Cutting insert and blade-tip-replaceable cutting tool

Abstract

Provided is a plate-shaped cutting insert (5) that is detachably attached to a distal end portion of a tool body that is rotated about a central axis (C), the cutting insert (5) comprising a pair of cutting edges (4) that are spaced apart from each other around the central axis (C). Each of the cutting edges (4) has an arc-shaped outer peripheral cutting edge (11) extending toward the radial inner side with decreasing distance toward the axial distal end, and an arc-shaped inner peripheral cutting edge (12) smoothly connected to the radial inner end of the outer peripheral cutting edge (11) and extending toward the axial base end with decreasing distance toward the radial inner side. A connection point (P) between the outer peripheral cutting edge (11) and the inner peripheral cutting edge (12) is positioned at the foremost distal end of the cutting insert (5), and the outer peripheral cutting edge (11) and the inner peripheral cutting edge (12) have the same radius of curvature (R) in at least regions immediately before and after the connection point (P).

Description

Cutting inserts and indexable cutting tools

[0001] This application claims priority to Japanese Patent Application No. 2024-042454, filed on March 18, 2024, the contents of which are incorporated herein by reference.

[0002] Generally, the surface of a press die used to form automobile body parts and the like is finished using an indexable cutting tool such as those disclosed in Patent Documents 1 and 2. Specifically, the surface of the press die is machined using the indexable cutting tool to a surface roughness Ra of about 2 to 3 μm, and then subjected to a grindstone polishing step and a paper polishing step in this order, until the surface roughness Ra of the die surface is 0.8 μm or less.

[0003] JP 2005-319558 A Japanese Patent No. 6683261 A

[0004] For example, in press dies used to mold large parts such as side panels, the polishing process alone after machining can take more than 200 hours. In particular, the grinding process can result in a loss of approximately 30 μm, which can ruin the design of the die. Furthermore, the grinding dust generated by the grinding process can scratch the die surface, resulting in repeated corrections and making the process complicated. To reduce the time and cost required to process the die surface, it is desirable to achieve a high-quality machined surface with sufficiently low surface roughness through machining, thereby reducing or even eliminating the grinding process.

[0005] In recent years, the machining environment has become more severe due to the increasing hardness of work materials and the increasing preference for dry machining due to environmental considerations. Conventional indexable cutting tools have not yet been able to achieve a high-quality machined surface that eliminates the need for grinding stone polishing processes through highly efficient machining.

[0006] An object of the present invention is to provide a cutting insert and an indexable cutting tool that can achieve a higher quality machined surface than conventional ones and can reduce or eliminate the grinding process.

[0007] In order to solve the above problems, the present invention provides the following means.

[0008] [Aspect 1 of the present invention] A plate-shaped cutting insert that is detachably attached to the tip of a tool body that is rotated about a central axis, and that includes a pair of cutting edges that are spaced apart from each other around the central axis, each cutting edge having an arc-shaped outer peripheral blade that extends radially inward as it approaches the tip in the axial direction, and an arc-shaped inner peripheral blade that is smoothly connected to the radial inner end of the outer peripheral blade and extends radially inward as it approaches the base end in the axial direction, the connection point between the outer peripheral blade and the inner peripheral blade being located at the forefront of the cutting insert, and the outer peripheral blade and the inner peripheral blade having the same radii of curvature at least before and after the connection point.

[0009] [Aspect 2 of the present invention] The cutting insert according to Aspect 1, wherein, in a tip view of the cutting insert seen from the tip side in the axial direction, a distance L between the connection point of one of the pair of cutting edges and the connection point of the other cutting edge satisfies 0.010D≦L≦0.200D, where D is a blade diameter dimension that is a diameter dimension of a rotation locus of the cutting edges around the central axis.

[0010] [Aspect 3 of the Present Invention] The cutting insert according to aspect 1 or 2, wherein the rake angle X1 of the inner cutting edge is −15°≦X1≦5°.

[0011] [Aspect 4 of the present invention] The cutting insert according to any one of Aspects 1 to 3, wherein, in a tip view of the cutting insert seen from the tip side in the axial direction, a thickness dimension M between the inner peripheral cutting edges of the pair of cutting edges along a direction perpendicular to the blade length direction in which the inner peripheral cutting edges extend satisfies 0.002D≦M≦0.015D, where D is a blade diameter dimension, which is a diameter dimension of a rotation locus of the cutting edges around the central axis.

[0012] [Aspect 5 of the present invention] The cutting insert according to any one of Aspects 1 to 4, wherein, in a plan view of the cutting insert seen from the plate thickness direction, a distance N between a radially inner end of the inner cutting edge and the central axis satisfies 0.002D≦N≦0.015D, where D is a blade diameter dimension that is a diameter dimension of a rotation locus of the cutting edge about the central axis.

[0013] [Aspect 6 of the present invention] The cutting insert according to any one of Aspects 1 to 5, further comprising a chisel portion disposed between radially inner ends of the inner cutting edges of the pair of cutting edges in a tip end view of the cutting insert seen from the tip end side in the axial direction.

[0014] [Aspect 7 of the present invention] The cutting insert according to any one of Aspects 1 to 5, wherein the cutting insert has a concave cutout portion disposed between radially inner ends of the inner peripheral cutting edges of the pair of cutting edges and recessed toward the base end in the axial direction, in a tip view of the cutting insert seen from the tip end side in the axial direction.

[0015] [Aspect 8 of the Present Invention] The cutting insert according to any one of Aspects 1 to 7, wherein the centers of the radii of curvature R of the outer peripheral cutting edge and the inner peripheral cutting edge are located on an imaginary line that passes through the connection point and is parallel to the central axis, and the radii of curvature R satisfy R≦D / 2, where D is a blade diameter dimension, and D is a diameter dimension of a rotation locus of the cutting edge around the central axis.

[0016] [Aspect 9 of the Present Invention] The cutting insert according to any one of Aspects 1 to 8, wherein the radius of curvature of the entire peripheral cutting edge and the radius of curvature of the entire inner peripheral cutting edge are the same.

[0017] [Aspect 10 of the Present Invention] The cutting insert according to any one of Aspects 1 to 8, wherein the radius of curvature of the peripheral cutting edge gradually increases from the connection point toward the radially outer side.

[0018] [Aspect 11 of the Present Invention] The cutting insert according to any one of Aspects 1 to 10, wherein a radially inner end of the inner cutting edge does not reach the central axis.

[0019] [Aspect 12 of the Present Invention] The cutting insert according to any one of Aspects 1 to 11, wherein the cutting length of the inner cutting edge of one of the pair of cutting edges is longer than the cutting length of the inner cutting edge of the other cutting edge.

[0020] [Aspect 13 of the Present Invention] The cutting insert according to any one of Aspects 1 to 10 and 12, wherein a radially inner end of the inner peripheral cutting edge of at least one of the pair of cutting edges reaches the central axis.

[0021] Aspect 14 of the present invention is an indexable cutting tool comprising: a tool body that can be rotated about the central axis; and the cutting insert according to any one of Aspects 1 to 13 that is detachably attached to a tip end of the tool body.

[0022] [Aspect 15 of the Invention] A plate-shaped cutting insert that is detachably attached to a tip end of a tool body that is rotatable about a central axis, the cutting insert comprising a pair of cutting edges that are spaced apart from each other about the central axis, each cutting edge having an arc-shaped outer peripheral blade that extends radially inward as it approaches the tip end in the axial direction, and an arc-shaped inner peripheral blade that is smoothly connected to the radially inner end of the outer peripheral blade and extends radially inward as it approaches the base end in the axial direction, the connection point between the outer peripheral blade and the inner peripheral blade being located at the forefront of the cutting insert, the radii of curvature of the outer peripheral blade and the inner peripheral blade being identical to each other, and the radially inner end of the inner peripheral blade not reaching the central axis.

[0023] According to the cutting insert and indexable cutting tool of the above-described aspects of the present invention, a high-quality machined surface with sufficiently low surface roughness can be achieved by machining. Therefore, the grindstone polishing process of the machined surface after machining can be reduced or even eliminated. The polishing process of the mold surface can be significantly shortened. Furthermore, since the mold does not suffer from damage to the design surface or scratches caused by abrasive dust, high-quality molds can be steadily procured. The processing time and processing cost of the mold surface can be significantly improved (reduced).

[0024] FIG. 1 is a perspective view showing an indexable cutting tool according to an embodiment of the present invention. FIG. 2 is a front view of the indexable cutting tool as viewed from the axial tip side. FIG. 3 is a plan view showing a portion (near the cutting edge) of the indexable cutting tool. FIG. 4 is a side view showing a portion (near the cutting edge) of the indexable cutting tool. FIG. 5 is a front view of the tool body as viewed from the axial tip side. FIG. 6 is a plan view showing a portion (near the insert mounting seat) of the tool body. FIG. 7 is a side view of a portion (near the insert mounting seat) of the tool body. FIG. 8 is a perspective view showing a cutting insert. FIG. 9(a) is a plan view of the cutting insert as viewed from the thickness direction in the radial direction, FIG. 9(b) is a side view of the cutting insert as viewed from the radial direction perpendicular to the thickness direction, and FIG. 9(c) is a front view of the cutting insert as viewed from the axial tip side. FIG. 10 is a plan view of a portion (near the cutting edge) of the cutting insert as viewed from the thickness direction. FIG. 11 is a schematic diagram showing a rotation trajectory obtained by rotating the cutting edge around the central axis. FIG. 12 is a front view of a portion of the cutting insert (near the tip center) as seen from the axial tip side and a plan view as seen from the thickness direction. FIG. 13 is a front view of a portion of the cutting insert (near the tip center) of a first modified example as seen from the axial tip side and a plan view as seen from the thickness direction. FIG. 14 is a schematic diagram showing a rotation trajectory obtained by rotating the cutting edge of the cutting insert of a second modified example around the central axis. FIG. 15 is a plan view of a portion of the cutting insert (near the cutting edge) of a third modified example as seen from the thickness direction. FIG. 16 is a front view of a portion of the cutting insert (near the tip center) of a fourth modified example as seen from the axial tip side and a plan view as seen from the thickness direction. FIG. 17 is a front view of a portion of the cutting insert (near the tip center) of a fifth modified example as seen from the axial tip side and a plan view as seen from the thickness direction. FIG. 18 is a graph showing the change in surface roughness Ra of the machined surface versus the feed rate fz per tooth in Test 1. Fig. 19 is a graph showing the change in surface roughness Ra of the machined surface with respect to the feed rate fz per tooth in Test 2. Fig. 20 is a graph showing the change in surface roughness Ra of the machined surface with respect to the feed rate fz per tooth in Test 3. Fig. 21 is a photograph of the state of the machined surface in Test 1. Fig. 22 is a photograph of the state of the machined surface in Test 2.FIG. 23 is a photograph showing the state of the processed surface in Test 3.

[0025] A cutting insert 5 and an indexable cutting tool 6 according to one embodiment of the present invention will be described with reference to Figures 1 to 12. The indexable cutting tool 6 of this embodiment is an indexable radius end mill, and has an external shape that appears to be similar to that of an indexable ball end mill. The indexable cutting tool 6 of this embodiment is suitable for finishing, by machining, the surface of a press die used to mold large parts such as automobile side panels. In this embodiment, the indexable cutting tool 6 may also be referred to simply as a cutting tool or tool.

[0026] 1 to 4 , an indexable cutting tool 6 includes a tool body 1, a cutting insert 5, and a fixing screw 8. The tool body 1 is columnar and can be rotated around a central axis C of the tool. The cutting insert 5 is plate-shaped and is detachably attached to one end (tip portion 2) of both ends (one end and the other end) of the tool body 1 in the extension direction of the tool body 1. The fixing screw 8 fixes the cutting insert 5 to the tool body 1.

[0027] In this embodiment, the direction in which the central axis C of the indexable cutting tool 6 extends, i.e., the direction along the central axis C, is referred to as the axial direction. In this embodiment, the central axis C of the indexable cutting tool 6 is the central axis of the tool body 1 and also the central axis of the cutting insert 5.

[0028] In the axial direction, the direction from one end of the tool body 1 where the cutting insert 5 is attached to the other end is referred to as the axial base end side or simply the base end side. In addition, in the axial direction, the direction from the other end of the tool body 1 to one end is referred to as the axial tip side or simply the tip side. In the following description, one end of the tool is referred to as the tip portion 2, and the other end is referred to as the base end. The tip portion 2 may also be referred to as the cutting portion.

[0029] Additionally, the direction perpendicular to the central axis C is called the radial direction. Within the radial direction, the direction approaching the central axis C is called the radially inner direction, and the direction away from the central axis C is called the radially outer direction. The direction rotating around the central axis C is called the circumferential direction. Within the circumferential direction, the direction in which the tool is rotated during cutting is called the tool rotation direction T, and the opposite rotation direction is called the opposite side to the tool rotation direction T or counter-tool rotation direction.

[0030] The tool body 1 is made of a metal such as steel, and has a generally cylindrical shape extending in the axial direction. The tool body 1 has an insert mounting seat 3 disposed at a tip end 2 of the tool body 1, an insert fixing screw hole 2c disposed at the tip end 2, and a shank portion that constitutes the portion of the tool body 1 other than the tip end 2. The shank portion is cylindrical and extends in the axial direction.

[0031] The base end of the shank of the tool body 1 is detachably attached to a spindle of a machine tool (not shown). The indexable cutting tool 6 is rotated by the spindle of the machine tool in a tool rotation direction T about a central axis C and moved radially and axially to perform milling, profiling, and other milling processes on the surface of a workpiece, such as the surface of a mold.

[0032] 5 to 7, the insert mounting seat 3 is recessed from the tip surface and outer peripheral surface of the tip portion 2 of the tool body 1 toward the inside of the tool. A cutting insert 5 is removably attached to the insert mounting seat 3. The insert mounting seat 3 has a slit-shaped fitting groove 7.

[0033] The fitting groove 7 opens to the tip surface and outer peripheral surface of the tool body 1 and penetrates radially through the tool body 1. The fitting groove 7 has a pair of planar inner walls 7a, 7b arranged parallel to each other about the central axis C and facing each other in the radial direction, and a bottom wall 7c arranged at the base end of the fitting groove 7 and connected to the base ends of the pair of inner walls 7a, 7b. The bottom wall 7c has a concave V-shaped cross section that is recessed toward the base end and faces the tip side in the axial direction.

[0034] The tool body 1 has a pair of tip half portions 2a, 2b that are spaced apart from each other in the radial direction at the tip portion 2 by providing a slit-shaped fitting groove 7 (insert mounting seat 3) in the tip portion 2. Each of the pair of tip half portions 2a, 2b has a substantially semicircular plate shape.

[0035] The insert fixing screw hole 2c extends radially inward from the outer surface of one of the pair of tip half portions 2a, 2b, and reaches the inside of the other tip half portion 2b through the fitting groove 7. The central axis of the insert fixing screw hole 2c extends radially in the tip portion 2, specifically in a direction perpendicular to the radial extension of the fitting groove 7.

[0036] Of the insert fixing screw holes 2c, the hole formed in one tip half 2a has an inner diameter dimension larger than the hole formed in the other tip half 2b. Furthermore, the inner circumferential surface of the hole formed in the other tip half 2b is provided with a female thread portion that screws into the male thread portion of the fixing screw 8. Of the insert fixing screw holes 2c, at least the hole formed in one tip half 2a is a through hole. In this embodiment, each of the holes in one tip half 2a and the other tip half 2b is a through hole.

[0037] The cutting insert 5 is made of a hard sintered body such as cemented carbide. As shown in Figures 8 and 9(a) to 9(c), the cutting insert 5 has a plate-shaped insert body 15, a rake face, a flank face, and a cutting edge 4 formed on the outer surface of the insert body 15, and a screw insertion hole 18 penetrating the insert body 15 in the plate thickness direction. The rake face faces the tool rotation direction T on the outer surface of the insert body 15. The flank face faces the axial tip side, the radial outward side, or both on the outer surface of the insert body 15. The cutting edge 4 is located on a ridge line connecting the rake face and the flank face.

[0038] The cutting insert 5 has a pair of sets of a rake face, a flank face, and a cutting edge 4 that are spaced apart from each other around the central axis C. That is, the cutting insert 5 is a two-blade cutting insert having a pair of cutting edges 4. Specifically, the cutting insert 5 of this embodiment has a front-to-back inverted symmetric shape (a 180° rotationally symmetric shape) with the central axis C as the center (axis of symmetry). Therefore, one cutting edge 4 of the pair of cutting edges 4 coincides with the other cutting edge 4 when the cutting insert 5 is rotated 180° around the central axis C.

[0039] The insert body 15 is generally flat. A pair of flat plane portions 16, 17 extending in a direction perpendicular to the thickness direction (surface direction) are formed on both surfaces (front and back surfaces) of the insert body 15 facing the thickness direction. In addition, a top surface 19 having a convex V-shaped cross section that protrudes toward the base end is formed on the surface of the insert body 15 facing the base end in the axial direction.

[0040] When this cutting insert 5 is placed in the fitting groove 7 of the insert mounting seat 3, the pair of flat surfaces 16, 17 abut against the pair of inner walls 7a, 7b of the fitting groove 7. In addition, the top surface 19 abuts against the bottom wall 7c of the fitting groove 7. This positions the cutting insert 5 relative to the fitting groove 7 (insert mounting seat 3).

[0041] The screw insertion hole 18 is a circular hole that penetrates the insert body 15 in the plate thickness direction and opens to one flat surface portion 16 and the other flat surface portion 17. That is, the screw insertion hole 18 penetrates the cutting insert 5 in the plate thickness direction. A fixing screw 8 for fixing the cutting insert 5 to the insert mounting seat 3 is inserted into the screw insertion hole 18.

[0042] More specifically, with the cutting insert 5 inserted into the fitting groove 7 of the insert mounting seat 3, a fixing screw 8 is inserted through the insert fixing screw hole 2c of one of the tip half body portions 2a, and this fixing screw 8 is inserted into the screw insertion hole 18 of the cutting insert 5 and screwed into the insert fixing screw hole 2c of the other tip half body portion 2b. This fastens and fixes the cutting insert 5 to the insert mounting seat 3.

[0043] Fig. 10 is an enlarged view of a portion (near the cutting edge 4) of the cutting insert 5 in Fig. 9(a), specifically, a plan view of the portion of the cutting insert 5 as viewed from the plate thickness direction. Fig. 11 is a schematic view of a rotation trajectory obtained by rotating the cutting edge 4 around the central axis C.

[0044] Each of the pair of cutting edges 4 extends from the axial tip to the radial outer end of the insert body 15. In the plan view of Fig. 10 or the schematic diagram of the rotation trajectory of Fig. 11, each cutting edge 4 has an arc-shaped outer peripheral cutting edge 11 that extends radially inward as it approaches the axial tip, an arc-shaped inner peripheral cutting edge 12 that is smoothly connected to the radial inner end of the outer peripheral cutting edge 11 and extends radially inward as it approaches the axial base end, and a linear side cutting edge 13 that is smoothly connected to the axial base end of the outer peripheral cutting edge 11 and extends in the axial direction.

[0045] The peripheral cutting edge 11 and the inner peripheral cutting edge 12 are smoothly connected so as to have a common tangent line TL at their connecting portion. In this embodiment, the connecting portion between the peripheral cutting edge 11 and the inner peripheral cutting edge 12 is called a connection point P. The connection point P between the peripheral cutting edge 11 and the inner peripheral cutting edge 12 is the axially most distal position of the cutting edge 4. In other words, the connection point P is located at the axially most distal position of the cutting insert 5. The connection point P is also located at the axially most distal position of the indexable cutting tool 6. The tangent line TL tangent to the cutting edge 4 at the connection point P extends along an imaginary plane extending in a direction (plane direction) perpendicular to the central axis C.

[0046] The peripheral cutting edge 11 and the inner cutting edge 12 have the same radius of curvature R at least before and after the connection point P. Note that the term "before and after the connection point P" refers to the area before and after the connection point P along the cutting edge length direction in which the cutting edge 4 extends. In other words, the term "before and after the connection point P" refers to the areas adjacent to both sides of the connection point P in the cutting edge length direction of the cutting edge 4 (the area adjacent to the peripheral cutting edge 11 side and the area adjacent to the inner cutting edge 12 side). More specifically, the term "before and after the connection point P" refers to a range of approximately ±10° from the connection point P in terms of a central angle centered on the center O of the radius of curvature R, for example, in a plan view of the cutting insert 5 shown in FIG. 10 .

[0047] In this embodiment, the radius of curvature R of the outer peripheral cutting edge 11 and the radius of curvature R of the inner peripheral cutting edge 12 are the same. More specifically, the radius of curvature R of the entire outer peripheral cutting edge 11 and the radius of curvature R of the entire inner peripheral cutting edge 12 are the same. In this embodiment, in the rotation trajectory obtained by rotating the cutting edge 4 around the central axis C as shown in Fig. 11, the radius of curvature R of the outer peripheral cutting edge 11 and the radius of curvature R of the inner peripheral cutting edge 12 are the same. In other words, the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 are formed so as to each constitute a part of a single arc having a constant radius of curvature R.

[0048] When the curvature radii R of the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 are different, as in the cutting edge shape of a conventional cutting insert (Patent Document 1), a grinding process is required for each cutting edge during insert manufacturing. In contrast, the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 of this embodiment (the present invention) have the same curvature radii R, so they can be formed in a single grinding process. Therefore, the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 are smoothly connected so that they share a common tangent line TL at their connecting portions. Therefore, no corners are formed at the boundary between the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12. As a result, the formation of trochoidal feed marks (surface irregularities) on the machined surface of the workpiece due to a built-up cutting edge to which chip deposits adhere is suppressed, and the surface roughness Ra can be kept to 0.8 μm or less.

[0049] 10 and 11, the center O of the radius of curvature R of the peripheral cutting edge 11 coincides with the center O of the radius of curvature R of the inner peripheral cutting edge 12. The centers O of the radii of curvature R of the peripheral cutting edge 11 and the inner peripheral cutting edge 12 are located on a virtual straight line VL that passes through the connection point P and is parallel to the central axis C.

[0050] In this embodiment, when the diameter dimension (maximum dimension) of the rotation locus of the cutting blade 4 around the central axis C is defined as the blade diameter dimension D, the radius of curvature R satisfies R≦D / 2, and preferably R<D / 2. The center O of the radius of curvature R is located radially outward and axially toward the tip side of the center A of the screw insertion hole 18, which is located on the central axis C. The blade diameter dimension D is, for example, 6 to 50 mm, and in this embodiment, is a predetermined value of 30 mm.

[0051] As shown in Fig. 9(b), in a side view of the cutting insert 5 seen from a radial direction perpendicular to the plate thickness direction, the peripheral cutting edge 11 has a curved shape that is convex toward the tool rotation direction T. Also, as shown in Fig. 9(c), in a front view (tip view) of the cutting insert 5 seen from the tip side in the axial direction, the peripheral cutting edge 11 has a curved shape that is convex toward the tool rotation direction T.

[0052] 12 is a front view (front view) of the cutting insert 5 near the tip center as viewed from the tip side in the axial direction, and a plan view of the cutting insert 5 near the tip center as viewed from the plate thickness direction. As shown in Fig. 12, in the tip view (front view) of the cutting insert 5 as viewed from the tip side in the axial direction, the inner peripheral cutting edge 12 extends linearly. Specifically, the section from the inner peripheral cutting edge 12 to the radially inner end of the outer peripheral cutting edge 11 connected to the inner peripheral cutting edge 12 via the connection point P (hereinafter, this section may be referred to as the inner peripheral portion of the cutting edge 4) extends linearly.

[0053] 12 , the radially inner end 12 a of the inner peripheral cutting edge 12 does not reach the central axis C. In other words, the radially inner end 12 a of the inner peripheral cutting edge 12 is positioned away from the central axis C, and more specifically, is positioned between the connection point P and the central axis C in the radial direction.

[0054] Furthermore, in a tip view of the cutting insert 5 seen from the tip side in the axial direction, the distance L between the connection point P of one cutting edge 4 and the connection point P of the other cutting edge 4 of the pair of cutting edges 4 is in the range of 0.010D≦L≦0.200D, where D is the diameter dimension of the rotation trajectory of the cutting edges 4 about the central axis C. More preferably, it is in the range of 0.015D≦L≦0.167D. More preferably, it is in the range of 0.030D≦L≦0.100D. For example, when D=30 mm, the distance L is preferably in the range of 0.45 mm≦L≦5.01 mm.

[0055] In addition, in the tip view shown in Figure 12, the angle α formed between the imaginary straight line SL connecting each connection point P of a pair of cutting edges 4 and the inner peripheral portion of the cutting edge 4 is, for example, 5°≦α≦15°, and in this embodiment, it is set to 10°.

[0056] 12, the thickness dimension M between the inner peripheral blades 12 of the pair of cutting blades 4 along a direction perpendicular to the blade length direction in which the inner peripheral blades 12 extend is set to 0.002D≦M≦0.015D, where D is the diameter dimension of the rotational locus of the cutting blade 4 about the central axis C. The thickness dimension M may also be expressed as the distance between a pair of connection points P along a direction perpendicular to the inner peripheral portion of the cutting blade 4. In this embodiment, the thickness dimension M is set to a range of 0.15 to 0.2 mm.

[0057] 12 , in a plan view of the cutting insert 5 seen from the plate thickness direction, the distance N between the radially inner end 12a of the inner peripheral cutting edge 12 and the central axis C is set to 0.002D≦N≦0.015D, where D is the diameter dimension of the rotation locus of the cutting edge 4 about the central axis C. More preferably, it is set to 0.004D≦N≦0.01D.

[0058] In this embodiment, the rake angle X1 of the inner cutting edge 12 is set to -15°≦X1≦5°.

[0059] The inner rake angle X1 is preferably in the range of -15° to 5°, and more preferably in the range of -7° to 0°. With conventional cutting edge rake angles (-20° to -30°), chips from cast iron workpieces tend to turn into powder and flow toward the flank, making it easier for built-up edges to form and increasing the surface roughness Ra of the machined surface. In this embodiment (present invention), the rake angle is set to an acute angle (-15° to +5°), i.e., a positive (positive angle), making it easier for chips to flow toward the rake face, suppressing built-up edges and reducing the surface roughness Ra of the machined surface. Furthermore, if the inner rake angle X1 is set to 5° or more, the cutting edge strength decreases due to the excessively acute angle.

[0060] In this embodiment, the rake angle X2 of the outer peripheral cutting edge 11 connected to the inner peripheral cutting edge 12 is also set to -15°≦X2≦5°.

[0061] The cutting insert 5 of this embodiment also includes a chisel portion 20 at the center of the tip of the cutting insert 5. In the tip view shown in Fig. 12 , the chisel portion 20 is disposed between the radial inner ends 12a of the inner peripheral cutting edges 12 of the pair of cutting edges 4. The chisel portion 20 is disposed on a ridge portion connecting the flank face of one cutting edge 4 to the flank face of the other cutting edge 4. In this tip view, the chisel portion 20 forms a straight line connecting the radial inner ends 12a of the pair of inner peripheral cutting edges 12 and passes through the central axis C.

[0062] In the tip view shown in Figure 12, the chisel angle β formed between the inner peripheral portion of the cutting edge 4 and the chisel portion 20 is 160° or less, and in this embodiment, it is 150°. In addition, the length of the ridge line of the chisel portion 20 (chisel width) is 0.7 mm or less. In this embodiment, the chisel width is 0.4 mm. In addition, the rake angle of the chisel portion 20 is a negative angle, and the absolute value of the rake angle is 83° or less.

[0063] The chisel portion 20 is a part that is hardly involved in cutting, but when steep oblique cutting or cutting in the protruding direction (Z direction) is required, it is expected that the chisel portion 20 will be involved in cutting. Therefore, the shape of the chisel portion 20 ensures the strength of the chisel portion and can prevent damage to the cutting edge during steep oblique cutting or cutting in the protruding direction (Z direction).

[0064] 9(a), (b) and 10, the side cutting edge 13 is connected to the peripheral cutting edge 11 via the outermost point S, which is the axial base end and radial outer end of the peripheral cutting edge 11. The side cutting edge 13 extends from the outermost point S toward the axial base end. Specifically, the side cutting edge 13 extends linearly in the direction opposite to the tool rotation direction as it approaches the axial base end.

[0065] In the embodiment described above, a connection point P between the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 is located at the tip of the cutting insert 5 and the indexable cutting tool 6. As shown in Figures 10 and 11, the outer peripheral cutting edge 11 has an arc shape extending radially outward from the connection point P toward the base end in the axial direction, and the inner peripheral cutting edge 12 has an arc shape extending radially inward from the connection point P toward the base end in the axial direction. The outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 are smoothly connected to each other, and the radii of curvature R are the same at least before and after the connection point P.

[0066] For example, unlike the present embodiment, if the radius of curvature R of the peripheral cutting edge 11 and the radius of curvature R of the inner cutting edge 12 are different before and after the connection point P, chips will adhere to the weld deposit near the connection point P between the peripheral cutting edge 11 and the inner cutting edge 12, making it easier for a built-up edge to form. When such a built-up edge occurs, trochoidal feed marks (surface irregularities) are more likely to appear on the die surface (the machined surface of the workpiece), making it impossible to keep the surface roughness Ra sufficiently small.

[0067] On the other hand, according to this embodiment, the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 are smoothly connected so as to form a portion of a single arc at least near the connection point P, and no sharp edges or inflection points are formed at the connection portion between the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12. This prevents the formation of a built-up edge near the connection point P between the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12, thereby preventing the machined surface from being roughened by such a built-up edge. In this embodiment, the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 have a constant radius of curvature R throughout their respective cutting lengths. Therefore, when manufacturing the insert, the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 can be easily formed in a single grinding process.

[0068] In this embodiment, the radially inner end 12a of the inner peripheral cutting edge 12 does not reach the central axis C. In other words, the radially inner end 12a of the inner peripheral cutting edge 12 is located radially between the central axis C and a connection point P between the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12. The tip center portion of the cutting insert 5 located on the central axis C is recessed toward the base end in the axial direction relative to the connection point P.

[0069] For example, unlike the present embodiment, if the tip center portion of the cutting insert 5 located on the central axis C is located at the very front end of the cutting insert 5, the cutting surface is likely to be torn due to contact with the tip center portion where the peripheral speed is essentially zero, and therefore it is not possible to achieve a high-quality cutting surface with sufficiently low surface roughness.

[0070] On the other hand, according to this embodiment, the tip center of the cutting insert 5 is concave, which prevents the tip center from coming into contact with the machined surface, thereby preventing defects such as chipping of the machined surface. Therefore, it is possible to achieve a high-quality machined surface with sufficiently low surface roughness.

[0071] Specifically, according to this embodiment, the surface roughness Ra of the machined surface after dry machining (finishing) can be reduced to, for example, 0.8 μm or less, thereby reducing or eliminating the need for a grindstone polishing process for the machined surface after machining.

[0072] As described above, according to this embodiment, a high-quality machined surface with sufficiently small surface roughness can be achieved by machining. Therefore, the grindstone polishing process of the machined surface after machining can be reduced or even eliminated. The polishing process of the mold surface can be significantly shortened. Furthermore, since the mold does not suffer from damage to the design surface or scratches caused by abrasive dust, high-quality molds can be procured stably. The processing time and processing cost of the mold surface can be significantly improved (reduced).

[0073] In this embodiment, in a tip view of the cutting insert 5 seen from the tip side in the axial direction, the distance L between the connection point P of one cutting edge 4 and the connection point P of the other cutting edge 4 of the pair of cutting edges 4 is preferably in the range of 0.010D≦L≦0.200D, more preferably 0.015D≦L≦0.167D, or more preferably 0.030D≦L≦0.100D, where D is the diameter dimension of the rotation trajectory of the cutting edges 4 about the central axis C.

[0074] In the above configuration, the distance L between the connection points P (the foremost positions of the cutting edges 4) of a pair of cutting edges 4 is kept small, at 0.200D or less. This makes it possible to minimize the occurrence of uncut portions at the center of the tip located between the connection points P of the cutting insert 5, and also stably reduces the surface roughness Ra of the mold surface, thereby improving the precision of the machined surface. The above configuration makes it easier to obtain particularly significant effects, particularly when cutting the bottom surface of a mold.

[0075] Furthermore, by ensuring that the distance L between each connection point P of a pair of cutting edges 4 is 0.010D or more, the connection point P (the most distal end position) of each cutting edge 4 is located away from the central axis C. Because the connection point P of each cutting edge 4 is not located on the central axis C, where the peripheral speed during cutting is substantially zero, chipping of the machined surface can be effectively suppressed, and the accuracy of the machined surface can be improved.

[0076] In this embodiment, when viewed from the tip of the cutting insert 5, the angle α formed between the imaginary line SL connecting the pair of connection points P and the inner peripheral portion of the cutting edge 4 is set to 5°≦α≦15°. By providing the angle α, the cutting edge 4 is centered up, thereby improving chip discharge performance. In addition, the tip of the cutting edge 4 can be formed thick, ensuring cutting edge strength.

[0077] In this embodiment, the rake angle X1 of the inner peripheral cutting edge 12 is -15°≦X1≦5°. When the rake angle X1 of the inner peripheral cutting edge 12 is -15° or greater, the tendency of powdery chips (swarf) generated, for example, when cutting a cast workpiece, to flow easily to the flank side is reduced. By making it easier for chips to flow to the rake face side, defects such as the formation of a built-up edge on the cutting edge 4 can be stably suppressed. This allows the surface roughness Ra of the machined surface to be stably kept small.

[0078] Furthermore, when the rake angle X1 of the inner peripheral cutting edge 12 is 5° or less, the cutting edge 4 is prevented from becoming too small, and the cutting edge strength is stably ensured. This allows high-precision cutting to be performed stably for a long period of time.

[0079] In this embodiment, the rake angle X2 of the peripheral cutting edge 11 connected to the inner peripheral cutting edge 12 is also -15°≦X2≦5°. Therefore, the peripheral cutting edge 11 also provides the same excellent effects as the inner peripheral cutting edge 12 described above.

[0080] In addition, in this embodiment, when the cutting insert 5 is viewed from the tip side in the axial direction, the thickness dimension M between each inner cutting edge 12 of a pair of cutting edges 4 along a direction perpendicular to the blade length direction in which the inner cutting edges 12 extend is 0.002D≦M≦0.015D, when the diameter dimension of the rotational trajectory of the cutting edge 4 around the central axis C is defined as the blade diameter dimension D.

[0081] In the above configuration, the thickness dimension M, which corresponds to the thickness between the inner peripheral cutting edges 12 of the pair of cutting edges 4 (the back metal of the inner peripheral cutting edges 12), is set to 0.002D or more, so that the strength of the cutting edges is stably ensured. Furthermore, because the thickness dimension M is set to 0.015D or less, the radial rake (radial rake angle) of the inner peripheral cutting edge 12 is prevented from becoming too large on the negative side. As a result, the sharpness of the inner peripheral cutting edge 12 is maintained at a good level.

[0082] In this embodiment, in a plan view of the cutting insert 5 seen from the plate thickness direction, the distance N between the radially inner end 12a of the inner peripheral cutting edge 12 and the central axis C is set to 0.002D≦N≦0.015D, where D is the diameter dimension of the rotation locus of the cutting edge 4 about the central axis C. More preferably, it is set to 0.004D≦N≦0.01D.

[0083] When the distance N is 0.002D or more, each of the inner peripheral cutting edges 12 is positioned away from the central axis C. Since each of the inner peripheral cutting edges 12 is not positioned on the central axis C, where the peripheral speed during cutting is substantially zero, chipping of the machined surface can be effectively suppressed, and the accuracy of the machined surface can be improved.

[0084] In addition, the distance N is kept small, at 0.015 D or less, which makes it possible to minimize the occurrence of uncut portions at the center of the tip end located on the central axis C of the cutting insert 5, and also to stably keep the surface roughness Ra of the mold surface small, thereby improving the precision of the machined surface.

[0085] In addition, the cutting insert 5 of this embodiment has a chisel portion 20 arranged between the radial inner ends 12a of each inner cutting edge 12 of the pair of cutting edges 4 when viewed from the axial tip side of the cutting insert 5.

[0086] In this case, the chisel portion 20 located at the tip center of the cutting insert 5 is recessed axially toward the base end side from the connection point P (the foremost position of the cutting edge 4) between the peripheral cutting edge 11 and the inner peripheral cutting edge 12. This makes it possible to suppress the influence on the processed surface properties of the tip center of the cutting insert 5 (chisel portion 20), where the peripheral speed during cutting is essentially zero.

[0087] In this embodiment, the center O of the radius of curvature R of each of the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 is located on a virtual line VL that passes through the connection point P and is parallel to the central axis C, and when the diameter dimension of the rotational locus of the cutting edge 4 around the central axis C is taken as the blade diameter dimension D, the radius of curvature R satisfies R≦D / 2. More preferably, R<D / 2.

[0088] In this case, the cutting insert 5 has a cutting edge shape of a radius end mill (ball-shaped radius end mill) that is similar to the shape of a ball end mill, thereby achieving the above-mentioned excellent effects.

[0089] The present invention is not limited to the above-described embodiment, and the configuration may be modified within the scope of the present invention, as described below. In the illustrations of the modified examples, the same components as those in the above-described embodiment are denoted by the same reference numerals, and the following mainly describes the differences.

[0090] 13 is a front view (anterior view) of the vicinity of the tip central portion of a cutting insert 5 according to a first modified example of the above-described embodiment, as viewed from the tip side in the axial direction, and a plan view of the vicinity of the tip central portion as viewed from the plate thickness direction of the cutting insert 5. In this first modified example, the cutting insert 5 has a notch 21 at the tip central portion of the cutting insert 5. As shown in FIG. 13 , in a tip view (front view, anterior view) of the cutting insert 5 as viewed from the tip side in the axial direction, the notch 21 is disposed between the radial inner ends 12 a of the inner peripheral cutting edges 12 of the pair of cutting edges 4, and has a concave shape recessed toward the base end side in the axial direction. In the illustrated example, the notch 21 has a parallelogram-shaped concave shape in tip view.

[0091] In the first modified example, a concave notch 21 recessed toward the base end in the axial direction is provided in the center of the tip of the cutting insert 5. This reduces the effect on the machined surface properties of the tip center (notch 21) of the cutting insert 5, where the peripheral speed during cutting is essentially zero. In addition, the presence of the notch 21 makes the difference in shape from conventional inserts clear to the naked eye, preventing workers from mistaking the new insert for a conventional product.

[0092] FIG. 14 is a schematic diagram showing a rotation locus obtained by rotating the cutting edge 4 of the cutting insert 5 of the second modified example of the above-described embodiment around the central axis C.

[0093] In this second modified example, the center O of the radius of curvature R of the peripheral cutting edge 11 and the center O of the radius of curvature R of the inner peripheral cutting edge 12 also coincide with each other. The centers O of the radii of curvature R of the peripheral cutting edge 11 and the inner peripheral cutting edge 12 are located on an imaginary line VL that passes through the connection point P and is parallel to the central axis C. Furthermore, when the diameter dimension of the rotational locus of the cutting edge 4 about the central axis C is defined as the blade diameter dimension D, the radius of curvature R satisfies R>D / 2. The center O of the radius of curvature R is located radially outward and axially proximal to the center A of the screw insertion hole 18 (not shown) that is located on the central axis C (see FIG. 10 ).

[0094] In the second modified example, the cutting insert 5 has a cutting edge shape of a so-called lenticular radius end mill, thereby achieving the above-mentioned excellent effects.

[0095] 15 is a plan view of a portion (near the cutting edge) of the cutting insert 5 of a third modified example of the above-described embodiment, viewed from the plate thickness direction. In this third modified example, in the plan view of the insert shown in FIG. 15, the radius of curvature of the inner peripheral cutting edge 12 is R2, and the radius of curvature of the outer peripheral cutting edge 11 is R1a, before and after the connection point P in the cutting edge 4's cutting length direction. In the third modified example, as in the above-described embodiment, the radii of curvature R1a and R2 of the outer peripheral cutting edge 11 and the inner peripheral cutting edge 12 are the same at least before and after the connection point P.

[0096] The radius of curvature R of the peripheral cutting edge 11 gradually increases radially outward from the connection point P. Specifically, the radius of curvature R of the peripheral cutting edge 11 is set to a radius of curvature R1a before and after the connection point P (near the connection point P), and becomes larger than the radius of curvature R1a (R2) as it moves radially outward from the vicinity of the connection point P and toward the base end in the axial direction. At the outermost point S of the peripheral cutting edge 11 (the connection point between the peripheral cutting edge 11 and the side cutting edge 13), the radius of curvature R of the peripheral cutting edge 11 is set to a maximum value of a radius of curvature R1b.

[0097] Furthermore, the center O of the radius of curvature R of the peripheral cutting edge 11 coincides with the center O of the radius of curvature R2 of the inner peripheral cutting edge 12 near the connection point P. Then, as one moves from the vicinity of the connection point P toward the radially outward direction (the base end side in the axial direction) along the cutting length direction of the peripheral cutting edge 11, the center O of the radius of curvature R of the peripheral cutting edge 11 moves from the center O of the radius of curvature R2 of the inner peripheral cutting edge 12 toward the center A of the screw insertion hole 18. In this modification, at the outermost peripheral point S of the peripheral cutting edge 11, the center O of the radius of curvature R1b of the peripheral cutting edge 11 coincides with the center A. Specifically, the radius of curvature R2 of the inner peripheral cutting edge 12 and the radius of curvature R1a of the peripheral cutting edge 11 are, for example, approximately 13.5 mm, and the radius of curvature R1b of the peripheral cutting edge 11 is, for example, approximately 15 mm.

[0098] The third modified example described above also provides excellent effects similar to those of the previously described embodiment and modified examples (hereinafter sometimes referred to as embodiment, etc.).

[0099] Figure 16 is a front view (front view) of the vicinity of the tip center of the cutting insert 5 of the fourth modified example of the above-mentioned embodiment, viewed from the axial tip side, and a plan view of the vicinity of the tip center, viewed from the plate thickness direction of the cutting insert 5.

[0100] In the fourth modified example, a chisel portion 20 is formed in the center of the tip end of the cutting insert 5. The center of (the ridge line of) the chisel portion 20 is disposed at a position shifted from the central axis C of the cutting insert 5. Accordingly, the cutting length of the inner cutting edge 12 of one of the pair of cutting edges 4 (the left cutting edge 4 in FIG. 16 ) is longer than the cutting length of the inner cutting edge 12 of the other cutting edge 4 (the right cutting edge 4 in FIG. 16 ). More specifically, as shown in FIG. 16 , in both a front view of the cutting insert 5 seen from the tip end side in the axial direction and a plan view of the cutting insert 5 seen from the plate thickness direction, the cutting length of the inner cutting edge 12 of one cutting edge 4 is longer than the cutting length of the inner cutting edge 12 of the other cutting edge 4.

[0101] In the fourth modified example, the radially inner end 12a of the inner peripheral cutting edge 12 of at least one of the pair of cutting edges 4 reaches the center axis C in the insert plan view shown in FIG.

[0102] Figure 17 is a front view (front view) of the vicinity of the tip center of the cutting insert 5 of the fifth modified example of the above-mentioned embodiment, viewed from the axial tip side, and a plan view of the vicinity of the tip center, viewed from the plate thickness direction of the cutting insert 5.

[0103] In the fifth modified example, a concave notch 21 recessed toward the base end in the axial direction is provided in the center of the tip of the cutting insert 5. The center of the notch 21 is positioned offset from the central axis C of the cutting insert 5. Accordingly, the cutting length of the inner cutting edge 12 of one of the pair of cutting edges 4 (the left cutting edge 4 in FIG. 17 ) is longer than the cutting length of the inner cutting edge 12 of the other cutting edge 4 (the right cutting edge 4 in FIG. 17 ). More specifically, as shown in FIG. 17 , in both a front view of the cutting insert 5 seen from the tip end in the axial direction and a plan view of the cutting insert 5 seen from the plate thickness direction, the cutting length of the inner cutting edge 12 of one cutting edge 4 is longer than the cutting length of the inner cutting edge 12 of the other cutting edge 4.

[0104] In the fifth modified example, the radially inner end 12a of the inner peripheral cutting edge 12 of at least one of the pair of cutting edges 4 reaches the center axis C in the insert plan view shown in FIG.

[0105] The fourth and fifth modifications described above also provide excellent effects similar to those of the above-described embodiment.

[0106] In the above-described embodiment and each modified example, the material of the substrate (insert body 15) of the cutting insert 5 may be, in addition to cemented carbide containing tungsten carbide (WC) and cobalt (Co), for example, cermet, high-speed steel, ceramics consisting of titanium carbide, silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, and mixtures thereof, cubic boron nitride sintered body, diamond sintered body, or an ultra-high pressure sintered body obtained by sintering a hard phase consisting of polycrystalline diamond or cubic boron nitride with a binder phase such as ceramic or an iron-group metal under ultra-high pressure. Furthermore, the tool body 1 may be manufactured from an alloy tool steel such as SKD61, or may be formed by joining an alloy tool steel such as SKD61 and a cemented carbide.

[0107] Furthermore, although the above-described embodiment and each modified example are indexable cutting tools in which the cutting insert 5 is attached to the insert mounting seat 3 of the tool body 1, the present invention may be applied to tools other than these. For example, the present invention may be applied to a solid-type end mill in which the cutting edge and the tool body are integrated.

[0108] The present invention may be combined with the various configurations described in the above-described embodiments and modifications, and may also include additions, omissions, substitutions, and other modifications of configurations, without departing from the spirit of the present invention. Furthermore, the present invention is not limited to the above-described embodiments, but is limited only by the scope of the claims.

[0109] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0110] As shown in Table 1, each cutting insert of Examples 1 to 5 of the present invention and conventional Comparative Examples 1 and 2 was attached to the insert mounting seat 3 of the tool body 1, and cutting was performed in Tests 1 to 3. The surface roughness Ra of the machined surface was measured for the feed rate fz per tooth under each cutting condition, and the state of the machined surface was photographed.

[0111]

[0112] The measured values ​​of surface roughness Ra for each test are shown in Table 2. The changes in surface roughness Ra are shown in the graphs of Figures 18 to 20. The surface roughness Ra was measured in both the tool feed direction and the direction perpendicular to the tool feed direction (pick direction).

[0113] Photographs of the state of the machined surface at a feed rate per tooth of fz = 0.25 mm / t under each cutting condition are shown in Figures 21 to 23. The vertical direction of the photograph is the tool feed direction, and the horizontal direction is the pick direction.

[0114]

[0115] Each test and example will be described below.

[0116] (Test 1: Theoretical roughness Rmax=1.0 μm) Each of the cutting inserts of Example 1 of the present invention and the conventional Comparative Example 1 was subjected to cutting under the following cutting condition 1.

[0117] <Cutting condition 1> Cutting insert cutting diameter D: 30 mm Radius of curvature R of cutting edge 4 in Example 1 (radius of curvature R of each of outer peripheral cutting edge 11 and inner peripheral cutting edge 12): 14.5 mm, with notch Radius of curvature R of cutting edge 4 in Comparative Example 1 (radius of curvature R of outer peripheral cutting edge 11): 15.0 mm Coolant: dry (air blown) Cutting speed: Vc = 1695.6 m / min Spindle rotation speed: n = 18000 min －１・Feed speed: Vf = 6000 to 16560 mm / min ・Feed per tooth: fz = 0.17 to 0.46 mm / t ・2-way: up & down cut ・Axial depth of cut ap x radial depth of cut ae: 0.05 mm x 0.35 mm ・Tool overhang length: OH = 130 mm ・Theoretical roughness Rmax = 1.0 μm ・Equipment used: MAKINO D800Z HSK-A100 (spindle power: 22 kW / 18.5 kW) ・Surface roughness measurement method: contact-type roughness meter (surface roughness measuring instrument SURFCOM FLEX-35B manufactured by Tokyo Seimitsu Co., Ltd.) ・Machinery surface imaging equipment: Digital Microscope VHX-7000 manufactured by Keyence Corporation (magnification 200x)

[0118] In Example 1 of the present invention, a cutting insert 5 according to the first modification of the above-described embodiment was prepared. That is, it was a notch-type insert having a notch 21 at the center of the tip of the cutting insert 5. The radius of curvature R (each radius of curvature R of the outer cutting edge 11 and the inner cutting edge 12) was designed to be 14.5 mm.

[0119] In Comparative Example 1, a cutting insert was prepared as a conventional example, in which each of the pair of cutting edges 4 did not have an inner peripheral cutting edge 12, and a chisel portion was provided connecting the radially inner ends of each of the outer peripheral cutting edges 11 of the pair of cutting edges 4. In other words, the cutting insert of the Comparative Example had its tip positioned on the central axis of the cutting insert. The radius of curvature R (the radius of curvature R of the outer peripheral cutting edge 11) was designed to be 15.0 mm.

[0120] 18, in Comparative Example 1, the surface roughness Ra (feed direction, pick direction) of the machined surface was 0.8 μm or more, and most of the surface roughness Ra was 1.0 μm or more. On the other hand, in Example 1, it was confirmed that the surface roughness Ra was suppressed to 0.4 μm or less.

[0121] 21, many trochoidal feed marks (surface irregularities) were observed on the machined surface of Comparative Example 1, making the machined surface opaque. On the other hand, the machined surface of Example 1 had fewer feed marks, and a good machined surface was obtained.

[0122] (Test 2: Theoretical roughness Rmax=2.0 μm) Using the cutting inserts of Example 2 and Example 3 of the present invention and the conventional Comparative Example 2, cutting was performed under Cutting Condition 2, which was the above-mentioned Cutting Condition 1 modified in the following respects.

[0123] <Cutting condition 2> Radius of curvature R of cutting edge 4 in Example 2 (radius of curvature R of each of outer peripheral cutting edge 11 and inner peripheral cutting edge 12): 14.5 mm, no notch Radius of curvature R of cutting edge 4 in Example 3 (radius of curvature R of each of outer peripheral cutting edge 11 and inner peripheral cutting edge 12): 13.5 mm, no notch Radius of curvature R of cutting edge 4 in Comparative Example 2 (radius of curvature R of outer peripheral cutting edge 11): 15.0 mm Axial cutting depth ap × radial cutting width ae: 0.05 mm × 0.5 mm

[0124] In Example 2 of the present invention, a cutting insert 5 having the tip shape shown in Figure 12 of the above-mentioned embodiment was prepared. That is, this cutting insert 5 is a notchless insert. In addition, the radius of curvature R (the radius of curvature R of each of the outer cutting edge 11 and the inner cutting edge 12) was designed to be 14.5 mm.

[0125] In Example 3 of the present invention, a cutting insert 5 having the tip shape shown in Figure 12 of the above-mentioned embodiment was prepared. That is, this cutting insert 5 is a notchless insert. In addition, the radius of curvature R (the radius of curvature R of each of the outer cutting edge 11 and the inner cutting edge 12) was designed to be 13.5 mm.

[0126] For conventional comparative example 2, a cutting insert similar to that of comparative example 1 used in test 1 was prepared.

[0127] 19, it was confirmed that the surface roughness Ra (feed direction, pick direction) of the machined surface was suppressed to 0.5 μm or less in both Examples 2 and 3. It was also confirmed that the surface roughness Ra was suppressed to 0.4 μm or less when the feed rate fz per blade was 0.31 mm / t or less.

[0128] In addition, Test 2 is a condition in which the cutting load is increased by setting a value higher than the theoretical roughness Rmax in Test 1. In other words, it can be seen that in Examples 2 and 3, a good machined surface can be obtained even if the cutting load is increased. In addition, the theoretical roughness is Rmax = (ae ２ / 8R) x 10３ This is shown by the formula:

[0129] As shown in FIG. 22, the feed marks were suppressed on the machined surfaces of Examples 2 and 3, and good machined surfaces were obtained.

[0130] Furthermore, it can be seen that feed marks can be significantly suppressed on the machined surface of Example 3. The distance L between each connection point P of a pair of cutting edges 4 of the insert of Example 3 is greater than that of Example 2. As a result, in Example 3, the connection points P of each cutting edge 4 are positioned farther away from the central axis C than in Example 2. In other words, in Example 3, the peripheral speed at the tip of the tool is faster, so the occurrence of chipping on the machined surface is suppressed and surface roughness is kept sufficiently small.

[0131] (Test 3: Presence or Absence of Notch) Using the cutting inserts of Examples 4 and 5 of the present invention, cutting was carried out under Cutting Condition 3, which was obtained by changing the above-mentioned Cutting Condition 1 in the following respects.

[0132] <Cutting condition 3> Radius of curvature R of cutting edge 4 in Example 4 (radius of curvature R of each of outer peripheral cutting edge 11 and inner peripheral cutting edge 12): 14.5 mm, with notch Radius of curvature R of cutting edge 4 in Example 5 (radius of curvature R of each of outer peripheral cutting edge 11 and inner peripheral cutting edge 12): 14.5 mm, without notch Axial cutting depth ap × radial cutting width ae: 0.1 mm × 0.35 mm Theoretical roughness Rmax = 1.0 μm

[0133] In Example 4 of the present invention, a notch-type cutting insert similar to that in Example 1 was prepared. The radius of curvature R (each radius of curvature R of the outer cutting edge 11 and the inner cutting edge 12) was designed to be 14.5 mm.

[0134] In Example 5 of the present invention, a cutting insert of the same unnotched type as in Example 2 was prepared. In addition, the radius of curvature R (the radius of curvature R of each of the outer cutting edge 11 and the inner cutting edge 12) was designed to be 14.5 mm.

[0135] As shown in Table 2 and Fig. 20, it was confirmed that the surface roughness Ra was suppressed to 0.6 µm or less in both Examples 4 and 5. It was also confirmed that the surface roughness Ra was suppressed to 0.5 µm or less when the feed rate fz per tooth was 0.31 mm / t or less.

[0136] In addition, Test 3 is a condition in which the cutting load is increased by setting the axial depth of cut ap to a value higher than that in Test 1. In other words, it can be seen that in Examples 4 and 5, a good machined surface can be obtained even when the cutting load is increased.

[0137] Furthermore, as shown in FIG. 23, the feed marks were suppressed on the machined surfaces of Examples 4 and 5, and good machined surfaces were obtained.

[0138] From Examples 4 and 5 in Test 3 above, it can be seen that good surface roughness and machined surface can be obtained in both the notched type and the non-notched type.

[0139] In the above-described Examples, Tests 1 to 3, it was confirmed that the surface roughness Ra could be suppressed to 0.8 μm or less. Therefore, the grindstone polishing process of the machined surface after machining can be reduced or eliminated.

[0140] The cutting insert and indexable cutting tool of the present invention can achieve a high-quality machined surface with sufficiently low surface roughness through machining. Therefore, the grindstone polishing process of the machined surface after machining can be reduced or even eliminated. This significantly shortens the polishing process of the mold surface. Furthermore, since the mold does not suffer from damage to the design surface or scratches caused by abrasive dust, high-quality molds can be steadily procured. The processing time and processing cost of the mold surface can be significantly improved (reduced). Therefore, the present invention has industrial applicability.

[0141] 1...Tool body, 2...Tip portion, 4...Cutting edge, 5...Cutting insert, 6...Indexable cutting tool, 11...Peripheral cutting edge, 12...Inner peripheral cutting edge, 12a...Radial inner end, 20...Chisel portion, 21...Notch portion, C...Central axis, D...Blade diameter dimension, L, N...Distance, M...Thickness dimension, O...Center of each curvature radius of the outer peripheral cutting edge and the inner peripheral cutting edge, P...Connection point, R...Radius of curvature, VL...Virtual straight line, X1...Rake angle of the inner peripheral cutting edge

Claims

1. A plate-shaped cutting insert that is detachably attached to the tip of a tool body that is rotatable about a central axis, and that has a pair of cutting edges that are spaced apart around the central axis, each cutting edge having an arc-shaped outer peripheral blade that extends radially inward as it approaches the tip in the axial direction, and an arc-shaped inner peripheral blade that is smoothly connected to the radial inner end of the outer peripheral blade and extends radially inward as it approaches the base end in the axial direction, the connection point between the outer peripheral blade and the inner peripheral blade being located at the forefront of the cutting insert, and the outer peripheral blade and the inner peripheral blade having the same radii of curvature at least before and after the connection point.

2. A cutting insert according to claim 1, wherein, in a tip view of the cutting insert seen from the tip side in the axial direction, a distance L between the connection point of one of the pair of cutting edges and the connection point of the other cutting edge is 0.010D≦L≦0.200D, where D is the diameter dimension of the rotation locus of the cutting edges around the central axis.

3. The cutting insert according to claim 1, wherein the rake angle X1 of the inner cutting edge is -15°≦X1≦5°.

4. A cutting insert as described in claim 1, wherein, in a tip view of the cutting insert seen from the tip side in the axial direction, a thickness dimension M between each inner peripheral blade of a pair of cutting edges along a direction perpendicular to the blade length direction in which the inner peripheral blade extends is 0.002D≦M≦0.015D, where D is the diameter dimension of the rotational locus of the cutting edge around the central axis.

5. A cutting insert according to claim 1, wherein, in a plan view of the cutting insert seen from the plate thickness direction, a distance N between the radially inner end of the inner cutting edge and the central axis satisfies 0.002D≦N≦0.015D, where D is the diameter dimension of the rotation locus of the cutting edge around the central axis.

6. A cutting insert according to claim 1, comprising a chisel portion disposed between the radially inner ends of the inner peripheral cutting edges of the pair of cutting edges in a tip view of the cutting insert from the tip side in the axial direction.

7. A cutting insert according to claim 1, wherein, in a tip view of the cutting insert seen from the tip side in the axial direction, the cutting insert has a concave notch portion disposed between the radial inner ends of the inner peripheral cutting edges of the pair of cutting edges and recessed toward the base end side in the axial direction.

8. The cutting insert according to claim 1, wherein the centers of the radii of curvature R of the outer peripheral cutting edge and the inner peripheral cutting edge are located on an imaginary line that passes through the connection point and is parallel to the central axis, and the radii of curvature R satisfy the relationship R≦D / 2, where D is the diameter dimension of the rotational locus of the cutting edge around the central axis.

9. The cutting insert according to claim 1, wherein the radius of curvature of the entire outer cutting edge and the radius of curvature of the entire inner cutting edge are the same.

10. The cutting insert according to claim 1, wherein the radius of curvature of the peripheral cutting edge gradually increases from the connection point toward the radially outer side.

11. The cutting insert according to claim 1, wherein the radially inner end of the inner cutting edge does not reach the central axis.

12. The cutting insert according to claim 1, wherein the cutting length of the inner cutting edge of one of the pair of cutting edges is longer than the cutting length of the inner cutting edge of the other cutting edge.

13. The cutting insert according to claim 1, wherein the radially inner end of the inner peripheral cutting edge of at least one of the pair of cutting edges reaches the central axis.

14. An indexable cutting tool comprising: a tool body that can be rotated around the central axis; and a cutting insert according to any one of claims 1 to 13 that is detachably attached to a tip end of the tool body.

Images