Rotatable tool for chip removing machining as well as a loose top and a basic body therefor

[0001] This Application claims priority under 35 U.S.C. §119 to Swedish Patent Application No. 1050288-8, filed on Mar. 26, 2010, which is incorporated by reference herein in its entirety.

[0002] The present invention relates generally to a rotatable tool for chip removing machining of the type that includes a basic body having front and rear ends between which there extends a first geometrical center axis around which the basic body is rotatable in a predetermined direction of rotation, and a loose top having front and rear ends between which a second geometrical center axis extends. The front end of the basic body includes a jaw, which is delimited by two drivers and an intermediate bottom and in which a part of the loose top is received. A centering pin protrudes axially rearward from the loose top and is inserted in an axial center hole, which mouths in the bottom of the jaw and in which a threaded hole mouths for a screw co-operating with the center pin.

[0003] The invention also relates generally to a loose top for the type of tool in question as well as a basic body. Tools of the kind in question are suitable for chip removing or cutting machining of workpieces of metal, such as steel, cast iron, aluminium, titanium, yellow metals, etc. The tools may also be used for the machining of composite material of different types.

[0004] More recently, drilling tools as well as milling tools have been developed, which, contrary to integral solid tools, are composed of two parts, viz. a basic body and a head detachably connected with the same and thereby being replaceable, in which head the requisite cutting edges are included. In such a way, the major part of the tool can be manufactured from a comparatively inexpensive material having a moderate modulus of elasticity, such as steel, while a smaller part, viz. the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the cutting edges a good chip-removing capacity, good machining precision and long service life. In other words, the head forms a wear part, which can be discarded after wear-out, while the basic body can be re-used several times. A now recognized denomination of such, cutting edge-carrying heads is “loose tops”, which henceforth will be used herein together with the concept “loose top tools”.

[0005] On rotatable tools of the loose top type, several requirements are made, one of which is that the loose top should be held centered in an exact and reliable way in relation to the basic body. Accordingly, each unintentional eccentricity between the center axis of the loose top and the center axis of the basic body should not be more than 0.01 mm. Another requirement or desire from the users' side is that the loose top should be mountable and dismountable in a rapid and convenient way without risk of incorrect mounting. Most preferably, mounting and dismounting should be possible to be carried out without the basic body necessarily having to be removed from the driving machine.

[0006] Drilling tools as well as milling tools (shank-end mills) of the loose top type are widely described in the patent literature and may be divided into a number of different categories depending on the ideas on which the designs are based. Thus, certain tools use loose tops having rear pins that entirely or partly (together with other coupling details) fulfill the task of centering the loose top in relation to the basic body. To this category belongs among others the tool disclosed in U.S. Pat. No. 6,012,881, which discloses a loose top drill in which a rear coupling part of the loose top is axially inserted in a jaw between two non-compliant drivers, the insides of which include axially extending, torque-transferring ridges that engage the corresponding chutes in the coupling part included in the loose top, besides which a centric pin protruding rearward from the coupling part is inserted in a center hole mouthing in the bottom of the jaw. With the pin, a screw mounted in a radial, threaded hole in the basic body co-operates, which has the purpose of locking the loose top in relation to the basic body. In that connection, the center pin is cylindrical and insertable at a close (form fitting) fit in a likewise cylindrical center hole, in order to, together with concave and convex contact surfaces of the drivers and the coupling part, respectively, fulfill the purpose of centering the loose top. However, the concurrent requirements of fit not only between the center pin and the hole, but also between the drivers and the coupling part, impose extreme, not to say unattainable, requirements of manufacturing precision. In addition, a conflicting problem will arise if good precision peradventure would be achieved, viz. that the mounting and dismounting of the loose top become difficult to carry out as a consequence of the centering pin requiring great force to be pushed into and pulled out of, respectively, the hole.

[0007] With the purpose of obviating the above-mentioned disadvantages of the tool according to U.S. Pat. No. 6,012,881, it has by EP 2266736 and EP 2266734 been proposed to form the centering pin of the loose top with, on one hand, an essentially semi-cylindrical contact surface concentric with the center axis of the loose top, and on the other hand a diametrically opposite clearance surface, the first-mentioned one of which can—by means of the screw of the tool—be pressed against the inside of a cylindrical centering hole in the basic body. In such a way, the centering pin can be given a cross-sectional area that is smaller than the cross-sectional area of the hole, the mounting and dismounting of the loose top being facilitated without the exact centering of the loose top being jeopardized. However, in this case, a 50% risk arises of incorrect mounting of the loose top in connection with the centering pin being inserted into the hole, in that the contact surface of the pin can be turned toward any half of the internal, cylindrical hole wall.

[0008] The present invention aims at obviating not only the serious disadvantages inherent in the loose top tool according to U.S. Pat. No. 6,012,881, but also the practical mounting inconveniences that have turned out to be associated with the tools and loose tops that are objects of EP 2266736 and EP 2266734.

[0009] An object of the invention to provide a loose top tool the loose top of which can, on one hand, be centered in an accurate way in relation to the basic body of the tool, and on the other hand be mounted in a simple way without risk of incorrect mounting. In other words, the operator should without mental effort be able to, in a foolproof way, mount the loose top in only one predetermined position.

[0010] Another object of the invention is to provide a loose top tool, the two main components of which, i.e., the basic body and the loose top, respectively, should be manufacturable in an effective and economical way while achieving the desired simplicity of the mounting.

[0011] Yet another object of the invention is to provide a loose top the centering pin of which is strong and robust.

[0012] In an embodiment, the invention provides a rotatable tool for chip removing machining, including a basic body having front and rear ends between which there extends a first geometrical center axis (C1) around which the basic body is rotatable in a predetermined direction of rotation, and a loose top having front and rear ends between which a second geometrical center axis (C2) extends. The front end of the basic body includes a jaw, which is delimited by two drivers and an intermediate bottom and in which a part of the loose top is received. A centering pin protrudes axially rearward from the loose top and is inserted in an axial center hole, which mouths in the bottom of the jaw and in which a threaded hole mouths for a screw co-operating with the center pin. The center hole includes a cylindrical support surface, which is concentric with the center axis (C1) of the basic body and against which the centering pin of the loose top is pressed by the screw. The centering pin of the loose top includes first and second diametrically opposed, external surfaces, each having an axial extension and a peripheral extension, the first external surface forming a contact surface that is pressed against the support surface of the center hole and extends tangentially between two axially extending boundary generatrices, which are situated along an imaginary circumscribed circle (S2) having a center (MP2) coinciding with the center axis (C2) of the loose top, and between which an arc angle (β) is less than 180°, the second external surface forming a clearance surface that lacks contact with the inside of the center hole as a consequence of the cross-sectional area of the centering pin being smaller than the cross-sectional area of the center hole. The circumscribed circle (S2) has a radius (r2) that is equally great as the radius (r1) of an inscribed circle (S1) along the cylindrical support surface of the center hole. A greatest radial distance (RD2) between the center axis (C2) of the loose top and a point on the clearance surface of the centering pin is greater than the radius (r2) of the circumscribed circle (S2) but smaller than a corresponding distance (RD1) between the center axis (C1) of the basic body and a clearance surface situated in the center hole and opposite the support surface.

[0013] In another embodiment, the invention provides a loose top for rotatable tools for chip removing machining, including front and rear ends between which a geometrical center axis (C2) extends. An axially rearwardly protruding centering pin includes first and second diametrically opposed, external surfaces, each having an axial extension and a peripheral extension, the first external surface forming a contact surface that extends peripherally between two axially extending boundary generatrices, which are situated along an imaginary circumscribed circle (S2) having a center (MP2) coinciding with the center axis (C2), and between which an arc angle (β) is less than 180°, the second external surface forming a clearance surface. A greatest radial distance (RD2) between the center axis (C2) and a point on the clearance surface is greater than the radius (r2) of the circumscribed circle (S2).

[0014] In yet another embodiment, the invention provides a basic body for rotatable tools for chip removing machining, including an envelope surface and front and rear ends, between which a geometrical center axis (C1) extends. The front end includes a jaw delimited by two drivers and an intermediate bottom in which an axially extending center hole mouths, a hole for a screw extending between the center hole and the envelope surface. The center hole is delimited by a cylindrical support surface, which is concentric with the center axis (C1) and along which a circle (S1) having a radius (r1) is inscribed, and by an opposite clearance surface situated outside the inscribed circle (S1).

[0027] In the drawings, the loose top tool according to an embodiment of the invention is exemplified in the form of a twist drill, which includes a basic body 1 and a loose top 2, in which the requisite cutting edges 3 are included. In its assembled, operative state according to FIG. 1, the tool is rotatable around a geometrical center axis designated C, more precisely in the direction of rotation R. The basic body 1 includes front and rear ends 4, 5 between which a center axis C1 specific to the basic body extends. In the backward direction from the front end 4, a cylindrical or rotationally symmetrical envelope surface 6 extends, in which two chip flutes 7 are countersunk, which in this case are helicoidal but which also may be straight. In the example, the chip flutes 7 terminate in a collar 8, separating the front part 9 of the basic body from a rear part 10, which has a diameter that is greater than that of the part 9, and is intended to be attached to a driving machine (not shown).

[0028] Also the loose top 2 includes front and rear ends 11, 12 and an own center axis C2, with which two envelope part surfaces 13 are concentric. Between the envelope part surfaces 13, two helicoidal chip flute sections are countersunk in the form of concave surfaces 14, which form extensions of the chip flutes 7 of the basic body 1 when the loose top is mounted to the basic body. If the loose top 2 is centered correctly in relation to the basic body 1, the individual center axes C1 and C2 coincide with the center axis C of the assembled tool.

[0029] Since the major part of the basic body 1 lacks interest in connection with the invention, henceforth only the front end portion thereof will be illustrated together with the loose top 2, more precisely on an enlarged scale.

[0030] As is seen in FIG. 3, a jaw 15 is formed in the front end 4 of the basic body 1 and that is delimited by two diametrically opposed drivers 16a, 16b as well as by an intermediate bottom 17, which in this case is in the form of a plane surface that extends perpendicular to the center axis C1. In this case, the drivers 16a, 16b are non-compliant lugs, contrary to elastically compliant branches. In the bottom 17, a center hole 18 mouths that extends axially into the basic body so far that a screw hole 19 (see FIGS. 5-7) can mouth therein. The screw hole 19 is formed with a female thread 20 and concentrically with a geometrical center axis C3 that extends radially inside the basic body, in this case perpendicular to the center axis C1 of the basic body. Note that the screw hole 19 is recessed in the material that is present between the two concave surfaces that form the chip flutes 7. In FIG. 7, it is furthermore seen that the screw hole 19 extends between an inner mouth 19a, which opens in the center hole 18, and an outer mouth 19b, which opens in the envelope surface 6 of the basic body. The female thread 20 of the screw hole 19 co-operates with a male thread 21 (see FIG. 6) of a screw 22, which in a rear end 23 includes a key grip 24 for a tool (not shown), by which the screw can be tightened into and loosened from, respectively, the screw hole 19. In this case, the front end of the screw 22 includes a plane end surface 25 as well as a conical surface 26, which forms a transition surface between the end surface 25 and the male thread 21.

[0031] In the shown, preferred example, the loose top 2 is a single, simple head, which laterally is delimited by two opposite, plane contact surfaces 27 in addition to the envelope part surfaces 13 and the concave surfaces 14 forming chip flute sections. In the backward direction, the head is delimited by a plane surface that extends perpendicular to the center axis C2 and forms the rear end 12 of the loose top. As pointed out previously, the two cutting edges 3 are included in the front portion of the loose top. More precisely, the two cutting edges 3 converge into a central tip 28 (see FIG. 4) situated along the center axis C2 of the loose top. The side contact surfaces 27 of the loose top co-operate with the insides of the drivers 16a, 16b, which are in the form of plane surfaces 29 (see FIG. 3). In the operative state of the tool, the rear end surface 12 of the loose top is held pressed against the plane surface that forms the bottom 17 of the jaw 15 at the same time as the two side contact surfaces 27 of the loose top are held pressed against the support surfaces 29 of the insides of the drivers. In this operative state, a centering pin 30 is inserted in the center hole 18 and that protrudes axially rearward from the rear end surface 12 of the loose top 2. In the example, the rear, free the end of said pin 30 is represented by a plane end surface 31, while the front end designated 32, which is an integrated part of the loose top, is represented by a circumferential boundary line between the pin 30 and the end surface 12.

[0032] Reference is now made to FIGS. 5-10, which more in detail illustrate the nature of the centering pin 30 and center hole 18.

[0033] The cross-sectional shape characteristic for the center hole 18 is best seen in FIGS. 7 and 9. A first part surface 33 included in the inside of the hole (i.e. the hole wall) is cylindrical and has an axial as well as a peripheral extension in order to serve as a support surface for the centering pin 30. The circle that defines the cylinder shape of the support surface 33 has a center MP1 coinciding with the center axis C1 of the basic body. The radius of the support surface 33 is designated r1, and the peripheral extension thereof is determined by two boundary generatrices 34a, 34b between which the arc angle α, in this case, is greater than 180°. Thus, in the concrete example according to FIGS. 7 and 9, a amounts to approx. 230°. In the inside of the center hole 18, there is further included a part surface in the form of a clearance surface generally designated 35, which, like the support surface 33, has an axial extension as well as a peripheral one. In this clearance surface 35, a plurality of part surfaces 36a, 36b, 36c are included, the part surface 36a of which is concave and in this case cylindrical, while the part surfaces 36b, 36c are plane and form transition surfaces between the support surface 33 and the part surface 36a. As is seen to the naked eye in FIG. 9, the radius (lacks designation) of the part surface 36a is smaller than the radius r1 of the support surface 33. In the example, the two part surfaces 36b and 36c converge toward the part surface 36a, whereby the center hole 18 is imparted a drip-like cross-sectional shape.

[0034] Further, in FIG. 9, S1 designates a circle that is inscribed along the cylinder surface 33 and therefore has the same radius r1 as cylinder surface 33. In FIG. 9, it is furthermore seen that a symmetry plane SP1 divides the center hole 18 into two mirror-symmetrical halves.

[0035] Reference is now made to FIGS. 8 and 10, which illustrate the geometrical design of the centering pin 30. Like the center hole 18, the pin 30 has a generally drip-like cross-sectional shape that essentially is determined by a contact surface designated 38 as well as by a clearance surface in its entirety designated 39. In this case, the contact surface 38 is cylindrical and extends between two boundary generatrices 40a, 40b, more precisely along an arc angle β that should be less than 180°. In the example according to FIG. 10, β amounts to approx. 140°. However, β may vary upward as well as downward from this value provided that the value is less than 180°.

[0036] In the clearance surface 39 of the centering pin, a plurality of part surfaces are included, viz. a pair of convex transition surfaces 41 and a pair of plane part surfaces 42 that converge in the direction from the contact surface 38 toward a common, convex back surface 43. In FIG. 8, it is seen that the angle of convergence γ between the surfaces 42 amounts to approx. 60°. The center MP2 of the circle that defines the cylindrical shape of the contact surface 38 coincides with the center axis C2 of the loose top 2. In other words, the surface 38 is concentric with the center axis of the loose top and has a radius r2. Thus, a circumscribed circle S2 along the external, cylindrical contact surface 38 has the radius r2.

[0037] In this connection, it should be pointed out that the contact surface 38 does not necessarily have to be cylindrical. Thus, the same may, for instance, have an elliptical shape provided that the radial distances or the radii r2 between the center MP2 (=C2) and the two boundary generatrices 40a, 40b are equally large.

[0038] A feature of the invention is that the cross-sectional area of the center hole 18 is larger than the cross-sectional area of the centering pin 30 in order for the external clearance surface 39 of the centering pin to clear from the internal clearance surface 35 in the center hole, as is clearly shown in FIG. 7. A characteristic feature is further that the circumscribed circle S2 of the contact surface 38 has the same radius r2 as the radius r1 of the inscribed circle S1 of the internal support surface 33 (i.e., r1 and r2 are equally great). In addition, a greatest radial distance RD2 between the center axis of the loose top, i.e., the center MP2 in FIG. 10, and a point on the clearance surface 39, on one hand, is greater than the radius r2 of the circumscribed circle S2, but, on the other hand, smaller than a greatest radial distance RD1 between the center axis C1 (=MP1 in FIG. 9) of the basic body and the internal clearance surface 35. As a consequence of this geometry, the centering pin can only be inserted into the center hole in one way, viz. with the comparatively slender back surface 43 facing the internal clearance surface 35 in the center hole. Should the loose top by misadventure be held rotated 180° in relation to the position according to FIG. 7, any insertion of the centering pin 30 into the center hole 18 is accordingly made impossible.

[0039] Also the cross-section of the centering pin 30 is divided into two mirror-symmetrical halves by a symmetry plane SP2.

[0040] As may be best seen in FIG. 3, in the centering pin 30, more precisely the clearance surface 39 thereof, a seat 44 is countersunk, which is spaced apart from the free end 31 of the pin and includes a bottom surface 45, which in this case is plane and transforms into two plane chamfer surfaces 46a, 46b, which individually form an obtuse angle with the bottom surface 45. The rear one of these chamfer surfaces, viz. the chamfer surface 46b (see FIG. 2), forms a stop surface for the screw 22. More precisely, the cone surface 26 of the screw will be pressed against the surface 46b, thereby preventing the centering pin from being pulled out of the center hole. Furthermore, the front end surface 25 of the screw will partially be pressed against the bottom surface 45 of the seat 44, more precisely with the purpose of applying a torque to the pin that aims at pressing the two side contact surfaces 27 of the loose top against the internal support surfaces 29 of the drivers 16a, 16b. In order to allow such a rotation, the screw is generally inclined in relation to the bottom surface 45 of the seat. Thus, in FIG. 7, it is seen how the center axis C3 of the threaded hole 19 and the symmetry plane SP1 through the center hole 18 form an angle δ with each other. In FIG. 7, the angle δ is exaggerated (amounts to approx. 20°) for the sake of clarity. In practice, the same may, however, be limited to the order of 2-5°.

[0041] In the preferred embodiment, the surface 43 of the centering pin extends from the free end of the pin (and runs—with the exception of the seat 44—all the way up to the rear end surface 12 of the loose top). In such a way, it is ensured that the centering pin 30 cannot even partially be incorrectly inserted into the center hole 18. Should the loose top be turned the wrong way, accordingly not even the free end of the centering pin could be brought into the center hole, and therefore the operator could quickly correct the error.

[0042] In FIG. 3, it is furthermore seen that the mouth of the center hole 18 includes a conical surface 47 having the purpose of facilitating the insertion of a correctly turned centering pin into the hole.

[0043] When the loose top 2 is to be mounted in the jaw 15 of the basic body 1, the screw 22 is unscrewed a bit from the screw hole 19 in order not to stick into the center hole 18. Therefore, if the loose top 2 is turned the right way, the centering pin 30 can freely be inserted into the center hole until the rear end surface 12 of the loose top is pressed against the bottom surface 17 of the jaw. In the next step, the screw 22 is tightened, a slight rotary motion (e.g. 2 to 5°) being applied to the loose top, as mentioned previously, which ensures that the two side contact surfaces 27 of the loose top are pressed against the internal support surfaces 29 of the drivers 16a, 16b. Simultaneously, the front, conical surface 26 of the screw 22 is pressed against the rear stop surface 46b in the seat 44. In such a way, also a (positive) axial force is applied to the loose top, by which the end surface 12 thereof in a mechanical way is pressed against the bottom surface 17 of the jaw. In this state, the loose top is locked in an operative state. During drilling, the torque applied to the loose top via the drivers 16a, 16b is strong enough to hold the drivers pressed against the side contact surfaces 27 without the help of the screw. In addition to preventing retraction of the loose top from the jaw (e.g. in connection with retraction of the drill from a drilled hole), accordingly the screw has foremost the purpose of initially clamping the loose top so that the loose top is not moved from its position in connection with the entering of a workpiece.

[0044] When the loose top 2 is to be replaced, the centering pin 30 thereof can easily be pulled out of the center hole 18 after the simple measure of unscrewing the screw 22 a bit from the screw hole 19. Thus, the retraction offers no frictional resistance because the cross-sectional area of the center hole 18 is larger than the cross-sectional area of the centering pin 30.

[0045] Should the operator in connection with mounting peradventure hold the loose top turned the wrong way, i.e., with the back surface 39 facing the support surface 33 in the center hole, instead of facing the clearance surface 35, any attempt to introduce the pin even a short distance into the hole is made impossible.

[0046] In addition to the invention obviating the risk of incorrect mounting of the loose top in a foolproof way, the same affords the advantage of a very meticulous centering of the loose top in relation to the basic body at the same time as mounting as well as dismounting of the loose top can be carried out in a fast and easy way. Another advantage of the invention, as a consequence of the simple axial locking of the loose top by the radial screw, is that the side contact surfaces of the loose top as well as the co-operating support surfaces of the insides of the drivers can be manufactured at high accuracy and by simple means. In particular, the receiving jaw in the basic body can be produced in a single simple working operation, e.g. by a slitting cutter or a grinding disc. In addition, the centering pin of the loose top can be made with extra great strength because much material (cemented carbide) can be incorporated into the same, more precisely as a consequence of the clearance surface thereof being accommodated in a clearance space situated radially outside the inscribed circle of the support surface (contrary to a genuinely cylindrical center hole, which necessitates a reduction of the cross-sectional area of the centering pin).

[0047] Reference is now made to FIG. 11, which illustrates a basic body 1 the center hole 18 of which has a design other than the drip-shaped center hole in the previously described embodiment. Thus, in this case, two plane part surfaces 36b, 36c, which are included in the clearance surface 35 together with a concave, cylindrical part surface 36a, are mutually parallel. This means that the center hole 18 obtains an oval cross-sectional shape and that not only the support surface 33 but also the part surface 36a becomes genuinely semi-cylindrical. The centering pin 30 outlined by dashed lines has, however, still the drip-like cross-sectional shape described previously. The oval cross-sectional shape of the hole 18 means in practice that the crescent-shaped gap between the internal clearance surface 35 and the external clearance surface of the centering pin 30 becomes larger than in the preceding case.

[0048] It is possible to modify the described tool within the scope of the invention. For instance, the loose top may be formed with a particular coupling part between the centering pin and the front head in which the cutting edges are included. Furthermore, the screw hole may be placed at an acute instead of a right angle to the center axis of the basic body, more precisely in a direction obliquely rearward/inward from the envelope surface in order to accentuate the axial force component applied to the centering pin by the screw. The front end of the tightening screw may also be given other shapes than the shown one, e.g. pointed, at the same time as the seat in the centering pin is modified to co-operate with the screw in an efficient way. For instance, the rear stop surface could be made concave instead of plane. As pointed out previously, the contact surface of the centering pin may further be given other shapes than cylindrical, e.g. elliptical. The clearance surfaces of the centering pin as well as of the center hole may also be given varying shapes.

[0049] While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims and their equivalents thereof. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.