Indexable insert for milling and milling cutter emplyoing the same

Abstract: An indexable insert for milling comprises a metal base (1) which is made of cemented carbide and a subcutting edge (3) consisting of a sintered CBN compact (2) and a flat drag type cutting edge (4) which are brazed to the metal base (1) or bonded thereto by integral sintering. A subcutting edge angle (β), a negative land angle () and a negative land width (L) are set at 30° to 60°, 30° to 45° and 0.05 to 0.40 mm respectively, while the subcutting edge (3) has a straight shape. Due to this structure, the indexable insert has excellent cutting performance particularly in face milling of parts which are made of gray cast iron, and the tool life can be extended.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an indexable insert which is clamped into a face milling cutter in face milling of cast-iron parts and a milling cutter employing the same, and more particularly, it relates to an indexable insert for milling which enables ultrahigh-speed milling of cast-iron parts while attaining a long tool life and a milling cutter employing the same. The indexable insert for milling according to the present invention attains a remarkable effect particularly when the same is applied to milling of gray cast iron. Description of the Background Art Cast-iron parts such as cylinder blocks and cylinder heads for automobile engines are generally face-milled by cemented carbide inserts, coated inserts, ceramic inserts etc. In general, negative land angles of 15° and 25° are well known in relation to conventional indexable inserts for milling which are made of cemented carbide and formed by a sintered cubic boron nitride compact (hereinafter referred to as "CBN compact") respectively. In face milling of cast-iron parts, cemented carbide and coated inserts are utilized at cutting speeds V of about 150 to 250 m/min., while a ceramics insert is utilized at a cutting speed V of about 400 m/min. in practice. If the cutting speeds are increased beyond these ranges, the tool life of the inserts is disadvantageously reduced to increase the working costs. In recent years, however, machine tools which are capable of high-speed rotation have been developed one after another, and awaited is provision of tools having cutting edges which can cope with such machine tools. Increase of the cutting speed leads to improvement of productivity, as a matter of course. It has been known in the art that the cutting speed of a cutting tool which is prepared from a sintered CBN compact can be increased to at least three times that of a ceramics insert. However, the sintered CBN compact is easy to chip since the same is inferior in toughness to other cutting tool materials due to its characteristics, and thermal crack is easily caused due to heat affection. When the sintered CBN compact is simply applied to a cutting edge, therefore, it may not be possible to attain a sufficient life. SUMMARY OF THE INVENTION An object of the present invention is to provide an indexable insert for milling which can excellently finish a machined surface of a workpiece while attaining a sufficient tool life particularly in high-speed cutting of at least 800 m/min., or at least 1000 m/min. and a milling cutter employing the same. According to the present invention a indexable insert being clamped into a milling cutter for face milling, comprises a metal base being made of cemented carbide containing WC and Co, and a cutting edge, consisting of a sintered cubic boron nitride compact, being brazed to or integrally sintered with said metal base, said cutting edge including a subcutting edge having a subcutting edge angle (β) of at least 30° and not more than 60°, and a flat drag type cutting edge being continuous thereto, and having a negative land angle () of at least 30° and not more than 45° and a negative land width (L) of at least 0.05 mm and not more than 0.40 mm, said subcutting edge having a straight shape. According to this structure of the indexable insert for milling, the cutting edge is hardly affected by heat due to the subcutting edge angle β of at least 30°, whereby occurrence of thermal crack is prevented. Further, increase of cutting resistance is prevented since the subcutting edge angle β is not more than 60°, whereby the indexable insert is maintained in excellent sharpness. Due to the negative land angle  of at least 30° and 45°, further, high-speed milling of at least 800 to 1000 m/min. is enabled. While a conventional sintered CBN compact tool has a standard negative land angle  of 25°, the aforementioned high-speed milling is enabled by increasing the negative land angle  beyond this standard. While it has generally been regarded as impractical to increase the negative land angle  since cutting resistance is excessively increased in this case, the present invention enables cutting with a large negative land angle  in milling at a high speed exceeding 800 m/min., since the strength of a workpiece is rapidly reduced, particularly when the workpiece is made of gray cast iron. In addition, the negative land width L is set to be at least 0.05 mm, whereby the subcutting edge and the flat drag type cutting edge are inhibited from chipping. The negative land width L is set to be not more than 0.40 mm, in order to form a cutting edge ridgeline 8 within the range of a thickness t of a CBN layer as shown in Fig. 19, since the thickness (t shown in Figs. 19 and 20) of a general CBN layer is about 0.8 mm. If the negative land angle  and the negative land width L are set at 45° and 1.2 mm respectively, for example, the negative land is so excessively increased in size that no actual cutting edge ridgeline 8 is formed on the sintered CBN compact as shown in Fig. 20. Further, the sintered CBN compact is so hard that it is difficult to grind the same as compared with cemented carbide etc. If the negative land width L is meaninglessly increased, therefore, the time for grinding the negative land is so extremely increased that the cost for working the indexable insert is disadvantageously increased as the result. The aforementioned object of the present invention is attained through the straight shape of the subcutting edge, and no desired effect can be attained if the subcutting edge has a curved shape. When the subcutting edge has a straight shape, the contact length between the same and a workpiece can be further reduced as compared with a curved subcutting edge in relation to the same depth of cut. The negative land width L of the cutting edge is preferably at least 0.075 mm and not more than 0.30 mm. The effect of inhibiting the cutting edge from chipping is further facilitated due to the lower limit of at least 0.075 mm, while the thickness of the CBN layer is further sufficiently ensured to further facilitate the effect of preventing increase of the time for grinding the negative land due to the upper limit of not more than 0.30 mm. The flat drag type cutting edge is preferably in the form of a circular arc having a radius of curvature of at least 200 mm and not more than 400 mm, in order to improve machined surface roughness due to such an arcuate shape of the flat drag type cutting edge. Fig. 21 shows an exemplary flat drag type cutting edge 4. The radius R of curvature of the arcuate tooth profile is set in the range of at least 200 mm and not more than 400 mm for the following reason: If the radius R of curvature of the arcuate tool profile exceeds 400 mm, the contact length between the flat drag type cutting edge 4 and a workpiece is further increased as compared with a cutting edge having a small radius of curvature to increase cutting resistance, leading to occurrence of a chatter phenomenon during cutting. If the radius R of curvature is smaller than 200 mm, on the other hand, machined surface roughness of the workpiece is hardly improved as compared with an indexable insert which is in the form of a straight flat drag. The aforementioned indexable insert for milling according to the present invention can also be employed along with a cemented carbide insert and a ceramics insert. In other words, indexable inserts of different materials can be applied to a milling cutter together. In milling with such different types of inserts, the indexable insert according to the present invention is employed as a wiper insert, to attain an effect of improving machined surface roughness of the workpiece. However, the cemented carbide and ceramics inserts are undurable for high-speed milling. In order to carry out high-speed cutting, therefore, all of the plurality of indexable inserts which are clamped on the cutter are preferably prepared from the inventive indexable inserts. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are a plan view and a right side elevational view showing an indexable insert employed in Examples of the present invention, and Fig. 1C is a sectional view taken along the line A - A in Fig. 1A; Fig. 2 is a perspective view showing a typical mode of an indexable insert for milling to which the present invention is applied; Figs. 3A and 3B are perspective views illustrating a substrate and a cutting edge of the indexable insert for milling shown in Fig. 2, in a state not brazed to each other; Fig. 4 is a perspective view illustrating a modification of the shape of a cutting edge of the indexable insert for milling shown in Fig. 2; Fig. 5 is a perspective view illustrating another modification of the shape of the cutting edge of the indexable insert for milling shown in Fig. 2; Fig. 6 is a perspective view showing an indexable insert for milling, to which the present invention is applied, in such an exemplary mode that a cutting edge is brazed to an upper surface around an end of an elongated metal base; Fig. 7 is a perspective view showing an indexable insert for milling, to which the present invention is applied, having a substrate and a cutting edge made of sintered CBN compact integrally sintered with each other; Fig. 8 shows enlarged plan views around subcutting edges, negative land angles, and damaged states of cutting edges after experiments in indexable inserts employed for respective experiments in Example 1 of the present invention; Figs. 9A and 9B are microphotographs showing a typical thermal crack caused in an edge and a typical example of a worn state of an edge respectively; Fig. 10 illustrates results of experiments in Example 2 of the present invention; Fig. 11 illustrates results of experiments in Example 3 of the present invention; Figs. 12A and 12B illustrate results of experiments in Example 4 of the present invention respectively; Fig. 13 is a perspective view showing a cylinder block employed as a workpiece for each experiment in Example 5 of the present invention; Figs. 14A, 14B and 14C are microphotographs showing the state of a cutting edge causing thermal crack in Example 5 of the present invention, the state of a cutting edge chipped off with progress of thermal crack, and the state of a cutting edge of a indexable insert worn after a cutting test in experiment No. 8 of Example 5 respectively; Figs. 15A and 15B are a plan view and a front elevational view of a face milling cutter on which the inventive indexable inserts for milling are clamped; Fig. 16 is an enlarged perspective view showing a portion around a subcutting edge for illustrating definition of a flank wear width w of a cutting edge; Figs. 17A, 17B and 17C are enlarged plan views showing portions around subcutting edges having a straight shape, an arcuate shape, and a combined shape of straight and arcuate ones respectively; Figs. 18A, 18B and 18C are enlarged plan views showing contact portions between a straight subcutting edge having a relatively small subcutting edge angle β, a straight subcutting edge having a relatively large subcutting edge angle β and an arcuate subcutting edge and workpieces respectively; Fig. 19 illustrates an exemplary cutting edge, worked with a negative land angle  and a negative land width L of proper values, having a cutting edge ridgeline 8 formed on a CBN layer; Fig. 20 illustrates a portion around a cutting edge of a tool, worked with a negative land angle  and a negative land width L larger than proper values, having a cutting edge 8 formed outside a CBN layer in an enlarged manner; Fig. 21 illustrates a typical flat drag type cutting edge of an indexable insert; Fig. 22 is a perspective view typically showing the state of each cutting experiment carried out in relation to the present invention; and Fig. 23A is a partially fragmented perspective view of the face milling cutter shown in Fig. 15A as viewed along arrow E, and Fig. 23B illustrates a portion B in Fig. 23A in an enlarged manner. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are now described with reference to the drawings. Fig. 2 shows a typical mode of an indexable insert for milling to which the present invention is applied. The indexable insert shown in Fig. 2 consists of a metal base 1 which is made of cemented carbide containing WC and Co, and a sintered CBN compact 2, forming a cutting edge, which is brazed to a portion close to an upper surface corner of the metal base 1. The CBN sintered compact 2 has a subcutting edge 3 on an upper edge of its corner portion, and a flat drag type cutting edge 4 is provided in continuation to this subcutting edge 3. Figs. 3A and 3B illustrate the metal base 1 and the sintered CBN compact 2 of the indexable insert shown in Fig. 2, which are not yet brazed to each other. As shown in Fig. 3B, the sintered CBN compact 2 is formed by a substrate 2a of cemented carbide and a CBN layer 2b which are integrally sintered with each other in a stacked state. Figs. 4 and 5 shows other modes of indexable inserts in which cutting edges consisting of sintered CBN compacts are brazed to only portions close to upper surface corners of metal bases. Fig. 6 shows a further indexable insert in which a sintered CBN compact 2 is brazed to only an upper surface portion around an end of a metal base 1, made of cemented carbide, which is in the form of an elongated block. While the sintered CBN compacts 2 are bonded to the metal bases 1 by brazing in all of the indexable inserts shown in Figs. 2 to 6, the present invention is also applicable to an indexable insert formed by a support layer 2a of cemented carbide and a CBN layer 2b consisting of a sintered CBN compact which are integrally sintered with each other, as shown in Fig. 7. Each of the aforementioned indexable inserts is clamped as a cutting edge into a face milling cutter shown in Figs. 15A and 15B. Referring to Figs. 15A and 15B, indexable inserts having the shape shown in Fig. 2 are clamped into the body 5 of the face milling cutter, so that the metal base 1 of each indexable insert is fixed to the cutter body 5 by a clamp wedge 6 and a clamp screw 7. Fig. 23A is a perspective view of the face milling cutter as viewed along arrow E in Fig. 15A, three-dimensionally showing the indexable inserts which are clamped into the cutter body. Fig. 23B is an enlarged view of an edge part B of the indexable insert shown in Fig. 23B, showing a subcutting edge angle β, a negative land angle , a negative land width L and a clearance angle α of the indexable insert respectively. Concrete Examples including experiments for verifying the function/effect of the indexable insert for milling according to the present invention are now described with reference to the drawings. In general, the cutting edge of a face milling cutter is formed by a subcutting edge 3 and a flat drag 4 as shown in Figs. 1A to 1C, and a negative land is provided as the case may be. It has been proved by the results of the following experiments that setting of a subcutting edge angle β, a negative land angle  and a negative land width L has an extremely important meaning when a sintered CBN compact is employed as the cutting edge. In the following Examples, indexable inserts of the mode shown in Fig. 2 were employed with planar and right side elevational shapes shown in Figs. 1A and 1B respectively. The subcutting edge angle β is defined in Fig. 1A, while the negative land angle  and the negative land width L are defined in Fig. 1C. A clearance angle which is defined by an angle α shown in Fig. 1B may be either 0° or an acute angle which is in the range of 5° to 20°. In each of the following Examples, the clearance angle α was set at 15°. In each of the indexable inserts employed in the following Examples, the subcutting edge 3 had a straight shape. The reason for this is explained as follows, on the basis of Figs. 17A to 17C and 18A to 18C: As to shapes of portions of cutting edges around subcutting edges, a straight shape, an arcuate shape and a combination of straight and arcuate shapes are employable as shown in Figs. 17A to 17C respectively. The arcuate subcutting edge shown in Fig. 17B has been mainly employed, and no straight cutting edge has been used in general. Comparing Figs. 18A and 18B with each other, however, it is understood that the contact length l between an arcuate subcutting edge shown in Fig. 18C and a workpiece is longer than that between a straight subcutting edge shown in Fig. 18A and a workpiece in relation to the same depth d of cut. In the case of a subcutting edge having a curved shape such as an arcuate shape, therefore, heat affection in cutting is so increased as to disadvantageously cause thermal crack. On the other hand, it is possible to minimize the contact length between the subcutting edge and the workpiece by straight forming the subcutting edge in accordance with the inventive indexable insert, thereby reducing heat affection and suppressing occurrence of thermal crack. While the flat drag type cutting edge 4 of the sintered CBN compact 2 may have a straight shape, arcuate flat drag type cutting edges having radii of curvature of 200 to 400 mm were employed in the following Examples, in order to improve machined surface roughness. Example 1 In Example 1, the negative land width L was fixed at 0.2 mm, while the subcutting edge angle β and the negative land angle  were varied to carry out experiments. In these experiments, two plates 12a and 12b of 25 mm in width and 150 mm in length which were made of gray cast iron (FC 250 in Japanese Industrial Standard) were employed and set as shown in Fig. 22. An indexable insert of experiment No. 1 shown in Fig. 8 was first clamped on a face milling cutter 13 of 200 mm in diameter shown in Fig. 22 to carry out cutting for 100 passes under conditions of a cutting speed of 1000 m/min., a feed rate of 0.15 mm/tooth, and a depth of cut of 0.5 mm, and a damaged state of the cutting edge was checked. Thereafter similar experiments were successively carried out on indexable inserts of experiments Nos. 2 to 12 shown in Fig. 8. Each experiment was stopped at 100 passes since it was possible to correctly compare damaged states of the various shapes of edges employed in the experiments after cutting for 100 passes. Not every cutting edge reaches the end of its life after cutting for 100 passes. In each intermittent cutting experiment shown in Fig. 22, the face milling cutter 13 was rotated along arrow G, while the plates 12a and 12b were fed along arrow F. Dimensions L1, W1 and W2 shown in Fig. 22 are 150 mm, 25 mm and 65 mm respectively. Fig. 8 shows states of the edges of the respective indexable inserts which were damaged as the results of these experiments. This figure shows thermal crack of the cutting edges and flank wear conditions, and Figs. 9A and 9B are enlarged views showing such thermal crack and flank wear respectively. Fig. 9A shows the state of thermal crack caused on a cutting edge. In general, the damaged state of the edge is changed from cracking to chipping as the number of such thermally cracked portions and the depths thereof are increased. Thus, a longer tool life can be attained as the number of such thermally cracked portions and the depths thereof are reduced. Fig. 9B shows flank wear which was caused on the cutting edge while rounding the edge. In this case, a longer tool life can be attained as the flank wear width is reduced, due to occurrence of no thermal crack. It is understood from the states of the edges damaged as the results of experiments shown in Fig. 8 that occurrence of thermal crack is so gradually suppressed that the number of thermally cracked portions is reduced as the negative land angle  is increased, regardless of the subcutting edge angle β. Then, it has been recognized by varying the subcutting edge angle β while retaining the negative land angle  at a constant value that the edge is easily affected by heat to cause thermal crack as the subcutting edge angle β is increased while occurrence of thermal crack is reduced and flank wear is increased as the subcutting edge angle β is increased. Among the indexable inserts of experiments Nos. 1 to 12 shown in Fig. 8, that of experiment No. 8 having the negative land angle  of 45° and the subcutting edge angle β of 45° exhibited small thermal cracking and small flank wear. The relation between the value of the subcutting edge angle β and heat affection in cutting is described as follows, by comparison of Figs. 18A and 18B: While the contact length l between the workpiece and the subcutting edge is relatively increased at a prescribed depth of cut d when the subcutting edge angle β is relatively small as shown in Fig. 18A, the contact length l between the subcutting edge and the workpiece at the same depth of cut d is remarkably reduced when the subcutting edge angle β is increased, as shown in Fig. 18B. Since heat affection in cutting is increased as the contact length l is increased, it is understood that the heat affection in cutting can be reduced as the subcutting edge angle β is increased. When the subcutting edge angle β was increased in practice, however, flank wear was increased while rounding the cutting edge due to increase of the actual chip thickness and cutting resistance to reduce sharpness, although occurrence of thermal crack was prevented. The state of the cutting edge damaged in this case has already been described in detail with reference to Fig. 9B. Noting the negative land angle , thermal crack is easily caused and the depth of the cracked portion is increased when this angle is 25°. On the other hand, thermal crack is hardly caused when the negative land angle  is 45°, and even if thermal crack is caused, progress thereof is retarded. Thus, it has been proved that the subcutting edge angle β is in the range of 30° to 60° and the negative land angle  is in the range of 30° to 45° in a proper edge shape minimizing occurrence of thermal crack and maintaining sharpness of the cutting edge. Example 2 In order to examine the relation between the subcutting edge angle β and the negative land angle  in further detail on the basis of the results of Example 1, experiments were made by varying combinations of these angles as shown in Fig. 10. The negative land width L was fixed at 0.2 mm. The same workpieces as those in Example 1 were employed, to check the numbers of pass times up to flank wear widths, defined by a width w shown in Fig. 16, of 0.2 mm under conditions of a cutting speed of 1500 m/min., a feed rate of 0.15 mm/tooth, and a depth of cut of 0.5 mm. A cutter of 200 mm in diameter, which was identical to that in Example 1, was so employed that each of indexable insert having various negative land angles  and subcutting edge angles β was clamped into this cutter for cutting the workpieces. Consequently, it has been verified that ranges enabling cutting for 180 to 200 passes are 30° to 60° for the subcutting edge angle β and 30° to 45° for the negative land angle  respectively. Further, it was possible to attain machined surface roughness of at least 6.3 Z under JIS (Japanese Industrial Standard) by face milling with such a cutting edge. In Example 2, the flank wear width reached 0.2 mm after cutting for 180 to 200 passes since only one indexable insert was clamped into the cutter body. In actual machining of a cylinder block for an automobile engine, for example, at least eight indexable inserts are generally clamped into a cutter. In practice, therefore, at least 8 x 200 passes, i.e., at least 1600 passes can be expected for the tool life. Example 3 Then, the following experiments were made in order to compare the life of the inventive indexable insert with those of conventional indexable inserts made of cemented carbide and ceramics. Indexable inserts of cemented carbide (K10), ceramics (Al 2 O 3 -TiC) and sintered CBN compacts were clamped on a face milling cutter of 200 mm in diameter respectively for cutting workpieces at a constant feed rate f of 0.15 mm/tooth and a constant depth of cut d of 0.5 mm and various cutting speeds Vm/min., to check numbers of cutting passes bringing flank wear widths of 0.2 mm. The workpieces were identical to those employed in Example 1. The sintered CBN compacts were prepared from those employed in experiments Nos. 1, 4 and 8 in Example 1. Fig. 11 shows the results of experiments in Example 3. As understood from Fig. 11, flank wear was so quickly caused that the flank wear width reached 0.2 mm after cutting for 5 passes in the indexable insert of cemented carbide cutting the workpiece at a cutting speed of 150 m/min. In the indexable insert of ceramics, the flank wear width reached 0.2 mm after cutting the workpiece for 10 passes at a cutting speed of 400 m/min. As to the sintered CBN compacts which were employed for cutting the workpieces at cutting speeds in the range of 400 to 1500 m/min., on the other hand, each indexable insert of the sintered CBN compact of experiments Nos. 1, 4 and 8 reached a flank wear width of 0.2 mm with 50 to 60 passes at the cutting speed of 400 m/min. which was identical to that for the ceramics indexable insert, and attained a tool life which was 5 to 6 times that of the ceramics indexable insert. The lives of these indexable inserts were further increased as the cutting speeds were increased, such that the indexable insert of the sintered CBN compact of experiment No. 8 was capable of cutting the workpiece for 200 passes at the cutting speed of 1500 m/min. and attained a life of 20 and 40 times those of the ceramics and cemented carbide indexable inserts respectively although the indexable inserts of the sintered CBN compacts of experiments Nos. 1 and 4 were chipped by thermal crack at 90 and 140 passes respectively. Example 4 Then, the negative land width L was varied in the range of 0.025 to 0.40 mm in six types of indexable inserts and the feed rate f was varied in the range of 0.05 to 0.30 mm/tooth as shown in Figs. 12A and 12B to check presence/absence of chipping of cutting edges, in order to analyze the proper value of the negative land width L. Workpieces identical to those in Example 1 were employed and each of the aforementioned six types of indexable inserts was clamped on a face milling cutter of 200 mm in diameter to cut the workpieces at a cutting speed of 1500 m/min. and a depth of cut of 0.5 mm. Fig. 12A shows the results of cutting tests which were made with edges having subcutting edge angles β of 45° and negative land angles  of 30°. When the negative land widths L were 0.025 mm and 0.05 mm, the cutting edges were chipped with respect to all feeds. When the negative land widths L were in excess of 0.075 mm, on the other hand, occurrence of chipping was suppressed. Fig. 12B shows the results of cutting tests which were made with edges having subcutting edge angles β of 45° and negative land angles  of 45°. While the cutting edges were not chipped at the negative land widths L of 0.05 mm when the feed rates f were 0.05 mm/edge and 0.10 mm/edge, chipping was caused when the feed rates were in excess of 0.15 mm/edge. Analyzing the data shown in Figs. 12A and 12B together, it is understood that the negative land width L must be at least 0.05 mm, and preferably at least 0.075 mm, in order to prevent chipping in the normal feed rate range of 0.05 to 0.30 mm/edge. As to the upper limit of the negative land width L, no cutting edges were chipped up to 0.4 mm, while the thickness of the CBN layer becomes thinner and the time for grinding the negative land is disadvantageously increased when the negative land width L is increased since the CBN layer of the sintered CBN compact is about 0.8 mm in thickness as already described with reference to Figs. 19 and 20. Therefore, the upper limit of the negative land width L must be not more than 0.4 mm, and preferably not more than 0.3 mm. Example 5 Then, 12 indexable inserts of experiment No. 1 of Example 1 were clamped into a face milling cutter of 250 mm in diameter, to cut an upper surface 11a of a cylinder block 11 shown in Fig. 13, which was an automobile engine part of cast iron, under conditions of a cutting speed of 1500 m/min., a feed rate of 0.15 mm/tooth and a depth of cut of 0.5 mm in practice. Table 1 shows the cutting conditions and the resulting tool lives. A similar cutting test was made with indexable inserts of experiment No. 8 of Example 1. In the results of the cutting tests for the cylinder blocks, a number of thermal cracks were caused in the indexable inserts of experiment No. 1 after cutting 450 workpieces as shown in Fig. 14A, and the cutting edges were chipped off by thermal crack as shown in Fig. 14B when the operation was further continued to cut 600 workpieces, to reach the ends of the lives. When the indexable inserts of experiment No. 8 were employed, on the other hand, no thermal crack was caused as shown in Fig. 14C and no chipping resulted from thermal crack while only rounded flank wear was observed in the cutting edges, and the cutting edges were possible to be re-ground after cutting 2500 workpieces. According to the inventive indexable insert for milling, as hereinabove described, it is possible to suppress chipping resulting from thermal crack by setting the subcutting edge angle, the negative land angle and the negative land width in prescribed ranges respectively while straight forming the subcutting edge. Reduction of sharpness and chipping of the cutting edge resulting from thermal crack are prevented also in high-speed milling of at least 800 m/min. or at least 1000 m/min. so that the tool life can be extended, while an excellent surface finish can be attained with machined surface roughness of a workpiece of at least 6.3 Z under JIS. Consequently, productivity can be remarkably improved in face milling of parts which are made of gray cast iron, in particular. According to the inventive milling cutter, on the other hand, the aforementioned indexable insert according to the present invention is applied to every one of a plurality of indexable inserts clamped thereon, whereby the indexable insert can make high-speed cutting and attain excellent machined surface roughness in milling. Consequently, it is possible to provide a milling cutter which remarkably contributes to improvement of productivity in milling of cast iron. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims: An indexable insert being clamped into a milling cutter for face milling, comprising: a metal base (1) being made of cemented carbide containing WC and Co, and a cutting edge (2), consisting of a sintered cubic boron nitride compact, being brazed to or integrally sintered with said metal base (1), said cutting edge (2) including a subcutting edge (3) having a subcutting edge angle (β) of at least 30° and not more than 60°, and a flat drag type cutting edge (4) being continuous thereto, and having a negative land angle () of at least 30° and not more than 45° and a negative land width (L) of at least 0.05 mm and not more than 0.40 mm, said subcutting edge (3) having a straight shape. The indexable insert for milling in accordance with claim 1, wherein said negative land width (L) of said cutting edge (2) is at least 0.075 mm and not more than 0.30 mm. The indexable insert for milling in accordance with claim 1, wherein said flat drag type cutting edge (4) being continuous to said subcutting edge (3) of said cutting edge (2) is in the form of a circular arc having a radius of curvature of at least 200 mm and not more than 400 mm. A milling cutter for face milling comprising a plurality of indexable inserts, wherein every one of said plurality of indexable inserts comprises: a metal base (1) being made of cemented carbide containing WC and Co; and a cutting edge (2), consisting of a sintered cubic boron nitride compact, being brazed to or integrally sintered with said metal base (1), said cutting edge (2) including a subcutting edge (3) having a subcutting edge angle (β) of at least 30° and not more than 60°, and a flat drag type cutting edge (4) being continuous thereto, and having a negative land angle () of at least 30° and not more than 45° and a negative land width (L) of at least 0.05 mm and not more than 0.40 mm, said subcutting edge (3) having a straight shape. The milling cutter in accordance with claim 4, wherein said negative land width (L) of said cutting edge (2) of each of said plurality of indexable inserts is at least 0.075 mm and not more than 0.30 mm. The milling cutter in accordance with claim 4, wherein said flat drag type cutting edge (4) being continuous to said subcutting edge (3) of said cutting edge (2) of each of said plurality of indexable inserts is in the form of a circular arc having a radius of curvature of at least 200 mm and not more than 400 mm. A milling cutter for face milling comprising a plurality of indexable inserts, wherein part of said plurality of indexable inserts comprises: a metal base (1) being made of cemented carbide containing WC and Co; and a cutting edge (2), consisting of a sintered cubic boron nitride compact, being brazed to or integrally sintered with said metal base (1), said cutting edge (2) including a subcutting edge (3) having a subcutting edge angle (β) of at least 30° and not more than 60°, and a flat drag type cutting edge (4) being continuous thereto, and having a negative land angle () of at least 30° and not more than 45° and a negative land width (L) of at least 0.05 mm and not more than 0.40 mm, said subcutting edge (3) having a straight shape. The milling cutter in accordance with claim 7, wherein said negative land width (L) of said cutting edge (2) is at least 0.075 mm and not more than 0.30 mm. The milling cutter in accordance with claim 7, wherein said flat drag type cutting edge (4) being continuous to said subcutting edge (3) of said cutting edge (2) is in the form of a circular arc having a radius of curvature of at least 200 mm and not more than 400 mm. The milling cutter in accordance with claim 7, wherein cemented carbide inserts and/or ceramics inserts are also employed as said plurality of indexable inserts.

Inventors: MIZUTANI MASANORI, MIZUTANI, MASANORI, Mizutani, Masanori, c/o Itami Works of Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1995
Main classification: B23C5
IPC codes: B23C5