion of the chip discharge flute 14 is ground using a normal grindstone, and the base material surface has a surface shape with directional striations. On the other hand, the rear end portion of the chip discharge flute 14 is formed using a normal grindstone, and then the surface is smoothed by mirror-grinding using an elastic grindstone or the like. The mirror-ground base material surface has a smooth surface with reduced striations.

[0026] The rearmost portion of the chip discharge groove 14 is not mirror-ground. This is because the rearmost portion of the chip discharge groove 14 becomes shallower toward the rear end and contributes little to chip discharge, while grinding this region imposes a high grinding load and affects productivity. Note that the chip discharge groove 14 may be mirror-ground all the way to the rear end.

[0027] In the first embodiment, the trailing edge 17a of the hard coating 17 is located rearward of the leading edge 14a of the mirror-ground region of the substrate. That is, the trailing portion of the hard coating 17 overlaps the leading portion of the mirror-ground region of the substrate. As a result, the wall surface of the chip flute 14 of the drill 1 is divided into four sections: a first section S1, a second section S2, a third section S3, and a fourth section S4, by combining two sections defined by whether the substrate is normally ground or mirror-ground and two sections defined by the presence or absence of the hard coating 17.

[0028] The first section S1 is located at the tip of the drill 1, and the wall surfaces of the chip discharge flutes 14 are configured such that a hard coating 17 is formed on the surface of a normally ground substrate. The second section S2 is located in a region from the center of the drill 1 toward the rear end, and the hard coating 17 is not formed, and the mirror-ground substrate surface is exposed on the wall surfaces of the chip discharge flutes 14. The third section S3 is located between the first section S1 and the second section S2, and the wall surfaces of the chip discharge flutes 14 are configured such that a hard coating 17 is formed on the surface of a mirror-ground substrate. The third section S3 is the region between the rear end edge of the hard coating 17 and the leading edge 14a of the mirror-ground region of the substrate. The fourth section S4 is located on the rear end side of the second section S2, and the hard coating 17 is not formed, and the normally ground substrate surface is exposed on the wall surfaces of the chip discharge flutes 14.

[0029] In the first section S1, the uneven shape due to the streaks on the normally ground substrate surface is superimposed on the isotropic undulations due to the microparticles of the hard coating 17, so the surface roughness in the first section S1 (first surface roughness) is the largest among the first section S1 to the fourth section S4. In the second section S2, the mirror-ground substrate surface is exposed, so the surface roughness in the second section (second surface roughness) is the smallest among the first section S1 to the fourth section S4. In the third section S3, the hard coating 17 is formed on the mirror-ground substrate surface, so the surface roughness (third surface roughness) is smaller than the first surface roughness in the first section S1 and larger than the second surface roughness in the second section S2. In the fourth section S4, the normally ground substrate surface is exposed, so the surface roughness is smaller than the first surface roughness in the first section S1 and larger than the second surface roughness in the second section S2.

[0030] The arithmetic mean roughness Sa of the substrate surface in the second section S2 is preferably 0.8 or 0.5 times the arithmetic mean roughness Sa of the hard coating 17 surface in the first section S1. The arithmetic mean roughness Ra of the substrate surface in the second section S2 is preferably 0.8 or 0.5 times the arithmetic mean roughness Ra of the hard coating 17 surface in the first section S1.

[0031] In the first section S1, the arithmetic mean roughness Sa or Ra of the surface of the hard coating 17 is preferably 1.6 μm or less, or 0.4 μm or less. If the surface roughness in the first section S1 is too large, chips may be difficult to remove near the cutting edge 15, which may increase cutting resistance.

[0032] The mirror-ground substrate surface in the second section S2 preferably has a small value of the interface developed area ratio Sdr defined in ISO 25178. The smaller the value of the interface developed area ratio Sdr, the closer the measurement target area is to a flat surface. The interface developed area ratio Sdr of the mirror-ground substrate surface in the second section S2 is preferably 0.2 or less, and more preferably close to 0.

[0033] The surface roughness of the hard coating 17 in the first section S1 is a value measured on the rake face connected to the cutting edge 15. The surface roughness in the second section S2, the third section S3, and the fourth section S4 is a value measured at the deepest position of the chip discharge groove 14 near the axial center of each section. Because the boundary between sections is easily affected by the surface roughness of the adjacent section, it is preferable to measure the surface roughness at the center of each section. The arithmetic mean roughness Sa, Ra, and the developed area ratio Sdr of the interface are determined, for example, by measuring a 50 μm × 50 μm area at three locations using a laser microscope (VK-X3000) equipped with a white light interferometer manufactured by Keyence Corporation at a cutoff value of 0.25 mm and a magnification of 100x, and then averaging the measured values ​​obtained.

[0034] In the drill 1 of this embodiment, the first section S1 has a length in the axial direction of at least 0.5×D or more. This is because if the portion with low surface roughness is too close to the cutting edge 15, the hard coating 17 is more likely to wear in the portion with low surface roughness. Although not particularly limited, the length of the first section S1 in the axial direction may be 0.7×D or more, or 1.0×D or more. Furthermore, although not particularly limited, the length of the first section S1 in the axial direction may be 10×D or less, 7×D or less, or 5×D or less. The axial length of the region where the hard coating 17 is formed is preferably 10×D or less. Because the surface roughness of the region where the hard coating 17 is formed increases, the longer the hard coating 17 extends toward the rear end, the more resistance is applied to chips in the chip flute 14, and the more the cutting resistance increases.

[0035] According to the drill 1 of the first embodiment having the above-described configuration, the hard coating 17 is formed only on the tip portion of the drill 1, and the mirror-ground substrate surface is exposed in the chip discharge groove 14 in the rearward second section S2. This allows chips moving in the chip discharge groove 14 in the second section S2 to move smoothly toward the rear end. Compared to a configuration in which a hard coating is formed over the entire chip discharge groove or a configuration in which the normally ground substrate surface is exposed in the rearward chip discharge groove, chip discharge performance can be improved.

[0036] Furthermore, in the drill 1 of the first embodiment, a hard coating 17 is formed on the surface of the substrate, which is normally ground, in the first section S1 of the tip. If the surface of the hard coating is smoothed, the hard coating is partially removed or thinned, making the hard coating more susceptible to wear on the rake face and other areas that are subject to heavy loads. In contrast, in the drill 1 of the first embodiment, the hard coating 17 is provided around the cutting edge 15, where chips are generated, without being smoothed, making it less susceptible to wear.

[0037] Furthermore, because the surface of a normally ground substrate has directional striations, the surface of the hard coating in the first section S1 also has a surface shape with directional striations. This makes it easier for the lubricant to penetrate to the cutting edge 15, thereby suppressing damage to the cutting edge 15. Furthermore, in this embodiment, since only a portion of the chip discharge groove 14 needs to be mirror-ground, the process time can be shortened compared to when the entire chip discharge groove 14 is mirror-ground, and productivity in tool manufacturing can be ensured.

[0038] Furthermore, the drill 1 of the first embodiment has a third section S3 in which the hard coating 17 is formed on the mirror-ground surface of the substrate, and therefore the region in which the hard coating 17 is formed is relatively long. As the tool diameter becomes smaller, it is preferable to shorten the region in which the hard coating 17 is formed. However, if the region in which the hard coating 17 is formed is too short, it becomes impossible to ensure a sufficient hard coating thickness. In this case, by coating the hard coating 17 on a portion of the mirror-ground surface of the substrate, as in this embodiment, the hard coating 17 can be formed to a predetermined thickness even in a small-diameter drill, thereby ensuring the life of the drill.

[0039] In the first embodiment, the axial position of the trailing edge 17a of the hard coating 17 may coincide with the axial position of the leading edge 14a of the mirror-ground region of the substrate. In this case, the drill 1 of the first embodiment has a configuration in which the first section S1 and the second section S2 are adjacent to each other and does not have a third section S3. In this case, the effects of providing the first section S1 and the second section S2 can be obtained.

[0040] Second Example Fig. 5 is a diagram showing a second example of the drill 1 according to the present embodiment. The description of the parts common to the first example shown in Fig. 4 will be omitted where appropriate. As shown in Fig. 5, the hard coating 17 is formed only on the tip end portion of the base material of the drill 1, and is not formed on the rear end portion of the base material. In the first example, the hard coating 17 is formed in a region of approximately one-fifth of the body 10 from the tip.

[0041] 5, the tip end portion of the chip discharge groove 14 is ground using a normal grindstone, and the substrate surface has a surface shape with directional striations. Meanwhile, the central portion of the chip discharge groove 14 has its surface smoothed by mirror grinding using an elastic grindstone or the like. The rearmost portion of the chip discharge groove 14 is not mirror ground.

[0042] In the second embodiment, the trailing edge 17a of the hard coating 17 is located closer to the tip end than the leading edge 14a of the mirror-ground region of the substrate. That is, the region where the hard coating 17 is formed and the mirror-ground region of the substrate do not overlap each other. As a result, the wall surface of the chip discharge flute 14 of the drill 1 in the second embodiment is also divided into four sections: a first section S1, a second section S2, a third section S3, and a fourth section S4.

[0043] The first section S1 is located at the tip of the drill 1, and the wall surfaces of the chip discharge groove 14 are configured such that a hard coating 17 is formed on the surface of a normally ground substrate. The second section S2 is located in a region from the center of the drill 1 toward the rear end, and the hard coating 17 is not formed, and the surface of the mirror-ground substrate is exposed on the wall surfaces of the chip discharge groove 14. The third section S3 is located between the first section S1 and the second section S2, and the surface of the normally ground substrate is exposed on the wall surfaces of the chip discharge groove 14. The fourth section S4 is located on the rear end side of the second section S2, and the hard coating 17 is not formed, and the surface of the normally ground substrate is exposed on the wall surfaces of the chip discharge groove 14.

[0044] In the first section S1, the uneven shape due to the streaks on the normally ground substrate surface is superimposed on the isotropic undulations due to the microparticles of the hard coating 17, so the surface roughness in the first section S1 (first surface roughness) is the largest among the first section S1 to the fourth section S4. In the second section S2, the mirror-ground substrate surface is exposed, so the surface roughness in the second section (second surface roughness) is the smallest among the first section S1 to the fourth section S4. In the third section S3, the normally ground substrate surface is exposed, so the surface roughness (third surface roughness) is smaller than the first surface roughness in the first section S1 and larger than the second surface roughness in the second section S2. In the fourth section S4, the normally ground substrate surface is exposed, so the surface roughness is smaller than the first surface roughness in the first section S1 and larger than the second surface roughness in the second section S2.

[0045] In the drill 1 of the second embodiment having the above configuration, the hard coating 17 is also formed only on the tip portion of the drill 1, and the mirror-ground substrate surface is exposed in the chip discharge flute 14 in the second section S2 behind it. This results in the drill 1 having excellent chip discharge properties, as in the first embodiment. Furthermore, in the drill 1 of the second embodiment, the hard coating 17 is also formed on the normally ground substrate surface in the first section S1 of the tip portion. This makes it less likely for the hard coating 17 to wear around the cutting edge 15 where chips are generated.

[0046] In the drill 1 of the second embodiment, the normally ground substrate surface extends beyond the area where the hard coating 17 is formed toward the rear end. When the drill tool diameter is large, the thickness of the hard coating 17 can be easily maintained even if the area of ​​the hard coating 17 is relatively short, making the configuration of the second embodiment easy to apply. The drill 1 of the second embodiment can be designed with regrinding in mind. Repeated regrinding gradually moves the mirror-ground region of the substrate closer to the tip. Therefore, if the normally ground surface of the substrate is short, repeated regrinding may result in a mirror-ground surface at the tip of the drill 1, accelerating wear of the hard coating on the rake face, etc. In contrast, the normally ground region of the substrate can be lengthened in the drill 1 of the second embodiment, making it less likely that the mirror-ground surface will be located near the cutting edge 15, even after repeated regrinding.

[0047] However, in the third section S3 where the surface of the substrate is exposed in normal grinding, the surface roughness is relatively large and tends to hinder the discharge of chips. Therefore, it is preferable that the axial length of the third section S3 in the second embodiment be 2 × D or less.

[0048] According to the present invention, it is possible to provide a drill that has improved chip removal performance while ensuring the wear resistance of the hard coating near the cutting edge.

[0049] REFERENCE SIGNS LIST 1 Drill 10 Body (tool main body) 11 Shank 13 Tip flank 14 Chip discharge groove 14a Tip edge of mirror-ground area 15 Cutting edge 16 Land 16A First margin 16B Second margin 16C Relief surface 17 Hard coating 17a Trailing edge of hard coating D Tool diameter O Central axis (axis line) Ra, Sa Arithmetic mean roughness S1 First section S2 Second section S3 Third section S4 Fourth section Sdr Interface developed area ratio T Tool rotation direction

Claims

1. A drill comprising a shaft-shaped tool body which rotates about a central axis, a chip discharge groove formed on the outer periphery of the tool body, and a cutting edge formed at the intersection ridge between the tip flank of the tool body and the chip discharge groove, wherein the tool body has a substrate and a hard coating formed in a region of a predetermined length from the tip of the substrate, and the chip discharge groove has, in the central axial direction, from the tip of the tool body, a first section having a first surface roughness, and a second section which is located more rearward than the first section and has a second surface roughness smaller than the first surface roughness, the first section having a length of at least 0.5 times the tool diameter D, and the hard coating is formed in the first section, while the hard coating is not formed in the second section.

2. The drill according to claim 1, wherein the surface roughness of the base material in the second section is smaller than the surface roughness of the base material in the first section.

3. The drill according to claim 1, wherein a surface of the hard coating having directional striations is exposed in the first section, and a smooth surface of a mirror-ground substrate is exposed in the second section.

4. The drill according to claim 1, further comprising a third section between the first section and the second section, the third section having a third surface roughness smaller than the first surface roughness and larger than the second surface roughness, and the hard coating being formed on the third section.

5. The drill according to claim 1, further comprising a third section between the first section and the second section, the third section having a third surface roughness smaller than the first surface roughness and larger than the second surface roughness, and the hard coating is not formed in the third section.

6. The drill according to claim 4 or 5, wherein the surface roughness of the base material in the second section and the third section is smaller than the surface roughness of the base material in the first section.

7. The drill according to claim 4 or 5, wherein a surface of the hard coating having directional striations is exposed in the first section, and a smooth surface of the mirror-ground substrate is exposed in the second section.

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