Cutting tool and cutting insert

Abstract: The present invention relates to a milling cutter. The cutter comprises a holder and at least two cutting inserts. The cutting inserts (10;50) have a generally octagonal basic shape and each includes at least three major cutting edges (16,55A). An edge face of each insert is provided with a step which is provided so as to constitute an exit zone for a fracture. At least one of the cutting inserts is a wiper insert (50) and every other cutting edge of the wiper insert is a major cutting edge (55A) joining a wiper edge (55B) situated between two major cutting edges. A first distance of the wiper insert between two diametrically opposed wiper edges (55B) is larger than a second distance of the wiper insert between two diametrically opposed major cutting edges (55A). The invention further relates to a cutting insert (50), per se.

Description: FIELD OF THE INVENTION The present invention relates to a cutting insert and a milling cutter according to the preambles of the independent claims. BACKGROUND OF THE INVENTION In tools for metal cutting having indexable cutting inserts of hard and wear resistant material the cutting edges are subjected to wear and sometimes some part of or the entire cutting edge collapses. If a small insert fractures only the cutting edge collapses without any risk for damage to the tool body. If a large insert fractures, the risk of damage to the tool body increases to an essential degree. If the cutting insert is not seated on a shim then a fracture of the insert often gives rise to damage of the tool body. To avoid such damages it is usual that the cutting insert rests on a shim of cemented carbide. A usual combination is a cutting insert having a thickness of about 5 mm and a shim having a thickness of 3-5 mm. This results in a total thickness of the cutting insert and the shim of 8-10 mm. In US-A-5 147 158 there is depicted a four-sided cutting insert having an integrated shim so as to provide a fracture zone. The cutting insert comprises an upper rake face, a lower face, and an edge clearance face interconnecting said upper and lower faces. The edge clearance face includes a step located intermediate the upper and lower faces. When the insert is in use the step defines a zone with a strain peak such that in the event of insert breakage, the propagation of the crack will be guided to the area of the step. The portion of the insert located below the step remains intact and thus protects the tool body. US-A-4,966,500 shows a milling cutter having octagonal or hexagonal cutting, inserts. Each cutting insert has a segmented (non-linear) major cutting edge, an end cutting edge and a radially inner cutting edge. The radially inner cutting edge is supposed to cut during coring of the work piece. The known insert lacks fracture zones so as to prevent an insert breakage from transferring into the seat or shim. Furthermore the known milling cutter cannot produce very fine surface finish. OBJECTS OF THE INVENTION One object of the present invention is to present a wiper insert and a milling tool that overcome the disadvantages discussed above. Another object of the present invention is to provide a wiper insert and a milling tool providing a very fine surface finish. Still another object of the present invention is to provide an economically favorable wiper insert providing at least three wiper edges. Still another object of the present invention is to provide a wiper insert having a built-in cutting depth. These and other objects are realized by a milling cutter and a wiper insert that has been given the characteristics of the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of the invention will become apparent from the following detailed description of a preferred embodiment thereof in connection with the accompanying drawings, and in which: Fig. 1A shows an embodiment of a milling cutter according to the present invention, in a side view. Fig. 1 B schematically shows two octagonal cutting inserts in sequence. Fig. 2A shows a plan view of a cutting insert of the milling cutter of Fig. 1. Fig. 2B shows a section along line II-II in Fig. 2A. Fig. 2C shows an insert in a side view. Fig. 2D shows a magnified portion of a lower comer of the Insert in Fig. 2B. Fig. 3A shows a plan view of a wiper insert according to the present invention. Fig. 3B shows a section along line III-III in Fig. 3A. Fig. 3C shows the wiper insert in a side view. Fig. 3D shows the wiper insert in a bottom view. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION In Fig. 1A is shown a milling cutter 30 according to the present invention, including a tool holder 31 and cutting inserts 10 and 50. The holder 30 is a milling cutter body which has a number of pockets 32 for receiving cutting inserts. Each pocket comprises a substantially planar base 40 and two shoulders (only one shoulder 42 being visible). The shoulders are substantially perpendicular to each other. Each shoulder includes a first surface, provided to abut a corresponding first portion 13a of the insert 10 or 50. A threaded boring is provided in the vicinity of the pocket 32 so as to receive a fastening screw 33 to force the insert towards the shoulders and the base, via a radially outer part of the screw head. When the insert is to be indexed the screw 33 is partially unscrewed until the cutting insert can be pulled by hand radially outwardly and the insert may be rotated by an increment corresponding to 45° counter-clockwise, so that a fresh secondary cutting edge 16E comes in position. Then the insert is pushed into the pocket and the screw is tightened again. The edge 16D is preferably passive and does not cut during milling. To provide for an excellent surface finish at least one wiper insert 50 according to the present invention is inserted into the milling cutter 30. The wiper insert 50 fits into any of the pockets 32 as described above in connection with the insert 10. The inserts 10, 50 will be located in the holder at a setting angle Q about 43°, to obtain a clearance angle of about 2° for the secondary cutting edge 16E of the insert 10 and about 0° for the wiper insert edge 55B. Fig. 1B schematically shows two octagonal cutting 10, 50 inserts in sequence, i.e. each insert profile is taken at the same position at the periphery of the cutter. The cutting insert 10 depicted in Figs. 2A to 2D has a generally octagonal basic shape and includes an upper face 11, a lower face 12 and an edge face 13 generally connecting the upper and lower faces 11 and 12, respectively. The cutting insert 10 has a positive geometry, i.e., an upper portion 13a of the edge face defines a clearance face and forms an acute angle with the upper face 11, the latter being a rake face. The upper face 11 includes a chip upsetting face located at the periphery of the insert 10. The upper face 11 furthermore, includes a chip face 15 sloping inwardly and downwardly from the chip upsetting face. The latter faces constitute the most peripheral part of the upper face 11. The intersecting line between the chip upsetting face and the edge face 13 forms the overall cutting edge 16, which is defined by eight cutting edges 16A-16H. The chip face 15 is concavely curved and extends inwardly from the chip upsetting face. A plurality of projections or preferably depressions 25 are formed in the chip face 15, the projections or depressions 25 being spaced apart along the transition edge 26. The depressions 25 preferably intersect the transition edge and extend inwardly therefrom in a direction non-perpendicularly to the associated cutting edge 16 as said insert is viewed in plan. Each depression thereby forms in plan, an angle in the range of 15° to 40° relative to a normal of the associated clearance face 13. The effect of such design is to minimize heat transfer from the chip to the insert. Respective pairs of adjacent cutting edges 16 meet to form a cutting corner 34 having an internal obtuse angle, about 135°. Each cutting edge is in a bottom view, generally parallel to the adjacent second portion 13b of the shim 22. Each cutting edge is concavely curved when seen in a side view, such that the mid section of the edge is closer to the lower side 12 than are the end sections thereof. A ridge 28 provided at each intersection of the chip faces 15 strengthens each comer 34 and forms the chip generated at the comer 34. The edge face 13 includes eight segments. An upper portion 13a of each segment of the edge face 13 forms a clearance angle α with a plane P oriented parallel to a center axis 20 of the insert (Fig. 2B). The clearance angle α is at least 20°, preferably at least 25°. The distance between two diametrically opposed major cutting edges 16C and 16G for example is depicted by numeral L1 in Fig. 2A. The distance L1 is the same for all combinations of diametrically opposed major cutting edges. The eight, planar edge surfaces 13 taper towards the lower face 12 and generally intersect at the center line 20 of the cutting Insert 10 below the lower face 12. Each segment of the edge face 13 is provided with an inward, downwardly facing shoulder forming a step 18. The upper or first portion 13a of the segment is located above the step 18 has a positive geometry in order to provide the necessary clearance angle. A lower or second portion 13b of the segment forms a peripheral edge of a shim 22 which is of integral one-piece construction with the upper part of the insert. The shim 22 forms the lower face 12 of the insert. The second portion 13b of each segment has a negative geometry, i.e. the second portion 13b is perpendicular to the lower face 12 and thus form no clearance angle. Each second portion 13b lies radially inside of an imaginary extension of the associated first portion. That is, the edge surface does not need to be ground if the clearance face is ground and thereby saving energy. Furthermore the step 18 will then be sufficiently large in the radial direction to provide for a favorable control of any cracks emanating during machining. The step 18 thus provides a sharp inner corner 19 that constitutes an exit zone for a fracture that by a possible break of the cutting edge controls the break and leaves the lower portion of the cutting insert undamaged. By the term "sharp" is here meant that the comer intentionally weakened by having a radius of 0 to 0.3 mm, preferably about 0.2 mm. The second or lower portion 13b thus protects the tool body. Directing attention to Fig. 2B and 2D the following is to be said. The height h 1 is 15 - 60% of the insert height h, preferably about 25%. This is possible since the anvil or shim constitutes an integral part of the cutting insert 10, i.e. the height h 1 corresponds to the height of the integrated shim 22. In order to achieve a good function of the cutting insert 10 it is necessary that the height h 1 does not constitute too big a part of the height h since in that case the strength of the cutting edge will be too low. That could result in fractures of the cutting edge also at small loads. The smallest radial distance d from the step 18 to the second portion 13b in the plane of the lower face 12 depends on the clearance angle. The distance in the cross section II-II in Fig. 2A, is in the interval 0.01 -1.0 mm, preferably in the magnitude of 0.1 mm. The second portion 13b is about parallel with the center axis 20. The step 18 forms an angle δ with a line perpendicular to the center axis 20. The angle δ is 0° to 30°, and preferably about 20°. Eight second portions 13b are formed around the periphery of the shim 22. Two adjacent portions 13b meet in a comer 23 at an internal obtuse angle, about 135°. The corners project radially relative to the rest of the second portions 13b. The periphery of the lower part describes a path of increasing and decreasing radial distances from the center axis 20 of the insert. Turning now to Figs. 3A to 3D a wiper insert 50 according to the present invention is shown having the same basic shape as the insert 10 described above and like features are depicted by like numerals hereinafter. The cutting insert 50 depicted in Figs. 3A to 3D has a generally octagonal basic shape and includes an upper face 11, a lower face 12 and an edge face 13 generally connecting the upper and lower faces 11 and 12, respectively. The cutting insert 50 has a positive geometry, i.e., an upper portion 13a of the edge face defines a clearance face and forms an acute angle with the upper face 11, the latter being a rake face. The upper face 11 includes a circumferential land 51 located generally at the periphery of the insert 50 (see Fig. 3A). The upper face 11 furthermore, includes a recess 52 defined by a star-shaped rim 53 sloping inwardly and downwardly from the land 51 to a bottom surface 54. The rim 53 describes a path of a generally sinusoidal curve. The rim is provided to safeguard the insert from being thrown outwardly if the frictional force between the fastening device and the bottom 54 is overpowered by centrifugal force. The depth Z of the recess 52 is about 0.1 mm. The bottom surface 54 is provided with four markings or arrows 59 to facilitate identification of the wiper edges 55B-58B. The intersecting line between the land 51 and the edge face 13 form four pairs of cutting edges 55-58, each pair comprising a major cutting edge 55A-58A and a wiper edge 55B-58B such that every other cutting edge is a major cutting edge 55A-58A joining a wiper edge 55B-58B situated between two major cutting edges. Stated another way, major cutting edges 55A-58A and wiper edges 55B-58B are alternately disposed on the perimeter of the cutting insert. A first distance L2 between two diametrically opposed wiper edges 56A and 58A for example, is larger than a second distance L3 between two diametrically opposed main cutting edges 55A and 57A. The first distance L2 is about 1 % to 5 % larger than the second distance L3, preferably about 1.5 % larger. The first distance L2 is about 0.1 % to 1 % larger than the distance L1 of the insert 10, preferably about 0.5 % larger. The edges 55B-58B are substantially straight when seen in a side view and coincide in a common plane K (Fig. 3C) but are preferably somewhat curved when seen in a plan view. The radius of said curve is about 600 mm. An imaginary circle C inscribed in the insert 50 touches each wiper edge and each major cutting edge partially extends radially inside of the circle C a distance X, Fig. 3A. Locating segments 13c connected to the upper portions 13a are provided at a clearance angle which is larger than the clearance angle of the upper portions 13a. The insert 50 is mounted to the tool body in generally the same manner as the insert 10. However, only two of the passive locating segments 13c connected to the wiper edges, for example 56B and 57B, will simultaneously bear against the shoulders of the tool body 31. By the term "passive" is here meant that the segment or connected cutting edge is not in cutting position. In the position shown in Fig. 1 B the major cutting edge 55A of the wiper insert 50 is aligned with the major cutting edge 16F of the insert 10, i.e. they coincide with an imaginary truncated cone. By having the active wiper edge 55B of the wiper insert extend more in the axial direction of the milling cutter, due to the longer distance L2, than the insert 10 the edge 55B will generate the surface in the work piece (not shown). Furthermore by having the distance L3 of the wiper insert shorter than the distance L1 of the insert 10 the edges 55A and 16F still can coincide in the common imaginary cone. The recess 52 is provided to cooperate with the screw head 33 to keep the insert 50 from being thrown radially outwardly from the cutter due to centrifugal forces when in use. By applying the teachings of the present invention a wiper insert and a milling tool providing a very fine surface finish is provided. Furthermore an economically favorable wiper insert having at least three wiper edges is provided. In addition, the wiper insert is easily mounted in the tool body since the insert has a built-in cutting depth. By providing the insert with an integral shim, defined by a sharp corner, the possible cracks will be stopped from propagating into the holder body. The present invention provides an economical solution to the user, by having eight cutting edges in combination with wiper edges and by saving holder bodies. In addition, the tool according to the present invention cuts in an easy manner through the workpiece metal which is especially important when a thin flange shall remain connected to the work piece of aluminum, for example. Although the present invention has been described in connection with a preferred embodiment thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the scope of the invention as defined in the appended claims. For instance, in the disclosed embodiment the cutting inserts 10, 50 are completely solid but may alternatively be provided with a through-hole intended to receive clamping screws that secures the cutting inserts to the tool holder.

Claims: A cutting insert (50) for milling, said cutting insert (50) having a generally octagonal basic shape and including eight cutting edges (16) created by the transition between an upper face (11) and an edge face (13) of the cutting insert (50), said upper face (11) defining a rake face and said edge face (13) defining a clearance face, said edge face (13) forming a first clearance angle (α) with a plane (P) parallel to a center axis (20) of the insert, characterized in that the cutting insert (50) is a wiper insert, having major cutting edges (55A-58A) and wiper edges (55B-58B) that are alternately disposed on the perimeter of the cutting insert, whereby a first distance (L2) between two diametrically opposed wiper edges (55B-58B) is larger than a second distance (L3) between two diametrically opposed major cutting edges (55A-58A) and the first distance (L2) is 1 % to 5 % larger than the second distance (L3), preferably about 1.5 % larger. The cutting insert according to claim 1. characterized in that the edge face (13) is provided with a step (18) which is provided so as to constitute an exit zone for a fracture, The cutting insert according to claim 1 or 2, characterized in that an imaginary circle (C) inscribed in the insert (50) touches each wiper edge (55B-58B) and each major cutting edge (55A-58A) partially extends radially inside of the circle (C). The cutting insert according to claim 1,2 or 3, characterized in that the upper face (11) includes a circumferential land (51) located generally at the periphery of the cutting insert (50) and that the upper face (11) includes a recess (52) defined by a star-shaped rim (53) sloping inwardly and downwardly from the land (51) to a bottom surface (54) and that a depth (Z) of the recess (52) is about 0.1 mm and that the intersecting line between the land (51) and the edge face (13) form four pairs of cutting edges (55A, 55B-58A, 58B). The cutting insert according to claim 2, characterized in that the step (18) is so designed that in a certain longitudinal cross-section of the cutting insert the distance from a centre line (20) of the cutting insert (50) to the periphery of the cutting insert (50), said distance being perpendicular to the centre line (20), is smaller below the step (18) than above the step (18), and that the distance h 1 from the lower face (12) of the cutting insert (50) to the step (18) is 15 - 60 % of the distance h from the lower face (12) to the cutting edge (16), said step being provided so as to constitute an exit zone for a fracture radially inside the clearance face. A milling cutter comprising a holder and at least two cutting inserts, whereof at least one of the cutting inserts is a wiper insert (50) according to anyone of claims 1 to 5.

Inventors: STRAND BENGT, STRAND, BENGT
Applicants: SECO TOOLS AB
Application year: 1999
Main classification: B23C5
IPC codes: B23C5