Throw away insert and face milling cutter

Abstract: Throw away insert (10) having four cutting edges with high edge strength without restriction to the cutting amount. One cutting edge is provided at each of two acute-angled corners of a parallelogram. The cutting edge has a major cutting edge (14) defined on one side of the parallelogram and a minor cutting edge (15) defined on the other side and forming an angle of about 90° with the major cutting edge. The lower half portion of the insert has the same structure and dimensions as the upper half portion, but is turned 180° and further 3 - 7° clockwise with respect to the upper half portion. Thus, two on the upper half and two on the lower half, four cutting edges in all are provided.

Description: The present invention relates to a throw away insert according to the precharacterising portion of claim 1 for milling having a cutting angle of 90° (90° herein used is to encompass angles approximate to 90°) and a face milling cutter carrying such inserts. Milling with a cutting angle of 90° (what is called corner cutting) is ordinarily carried out using throw away inserts having a basically regular triangular shape as shown in Fig. 13 or throw away inserts having a basically parallelogrammic shape as shown in Fig. 14. Also, as shown in Fig. 15, there is known an insert basically square in shape and having minor cutting edges protruding from the four corners (EP-A 0 156 780). These throw away inserts are mounted on the outer periphery of the cutter body near its tip so that each of their major cutting edges 14 extends parallel to the axis of the cutter or is slightly inclined relative to the cutter axis in such a way that it gets nearer to the center of the cutter at its rear end. Cutting is done with one of the major cutting edges 14 and a minor cutting edge 15 contiguous to the major cutting edge, the corner angle α2 therebetween being about 90° (those shown in Figs. 13 and 14 ordinarily have a corner angle of 89° and the one shown in Fig. 15 has a corner angle several degrees still smaller). Further, since it is necessary that the cutting edges of these inserts have a positive radial rake and axial rake, their side faces have to have a relief angle. Thus, these inserts are a positive type in which the side faces intersect the top surface as the rake face at an acute angle. In case of the above-described conventional throw away inserts, if they are turned over, the positions of the cutting edges are reversed even if they are a negative type. Thus, cutting edges can be formed only on one side. This means that the numbers of cutting edge corners which can be formed on the inserts of Figs. 14 and 13 are limited to 2 and 3, respectively. The insert shown in Fig. 13 is used more extensively than the insert of Fig. 14 because the former has one more cutting edge corners and thus is economically advantageous. But this insert, having a basically regular triangular shape, is insufficient in strength because its corners are thin. It is possible to increase its strength to a level comparable to other inserts by increasing the widths of the minor cutting edges 15. But wider edges 15 mean that the distance L between the center of the insert and the minor cutting edges is small. Thus, the support points X, Y and Z are arranged in an unbalanced manner, so that the insert cannot be supported stably. Thus, it is difficult to increase the strength by widening the minor cutting edges. On the other hand, the insert of Fig. 15 has sufficiently high edge strength and has four cutting edge corners. But this insert has a major drawback in that the cutting amount is limited to d due to the interference by the adjacent minor cutting edges. Further, since these inserts have side faces inclined (with respect to a plane perpendicular to the top surface) at an angle of 11 - 20° or greater, side faces cannot be supported stably on the support surface. Thus, it is difficult to increase the mounting accuracy of the cutter edges. A conventional face milling cutter cannot dispose of chips forcibly and thus the curling diameter of chips tend to be large. Thus, in order to curl such chips, as shown in Fig. 16, it was necessary to provide a concave chip pocket 4 in each back metal portion 3 defined between adjacent gullets 2 formed in a cutter body 1 of a conventional face milling cutter. Such pockets lower the strength of the cutter body, so that the body is more likely to be deformed. This may lead to reduced machining accuracy or chattering during cutting. The body surface is subjected to black oxide surface treatment (coloring with caustic soda, what is called black oxide finish). But this treatment cannot sufficiently increase the corrosion resistant or the surface hardness. Thus, the body can be damaged easily by contact with chips. It is therefore an object of the present invention to provide an economical and reliable throw away insert on which cutting edges can be formed on four corners without decreasing the strength of cutting edges and limiting the range of cutting amount and a high-performance face milling cutter having a plurality of such inserts mounted thereon. A throw away insert for use in a face milling cutter according to the present invention comprises an upper half portion and a lower half portion which are integral with and identical in shape and dimension to each other and have a substantially parallelogrammic shape and have two cutting edges at two acute-angled corners of the parallelogram, the cutting edges being arranged point-symmetrically with respect to the center of the upper half portion and the lower half portion and each comprising a major cutting edge defined on one side of the parallelogram and a minor cutting edge defined on the other side thereof and forming a ridge at a position displaced outwardly from the contour of the parallelogram and contiguous to the major cutting edge, the corner angle between the major cutting edge and the minor cutting edge being substantially 90°, the lower half portion being positioned with respect to the upper half portion as if turned over by 180° and turned clockwise as viewed from top by 3 - 7° with respect to the upper half portion, the minor cutting edges being acute-angled edges. The top surface and bottom surface have breaker grooves that are arranged in juxtaposition with the major cutting edges, each major cutting edge sinking gradually downwardly from its one end near the minor cutting edge and at its mid-portion beginning to curve upwardly so that it becomes flush again with the top or bottom surface of the insert. The face milling cutter according to the present invention has mounted thereon a plurality of inserts of the above-described type, each throw away insert being mounted so that each of the major cutting edges is disposed on the outer periphery of the cutter and each of the minor cutting edges are disposed on the front side thereof, that each major cutting edge is arranged parallel to the cutter axis or slightly inclined relative to the cutter axis in such a way that it gets nearer to the center of the cutter at its rear end than at its front end, and that each cutting edge has a radial rake of -17 to -10° and an axial rake of 0 to +8°. As the throw away inserts used have breaker grooves, it is not necessary to provide deep chip pockets in the back metal portions because the curling radius of chips can be reduced sufficiently by the breaker grooves. Thus, in order to increase the strength of the body, each back metal portion may be shaped such that its outer peripheral surface has an arcuate convex shape so that it is nearer to the center of the cutter at its portion nearer to its rear end with respect to the direction of rotation of the cutter. Also, if black oxide finish is not desirable, a wear-resistant, corrosion-resistant plating layer may be provided on the surface of the cutter body. The throw away insert of the present invention comprises the following advantages: (1) The top and bottom surfaces are arranged such that even if the insert is turned over, the position of the cutting edges will not change. Thus, four cutting edges each having a wide minor cutting edge and having a cutting angle of 90° can be formed on diagonally opposite corners of the top and bottom surfaces. (2) Since no cutting edges are formed on the obtuse-angled corners of the top and bottom surfaces, the cutting amount can be selected freely by setting the radial rake at a negative value. (3) Since the upper and lower half portions are angularly displaced from each other by 3 - 7° at a point of 1/2 of the thickness of the insert, it is possible to combine positive type minor cutting edges for better sharpness in cutting with negative type major cutting edges for increased strength. (4) Since the cutting edge corners protrude outwardly beyond the obtuse-angled corners of the opposite surface, cutting can be done without interference even if the side faces of the obtuse-angled corners are perpendicular to the top and bottom surfaces. The insert can be seated stably by having the side faces perpendicular. (5) The use of inserts having breaker grooves will eliminate the necessity of providing deep chip pockets in the back metal portions because the grooves serve to reduce the curling radius of chips. Also, since each major cutting edge is arranged such that its longitudinal position is adjustable, it is possible to increase the cutting stability by controlling the thrust force during cutting (which acts to push or lift the work). We will describe this operation in detail hereinbelow. Since inserts are mounted on the milling cutter of the present invention so that their radial rake and axial rake will be -17 to -10° and 0 to +8°, respectively, both major cutting edges and minor cutting edges have suitable relief angles. As described above, the throw away insert of the present invention is basically parallelogrammic in shape and has cutting edges having a cutting angle of 90° at the acute-angled corners. Such cutting edges are sufficiently strong. Two cutting edge corners are formed on the top surface and the bottom surface. Such an insert is economically advantageous because four corners can be used for cutting and its use is not limited in the amount of cut. Thanks to the above-described advantages, the face milling cutter according to the present invention can machine stably with high accuracy, so that it is highly durable and economically advantageous when compared with conventional cutters. Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: Fig. 1 is a perspective view showing one embodiment of the throw away insert of the present invention; Fig. 2 is a plan view of the insert of Fig. 1; Fig. 3 is a side view of the major cutting edge side of the same; Fig. 4 is a side view of the minor cutting edge side of the same; Fig. 5(a) is a sectional view taken along line V-V of Fig. 1 and Fig. 5(b) is a sectional view of another embodiment at the same position; Fig. 6 is a view showing the insert with its upper and lower portions separated from each other; Fig. 7 is a front view showing a portion of the face milling cutter of the present invention; Fig. 8 is a sectional view of the same; Fig. 9 is a side view of the insert mounting portion of the same; Fig. 10 is a perspective view for explaining the insert mounting position of the same; Fig. 11 is an explanatory view showing how the axial rake and the thrust force of the main cutting edge are controlled; Fig. 12 is a view showing the insert mounting position when the radial rake is zero; Fig. 13 is a front view showing a conventional throw away insert; Fig. 14 is a front view showing another conventional throw away insert; Fig. 15 is a front view showing still another conventional throw away insert; and Fig. 16 is a front view showing a part of a conventional face milling cutter. Figs. 1-5 show one embodiment of a throw away insert of the present invention. This throw away insert 10 has a top surface 11 which is a basically parallelogram whose acute-angled corners have an apex angle α1 of 80 - 88°, as shown in Fig. 2. The relation between the distances of its sides A and B may be set at A = B , A > B or A < B. On the top surface 11 is provided two cutting edges having a cutting angle (corner angle α2) of about 90° and arranged point-symmetrically with respect to the center of the top surface. Each cutting edge comprises a major cutting edge 14 defined on one (B in Fig. 2) of the two sides forming an acute corner of the parallelogram and a minor cutting edge 15 which is a several-millimeter-long ridge offset slightly outwardly from the other side and connected at one end to the major cutting edge 14 and at the other end to a mid-point of the side A through an inwardly tapered ridge. As shown in Fig. 3, the level of the major cutting edge 14 with respect to the top surface gradually lowers from its end contiguous to the minor cutting edge 15 and at a point near the center of the side B, it curves upwards until it comes back to the same level as the top surface. The edges 14 and 15 are connected together through an arcuate portion having a small radius of curvature in the embodiment. But they may be connected through a small chamfered portion. Side faces 12 of the major cutting edges 14 and side faces 13 contiguous to the side faces 12 through the obtuse-angled corners are perpendicular to the top surface 11, while side faces 16 as flanks of the minor cutting edges 15 are inclined by an angle larger than a predetermined axial rake so as to intersect the top surface 11 at an acute angle. Side faces 17 between the side faces 12 and 16 are also inclined slightly so that the corners of the cutting edges define acute-angled edges. Side faces 13, not flush with the side faces 16, are connected to the latter through tapered surfaces 18. As shown in Fig. 5(b), each side face 12 may have its upper portion (an area within the range of 1/6W - 1/4W wherein W is the thickness of the insert) tapered to provide an obtuse-angled (r = 95 - 100°) major cutting edge 14. In the top surface 11 are further provided breaker grooves 19 extending along the major cutting edges 14. The grooves 19 have a depth t (Fig. 5a) which is constant in the longitudinal direction. In the width direction, the grooves 19 curve with a radius of curvature R1 from edge 14 to the deepest point and then curve upwards with a larger radius of curvature R2. Tapered surfaces 20, 21 (Figs. 3 and 4) may be omitted so that the bottom end portions of the side faces 16, 17 rise from the side faces 12a, 13a at a right angle at the point 1/2W. The lower half portion of the insert borders its upper half portion along the 1/2W line and is shaped and dimensioned identically with the upper half portion. Namely, as shown in Fig. 6, two inserts both having the thickness of 1/2W are prepared. One of them which is to be the lower half portion (the one on the right in Fig. 6) is turned by 180° about its axis c with respect to the upper half and then turned clockwise in the figure by 3 - 7° about the center O2 of surface. In this state, the two inserts are put one on the other and bonded together with their surface centers O1, O2 aligned with each other. The insert according to the present invention has such a structure as if made in the abovesaid manner. Since the upper and lower half portions have the same structure, the elements on the lower half which are identical to the elements on the upper half are denoted with the same numerals but with the letter a attached thereto to distinguish them from each other. In the illustrated embodiment, side faces 13 and 13a are provided in the same plane but they may be slightly offset from each other. Next, referring to Figs. 7-12, we will describe a preferred embodiment of the face milling cutter according to the present invention. In Figs. 7-9, numeral 1 designates a cutter body. Gullets 2 are formed in the body along its outer edge with a predetermined pitch. Back metal portions 3 are provided between the adjacent gullets 2. A locator 5 is bolted to the rear (with respect to the direction of rotation) wall of each gullet 2. A presser metal 6 is press-fitted in a wedge groove formed in the front part (with respect to the direction of rotation) of each gullet 2. The throw away insert 10 of the present invention is set in each gullet 2. If it is intended to use the cutting edges on the top surface, the insert is fixed in position by bringing its bottom surface 11a and side faces 12, 13 on one side into contact with the seating groove of the locator 5. Then, it is clamped by pulling the presser metal 6 with a tightening screw 7. In this state, the insert 10 should be positioned so that one of its major cutting edges 14 used for cutting is parallel to the axis C of the cutter or slightly inclined with respect to the axis C in such a way that its rear end is nearer to the cutter than at its front end. With this arrangement, the foremost end P of the cutting edges on the bottom surface is located behind the minor cutting edge 15, so that the end P will not interfere with the workpiece. Also, as shown in Figs. 7 and 10, the radial rake β should be set at -17 to -10° and as shown in Figs. 9 and 11, the axial rake Θ of the rake face should be set at 0 to +8°. If β = 0°, as shown in Fig. 12, the outermost end Q of the cutting edge on the bottom surface will be disposed outwardly of the rear end of the major cutting edge on the upper side. But by inclining the insert 10 by 10 - 17° from the position of Fig. 10 in such a direction that its side on the left of the figure is raised, the point Q will be moved inwardly until it is located nearer to the center of the cutter than the rear end of the major cutting edge 14. In this state, the major cutting edge has a suitable relief angle, so that any portion of the insert other than the cutting edge will not interfere with the workpiece. Further, by setting the angle Θ at 0 to +8°, the minor cutting edge 14, too, has a suitable relief angle as shown in Fig. 11. If the axial rake Θ is too large, the workpiece would be lifted up by the thrust force during cutting. If it is too small, it would be pressed against the cutter with a strong force. In either case, the quality of the product or the strength of the cutting edge will be badly influenced. As shown in Fig. 11, the major cutting edge of the insert according to the present invention has an axial rake which changes from positive to negative toward its rear end so that the thrust force can be controlled to an optimum value at any depth of cut which may change from d1 to d4. A conventional insert has a straight major cutting edge, so that the thrust force increases in proportion to the depth of cut. Thus, it was impossible to control the thrust force to an optimum value according to the depth of cut. But in the present invention, up to the depth d3, the work pressing force is kept smaller than when cutting with a conventional straight major cutting edge because the axial rakes Θ1 - Θ3 of the major cutting edge are larger than the axial rake of the rake face 11. Also, the lifting force produced when the depth of cut d4 is smaller than when cutting with a conventional straight major cutting edge because part of the lifting force that acts on the workpiece is negated by the pressing force that acts on the rear end of the edge having a negative angle Θ4. Further, the back metal portions 3 of the cutter of the present invention have a unique shape not found in conventional cutters. Namely, as shown in Fig. 7, each back metal portion has its outer peripheral surface shaped as a convex arcuate surface which is tapered rearwardly (with respect to the direction of rotation) toward the center of the cutter and has no conventional concave chip-breaking pocket. With this arrangement, the thickness of each back metal portion (dimension between S - T) can be approximately doubled when compared with a conventional cutter of the same size. Thus, the rigidity of the body is high, so that the body is deformed less when the pressers 6 are press-fitted or when subjected to cutting forces. Thus, the mounting accuracy of the cutting edge improves and cutting can be done stably with less chattering. This improves the life of the inserts and the machining accuracy. Further, the cutter body 1 is subjected to elecrtoless plating (what is called Kanigen plating), so that its corrosion resistance and surface wear resistance are higher than conventional cutters.

Claims: A throw away insert (10) for use in a face milling cutter comprising an upper half portion and a lower half portion which are integral with and identical in shape and dimension to each other and have a substantially parallelogrammic shape and have two cutting edges (14, 15) at two acute-angled corners of the parallelogram, said cutting edges being arranged point-symmetrically with respect to the center of said upper half portion and said lower half portion and each comprising a major cutting edge (14) defined on one side (B) of the parallelogram and a minor cutting edge (15) defined on the other side (A) thereof and forming a ridge at a position displaced outwardly from the contour of the parallelogram and contiguous to the major cutting edge (14), the corner angle (α 2 ) between said major cutting edge (14) and said minor cutting edge (15) being substantially 90°, said minor cutting edges (15) being acute-angled edges, said lower half portion being positioned with respect to said upper half portion as if turned over by 180°, characterized in that said lower half portion is positioned with respect to said upper half portion as if additionally turned clock-wise as viewed from top by 3 - 7° with respect to said upper half portion and that said upper half portion and said lower half portion have breaker grooves (19) arranged in juxtaposition with said major cutting edges (14), said each major cutting edge (14) sinking gradually downwardly from one end thereof near said minor cutting edge (15) and at its mid-portion beginning to curve upwardly so that it becomes flush again with the top or bottom surface (11) of the insert (10). A face milling cutter formed with a plurality of gullets (2) in outer periphery thereof and having a plurality of throw away inserts (10) mounted in said gullets (2), wherein said each throw away insert (10) is mounted so that each of said major cutting edges (14) is disposed on the outer periphery of the cutter and each of said minor cutting edges (15) are disposed on the front side thereof, that said each major cutting edge (14) is arranged parallel to the cutter axis (C) or slightly inclined relative to the cutter axis (C) in such a way that it gets nearer to the center of the cutter at its rear end than at its front end, and that said each cutting edge (14, 15) has a radial rake (β) of -17 to -10° and an axial rake (Θ) of 0 to +8°, characterized in that said throw away inserts (10) are of the kind as claimed in claim 1. A face milling cutter as claimed in claim 2, characterized in that said each major cutting edge (14), instead of being arranged parallel to the cutter axis (C), is arranged slightly inclined relative to the cutter axis (C) in such a way that it gets nearer to the center of the cutter at its rear end than at its front end. A face milling cutter as claimed in claim 2 or 3 wherein each back metal portion (3) defined between said gullets (2) of the cutter body (1) has an outer peripheral surface having an arcuate convex shape so that it is nearer to the center of the cutter at its portion nearer to its rear end with respect to the direction of rotation of the cutter. A face milling cutter as claimed in any of claims 2 to 4 wherein a plating layer is provided on the surface of the cutter body (1) to increase the wear resistance and the corrosion resistance of the body (1).

Inventors: MAEDA YOSHIKI, MORI YOSHIKATSU, NODA MITSUAKI, YAUCHI NARUHITO, MAEDA, YOSHIKI, MORI, YOSHIKATSU, NODA, MITSUAKI, YAUCHI, NARUHITO, Maeda, Yoshiki, c/o Itami Works, Mori, Yoshikatsu, c/o Itami Works, Noda, Mitsuaki, c/o Itami Works, Yauchi, Naruhito, c/o Itami Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1993
Main classification: B23C5
IPC codes: B23C5