CUTTING INSERT, CUTTING TOOL AND METHOD

Abstract: The present invention relates to a cutting tool preferably for milling. The tool comprises a holder (30) and a cutting insert (10). The cutting insert (10) has a generally circular basic shape and includes at least one cutting edge (16) created by the transition between an upper face (11) and an edge face (13) of the cutting insert (10). The upper face (11) is a rake face, and the edge face (13) is a clearance face. The holder and cutting insert have first means (37, 13b) for preventing rotation of the insert about a center axis (20). The clearance face is conical and converges towards a lower face of the insert. The first means for preventing rotation is provided distant from and radially short of the clearance face and at a part of the insert that is closer to the lower face than to the upper face so as to constitute an indication of fracture radially inside the conical clearance face. The holder and insert comprise second means (35, 13a) for preventing rotation of the insert. Also, said step constitutes an indication of fracture, and thereby a breakage of the cutting edge is better controlled. The present invention further relates to a cutting insert and a method for preventing rotation of the cutting insert about its center axis in a holder for chip forming machining.

Description: CUTTING INSERT, CUTTING TOOL AND METHOD BACKGROUND OF THE INVENTION The present invention relates to a cutting tool for chip forming machining preferably for milling, comprising a holder and a cutting insert, said cutting insert having a generally circular basic shape and including at least one cutting edge created by the transition between an upper face and an edge face of the cutting insert, said upper face being a rake face and said edge face being a clearance face, said holder and cutting insert having first means for preventing rotation of the insert about a center axis. The invention further relates to a cutting insert and a method for preventing rotation of a cutting insert. 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 has no shim an insert fracture often gives rise to damages of the tool body. In order 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 we have disclosed a polygonal cutting insert having an integrated shim so as to provide a fracture zone. US-A-5,346,336 shows a circular metal cutting insert provided with five equidistant flat facets on the clearance surface. The facets are provided to constitute defined positions when indexing the cutting insert. Each facet forms an acute angle relative to the central axis of the cutting insert. A retention screw secures the insert to the insert seat of a holder. In the known tool friction is not a major contributor against unwanted rotation of the insert. The locating surfaces of the insert will be worn such that the position of the insert in the seat will be increasingly less accurate during use. Furthermore the known insert lacks fracture zones so as to prevent an insert breakage from transferring into the seat or shim. OBJECTS OF THE INVENTION One object of the present invention is to present a cutting insert that overcomes the disadvantages discussed above. Another object of the present invention is to provide a cutting insert, a cutting tool and a method including means adapted for a double retention against rotation of the insert. Still another object of the present invention is to provide a cutting insert having an integrated shim performing three different functions, namely fracture, rotational and indexing control. These and other objects are realised by a cutting insert that has been given the characteristics of the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the cutting tool according to the present invention is described below, reference being made to the accompanying drawings wherein; Fig. 1 shows a bottom view of the cutting insert according to the present invention; Fig. 2 shows a section along ll-ll in Fig. 1 ; Fig. 3 shows an insert site in a cutter body according to the present invention in a top view; Fig. 4 shows a cross section along IV-IV in Fig. 3; Fig. 5 shows a cross section along V-V in Fig. 3; Fig. 6 shows a milling cutter according to the present invention in a side view; Fig. 7 shows a cross section through the cutter in Fig. 6. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The cutting insert 10 in Figs. 1 and 2 has a generally circular basic shape and includes an upper face 1 1 , a lower face 12 and an edge face 13 generally connecting the upper and lower faces 1 1 , 12 respectively. The cutting insert 10 has a positive geometry, i.e. the edge surface 13 forms an acute angle with the upper face 1 1. The upper face 1 1 is a rake face and the edge face 13 is a clearance face. The upper face 1 1 includes a circular chip upsetting face 14 located at the periphery of the cutting insert 10. Further the upper face 1 1 includes a chip face 15 sloping inwardly and downwardly from the circular chip upsetting face 14. Alternatively the upper face may have other shapes such as a planar shape. The faces 14 and 15 constitute the most peripheral part of the upper face 1 1 . The intersecting line between the chip upsetting face 14 and the edge face 13 forms the circular cutting edge 16. The edge surface 1 3 coincides with a part of an imaginary cone C. The apex of the cone intersects the center line 20 of the cutting insert 10 below the lower face 12. In the disclosed embodiment the cutting insert 10 is provided with a through hole 1 7 intended to receive a clamping screw (not shown) that secures the cutting insert 10 to the tool holder. The edge surface 13 is provided with a step 18. A first portion 13a of the edge surface 13 that is located above the step 18 has a positive geometry in order to provide the necessary clearance angle of the tool while a second portion 13b of the edge surface 13 has a negative geometry, i.e. the second portion 13b is perpendicular to the lower surface 12 and has a clearance angle of 0°. The second portion 13b of the edge surface 13 is radially short of the above-mentioned imaginary cone. That is, the edge surface does not need to be ground if the clearance face is ground and thereby it saves energy. Furthermore the step will then be sufficiently large in the radial direction to provide for a favourable control of any cracks emanating during machining. The step 18 provides a sharp inner corner 19 that constitutes an indication of fracture that by a possible break of the cutting edge controls the break and leaves the lower portion of the rutting insert undamaged. The second or lower portion 13b thus protects the tool body. Concerning the measures shown in Fig. 2 the following is to be said. The height h, is 15 - 60% of the height h, preferably about 25%. This is possible since the "shim" constitutes an integral part of the cutting insert 10, i.e. the height h, 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, does not constitute too big a part of the height h since in that case the strength of the cutting edge will be low. That could result in fractures of the cutting edge also at small loads. The radial distance e from the step 18 to the cone C in the plane of the lower face 12 depends on the clearance angle. The distance e in the cross section ll-ll, is in the interval 0,2 - 2,5 mm, preferably in the interval 0,2 - 1 ,5 mm. A preferred value of e is in the magnitude of 0,5 mm. The distance e in another cross section according to line A-A in Fig. 1 , is in the interval 0, 1 - 1 ,5 mm, preferably in the interval 0,2 - 1 ,0 mm. The step however, is spaced at all sections from the imaginary cone C to provide for breakage control all around the insert. The second portion 13b further connects to a bevel portion 21 to facilitate the entry of the insert into a pocket of a holder, described in detail below. A number of second portions 13b are formed at the periphery of the shim 22. There can be provided 5 - 12 portions on the shim, preferably 7 - 9 portions. A preferred value is 8 portions. In the latter case two diametrically opposed second portions 13b are parallell, as can be seen in Fig. 1 . Two adjacent portions 1 3b meet in a corner 23 at an intemal obtuse angle, about 135°. When the insert is mounted in a holder, the corners 23 will prevent undesired rotation of the insert. It is therefore important that the corners project radially relative to the rest of the second portions 13b. Said rest of the second portions 13b shall indicate the proper position of the insert but shall participate in preventing the insert from rotation only when the cutting forces exceed the frictional force. The periphery of the lower part describes a path of increasing and decreasing distance from the center axis of the insert. The second portions may for example be parts of a star-shaped form or similar as long as the corners project radially relative to the rest of the second portions. Turning now to Figs. 3, 4 and 5 a tool holder 30 is shown. The holder 30 is a milling cutter body which has a number of pockets 31 for receiving cutting inserts. Each pocket comprises a substantially planar base 32 and upstanding shoulders 33, 34. Each shoulder discloses an upper concave surface 35, provided to abut the first portion 1 3a of the insert, and an undercut portion 36. The surfaces 35 are provided to coincide with the previously discussed cone C. At least one shoulder 33 has a lower portion 37, provided to abut a corner 23 of the shim 22. A threaded boring 38 is provided close to but not in the center of the base 32, so that a fastening screw in a known manner will force the insert towards the shoulder and the base. The boring has a center axis 39 which substantially perpendicular to the base 32. The lower portion 37 is preferably straight in a plane parallell to the base and forms an acute angle s with the center line 39 in a direction upwards from the base, as seen in Fig. 4. This is to secure a distinct position for the point or line of contact between the corner 23 and the portion 37. The shoulders 33, 34 may be interrupted by a slot 40. The function of the slot is to make the surfaces 35 of the shoulders less sensitive to tolerance faults during manufacturing thereof. In Figs. 6 and 7, the cutting tool is assembled and especially from Fig. 7 it is apparent how the cutting insert 10 and the holder 30 cooperate. The insert is placed in the pocket 31 with one of its second portions 13b facing towards the lower portion 37. Then the lower face 12 abuts the base 32. A screw 41 is inserted through the hole 1 7 of the insert and threaded into the bore 38 of the holder and tightened. Since the axis 20 of the insert is radially displaced relative to the axis 39 of the boring 38 a distance I in direction away from the shoulders 33, 34 the screw will cam the insert towards the shoulders and towards the base. When the screw has been tightened the clearance face 1 3a of the insert will exert a pressure against the upper concave surfaces 35. The friction caused thereby is in most cases sufficient to secure the insert from rotation during milling. The first means 37, 13b for preventing rotation of the insert comprises at least two possible positions, a first position wherein a gap p is arranged between the lower part 13b of the insert and the first wall 37 which gap is zero millimeters and a second position wherein the gap p is larger, about 0,1 - 0,3 mm. However when the cutting forces exceed the frictional force the insert will rotate until the corner 23 abuts the lower portion 37 of the shoulder 33. Then the insert is prevented from further rotation. When the insert is to be changed for a new insert the above¬ mentioned procedure is reversed. When the insert is to be indexed the screw 41 is partially unscrewed until the lower face 12 can be lifted above the lower portion 37 and the insert may be rotated a step corresponding to 35° when having 8 positions at the shim. Then the screw is tightened again. Although the described embodiment of the invention refers to a milling cutter the invention is also applicable on turning tools using a round insert. The invention is in no way restricted to the embodiment described above but can be varied freely within the scope of the appending claims.

Claims: CLAIMS 1 . Cutting insert (10) for chip forming machining preferably for milling, said cutting insert (10) having a generally circular basic shape and including at least one cutting edge (16) created by the transition between an upper face (1 1 ) and an edge face (13) of the cutting insert (10), said upper face (1 1 ) being a rake face and said edge face (13) being a generally conical clearance face, said clearance face being part of a second means for preventing rotation about a center axis (20) of the insert during use, c h a r a c t e r i z e d in that the edge face (13) is provided with a step (18) which is provided so as to constitute an indication of fracture radially inside the generally conical clearance face and that the insert comprises a portion (1 3b), connected to said step (18), being part of a first means for preventing rotation about a center axis of the insert during use and that the first means for preventing rotation is provided distant from and radially short of the clearance face and extemally at a part (22) of the insert that preferably is closer to the lower face (12) than to the upper face (1 1 ), said step being provided so as to constitute an indication of fracture and that the step (18) is so designed that in a certain Iongitudinal cross-section of the cutting insert the distance from a centre line (20) of the cutting insert (10) to the periphery of the cutting insert (10), 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, from the lower face (12) of the cutting insert (10) to the step (18) is 15 - 60 % of the distance h from the lower face (12) to the cutting edge (16). 2. Cutting insert according to claim 1 , c h a r a c t e r i z e d in that the clearance face (1 3a) is substantially conical and tapers towards a lower face (12) of the insert and that the second means prevents rotation of the insert by friction and that the step (18) is spaced at all sections from the clearance face (13a) to provide for breakage control all around the insert. 3. Cutting insert according to any of the preceding claims, characterized in that the edge face portion (13b) below the step (18) is perpendicular to the lower face (12). 4. Cutting insert according to any of the preceding claims, characterized in that the step (18) forms a sharp corner (19) with the edge face portion (13b) below the step (18). 5. Cutting tool for chip forming machining preferably for milling, comprising a holder and a cutting insert, said cutting insert (10) having a generally circular basic shape and including at least one cutting edge (16) created by the transition between an upper face (11) and an edge face (13) of the cutting insert (10), said upper face (11) being a rake face and said edge face (13) being a clearance face, said holder and cutting insert having first means (37,13b) for preventing rotation of the insert about a center axis, characterized in that the clearance face is conical and converges towards a lower face of the insert and that the first means (37,13b) for preventing rotation is provided distant from and radially short ofthe clearance face and externally at a part of the insert that preferably is closer to the lower face than to the upper face so as to constitute an indication of fracture radially inside the conical clearance face and that the holder and insert comprise second means (35,13a) for preventing rotation ofthe insert and that the first means for preventing rotation of the insert comprises cooperation between the lower part (13b) ofthe insert (10) and a first wall (37) of the holder and that the second means for preventing rotation of the insert comprises cooperation between the conical clearance surface (13a) of the insert and a second wall (35) of the holder (30) and that the first (37,13b) and second means (35,13a) for preventing rotation of the insert are axially and radially spaced with respect to the center axis (20) of the insert. 6. Cutting tool according to claim 5, c h a r a c t e r i z e d in that the first means (37,13b) for preventing rotation of the insert comprises at least two possible positions, a first position wherein a gap (p) is arranged between the lower part of the insert and the first wall which gap is zero millimeters and a second position wherein the gap (p) is larger. 7. Method for preventing rotation of a cutting insert about its center axis in a holder for chip forming machining preferably for milling, said cutting insert (10) having a generally circular basic shape and including at least one cutting edge (16) created by the transition between an upper face (1 1 ) and an edge face (13) of the cutting insert (10), said upper face (1 1 ) being a rake face and said edge face (13) being a clearance face, said holder and cutting insert having first means (37,13b) for preventing rotation of the insert about its center axis, c h a r a c t e r i z e d in that the method comprises the steps of - providing a holder having an insert site with a lower first wall and an upper curved second wall, said first and second walls being spaced apart in axial and radial direction of the center axis of the insert, - providing an insert having a lower part and a curved clearance face, the external periphery of said lower part describing a path of increasing and decreasing distance from the center axis of the insert, - mounting said insert to said insert site so as to fit said lower part in response to said first wall, - tighten the insert to said site by tightening means so as to press said curved clearance face against said curved second wall and - allowing the insert to rotate a small fraction during use in response to cutting forces larger than the frictional force between said curved clearance face and said curved second wall, thereby closing a gap (p) between said lower part of the insert and said first wall.

Inventors: EKLUND SVEN, RIVIERE BERTRAND, STEDT ROBERT, EKLUND, SVEN, RIVIERE, BERTRAND, STEDT, ROBERT
Applicants: EKLUND SVEN, RIVIERE BERTRAND, SECO TOOLS AB, STEDT ROBERT, EKLUND, SVEN, RIVIERE, BERTRAND, STEDT, ROBERT
Application year: 1996
Main classification: B23C5
IPC codes: B23C5