Metal-cutting insert having a round cutting edge

Abstract: A metal cutting insert (10) comprises a top surface (12) which is intersected by a frusto-conical edge surface (16) to form therewith a round cutting edge (18). Five equidistantly spaced flat facets (26) are formed in the edge surface (16). Upper ends of the facets are spaced below the cutting edge. A fixing screw secures the insert to the seat of a holder, such that two of the facets are pressed against respective flat contact areas formed on a locating surface of the holder. Each facet forms an acute angle relative to a central axis (25) of the insert.

Description: Background of the Invention The present invention relates to metal cutting inserts having a round cutting edge according to the precharacterising portion of claim 1. Cutting inserts are conventional which possess a top (chipbreaking) surface and a frusto-conical edge (clearance) surface which intersects the top surface to form therewith a round cutting edge. Such inserts, usually formed of a cemented carbide, are used in copy turning operations and making rounded fillets. A central through-hole in the insert receives a fixing screw which is connectible to a threaded hole in a holder to anchor the insert in place. The holder defines a tip seat or pocket in which the insert is received. The pocket includes an upstanding locating wall which is generally V-shaped when viewed in plan, whereby the frusto-conical edge surface makes contact with that locating wall at two locations. During a cutting operation, only a portion of the round cutting edge actually cuts the workpiece. When that portion is worn, the insert is indexed to present a fresh portion of the cutting edge for cutting. It is important that, during the cutting operation, the insert remain fixed against rotation (slippage) about its center axis relative to the holder. However, due to the curvature of the edge surface of the insert, such slippage may occur, despite being pressed tightly against the locating wall by the fixing screw. It is a goal of the present invention to prevent such slippage without weakening the cutting edge. In Raupp, Jr. et al U.S. Patent 4,273,479, an indexable broaching insert is described which includes a cylindrical edge surface that intersects a top (clearance) surface to form a round cutting edge. The cylindrical surface includes four equi-spaced flats which make engagement with a locating wall for resisting slippage of the broaching insert during cutting. The flats are oriented parallel to the center axis of the insert and intersect the cutting edge to convert what would otherwise be a round cutting edge into a polygonal cutting edge. Such a polygonal cutting edge, however, would not be suitable for copy turning and fillet making. The patent document DE-C-36 42 514 representing the closest prior art discloses an indexable cutting insert for ball nose end mills comprising facets on its clearance surfaces. However, these facets are curved and not well adapted for holding the insert stably against a flat support surface in an insert seat. Insert seats with curved support surfaces are considerably more complicated to manufacture. Summary of the Invention The present invention relates to a metal cutting insert. The cutting insert comprises a top chipbreaking surface, a bottom mounting surface, and a generally frusto-conical edge clearance surface extending between the top and bottom surfaces. The edge surface intersects the top surface to form therewith a round cutting edge. A plurality of circumferentially spaced flat facets are formed in the edge surface. Upper ends of the facets are spaced below the cutting edge. Each facet is inclined at an acute angle relative to a center axis of the insert. Each facet has a height which is no greater than 50-70 percent, preferably 60 percent, of a height of the insert. Preferably, the acute angle is from 10 to 20°, most preferably about 15°. There are preferably five facets spaced equidistantly apart. The insert preferably includes a central throughhole for receiving a fixing fastener that secures the insert to the holder. The top surface of the insert preferably includes a round land situated radially inwardly of the cutting edge. The holder includes a seating surface for supporting the insert, and an upstanding wall having a locating surface. The locating surface is inclined at about the same angle as the acute angle of the facets. The locating surface includes first and second flat contact areas which form an obtuse angle therebetween. Those contact areas are contacted by two of the facets. A fixing screw secures the insert to the holder and presses the two facets against the contact areas. Brief Description of the Drawing 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 in which like numerals designate like elements, and in which: FIG. 1 is a top plan view of a metal cutting insert according to the present invention; FIG. 2 is a side elevational view of the insert depicted in FIG. 1; FIG. 3 is a bottom plan view of the insert depicted in FIG. 1; FIG. 4 is a top plan view of an insert mounted in a holder; FIG. 5 is a cross-sectional view taken along the line 5-5 in FIG. 4; and FIG. 6 is a cross-sectional view taken along the line 6-6 in FIG. 4. Detailed Description of a Preferred Embodiment of the Invention A turning insert 10 comprises a top face 12, a bottom supporting surface 14, and a frusto-conical edge surface 16. The top surface acts as a chipbreaking surface. The intersection of the top surface and edge surface defines a round cutting edge 18. A round land 20 is situated inside of the cutting edge 18. The center portion of the top surface is recessed with respect to the land 20. A through-hole 24 extends centrally through that center portion for receiving a fixing screw. The edge surface 16 is inclined at an acute angle A relative to a line parallel to the center axis 25 of the insert, and thus is inclined at that same angle relative to the axis itself. That angle A is preferably about 7°. Formed in the edge surface 16 is a plurality of facets, i.e., planar surfaces 26 which extend from the bottom surface toward the top surface. There are preferably five identical and equidistantly spaced facets. Each facet forms an angle B relative to a line extending parallel to the central axis 25 and thus forms the angle B relative to the axis 25 itself. The angle B is preferably in the range of 10-20°, most preferably about 15°. The height H' of each facet 27 is about 50 to 70 percent of the height H of the insert, most preferably about 60 percent. Thus, the facets terminate short of (i.e., below) the cutting edge 18 so that the cutting edge remains round, and so that sufficient material remains disposed below the cutting edge to reinforce the latter. The insert 10 is to be mounted in a pocket of a holder 30. The pocket has a seating surface 32 on which a shim 34 rests, with the insert 10 seated on the shim 34. A receiving hole 36 extends into the seating surface 32 and defines a center axis 38. An upstanding wall 39 extends from the seating surface 32 and includes a locating surface 40. The locating surface 40 is generally V-shaped as viewed in plan (see FIG. 4) in that it is formed by two flat contact areas or sides 42 that form an obtuse angle C therebetween which is preferably about 108°. That angle C corresponds substantially to an angle D formed between two successive facets of the insert (see FIG. 3). Each side 40 forms an angle E relative to the axis 38 which is slightly smaller (e.g., by about 20') than the angle B to ensure that initial contact between the insert and locating wall occurs adjacent the upper end of the facet rather than at the lower end thereof (which could cause the insert to break). Since the insert is formed of a harder material than the holder, the upper end of the facet will eventually be pressed into the softer locating surface to achieve a flush surface-to-surface contact therebetween. It will be appreciated that when an insert 10 is seated on the shim 34 and is pressed against the locating surface 40 such that two flat facets 26 contact respective ones of the flat sides 42, slippage of the insert will be effectively resisted during cutting, i.e., rotation of the insert about its axis 25 will be resisted. In order to ensure that the insert is pressed firmly against the locating surface 40, the axis 38 of the hole 36 is (as is conventional) offset very slightly relative to the axis 25 of the insert through-hole, so that the fixing screw 50 will be offset by that amount toward the locating surface 40. Hence, the fixing screw will tend to push the insert against the locating surface. It will be appreciated that the faceted insert according to the present invention effectively resists slippage of the insert during cutting while presenting a round cutting edge 18 which is amply reinforced from below. 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, modifications, substitutions, and deletions not specifically described may be made without departing from the scope of the invention as defined in the appended claims.

Claims: An indexable metal -cutting insert comprising: a top chipbreaking surface (12); a bottom mounting surface (14): a generally frusto-conical edge clearance surface (16) extending between said top and bottom surfaces and intersecting said top surface to form therewith a cutting edge (18); and a plurality of circumferentially spaced facets (26) formed in said edge surface (16), upper ends of said facets (26) being spaced below said cutting edge (18), each facet (26) being inclined at an acute angle (B) relative to a center axis of the insert; characterized in that the cutting edge (18) is round, the facets (26) are flat and each facet (26) has a height (H') that is from fifty to seventy percent of the height (H) of the insert. A metal-cutting insert according to claim 1, wherein said height (H') of each facet (26) is sixty percent of said height (H) of said insert. A metal-cutting insert according to claim 1, wherein said acute angle (B) is from ten to twenty degrees. A metal-cutting insert according to claim 3, wherein said acute angle (B) is fifteen degrees. A metal-cutting insert according to claim 1, wherein there are five said facets (26) spaced equidistantly circumferentially apart. A metal-cutting insert according to claim 1, wherein said top surface (12) includes a round land (20) situated radially inwardly of said cutting edge (18). A metal-cutting insert according to claim 1, wherein said insert includes a central through-hole (24) for receiving a fixing fastener.

Inventors: RESCIGNO PASQUALE F, RESCIGNO, PASQUALE F.
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1993
Main classification: B23C5
IPC codes: B23B27