DRILLING AND THREADING TOOL

Abstract: According to the invention a combined drilling and threading tool is provided which includes an elongated shank and a conical front portion (11) provided at its front end, said portion being provided with symmetrically shaped cutting edges (22,23) and symmetrically twisted ribs (13,14) arranged axially rearwardly thereof. Cutting teeth (18) for threading operations are provided on these ribs with a certain mutual spacing (d) and radial clearance and axial clearances. The symmetrically shaped cutting edges shall be configured such that the inner parts have contour differing from the contour of the outer parts thereof. Further according to the invention a method for drilling and threading is provided which, inter alia, includes rotation of the tool simultaneously as it is circularly interpolated one turn around and one pitch axially forwardly or rearwardly.

Description: The present invention relates to a combined drilling and threading tool comprising a shank having a conical front portion defined by a plurality of hole-cutting edges said hole-cutting edges extending symmetrically in relation to a central axis of rotation of said shank, said shank including clearance surfaces extending rearwardly of radially inner portions of said cutting edges, a plurality of ribs disposed rearwardly of respective ones of said hole-cutting edges, said ribs extending symmetrically relative to said axis and being spaced apart by chip flutes extending symmetrically relative to said axis, each rib including leading and trailing edges with respect to a direction of rotation of said tool, a plurality of thread-cutting teeth being disposed along each of said ribs, whereby said cutting teeth do not project radially beyond a circumference having center on the axis of the tool and passing through the outermost points of the cutting edges, said teeth on each rib being spaced apart by equal distances in said front-to-rear direction. Such a tool is known from EP-A-0 237 035, which falls under Art. 54(3) EPC. In order to conduct machining operations, such as drilling and thread milling, different types of tools have been commonly used which have been optimized for each machining. The drilling operation has been conducted by means of a tool such as a drill provided with hard metal inserts or a solid drill of hard metal or speed steel. In order to be able to cut an internal screw thread after drilling it is necessary to change tool in the machine. A tool, such as a tap or a boring bar, is mounted instead of the drill. In the latter case the boring bar has been provided with a radially projecting thread cutting insert at its front end which has a cutting edge having the thread profile corresponding to the thread to be machined in the work piece. For reducing of costs in the work shops it is most desirable to use a combination tool which provides for several different operations in succession without downtime for tool exchange in the machine. The tool for drilling and threading a hole in a workpiece disclosed in EP-A-0237035 comprises a straight body with a drill point at the forward end, a shank at the rearward end and thread-forming teeth on elongated ribs. Straight flutes for chip evacuation purposes are linearly extending axially between said ribs and the cutting teeth obviously do not decrease a size in the axial direction away from the direction of rotation. As a result thereof it is not provided sufficient radial clearance between the teeth and the thread being cut. Since the ribs are not spirally curved there will not occur a successive or gradual engagement of the teeth with the workpiece and, as a consequence thereof, a desireable successive reduction of cutting forces cannot be achieved. The objects of the present invention are to provide an improved combined drilling and threading tool of a new type which provides for drilling and threading with large feed and cutting speeds and that simultaneously provides for a favourable chip flow. This is achieved with a tool according to claim 1. A preferred embodiment of the invention will be described hereinafter. Fig. 1 shows a side view of a drilling and threading tool according to the invention. Fig. 2 shows a section along the line II-II in Fig. 1. Fig. 3 shows a partially enlarged side view of the tool in Fig. 1. Fig. 4 shows an end view of the tool in Fig. 3. Fig. 5 shows a cross-section along the line V-V in Fig. 3. Figs. 6 a-f show an axial section of a work piece which is drilled and threaded by a tool according to Figs. 1-5, and Figs. 7 a-d show corresponding machining by means of an alternative tool according to the invention. The tool shown in Figs 1-3 includes an elongated cylindrical shank 10 of hard metal, steel or speed steel, whose front end is formed by a conical front portion 11, which transfers into cylindrical guiding surfaces 12a and 12b having a small axial extension, usually less than 1 mm. The part of the drill shank 10 which is close behind the guiding surfaces 12a and 12b is formed by two spirally twisted ribs 13 and 14 between which likewise twisted grooves 15, 16 are formed for chip conveyance. The ribs 13, 14 are twisted at an angle about 0 o to 40 o relative to the center axis 17 of the tool, preferably 25 o to 35 o . When said angle is 0 o the ribs are straight and parallel with said center axis. As can be seen in Figs. 1 and 4 the tool is completely symmetrically shaped relative to the center axis 17. Each of the twisted ribs 13, 14 is provided with a row of thread shaped teeth 18 without pitch and with mutally identical profiles and spacing d, said toothed ribs extending to the front portion 11. These teeth 18 are shaped such that they occupy a small part of the envelope surface of the rib 13, 14, seen in the circumferential direction. This has been found important in order to achieve a good chip flow without problems in the hole which is to be machined in a work piece. The teeth 18 adapted for threading are cut straight in the circumferential direction regarding the row of teeth but they have clearances in the radial and the axial directions, while the rest of the envelope surfaces of the ribs 13, 14 have smooth cylindrically rounded contours. The circumferential extension of each tooth is at least half the axial extension of the tooth and maximum twice as large. Preferably these extensions are equal. When the tool is rotated around its axis the tips of the teeth lying in the same plane normal to the center axis of the tool describe a circular path. All said paths form a cylinder. Alternatively the paths may form a cone whose apex is directed towards the shank such that during machining the thread will become cylindrical although the tool may be elastically bent. The cone angle is maximum 1 o . Thus the diameter of the circular path described by the teeth closest to the front portion 11 is larger than the diameters of the paths described by axially rearwardly positioned teeth. The tool may be provided with one or more axially oriented fluid passages (not shown), which are symmetrically shaped in both of the ribs and terminate in the front cone surface 19 at each side of the center line 17. As mentioned above the transition between the front portion or the drill tip 11 and the axially rearwardly extending tool part is formed by cylindrical guiding surfaces 12a and 12b, while the front surface 19 of the drill tip has a conical basic shape. In some applications though the cylindrical surfaces 12a and 12b may be replaced by a fully shaped profile which is equal to the other teeth 18 in all respects. In the front surface 19 of the tool the cutting edge portions 20 and 21 are formed, each of which forms a cutting angle α relative to a normal to the center line 17. The cutting edges 22 and 23 meet in the center of the tool. The cutting edges 22 and 23 are shaped such that the radially inner cutting edge portions are curved and thus forming a mainly S-shaped contour while the radially outer cutting edge portions have a mainly straight contour. The outer edge portion may have a mainly straight contour or be broken at an angle according to Fig. 4. According to a preferred embodiment the curvature of the inner cutting edge portions are larger adjacent to the center line 17 of the tool than further away from it. Furthermore, at the radially inner cutting edge portions there are provided special recesses 24 and 25 at each side of the center line 17 in order to create clearance surfaces at the starting points of the cutting edges 22 and 23 at the center of the tool. These recesses 24, 25 are formed by removing material in the front parts of the twisted passages 15, 16 such that an enlarged clearance surface 26 is created in connection with each cutting edge and adjacent to or in the center of the tool. This clearance surface is preferably provided with a clearance angle which is approximately 0 degrees. The border portion 27 resp 28 which is created thereby between these recesses 24, 25 and the bottom portions of the twisted chip flutes 15, 16 may serve as chip guides. The profile of the chip flutes 15, 16 may be varied seen in a cross-section perpendicular to the center line 17 together with the cutting angle α, wherein the radially outer portions of the cutting edges may be given a more or less positive or negative basic shape. The geometry of the chip flute determines the cutting geometry of the thread tooth. Thus it is possible to optimize the cutting process as desired. Furthermore, it may be advantageous in some applications to shape the chip surface of each cutting tooth 18 with a recess such that a better cutting geometry for threading is achieved; this is to additionally alter the cutting geometry which has been achieved by the twisted contour and the geometry of the chip flutes 15, 16. The function and mode of operation of the tool according to the invention are more closely described in connection with Figs. 6 a-f. It is assumed that a threaded holed 29 shall be provided in a work piece 30 by means of the tool. In a first operation a hole is drilled by means of the tool until a desired depth a is reached, as shown in Fig. 6a. The tool is thereafter retracted a small distance b rearwardly according to Fig. 6b, said distance b preferably fulfilling the condition b > tan α. In the next sequence the tool is laterally displaced a distance c corresponding to a full or a part of the thread profile depth according to Fig. 6c, whereafter the tool is rotated having its teeth 18 actively engaging the inner wall of the drilled hole 28 or preferably simultaneously rotating about its own tool axis 17. Simultaneous with these relative movements the tool is also axially fed such that the threading operation is completed while having the teeth in engagement around the whole circumference during spiral feed of one complete orbit, this has been repeated until the distance c corresponds to the complete thread profile depth according to Fig. 6d. The axial feed during an orbit shall be equal to the distance d , which corresponds to the thread pitch. Then the tool is allowed to return to the center of the hole, Fig. 6e, and is retracted, Fig. 6f. More particularly the method for drilling and threading includes the steps of providing a tool having an elongated shank, a conical front portion which is provided with cutting edges and having ribs positioned therebetween which are provided with cutting teeth projecting radially beyond said ribs but terminating radially equal to or short of the radially outermost parts of said cutting edges, inserting said tool into a machine being able to conduct rotation of the tool, circular interpolation and simultaneous pitch feed, rotating and axially feeding said tool towards and into a work piece thereby forming a hole, having a center line, by the use of said cutting edges, retracting said tool a first distance, offsetting the center axis of said tool a second distance relative to the center line of said hole, rotating said tool and conducting circular interpolation of said tool one turn around the center line of said hole having cutting teeth in engagement with said hole and moving said tool one pitch axially forwardly or rearwardly, allowing said tool to become coaxial with said hole and retracting said tool from said work piece. The rotary speed of the tool during drilling of the hole may be equal to or different from the rotary speed of the tool during threading. Such a combination of circular feed (the circumference of the thread) and axial feed (the pitch of the thread) is easily done in the machines available and which offer the possibility of a combination of circular interpolation with simultaneous axial pitch feed, i.e. one turn around and one pitch movement. It should be understood that it is possible to cut both an internal and an external screw thread on a work piece by means of the present tool. In the case shown in Fig. 6 it is assumed that the work piece 30 is stationary while only the tool is made to rotate. However, it is possible to allow also the work piece to rotate and/or to be circularly fed relative to the tool and displace the tool and the work piece axially relative to each other a distance which is equal to the pitch of the thread during a turn. According to an alternative embodiment, more closely shown in Figs. 7 a-d, the tool according to the invention may be provided with obliquely oriented bevel edges 31a and 31b, which are symmetrically positioned on each side of the axis 17 and axially rearwardly of the tool part that is provided with thread teeth 18. The bevel edges 31a and 31b connect to the tool shank 32 positioned rearwardly thereof. The bevel edges 31a and 31b should be obliquely oriented at an angle relative to the center axis 17, which is 30 o - 60 o , preferably 45 o . During holemaking in a work piece 30 by means of this tool the hole 29 is first drilled through. The tool is thereafter forwardly fed an additional distance such that the bevel edges 31a and 31b can engage the entrance side of the work piece and in direct connection with termination of the drilling operation said edges conducting chamfering of the rim of the hole, see Fig. 7a. The tool is retracted axially thereafter a distance, see Fig. 7b, such that the bevel edge runs free from the hole 29, whereafter the tool is displaced laterally a distance c, which corresponds to the whole or parts of the desired thread profile depth according to Fig. 7c, whereafter the tool is rotated having its teeth 18 in active engagement with the inner wall of the drilled hole 29 or simultaneously rotating around its own tool axis 17. Simultaneous with these relative movements the tool is however axially fed such that the threading operation is completed whilst having the teeth in engagement around the whole circumference during a complete spiral feed of a turn, this has been repeated until the distance c corresponds to the complete thread profile depth according to Fig. 7d. The pitch during the axial feed shall during a turn be equal to the distance d which corresponds to the thread pitch. The tool is then centered in the hole and retracted. The tool according to the invention may be produced from tool steel, hard metal or speed steel or combinations thereof. In the case of the tool body being made of tool steel the front drill tip and the cutting edges of the tooth profile should be configured on hard metal wafers which are brazed or clamped to the tool body. According to an alternative embodiment the tool may be manufactured from a compound body consisting of peripheral material portions and core portion. For example the peripheral material portions may be made of a material which includes hard substances in a substrate of speed steel while the core portions are made of a tougher material, preferably tool steel or speed steel. With such division of materials it is possible to machine the material and grind the necessary sharp cutting edges for the drilling and threading tool. By means of the above mentioned tool it is thus possible to conduct three different maching operations; drilling, chamfering and threading without having to exchange the tool in the machine spindle (not shown) where it is inserted. Important down time periods can thereby be eliminated for the benefit of a more cost reducing machining compared with previously known tools.

Claims: A combined drilling and threading tool comprising a shank (10) having a conical front portion (11) defined by a plurality of hole-cutting edges (20, 21) said hole-cutting edges extending symmetrically in relation to a central axis of rotation (17) of said shank, said shank including clearance surfaces (26) extending rearwardly of radially inner portions of said cutting edges, a plurality of ribs (13, 14) disposed rearwardly of respective ones of said hole-cutting edges, said ribs extending symmetrically relative to said axis and being spaced apart by chip flutes (15, 16) extending symmetrically relative to said axis (17), each rib including leading and trailing edges with respect to a direction of rotation of said tool, a plurality of thread-cutting teeth (18) being disposed along each of said ribs, whereby said cutting teeth do not project radially beyond a circumference having center on the axis of the tool and passing through the outermost points of the cutting edges, each of said ribs (13, 14) is spirally curved in a front-to-rear direction, said teeth (18) on each rib being spaced apart by equal distances (d) in said front-to-rear direction, each said tooth (18) having a dimension in a direction parallel to said axis, said dimension decreasing in size away from said direction of rotation for providing said teeth with axial clearance relative to threads being cut by the tool, each said tooth (18) having a radial dimension in a radial direction of the shank that decreases in size in a direction away from the direction of rotation for providing said teeth with radial clearance relative to threads being cut by the tool, and each of said teeth terminating at a location intermediate said leading and trailing edges of its associated rib. Drilling and threading tool according to claim 1, characterized in that the transition between the conical front portion (11) of the tool and the axially and rearwardly formed tool part for threading includes axially extending cylindrical guiding surfaces (12a,12b) having a small axial extension and having the same or somewhat larger diameter than the diameter of the tool part for threading, each said guiding surface being provided as a guiding rib. Drilling and threading tool according to claim 2, characterized in that the guiding surfaces (12a, 12b) are replaced by full teeth profiles which are equal to the other teeth (18) in all respects. Drilling and threading tool according to claim 1, characterized in that the tool is provided with axially extending passages for conveyance of fluid towards the front portion (11) of the tool. Drilling and threading tool according to claim 1, characterized in that the radially inner portions of the cutting edges (22,23) are symmetrically curved towards the center of the tool while the radially outer portions of the cutting edges are mainly straight. Drilling and threading tool according to claim 5, characterized in that each cutting edge portion in or adjacent to the center of the tool has a clearance angle which is about zero degrees. Drilling and threading tool according to claim 6, characterized in that each cutting edge portion has an increasing degree of curvature towards the center of the tool. Drilling and threading tool according to claim 1, characterized in that the ribs (13,14) provided with cutting teeth (18) are twisted by an angle which is less than 40 o , preferably 25 o to 35 o relative to the center axis (17) of the tool and in that the part provided for the threading operation describes a cylinder or a cone whose apex is directed towards the shank (10) and whose cone angle is maximum 1 o . Drilling and threading tool according to claim 1, characterized in that the cutting teeth (18) provided for threading are straightly cut so that they are circumferentially aligned but with radial and axial clearances and in that they, preferably to a small extent, occupy the envelope surface of the ribs (13,14) while said envelope surface otherwise having a smooth cylindrically rounded contour. Drilling and threading tool according to claim 1, characterized in that the tool is provided with chamfer edges (31a, 31b) obliquely oriented relative to its center axis (17) and symmetrically positioned on both sides of the center axis, which edges form transitions between the front part of the tool provided with cutting teeth (18) and a shank part (32) positioned axially rearwards thereof. Drilling and threading tool according to claim 10, characterized in that the chamfer edges (31a, 31b) are obliquely oriented at an angle relative to the center axis (17), which is 30 o to 60 o , preferably about 45 o .

Inventors: BERGSTROEM, BERT, LINDBERG, HANS, Bergström, Bert
Applicants: SANDVIK AKTIEBOLAG
Application year: 1988
Main classification: B23G5
IPC codes: B23G5, B23G1