INDEXABLE INSERT FOR FINISH MILLING AND CUTTER BODY THEREFOR

Abstract: A double-sided milling cutting insert has two plane-parallel, equal chip or main surfaces (2), which are turned 90 DEG relative to each other. Each main surface comprises four operative cutting corners. In connection to each cutting corner are two bevelled surfaces (5, 6) which are angled relative to the plane of the main surface. The four side surfaces (3) are pairwise angled in order to make possible a positive cutting geometry. By the two bevelled surfaces (5, 6) the occurrence of edge chippings in the work-piece has been considerably reduced.

Description: INDEXABLE INSERT FOR FINISH MILLING AND CUTTER BODY THEREFOR The present invention relates to a double-sided milling cutting insert for fine machining, which is made by form- pressing and sintering of a powder that can form a cutting insert material and comprises two substantially similar main surfaces, which are turned by 90° relative to each other, and four substantially similar side surfaces that extend between these main surfaces. Such milling cutting inserts are fastened in a way known per se in rotative milling tools. Usually, this fastening is made on insert-carrying cassettes, as for instance described in the Swedish patent application 9300889-4 and in DE-A- 4 013 717. In order to achieve the required surface smoothness, it is necessary to locate the cutting insert as exactly as possible in the desired position, particularly in the axial direction. If the axial positioning is insatisfactory, then so called axial play arises, which causes a deteriorated surface smoothness. In for instance motor industry, close pitch milling cutters are used for the milling of engine blocks, cylinder blocks and similar parts. For those applications, very high requirements are set on the surface smoothness and R a -values of maximally 1,5 μm, R z -values of between 10 and 15 μm and WT-values of between 5 and 8 μm are often required. It has been succeeded to attain those tolerances by the construction described in the above mentioned Swedish patent application. A problem that often arises at fine milling applications are the so called edge chippings, i.e., chips are torn off from the periphery of the produced surface when it is being machined. Of course, edge chippings may also occur around the edges of recesses on the machined surface, such as around the edge of a boring. Another problem that often arises is the milling of thin-walled workpieces. As known, for the sake of saving material and weight, certain parts of a workpiece may be relatively thin-walled, such as between 3 and 5 mm. These portions tend to yield to some extent to the pressure from the milling tool, which in turn results in a certain waviness on the produced surface . A further problem with several of the fine milling cutting inserts on the market is the fact that they to a high degree require a costly precision grinding in order to attain the highly set tolerance requirements. Of course, this increases the production costs most considerably. As an example of this, one may again mention DE-A-4 013 717, and the known cutting insert reproduced in Figure 4. Another inconvenience of the inserts disclosed in DE-A-4 013 717 is that they are slightly rhombic. This has the consequence that only two of the four corners on each side may become operatively active. Thus, this disadvantage doubles the already previously considerable production cost per cutting corner. Thus, a first object of the present invention is to provide a milling cutting insert that reduces the size and the number of edge chippings to a minimum. A second object of the present invention is to achieve fine and smooth surfaces also on thin-walled workpieces. Still another object of the present invention is to reduce the axial pressure of the milling tool against the workpiece. A still further object of the present invention is to minimize the costs for the production of the milling cutting insert. These and further objects have been achieved in a surprising way by forming a milling cutting insert with the features as defined in the characterizing portion of claim 1. For illustrative but non-limiting purposes, the invention will be described in more detail with reference to the appended drawings which show preferred embodiments. Figure 1 shows a perspective view obliquely from above of a cutting insert according to the invention. Figure 2 shows a top view of the cutting insert according to Figure 1. Figure 3 shows a side view of the cutting insert according to Figure 1. Figure 4 shows another embodiment of a cutting insert according to the invention. Figure 5 shows a top view of the cutting insert « according to Figure 4. Figure 6 shows a side view of the cutting insert . 5 according to Figure 4. Figure 7 shows a known insert in a perspective view obliquely from below. Figure 8 shows how the cutting insert according to the invention is mounted in a milling cutter body according to the 10 Swedish patent application 9300889-4. In figures 1 to 3 and 4 to 6, respectively, an indexable cutting insert of square basic shape is illustrated which in its entirety is designated by reference numeral 1. It is double-sided and has two similar main surfaces 2, which are 15 substantially plane-parallel to each other and are turned 90° relative to each other. Between the two main surfaces 2 of the insert extend four substantially identical side surfaces 3. In order to confer a positive cutting geometry to the cutting insert when it is mounted in the milling cutter body (cf. figure 20 8) , two side surfaces 3 opposed to each other are inclined in such a way that they form an acute angle with the associated main surface 2. Suitably, this angle can lie within the range of 60 to 85°, preferably between 70 and 80°. Each main surface essentially consists of an abutment 25 surface 4, which is wholly plane and functions as an abutment surface against the corresponding support surface in the insert pocket of the milling cutter body. An essential feature of the present invention is that in connection to each cutting corner there are two bevelled surfaces 5 and 6, which extend 30 substantially along the adjacent edge surface 3, that forms an obtuse angle with the associated main surface. The bevelled surface 5 may extend from one corner 12 to next corner 12, as illustrated in figures 1 and 2; however, it may also be interrupted into two separate areas by the support surface 4 35 extending the whole way to the two edge surfaces 3 with which it forms an .obtuse angle. Further, each corner portion of the bevelled surface 5 that directly adjoins to the bevelled surface 6 may be divided into three facet surfaces in order to increase the clearance when the insert is mounted in a milling cutter body. Thus, said portion may be divided into two plane facet surfaces 13 and 14, which connect to each other via a concave radius 15. Facet surface 13 occupies a constant level difference relative to the plane of support surface 4, while facet surface 14 is inclined upwardly towards said plane, its width diminishing in the direction towards the middle of the insert's side . The narrower end of facet surface 14 transposes via a convex radius 16 into a plane portion 17, which is substantially plane-parallel with the support surface 4 and located on a level somewhat below the plane of the support surfaced. When support surface 4 extends to the very edge surface 3, of course the portions 16 and 17 are dropped. In the same way as bevelled surface 5, also bevelled surface 6 may be divided into two facet surfaces 18 and 19, which are interconnected via a concave radius transition 20. Also in this case, the reason for this division into facets is to increase the clearance of the part of the bevelled surface that is situated next to the cutting edge. In the same way as facet surface 13, facet surface 18 is inclined with a constant distance from the plane of the support surface 4, while facet surface 19 is inclined upwardly towards said plane to connect to said support surface along a break-line 21. The bevelled surface 5 located at the cutting corner is adjacent to the edge surface 3 that forms an acute angle with the associated main surface along a secondary cutting edge 7, whose primary function is to impede edge chippings. In order to fulfil this function, the angle between facet surface 13 and the plane of the support surface 4 should be within 5 and 45°, suitably between 10 and 40° and preferably between 25 and 35°. As mentioned above, bevelled surface 6 is situated next to bevelled surface 5. The angle of its facet surface 18 to the support surface 4 is between 1 and 20°, suitably between 3 and 17° and preferably between 5 and 15°. The bevelled surface 6 connects to the adjacent edge surface 3 along a main cutting edge 8. Referring now to figures 4 to 6, two parallel lands 9' extend between two nearly located bevelled surfaces 6, which lands also adjoin to their respective parallel land cutting edge 10' , and to a middle portion 26 located between said parallel lands 9' . Usually the parallel lands 9 and the middle portion 26 are situated somewhat lower than support surface 4, wherefore transposition surfaces 27 and 28 are arranged between on one side the parallel lands and the middle portion and on the other side the support surface 4. Each prallel land 9 has a slight bend or curvature perpendicularly to the support surface 4. According to the embodiment shown in figures 1 to 3, a parallel land 9 extends between two adjacent bevelled surfaces 6, which land also adjoins to the parallel land cutting edge 10 and the support surface 4. The parallel land 9 has a slight bend or curvature perpendicularly to the support surface 4. The radius of this curvature is rather large and according to the embodiment shown in figures 1 to 3 it may suitably be between 500 and 1200 mm, preferably between 600 and 1000 mm. According to the embodiment shown in figures 4 to 6, the corresponding radius of curvature of the parallel lands 9' may suitably be between 20 and 600 mm, preferably between 70 and 400 mm. The highest points of the curved parallel lands 9' are located above the middle portion 26, however preferably below the support surface 4, in order to simplify the production of the insert seats in the milling cutter body. Since the parallel land cutting edges 10, 10' are very short in comparison with the directly pressed radii (cutting edge 10 is suitably between 6 and 8 mm, each edge 10' suitably between 1,8 and 3,2 mm) , the chordal height of the parallel land cutting edge becomes correspondly small. For example, for a cutting edge 10', it is within the range 3 to 7 μm, and for the edge 10 within the range 6 to 10 μm. The highest point of the parallel land cutting edge is at its middle, where the secondary land 9 has its smallest width. However, it does not fully reach up to the plane of support surface 4. Suitably, there is a step of between 2 and 100 μm between the highest point H of the parallel land and the support surface, preferably between 5 and 35 μm. These measures are also valid for the vertical distance the middle portion 26 and the support surface 4. By the curved parallel lands and the bevel-strengthened corners, no grinding of these surfaces is necessary for achieving the desired surface smoothness on the workpiece. The only surfaces that may require an after-grinding in order to attain exact positioning in the insert seats with a minimal radial play, are edge surfaces 3. The main surfaces or clearance surfaces 2 can be directly pressed in their entirety, "main surface" or "clearance surface" meaning the entirety of surfaces 4, 5, 6, 9, 16 and 17 (figure 2), and 4, 5, 6, 9', 10', 26, 27, 28 (figure 5) . However, edge surfaces 3 are easy to grind since they are wholly plane and have fully open ends. By the simplified production, the production costs have been brought down to about a fifth of those that arise when producing cutting inserts according to DE-A-4 013 717. Even so, it has been managed to bring down the radial play to between 0,01 and 0,02 mm. This may be compared with the radial play in DE-A-4 013 717 and of the cutting insert according to figure 4, where it amounts to about 0,1 mm. Suitably, the cutting inserts according to the invention are provided with a centrally located through-hole 11 for the insertion of a suitable fastening device, such as a screw, locking pin or similar, cf. figure 8. Returning to figure 7, this figure illustrates a known cutting insert 1' . As according to the invention, the main or relief surfaces 2' are plane-parallel and turned 90° relative to each other. Said surfaces 2' are joined via four substantially equal side surfaces 3', which are angled to the surfaces 2' in the way described above for the invention. Further, the side surfaces 3' are slightly curved outwards. A bevelled surface 5' is provided along the breaklines that form an obtuse angle between main surface 2' and side surface 3' . This bevelled surface is angled at about 30° relative to the continuation of the plane of the main surface 2' . Figure 8 shows a milling cutter body according to the Swedish patent application 9300889-4. According to the invention, the cutting inserts 1 are fastened in the cassettes 22, which in turn are mounted in grooves in the milling cutter body 23 by fastening screws 24. The axial height is finely adjusted by an eccenter tap 25. In spite of the apparent simplicity of the cutting inserts according to the invention, a number of surprising advantages have been attained. Thus, the occurrence of edge chippings has been reduced quite considerably. This is clearly illustrated in the following examples: Example 1 A series of equal details (engine blocks of cast iron) were submitted to a fine milling operation. The fine-milled surfaces had a number of borings, such as 80 mm cylinder holes and 15 mm holes for cooling water. The used milling cutter body was of the configuration as described in the above mentioned Swedish patent application, had a diameter of 250 mm and was in both tests fully mounted with 30 cutting inserts in the 30 existing insert seats. At both tests, the following cutting data were used: Cutting speed: 157 m/min Feed per insert: 0,22 mm Cutting depth: 0,5 mm In the first test, cutting inserts were used according to figure 4 and in the second test cutting inserts were used according to figures 1-3. Test 1 Test 2 Size of edge chippings after 1000 work pieces 0,8-0,9 mm 0,4 mm Size of edge chippings after 2189 work pieces unusable 0,8 mm Thus, this example shows the superiority of the cutting insert according to the invention for avoiding edge chippings, in comparison with a similar known insert . Also tests with cutting inserts according to the figures 4 to 6 resulted in considerably fewer and smaller edge chippings than the insert according to figure 7, with otherwise the same process parameters . Further, by the provision of totally eight operative cutting inserts, the insert has a very high functional level to a low production cost. Moreover, by its symmetrical form, it can be used both on right-hand and left-hand rotating milling cutter bodies . Example 2 Comparative measurements were also undertaken between a cutting insert according to figure 7 and a cutting insert according to the present invention (the embodiment shown in figures 4 to 6) with regard to the magnitude of the arisen axial forces when machining cast iron. The same milling cutter body and the same cutting depth (0,5 mm) were used as in Example 1. The cutting speed was 150 m/min. The axial force arising at each test was measured and the results are presented below: Tooth feed Test 1 Test 2 (Invention) (Fig. 7) 0,1 mm/tooth 35 N 145 N 0,2 mm/tooth 100 N 230 N 0,3 mm/tooth 135 N 320 N Thus, the cutting inserts according to the invention exert a considerably lower axial force, which is very essential at the machining of thin-walled work-pieces in order to avoid vibrations and yielding of thin-walled parts. Both embodiments according to the invention (i.e., on one hand the embodiment according to figures 1 to 3 and on the other hand the one according to figures 4 to 6) contribute to considerably less edge chippings. Further, the embodiment according to figures 1 to 3 produces somewhat smoother surfaces than the embodiment according to figures 4 to 6, while the latter influence more advantageously on the decrease of the tool's axial pressure against the work-piece. Thanks to these somewhat differentiated, further properties of the two embodiments, a milling tool may advantageously be mounted with both types of cutting inserts, whereby the beneficial effects of both cutting inserts are profitted of simultaneously. In order to optimize the beneficial properties of the two types, the proportion between the numbers of the two types may be varied. Tests have shown that the number of cutting inserts according to figures 4 to 6 preferably should exceed the number of inserts according.to figures 1 to 3. In particular, the number of cutting inserts according to figures 4 to 6 should be more than four times more than the number of inserts according to figures 1 to 3. Particularly for thin-walled work-pieces, such a proportion has given very good results.

Claims: CLAIMS 1. Double-sided milling cutting insert of a substantially square basic shape comprising two substantially similar main surfaces and four substantially similar side surfaces extending between these two main surfaces, two opposed side surfaces (3) forming an acute angle with the one main surface and the other two opposed side surfaces forming an obtuse angle with the same main surface, the two side surfaces forming an acute angle with said one main surface forming an obtuse angle with the other main surface and the two side surfaces forming an obtuse angle with said one main surface, forming an acute angle with said other main surface, c h a r a c t e r i z e d in that each of the two main surfaces (2) comprises four operative cutting corners and that in connection to each cutting corner are two bevelled surfaces (5,6) which are both angled relative to the plane of the main surface. 2. Milling cutting insert according to claim 1, c h a r a c t e r i z e d in that each of the two main surfaces (2) comprises a support surface (4) . 3. Milling cutting insert according to claim 2, c h a r a c t e r i z e d in that a first bevelled surface (5) located closest to the cutting corner forms a secondary cutting edge (7) along the intersection line with the side surface that forms an acute angle with the associated main- surface, and in that a second bevelled surface (6) adjacent to said first chamfered surface (5) forms a main cutting edge (8) with said side surface. 4. Milling cutting insert according to claim 3, c h a r a c t e r i z e d in that on each of the two main surfaces (2) are two parallel lands (9) , both being delimited by the support surface (4) , two other bevelled surfaces .(6) and the intersection line between the main surface and a side surface with which the main surface forms an acute angle, said intersection line forming a parallel land cutting edge (10) . 5. Milling cutting insert according to any of claims 1 to 3 , c h a r a c t e r i z e d in that two parallel lands (9') connect to the cutting edge line that extends between two near cutting corners on the same main surface, a parallel land cutting edge (10') associated to each of the two parallel lands forming a part of said cutting edge line. 6. Milling cutting insert according to claim 5, c h a r a c t e r i z e d in that each parallel land (9') is delimited in the direction towards the middle of the cutting insert by the support surface (4) , possibly via a transposition surface (27, 28) , and in that a middle portion (26) is foreseen along said cutting edge line between the two parallel lands (9') . 7. Milling cutting insert according to any of claims 4 to 6, c h a r a c t e r i z e d in that each secondary land (9,9') is curved in such a way that it has a radius in the direction towards the plane of the support surface (4) . 8. Milling cutting insert according to any of claims 5 to 7, c h a r a c t e r i z e d in that the two parallel lands (9') reach closer to the plane of the support surface (4) than the intermediate middle portion (26) . 9. Milling cutting tool for chipbreaking machining comprising a milling cutter body (23) being rotatable in a direction of rotation and a number of cutting insert-carrying cassettes (22) , each cassette (22) being placed in a groove in the periphery of the milling cutter body and arranged to be fixed in the groove by one or more fastening screws (24) , each fastening screw being arranged in a through hole in the cassette (22) , which holes are somewhat elongated in the longitudinal direction of the cassette to enable a certain axial movement of the cassette in the groove, c h a r a c t e r i z e d in that one or several cutting inserts according to claims 1 to 8 are mounted in its cassettes (22) . 10. Milling cutter tool according to claim 9, c h a r a c t e r i z e d in that it comprises at least one cutting insert according to claim 4 and at least one cutting insert according to claim 5.

Inventors: HESSMAN INGEMAR, ROMAN STEFAN, HESSMAN, INGEMAR, ROMAN, STEFAN
Applicants: HESSMAN INGEMAR, ROMAN STEFAN, SANDVIK AB, HESSMAN, INGEMAR, ROMAN, STEFAN
Application year: 1995
Main classification: B23C5
IPC codes: B23C5