Ball end mill

Abstract

To provide a ball end mill which can improve quality of a processed surface.SOLUTION: In a ball end mill 1 with two blades, cutting blades 51, 52 are formed at an intersection ridge line between groove wall surfaces 81a, 82a of gash grooves 81, 82 and flanks 61, 62, and the flanks 61, 62 have first flanks 61a, 62a forming a chisel edge 10a, and second flanks 61b, 62b which extend in a rotation direction rearward from the first flanks 61a, 62a at a larger relief angle. When being viewed from an axial tip side, if an intersection point between one cutting blade 51, 52 of two cutting blades and the chisel edge 10a is so made as to be a first intersection point, a straight line, which is orthogonal to a cutting blade tangential line at the first intersection point and passes an axis line is so made as to be a reference line, and an intersection point between a flank boundary line between the first flank and the second flank of the other cutting blade of the blades and the groove wall surface is so made as to a second intersection point when viewed from an axial tip side, the second intersection point is positioned on a rotation angle front side with respect to the reference line, and the flank boundary line of the other cutting blade crosses a cutting blade tangential line of one cutting blade.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a ball end mill. [Background technology]

[0002] Most of the chips generated by the cutting edge of a ball end mill are smoothly discharged from the gash groove adjacent to the cutting edge. However, chips generated near the rotation axis where the chisel edge is formed tend to become trapped in a space closed by the chisel edge, flank face, and machining surface. Patent Document 1 discloses that when machining extremely hard materials such as cemented carbide, chip clogging near the rotation axis can lead to damage near the center of the tool. Therefore, as a way to improve chip discharge from the inner end of the bottom cutting edge, a configuration is disclosed in which the inner end of the bottom cutting edge is made to be the starting point of the groove wall surface of the gash groove. Furthermore, Patent Documents 2 and 3 disclose that in order to obtain good machined surface roughness, the cutting edge thickness angle, chisel width, chisel angle, etc. are set within predetermined ranges, and a flank adjacent to the arc-shaped cutting edge is provided with a relatively small predetermined flank angle and flank width. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-83245 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-342835 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-088232 Summary of the Invention [Problem to be solved by the invention]

[0004] Even if the groove wall shape of the gash groove is devised as in Patent Document 1, when machining extremely hard workpieces such as cemented carbide, chip discharge performance sufficient to prevent chipping near the rotation axis due to clogging of extremely hard chips can be obtained, but in finish machining where high machined surface quality is desired, welding can occur on the machined surface, which can deteriorate the machined surface quality. Also, even if the shape of the chisel edge, the clearance angle and width of the flank face, etc. are adjusted as in Patent Documents 2 and 3, welding or gouging can occur on the machined surface.

[0005] The present invention was made against this background, and one of its objects is to provide a ball end mill that can improve the quality of the machined surface in a ball end mill that forms a chisel edge. [Means for solving the problem]

[0006] According to one aspect of the present invention, A ball end mill having two cutting blades and an end mill body that is rotated around an axis, the cutting edge is formed on an intersection ridge between a groove wall surface of the gash groove and a flank located rearward in the rotational direction of the gash groove, the flank having a first flank forming a chisel edge and a second flank extending rearward in the rotational direction from the first flank at a larger flank angle; When viewed from the axial tip side, an intersection point between one of the cutting edges and the chisel edge is defined as a first intersection point, a straight line perpendicular to the cutting edge tangent at the first intersection point and passing through the axis is defined as a reference line, and an intersection point between a flank boundary line between the first flank and the second flank of the other of the cutting edges and the groove wall surface is defined as a second intersection point. the second intersection is located forward of the reference line in the direction of rotation, The flank boundary line of the other cutting edge intersects with the cutting edge tangent line of the one cutting edge. A ball end mill is provided.

[0007] That is, the area (chisel edge front area) surrounded by the chisel edge, the cutting edge located forward in the direction of rotation of the chisel edge, and the reference line is composed only of the first flank and does not include the boundary between the first flank and the second flank (flank boundary line). With this configuration, an edge portion that is convex toward the machining surface is not formed in the chisel edge front area, which is near the area where chips are generated during cutting and is prone to clogging. This makes it difficult for chips to be pressed against the machining surface near the rotation axis, thereby suppressing the occurrence of welding. At the same time, the cutting edge (axial cutting edge) that defines the chisel edge front region is originally close to the rotation axis, making it difficult to increase cutting speed, but by configuring the chisel edge front region from only the first flank, the cutting angle of the axial cutting edge becomes relatively smaller compared to when the second flank is present in the chisel edge front region. This improves the cutting ability of the axial cutting edge and also suppresses the occurrence of gouging. Furthermore, by having the flank boundary line of one cutting edge intersect with the cutting edge tangent at the intersection (first intersection) with the chisel edge of the other cutting edge, the width of the first flank of the cutting edge located on the outer periphery in the direction perpendicular to the blade can be narrowed, thereby preventing cutting resistance from becoming greater than necessary and ensuring the cutting performance of the cutting edge that mainly performs cutting. The synergistic effect of suppressing welding and gouging, and good cutting performance at the cutting edge, can comprehensively improve the finished surface properties (roughness and quality) in finishing processes.

[0008] When viewed from the axial tip side, the tip surface may have a portion in which the total flank width in the extension direction of the reference line between the first flank of the one cutting edge and the first flank of the other cutting edge, which are adjacent to each other at the chisel edge, decreases from the second intersection point toward the radial outer periphery. When viewed from the axial tip side, the flank boundary line may have a portion that extends from the second intersection point toward the radial outer periphery in a direction approaching the chisel edge. When viewed from the axial tip side, the rake angle in the direction perpendicular to the cutting edge of the rake face of the axial side cutting edge, which is located radially outer than the reference line and radially axially closer than the first intersection point, may be a negative angle. The rake angle may be equal to or greater than -30° and equal to or less than -15°, and the clearance angle of the first flank may be equal to or greater than 5° and less than 10°. [Effects of the Invention]

[0009] According to one aspect of the present invention, a ball end mill capable of improving the quality of the machined surface is provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a side view showing a ball end mill according to an embodiment. [Figure 2] FIG. 2 is a partial side view showing the tip portion of the ball end mill according to the embodiment. [Figure 3] FIG. 3 is a plan view of the tip surface of the ball end mill as viewed in the axial direction. [Figure 4] FIG. 4 is an enlarged plan view showing the vicinity of the chisel of the first embodiment. [Figure 5] FIG. 5 is an enlarged plan view showing the vicinity of the chisel of the second embodiment. [Figure 6] FIG. 6 is a partial cross-sectional view of the chisel portion taken along the reference line L in FIGS. [Figure 7] FIG. 7 is a reference diagram showing the chisel edge region of another configuration for comparison. [Figure 8] FIG. 8 is a reference diagram showing a cross section taken along the reference line L shown in FIG. [Figure 9] FIG. 9 is a photograph of the processed surface in the example and the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] First Embodiment Fig. 1 is a side view showing a ball end mill of an embodiment. Fig. 2 is a partial side view showing the tip portion of the ball end mill of an embodiment. Fig. 3 is a plan view of the tip surface of the ball end mill as seen in the axial direction. Fig. 4 is an enlarged plan view showing the vicinity of the chisel of the first embodiment.

[0012] The ball end mill 1 of this embodiment is integrally formed from a base metal made of a hard material such as cemented carbide into a multi-stage, approximately cylindrical shape centered on axis O, as shown in FIG. 1. The rear end (upper portion in FIGS. 1 and 2) of the ball end mill 1 is a large-diameter, cylindrical shank 2. The front end (lower portion in FIGS. 1 and 2) of the ball end mill 1 is an approximately cylindrical end mill body 3 with a smaller diameter than the shank 2. The shank 2 and the end mill body 3 are connected by a tapered neck 4 that is a tapered truncated cone centered on axis O. In this specification, the direction parallel to the axis O is sometimes simply referred to as the "axial direction," the direction perpendicular to the axis O is sometimes simply referred to as the "radial direction," and the circumferential direction centered on the axis O is sometimes simply referred to as the "circumferential direction." Furthermore, when viewed in the axial direction, the outer side in the radial direction is sometimes referred to as the outer circumferential side, and the inner side in the radial direction is sometimes referred to as the inner circumferential side or the axial center side.

[0013] The ball end mill 1 has a shank 2 that is held by the spindle of a machine tool and rotated around an axis O in the end mill rotation direction T, while being fed in a direction intersecting the axis O, whereby cutting is performed on a workpiece by cutting edges 51, 52 formed on the end mill body 3. In other words, the rotation axis of the ball end mill 1 coincides with the axis O.

[0014] The ball end mill 1 is a two-blade ball end mill having two cutting edges 51 and 52. The cutting edge 51 has a bottom cutting edge 51a and a peripheral cutting edge 51b. The cutting edge 52 has a bottom cutting edge 52a and a peripheral cutting edge 52b.

[0015] Two chip discharge grooves 71, 72 are formed on the outer periphery of the end mill body 3 in a rotational symmetry with respect to the axis O. The chip discharge grooves 71, 72 open on the tip flanks 61, 62 located at the tip of the ball end mill 1 and extend toward the rear end. The chip discharge grooves 71, 72 are formed so as to be twisted in a spiral shape around the axis O in the opposite direction to the end mill rotation direction T as they approach the rear end of the ball end mill 1. The ball end mill 1 is formed in a shape that is rotationally symmetrical by 180° with respect to the axis O.

[0016] Concave gash grooves 81 and 82 are formed at the tip of each of the two chip discharge grooves 71 and 72, respectively, so as to extend toward the inner periphery of the ball end mill 1 as they approach the tip. The bottom cutting edges 51a and 52a of the cutting edges 51 and 52 are formed at the ridges where the groove wall surfaces 81a and 82a of the gash grooves 81 and 82, facing the end mill rotation direction T, intersect with the tip flanks 61 and 62. As shown in FIG. 2 , the bottom cutting edges 51a and 52a have a convex hemispherical shape whose rotation locus around the axis O is centered on the axis O. In this embodiment, the diameter D of the convex hemispherical surface formed by the rotation locus of the bottom cutting edges 51a and 52a is 2 mm or less. While there is no particular lower limit for the diameter D, it is preferable that the diameter D be 0.1 mm or greater. A diameter D of 0.1 mm or greater allows for precise control of the width and shape of the chisel portion 10, described below.

[0017] The outer peripheral surface of the end mill body 3 is formed with outer peripheral flanks 91, 92 extending along the chip discharge grooves 71, 72. The outer peripheral flanks 91, 92 are adjacent to the chip discharge grooves 71, 72 on the opposite side in the end mill rotation direction T. The outer peripheral cutting edge 51b of the cutting edge 51 is formed at the ridge line where the outer peripheral flank 91 intersects with the wall surface of the chip discharge groove 71 facing the end mill rotation direction T on the rear end side of the gash groove 81. The outer peripheral cutting edge 52b of the cutting edge 52 is formed at the ridge line where the outer peripheral flank 92 intersects with the wall surface of the chip discharge groove 72 facing the end mill rotation direction T on the rear end side of the gash groove 82. The peripheral cutting edges 51b and 52b have a rotation locus around the axis O that forms a cylindrical surface with a diameter equal to the diameter D of the convex hemispherical surface formed by the rotation loci of the bottom cutting edges 51a and 52a. The peripheral cutting edges 51b and 52b are provided as needed. In other words, the cutting edge of the ball end mill 1 may be configured to consist of only the bottom cutting edge.

[0018] 3 and 4, when viewed from the axial tip side, the two gash grooves 81, 82 pass each other on opposite sides without overlapping with each other across the axis O. As shown in Fig. 2, the inner peripheral portion of the groove wall surface 81a of the gash groove 81 facing the end mill rotation direction T extends across the axis O in a side view seen from the direction opposing the groove wall surface 81a. The same is true for the groove wall surface 82a of the gash groove 82.

[0019] The tip flank 61 has a first flank 61a adjacent to the bottom cutting edge 51a on the rear side in the end mill rotation direction T, and a second flank 61b adjacent to the first flank 61a on the rear side in the end mill rotation direction T. The tip flank 62 has a first flank 62a adjacent to the bottom cutting edge 52a on the rear side in the end mill rotation direction T, and a second flank 62b adjacent to the first flank 62a on the rear side in the end mill rotation direction T. The rotation trajectories of the tip flanks 61, 62 are further inward than the rotation trajectories of the bottom cutting edges 51a, 52a. The clearance angles of the second flanks 61b, 62b are larger than the clearance angles of the first flanks 61a, 62a.

[0020] A chisel portion 10 is formed between two gash grooves 81, 82 that extend opposite each other on the inner peripheral portion of the tip of the end mill body 3. A chisel edge 10a that intersects with the axis O is formed on the chisel portion 10 as an intersecting ridge line formed by the intersecting two tip flank faces 61, 62 that are connected to the two end cutting edges 51a, 52a on the opposite side of the end mill rotation direction T. In this embodiment, the chisel edge 10a is formed on the intersecting ridge line formed by the two first flank faces 61a, 62a.

[0021] In this embodiment, when viewed from the axial tip side, the bottom cutting edges 51a, 52a are arranged so as to be positioned on the side of the end mill rotation direction T relative to a line that is parallel to the bottom cutting edges 51a, 52a and passes through the axis O, which is a so-called center-up arrangement. The center-up arrangement of the bottom cutting edges 51a, 52a is preferable because it improves chip release at the bottom cutting edges 51a, 52a.

[0022] 4, the bottom cutting edges 51a and 52a intersect with the chisel edge 10a at first intersections P11 and P21, respectively. The bottom cutting edge 51a has a major cutting edge 51A extending from the first intersection point P11 toward the outer periphery and an axis-side cutting edge 51B extending from the first intersection point P11 toward the axis O. The bottom cutting edge 52a has a major cutting edge 52A extending from the first intersection point P21 toward the outer periphery and an axis-side cutting edge 52B extending from the first intersection point P21 toward the axis O.

[0023] In this specification, the main cutting edge 51A refers to the intersection ridge between the groove wall surface 81a of the gash groove 81, which is located in front of the main cutting edge 51A in the end mill rotation direction T and functions as a rake face, and the first flank surface 61a, which is located behind the main cutting edge 51A in the rotation direction and forms a rotation trajectory closer to the axis O than the rotation trajectory of the main cutting edge 51A. The axial center cutting edge 51B is connected to one of the main cutting edges 51A but is formed at the intersection ridge between the groove wall surface 81a and the first flank surface 62a of another main cutting edge 52A that is different from the one main cutting edge 51A. The same applies to the main cutting edge 52A and the axial center cutting edge 52B of the bottom cutting edge 52a.

[0024] 4 is a straight line that is perpendicular to the tangents to the cutting edge at the first intersection points P11 and P21 (cutting edge tangents L1 and L2) when viewed from the axial tip side and passes through the axis O. In this embodiment, the reference line that passes through the axis O and is perpendicular to the cutting edge tangent at the first intersection point P11 and the reference line that passes through the axis O and is perpendicular to the cutting edge tangent at the first intersection point P21 coincide with each other when viewed from the axial tip side, and is therefore shown as a single reference line L. The reference line that passes through the axis O and is perpendicular to the cutting edge tangent at the first intersection point P11 and the reference line that passes through the axis O and is perpendicular to the cutting edge tangent at the first intersection point P21 may be straight lines that do not overlap each other when viewed from the axial tip side.

[0025] Next, a characteristic feature of this embodiment will be described. A second intersection P12, which is the intersection of a flank boundary line 62c, which is the boundary line between the first flank 62a and the second flank 62b, and the groove wall surface 81a of the gash groove 81, is located forward of the reference line L in the end mill rotation direction T. In other words, the region surrounded by the chisel edge 10a, the reference line L, and the axial-side cutting edge 51B (chisel edge front region A1) is formed only by the first flank 62a and does not include the second flank 62b. That is, the chisel edge front region A1 does not include the boundary between the first flank 62a and the second flank 62b (flank boundary line 62c). Similarly, the chisel edge front region A2 does not include the boundary between the first flank 61a and the second flank 61b (flank boundary line 61c).

[0026] With this configuration, the ball end mill 1 of this embodiment does not have a boundary between relief faces with different relief angles in the chisel edge forward regions A1, A2, and no edge portion that is convex toward the machining surface is formed in the chisel edge forward regions A1, A2, so chips are less likely to be pressed against the machining surface near the axis O, and welding can be suppressed. At the same time, because the ball end mill 1 of this embodiment does not include the second flanks 62b, 61b in the chisel edge front regions A1, A2, the cutting angle α of the axial side cutting edges 51B, 52B of this embodiment can be made relatively smaller compared to when the second flanks 62b, 61b are included. As a result, in this embodiment, the cutting performance of the axial side cutting edges 51B, 52B is improved and the occurrence of gouges can also be suppressed.

[0027] 4, the flank boundary line 61c of one end cutting edge 51a intersects with the cutting edge tangent line L2 at the intersection point (first intersection point P21) with the chisel edge 10a of the other end cutting edge 52a. Similarly, the flank boundary line 62c of the other end cutting edge 52a intersects with the cutting edge tangent line L1 at the intersection point (first intersection point P11) with the chisel edge 10a of the one end cutting edge 51a.

[0028] With this configuration, the ball end mill 1 of this embodiment can ensure a large area occupied by the first relief face on the axial side, while narrowing the width of the first relief face in the direction perpendicular to the blade on the more outer periphery side, thereby preventing cutting resistance from becoming greater than necessary and ensuring the cutting performance of the cutting edge, which is the main contributor to cutting.

[0029] Also, as shown in Figure 4, in this embodiment, when viewed from the axial tip side, the total flank width W in the direction in which the reference line between the first flank 61a of one end cutting edge 51a and the first flank 62a of the other end cutting edge 52a, which are adjacent to each other at the chisel edge, extends has a portion that decreases from the second intersection point toward the radial outer periphery.

[0030] With this configuration, the ball end mill 1 of this embodiment can ensure a large area occupied by the first relief face on the axial side, while narrowing the width of the first relief face in the direction perpendicular to the blade on the more outer periphery side, thereby preventing cutting resistance from becoming greater than necessary and ensuring the cutting performance of the cutting edge, which is the main contributor to cutting.

[0031] Second Embodiment As a modification of the first embodiment, a second embodiment is shown in FIG. 5. In the first embodiment, there were no clear bends at the flank boundary lines 61c and 62c, but in the second embodiment, clear bends (connection positions) 61C and 62C are present. The other shapes are the same as in the first embodiment. The same reference numerals are used to designate the same parts as in the first embodiment.

[0032] 5, the flank boundary line 61c, which is the boundary line between the first flank 61a and the second flank 61b, has a first portion 61A extending along the bottom cutting edge 51a adjacent to the first flank 61a, and a second portion 61B that is continuous with the inner peripheral end of the first portion 61A and extends in a direction away from the bottom cutting edge 51a to reach the edge of the gash groove 82. The flank boundary line 62c between the first flank 62a and the second flank 62b on the tip flank 62 has a first portion 62A and a second portion 62B, similar to the flank boundary line 61c.

[0033] The first portion 61A of the flank boundary line 61c extends substantially parallel to the major cutting edge 51A of one end cutting edge 51a. In this embodiment, the inner peripheral end of the first portion 61A extends slightly inward (toward the axis O) from the first intersection point P11, which is the inner peripheral end of the major cutting edge 51A. The flank boundary line 61c bends at a connection position 61C between the first portion 61A and the second portion 61B. The second portion 61B extends from the connection position 61C in a direction away from one end cutting edge 51a. The second portion 61B extends toward the gash groove 82 adjacent to the end cutting edge 52a on the opposite side. As in the first embodiment, the second portion 61B intersects with the gash groove 82 (groove wall surface 82a) on the opposite side at a second intersection point P22.

[0034] The flank boundary line 62c has a similar configuration to the flank boundary line 61c. That is, the first portion 62A extends along the main cutting edge 52A of the bottom cutting edge 52a. The flank boundary line 62c bends at a connection position 62C located near the first intersection point P21. The second portion 62B extends toward the gash groove 81 on the opposite side and intersects with the edge of the gash groove 81 (groove wall surface 81a) at a second intersection point P12.

[0035] In this embodiment, the second intersection point P12 of the bottom cutting edge 51a is also located forward of the reference line L in the end mill rotation direction T. The second intersection point P22 of the bottom cutting edge 52a is located forward of the reference line L in the end mill rotation direction T. According to this configuration, the chisel edge front region A1, which is surrounded by the chisel edge 10a, the gash groove 81 (in other words, the axial-side cutting edge 51B), and the reference line L, is composed of only the first flank face 62a. In other words, the second flank face 62b does not enter the chisel edge front region A1. This makes it possible to suppress welding to and tearing of the workpiece, as in the first embodiment. The chisel edge front region A2, which is surrounded by the chisel edge 10a, the gash groove 82 (in other words, the axial-side cutting edge 52B), and the reference line L, is also similarly composed of only the first flank face 61a.

[0036] Next, a configuration specific to the second embodiment will be described. 5, in this embodiment, a connection position 61C between the first portion 61A and the second portion 61B of the flank boundary line 61c of one bottom cutting edge 51a is located, in the direction in which the reference line L extends (the vertical direction in the figure), between a third intersection point P23, which is the intersection point between the other bottom cutting edge 52a and the reference line L, and the axis O. Also, a connection position 62C between the first portion 62A and the second portion 62B of the flank boundary line 62c of the other bottom cutting edge is located, in the direction in which the reference line L extends, between a third intersection point P13, which is the intersection point between the one bottom cutting edge 51a and the reference line L, and the axis O.

[0037] As a result, on the inner peripheral side, the flank boundary line is not included in the front region of the chisel edge, thereby suppressing the occurrence of welding, and on the outer peripheral side, the width of the first flanks 61a, 62a in the direction perpendicular to the cutting edge can be narrowed, preventing cutting resistance from increasing more than necessary and ensuring the cutting performance of the main cutting edges 51A, 52A, which mainly perform cutting, thereby improving the overall quality of the machined surface.

[0038] In the ball end mill 1 of this embodiment, the width W1 of the first flank 61a in the direction along the reference line L at the position of the first intersection P11 is smaller than the width W2 of the gap between the cutting edge tangent line L1 of the bottom cutting edge 51a and the cutting edge tangent line L2 of the bottom cutting edge 52a in the direction along the reference line L. In this embodiment, the width of the first flank 62a in the direction along the reference line L at the position of the first intersection P21 is approximately equal to the width W1 of the first flank 61a described above, and is similarly smaller than the width W2. With this configuration, the width of the first clearance surfaces 61a, 62a of the cutting edges located on the outer periphery in the direction perpendicular to the blade can be made narrower, thereby reducing cutting resistance and improving the cutting performance of the main cutting edges 51A, 52A.

[0039] The width W2 of the gap between the cutting edge tangents L1, L2 in the direction along the reference line L roughly coincides with the width of the chisel portion 10. The width of the chisel portion 10 is the width of the thinnest part between the groove wall surfaces 81a, 82a facing the end mill rotation direction T in the two gash grooves 81, 82 that pass each other, as viewed from the axial tip side. Therefore, the width W1 of the first flank surfaces 61a, 62a may be smaller than the width of the chisel portion 10.

[0040] In this embodiment, when viewed from the axial tip side, the second portion 61B extends in a direction away from the chisel edge 10a as it approaches the gash groove 82. Furthermore, the second portion 62B extends in a direction away from the chisel edge 10a as it approaches the gash groove 81. With this configuration, the first flank surface 61a becomes wider as it approaches the chisel edge front region A2, and the first flank surface 62a becomes wider as it approaches the chisel edge front region A1. This ensures a larger area for the thicker portion of the chisel portion 10. This increases the strength of the axial side cutting edges 51B, 52B.

[0041] In this embodiment, when viewed from the axial tip side, in a region of the flank boundary line 61c that is more inward than the connection position 61C between the first portion 61A and the second portion 61B, the total flank width W of the two first flanks 61a, 62a in the direction of the reference line L increases toward the inner periphery in the section from the connection position 61C to the second intersection point P22. Similarly, in a region of the flank boundary line 62c that is more inward than the connection position 62C between the first portion 62A and the second portion 62B, the total flank width W of the two first flanks 61a, 62a in the direction of the reference line L increases toward the inner periphery in the section from the connection position 62C to the second intersection point P12. This allows a larger area to be secured for the thicker portion of the chisel portion 10. This increases the strength of the axial-side cutting edges 51B, 52B.

[0042] The effects common to the first and second embodiments will now be described in more detail. Fig. 6 is a partial cross-sectional view of the chisel portion 10 taken along the reference line L in Fig. 4 or 5. Fig. 7 is a reference diagram showing a chisel edge region of another configuration for comparison. Fig. 8 is a reference diagram showing a cross section taken along the reference line L shown in Fig. 7.

[0043] In any embodiment, in the present invention, the chisel edge front regions A1 and A2 are configured to consist only of the first flank surfaces 62a and 61a, respectively, and therefore, as shown in Figure 6, the chisel edge front regions A1 and A2 are regions whose surfaces are flat.

[0044] 7, in the comparative example, when the boundary line between the first flank and the second flank is a flank boundary line 61v extending along the bottom cutting edge 51a and a flank boundary line 62v extending along the bottom cutting edge 52a, the intersection of the flank boundary line 61v and the edge of the gash groove 82 is a second intersection point P22v located rearward in the rotational direction from the reference line L. Also, the intersection of the flank boundary line 62v and the edge of the gash groove 81 is a second intersection point P12v located rearward in the rotational direction from the reference line L.

[0045] In the configuration shown in FIG. 7, the chisel edge front region A1 includes a portion consisting of the first flank 62a and a portion consisting of the second flank 62b. The chisel edge front region A2 includes a portion consisting of the first flank 61a and a portion consisting of the second flank 61b. In other words, the chisel edge front region includes a boundary portion between flanks with different clearance angles. In this configuration, as shown in FIGS. 7 and 8, an edge portion consisting of flank boundary lines 62v, 61v is formed within the chisel edge front regions A1 and A2, and is convex toward the machining surface. The chisel edge front regions A1 and A2 are located near the area where chips are generated during cutting and are easily blocked by the chisel edge 10a and the machining surface. Therefore, if an edge portion exists within the chisel edge front regions A1 and A2, the chips will be caught on the edge portion and pressed against the machining surface near the axis O, which can easily cause welding to the workpiece.

[0046] In contrast to the above, in the ball end mills 1 of the first and second embodiments, there is no boundary between relief faces with different relief angles in the chisel edge front regions A1, A2, and the chisel edge front regions A1, A2 are flat surfaces, so that chips are less likely to be pressed against the machining surface near the axis O, and welding can be suppressed.

[0047] At the same time, the axial-side cutting edges 51B, 52B are originally close to the rotation axis (axis O) and the cutting speed is difficult to increase, but by configuring the chisel edge front regions A1, A2 from only the first flank faces 62a, 61a, the cutting angle of the axial-side cutting edges 51B, 52B (angle α in FIG. 6) becomes relatively smaller compared to the case where the second flank faces 62b, 61b are present in the chisel edge front regions A1, A2 (angle β in FIG. 8). As a result, in this embodiment, the cutting ability of the axial-side cutting edges 51B, 52B is improved and the occurrence of gouges can be suppressed.

[0048] Furthermore, in the ball end mill 1 of this embodiment, the flank boundary line of one cutting edge intersects with the cutting edge tangent at the intersection (first intersection) with the chisel edge of the other cutting edge. Therefore, on the inner peripheral side, the front region of the chisel edge does not include the flank boundary line, thereby suppressing the occurrence of welding, while on the more outer peripheral side, the width of the first flank in the direction perpendicular to the cutting edge can be narrowed, which prevents cutting resistance from increasing more than necessary and ensures the cutting performance of the cutting edge that mainly performs cutting.

[0049] According to the ball end mill 1 of this embodiment, the above-mentioned synergistic effects of suppressing adhesion, suppressing ripping, and ensuring machinability can improve the finished surface properties (roughness and quality) in finish processing.

[0050] In this embodiment, the width of each of the first clearance faces 61a, 62a of the main cutting edges 51A, 52A in the direction perpendicular to the blade is not particularly limited, but can be, for example, within the range of 1% to 15% of the diameter D of the arcuate rotation trajectory of the bottom cutting edges 51a, 52a.

[0051] In the first and second embodiments, the groove wall surfaces 81a, 82a of the gash grooves 81, 82 facing the end mill rotation direction T are inclined surfaces that incline toward the end mill rotation direction T as they move away from the bottom cutting edges 51a, 52a toward the center of the convex hemispherical surface formed by the rotation trajectories of the bottom cutting edges 51a, 52a. As a result, the rake angle perpendicular to the blade is negative for both the axial center cutting edge 51B of the bottom cutting edge 51a located between the first intersection point P11 and the third intersection point P13, and the axial center cutting edge 52B of the bottom cutting edge 52a located between the first intersection point P21 and the third intersection point P23. In this specification, the rake angle perpendicular to the cutting edge of the axial side cutting edges 51B, 52B is the angle between the rake face and a straight line that passes through the axial side cutting edges 51B, 52B and is parallel to the axis O in a cross section that passes through the axial side cutting edges 51B, 52B and is parallel to the reference line L and the axis O.

[0052] In the first and second embodiments, the rake angle of the axis-side cutting edges 51B, 52B in the direction perpendicular to the cutting edge is within a range of -30° to -15°. With this configuration, the cutting edge angle α of the axis-side cutting edges 51B, 52B can be set to a cutting edge angle that provides a good balance between cutting performance and cutting edge strength for the axis-side cutting edges 51B, 52B, which rotate at a slow speed and are located near the closed space. This makes it possible to further suppress chipping or fracture of the axis-side cutting edges 51B, 52B while further suppressing ripping and welding.

[0053] If the rake angle of the axial cutting edges 51B, 52B in the chisel portion 10 is greater than -15° toward the positive angle side, the cutting angle of the end cutting edges 51a, 52a will be small, resulting in insufficient cutting edge strength and increased susceptibility to chipping and breakage. On the other hand, if the rake angle of the axial cutting edges 51B, 52B is greater than -30° toward the negative angle side, cutting performance will deteriorate and chipping will be more likely to occur. The rake angles of the end cutting edges 51a, 52a may gradually change from the chisel portion 10 toward the peripheral cutting edges 51b, 52b. In the portion outer circumferentially closer to the first intersection points P11, P21 of the end cutting edges 51a, 52a, the rake angles of the end cutting edges 51a, 52a may be negative or positive.

[0054] In the present invention, the clearance angles of the first flanks 61a, 62a near the chisel and the clearance angles of the second flanks 61b, 62b are not particularly limited. For example, the clearance angles of the first flanks 61a, 62a may be within a range of 5° or more and less than 10°. The clearance angles of the second flanks 61b, 62b may be within a range of 10° or more and 30° or less. With this configuration, chips are less likely to be pressed by the machining surface near the rotation axis, which further reduces the occurrence of welding near the rotation axis, and also improves the balance between the cutting ability of the cutting edge angle near the rotation axis and the strength of the cutting edge, thereby further reducing the occurrence of ripping.

[0055] At least the surface of the end mill body 3 at the tip of the ball end mill 1 may be coated with a hard coating. The hard coating may also cover the entire surface of the ball end mill 1, including the shank 2 and tapered neck 4. The hard coating is applied, for example, using a physical vapor deposition method, which uses a relatively low coating temperature. Of the physical vapor deposition methods, it is desirable to apply the hard coating using arc ion plating, which has excellent coating adhesion.

[0056] The hard coating is preferably a nitride or carbonitride, which has excellent heat resistance and wear resistance. Specifically, it is preferable to use a hard coating made of a nitride or carbonitride having the highest content of Al, which is a heat resistance and wear resistance, and a total Al and Cr content of 90 atomic % or more. Furthermore, among such hard coatings, it is preferable to use a hard coating with a finer coating structure. This hard coating has a higher hardness than the base material from which the ball end mill 1 is formed.

[0057] A suitable hard coating is, for example, the hard coating described in Japanese Patent No. 6410797. The hard coating described in the above patent publication is composed of a nitride or carbonitride containing aluminum (Al) at 50 atomic % to 68 atomic % (inclusive), chromium (Cr) at 20 atomic % to 46 atomic % (inclusive), and silicon (Si) at 4 atomic % to 15 atomic % (inclusive) relative to the total amount of metal (including metalloid) elements. The atomic ratio (A) of metal (including metalloid) elements (atomic %) and the atomic ratio (B) of nitrogen (atomic %) satisfy the relationship 1.03≦B / A≦1.07, where the total of metal (including metalloid) elements, nitrogen, oxygen, and carbon is taken as 100 atomic %. In the intensity profile obtained from an X-ray diffraction pattern or a selected-area diffraction pattern obtained using a transmission electron microscope, the peak intensity due to the (200) or (111) plane of the face-centered cubic lattice structure exhibits the maximum intensity.

[0058] The thickness of the hard coating is preferably 1.0 μm or more, and more preferably 2.0 μm or more. The thickness of the hard coating is preferably 3.0 μm or less. Furthermore, as also described in Japanese Patent No. 6410797, a protective coating may be formed on the hard coating. The protective coating described in the above patent publication is made of a nitride or carbonitride containing 50 atomic % or more of Ti and 1 atomic % to 30 atomic % of Si relative to the total amount of metal (including semi-metal) elements. By applying such a protective coating, wear resistance can be further improved even for high-hardness workpiece materials.

[0059] The hard coating can also be formed using the method described in Japanese Patent No. 6410797. In the method described in the patent publication, an alloy target having an aluminum (Al) content of 55 atomic % to 70 atomic % (based on the total amount of metal (including semi-metal) elements), a chromium (Cr) content of 20 atomic % to 35 atomic % (based on the total amount of metal (including semi-metal) elements), and a silicon (Si) content of 7 atomic % to 20 atomic % (based on the total amount of metal (including semi-metal) elements) is placed on the cathode, and the coating is formed by arc ion plating. The coating is formed under the following conditions: a bias voltage applied to the substrate is −220 V to −60 V and a cathode voltage is 22 V to 27 V; or a bias voltage applied to the substrate is −120 V to −60 V and a cathode voltage is 28 V to 32 V. [Example]

[0060] Next, the present invention will be explained more specifically with reference to examples. In this example, a ball end mill based on the above-described embodiment and a ball end mill based on the comparative example described with reference to Figures 7 and 8 were fabricated. That is, in the ball end mill of the example, the second flank does not extend into the chisel edge front region. In the ball end mill of the example, the flank boundary line between the first flank and the second flank changes direction near the axis O, and the intersection of the flank boundary line and the wall surface of the gash groove is located forward of the reference line L in the rotation direction. Furthermore, the cutting edge tangent at the intersection point between one end cutting edge and the chisel edge intersects with the flank boundary line of the other end cutting edge. On the other hand, in the ball end mill of the comparative example, the second flank extends into the inside of the chisel edge front region. Common tool specifications are shown below.

[0061] (Tool specifications) Blade diameter: 0.6mm First clearance angle: 9° Second clearance angle: 17°

[0062] Contour machining of the bottom surface was performed on the ball end mills of the Example and Comparative Example under the cutting conditions below, and the state of the machined surface was observed. Figure 9 shows a photograph of the observation results. As shown in Figure 9, welding was observed throughout the machined surface of the workpiece cut with the ball end mill of the Comparative Example. In addition, ripping occurred at the points where the direction of the contour machining changed. On the other hand, neither welding nor ripping occurred on the machined surface of the workpiece cut with the ball end mill of the Example. In addition, the surface roughness of the machined surface cut with the ball end mill of the Example was smaller.

[0063] (Cutting conditions) Rotation speed n: 40,000 min -1 ·Rotation speed Vc: 75m / min ·Cutting speed Vf:800mm / min Feed per tooth fz: 0.01mm / t Axial depth of cut ap: 0.005 mm Radial cutting depth ae: 0.01 mm ·Work material: VANADIS23(64HRC) Processing location: Bottom processing Coolant: Mist blow [Explanation of symbols]

[0064] 1...ball end mill, 3...end mill body, 10a...chisel edge, 51, 52...cutting edge, 52A, 52B...shaft side cutting edge, 61a, 62a...first flank, 61b, 62b...second flank, 61c, 61v, 62c, 62v...flank boundary line, 81, 82...gash groove, 81a, 82a...groove wall, L...reference line, L1, L2...cutting edge tangent, O...axis, P11, P21...first intersection, P12, P22, P12v, P22v...second intersection, W...total flank width, W1, W2...width

Claims

1. A ball end mill having two cutting blades and an end mill body that is rotated around an axis, the cutting edge is formed on an intersection ridge between a groove wall surface of the gash groove and a flank located rearward in the rotational direction of the gash groove, the flank having a first flank forming a chisel edge and a second flank extending rearward in the rotational direction from the first flank at a larger flank angle; When the end mill body is viewed from the axial tip side, an intersection between one of the cutting edges and the chisel edge is defined as a first intersection point, a straight line perpendicular to the cutting edge tangent at the first intersection point and passing through the axis is defined as a reference line, and an intersection between a flank boundary line between the first flank and the second flank of the other of the cutting edges and a groove wall surface of the gash groove of the one cutting edge is defined as a second intersection point. the second intersection point is located forward of the reference line in the direction of rotation, the flank boundary line of the other cutting edge intersects with the cutting edge tangent line of the one cutting edge, A ball end mill characterized in that, when the end mill body is viewed from the axial tip side, the tip surface of the end mill body has a portion in which the total flank width in the extension direction of the reference line of the first flank of one cutting edge and the first flank of the other cutting edge, which are adjacent to each other at the chisel edge, decreases from the second intersection point toward the radial outer periphery.

2. The ball end mill of claim 1, characterized in that, when the end mill body is viewed from the axial tip side, the relief boundary line has a portion that extends from the second intersection point toward the radial outer periphery in a direction approaching the chisel edge.

3. A ball end mill as described in claim 1 or 2, characterized in that when the end mill body is viewed from the axial tip side, the rake angle in the direction perpendicular to the cutting edge of the rake face of the axial side cutting edge, which is located radially outer than the reference line and radially axially closer than the first intersection point, is a negative angle.

4. The ball end mill according to any one of claims 1 to 3, characterized in that the rake angle is greater than or equal to -30° and less than or equal to -15°, and the clearance angle of the first clearance face is greater than or equal to 5° and less than 10°.

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