Widin, Leif (Klangvägen 40, Sandviken, S-811 32, SE)
| 1. | Drilling tool comprising on one hand a basic body (2), which has two chip channels (8, 8') extending rearwards from a front end (7) and is rotatable around a geometrical centre axis (C), which is intersected by two imaginary diametrical planes (Pl, P2) extending axially and perpendicularly to each other, and on the other hand two replaceable and indexable bits (3,4) in the form of a centre bit (3) and a periphery bit (4), which are mounted in pockets (9,10) formed in the front end of the basic body adjacent to the chip channels (8, 8'), more precisely in a first pocket or centre pocket (9) adjacent to the centre axis (C) and a second pocket or periphery pocket (10) adjacent to the periphery of the basic body, respectively, having the bits and pockets spacedapart from each other along a first diamet rical plane (Pi), the centre bit (3) being located in a posi tion in which an operative cutting edge (12) intersects said second diametrical plane (P2), c h a r a c t e r i z e d in that at least the centre bit (3) includes four cutting edges (12) of one and the same shape and comprises two part edges (20,21) spacedapart by a transition edge portion (22), a first part edge (20) of whichwhich in an operative state is distanced from the centre axis (C) is located axially before a second part edge (21), positioned closer to the centre axis, and that an operative cutting edge on the periphery bit (4) is at least partly positioned in an imaginary cross plane (TP2) perpendicular to the centre axis, which cross plane is located somewhere between analogous cross planes (TP1, TP3) for the two part edges (20,21) of the centre bit (3) in order to enter a workpiece (11), on one hand after the first, radially outer part edge (20) of the centre bit, and on the other hand before the second part edge (21) of the centre bit. |
| 2. | Drilling tool according to claim 1, c h a r a c t e r i z e d in that the part edge (21, alternatively 23) included in the operative cutting edge (12) of the centre bit (3) that intersects said second diametrical plane (P2), extends at an angle of at least 60° and at most 75°, suitably at least 65° and at most 70° to said plane. |
| 3. | Drilling tool according to claim 1 or 2, c h a r a c t e r i z e d in that the first part edge (20), together with the transition edge portion (22) on the individual cutting edge (12) of the centre bit, has a length (B1) of at least 10 % and at most 60 %, suitably at least 20 % and at most 50 %, of the total length (B) of the cutting edge in order to protect, along a radial distance of the corresponding length, an inner and front, inoperative corner edge (27) on a periphery cutting edge (13) following during rotation of the drill. |
| 4. | Drilling tool according to any one of the preceding claims, c h a r a c t e r i z e d in that the cutting edges (12) on the centre bit (3) are tangent to an imaginary inscribed circle (IC), with a diametrical plane (RP1), serving as a reference plane extending parallel to the second part edge (21) and imaginary dividing the bit into two halves that are mirror invertedly symmetrical in an inverse state, and that the two part edges (20,21) of the individual cutting edge transform into each other via a transition edge portion (22), which is defined by on one hand a convex curve (24) adjacent to the first part edge (20), and on the other hand a concave curve (25) adjacent to the second part edge (21), an imaginary or actual, straight tangent line between the curves extending at an angle of at least 10° and at most 30° to the reference plane (RP1). |
| 5. | Drilling tool according to claim 4, c h a r a c t e r i z e d in that a distance difference (al) between the inscribed circle (IC) and the first part edge (20), brought about by the transition edge portion (22), amounts to at least 2 % and at most 15 %, suitably at most 5 % of the diameter (DIC) of the inscribed circle. |
| 6. | Drilling tool according to claim 4 or 5, c h a r a c t e r i z e d in that the individual cutting edge (12) on the centre bit (3), in addition to said first and second part edges (20, 21), includes a third part edge (23), which extends at another angle () to the reference plane (RP1) than a nearby part edge (21). |
| 7. | Drilling tool according to any one of the preceding claims, c h a r a c t e r i z e d in that the first part edge (20) on the individual cutting edge (12) of the centre bit (3) is at least partly formed on a bracketlike projection (26), the thickness (T1) of which is smaller than the thickness (T) of the bit, in order to leave a support surface (15A) between the projection (26) and a bottom side (16) of the bit, which sup port surface extends in the extension of a support surface (15) adjacent to the second part edge (21), and which, along with the lastmentioned support surface, is pressable against a radial or axial support in the centre pocket when the cutting edge (12) in question is inoperative. |
| 8. | Indexable drill bit having a quadrangular basic shape and four similar cutting edges (12), c h a r a c t e r i z e d in that the cutting edges (12) individually comprises first and second part edges (20,21) transforming into each other via a transition edge portion (22), and that the second part edges (21) of the four cutting edges are tangent to an imaginary inscribed circle (IC) having a diametrical plane serving as a reference plane (RPl), which extends parallel to a second part edge (21) and imaginary divides the bit into two halves, which are mirrorinvertedly symmetrical in an inverse state, the first part edge (20) being at least partly positioned at a greater distance from said reference plane (RP1) than the sec ond part edge (21), and the transition edge portion (22) being defined by a convex curve (24) adjacent to the first part edge (20), as well as a concave curve (25) adjacent to the second part edge (21), an imaginary or actual, straight tangent line between the curves extending at an angle of at least 10° and at most 30° to the reference plane (RP1). |
| 9. | Drill bit according to claim 8, c h a r a c t e r i z e d in that a distance difference (al) between the inscribed circle (IC) and the first part edge (20), brought about by the transi tion edge portion (22), amounts to at least 2 % and at most 15 %, suitably at most 5 % of the diameter (DIC) of the inscribed circle. |
| 10. | Drill bit according to claim 8 or 9, c h a r a c t e r i z e d in that the individual cutting edge (12) in addition to the first and second part edges (20,21) includes a third part edge (23), which extends at another angle (fui) to the reference plane (RP1) than a nearby part edge (21). |
| 11. | Drill bit according to any one of claims 810, c h a r a c t e r i z e d in that the first part edge (20) is formed on a bracketlike projection (26), the thickness (T1) of which is smaller than the total thickness (T) of the bit in order to leave a support surface (15A) between the projection and a bot tom side of the bit, which extends in extension of a support surface (15) adjacent to the second part edge (21). |
Drills of the above generally mentioned kind, denomi- nated short hole drills by those skilled in the art, are most commonly used for blast drilling holes in workpieces of metal, such as steel, aluminium or the like. However, the tools can also work in other materials than metal.
In a second aspect, the invention also relates to an indexable bit having a quadrangular basic shape and four simi- lar cutting edges, suited for short hole drills. An important feature of this bit is the very fact that the four cutting edges are similar. In this way, the bit may be readjusted or indexed to four different positions in the appurtenant pocket in order to enable utilization of no less than four cutting edges before the bit finally has to be discarded.
Prior Art In previously known short hole drills of the type that uses bits having four similar, operatively serviceable cutting edges, one of the bits-most commonly the centre bit- is arranged with the operative cutting edge thereof placed axi- ally in front of the operative cutting edge of the second bit
(periphery bit). When a hole is to be recessed in a workpiece, the entire operative cutting edge of the centre bit is brought to enter or cut into the workpiece before the cutting edge of the periphery bit. It has certainly always been an aim within the technique in question to reduce the axial distance differ- ence between the operative cutting edges of the periphery bit and the centre bit to a minimum, within the scope of given feed rates, but nonetheless the cutting edges of the two bits in its entirety enter the workpiece in two different steps. This results in the cutting forces becoming large at entering. The two spaced-apart bits use great force to"clutch"the material in the workpiece, the drill being applied large radial and tan- gential forces disturbing the balance of the drill. Imbalance of this type is particularly troubling when the drill has a length that is 4 to 5 times larger than the diameter, or more.
Objects and Features of the Invention The present invention aims at obviating the above- mentioned disadvantages of previously known drills and at pro- viding an improved drill. Therefore, a primary object of the invention is to provide a drill that is exposed to only moder- ate cutting forces at entering a workpiece, while guaranteeing good balance. An additional object is to provide a drill, the centre and periphery bits of which separately generates whole chips in connection with the chip removing. Furthermore, one of the bits, viz. the centre bit, should be able to protect the inner corner between the operative cutting edge of the periph- ery bit and an inoperative cutting edge turned towards the cen- tre of the drill, so that the cutting edge that is inoperative for the time being is kept intact until the same is made active by indexing.
According to the invention, at least the primary object of the drill according to the invention is attained by the features defined in the characterizing clause of claim 1.
Preferred embodiments of the drill are furthermore defined in the dependent claims 2-7.
Furthermore, the invention relates to an indexable bit, which is suited to be used in the drill. The primary fea- tures of said bit are seen in the independent claim 8. Pre-
ferred embodiments of the bit according to the invention are furthermore defined in the dependent claims 9-11.
Summary of the Invention As is seen in the subsequent detailed description, the invention is based on the intention to form at least the four cutting edges of the centre bit with at least two part edges, which are spaced-apart by a transition edge portion, and a radially outer part edge of which is at least partly posi- tioned axially before the radial inner part edge or edges, the operative cutting edge of the periphery bit being at least partly positioned on one hand axially behind the radially outer part edge of the centre bit, and on the other hand axially before the inner part edge or edges of the operative cutting edge of the centre bit. Expressed in other words, an imaginary, geometrical cross plane extending perpendicularly to the centre axis of the drill and being tangent to the front portion of the operative cutting edge of the periphery bit can be said to intersect the transition edge portion between the radially outer and inner part edges, respectively, of the centre bit. By the geometry and the location of the bits according to the invention, it is attained that only a certain part of the cen- tre bit in a first step cuts into the workpiece in connection with the entering of the drill, and then at least a part of the operative cutting edge of the periphery bit cuts into the work- piece before other portions of the operative cutting edge of the centre bit engage with the workpiece. The advantageous con- sequence from this is that the centre bit'is initially applied only moderate cutting forces, and as soon as the operative cut- ting edge of the periphery bit begins to cut into the work- piece, a balancing is achieved by the forces that act on the diametrically opposed bits before the inner part edge of the centre bit finally enters the workpiece. Thus, contrary to prior art, the operative cutting edge of the centre bit does not in its entirety clutch the workpiece before the operative cutting edge of the periphery bit is brought to engagement with the material.
According to the claims 4 and 8, which define a pre- ferred geometry of the transition edge portion between the part
edges of the centre bit, the effect is attained that the released chip remains whole, in spite of the fact that the same is produced by two grade separated part edges. In blast drill- ing of a hole, whole chips may be handled in a considerably simpler way than split chips.
Additional Elucidation of Prior Art By US 5 971 676 (Kyocera Corporation), a U-drill is previously known, the centre and periphery bits of which have cutting edges that are formed with two part edges, spaced-apart by a transition portion, which in an active state are located axially spaced-apart. However, in this case, the cutting edge of the periphery bit is not arranged to enter the workpiece in a step following the fact that entering of a first part edge of the centre bit has taken place, but prior to entering of the radially inner part edge or edges of the centre bit takes place. Furthermore, US 5 971 676 primarily aims at bringing about a partition of the released chips, rather than facilitat- ing entering.
Brief Description of the Appended Drawings In the drawings: Fig 1 is a first perspective view of a drill according to the invention, the drill being shown with the tip thereof turned upwards and the centre bit thereof removed from an appurtenant pocket, while the periph- ery bit being shown in a mounted state, Fig 2 is a second perspective view showing the periphery bit in a released state and the centre bit in a mounted state, Fig 3 is a simplified perspective view illustrating how the centre bit of the drill partially enters a workpiece in an initial stage of the entire entering phase of the drill, Fig 4 is a perspective view corresponding to fig 3, showing how the periphery bit-following further rotation of the drill-has commenced its entering of the work- piece,
Fig 5 is a schematic view only illustrating the centre and periphery bit, respectively, of the drill, two geo- metrical diametrical planes intersecting each other, as well as the bore diameter for the drill being illustrated by dash-dotted lines, Fig 6 is a schematic and hypothetical view showing the two bits of the drill at one and the same side (to the left) of the centre axis of the drill, the centre bit being shown closer to the observer than the periphery bit, the two bits being of mirror-inverted shape, which is feasible, although not preferred, Fig 7 is a geometrical view, which in the same way as fig 6 shows the two bits on one and the same side of the centre axis of the drill, the centre bit being clos- est to the observer, the bits being of different design according to a preferred embodiment of the invention, Fig 8 is a perspective view of a preferred embodiment of a bit according to the invention, Fig 9 is a planar view from above of the bit according to fig 8, and Fig 10 is a side view of the bit according to figs 8 and 9.
Detailed Description of Preferred Embodiments of the Invention In figs 1 and 2, a tool in the form of a drill is shown, generally designated 1, which includes a basic body 2 as well as two bits 3,4. The basic body 2 is in the example formed with a rear fastening part 5 and a front, long narrow shank 6 of cylindrical basic shape. The length of the shank 6 may vary most considerably. For the sake of clarity, the shank is shown with a limited length. However, in practice, the invention is applicable to drills with considerably longer shanks, e. g. of a length of at least 3 x D (= the diameter of the drill). From the front end or tip, designated 7, in which the bits are arranged, two chip channels 8, 8'extend, which advantageously are helicoidal. Adjacent to the front ends of said chip channels, pockets 9,10 are formed for receipt of the bits 3,4. More precisely, a first pocket 9 is located near the geometrical centre axis of the drill for receipt of the bit 3,
which constitutes a so-called centre bit. The bit 4, which forms a periphery bit, is mountable in a second, peripherically positioned pocket 10.
In figs 3 and 4, the drill is shown together with a schematically outlined workpiece 11. In these drawing figures, the geometrical centre axis C of the drill is outlined by dash- dotted lines. In all of the figures 1-4, the direction of rota- tion of the drill is illustrated by means of the arrow A.
In fig 5, only the two bits 3,4 of the drill are shown, more precisely inserted into a geometrical figure con- sisting of dash-dotted lines, which explains the geometrical location of the bits in relation to the centre axis C. More precisely, the figure shows how the centre axis C is inter- sected by two imaginary diametrical planes PI, P2, which extend axially along the longitudinal direction of the drill and per- pendicularly to each other. The diameter of the drill is desig- nated D, i. e. substantially the diameter that a hole drilled in the workpiece obtains, and that is determined by the distance between the centre axis C and the outermost part of a cutting edge on the periphery bit 4.
As is seen in fig 5, the bits 3,4 (and thereby the appurtenant pockets 9,10) are spaced-apart from each other along the first diametrical plane P1. More precisely, the periphery bit 4 is entirely separated from the second diametri- cal plane P2, which intersects the centre axis C, while the major part of the centre bit 3 is situated on the opposite side of the diametrical plane P2. However, a smaller portion of the centre bit 3 intersects the plane P2 (see the measure g), involving that a short, inner portion of the operative cutting edge of the centre bit intersects the diametrical plane P2. It should also be noted that the radially outer part of the centre bit 3 is situated at a considerable distance from the hole cir- cle HC. However, as is clearly seen in fig 5, the width and location of the bits are of such a kind that the operating ranges of the bits overlap each other during rotation of the drill. It should also be noted that the centre bit 3 is essen- tially positioned behind the diametrical plane Pl seen in the direction of rotation A. Furthermore, the centre bit 3 is gen- erally inclined in relation to the plane P1, more precisely in
a way so that the operative cutting edge 12 of the bit is inclined at a certain, moderate angle to the plane P1. However, an analogous, operative cutting edge 13 on the periphery bit 4 is in the preferred embodiment located in front of the diamet- rical plane P1, seen in the direction of rotation. Furthermore, the cutting edge 13 is approximately parallel to the plane P1.
Reference is now made to figs 8-10, illustrating a preferred embodiment of a drill bit according to the invention.
This bit may advantageously be used as a centre bit in the described drill, although it is also feasible to (in a mirror- inverted embodiment) use the same as a periphery bit. As is seen in figs 8-10, the bit is of quadrangular basic shape and includes four similar cutting edges, generally designated 12.
The individual cutting edge 12 is situated in the area between a top side 14 and an individual side surface 15, which extends between the top side 14 and the area of a plane bottom side 16.
Four corners on the bit are generally designated 17. In the example, the top side 14 of the bit is illustrated in the form of a plane surface. In practice, the same top side may, how- ever, be formed with highly varying topography and, among other things, include chip breakers of different types. It should furthermore be pointed out that the bit has a centre hole 18 for a screw 19 (see fig 1), by means of which the bit may be fixed in the appurtenant pocket in the basic body.
A characteristic feature of the bit illustrated in figs 8-10 is that each individual cutting edge 12 is formed with first and second part edges 20, 21, transforming into each other via a transition edge portion 22. In the shown, preferred example, the individual cutting edge 12 also includes a third part edge 23, extending at an obtuse angle to the second part edge 21.
An inscribed circle, designated IC, (the centre of which is designated S) is tangent to the second part edges 21 of each of the four cutting edges. In said inscribed circle, the diameter of which is designated DIC, two diametrical planes RP1 and RP2, respectively, serving as reference planes, are inserted that separately extends parallel to the part edges 21 in opposite pairs of cutting edges. Each such reference plane imaginary divides the bit into two halves, which are mirror-
invertedly symmetrical in an inverse state. Thus, the reference plane RP1 separates a lower half 3A from an upper half 3B in fig 9. If one of said halves 3A, 3B would be hypothetically inverted, i. e. moved with the right part thereof to the left, the two halves would become mirror-invertedly symmetrical.
As is clearly seen in fig 9, the first part edge 20 is at least partially located at a larger distance from the reference plane RP1 than the second part edge 21. Therefore, if the bit according to fig 9 would enter a workpiece (not shown) positioned above the drawing figure, the part edge 20 would at least partly come into engagement with the workpiece before the second part edge 21. The transition edge portion 22 between the part edges 20,21 is defined by a convex curve 24 (see at the bottom of fig 9) adjacent to the first part edge 20, as well as a concave curve 25 adjacent to the second part edge 21. In this connection, an imaginary or actual, straight tangent line extends between the curves 24,25 at a certain angle a to the reference plane RP1 (and the reference plane RP2, respec- tively). Said angle a should amount to at least 10° and at most 30°, suitably at least 13° and at most 25°. In the shown exam- ple, the angle a is approx. 15°.
Via the transition edge portion 22, a radial distance difference al is provided between the first part edge 20 and the inscribed circle IC. In practice, said distance difference should amount to at least 2 % and at most 15 %, suitably at most 5 % of the diameter DIC of the inscribed circle. The dis- tance between the reference plane RP1 and the second part edge 21, parallel to the same, is designated a2. This measure a2 equals the radius of the inscribed circle IC.
Although it is feasible, per se, to allow the second part edge 21 to extend continuously all the way up to a corner 17, a third part edge 23 has, as mentioned above, been formed between the corner 17 and the part edge 21. The angle ß between the part edge 23 and an imaginary extension of the part edge 21 may vary most considerably, but should amount to at least 1° and at most 30°, suitably at least 10° and at most 20°. In the example, the angle ß amounts to approx. 16°.
As is seen in figs 8 and 10, the first part edge 20 to a given cutting edge 12 and the third part edge 23 to a
nearby cutting edge 12 is formed on a common, bracket-like pro- jection 26, the thickness T1 of which is smaller than the total thickness T of the bit. Therefore, in the area below the pro- jection or shoulder 26, a partial support surface 15A is left, which extends in the extension of the main support surface that is formed by the side surface 15 that extends from the top side 14 of the bit to the area of the bottom side 16 of the bit. By the fact that the part edges 20,23 are formed on a projection of limited thickness-contrary to a projection extending all the way from the top side to the area of the bottom side-a substantially L-shaped support surface of optimum area is obtained. The thickness Tl should amount to 25-40 % of the total thickness T of the bit. In the example, the thickness T1 amounts to approx. 33 % of the total thickness T. Thus, along considerably more than half of the height of the bit, a lower support surface extends along the entire width of the bit.
In connection with figs 8-10, it should furthermore be noted that the transition or corner 17 between nearby part edges 20,23 consists of a convexly rounded edge portion 27 of a suitable radius. In this connection, it should also be men- tioned that the individual part edge 20,21 and 23, respec- tively, either may be straight, as is shown in figs 8-10, or slightly arched with at least partially convex or concave basic shape. For instance, the part edge 20 may be of a convex basic shape and the part edge 21 of a concave basic shape, and the possibly occurring third part edge 23 may be convex.
In fig 9, B designates the length of the cutting edge 12 between two corners 17. The measure Bl designates the total length of the part edge 20 and the transition portion 22, such as said length is counted from a corner 17 to the point where the transition portion 22 transforms into the second part edge 21. As is visible for the naked eye in fig 9, the measure B1 is smaller than half of the measure B. In the example, B1 amounts to 43 % of B. In practice, the length B1 should amount to at least 10 % and at most 60 %, suitably at least 20 % and at most 50 %, of the length B, in order to protect, along a radial dis- tance of the corresponding length, an inner and front, inopera- tive corner edge on a periphery cutting edge following during rotation of the drill.
Reference is now made to fig 6, schematically illus- trating the primary function of the invention, more precisely by means of two mirror-invertedly similar bits, the centre bit 3 of which, shown by an unbroken contour line, is positioned closest to the centre axis C of the drill, while a periphery bit 4, shown by a partly dashed contour line, is hypothetically assumed to be placed immediately behind the centre bit, seen in the direction of rotation (in practice, the periphery bit 4 is, however, located displaced approximately a half revolution in relation to the centre bit, see fig 5). In fig 6,12-OP3 desig- nates an operative cutting edge on the centre bit 3, while an operative cutting edge on the periphery bit 4 is designated 12- OP4. The other three cutting edges 12 on the respective bit are inoperative. As is seen in the figure, the different part edges of the operative cutting edges 12-OP3 and 12-OP4 are positioned in axially spaced-apart cross planes (generally designated TP), which extend perpendicularly to the centre axis C. Thus, the first, radially outer part edge 20-3 of the operative cutting edge of the centre bit 3 is positioned in a first cross plane TP1. In the cross plane TP2 being behind, the first, radially inner part edge 20-4 of the operative cutting edge of the periphery bit 4 is located. The second part edge 21-3 of the centre bit 3 is located in the next cross plane TP3. Finally, the second part edge 21-4-which is situated radially outside the part edge 20-4-of the periphery bit 4 is positioned in a fourth cross plane TP4. In practice, the axial distance differ- ence between the different cross planes varies depending on the feeding in question for the individual drill. Generally, it may be said that said distance difference has to amount to at least 50 % of the feeding. Suppose that the feeding is to amount to 0,4 mm/revolution. Then, the distance difference between for instance the cross planes TP1 and TP2 has to amount to at least 0,20 mm. In practice, the distance difference should however be selected somewhat larger, e. g. to 60-90 %, suitably approx.
75 % of the feeding per revolution.
When the drill enters a workpiece, the radially outer part edge 20-3 of the centre bit 3 in a first step engages the material. Following further rotation of the drill, the inner part edge 20-4 of the periphery bit 4 then engages the material
in a second step. In a third step, the entire cutting edge 12- OP3 engages the material by also the part edge 21-3, positioned in the cross plane TP3, cuts in. Only in a fourth step, the outer, second part edge 21-4 of the periphery bit cuts into the material. By the fact that the different part edges on the bits 3,4 cuts into the material in different stages of the entering phase, a reduction of the magnitude of the individual cutting forces is guaranteed, as well as a distribution of the forces to four radially different, ring-shaped areas.
In connection with fig 6, it should furthermore be pointed out that the inner corner edge 27-which in this indexing position is inoperative-of the periphery bit 4 shown at the top is in a protected position behind the outer, opera- tive corner area of the centre bit 3, seen in the direction of rotation of the bits. In other words, said corner edge 27 will go free in a groove recessed by the part edge 20-3 in the work- piece. Therefore, when the corner edge, following indexing of the bit, forms an operative corner edge of the type that is shown at 27', the same is undamaged and fresh.
Reference is now made to fig 7, which shows an alter- native, and in practice preferred embodiment, according to which the centre and periphery bits 3,4 are of different designs. More precisely, the centre bit 3 is formed in princi- pally the same way as the bit according to figs 8-10 so far that the four cutting edges 12 of the bit include first and second part edges, while the periphery bit 4 is genuinely square so far that each one of the four cutting edges 13 con- sists of substantially straight, continuos edges. In the same way as in figs 8-10, the different part edges of the centre bit 3 are designated 20,21 and the transition edge portions are designated 22. The inscribed circle for the centre bit 3 is designated IC3, while the corresponding inscribed circle for the periphery bit is designated IC4. R designates the radius of the drill, such as the same is represented by the radial dis- tance between the centre axis C and the outer corner edge 27-4 on the periphery bit 4. It is axiomatic that said radius R determines the diameter of the recessed hole (D = 2R). The radius R3 of the centre bit 3 is determined by the distance
between the centre axis C and the upper, outer corner edge 27-3 of the centre bit.
The operative cutting edge 13 (at the top in fig 7) is inclined and extends at an angle X to the centre axis C of the drill. Said angle should amount to at least 91° and at most 94°, suitably at least 92'and at most 93°. This means that the outer, inoperative cutting edge 13 of the periphery bit 4, extending axially rearwards from the outermost, active corner edge 27-4, obtains a clearance angle within the range of 1-4°, suitably 2-3°.
6 designates the angle between the active first part edge 20-3 of the centre bit 3 and the centre axis C of the drill. Said angle should amount to at least 90° and at most 93°, and suitably be within the range of 91-92°. The angle E between the centre axis C of the drill and the transition edge portion 22 between the part edges 20-3 and 21-3 should amount to at least 60° and at most 70° (observe that the angle E, which defines the geometrical position of the transition edge portion 22 in the mounted state of the centre bit, should not be mistaken for the angle a in fig 9).
The angle A between the centre axis C and the second part edge 21-3 of the centre bit may advantageously amount to at least 84° and at most 87°.
Reference is now made to fig 3, which illustrates a centre bit 3 according to fig 7 during initial entering of the workpiece 11, as well as fig 4, which shows a periphery bit 4 according to fig 7, likewise during initial entering of the workpiece. As has been described above, the entering of the two bits of the workpiece takes place in several different steps.
In the first step, which is shown in fig 3, a first part edge 20 on the operative cutting edge of the centre bit 3 has com- menced the separation of a chip while forming a groove 28 in the workpiece. Said groove is radially distanced from the cen- tre axis of the drill. In the next step, which is shown in fig 4, the periphery bit 4 has commenced its entering of the work- piece. By the fact that the active cutting edge 13 of the periphery bit 4 is somewhat inclined, the radially inner corner thereof will initially be housed in the recessed groove 28, and then the radially outer part of the cutting edge 13 will suc-
cessively commence recession of a second groove 29 in the work- piece. Following further rotation and simultaneous axial feed- ing of the drill, also the second part edge 21 on the centre bit 3 cuts into the material (this step is not shown in figs 3 or 4), both bits entering the workpiece completely. When the operative cutting edge of the centre bit in its entirety has cut into the material, i. e. entering of both the part edges 20- 3,21-3 has taken place, a continuous chip is separated, in that the transition edge portion 22, as a consequence of the chosen angle s, forms a gentle or flat transition between the part edges.
List of Reference Designations 1 = Drill 2 = Basic body 3 = Centre bit 4 = Periphery bit 5 = Fastening part 6 = Shank 7 = Drill tip 8, 8'= Chip channels 9 = Centre pocket 10 = Periphery pocket 11 = Workpiece 12 = Cutting edge on centre bit 13 = Cutting edge on periphery bit 14 = Top side 15 = Side surface 15A = Partial support surface 16 = Bottom side 17 = Corner 18 = Hole 19 = Screw 20 = First part edge 21 = Second part edge 22 = Transition edge portion 23 = Third part edge 24 = Convex curve 25 = Concave curve 26 = Corner projection 27 = Corner edge 28 = Groove from centre bit 29 = Groove from periphery bit
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