DRILL AND CUTTING INSERT THEREFOR

Abstract: The invention relates to a drill and inserts therefor, for drilling in metal with improved performance in the smaller diameter range. The drill consists of a drill shaft (10), the front end of which is provided with radially outer and inner inserts (16, 17). Each insert has an elongated rhomboid shape, in which the short sides are broken in the middle to form a V with an internal obtuse angle ( delta ). The inserts are arranged in such a way that the longitudinal axis of the radially outer insert (16) is inclined at an angle ( alpha ) from the axis of the drill (CL), which angle differs from the angle ( beta ) of inclination of the radially inner insert (17).

Description: Drill and cutting insert therefor The present invention relates to a drill for drilling of preferably metallic work pieces and a cutting insert for use in this drill. The drill consists of a drill shank having at least two chip flutes axially formed therein and at least two cutting insert sites to receive lock means and cutting inserts with mutually identical shaping, the active cutting edge of one cutting insert of which projects sidewardly such that the diameter of the bore is determined thereby during flow of chips through one chip flute. The active cutting edge of the other cutting insert is arranged radially inside the periphery of the drill shank for flow of chips through the other chip flute. German Patent 2730418 discloses a drill equipped with two 6-cornered inserts of mutually identical shaping received in two cutting insert sites in the front portion broken such that one bisector of each pair of edges extends parallel with the central axis of the drill. Another type of drill is disclosed in EP-A-181 844. The inserts of this drill have mainly rectangular shaping where each end surface of the drill is confined by two cutting edges that intersect at an obtuse angle and wherein the radially outer insert of the drill is inclined such that the size of the hole to be drilled is defined thereof whereas the longitudinal axis of the radially inner insert extends parallel with the central axis of the drill whilst provided to overlap the same. With drills of the aforesaid type it is difficult to achieve holes with small diameters. Further, it is difficult to use such drills for stack drilling, i.e. CONFIRMATION COPY drilling of a plurality of plates at the same time. During such type of stack drilling there is usually formed a breakout slug. The slug will rotate with the drill and at large feed force there is a risk for cutting edge and/or drill breakdown. The object of the present invention is to solve the above- mentioned problems. The invention will be more closely described below in connection with the following drawings which show a preferred embodiment of the invention. Fig. 1 shows a side view of the forward cutting portion of a drill shank of the invention. Fig. 2 shows a top view of the drill shown in Fig. 1. Fig. 3 is an enlarged side view of the drill in Fig. 1. Fig. 4 is a perspective view of the drill in Fig. 1-3. Fig. 5 is a sectional view along the line V-V in Fig. 3 Fig. 6 is an enlarged side view of a cutting insert as shown with the drill in Fig. 3-4. Figs. 1-4 show a portion of a cylindrical drill shank 10 which at its most forward part is provided with two cutting insert sites 11 and 12, positioned mainly on each sides of the center line CL of the drill. Axial flutes or passages for the flow of chips extend to said insert sites and are depicted by 13 and 14. These flutes can be straight or helical. The drill shank 10 also includes channels 15 for submitting cooling medium to the front cutting end portion of the drill. Both of the cutting insert sites 11 and 12, are centrally provided with holes to receive threaded lock screws 18 for locking of centrally apertured cutting inserts 16, 17 in the sites. The sites consist of a central site 11 which is arranged radially inside the periphery of the drill shank 10 and a peripheral site 12 which terminates in the periphery of the drill shank. The cutting insert sites 11 and 12 are shaped in the drill shank 11 such that they disclose different axial clearance angles and together achieve drilling of the complete hole diameter. The radially outer insert 16 is inclined at a certain angle such that the diameter of the drilled hole is defined thereof. There is a core portion M provided between said inner and outer inserts 16, 17 of such width so that sufficient strength of the drill is achieved. The dimension of the drill is defined by a diameter D. Each cutting insert site comprises a tangential support surface 19, an axial support surface 20 and a radial support surface 21. Each insert has an elongated polygonal shape, a rhomboidic shape comprising an upper face 23 and a lower face 22 which are mainly parallel with each other. The faces 22, 23 are joined by two opposite end faces which correspond to short sides of a basic rectangle. The lower face 22 is smaller than the upper face 23 and therefore the cutting insert achieves a positive clearance angle. The intersection lines of said short sides with the upper face 23 form two main cutting edges 24, 25, where each of these edges is broken on its middle and forms a V with internal obtuse angle δ, only one of which cuts the workpiece at each drilling operation. Each of these edges is rounded off at parts forming nose portions 26, 27 and 28, 29 between which the largest width x is defined. The shortest distance between the long side surfaces of the insert is defined by W 2 . As appears from Fig. 3 both inserts 16 and 17 are rhomboidic in shape whilst located at a certain inclined angle from the longitudinal axis (CL) of the drill. The centrally located insert 17 projects axially a certain distance in front of the peripheral insert 16 in order to ensure that the radial forces are more favorably directed so that the desired diameter is achieved. The outer insert 16 is inclined at an angle a and the inner insert 17 is inclined at an angle β in relation to the longitudinal axis CL of the drill . The angle α shall be smaller than the angle β in drills for solid drilling. The angle β should preferably amount to a value of 4-8° whereas the angle o. should have a value in the range 1-3°. In drills for stack drilling for making a hole in a number of plates, the angle α. should be somewhat larger than the angle β because in this type of drilling applications there is usually formed a breakout slug which requires a larger degree of overlap between the inserts. In Fig. 6 the rhomboidic shape of the insert 16, 17 is shown in more detail. A straight side surface 30 extends longitudinally between the nose portions 26 and 28. The radius of diagonally opposed nose portions 26 and 29 is somewhat larger than the radius of the other pair of diagonally opposed nose portions 27 and 28. In similarity with the insert in EP-A-181 844 this insert could be viewed as being composed of two partially overlapping triangles Tl and T2 wherein each side surface of said triangle is broken in the middle so that each cutting edge forms an obtuse angle at the break. However, the insert has a configuration such that the nose portions 26 and 27 do not fully coincide with the contour of the nose portions of the projected surface of the triangle. The side surface located between two adjacent nose portions coincides with the edge 24 of said projected triangle, but the ends forming rounded off nose portions 28 and 29 are laterally displaced from the projected surface of its associated triangle T2 in one direction whereas the opposite pair of rounded off nose portions 26 and 27 are similarly laterally displaced in the opposite direction from the nose portions of their associated triangle Tl. The third corner portion of each triangle is arranged relatively centrally in the cutting insert at a distance d from each other. This distance is shorter than the entire length L of the insert such that d becomes 0.5 to 0.9 preferably 0.7 to 0.9 times the length L. As appears from Fig. 6 the lower nose portion 28, to the left, is located at a certain distance laterally inside the lowermost left corner of the triangle T2 whereas the lower nose portion 29, to the right, is located at a certain distance laterally outside the lowermost right corner of same triangle T2. This configuration of the insert is achieved due to the fact that a bisector SI drawn through the opposite obtuse-angled corners of triangles Tl and T2 forms an angle with the bisector S2 drawn through the centrally located corners between which distance d is defined. The size of the angle o should preferably be selected to be in the area 2-6°. In addition thereto the drill is preferably designed such that the radially inner insert 17 has such radial orientation that it becomes located offset from a 180 degree relationship with the radially outer insert 16 by an angle θ wherein θ is of such magnitude that the drill is radially balanced against the radial cutting forces acting on the drill shaft 10. Usually, the angle θ is in the range of 5-7°. By having provided a drill with inserts having a rhomboidic shape it has become possible to reduce the diameter of the drill without negatively affecting the thickness of the core portion since this shape of the insert will enable the insert to appear at a certain inclination from the longitudinal axis of the drill. With a drill such as defined and described in EP-A-181 844 a diameter range of 17.5-26 mm could be used whereas a drill such as defined in this application will enable successful usage of drills having a diameter in the range of 12.7-26 mm.

Claims: Claims 1. Drill comprising a drill shank (10) having at least two chip flutes (13, 14) axially formed therein and at least two cutting insert sites (11, 12) to receive mainly elongated polygonal cutting inserts (16, 17) with mutually identical shaping, the active cutting edge (24) of one cutting insert (16) of which projects sidewardly such that the diameter (D) of the bore is determined thereby during flow of chips through one chip flute (13) and the active cutting edge of the other cutting insert (17) is arranged radially inside the periphery of the drill shank (10) for flow of chips through the other chip flute (14) , active cutting edges being provided at the short side surfaces of said cutting inserts, said side surfaces are broken in the middle so that each cutting edge forms an internal obtuse angle (δ) at the break, and said radially outer insert (16) being inclined at an angle (α) in relation to the longitudinal axis (CL) of the drill, c h a r a c t e r i z e d in that each insert (16, 17) has rhomboidic basic shape and is arranged such that the longitudinal axis of the radially outer insert (16) is inclined at an angle (o.) from the longitudinal axis (CL) that is smaller than the angle (β) of inclination between the longitudinal axis of the radially inner insert (17) and said drill axis (CL) . 2. Drill as defined in claim 1, c h a r a c t e r i z e d in that the angle (β) amounts to a value in the range 4-8°. 3. Drill as defined in claim 1 or 2, c h a r a c t e r i z e d in that the radially inner insert (17) projects axially a certain distance in front of said radially outer insert (16) . 4. Cutting insert for a drill of the kind described in claim 1 with elongated polygonal basic shape with long and short side surfaces, comprising an upper and a lower face (22, 23) which are mainly parallel, and with side surfaces joining them, said lower face (22) is smaller than the upper face (22) such that the cutting insert (21) achieves a positive cutting geometry, cutting edges (24, 25) formed at parts of the lines of intersection between the side surfaces and the upper face (23) , said cutting edges (24, 25) being limited to the short side surfaces of the cutting insert, said surfaces are broken in the middle so that each cutting edge forms an internal obtuse angle (δ) at the break and in that the shortest distance (w 2 ) between the long side surfaces (30) of the cutting insert is shorter than the distance (w between the ends of respective cutting edge (24, 25) , c h a r a c t e r i z e d in that the insert has rhomboidic basic shape at which the ends of the short side surfaces are rounded off at parts forming nose portions (26, 27) and (28, 29) which define the largest width (w x ) of the insert. 5. Cutting insert according to claim 4, c h a r a c t e r i z e d in that the radius of each of two diagonally opposed nose portions (26) and (29) is somewhat larger than the radius of the other diagonally opposed nose portions (27) and (28) . 6. Cutting insert according to claim 4 or 5, c h a r a c t e r i z e d in that its projected surface is composed of two partially overlapping triangles (Tl, T2) , each said triangle being confined by side surfaces that are broken in the middle so that each edge forms an internal obtuse angle (δ) at the break. 7. Cutting insert according to claim 6, c h a r a c t e r i z e d in that it is composed of two opposed partially overlapping triangles (Tl, T2) having side surfaces that are broken, at which the ends forming rounded off nose portions (28) and (29) are laterally displaced from the projected surface of its associated triangle (T2) in one direction whereas the opposite pair of rounded off nose portions (26) and (27) are similarly laterally displaced in the opposite direction from their associated triangle (Tl) . 8. Cutting insert according to any of the claims 4-7, c h a r a c t e r i z e d in that one corner of each triangle (Tl, T2) is arranged relatively centrally in the cutting insert at a distance (d) from each other such that (d) is 0.5 to 0.9 times distance (L) of the insert. 9. Cutting insert according to claim 8, c h a r a c t e r i z e d in that distance (d) amounts to a value that is 0.7-0.9 times distance (L) of the insert. 10. Cutting insert according to any of the claims 4-9, c h a r a c t e r i z e d in that it is having a configuration such that a bisector (SI) drawn through the opposite obtuse-angled corners of triangles (Tl) and (T2) forms an angle ( Q) of 2-6° with a bisector (S2) drawn through the centrally located corners of the projected triangles between which the distance (d) is defined. 11. Drill for drilling through stacked workpieces, comprising a drill shank (10) having at least two chip flutes (13, 14) axially formed therein and at least two cutting insert sites (11, 12) to receive mainly elongated polygonal cutting inserts (16, 17) with mutually identical shaping, the active cutting edge (24) of one cutting insert, (16) of which projects sidewardly such that the diameter (D) of the bore is determined thereby during flow of chips through one chip flute (13) and the active cutting edge of the other cutting insert (17) is arranged radially inside the periphery of the drill shank (10) for flow of chips through the other chip flute (14) , active cutting edges being provided at the short side surfaces of said cutting inserts, said side surfaces are broken in the middle so that each cutting edge forms an internal obtuse angle (δ) at the break, and said radially outer insert (16) being inclined at an angle (α.) in relation to the longitudinal axis (CL) of the drill, c h a r a c t e r i z e d in that each insert (16, 17) has rhomboidic basic shape and is arranged such that the longitudinal axis of the radially outer insert (16) is inclined at an angle (α) from the longitudinal axis (CL) that is larger than the angle (β) of inclination between the longitudinal axis of the radially inner insert (17) and said drill axis (CL) .

Inventors: SJOELANDER AAKE, TUKALA TOMMY, SJOELANDER, AAKE, TUKALA, TOMMY
Applicants: SANDVIK AB
Application year: 1995
Main classification: B23B51
IPC codes: B23B27, B23B51, B23C5