Double negative cutting insert for tools for chip removing machining

Abstract: A double negative cutting insert (1) has opposite top and bottom sides between which transverse flank surfaces (9) extend, adjacent to which cutting edges (12, 16) are formed. More precisely, there are at least four useful major cutting edges (12), which separately extends from an active operating corner (10) towards a nearby first inactive corner (11). The individual major cutting edge (12) co-operates with a minor cutting edge (16) with the purpose of face machining the surface, that is left after material removal. The cutting insert has an octagonal basic shape, the top and bottom sides thereof having an identical shape with four spaced-apart supporting projections (18), which are placed near the four inactive corners (11) and have plane abutment surfaces (19) located in a common plane. An individual land (20) adjacent to the major cutting edges (12) has a concavely arched shape in order to provide a positive cutting geometry during operation. Thanks to the octagonal basic shape, the active corners of the cutting inserts may be made strong.

Description: This invention relates to a double negative cutting insert intended for tools for chip removing machining of the type that comprises a body with a flat, polygonal basic shape which has opposite upper and lower sides with identical shape, which are turned from an imaginary neutral plane through the body and between which transverse flank surfaces extend adjacent to which material-removing major cutting edges are formed, separately extending from the area of a corner and co-operating with a minor cutting edge formed adjacent to a parallel land, which minor cutting edge has the purpose of planar machining the surface of a work piece which is left after material removal. Such a cutting insert is known, for example, from US-A-4 475 851. Background of the Invention Double negative and double-positive cutting inserts have advantages as well as disadvantages. The double negative cutting inserts of conventional type are advantageous because cutting edges may be formed adjacent to the two opposite sides of the cutting insert body. In this way, the cutting insert may be formed with a generally doubled number of cutting edges in comparison with double-positive cutting inserts of the corresponding type. This means that a large number of cutting edges may be used before the individual cutting insert has to be discarded. This is something, which in turn reduces the purchase costs of the user. However, a disadvantage of the double negative cutting inserts is that they have mediocre cutting performance. This is something, which in total means impaired operating economy. Double-positive cutting inserts, on the other hand, have good cutting performance and enables quick and efficient working, but such cutting inserts may be formed with cutting edges only along the top side thereof. This is something, which limits the number of feasible cutting edges. Previously, cutting inserts have been developed which combine the conventional advantages of the double negative and double-positive cutting inserts, respectively, without the defects of the above-mentioned disadvantages; more precisely thanks to the cutting insert being endowed with a double negative basic shape at the same time as the top and bottom sides of the cutting inserts are formed with countersinks adjacent to the cutting edges in order to bring about a positive cutting geometry during operation. Previously known combination cutting inserts of this type have, however, had a square or hexagonal shape. As an example of such a combination cutting insert, a cutting insert should be mentioned which is commercially available under the trade mark TRIGON and is marketed by Greenleaf Corporation, Greenleaf Drive, Saegertown, PA 16433, US. An aggravating disadvantage of square as well as hexagonal cutting inserts is, however, that the corner angle at each active working corner becomes moderate. Thus, in square cutting inserts, the corner angle is 90° and in the hexagonal cutting inserts of the type TRIGON, the corner angle is even smaller. This means that the active corner becomes considerable acute, whereby the corner portion of the cutting insert risks to be broken up if it is submitted to considerable forces during operation. Aims and Features of the Invention The present invention aims at obviating the above-mentioned inconveniences of previously known combination cutting inserts and at providing an improved cutting insert. Therefore, a primary object of the invention is to provide a cutting insert, which may be formed with a large number of cutting edges adjacent to active corners of a strong and robust character, and that in spite of this should be able to be fixed in a steady and reliable way in a corresponding insert seat. Although the cutting insert according to the invention should be able to be used for a wide variety of tools for chip removing machining, such as turning tools, the same is primarily intended for milling tools, in particular face mills and side-milling cutters, regardless whether these for the mounting of the cutting inserts make use of cassettes or not. In a particular aspect, the invention aims at providing a cutting insert which is optimized for milling tools of the type that is either right-hand cutting or left-hand cutting, the purpose being to bring about the largest feasible number of useful strong cutting edges for either of these two working cases. In another aspect, the invention aims at providing a cutting insert which is neutral to the extent that one and the same cutting insert should be able to be used for either right-hand cutting working or left-hand cutting working; all with the purpose of obviating the need of separate stock-keeping and distribution of two different types of cutting inserts. Another aim of the invention is to create a cutting insert, which may be manufactured in a simple way, above all in respect of the ability to release from a mould after moulding. Another aim is to create a cutting insert which may be formed with an aperture intended for a fixing screw, which aperture has a diameter which is large in relation to the dimension of the cutting insert, the outermost aim being to enable manufacture of small cutting inserts with many cutting edges which may be fixed by means of simple, conventional fixing screws. According to the invention, at least the primary aim is attained by the features defined in claim 1. Preferred embodiments of the cutting insert according to the invention are furthermore defined in the dependent claims. Further Elucidation of Prior Art In US 3497933, US 4954021 and US 4966500, different embodiments of cutting inserts are shown having a generally octagonal basic shape; i.e. cutting inserts which in one way or the other way have eight edges. None of these known cutting inserts has, however, any supporting projections of the type characterizing the present invention and neither the unique geometry, which is seen in the subsequent claims. Brief Description of the Appended Drawings In the drawings: Fig 1 is a perspective view of a schematically shown cutting insert according to the invention fixed in a cassette which is assumed be included in a milling cutter in the form of a face mill (not shown), Fig 2 is a planar view of the milling cassette together with the cutting insert according to fig 1, Fig 3 is an end view of the same cassette and cutting insert, Fig 4 is a side view of the cassette and the cutting insert, Figs 5, 6 are schematic illustrations of a vital difference between, on one hand, a square-shaped cutting insert and an octagonal cutting insert according to the invention, Fig 7 is a very enlarged perspective view showing a first embodiment of a cutting insert according to the present invention, Fig 8 is a side view of the cutting insert according to fig 7, Fig 9 is a planar view of the same cutting insert, Fig 10 is a partial, further enlarged planar view of only a corner portion of the cutting insert according to figs 7-9, Fig 11 is a perspective view corresponding to fig 7 showing an alternative embodiment of a cutting insert which is optimized for only right-hand cutting working, Fig 12 is a partial, enlarged planar view showing only a corner portion of the cutting insert according to fig 11, Fig 13 is a perspective view showing a cutting insert optimized for left-hand cutting working, Fig 14 is a partial, enlarged planar view showing only a corner portion of the cutting insert according to fig 13, Fig 15 is a planar view of an additional alternative embodiment of the cutting insert according to the invention, and Fig 16 is a planar view shown for comparative purpose of a cutting insert according to fig 9. Detailed Description of Preferred Embodiments of the Invention Before the cutting insert according to the invention is described in detail, reference is made to figs 1-4, which schematically illustrate the co-operation between a cutting insert and a cassette. In these drawing figures, the cutting insert is generally designated 1, while the cassette is designated 2. In the cassette, a seat 3 is formed in which the cutting insert is fixed by means of a screw (not shown) tightenable in an aperture (cutting inserts without apertures may be fixed in another way). The cassette 2 is in turn mounted in a peripheral recess in a milling tool, e.g. a head of a face mill (not shown). As is seen in figs 1 and 2, only one of the totally eight corners of the cutting insert is active during operation, viz . the corner designated 10 which protrudes towards the side in relation to the cassette. In fig 3 is shown how the flat cutting insert 1 is oblique in relation to a reference plane which in figs 1-4 is illustrated by means of two spaced-apart, thin dowels 6 extending parallel to the centre axis around which the milling cutter, and thereby the cutting insert, rotates. The angle of inclination illustrated in fig 3 is denominated "axial rake angle" by those skilled in the art. In relation to an imaginary tangent along the periphery of the milling disc, the cutting insert is in practice also radially oblique in a so-called "radial rake angle", which is negative in order to guarantee clearance. In practice, these setting angles are moderate and may be within the range of 5-15°. Reference is now made to figs 7-10, which in detail illustrate a first so-called neutral embodiment of a cutting insert 1 according to the invention. The cutting insert consists of a solid body made of a hard and non-abrasive material, e.g. moulded and sintered cemented carbide, ceramics or the like. This cutting body has a flat, octagonal basic shape and a top side 7 and a bottom side 8. The concept's top and bottom sides, respectively, are related only to the position in question of the cutting insert in the cassette. Thus, these opposite sides of the cutting insert have an identical shape, and therefore the cutting insert may be turned with an arbitrary side turned upwards in the cassette. Between these the top and bottom sides, flank surfaces generally designated 9 extend. In fig 8 is seen that the top and bottom sides 7, 8 are turned away from an imaginary neutral plane P, which extends centrally through the cutting body. A central, geometric axis C extends perpendicularly to this neutral plane P. As a consequence of the octagonal basic shape thereof, the cutting body 1 includes in total eight corners, of which four are designated 10, 10' 10", 10"' while the four other corners are designated 11, 11', 11", 11'''. Of these corners, each one of the four first-mentioned ones may be actively acting during a working operation, while the other four corners always are of an inactive character. Each individual corner is developed thanks to two nearby flank surfaces 9 extending in an obtuse angle against each other. At each active corner, this obtuse angle is designated α, while the corresponding obtuse angle at an inactive corner is designated β (see fig 9). Adjacent to each individual flank surface 9, two major cutting edges, generally designated 12, are formed between the individual flank surface and each one of the top and bottom sides of the cutting insert. Each one of two flank surfaces 9 meeting at an active corner transforms into parallel lands 13, between which there is a transition surface 14. In the example shown, this transition surface 14 is plane, between the transition surface and the individual parallel land there being an utterly narrow radial transition 15 (the transition surface may also be in its entirety softly rounded). At axially opposite ends, the parallel lands 13 have comparatively short edges in the form of minor cutting edges 16, 16' and 16", 16"'. These minor cutting edges have the purpose of, in a conventional way, face machining the surface on a work piece, which is left after material removal. Suppose that the major cutting edge 12" carries out material removal. Then, the minor cutting edge 16 will face machine the surface left behind on the work piece in order to make this surface smooth. These minor cutting edges may also be denominated "wiper edges". Where two flank surfaces 9 meet each other adjacent to an inactive corner 11, the transition section may take place via a marked break line 17. However, at the inactive corners the flank surfaces may also intersect each other via an arched and softly rounded surface. The top and bottom sides 7, 8 of the cutting insert have one and the same shape. Characteristic of this shape is the existence of four supporting projections 18, which are placed near each one of the four inactive corners 11-11'''. The cutting insert is symmetrical so far that all inactive corners (like the active corners) are equidistantly spaced-apart. This entails that also the supporting projections 18 are equidistantly spaced-apart. In the embodiment according to fig 9, the individual supporting projection is located right in front of each inactive corner 11 (the central plane through the corner intersects the supporting intermediate plane of the projection). Each individual supporting projection has a plane abutment surface 19, which is located in a plane common for all abutment surfaces of the supporting projections, which is parallel to the neutral plane P. In the area between the pair of nearby supporting projections, a land 20 extends, which ends at an active corner 10 and the edges adjacent thereto. Characteristic of these lands is that the same have a concavely arched shape in order to endow the cutting insert positive cutting geometry during operation. In the illustrated, preferred embodiment, each land 20 extends in the main continuously, i.e. with a smooth surface, from the area of each active corner up to two nearby supporting projections. However, it is also feasible to reduce the dimension or area of the arched land, so far that the land is given a limited width in the area closest to the cutting edges, then transforming into an elevated, for instance plane surface. This plane, elevated surface may advantageously without interruption transform into the nearby plane abutment surfaces 19 of the supporting projections. According to a preferred embodiment of the invention, the individual supporting projection 18 has side surfaces 21 inclining obliquely upwards/inwards towards the abutment surface 19. Furthermore, the supporting projection is so formed that it diminishes in the direction towards the centre of the cutting insert. This means that also the abutment surface 19 diminishes inwards. As is best seen in fig 8, the plane, in which the abutment surfaces 19 of the supporting projections are positioned in common, is located at a larger distance from the neutral plane P than the portions of the cutting edges 12, 16 positioned farthest from this plane. In other words, the projections protrude somewhat longer from the neutral plane than the cutting edges. This is something that entails that the supporting projections may be applied against a plane bottom in the appurtenant insert seat, e.g. the seat 3 in a cassette according to figs 1-4. However, it is also feasible to locate the abutment surfaces of the supporting projections at a lower level than the cutting edges, requiring that the bottom of the insert seats is formed with peripheral, flute-like recesses in which the cutting edges may be housed without coming in contact with the supporting surface. As is furthermore seen in fig 8, the individual major cutting edge 12 is, together with the adjacent minor cutting edge 16, oblique relatively the neutral plane P. More precisely, these edges are sloping from a highest point in the area of the corner transition surface 14 towards a lower point closer to each inactive corner 11. It is to advantage if the angle of inclination is within the range of 5-15°. In the preferred embodiments, which are shown in the drawings, the cutting insert is formed with a central aperture 22. On each opposite side of the cutting insert, the same aperture is delimited by a ring-shaped bulge 23, which projects a distance from the surrounding land 20 and has a plane abutment surface 24 which is parallel to the neutral plane. In the exemplified embodiments, this abutment surface 24 is positioned at a lower level than the abutment surfaces 19 of the supporting projections. Nevertheless, the abutment surface 24 may be used for supporting either directly against the bottom of an insert seat, which has been formed in view of the shape of the supporting projections and the ring bulge together with the level difference therebetween, or against a shim plate formed in a similar way which constitutes a separate unit between the bottom side of the cutting insert and the bottom of the insert seat. However, it is also feasible to position the ring-shaped abutment surface 24 of the bulge in flush with the abutment surfaces 19 of the supporting projections, i.e. in a plane which is common for all abutment surfaces of the four supporting projections and the ring bulge. From general theory of geometry is known that the sum of the corner angles in an octagon amounts to 1080°. Generally, the rule is also valid that the corner angle at a given corner increases if the corner angles at nearby corner decrease. In the embodiment illustrated in fig 9, the corner angle α at each active operative corner 10 amounts to 110°, from which follows that the corner angle at the inactive corners 11 amounts to 160°. Said angles are related to the flank surfaces 9 of the cutting insert. However, at each active corner, there are also the two parallel lands 13. Said parallel lands extend in turn in a certain, small angle towards the respective flank surfaces 9 while forming an angle of clearance λ (see fig 10). In practice, the angle of clearance λ may be within the range of 0,5-5°. Within the scope of the invention, it is possible to vary the above-mentioned corner angles α and β, respectively, within rather wide limits. However, according to the invention, angles within the range of 110-160° or suitably 120-150° are preferred. In a feasible embodiment, the cutting insert may be regular so far that all corner angles are equally large and amount to 135°. From this angle value, the corner angle of the active corners may vary upwards as well as downwards. In doing so, the so-called overhang of each individual active corner portion increases with descending corner angle at this portion and vice versa . In order to elucidate the size of the overhang, a comparison has been made in figs 5 and 6 between a conventional, square cutting insert and an octagonal cutting insert according to the invention. If the square cutting insert according to fig 5 was provided with supporting projections according to the invention, the overhang "a" would be rather large because each corner portion outside two co-operating supporting projections would be comparatively acute, inasmuch as the comer angle is only 90°. In fig 6 a regular, octagonal cutting insert according to the invention is shown, the corner angle at the active the corners amounting to 110°. Because this angle is substantially more obtuse than 90°, the overhang "b" - for a given setting angle - becomes considerably smaller than in the square cutting insert according to fig 5. Thus, a substantial advantage of the octagonal basic form of the cutting insert according to the invention is that the overhang of the corner portion from nearby supporting projections becomes moderate, provided that the corner angles are within the above-mentioned angle range of 110-160°. Therefore, cutting inserts that are to work under large stress should be formed with large corner angles at the active corners. It should be pointed out that all flank surfaces 9 are plane and extend perpendicularly to the neutral plane. However, cutting inserts for lighter work may be formed with a slightly marked, central waist. The Function of the Cutting Insert According to the Invention When the individual cutting insert is to work a work piece, a suitable, active corner on either of the opposite top and bottom sides of the cutting insert is selected, and then the cutting insert is fixed in a cassette to a milling cutter in the way that is illustrated in figs 1-4. When the cutting insert is fixed, the same is position determined in relation to the milling cutter and the rotation axis thereof automatically by virtue of the shape of the cassette and insert seat. More precisely, the cutting insert is located in a certain axial rake angle as well as in a certain radial rake angle. The last-mentioned angle has to be negative as a consequence of the cutting insert having a double negative basic shape. Otherwise, the flank surface adjacent to the major cutting edge would not go free from the work piece. Suppose that the side 7 is turned upwards and that the corner 10 is selected for executing the working. In doing so, the major cutting edge 12" together with the minor cutting edge 16" will effect the material-removing working, while the second minor cutting edge 16 at the same corner serves as a wiper edge which gives the worked surface smoothness. The cutting insert is, per se , fixed with a negative radial rake angle, but since the land 20 inside the edges 12", 16" has a concavely arched shape, said edges will, in practice, act with a positive cutting geometry inasmuch as the edges "hollows" out the material rather than to only shear the same loose. In addition to the fundamental advantage that the active corners becomes strong, the cutting insert according to the invention also has the advantage that high cutting performance is attained thanks to the positive cutting geometry during operative work at the same time as the cutting insert uses the advantages of the double negative cutting insert in respect of the possibility of making many cutting edges on one and the same cutting insert. Thus, as is seen in the previous description, the octagonal cutting insert according to the invention includes, according to figs 7-10, in total sixteen major cutting edges (appurtenant minor cutting edges uncounted), more precisely eight major cutting edges on one of the sides and eight major cutting edges on the other side. Only eight of these sixteen major cutting edges may, per se, be used for one and the same type of milling operation, i.e. either right-hand cutting or left-hand cutting, but thanks to the cutting insert being neutral inasmuch as the same is entirely symmetrical, the advantage is won that one and the same cutting insert may, if required, be used for both types of working operations. In other words, two different types of right-hand and left-hand cutting inserts, respectively, need not be stock-kept. A particular advantage of the cutting insert according to the invention is that the same may be formed with a central aperture, the diameter of which is large in comparison with the dimension of the cutting insert without because of this the corners of the cutting insert becoming weak and without the support of the cutting insert against the bottom of the insert seat becoming unreliable. In other words, the cutting insert may, if required, be formed with minimal dimensions and in spite of this be fixed by means of simple fixing screws. Reference is now made to figs 11 and 12, which illustrate a cutting insert intended for only right-hand cutting milling operations. In this case, the two parallel lands 13a, 13b adjacent to a transition surface 14 at an active corner are not of even thinness, but wedge-shaped tapering. Thus, the parallel land 13a is pointed in the direction downwards and transforms at the upper edge thereof in a minor cutting edge 16a adjacent to the top side 7 of the cutting insert. The second parallel land 13b has the tip thereof turned upwards and transforms into a minor cutting edge 16b adjacent to the bottom side 8 of the cutting insert. In doing so, there is only one active major cutting edge 12a adjacent to a co-operating minor cutting edge 16. It should be observed that the major cutting edge 12a extends continuously from the nearby supporting projection 18 up to the transition surface 14. In other words, each minor cutting edge in extension of the major cutting edge 12a is missing. In fig 13 and 14 an analogous embodiment is shown intended for left-hand cutting working. In this case, the wedge-shaped parallel lands 13a and 13b extend in the converse way, i.e. with the tip of the surface 13a pointing upwards and the tip of the surface 13b pointing downwards. In doing so, an actively cutting major cutting edge 12a is formed near the parallel land 13a, and a wiping minor cutting edge 16a is formed on the other side of the corner. The same minor cutting edge 16a transforms into an edge 12b, which do not form any major cutting edge. In fig 15 an alternative embodiment is shown according to which the individual supporting projection 18 is located laterally displaced in relation to each nearby, inactive corner 11. More precisely, the supporting projection is, in all essentials, in its entirety located besides the central plane, which intersects diametrically opposite inactive corners. In the example, the cutting insert is optimized for right-hand cutting milling operations (compare fig 11 and 12). By the fact that the supporting projection 18 is laterally displaced in relation to the corner 11, is gained that the operative major cutting edge 12b receives a large length L. In practice, the cutting edge length L may be made about 50 % larger than the same cutting edge length L 1 (see fig 16) of the above described cutting inserts which have the supporting projection thereof centrally placed in relation to each appurtenant inactive corner. It should be pointed out that all edges in practice should be reinforced in a conventional way.

Claims: Double negative cutting insert for tools for chip removing machining, including a body (1) with a flat, polygonal basic shape which has opposite upper and lower sides (7, 8) with identical shape, which are turned from an imaginary neutral plane (P) through the body and between which transverse flank surfaces (9) extend adjacent to which operatively serviceable, material-removing major cutting edges (12) are formed, each of which separately extending from the area of a comer (10) and co-operating with a minor cutting edge (16) formed adjacent to a parallel land (13) with the purpose of face machining the surface of a work piece which is left after material removal, characterized in, that the cutting insert has an octagonal basic shape in order to include eight comers (10, 11) having obtuse corner angles, of which four (10) are actively engageable and associated with at least one major cutting edge (12) and a co-operating minor cutting edge (16), while the other four (11) are inactive, that each one of said top and bottom sides (7, 8) has four spaced-apart supporting projections (18) which are placed near the four inactive comers (11) and have plane abutment surfaces (19) located in a common plane, which is parallel to the neutral plane (P), each one of four lands (20) in the area between an active corner and two nearby supporting projections (18) on the top side as well as the bottom side of the cutting insert having a concavely arched shape to provide a positive cutting geometry during operation. Cutting insert according to claim 1, characterized in, that the corner angle (α and β, respectively) between two flank surfaces (9), meeting each other in a corner, is within the range of 110-160°, preferably 120-150°. Cutting insert according to claim 1 or 2, characterized in, that two major cutting edges (12) are formed at each active corner (10) adjacent to each one of said top and bottom sides (7,8), one of which major cutting edges is right-hand cutting and the second left-hand cutting, parallel lands (13) being formed on both sides of a transition surface (14) at each individual active corner (10), which lands separately has minor cutting edges (16) at each one of axially opposite ends and which extend into flank surfaces (9), which at axially opposite ends have said major cutting edges (12). Cutting insert according to any one of the preceding claims, characterized in, that the concavely arched land (20) extends mainly continuously, i.e. with a smooth surface, from the area of each active corner (10) up to two nearby supporting projections (18). Cutting insert according to any one of the preceding claims, characterized in, that the individual supporting projection (18) has side surfaces (21) inclining obliquely upwards/inwards against the proper abutment surface (19). Cutting insert according to any one of the preceding claims, characterized in, that the supporting projection (18) and the abutment surface (19) thereof slender in the direction towards the centre of the cutting insert. Cutting insert according to any one of the preceding claims, the same comprising a central aperture (22) for a fixing screw, characterized in, that a ring-shaped bulge (23) is formed around the hole (22 on each one of said top and bottom sides (7, 8), which bulge projects from the land (20) and has a plane abutment surface (24). Cutting insert according to claim 7, characterized in, that the flat abutment surface on the ring bulge (23) is situated in the same plane as the abutment surfaces (19) of the supporting projections (18). Cutting insert according to any one of the preceding claims, characterized in, that the plane in which the abutment surfaces (19) of the supporting projections is commonly positioned is located at a larger distance from the neutral plane (P) than the portions of the cutting edges (12, 16) positioned farthest therefrom. Cutting insert according to any one of the preceding claims, characterized in, that each one of the four supporting projections (18) is laterally displaced in relation to said inactive corners (11).

Inventors: ASTROEM WILLY, BEJERSTAL PER, NILSSON ANDERS, QVARTH INGEMAR, SCHERBARTH STEFAN, ASTROEM, WILLY, BEJERSTAL, PER, NILSSON, ANDERS, QVARTH, INGEMAR, Aström, Willy
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 2000
Main classification: B23B27
IPC codes: B23C5, B23B27