Rotary cutting tools

Abstract

This end mill has a flute groove shape that is easy to form while ensuring sufficient cutting edge rigidity when machining heat-resistant alloys. [Solution] Each flute groove has a fastest descent curve structure in which, in a cross-sectional view perpendicular to the axis O1 of the end mill body, the curvature is maximum near the bottom of the groove, starting from the cutting edge of the outer cutting edge 132a, and the curvature from near the bottom of the groove to the outer circumference 10a is substantially zero.

Description

Technical Field

[0001] The present invention relates to rotary cutting tools such as end mills and drills.

Background Art

[0002] For example, when machining a heat-resistant alloy such as INCONEL (registered trademark) using a rotary cutting tool made of cemented carbide, the chips become sticky and the residence time in the flute groove for discharging them becomes long. Therefore, heat accumulates in the tool body, the cutting resistance becomes excessive, and there is a problem that the durability is impaired.

[0003] As a technique for solving such a problem, in the end mill disclosed in Patent Document 1, the rake angles of a pair of alternately positioned bottom edges and the rake angles of the outer peripheral edges are made different from each other, and the rake angle of the outer peripheral edge continuous with the bottom edge having a small rake angle is made larger than the rake angle of the outer peripheral edge continuous with the bottom edge having a large rake angle, thereby smoothing the distribution of the cutting resistance around the axis of the entire tool.

[0004] Further, in the end mill disclosed in Patent Document 2, the diameter of the rotation locus of one of the plurality of outer peripheral edges is made larger than the diameter of the rotation locus of the other outer peripheral edges over the entire region along the axial direction in which the outer peripheral edges are formed. Also, it is disclosed that the clearance angle (θ) of the one outer peripheral edge is set smaller than the clearance angles of the other outer peripheral edges, and a plurality of grinding stripes enabling finish machining are formed on the clearance surface.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the end mill disclosed in Patent Document 1, the rake angles (axial rake angles) of the multiple cutting edges are not the same, resulting in different shapes for the helical grooves (= flute grooves) used to discharge chips. This makes it impossible to reduce the number of grinding passes required to form each helical groove. The same applies to the end mill disclosed in Patent Document 2, which enlarges only the outer diameter of one outer cutting edge and forms grinding grooves on the relief surface of that outer cutting edge. Furthermore, neither of the end mills disclosed in Patent Documents 1 and 2 takes into account the need to suppress heat buildup in the end mill body due to the accumulation of chips generated during cutting. One object of the present invention is to provide a rotary cutting tool having a flute groove structure that is easy to form and suppresses chip accumulation. Other issues of the present invention will become apparent from this disclosure. [Means for solving the problem]

[0007] One aspect of the present invention is a rotary cutting tool comprising: a flute groove formed on the outer circumference of an end mill body rotatable around an axis and extending from the tip side to the base side of the end mill body; and an outer cutting edge formed on the intersecting ridge line between the rake face facing forward in the rotational direction in the flute groove and the outer circumferential surface of the end mill body, wherein each flute groove has a fastest descent curve structure in which the curvature is maximum near the bottom of the groove, starting from the cutting edge of the outer cutting edge, and the curvature from near the bottom of the groove to the outer circumference is substantially zero. The aforementioned flute grooves can be configured as the brachistochrone curve structure with N (where N is a natural number of 2 or more) grooves having the same rake angle. [Effects of the Invention]

[0008] According to the above embodiment, a rotary cutting tool having flute grooves with a structure that suppresses chip accumulation can be easily realized. [Brief explanation of the drawing]

[0009] [Figure 1]This is a view of the main parts showing an example of the structure of an end mill according to this embodiment. [Figure 2] Schematic diagram of the A-A' section in Figure 1. [Figure 3] Diagram illustrating the chip discharge channel in Figure 1. [Figure 4] An explanatory diagram of the chip discharge channel in a cross-sectional view perpendicular to the axis in Figure 1. [Figure 5] Diagram illustrating the key shape of a composite R-shaped grinding wheel. [Figure 6] A view of the main parts showing an example of the structure of an end mill related to a comparative example. [Figure 7] Diagram illustrating the chip discharge channel in Figure 6. [Figure 8] Diagram illustrating the chip discharge channel in a cross-sectional view perpendicular to the axis of Figure 6. [Modes for carrying out the invention]

[0010] The following describes an example of an embodiment in which the present invention is applied to an end mill, which is an example of a rotary cutting tool. The end mill of this embodiment is a radius end mill suitable for cutting workpieces made of heat-resistant alloys such as INCONEL®. However, it can be similarly applied to other types of end mills and drills, such as square end mills and ball end mills.

[0011] Figure 1 is an external view of the main parts showing an example of the structure of the end mill 1 according to this embodiment, and Figure 2 is a schematic diagram of the cross-sectional view along line A-A' in Figure 1. Figure 3 is an explanatory diagram of the chip discharge channel in Figure 1, and Figure 4 is an explanatory diagram of the chip discharge channel in Figure 2. As shown in Figure 1, the end mill 1 of this embodiment is constructed by integrally molding a shank portion 11, a neck portion 12, and a body portion 13 onto a round bar-shaped end mill body 10 made of cemented carbide that can rotate around an axis O1. In this example, the diameter D1 of the end mill body 10 is assumed to be φ6, but this value is merely an example, and other values ​​are also acceptable.

[0012] The shank portion 11 is a portion to be attached to a machine tool not shown. The outer diameter of the shank portion 11 is formed in accordance with the size of the attachment mechanism on the machine tool side. The neck portion 12 is the rising portion of the body portion 13 in the end mill body 10, and the outer diameter may be smaller than that of the shank portion 11, but in this embodiment, an example in which the shank portion 11 and the neck portion 12 have the same diameter and are integrated is shown.

[0013] The body portion 13 is a portion also called the "cutting edge portion" or "blade portion", and is formed including a bottom blade, an outer peripheral blade, a corner R (Radius) blade, a gash, and a flute groove. The flute groove functions as a flow path for discharging chips generated during cutting. The flute groove also serves to disperse a part of the force during cutting with the outer peripheral blade or the like. This embodiment and FIGS. 1 to 4 mainly show this flute groove portion.

[0014] [Directions and the like used in this embodiment] In the following description, rotation about the axis O1 of the end mill body 1 is referred to as "rotation about the axis", and the direction along the axis O1 is referred to as the "axial direction". Also, among the axial directions, the direction from the shank portion 11 toward the body portion 13 is referred to as the tip side, and the direction from the body portion 13 toward the shank portion 11 is referred to as the base end side. Further, the direction orthogonal to the axis O1 is referred to as the radial direction, and among the radial directions, the direction approaching the axis O1 is referred to as the inner side of the radial direction, and the direction away from the axis O1 is referred to as the outer side of the radial direction. Also, among the directions of orbiting around the axis, the direction in which the end mill 1 is rotated during cutting is referred to as the tool rotation direction, the direction pointing in the tool rotation direction is referred to as the front, and the reverse is referred to as the rear. Other directions and the like are defined as appropriate.

[0015] [Configuration (parts) of the body portion] The body part 13 is formed on the outer periphery 10a at the tip side of the end mill body 10 using a grinding wheel or the like, and has four full flute grooves and four outer peripheral cutting edges that extend from the tip side in the axial direction of the end mill body 10 toward the base end side. Since the shapes and structures of all the full flute grooves and the outer peripheral cutting edges are the same, in the following description, for convenience, the parts related to the adjacent full flute grooves 131a and 131b, the outer peripheral cutting edges 132a and 132b, and the lands 133a and 133b will be described respectively.

[0016] The outer peripheral cutting edge 132a is formed at the intersection ridge line between the rake face 1311a facing the front in the rotation direction T1 in the full flute groove 131a and the outer peripheral surface 10a of the end mill body 10. The outer peripheral cutting edge 132b is formed at the intersection ridge line between the rake face 1311b facing the front in the rotation direction T1 in the full flute groove 131b and the outer peripheral surface 10a of the end mill body 10. Lands 133a and 133b exist behind the outer peripheral cutting edges 132a and 132b respectively.

[0017] [Full Flute Grooves and Outer Peripheral Cutting Edges] Referring to FIGS. 1 to 3, in a cross-sectional view (A-A' curve: FIG. 2) orthogonal to the axis O1 of the end mill body 10, the full flute groove 131a is formed such that the groove space (pocket) from the cutting edge of the outer peripheral cutting edge 132a toward the outer peripheral surface 10a has a brachistochrone structure. There is a well-known term "brachistochrone" in the field of analytical mechanics. The "brachistochrone" is, for example, the curve that reaches the goal fastest among the courses connecting two points with a height difference, and the "brachistochrone structure" refers to the structure that realizes a bundle of such "brachistochrones" with a concave surface.

[0018] Specifically, as shown in Figure 2, the "fastest descent curve structure" is a concave curved structure that includes a curve 141a with curvature determined by the rake angle γ, starting from the cutting edge of the outer cutting edge 132a and extending toward the groove bottom (near the deepest part of the pocket), a curve 141b with maximum curvature near the groove bottom, and a nearly straight line 141c where the curvature is virtually zero (radius of curvature is almost infinite) between the groove bottom and the outer circumference 10a. The nearly straight line 141c refers to a straight line or a concave curve that can be considered a straight line. That is, between the groove bottom and the outer circumference, it refers to a line where the force f11 acting on the chip (described later) does not change in the direction of the axis O1 after initially changing away from the axis O1. It is expressed as "nearly straight" because it does not need to be strictly a straight line. By using such a "fastest descent curve structure," the chip accelerated near the groove bottom will move toward the outer circumference 10a at the fastest speed without being decelerated. In the illustrated example, the groove can be a roughly straight line that extends diagonally downward from near the bottom of the groove (towards an axis perpendicular to the line connecting the cutting edge and the axis).

[0019] The "fastest descent curve structure" can be described as a structure in which, when viewed from the outer circumference 10a, the curvature of the curve 141a from near the outer circumference 10a to near the groove bottom, and then to the cutting edge of the outer peripheral blade 132a extending towards the base end of the end mill body 10, is smaller than the curvature of the curve 141b near the groove bottom.

[0020] The specific shape and size of the "fastest descent curve structure" can be determined using a known numerical calculation application, with parameters such as the size and weight of the workpiece processed by the end mill per unit time, the frequency of chip generation, and the pocket size.

[0021] As shown in Figures 3 and 4, in a flute groove 131a of this shape, the force f11 acting on the chips generated when the workpiece is cut by the outer cutting edge 132a during machining is accelerated to the bottom of the flute groove 141a and guided in the direction of the outer circumference 10a at the fastest speed and shortest distance. This suppresses heat buildup in the end mill body 10 due to chips accumulating in the pockets of the flute groove 131a.

[0022] The flute groove 131a extends spirally in the circumferential direction along the axis O1 of the end mill body 10, from the tip side along the axis O1 towards the base side, and in the opposite direction to the tool rotation direction T1. The end portion 135 of the flute groove 131a is curved and cuts up to the outer circumference of the neck portion 12. In other words, since the cut-up portion of the flute groove 131a is a curved region 135, there are no sharp corners in the cut-up portion, which effectively prevents breakage of the end mill 1.

[0023] [land] A land 133a, having the same diameter as the outer surface 10a, exists in the portion of the outer cutting edge 132a that is connected to the rear in the tool rotation direction T1. The land 133a is a plane with a small curvature and width located rear in the tool rotation direction T1, and is distinct from the "margin" described in Patent Document 2. The "margin" is a narrow contact surface formed between the cutting surface of the end mill and the cutting edge, and is formed for the purpose of adjusting friction. In contrast, the land 133a is a relatively flat surface of the cutting edge portion, and is the flat portion of the cutting edge itself. The land 133a is the surface that the cutting edge is machined from, and is formed for the purpose of maintaining the sharpness, strength, and durability of the cutting edge. This makes it possible to significantly increase the rigidity of the cutting edge portion of the outer cutting edge 132a compared to a structure in which it does not exist. A similar explanation is also applicable to the land 133b. In reality, after the flute groove is formed, a relief surface is formed on the rear side of the land 133a in the direction of rotation. In this case as well, the land width (d2, described later) will include the width of the relief surface.

[0024] [End mill rigidity] The rigidity of the end mill 1 is affected by cutting conditions and the hardness of the workpiece, but when the workpiece is a difficult-to-machine material such as a heat-resistant alloy, parameters such as the rake angle γ and helix angle β of the outer cutting edge 132a shown in Figure 2 (see Figure 1), the diameter D1 of the end mill body 10, the core thickness d1, and the land width d2 have a significant influence. According to repeated measurements by the inventors, the parameters for obtaining the desired rigidity (in terms of lifespan) when cutting a heat-resistant alloy are a rake angle γ of -10° to +10° and a helix angle β of 40° to 50°. In addition, the core thickness d1 is 45% or more and less than 65% of D1, and the land width d2 is 10% or more of the outer circumference 10a. A more significant effect was observed when the land width d2, which is the length of the land 133a that is included in the outer circumference 10a, was in the range of 15% to 30%. By selecting these values ​​as parameters, it was possible to increase the rigidity when cutting a heat-resistant alloy using the end mill 1 of this embodiment and extend the lifespan compared to using parameters other than those specified. The basis for these values ​​is as follows.

[0025] When the rake angle γ is below -10°, the machinability of heat-resistant alloys decreases, leading to increased heat generation. This heat generation reduces wear resistance, causing welding or abnormal wear of the cutting edge. Furthermore, when the rake angle is above 10°, insufficient cutting edge rigidity reduces fracture resistance, leading to chipping or abnormal wear of the cutting edge.

[0026] When the helix angle β is 40° or less, the cutting load cannot be adequately distributed, causing overheating due to the load on the cutting edge and resulting in a shortened tool life. Furthermore, at helix angles β of 50° or more, the twisting action strongly pulls the workpiece and cutting tool together, causing abnormal wear due to overheating. In particular, when machining thin plates, bending and tool biting can occur, degrading the machined surface and significantly reducing machining accuracy.

[0027] If the core thickness d1 is less than 45% of D1, it is not possible to form a tool cross-section that is approximately straight (chip accumulation cannot be avoided). Also, if it exceeds 65%, chip evacuation performance becomes significantly worse.

[0028] When the land width d2 is less than 10% of the circumference, the outer edge chips quickly break due to insufficient rigidity of the cutting edge thickness when cutting heat-resistant alloys. At a land width greater than d2, especially 15%, sufficient rigidity for practical use could be ensured. However, at land widths of 30% or more, although rigidity increases, the pocket space of the flute groove narrows, resulting in significantly poor chip evacuation.

[0029] [Unequal division] The first flute groove 131a and the second flute width 131b described above may be equally divided, but it was confirmed that the same effect as above can be obtained even if they are unequally divided, as long as the division is in the range of 3° to 10°, compared to the case of equal division. The unequal division interval shifts the phase of the vibration, canceling out the periodicity of the vibration. However, if the unequal division is less than 3°, the effect of canceling out the periodicity of the vibration is weak, and chatter vibration cannot be sufficiently suppressed. Furthermore, it was confirmed that if it exceeds 10°, the amount of material removed increases due to the amount of feed being biased to one side, making it easier to cause chipping, and the chip evacuation efficiency in one of the flute grooves decreases.

[0030] [Example of end mill manufacturing method] The shape of all flute grooves, including the flute groove 131a which has curves 141a, 141b, and a nearly straight line 141c forming the velocitate descent curve structure shown in Figures 2 and 4 in cross-sectional view, can be formed by multiple grinding passes, provided that manufacturing costs are not considered. When forming the flute grooves of an end mill 1, curved concave surfaces are generally formed with a flat grinding wheel, but straight planes are difficult to form with a flat grinding wheel. Furthermore, forming grooves that form the velocitate descent curve in cross-sectional view according to the design value requires numerous forming operations, making mass production difficult.

[0031] Therefore, in this embodiment, the flute groove was formed by a single-pass cutting process using a composite R-shaped grinding wheel, the main part shape of which is shown in Figure 5. The composite R-shaped grinding wheel 200 shown in Figure 5 has three types of convex surfaces 201, 202, and 203 that correspond to the curves 141a, 141b, and the approximately straight line 141c, respectively. The grinding wheel is a diamond grinding wheel. When forming a flute groove that combines curves and straight lines in a cross-sectional view (A-A') perpendicular to the axis O1 of the end mill body 10 using a flat grinding wheel or the like, the processing must be repeated many times. However, by using the composite R-shaped grinding wheel 200, it was possible to form the flute groove simply, quickly, and with high precision.

[0032] [Comparative Example] The applicant attempted to create a comparative end mill, manufactured using a flat grinding wheel instead of the composite R-shaped grinding wheel 200 shown in Figure 5, so that it would have almost the same shape and structure as the end mill 1 of this embodiment. Figure 6 shows the main components of this comparative end mill 2. Figure 7 is an explanatory diagram of the chip discharge channel in the comparative end mill 2, and Figure 8 is an explanatory diagram of the chip discharge channel in the comparative end mill 2. The same reference numerals are used for parts that are the same as those in the end mill 1 of this embodiment, and different reference numerals are used only for parts that are different.

[0033] In the comparative example, even if the end mill 2 is shaped such that the groove space (pocket) extending from the cutting edge to the outer surface 10a forms a brachistochrone curve structure in a cross-sectional view perpendicular to the axis O1 of the end mill body 10 (A-A' curve: Figure 6), when formed using a flat grinding wheel, the flute groove 231a is not necessarily the same as the flute groove 131a shown in Figure 1, as shown in Figure 6. In other words, due to the nature of flat grinding wheels, which are originally designed to shape curved surfaces, a slightly raised portion inevitably occurs at the bottom of the pocket from the tip to the base, extending from the deepest part. Furthermore, even if the shape and size of the flat grinding wheel are adjusted, grinding unevenness 234a occurs at the rear of the land 133b in the direction of tool rotation, due to its shape and size.

[0034] Therefore, as shown in Figures 7 and 8, the rotation of the tool during cutting generates a force f21 acting on the chips heading towards the raised portion of the flute groove 231a, and a force f22 acting on the chips heading from that portion towards the grinding unevenness 234a. These forces are smaller and have a different direction (curvature) than the force f11 shown in Figure 3, resulting in a longer chip retention time compared to the flute groove 131a formed by this embodiment. While it is possible to avoid this phenomenon by repeatedly processing the raised areas and uneven grinding 234a mentioned above, this process is extremely time-consuming.

[0035] In this embodiment, since the composite R-shaped grinding wheel 200 is used to form the groove in a single process, it is not only advantageous in terms of manufacturing costs, but it also avoids the reduction in surface machining accuracy that can occur when machining the same part repeatedly, resulting in the advantage of being able to machine the groove surface with high precision. [Explanation of Symbols]

[0036] 1 End mill 10 End mill body 131a, 131b Flute groove 132a,132b Peripheral blade 133a, 133b Land 200 Compound R-shaped grinding wheel O1 axis D1 End mill body diameter

Claims

1. A flute groove formed on the outer circumference of the end mill body, which is rotatable around its axis, and which extends from the tip side to the base side of the end mill body, The end mill comprises an outer cutting edge formed on the intersecting ridge between the rake face facing forward in the rotational direction in the flute groove and the outer circumferential surface of the end mill body, Each flute groove has a fastest descent curve structure in which the curvature is maximum near the bottom of the groove, starting from the cutting edge of the outer blade, and the curvature from near the bottom of the groove to the outer circumference is substantially zero. Rotary cutting tool.

2. The aforementioned fastest descent curve structure has a curvature from the outer circumference to the vicinity of the groove bottom, and then to the cutting edge extending towards the base end, which is smaller than the curvature near the groove bottom. The rotary cutting tool according to claim 1.

3. The aforementioned flute groove has the brachistochrone curve structure of N (where N is a natural number of 2 or more) grooves having the same rake angle, The rotary cutting tool according to claim 1.

4. A land exists in the portion of the outer peripheral cutting edge that is connected to the rear in the direction of tool rotation, and the width of the land along the outer circumference exceeds 10% of the outer circumference. The rotary cutting tool according to claim 1.

5. In a cross-sectional view perpendicular to the axis of the end mill body, the core thickness is 45% or more and less than 65% of the outer diameter. The rotary cutting tool according to claim 1.

6. The aforementioned flute grooves are formed in a single pass using a composite R-shaped grinding wheel having multiple convex surfaces. A rotary cutting tool according to any one of claims 1 to 5.

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