DRILL WITH IMPROVED CUTTING INSERT FORMATION

Abstract: A drill comprising a tip-forming end including at least two cutting edges (8, 8') and at least two chip flutes (7, 7') extending from the tip end (2) towards the second end (3), having the purpose of forming and removing chips from the corresponding cutting edge. In accordance with the invention the drill tip is composed of cutting edges on mutual sides of the axial centre line having a straight unbroken contour, the central portions of which extend axially beyond the peripheral portions whilst the nose radiused surface on each cutting edge at the periphery is provided with a smoothly curved radius comprising one or several circular segments with mutually different radii (R1, R2, R3) where adjacent radii always appear with different size.

Description: Drill with improved cutting insert formation Field of the invention The present invention relates to a drill comprising a substantially solid shank with a first tip-forming end including at least two cutting edges for cutting machining of metallic workpieces in order to make holes in these, an opposite second end, and a number of flutes corresponding to the number of cutting edges, extending from the tip end towards the other end, having the purpose of forming and removing chips from the corresponding cutting edge. Background of the invention and description of the Prior Art The flutes of, e.g., a twist drill, have two important functions, viz. on the one hand the forming of chips (including chipbreaking) and on the other hand the removal of these formed chips. The chip formation occurs in a space immediately behind each cutting edge, regardless of whether the edge is formed by a sharpening of the drill top as such, or by a separate cutting insert which is fastened on the tip, mechanically or, by, e.g., brazing. In this front space of a flute, the initially bandlike material that is separated off by the cutting edge from a workpiece, is forced to curl and break, thus creating separate chips with a particular form and size. When designing drills of the abovementioned type there is often difficulty in achieving desired high cutting speeds since thermal cracks often appear at the active cutting corner where such thermal cracks usually appear. It is a first object of the invention to accomplish a new type of drill that enables achieving higher cutting speeds during metal drilling. It is a second object of the invention to accomplish a new type if drill wherein the inserts have such optimized formation that high cutting speed becomes possible without impairing the surface finish of the hole wall in the hole to be drilled. It is a third object to provide a drill wherein the drill inserts have been given such optimized formation that increased cutting speed becomes possible during drilling while maintaining good lifetime of the drilling tool. These and other objects are being achieved by means of a drill that has been given the characterizing features that are outlined in the appending claims. Brief description of the drawings In the drawings; Fig. 1 is a perspective view of a drill according to the present invention. Fig. 2 is a schematical longitudinal view of a drill as shown in Fig. 1. Fig. 3 is an enlarged perspective view of the drill tip as in Fig. 2. Fig. 4 is an enlarged side view of the nose portion of the drill insert as in Fig. 3. Fig. 5 is an exploded view of an alternative embodiment of the invention. Fig. 6 is a cross-sectional view of the embodiment in Fig. 5. Fig. 7 is a perspective view showing an alternative embodiment. Fig. 8 is a somewhat enlarged side view showing a drill pursuant to Fig. 7. Fig. 9 is an enlarged side view of the nose portion of a drilling insert according to an alternative embodiment. Fig. 10 is a cross-sectional view of a drilling insert according to an alternative embodiment of the invention. Detailed description of preferred embodiments of the present invention In the drawings 1 generally designates a solid shank of a suitable tool steel with a first end or tip 2 and an opposite second end 3. At this second, rear end the shank of the shown embodiment has a thicker part 4 suitable for insertion into a holder. From a core two lands 6, 6' protrude, which delimit two flutes 7, 7'. In the illustrated embodiment the drill is a twist drill on which the lands 6, 6' as well as the flutes 7, 7' extend helically around the longitudinal axis of the shank. In practice, these flutes in known manner have a suitably uniform pitch angle within the range 20-30°. As alternative embodiments these flutes 7, 7' could be straight. At the tip 2, two cutting edges 8, 8' are provided, which in this case are formed on special cutting inserts 9, 9' that have been fixed to the tip in a suitable way, for instance by soldering. However, these cutting edges 8, 8' could equally well be formed by grinding of suitably formed surfaces on the shank per se or on a part, which is disconnectable with the shank. It should also be noted that the shank 1 can be provided with one or more axial ducts (see Fig. 3) which lead to two openings 10, 10' in the proximity of the cutting edges 8, 8'. In a known manner, cooling medium can be fed through these ducts to the drill tip. The term "solid shank" as used in the present description and in the following claims has the purpose of distinguishing the drill according to the invention from such drills that include pipe-formed shanks, but does not exclude such shanks which have one or several ducts for a cooling medium in an otherwise solid body. In accordance with the invention the end portions of the edge surfaces of the insert are in the form of curved nose portions 11 preferably provided on corner portions that extend from the remainder of the insert. The nose radius surface of each such corner portion 11 shall be composed of one singular circle segment with a unitary radius, or of a plurality of circular segments which intersect with the top surface of the insert at intersection lines that constitute edge portions with several different radii Ri, R and R as shown in Fig. 4. When several radii are provided adjacent segments always appear with different radius portions along said edge portions. The radius R 2 shall always be substantially larger than Ri which provides a surface finish improving "wiper-effect" along the edge d. This applies both during axial drilling forwardly into the workpiece as well as during axial displacement of the drill in opposite direction. The radius R 3 is somewhat smaller than both Ri and R 2 . The straight edges 8, 8' of the drilling tip are so formed that they mutually meet at an obtuse angle in the drill centre 12 where they together provide an S-shaped contour and positive rake surfaces along the cutting edges. The curvature of these cutting edges 8, 8' is larger in proximity of the drill centre line CL than further away therefrom. The rake surfaces are provided at zero rake angle in immediate vicinity of said centre line. This S-shaped contour in combination with said zero degree rake angle in the centre will contribute to formation of short comma-shaped chips, which can easily be taken away. The formation of the cutting edges 8, 8' and the formation of said rake surfaces is described more in detail in Swedish patent SE-A-7812393 which is hereby incorporated by reference. Fig. 5 shows an alternative drill according to the invention comprising a drill body 13 and a separate drill tip 14. The front surface 15 of said drill body 13 is planar and perpendicularly oriented in relation to the centre line CL of the drill. A centrally provided flushing channel 16 extends from the anchoring end to a position located axially inside the front surface 15. The channel 16 is communicating with two diametrically opposed channels 17A and 17B, which terminate at the periphery of said front surface 15. Each of the short channels 17A and 17B is provided at an acute angle from the centre line CL which has a value between 15° and 20°. The drill tip 14 has a cylindrical basic shape with partially conical upper surface. The drill tip 14 also comprises chip flutes, which are provided as extensions of the chip flutes 7, 7'. On its underneath side a central recess 18 is provided (see Fig. 6) to receive a resilient tap 19 of the drill body 13. The drill tip 14 has also been provided with two bores 19A and 19B which are arranged for the receipt of screws 20A and 20B which are received in the drill tip and arranged to be threadably engaged in the drill body 13 in correspondingly provided bores 21 A and 21 B . In an alternative embodiment of the invention the drill is provided as a straight solid drill body, the front portion of which comprises two symmetrical cutting edges which meet at the drill centre, the peripheral portions of which are provided with a nose radius surface according to the invention. The drill body in this case consists of a composite cemented carbide body comprising a core and a surrounding tube, said core consisting of a relatively tough cemented carbide grade and the tube consisting of a more wear resistant cemented carbide grade. The core portion consists of a WC-Co-grade comprising 5-20, preferably 10-15 weight-% Co, whereas the external tube portion consists of 8-20 weight-% of one or more carbides and/or carbonitrides in group 4-6, preferably Ti, Ta or Nb. The difference in Co-content between the core and the tube ought to be 2-4 weight-%. There ought to be a transition zone of 300-500 μm width measured as change in Co-content between the core and the tube. For the purpose of avoiding problems in the manufacture of such composite drill body and thermal cracks connected therewith the tube hollow ought to be made by powder metallurgical means comprising pressing in two steps according to the method described in Swedish patent 9604779-0 the content of which is incorporated herewith. In order to get good surface finish in the hole to be drilled it has been found that the different radii of the radiused edges when selecting formation of nose radius portion 11 comprising several circular segments ought to be selected in the following range: Ri: 0.1-2.0 mm, preferable 0.2-1.6 mm R 2 : 1.6-10.0 mm R 3 : 0.1-1.0 mm When selecting size of the singular radius this ought to be in the range of 0.1-10 mm. When using drills with a basic geometry and a nose radius surface according to the invention it has been found possible during hole drilling in low alloyed steel to double the cutting speed. In one instance a cutting speed of lOOm/min was used whilst it was possible, after switching to an insert with a nose radius formation according to the invention, to achieve a cutting speed of 250 m/min. At the same time it was found possible to maintain the lifetime of the drill which indicates the superiority that can be achieved with a drill insert having a nose radius formation according to the invention. The drill shaft 1 in Figs. 7-8 has two flushing channels 7, 7' located adjacent to the insert pockets which can be helically shaped or be straight whilst being arranged on mutual sides of the centre line CL. The drill shaft 1 can also include channels (not shown) for transmitting flushing fluid to the cutting end of the drill. The insert pockets are designed for the receipt of two inserts 9, 9' which have a central hole 9" and is intended to be clamped by means of a central screw that is threadably engaged in the drill shaft. One of the inserts is a central insert 9' whereas the other insert is an outer insert 9 that is inclined in relation to the centre line CL at an angle α such that the size of the hole is defined therefrom. Between the outer and the inner inserts there is a waist portion M that has sufficient width such that necessary strength is achieved. The dimensions of the drill are defined by a diameter D. Each insert pocket comprises a tangential support surface 22a, an axial support surface 22b and a radial support surface 22c. Each insert has polygonal basic shape, in this case a square basic shape with two opposite flat surfaces 23 which are connected by four to each other perpendicularly arranged edge surfaces. The bottom surface of the insert is smaller than the upper surface 23 such that the insert appears with a positive clearance angle. The intersection lines between the side surfaces and the upper surface provide main cutting edges 24, 25 where each main edge is straight unbroken and smoothly rounded at its ends to the nose portion 26, 27. As appears from Fig. 8 the outer insert 9 is inclined outwards in relation to the central axis CL. The central insert 9' could be located a bit axially in front of the outer insert 9 in order to promote a better guidance of the radial forces such that desired diameter can be maintained. The outer insert is inclined at an angle α in relation to a line L that is parallel with central axis CL. The size of angle α ought to be 0.5-5°. In accordance with the invention the end portions of the edge surfaces are in the form of curved nose portions 26, 27 provided at corner portion wherein each such corner portion is confined by a plurality of circular segments which intersect with the upper insert surface to provide edge portions with several different radii Rj, R 2 and R 3 as shown in enlarged section in Fig. 9. Adjacent segments always appear with different radius portions along said edge portions. The radius R 2 shall always be substantially larger than R], which contributes to a surface finish improving "wiper"-effect along the secondary edge 28. This applies both at machining by axial drilling forwards into the workpiece as well as during axial displacement of the drill in opposite direction. The radius R 3 is somewhat smaller than both Ri and R 2 . The primary curved edge 26 should have a radius R \ that is somewhat smaller at a position adjacent the main cutting edge 24 compared with radius R 2 of the secondary cutting edge 28. Further, there should also be arranged a curved edge with a somewhat smaller radius R-t as compared with the radius Ri that is provided between the curved nose edge 26 and the secondary cutting edge 28. Fig. 10 shows an embodiment of the insert 9 of the invention where the insert has a double-positive geometry. This means that the insert has equally large top and bottom surfaces 23a, 23b both of which however intersect with the edge surfaces 29a, 29b at an acute angle β. The edge surfaces 29a, 29b are thus composed of inwards-downwards directed edges, which meet at an obtuse angle γ such that a clear waist portion 31 is provided. This enables the insert to be used as indexable insert whilst using twice the number of edges compared with a single-sided insert. The central hole 30 of the insert has a cross-section that diminishes from the top surface 23 a and inwards and similarly the hole has a diminishing cross-section from the bottom surface 23b and inwards towards the centre of the insert. As regards radii Ri and R 2 the selected dimension depends from the diameter of the drill. For drill diameters 13-18 mm Rj should be 0.2-0.4 mm and R 2 should be 1.6-4.0 mm. For drills in diameter area 18-58 mm should be 0.4-1.6 mm and R 2 should be 2.5-10 mm. Radius j should be 0-1.0 mm. The length of secondary edge 28 should be in the range 0.1-0.5 mm.

Claims: Claims 1. Drill comprising an essentially solid shaft (1) with a first tip end portion (2) provided with at least two cutting edges (8,8') which together generate the drilled hole in a workpiece, an opposite second end (3) and a number of flutes (7, 7') corresponding to the number of cutting edges that extend from the tip end (2) in direction towards the opposite end (3) having the purpose of forming and removing chips from the corresponding cutting edge, characterized in the following in combination, a) the drill tip is composed of cutting edges on mutual sides of the axial centre line CL the central portions of which extend axially in front of the peripheral portions, and b) the nose radius portion on each cutting edge at the periphery is shaped as a smoothly curved radius which at the periphery is confined by a number of circular segments with mutually differently sized radii (R ls R 2 , R 3 ) where adjacent radii always appear with different size. 2. Drill according to claim 1, characterized in, that the shaft in a manner known per se is composed of a main portion (13) and a releasably connectable front portion (14) on which the cutting edge-edges (8, 8') are provided. 3. Drill according to claim 1 or 2, characterized in, that the cutting edges (8, 8') are provided on special insert bodies (9, 9') of cemented carbide which has been provided on the solid drill shaft by soldering. 4. Drill according to any of the claims 1-3, characterized in, that it is in the shape of a straight solid drill the front portion of which comprises two symmetrically arranged cutting edges which meet under an obtuse angle whereby the end portions of such cutting edges at the periphery are in the shape of a nose radius surface portion with one or several circular segments. 5. Drill according to claim 1, characterized in, that the drill tip is provided with two polygonally shaped inserts with straight cutting edges having non-broken contour where the nose radius at the periphery is provided as a portion (11,11') that extends radially from the remainder of the insert. 6. Drill according to claim 1, characterized in, that the radii (R 1; R 2 , R 3 ) are so shaped that the central radius (Ri) that provides the smooth transition between two edge surfaces extending to the cutting corner are essentially smaller than the portion that is located adjacent as a surface finish improving wiper-radius (R 2 ). 7. Drill according to any of the claims 1-6, characterized in, that the solid drill comprises a core of a tough cemented carbide grade containing 5-20 weight-%) Co and an outer tube shaped portion which comprises the curved nose radius portion (11, 11') consisting of a more wear resistant cemented carbide grade containing 8-20 weight-% of one or several carbides and/or carbonitrides of Ti, Ta or Nb. 8. Drill comprising an essentially solid shaft (1) with a first tip end portion (2) provided with two cutting edges (8, 8') which together generate the drilled hole in a workpiece, an opposite second end (3) and a number of flutes (7, 7') corresponding to the number of cutting edges that extend from the tip end (2) in direction towards the opposite end (3) having the purpose of forming and removing chips from the corresponding cutting edge, characterized in the following in combination, a) the drill tip is composed of straight cutting edges symmetrically arranged on mutual sides of the centre line CL the central portions of which extend axially in front of the peripheral portions, which meet at the drill centre under an obtuse angle, and b) the nose radius portion on each cutting edge at the periphery is shaped as a smoothly curved radius on a portion (11) that extends radially from the remainder of the insert and which is confined by one or several circular segments. 9. Drill according to claim 8, characterized in, that it is in the shape of a straight solid drill the front portion of which comprises two symmetrically arranged cutting edges which meet under an obtuse angle whereby the end portions of such cutting edges at the periphery are in the shape of a nose radius surface portion with one or several circular segments.

Inventors: EDVARDSSON JAN, SJOELANDER AAKE, WIMAN JOERGEN, EDVARDSSON, JAN, SJOELANDER, AAKE, WIMAN, JOERGEN
Applicants: EDVARDSSON JAN, SANDVIK AB, SJOELANDER AAKE, WIMAN JOERGEN, EDVARDSSON, JAN, SANDVIK AKTIEBOLAG, SJOELANDER, AAKE, WIMAN, JOERGEN
Application year: 2001
Main classification: B23B51
IPC codes: B23B51