Ball nose end mill

Abstract

A ball end mill has a main body (1), a chip flute (5), a first center groove (7), and a bottom edge (8). A second center groove (9) is formed on at least a first wall surface (7a) of the first center groove (7) at a distance from the bottom edge (8). The second center groove (9) has a minimum radius of curvature in an axial orthogonal cross section that is greater than a minimum radius of curvature of a groove bottom (7c) of the first center groove (7) in the axial orthogonal cross section.

Description

Technical Field

[0001] This invention relates to a ball end mill.

[0002] The present invention relates to a ball end mill, wherein a chip evacuation groove is formed on the outer periphery of the front end of the end mill body rotating about an axis in the direction of end mill rotation. The chip evacuation groove opens on the front flank face of the end mill body and extends toward the rear end in the axial direction. A groove-shaped center groove is formed on the front end of the chip evacuation groove in such a way that the bottom surface of the chip evacuation groove is cut toward the inner periphery of the end mill body. A bottom cutting edge is formed on the edge of the outer periphery of the front end of the wall of the center groove facing the direction of end mill rotation. The rotation trajectory of the bottom cutting edge about the axis is formed into a hemispherical shape having a center on the axis and protruding toward the front end. Background Technology

[0003] As such a ball end mill, Patent Document 1 describes the following: In a ball end mill having a ball-shaped cutting edge (bottom cutting edge) made of a single tool material, when the cutter shape is projected onto a plane including the end mill's rotation axis (axis) in a direction perpendicular to the line segment connecting the cutting edge (the bottom cutting edge) located at 45° relative to the end mill's rotation axis from the center of the ball-shaped cutting edge, the rake angle at that position is 5° or more, and the relationship between the radius R of the bottom cutting edge (center groove) arc of the end mill and the end mill's diameter D is R = (0.15 to 0.3) × D.

[0004] Patent Document 1: Japanese Patent Application Publication No. 5-042410

[0005] However, in the ball end mill described in Patent Document 1, where the bottom edge radius R of the bottom edge, which is located at 45° relative to the end mill's rotation axis, is set to R = (0.15~0.3)×D relative to the end mill's diameter D to enlarge the chip groove, and the rake angle is set to 5° or more to improve machinability, the strength may decrease due to the smaller tool angle of the bottom edge, resulting in chipping or damage in the bottom edge.

[0006] Furthermore, the center groove extends in the front flank face of the end mill body with a constant bottom edge radius and rake angle to the vicinity of the end mill's rotation center through which the axis passes. In this structure, even near the end mill's rotation center where a large cutting load is applied due to the circumferential velocity approaching zero, the strength of the bottom edge decreases, making it prone to chipping or breakage.

[0007] On the other hand, by reducing the rake angle of the bottom edge while simultaneously reducing the chip flute and increasing the tool angle of the bottom edge, the strength of the bottom edge can be ensured. However, in this structure, especially when the end mill body rotates at high speed for cutting or when cutting with a large depth of cut, chip removal is impaired, leading to chip clogging and thus increased cutting resistance.

[0008] This invention was made in this context, and its purpose is to provide a ball end mill with a long end mill life.

[0009] The present invention provides a ball end mill that ensures good chip removal to suppress the increase in cutting resistance caused by chip clogging, and suppresses the decrease in the strength of the bottom edge to prevent chipping or damage in the bottom edge. Summary of the Invention

[0010] This invention was made in this context, and its purpose is to provide a ball end mill with a long end mill life.

[0011] The present invention provides a ball end mill that ensures good chip removal to suppress the increase in cutting resistance caused by chip clogging, and suppresses the decrease in the strength of the bottom edge to prevent chipping or damage in the bottom edge.

[0012] According to one aspect of the present invention, a ball end mill includes: an end mill body that rotates about an axis in the direction of end mill rotation; a chip flute located on the outer periphery of the front end of the end mill body, opening on the front flank face of the end mill body and extending towards the rear end in the axial direction; a recessed first center groove located at the front end of the chip flute, formed by slotting the bottom surface of the chip flute towards the inner periphery of the end mill body; and a bottom cutting edge located on the edge of the first center groove on the outer periphery of the front end of a first wall surface facing the direction of end mill rotation, wherein the rotation trajectory of the bottom cutting edge about the axis is formed as a hemispherical surface having a center on the axis and protruding towards the front end. The first center groove has the first wall surface, a bottom surface facing the outer periphery of the front end of the end mill body, and a groove bottom extending between the bottom surface and the first wall surface. A recessed second center groove is formed on at least the first wall surface of the first center groove, spaced apart from the bottom cutting edge and further slotted into the first wall surface. The minimum radius of curvature of the second central groove on the cross section orthogonal to the axis is greater than the minimum radius of curvature of the bottom of the first central groove on the cross section orthogonal to the axis.

[0013] In this ball end mill, a second center groove, further grooved into the first wall surface of the recessed first center groove formed at the front end of the chip evacuation groove and facing the direction of rotation of the end mill, is formed. This second center groove is spaced apart from the bottom cutting edge on the outer periphery of the front end of the first wall surface of the first center groove facing the direction of rotation of the end mill.

[0014] Therefore, near the rotation center of the end mill or on the outer periphery of the end mill body, the tool angle of the bottom cutting edge can be largely ensured by the first wall surface of the first center groove. Furthermore, the size of the chip groove can be largely ensured by the second center groove, formed by slotting at least the first wall surface of the first center groove. Thus, it is possible to suppress the decrease in bottom cutting edge strength and the resulting chipping or breakage due to a small tool angle, and to improve chip removal performance to suppress the increase in cutting resistance caused by chip clogging.

[0015] Furthermore, the minimum radius of curvature of the second central groove on a section orthogonal to the axis, i.e., the radius of curvature of the bottom arc, is greater than the minimum radius of curvature of the bottom of the first central groove on a section orthogonal to the axis. This structure prevents stress concentration on the bottom edge due to the formation of the second central groove and ensures the rigidity of the bottom edge. As a result, the end mill's life is extended.

[0016] When the second central groove extends from the first wall of the first central groove across the bottom surface of the groove and covers the entire bottom surface, when viewed from a direction opposite to the first wall along a straight line passing through the intersection of the bottom of the groove and the second central groove and perpendicular to the first wall, the angle formed by the center of the hemisphere formed by the rotation trajectory of the bottom edge about its axis, through the intersection of the bottom of the groove and the second central groove, relative to the axis extending from the center of the bottom edge towards the front end, is preferably in the range of 40° to 85°. This angle may also be 45° or more, 50° or more, or 55° or more. Alternatively, the angle may be 80° or less, 75° or less, or 70° or less.

[0017] When the second center groove extends from the first wall of the first center groove across the bottom of the groove and covers the bottom surface, the intersection of the bottom of the groove and the second center groove becomes one of the boundary portions of the front end side of the first and second center grooves in the axial direction. If the angle formed by the straight line passing through this intersection from the center of the bottom cutting edge with respect to the axis extending from the center of the bottom cutting edge towards the front end side is less than 40°, the second center groove is too close to the rotation center of the end mill, and the bottom cutting edge may become prone to chipping or breakage.

[0018] Conversely, if the angle formed by the straight line passing through the intersection from the center of the bottom cutting edge with respect to the axis extending from the center of the bottom cutting edge toward the front end is greater than 85°, then most of the rake face of the bottom cutting edge is occupied by the first wall of the first center groove, which may make it difficult to improve chip removal.

[0019] Relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom cutting edge around the axis, the minimum radius of curvature of the second center groove on the section orthogonal to the axis is preferably in the range of 0.03×R to 0.2×R. If the minimum radius of curvature of the second center groove is less than the above range, the second center groove becomes too small, which may make it difficult to improve chip removal performance. If the minimum radius of curvature of the second center groove is greater than the above range, the first center groove is slotted too large, which may make it difficult to ensure the strength of the bottom cutting edge. Relative to the radius R, the minimum radius of curvature can also be set to 0.05×R or more, or 0.07×R or more. Relative to the radius R, the minimum radius of curvature can also be set to 0.15×R or less, or 0.12×R or less.

[0020] Furthermore, relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom cutting edge around the axis, the minimum radius of curvature of the bottom of the first central groove on a section orthogonal to the axis is preferably in the range of 0.005×R to 0.15×R. If the minimum radius of the bottom of the first central groove is less than the above range, chips may get stuck at the bottom of the groove, causing blockage. If the minimum radius of the bottom of the first central groove is greater than the above range, the strength of the bottom cutting edge at the front end in the axial direction compared to the second central groove may be compromised. The minimum radius of curvature relative to radius R can also be set to 0.01×R or more, or 0.05×R or more. The minimum radius of curvature relative to radius R can also be set to 0.12×R or less, or 0.1×R or less.

[0021] The first wall surface on the outer periphery of the front end of the end mill body, between the intersection line of the first wall surface and the second center groove and the bottom cutting edge, can be a chamfered surface. Regarding the radial width of this chamfered surface relative to the center of the hemisphere formed by the rotation trajectory of the bottom cutting edge around the axis, the width on the front end side in the axial direction is preferably greater than the width on the rear end side in the axial direction. Therefore, the strength of the bottom cutting edge can be maintained on the front end side in the axial direction where a large cutting load is applied. On the rear end side in the axial direction where the bottom cutting edge generates a large amount of chips due to its large radius of rotation around the axis, chip removal from the bottom cutting edge can be improved.

[0022] When the chamfered surface is as described above, it is preferable that the radial width of the chamfered surface relative to the center of the hemisphere formed by the rotational trajectory of the bottom edge about the axis is constant on the rear end side in the axial direction. This prevents unnecessary damage to the strength of the bottom edge on the rear end side in the axial direction of the bottom edge.

[0023] The radial width of the chamfered surface, which is constant on the rear end side in the axial direction, relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom cutting edge around the axis, is preferably in the range of 0.03×R to 0.25×R. If the width of the chamfered surface is less than 0.03×R, the strength of the bottom cutting edge may be compromised. If the width of the chamfered surface is greater than 0.25×R, the cutting resistance may increase. The width may also be 0.06×R or more, or 0.09×R or more. The width may also be 0.2×R or less, or 0.15×R or less.

[0024] Alternatively, the structure can be configured such that, when viewed from a direction opposite to the first wall along a straight line passing through the intersection of the bottom of the aforementioned groove and the second central groove and orthogonal to the first wall surface, the intersection line of the first wall surface and the second central groove on the front end side of the end mill body forms a convex curve protruding towards the front end side in the direction of the aforementioned axis. In this case, the angle formed by the tangent line originating from the center of the hemisphere formed by the rotation trajectory of the bottom cutting edge around the axis and tangent to the convex curve formed by the intersection line on the front end side of the end mill body, relative to the aforementioned axis extending from the center of the bottom cutting edge towards the front end side, is preferably in the range of 35° to 75°. This angle can also be set to 40° or more, 45° or more, or 50° or more. The angle can also be set to 70° or less, 65° or less, or 60° or less.

[0025] The aforementioned tangent at this point represents the position around the center of the bottom cutting edge, extending from the axis closer to the front end than the center of the bottom cutting edge to the front end of the chamfered surface. If the angle formed by this tangent is less than 35°, the chamfered surface is too close to the rotation center of the end mill, potentially reducing the sharpness of the bottom cutting edge. Furthermore, if the angle formed by this tangent exceeds 75°, the chamfered surface is formed from the rear end side in the axial direction, which may not improve chip removal at the front end side.

[0026] Alternatively, the structure can be configured such that the first and second center grooves are twisted toward the side opposite to the rotation direction of the end mill as they move toward the rear end of the axis. In this case, it is preferable that the twist angle of the second center groove twist line relative to the axis is greater than that of the first center groove twist line relative to the axis. Specifically, the second center groove twist line is formed at the groove bottom position where the arc of the second center groove with the smallest radius of curvature on a cross section orthogonal to the axis is closest to the axis, and the first center groove twist line is formed at the groove bottom position where the arc of the first center groove with the smallest radius of curvature on a cross section orthogonal to the axis is closest to the axis.

[0027] Therefore, the second center groove is formed such that it twists at a greater angle than the first center groove towards the rear end side in the axial direction, opposite to the rotation direction of the end mill. Consequently, on the rear end side in the axial direction, the second center groove can be formed to be deeper than the first wall surface of the first center groove. This allows for further improved chip removal. It also increases the drop from the bottom cutting edge to the second center groove, thus improving chip breaking performance.

[0028] The difference between the torsion angle of the second center groove relative to the axis and the torsion angle of the first center groove relative to the axis is preferably within the range of 2° to 15°. If the difference in torsion angle is less than 2°, it may be impossible to form a second center groove that is deeper than the first wall surface of the first center groove, thus failing to sufficiently improve chip removal or chip breaking performance. If the difference in torsion angle exceeds 15°, the second center groove becomes too deep on the rear end side, potentially reducing the strength of the bottom cutting edge or the end mill body. The difference in torsion angle can also be set to 4° or more, or 6° or more. The difference in torsion angle β-α can also be set to 12° or less, or 9° or less.

[0029] Preferably, relative to the straight line connecting the axis and the bottom position of the first center groove on a cross section that intersects the second center groove and is orthogonal to the axis when the first center groove twist line is extended toward the rear end of the axis in a state of twist angle relative to the axis, the straight line connecting the axis and the bottom position of the second center groove on the same cross section is offset in the direction of end mill rotation with the axis as the center.

[0030] That is, on the same cross section orthogonal to the axis, relative to the straight line connecting the axis and the bottom position of the first central groove, the straight line connecting the axis and the bottom position of the second central groove is offset in the direction of end mill rotation with the axis as the center, thereby creating a phase difference between the bottom positions of the first and second central grooves. Therefore, especially when machining with a large depth of cut, the bottom position of the second central groove, which produces thicker chips that bend from the bottom edge on the rear end side in the axial direction, can be moved away from the bottom edge.

[0031] Therefore, thicker chips can be guided into a larger chip-receiving groove formed by the second central groove, and effectively broken by bending it at the bottom of the groove. Thus, with this configuration, the chip-receiving groove formed by the second central groove can be effectively utilized, enabling stable chip handling even with a large cutting depth.

[0032] The angle between the straight line connecting the aforementioned axis and the bottom position of the first central groove on the aforementioned cross section and the straight line connecting the aforementioned axis and the bottom position of the second central groove is preferably within the range of 5° to 30°. If the angle is less than 5°, the bottom position of the second central groove may not be sufficiently far from the cutting edge, thus hindering the effective utilization of the chip groove. If the angle exceeds 30°, conversely, the bottom position of the second central groove is too far from the cutting edge, potentially leading to an increase in cutting resistance. The angle may also be 7° or more, 10° or more, or 12° or more. The angle may also be 25° or less, or 20° or less.

[0033] As described above, according to one aspect of the present invention, a ball end mill with a long end mill life is provided.

[0034] The ball end mill according to an embodiment of the present invention, by forming a second center groove spaced apart from the bottom cutting edge, improves chip removal even when cutting with a large depth of cut, thereby suppressing the increase in cutting resistance caused by chip clogging. The ball end mill according to an embodiment of the present invention ensures the tool angle of the bottom cutting edge, suppressing the decrease in the strength of the bottom cutting edge and suppressing the occurrence of chipping or breakage. Attached Figure Description

[0035] Figure 1 This is a perspective view showing the front end of the body of an end mill according to one embodiment of the present invention.

[0036] Figure 2 Viewed from the front side along the axis. Figure 1 The front view of the embodiment shown.

[0037] Figure 3 It is along Figure 2 The side view viewed along the X direction of the arrow in the image.

[0038] Figure 4 yes Figure 3 An enlarged side view of the front end of the end mill body.

[0039] Figure 5 yes Figure 4 VV sectional view in the image.

[0040] Figure 6 yes Figure 4 WW cross-sectional view in the image.

[0041] Figure 7 yes Figure 4 XX section view in the image.

[0042] Figure 8 yes Figure 4 YY sectional view.

[0043] Figure 9 yes Figure 4 ZZ sectional view. Detailed Implementation

[0044] Figures 1-9 This illustrates one embodiment of the present invention. In this embodiment, the end mill body 1 is formed from a hard material such as cemented carbide into a generally cylindrical shape centered on axis O. The rear end of the end mill body 1 ( Figure 1 The middle part is the upper right side. Figure 3 The middle part (right side) is set as the handle 2 that maintains a cylindrical shape, and the front end ( Figure 1 The middle section is the lower left side. Figure 3 The middle part (left side) is designated as the cutting edge 3.

[0045] In this specification and accompanying drawings, the numerical range indicated by "~" refers to the range encompassed by the values ​​recorded before and after "~" as the lower and upper limits. Furthermore, in the detailed description of the invention, constituent elements with substantially the same functional structure are omitted from repeated description by assigning the same symbols.

[0046] In the ball end mill of this embodiment, when the shank 2 is held by the spindle of the machine tool, the end mill body 1 rotates around the axis O along the end mill rotation direction T, and feeds in a direction perpendicular to the axis O or in a direction inclined to the axis O and the direction perpendicular to the axis O. Thus, the cutting edge formed on the cutting edge 3 performs groove machining or step surface machining on the workpiece.

[0047] A chip removal groove 5 is formed on the outer periphery of the cutting edge portion 3. The chip removal groove 5 opens on the front end face 4 of the cutting edge portion 3 and twists around the axis O along a side opposite to the rotation direction T of the end mill, and extends towards the rear end side in the direction of the axis O. In this embodiment, four chip removal grooves 5 are formed at intervals in the circumferential direction. These chip removal grooves 5 terminate on the outer periphery of the front end of the shank portion 2.

[0048] On the outer peripheral edge of the wall surface of these chip grooves 5 facing the rotation direction T of the end mill, an outer peripheral cutting edge 6 is formed, which serves as the rake face of the aforementioned cutting edge. In this embodiment, the outer peripheral cutting edge 6 is formed such that its rotation trajectory around the axis O is cylindrical with the axis O as its center. Similar to the chip grooves 5, the outer peripheral cutting edge 6 twists around the axis O along the side opposite to the rotation direction T of the end mill as it moves toward the rear end side in the direction of the axis O.

[0049] On the other hand, a first central groove 7 in the shape of a groove is formed on the front end of the chip removal groove 5 by slotting the wall surface of each chip removal groove 5 toward the inner peripheral side of the end mill body 1 in the direction of rotation T of the end mill and the wall surface toward the side opposite to the direction of rotation T of the end mill. The first central groove 7 includes: a first wall surface 7a, toward the direction of rotation T of the end mill; a bottom surface 7b, toward the outer peripheral side of the end mill body 1; a groove bottom 7c, extending between the first wall surface 7a and the bottom surface 7b; and a second wall surface 7d, in this embodiment, toward the side opposite to the direction of rotation T of the end mill. The width of the first central groove 7 between the first wall surface 7a and the second wall surface 7d gradually increases as it moves toward the outer peripheral side of the end mill body 1.

[0050] Furthermore, a bottom cutting edge 8 is formed on the outer peripheral edge of the first wall surface 7a, which intersects with the aforementioned front flank face 4 of the end mill body 1. The rotation trajectory of the bottom cutting edge 8 about the axis O is formed into a hemispherical shape with a center P on the axis O and protruding towards the front end. The end mill body 1 has a plurality of bottom cutting edges 8. The outer peripheral end of each bottom cutting edge 8 is smoothly connected to the front end of the aforementioned outer peripheral cutting edge 6 formed on the outer peripheral edge of the chip groove 5.

[0051] In this embodiment, as Figure 2 As shown, among the four bottom edges 8 formed, two bottom edges that are separated by one bottom edge in the circumferential direction (in... Figure 2 The bottom cutting edge 8 extending vertically is a long bottom cutting edge 8a, the front end face 4 of which extends to the vicinity of the end mill's rotation center C through which the axis O passes. The remaining two bottom cutting edges, spaced one bottom cutting edge apart in the circumferential direction (in... Figure 2 The bottom cutting edge 8 extending in the left-right direction is a short bottom cutting edge 8b. The front end face 4 of the short bottom cutting edge 8b extends from the outer peripheral end to a position that is more spaced apart from the rotation center C of the end mill compared to the long bottom cutting edge 8a.

[0052] In addition, in this embodiment, such as Figure 6 As shown, the first central groove 7 is formed on a cross section orthogonal to the axis O, with its first wall surface 7a, second wall surface 7d, and bottom surface 7b extending in a generally straight line. The groove bottom 7c is formed as a concave curve, such as a concave arc with a very small radius of curvature, tangent to the first wall surface 7a and the bottom surface 7b. Relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom edge 8 about the axis O, the minimum radius of curvature R1 of the groove bottom 7c on the cross section orthogonal to the axis O is preferably in the range of 0.005×R to 0.15×R. Relative to the radius R, the minimum radius of curvature R1 can also be set to 0.01×R or more, or 0.05×R or more. Relative to the radius R, the minimum radius of curvature R1 can also be set to 0.12×R or less, or 0.1×R or less.

[0053] In addition, such as Figure 4 As shown, a second central groove 9, in the form of a groove, is formed on at least the first wall surface 7a of the first central groove 7, spaced apart from the bottom cutting edge 8 in the circumferential direction and further grooved into the first wall surface 7a. For example... Figure 8 As shown, in this embodiment, the second center groove 9 is formed to extend from the first wall surface 7a of the first center groove 7 across the bottom 7c of the first center groove 7 that extends between the first wall surface 7a and the bottom surface 7b on a cross section orthogonal to the axis O, and then to the second wall surface 7d and connected to the front end relief face 4 adjacent to the end mill in the rotation direction T.

[0054] Here, as Figures 6-9 As shown, the bottom surface 9a of the second central groove 9 facing the outer periphery of the end mill body 1 is formed in a concave curve shape on a section orthogonal to the axis O. For example... Figure 9 As shown, the concave curve formed by the bottom surface 9a of the second central groove 9 on a section orthogonal to the axis O has a minimum radius of curvature R2 at the approximate point of tangency between the circle tangent to the bottom surface 9a with the axis O as the center and the bottom surface 9a. This minimum radius of curvature R2 is preferably in the range of 0.03×R to 0.2×R relative to the radius R. The minimum radius of curvature R2 can also be set to 0.05×R or more, or 0.07×R or more, relative to the radius R. The minimum radius of curvature R2 can also be set to 0.15×R or less, or 0.12×R or less, relative to the radius R.

[0055] Furthermore, the minimum radius of curvature R2 of the second central groove 9 on the cross section orthogonal to the axis O is greater than the minimum radius of curvature R1 of the bottom 7c of the first central groove 7 on the cross section orthogonal to the axis O.

[0056] Furthermore, a second central groove 9, formed as a concave curve in a cross-section orthogonal to axis O, extends from the first wall surface 7a of the first central groove 7 across the bottom 7c and extends to the bottom surface 7b, intersecting with the first wall surface 7a and the bottom surface 7b, which are formed as straight lines in the aforementioned cross-section. Through this structure, as... Figure 3 and Figure 4 As shown, when viewed from the direction opposite to the first wall surface 7a, the intersection line L1 of the first wall surface 7a and the second central groove 9 on the front end side of the end mill body 1, and the intersection line of the bottom surface 7b and the second central groove 9, form a convex curve-shaped mountain shape protruding towards the front end side in the direction of axis O. The intersection Q of the groove bottom 7c and the second central groove 9 is formed into a valley shape sandwiched between the mountain shape formed by the intersection line L1 of the first wall surface 7a and the bottom surface 7b.

[0057] In this embodiment, it is preferred that... Figure 4 The angles θ1 and θ2 shown are within the specified angle range. Figure 4 This diagram is taken from a direction opposite to the first wall surface 7a, along a straight line passing through the intersection Q of the bottom groove 7c and the second central groove 9 and orthogonal to the first wall surface 7a. Angle θ1 is the angle formed by the tangent M1, which originates from the center P of the bottom cutting edge 8 and is tangent to the convex curve formed by the intersection edge L1 of the first wall surface 7a and the second central groove 9 in the front end region of the end mill body 1, relative to the axis O extending from the center P of the bottom cutting edge 8 towards the front end. Angle θ1 is preferably in the range of 35° to 75°. Angle θ1 can also be set to 40° or more, 45° or more, or 50° or more. Angle θ1 can also be set to 70° or less, 65° or less, or 60° or less.

[0058] Angle θ2 is the angle formed by the straight line M2 passing through the intersection Q of the bottom edge 8, the bottom of the groove 7c, and the second central groove 9, when viewed from the direction opposite to the first wall surface 7a, relative to the axis O extending from the center P of the bottom edge 8 towards the front end. Angle θ2 is preferably in the range of 40° to 85°. Angle θ2 can also be set to 45° or more, 50° or more, or 55° or more. Angle θ2 can also be set to 80° or less, 75° or less, or 70° or less.

[0059] In the first wall surface 7a of the first center groove 7, the area radially sandwiched between the second center groove 9 and the bottom cutting edge 8 is a chamfered surface 7e. More specifically, the portion of the first wall surface 7a between the protruding end of the intersecting ridge line L1 formed by the first wall surface 7a of the first center groove 7 and the second center groove 9 in the direction of the axis O is a chamfered surface 7e, which extends from the protruding end on the outer periphery of the end mill body 1 towards the rear end and the bottom cutting edge 8 spaced apart from the second center groove 9. This chamfered surface 7e is formed by chamfering the ridge line portion of the bottom cutting edge 8 when the second center groove 9 and the front flank face 4 are extended along the rotation direction T of the end mill while remaining unchanged. Therefore, the portion of the first wall surface 7a further towards the rear end in the direction of the axis O than the aforementioned tangent M1 is this chamfered surface 7e.

[0060] Regarding the radial width H of the chamfered surface 7e relative to the center P of the bottom edge 8, the width H on the front end side in the direction of axis O is greater than the width H on the rear end side in the direction of axis O. In this embodiment, the width H of the chamfered surface 7e is widest along the tangent M1, gradually narrows towards the rear end side, and maintains a constant size with a narrow width on the rear end side in the direction of axis O.

[0061] The intersection line of the first wall surface 7a and the second center groove 9 on the outer periphery of the end mill body 1 forms a convex curve protruding towards the bottom cutting edge 8. The width H of the chamfer surface 7e gradually decreases in proportion to narrow towards the rear end on the front end side in the direction of axis O, and remains constant on the rear end side in the direction of axis O as described above. Relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom cutting edge 8 around axis O, the radial width H of the chamfer surface 7e on the rear end side of the bottom cutting edge 8, which is thus constant, relative to the center P of the bottom cutting edge 8, is preferably in the range of 0.03×R to 0.25×R. The width H may also be 0.06×R or more, or 0.09×R or more. The width H may also be 0.2×R or less, or 0.15×R or less.

[0062] Furthermore, in this embodiment, the first center groove 7 and the second center groove 9 are formed to cooperate with the twisting of the chip removal groove 5, twisting towards the side opposite to the rotation direction T of the end mill as the rear end side moves toward the axis O. Therefore, the first wall surface 7a, the second wall surface 7d, and the bottom surface 7b of the first center groove 7, which are formed in a generally straight shape on a cross section orthogonal to the axis O, are formed in a twisted shape in the direction of the axis O.

[0063] Furthermore, in this embodiment, the torsion angle β of the second central groove torsion line N2 is set to be greater than that of the second central groove torsion line N2. Figure 4 The twist angle α of the first central groove twist line N1 shown is the largest angle.

[0064] The first central groove twist line N1 is a twist line formed by connecting the bottom position 7f of the first central groove 7 along the axis O. For example... Figure 6 As shown, the bottom position 7f of the first central groove 7 is the position where the arc of the minimum radius of curvature R1 on the cross section orthogonal to the axis O is shortest from the axis O. The twist angle α is the twist angle of the twist line N1 of the first central groove relative to the axis O.

[0065] The second central groove twist line N2 is a twist line formed by connecting the bottom position 9b of the second central groove 9 along the axis O. For example... Figure 9 As shown, the bottom position 9b of the groove is the position where the arc of the second central groove 9, which forms the minimum radius of curvature R2 on the cross section orthogonal to the axis O, is closest to the axis O. The twist angle β is the twist angle of the twist line N2 of the second central groove relative to the axis O.

[0066] In this embodiment, the difference β-α between the torsion angle β of the second central groove torsion line N2 relative to the axis O and the torsion angle α of the first central groove torsion line N1 relative to the axis O is preferably in the range of 2° to 15°. The difference β-α can also be set to 4° or more, or 6° or more. Alternatively, the difference β-α can be set to 12° or less, or 9° or less.

[0067] Furthermore, in this embodiment, such as Figure 9 As shown, lines F1 and F2, which define the bottom positions of the center grooves on orthogonal sections of the same axis, are offset about axis O in the direction of end mill rotation T. Lines F1 and F2 are defined as follows: Figure 9 On the cross section shown, which intersects with the second central groove 9 and is orthogonal to the axis O.

[0068] The straight line F1 is the straight line connecting the bottom position 7f of the first groove on the cross section with the axis O on the cross section when the first central groove torsion line N1 is extended towards the rear end of the axis O with a torsion angle α relative to the axis O.

[0069] The straight line F2 is a straight line that connects the axis O on the same cross section as the defined straight line F1 and the aforementioned groove bottom position 9b of the second central groove 9.

[0070] In this embodiment, such as Figure 9 As shown, the phase difference between these straight lines F1 and F2, that is, the intersection angle γ between the straight line F1 connecting the axis O and the bottom position 7f of the first central groove 7 and the straight line F2 connecting the axis O and the bottom position 9b of the second central groove 9 on the above-mentioned cross section, is preferably in the range of 5° to 30°. The intersection angle γ can also be set to 7° or more, 10° or more, or 12° or more. The intersection angle γ can also be set to 25° or less or 20° or less.

[0071] In the ball end mill configured in this way, a second center groove 9 is formed on the first wall surface 7a of the groove-shaped first center groove 7 at least in the direction of rotation T of the end mill, in such a way that the first wall surface 7a of the first center groove 7 is further grooved. The second center groove 9 is spaced apart from the bottom cutting edge 8 formed on the edge portion of the front outer periphery of the first wall surface 7a.

[0072] Therefore, the size of the chip-receiving groove for accommodating chips can be largely ensured by the second center groove 9 formed by slotting the first wall surface 7a of the first center groove 7. On the other hand, the tool angle of the bottom cutting edge 8 can also be largely ensured by the first wall surface 7a of the first center groove 7 near the rotation center C of the end mill or on the outer peripheral side of the end mill body 1. Therefore, the decrease in strength of the bottom cutting edge 8 due to a small tool angle can be prevented, and the chip removal performance can be improved to suppress the increase in cutting resistance caused by chip clogging.

[0073] Furthermore, in this embodiment, the second central groove 9 extends beyond the groove bottom 7c of the first central groove 7, which lies between the first wall surface 7a facing the rotation direction T of the end mill and the bottom surface 7b facing the outer periphery of the end mill body 1, and further reaches the second wall surface 7d facing the side opposite to the rotation direction T of the end mill. Therefore, the chip-receiving capacity based on the second central groove 9 can be further increased, thereby achieving good chip removal performance.

[0074] Furthermore, the minimum radius of curvature R2 of the second center groove 9 on the section orthogonal to axis O is greater than the minimum radius of curvature R1 of the first center groove 7, i.e., the minimum radius of curvature R1 of the groove bottom 7c on the section orthogonal to axis O. Therefore, even with the formation of the second center groove 9 having a large radius of curvature at the bottom, the tip strength of the bottom cutting edge 8 can be ensured because the radius of curvature of the groove bottom 7c at the tool tip is small. Thus, the rigidity of the bottom cutting edge 8 can be ensured while preventing stress concentration at the bottom of the groove due to the formation of the second center groove 9. As a result, the end mill life can be extended.

[0075] Furthermore, in this embodiment, as described above, when the second center groove 9 extends from the first wall surface 7a of the first center groove 7 across the groove bottom 7c and covers the bottom surface 7b facing the outer periphery of the end mill body 1, a straight line M2 is defined that runs from the center P of the bottom cutting edge 8 through the groove bottom 7c and intersects with the second center groove 9 at the intersection Q. In this case, the angle θ2 formed by the straight line M2 relative to the axis O extending from the center P of the bottom cutting edge 8 towards the front end is preferably in the range of 40° to 85°. If the angle θ2 is within the above range, further good chip removal can be ensured, and chipping or damage to the bottom cutting edge 8 can be reliably prevented.

[0076] That is, if the angle θ2 formed by the straight line M2 passing through the intersection Q from the center C of the bottom cutting edge 8 with respect to the axis O extending from the center P of the bottom cutting edge 8 towards the front end is less than 40°, then the second center groove 9 will be closer to the rotation center C of the end mill, and the wall thickness of the end mill's front end will decrease. The bottom cutting edge 8 may become more prone to chipping or breakage.

[0077] On the other hand, if the angle θ2 formed by the straight line M2 passing through the intersection Q from the center P of the bottom cutting edge 8 with respect to the axis O extending from the center P of the bottom cutting edge 8 toward the front end is greater than 85°, then most of the rake face of the bottom cutting edge 8 is occupied by the first wall surface 7a of the first central groove 7, which may make it difficult to improve chip removal.

[0078] Furthermore, in this embodiment, the minimum radius of curvature R2 of the second central groove 9 on a section orthogonal to the axis O is preferably in the range of 0.03×R to 0.2×R, relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom cutting edge 8 around the axis O. This ensures even better chip removal performance. Furthermore, chipping or breakage of the bottom cutting edge 8 is further suppressed.

[0079] That is, if the minimum radius of curvature R2 of the second central groove 9 is less than the above range, the space at the bottom of the second central groove 9 becomes smaller, which may make it difficult to improve chip removal. On the other hand, conversely, if the minimum radius of curvature R2 of the second central groove 9 is greater than the above range, the first central groove 7 is slotted more extensively. If the wall thickness on the tool tip side becomes too thin, the strength of the bottom cutting edge 8 may not be guaranteed.

[0080] Furthermore, in this embodiment, relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom cutting edge 8 around the axis O, the minimum radius of curvature R1 of the groove bottom 7c of the first central groove 7 on the cross section orthogonal to the axis O is preferably in the range of 0.005×R to 0.15×R. This ensures good chip removal. Furthermore, chipping or damage to the bottom cutting edge 8 is further suppressed.

[0081] That is, if the minimum radius R1 of the bottom 7c of the first central groove 7 is less than the above range, chips may get stuck at the bottom 7c and cause blockage. On the other hand, conversely, if the minimum radius R1 of the bottom 7c of the first central groove 7 is greater than the above range, the wall thickness on the front side closer to the axis O than that of the second central groove 9 becomes thinner. If the wall thickness on the front side of the tool becomes too thin, the strength of the bottom cutting edge 8 may be damaged, resulting in chipping or breakage.

[0082] Furthermore, the first wall surface 7a of the first central groove 7 on the outer periphery of the front end of the end mill body 1, which intersects with the bottom surface 9a of the second central groove 9 and the bottom cutting edge 8, is a chamfered surface 7e as described above. This chamfered surface 7e is formed by chamfering the portion of the bottom cutting edge 8 where the second central groove 9 and the front flank face 4 intersect, extending them along the rotation direction T of the end mill while keeping them unchanged. In this embodiment, the width H of the chamfered surface 7e radially relative to the center P of the bottom cutting edge 8 is greater on the front end side in the direction of axis O than on the rear end side in the direction of axis O.

[0083] Therefore, according to this embodiment, the strength of the bottom cutting edge 8 can be maintained on the front end side in the direction of axis O where a large cutting load is applied. Furthermore, good chip removal can be promoted on the rear end side of the bottom cutting edge 8, where a large amount of chips are generated due to the large radius of rotation around axis O.

[0084] Furthermore, in this embodiment, when the radial width H of the chamfered surface 7e relative to the center P of the bottom cutting edge 8 is greater on the front end side in the direction of axis O than on the rear end side in the direction of axis O, the radial width H of the chamfered surface 7e relative to the center P of the bottom cutting edge 8 remains constant in the rear end side portion in the direction of axis O. Therefore, the width H of the chamfered surface 7e in the rear end side portion in the direction of axis O does not become excessively narrow, thus preventing unnecessary damage to the strength of the bottom cutting edge 8.

[0085] Furthermore, in this embodiment, the radial width H of the chamfered surface 7e on the rear end side of the bottom cutting edge 8 relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom cutting edge 8 around the axis O is preferably in the range of 0.03×R to 0.25×R. This maintains the strength of the bottom cutting edge 8 and prevents an increase in cutting resistance.

[0086] That is, if the radial width H of the chamfered surface 7e on the rear end side of the bottom cutting edge 8 relative to the center P of the bottom cutting edge 8 is less than 0.03×R, the chamfered surface 7e becomes too small and the strength of the bottom cutting edge 8 may be compromised. On the other hand, if the width H is too large and exceeds 0.25×R, the chamfered surface 7e becomes too large and may lead to an increase in cutting resistance.

[0087] Furthermore, in this embodiment, when viewed from the direction opposite to the first wall surface 7a of the first central groove 7 along a straight line passing through the intersection Q of the groove bottom 7c and the second central groove 9 and orthogonal to the first wall surface 7a, the tangent line M1 that is tangent to the convex curve L1 formed by the intersection of the first wall surface 7a on the front end side of the end mill body 1 and the bottom surface 9a of the second central groove 9, which protrudes towards the front end side, preferably intersects the axis O extending from the center P of the bottom cutting edge 8 towards the front end side at an angle θ1 within the range of 35° to 75°. By setting the angle θ1 within the above range, the sharpness of the bottom cutting edge 8 can be maintained, and good chip removal can be achieved.

[0088] That is, if the angle θ1 formed by the tangent M1 relative to the axis O extending from the center P of the bottom cutting edge 8 towards the front end is less than 35°, the chamfered surface 7e will extend to the vicinity of the end mill's rotation center C, potentially reducing the sharpness of the bottom cutting edge 8. On the other hand, if the angle θ1 formed by the tangent M1 relative to the axis O extending from the center P of the bottom cutting edge 8 towards the front end exceeds 75°, the chamfered surface 7e will be formed too far from the rear end side in the direction of the axis O, potentially failing to improve chip removal on the front end side.

[0089] On the other hand, in this embodiment, the first center groove 7 and the second center groove 9 are formed in the same way as the chip removal groove 5, twisting towards the side opposite to the rotation direction T of the end mill as they face the rear end side in the direction of axis O. Therefore, as the end mill body 1 rotates during cutting, the chips generated by the bottom cutting edge 8 are fed into the chip removal groove 5 on the rear end side and discharged, thus achieving good chip removal performance.

[0090] Furthermore, in this embodiment, the first central groove 7 and the second central groove 9 are formed with different torsion angles relative to each other, such that the torsion angle β of the second central groove torsion line N2 relative to the axis O is greater than the torsion angle α of the first central groove torsion line N1 relative to the axis O. The second central groove torsion line N2 is formed by the groove bottom position 9b of the arc that forms the minimum radius of curvature R2 on the cross section orthogonal to the axis O, which is connected to the second central groove 9 along the axis O direction. The first central groove torsion line N1 is formed by the groove bottom position 7f of the arc that forms the minimum radius of curvature R1 on the cross section orthogonal to the axis O, which is connected to the groove bottom 7c of the first central groove 7 along the axis O direction.

[0091] Therefore, the second center groove 9 is formed such that it twists relative to the axis O at a twist angle β that is larger than the twist angle α of the first center groove 7, while simultaneously moving towards the rear end side in the direction of the axis O and toward the side opposite to the rotation direction T of the end mill. As a result, on the rear end side in the direction of the axis O, the second center groove 9 can be formed to be deeper than the first wall surface 7a of the first center groove 7.

[0092] Therefore, according to this embodiment, the capacity of the chip-receiving groove based on the second central groove 9 can be further increased to achieve further improved chip removal performance. Furthermore, the drop from the bottom cutting edge 8 to the bottom surface 9a of the second central groove 9 can also be increased to improve chip breaking performance.

[0093] In this embodiment, the difference β-α between the torsion angle β of the second center groove torsion line N2 relative to the axis O and the torsion angle α of the first center groove torsion line N1 relative to the axis O is preferably within the range of 2° to 15°. By setting the difference β-α within the above range, chip removal or chip breaking performance can be further reliably improved, and the strength of the bottom cutting edge 8 or the end mill body 1 can be prevented from decreasing.

[0094] That is, if the difference between the torsion angles β and α, β-α, is less than 2°, it may be impossible to form a second central groove 9 that is deeper than the first wall surface 7a of the first central groove 7, thus failing to sufficiently improve chip removal or chip breaking performance. On the other hand, if the difference between the torsion angles β and α, β-α, is greater than 15°, the second central groove 9 will be engraved too deeply on the rear end side in the direction of axis O, which may reduce the strength of the bottom cutting edge 8 or the cutting edge 3 of the end mill body 1.

[0095] On the other hand, in this embodiment, relative to the straight line F1 that connects the axis O and the bottom position 7f of the first center groove 7 on the cross section that intersects the second center groove 9 and is orthogonal to the axis O when the aforementioned first center groove torsion line N1 is extended toward the rear end side in the direction of the axis O with a torsion angle α relative to the axis O, the straight line F2 that connects the axis O and the bottom position 9b of the second center groove 9 on the same cross section is misaligned with the axis O in the end mill rotation direction T.

[0096] Therefore, according to this embodiment, the straight line F2 connecting the axis O and the bottom position 9b of the second central groove 9 is offset from the straight line F1 connecting the axis O and the bottom position 7f of the first central groove 7 in the end mill rotation direction T, with the axis O as the center. That is, a phase difference is set between the bottom position 7f of the first central groove 7 and the bottom position 9b of the second central groove 9. Especially when cutting with a large depth of cut, the thicker chips generated by the bottom cutting edge 8 on the rear end side in the direction of the axis O bend at the bottom position 9b of the second central groove 9. According to the above structure, the position where the chips bend, i.e., the bottom position 9b, can be moved away from the bottom cutting edge 8.

[0097] Therefore, thicker chips generated by a larger cutting depth can be guided into a larger chip groove formed by the second central groove 9, and bent at the bottom position 9b of the groove, thereby effectively breaking them apart. Thus, in this embodiment, the chip groove formed by the second central groove 9 can be effectively utilized. As described above, even with a large cutting depth, stable processing of thicker chips can be promoted.

[0098] Furthermore, in this case, in this embodiment, the intersection angle γ of the straight line F1 connecting the axis O and the bottom position 7f of the first central groove 7 and the straight line F2 connecting the axis O and the bottom position 9b of the second central groove 9 on the aforementioned cross section is preferably within the range of 5° to 30°. By keeping the intersection angle γ within the aforementioned range, the effective utilization of the chip groove based on the second central groove 9 as described above can be reliably achieved, while preventing unnecessary increases in cutting resistance caused by chip clogging, etc.

[0099] That is, if the cross angle γ is less than 5°, the bottom position 9b of the second central groove 9 may not be able to move sufficiently away from the bottom cutting edge 8, thus failing to achieve effective utilization of the chip groove based on the second central groove 9. On the other hand, conversely, if the cross angle γ exceeds 30°, the bottom position 9b of the second central groove 9 will be too far away from the bottom cutting edge 8, causing the chips to become longer and rub against the bottom surface 9a of the second central groove 9, resulting in chip clogging and potentially increasing cutting resistance.

[0100] Based on the above implementation methods, the following methods can also be mastered.

[0101] (1) A ball end mill, comprising: an end mill body that rotates about an axis in the direction of rotation of the end mill; a chip flute located on the outer periphery of the front end of the end mill body and opening on the front flank face of the end mill body and extending toward the rear end side in the axial direction; a groove-shaped first center groove located at the front end of the chip flute and formed by slotting the lower bottom surface of the chip flute toward the inner periphery of the end mill body; and a bottom cutting edge located on the edge portion of the first center groove on the outer periphery of the front end of a first wall surface facing the direction of rotation of the end mill, wherein the rotation trajectory of the bottom cutting edge about the axis is formed as a hemispherical surface having a center on the axis and protruding toward the front end side.

[0102] A second central groove, which is spaced apart from the bottom edge and further grooved into the first wall surface, is formed in the shape of a groove.

[0103] The first wall surface, between the intersection line of the first wall surface and the second center groove on the outer periphery of the front end of the end mill body and the bottom cutting edge, is a chamfered surface.

[0104] With respect to the radial width of the chamfered surface relative to the center of the hemisphere formed by the rotational trajectory of the bottom edge about the axis, the width on the front end side in the axial direction is greater than the width on the rear end side in the axial direction.

[0105] (2) A ball end mill, comprising: an end mill body that rotates about an axis in the direction of rotation of the end mill; a chip flute located on the outer periphery of the front end of the end mill body and opening on the front flank face of the end mill body and extending toward the rear end side in the axial direction; a groove-shaped first center groove located at the front end of the chip flute and formed by slotting the lower bottom surface of the chip flute toward the inner periphery of the end mill body; and a bottom cutting edge located on the edge portion of the first center groove on the outer periphery of the front end of a first wall surface facing the direction of rotation of the end mill, wherein the rotation trajectory of the bottom cutting edge about the axis is formed as a hemispherical surface having a center on the axis and protruding toward the front end side.

[0106] The first central groove has the first wall surface, a bottom surface facing the outer periphery of the front end of the end mill body, and a groove bottom extending between the bottom surface and the first wall surface.

[0107] A second central groove, which is spaced apart from the bottom edge and further grooved into the first wall surface, is formed in the shape of a groove.

[0108] The first center groove and the second center groove are formed to twist toward the side opposite to the rotation direction of the end mill as they move toward the rear end side toward the axial direction.

[0109] The twist angle of the second central groove twist line relative to the axis is greater than that of the first central groove twist line relative to the axis. The second central groove twist line is formed by connecting the second central groove along the axial direction to the groove bottom position where the arc with the smallest radius of curvature on the cross section orthogonal to the axis is the shortest distance from the axis. The first central groove twist line is formed by connecting the bottom of the first central groove along the axial direction to the groove bottom position where the arc with the smallest radius of curvature on the cross section orthogonal to the axis is the shortest distance from the axis.

[0110] Symbol Explanation

[0111] 1 End mill body

[0112] 2 handles

[0113] 3 Cutting edge

[0114] 4. Front flank face

[0115] 5 chip removal slots

[0116] 6 peripheral blades

[0117] 7. First central slot

[0118] 7a First wall of the first central groove 7

[0119] 7b Bottom surface of the first central groove 7

[0120] 7c The bottom of the first central groove 7

[0121] The second wall of the first central groove 7d

[0122] 7e chamfered surface

[0123] The bottom position of the first central groove 7f

[0124] 8-inch bottom edge

[0125] 8a Long Bottom Edge

[0126] 8b short bottom edge

[0127] 9. Second central slot

[0128] 9a Bottom surface of the second central groove 9

[0129] 9b The bottom position of the second central groove 9

[0130] The axis of the end mill body 1

[0131] T-end mill rotation direction

[0132] C-end mill rotation center

[0133] The center of the hemisphere formed by the rotation trajectory of the bottom edge 8 around axis O.

[0134] The intersection of the bottom 7c of the first central groove 7 and the second central groove 9.

[0135] The radius of the hemisphere formed by the rotation trajectory of the bottom edge 8 around axis O

[0136] The minimum radius of curvature of the bottom 7c of the first central groove 7 of R1 on a section orthogonal to axis O.

[0137] The minimum radius of curvature of the bottom surface 9a of the second central groove 9 of R2 on the section orthogonal to the axis O.

[0138] The radial width of chamfered face 7e relative to the center P of the bottom edge 8

[0139] The intersection line of the first wall surface 7a and bottom surface 7b on the front end side of the L1 end mill body 1 with the second central groove 9

[0140] M1 is a tangent line originating from the center P of the bottom edge 8 and tangent to the convex curve formed by the intersection line L1 of the first wall surface 7a and the second central groove 9.

[0141] The angle formed by θ1 tangent M1 relative to the axis O on the front side, which is closer to the center P of the bottom edge 8.

[0142] M2 is a straight line from the center P of the bottom edge 8 through the intersection Q of the bottom groove 7C and the second center groove 9.

[0143] The angle formed by line M2 relative to the axis O on the front side, which is closer to the center P of the bottom edge 8.

[0144] N1 First Central Groove Torsion Line

[0145] N2 Second Central Groove Twist Line

[0146] α The twist angle of the first central groove twist line N1 relative to the axis O

[0147] The twist angle of the second central groove twist line N2 relative to the axis O

[0148] F1 will be the straight line connecting axis O on the cross section that intersects with the second central slot 9 and is orthogonal to axis O, and the bottom position 7f of the first central slot 7.

[0149] F2 will connect the axis O on the same cross section as straight line F1 and the bottom position 9b of the second central groove 9.

[0150] γ The angle of intersection of lines F1 and F2

Claims

1. A ball end mill, comprising: The end mill body rotates around its axis in the direction of rotation of the end mill; The chip removal groove is located on the outer periphery of the front end of the end mill body, and opens on the front flank face of the end mill body and extends to the rear end side in the axial direction. A first, recessed central groove is located at the front end of the chip removal groove, and the bottom surface of the chip removal groove is slotted into the inner circumferential side of the end mill body; and The bottom cutting edge is located on the outer periphery of the front end of the first wall surface of the first central groove facing the rotation direction of the end mill. The rotation trajectory of the bottom cutting edge about the axis is formed into a hemispherical shape with a center on the axis and convex towards the front end. The first central groove has the first wall surface, a bottom surface facing the outer periphery of the front end of the end mill body, and a groove bottom extending between the bottom surface and the first wall surface. A second central groove, which is spaced apart from the bottom edge and further grooved into the first wall surface, is formed in the shape of a groove. The minimum radius of curvature of the second central groove on the cross section orthogonal to the axis is greater than the minimum radius of curvature of the bottom of the first central groove on the same cross section.

2. The ball end mill according to claim 1, wherein, The second central groove extends from the first wall surface of the first central groove, across the bottom of the groove, and covers the entire bottom surface. When viewed from a direction opposite to the first wall along a straight line passing through the intersection of the bottom of the first central groove and the second central groove and orthogonal to the first wall, the angle formed by the straight line passing through the intersection from the center of the hemisphere formed by the rotation trajectory of the bottom edge about the axis is in the range of 40° to 85° with respect to the axis extending from the center of the bottom edge towards the front end.

3. The ball end mill according to claim 1 or 2, wherein, The minimum radius of curvature of the second central groove on a section orthogonal to the axis is in the range of 0.03×R to 0.2×R, relative to the radius R of the hemisphere formed by the rotational trajectory of the bottom edge around the axis.

4. The ball end mill according to claim 1 or 2, wherein, The minimum radius of curvature of the bottom of the first central groove in a section orthogonal to the axis is in the range of 0.005×R to 0.15×R, relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom edge about the axis.

5. The ball end mill according to claim 1 or 2, wherein, The first wall surface on the outer periphery of the front end of the end mill body, between the intersection line of the first wall surface and the second central groove and the bottom cutting edge, is a chamfered surface. With respect to the radial width of the chamfered surface relative to the center of the hemisphere formed by the rotational trajectory of the bottom edge about the axis, the width on the front end side in the axial direction is greater than the width on the rear end side in the axial direction.

6. The ball end mill according to claim 5, wherein, The radial width of the chamfered surface relative to the center of the hemisphere formed by the rotational trajectory of the bottom edge about the axis is constant on the rear end side in the axial direction.

7. The ball end mill according to claim 6, wherein, The radial width of the chamfered surface, which is constant on the rear end side in the axial direction, relative to the radius R of the hemisphere formed by the rotation trajectory of the bottom edge around the axis, is in the range of 0.03×R to 0.25×R.

8. The ball end mill according to claim 5, wherein, When viewed from a direction opposite to the first wall along a straight line passing through the intersection of the bottom of the groove and the second central groove and orthogonal to the first wall, the intersection line of the first wall and the second central groove on the front end side of the end mill body forms a convex curve protruding towards the front end side in the axial direction. The angle between the tangent line formed by the center of the hemisphere formed by the rotation trajectory of the bottom cutting edge around the axis and the convex curve formed by the intersecting edge line on the front end side of the end mill body and the axis extending towards the front end side from the center of the hemisphere formed by the rotation trajectory of the bottom cutting edge around the axis is in the range of 35° to 75°.

9. The ball end mill according to claim 1 or 2, wherein, The first center groove and the second center groove are formed to twist toward the side opposite to the rotation direction of the end mill as they move toward the rear end side toward the axial direction. The twist angle of the second central groove twist line relative to the axis is greater than that of the first central groove twist line relative to the axis. The second central groove twist line is formed by connecting the second central groove along the axial direction to the groove bottom position where the arc with the smallest radius of curvature on the cross section orthogonal to the axis is shortest from the axis. The first central groove twist line is formed by connecting the bottom of the first central groove along the axial direction to the groove bottom position where the arc with the smallest radius of curvature on the cross section orthogonal to the axis is shortest from the axis.

10. The ball end mill according to claim 9, wherein, The difference between the torsion angle of the second central groove torsion line relative to the axis and the torsion angle of the first central groove torsion line relative to the axis is in the range of 2° to 15°.

11. The ball end mill according to claim 9, wherein, Compared to the straight line connecting the axis of the first central groove to the bottom position of the first central groove on a cross section that intersects the second central groove and is orthogonal to the axis when the twist line of the first central groove is extended toward the rear end of the axis in a state of twist angle relative to the axis, the straight line connecting the axis of the first central groove to the bottom position of the second central groove on the same cross section is offset in the direction of rotation of the end mill with the axis as the center.

12. The ball end mill according to claim 11, wherein, The angle between the straight line connecting the axis and the bottom of the first central groove on the cross section and the straight line connecting the axis and the bottom of the second central groove is in the range of 5° to 30°.

Images