DRILL AND CUTTING INSERT THEREFORE

Abstract: A drill for chipforming machining comprising a drill body (11), two cutting inserts (12,13) and screws (14,15). The cutting inserts are screwed to cutting insert sites (18,19) in the axially forward end of the drill (10). The screws force the cutting inserts radially outwardly during tightening until a shoulder (28) at the lower side of each cutting insert abuts against a vertical support surface (22) on the drill body (11). The cutting inserts are tangentially mounted, i.e. they have holes for accomodating the screws, which terminates in the chip surfaces of the cutting inserts.

Description: The present invention relates to a drill according to the precharacterizing portion of claim 1 and a cutting insert therefor according to the precharacterizing portion of claim 10, see DE-A-3611999. The drill comprises a shank, a portion provided with lands and chip flutes, at least one cutting insert site in its axially forward end, provided with a hole, at least one cutting insert, provided with a hole, and tightening means, arranged to be received by said holes in order to hold the cutting insert in said cutting insert site. The drill has a direction of rotation relative to a work piece and has a center axis. The cutting insert has a cutting edge along the line of intersection of a chip surface and a clearance surface of the cutting insert, said clearance surface being arranged behind the cutting edge in the direction of rotation of the drill. Said cutting edge extends from the vicinity of the center axis of the drill towards the periphery of the drill. The cutting insert has a lower side opposite to the clearance surface. A drill is shown in EP-A2-0172148 wherein the hole of the cutting insert terminates in the chip surface of the cutting insert. This means that the insert must be relatively large to be able to accommodate the hole, which negatively influences the strength of the drill body, when the drill body has a small diameter. Furthermore the known drill is sensitive to tolerance faults. A drill tool comprising a shank with cutting inserts detachably secured to the drill shank is disclosed in DE-A-3611999. With this drill a pair of inserts are secured to the insert site of the shank by tightening means extending through a hole which terminates in the clearance surface of the cutting insert. Said holes of the insert are provided by means of a lateral protrusion. By providing such a laterally disposed surface the radial dimension of the insert site, and thus of the insert, is reduced. An object of the present invention is to provide a drill having easily exchangeable cutting inserts. Another object of the present invention is to provide cutting inserts for the drill that demand little hard material to be produced. Still another object of the present invention is to provide a drill whose cutting inserts are rigidly fixed to the drill body. Still another object of the present invention is to provide a drill whose cutting inserts can be covered by wear resistent layers. Still another object of the present invention is to provide a drill whose cutting inserts can consist of a very hard material. Still another object of the present invention is to provide a drill being relative unsensitive to tolerance faults. These and other objects have been achieved by forming the drill in accordance with claim 1 and the insert in accordance with claim 10. The invention will be more closely described hereinafter in connection with the accompanying drawings. Fig. 1 shows a drill in an exploded view. Figs. 2 to 5 show a cutting insert in a top view and in three different side views according to the arrows 3, 4 and 5 in Fig. 2, respectively. Figs. 6 and 7 show the drill in a top view and a side view, respectively. The drill 10, shown in Fig. 1, according to the present invention comprises a drill body 11, two cutting inserts 12, 13 and two screws 14, 15. The drill body 11 has a cylindrical shank, not shown. Two lands 16 are connected to the shank. The drill body has screw shaped or straight chip flutes 17. Two identical cutting insert sites 18, 19 are arranged in the forward end of the drill. Therefore only one site will be described hereinafter. The site 18 or 19 comprises a planar axial support surface 20, a planar tangential support surface 21 and a planar radial support surface 22. The words "axial", "tangential" and "radial" relates to the cutting forces that each support surface is supposed to receive. The axial and tangential support surfaces 20, 21 enclose an angle γ of about 60 to 90 degrees. The axial and radial support surfaces 20, 22 enclose an angle β in the magnitude of 60 to 105 degrees. The axial support surface 20 forms an acute angle with the center axis CL₁ of the drill. The intersection of the tangential support surface 21 and the front surface 23 of the drill is essentially parallel with the intersection of the axial support surface 20 and the chip flute 17. The last-mentioned intersection coincides with or is essentially parallel with a normal N to the center axis CL₁ and is arranged essentially perpendicular to the radial support surface. The radial support surface 22 extends from the axial support surface and axially rearwards. The axial support surface is provided with a threaded bore whose center axis CL₂ or CL₃ is arranged parallel with the center axis CL₁ of the drill. The center line of each hole is arranged at a certain distance from the tangential support surface 21 and the radial support surface 22. Furthermore the drill is provided with fluid passages. The cutting inserts 12, 13, which also are shown in Figs. 2 to 5, are identical and therefore only one insert 12 will be described hereinafter. The insert, which has the basic shape of a tetrahedron, has a chip surface 24, which forms an acute internal angle with a first clearance surface 25, a rear first support surface 26, a lower second support surface 27, a lower third support surface 28, a second clearance surface 29, a free lower surface 30 and a free radially inner surface 31. The rear support surface 26 is parallel with and is arranged at a distance from the normal N. A cutting edge 32 is arranged at the intersection of the chip surface 24 and the first clearance surface 25 and extends from the center axis CL₁ towards the periphery. The last-mentioned surfaces 24 and 25 are formed such that the outer portion 32′ of the cutting edge 32 is straight over the major part of the insert and has a constant curvature at the radially, relative to the center axis of the drill, inner portion 32″. The inner portion 32″ also climbs upwardly relative to the clearance surface 25. The outer portion 32′ of the cutting edge is essentially parallel with the lower second support surface 27, which is planar. The lower third support surface 28 consists of a step between the lower second support surface 27 and the free lower surface 30. The height of the step is about 10 to 40% of the height of the insert. The support surface 28 is planar, essentially perpendicular to the normal N and is perpendicular to or forms an acute angle α with the support surface 27. The angle α is between 75 and 115 degrees. The insert 12 has a hole which extends from the lower second support surface 27 to the first clearance surface 25. The center axis of the hole is parallel with the free inner surface 31, the rear first support surface 26 and the second clearance surface 29. The center axis CL₄, CL₅ is arranged at a certain distance from the rear first support surface 26 and from the lower third support surface 28. The length L of the insert is between 8 and 40 mm while the length l of the free lower surface 30 is about 0.1 to 0.3 L. When mounting the drill one of the inserts 12 is put such that its support surfaces 26, 27 and 28 engages the corresponding surfaces 21, 20 and 22 of the drill body 11, i.e. the insert is tangentially mounted. Thereafter a screw 14 is inserted into the hole of the insert whereafter it engages the threads of the bore in the drill body. When the screw is tightened the insert is forced to a stable abutment against the surfaces 20, 21 22. The center axis CL₂ of the bore is provided radially outwards of the center axis CL₄ of the insert hole and therefore the screw is prestressed during tightening, such that the insert is forced in a direction S radially outwards, which direction forms an acute angle with the normal N to the center axis of the drill. The diametrically opposite insert 13 is mounted in a corresponding manner, such that the cutting edges 32 essentially meet at the center axis CL₁. The ends of the cutting edges 32 meet or are provided close to each other at the center of the drill, which is shown in Figs. 6 and 7. The cutting edges form a mainly S-shaped edge portion, which is more closely described in US-A-4.132.493, hereby incorporated into the description. The process is reversed at replacement of the inserts. It should be understood that the drill can be modified without departing from the scope of the claims. A drill formed according to the present invention provides for a plurality of advantages. The inserts are easily exchangeable. The inserts can be made relatively small and therefore demand little hard material for their production. The inserts are rigidly held within the sites of the drill body. Compared to conventional brazed drills the drill according to the present invention can be provided with inserts having more wear resistant but more heat sensitive hard material and coatings.

Claims: 1. Drill comprising a shank, a part provided with lands (16) and chip flutes (17), at least one cutting insert site (18, 19) in its axially forward end, provided with a hole, at least one cutting insert (12, 13), provided with a hole, and tightening means arranged to be received by said holes in order to hold the cutting insert in said cutting insert site, said drill having a direction of rotation relative to a work piece and having a center axis (CL), said cutting insert having a cutting edge (32) arranged along the line of intersection of a chip surface (24) and a clearance surface (25) being provided behind the cutting edge in the direction of rotation of the drill, said cutting edge (32) extending from the vicinity of the center axis (CL) of the drill to the periphery of the drill, said cutting insert having a lower side provided opposite to the clearance surface (25), and said tightening means extending through a hole which terminates in the clearance surface (25) of the cutting insert, characterized in that said insert is of L-shape as viewed in a direction toward said chip surface and that the lower side of the cutting insert has two spaced apart surfaces (27, 30) providing a step surface (28) therebetween whereby said step surface (28) is formed in the axially rearwardly facing lower surface (27) of the insert, and is adapted to engage a surface (22) of the cutting insert site (18, 19). 2. Drill according to claim 1, characterized in that each cutting insert (12, 13) has the basic shape of a tetrahedron and comprises three support surfaces (26, 27, 28) provided to abut against corresponding surfaces (20, 21, 22) in the cutting insert site (18, 19), whereof a fist rear support surface (26) of the cutting insert is provided to transmit tangential cutting forces to a first surface (21) of the cutting insert site, a second lower support surface (27) is provided to transmit axial cutting forces to a second lower surface (20) of the cutting insert site and a third lower support surface (28) is provided to transmit radial cutting forces to a third surface (22) of the cutting insert site. 3. Drill according to claim 2, characterized in that each tightening means consists of a screw (14, 15) which forces all support surfaces (26, 27, 28) of the cutting insert against the corresponding surfaces (20, 21, 22) of the cutting insert site. 4. Drill according to claims 1 or 2, characterized in that the hole of each cutting insert (12, 13) is displaced relative to the hole of the cutting insert site (18, 19) such that the tightening means (14, 15) by prestressing forces the cutting insert in a direction (S) radially outwardly, said direction (S) forming an acute angle with a normal (N) to the center axis (CL₁) of the drill, 5. Drill according to claim 2, characterized in that the first rear support surface (26) of each cutting insert is parallel with and arranged at a distance from a normal (N) to the center axis (CL₁) of the drill, and in that the the third lower support surface (28) is essentially planar and perpendicular to said normal (N). 6. Drill according to claim 2, characterized in that the third lower support surface (28) consists of said step between the second lower support surface (27) and a free lower surface (30), said third lower support surface being perpendicular to or forming an angle (α) with the second lower support surface (27). 7. Drill according to claim 2, characterized in that the first surface (21) of the cutting insert site is planar and forms a first acute angle (γ) with the second lower surface (20) of the cutting insert and in that the line of intersection of a chip flute (17) and the second lower surface (26) forms a second angle (β) with the third lower surface (22) and in that the third lower surface (22) is essentially parallel with the center axis (CL₁) of the drill. 8. Drill according to claim 2, characterized in that the third surface (22) of the cutting insert site is parallel with a center axis (CL₁) of the drill and extends in direction axially rearwardly from the second lower surface (20) of the cutting insert site. 9. Drill according to claim 1, characterized in that the drill (10) has two identical, diametrically opposed cutting insert sites (18, 19) which receive two identical cutting inserts (12, 13), said inserts having cutting edges meeting essentially at the center axis of the drill and in that the formed cutting edge is S-shaped in the vicinity of the center axis of the drill and is essentially straight in its other parts. 10. Cutting insert for a drill having a cutting edge (32) along the line of intersection of a chip surface (24) and a clearance surface (25) of the cutting insert (10), said cutting insert including a hole, whereby a lower side of the cutting insert is arranged opposite to the clearance surface (25) in which the hole terminates, characterized in that the lower side includes a step (28) between adjacent surfaces (27, 30) of said lower side, whereby said insert is of L-shape as viewed in a direction toward said chip surface.

Inventors: LINDBERG, HANS
Applicants: SANDVIK AKTIEBOLAG
Application year: 1988
Main classification: B23B51
IPC codes: B23B51