Endmill and cutting method

Abstract: An endmill having a shank (1) and a milling head (6) having cutting teeth continuous from a peripheral surface toward a forward tip face, wherein the endmill has a through hole (10) extending from the forward tip face of the head (6) to the shank (1) past the head (6) along a rotation axis of the endmill. The head (6) and shank (1) comprise separate component parts and the head (6) is mounted on the shank (1) by inserting a screw member (13) from the tip end face into the hole (10) and fixing the screw member (13) to the shank (1).

Description: The present invention relates to an endmill according to the preamble of claim 1 and a cutting method using the endmill and, in particular, a cutting method using a ball endmill and radius endmill. In general, an endmill is mounted on a main spindle of a NC machining system such as a machining center, and used to create a surface having a three-dimensional complicated shape such as a surface of a metal mold. The endmill is one kind of a milling cutter. Of the endmill, a ball endmill and radius endmill have teeth at a forward tip end in a manner to be continuous from an outer peripheral surface thereof to a tip end and that the tip end also serves a cutting end. The endmill of such kinds are broadly classified into a solid ball endmill comprised of a single unitary tool of a shank and teeth and a tip-fixed type tool with a teeth-constituting tip brazed or bolted to a tool body. However, conventional ball endmill or radius endmill has problems as will be set out below. First, since the cutting speed becomes a substantially zero to a very low speed in the neighborhood of a rotation center of the tip end, there is a risk that no better cutting will be done and a work surface be scraped or peeled. Further, the pitch of the cutting teeth is narrowed in the neighborhood of the rotation center of the tip end and there is a risk that no better removal of chips will be done and a work surface be damaged. Further, the solid endmill is expensive because this tool as a whole is made of a rare material. And there is room for improvement in the savings of materials used. Still further, in a tip-fixed type endmill, for example, plural tips are mounted on the tool body and rotated at high speeds, thus resulting in a variation in the mounting accuracy of the tip end, as well as in the shaping accuracy of the tip itself, exerting a greater adverse effect on the machining accuracy. The tip-fixed type ball endmill is larger in size and there is a limitation on the rotation speed and hence an inherent limitation on a range in which cutting is made. DE 195 11 551 A1 discloses an endmill according to the preamble of claim 1. The present invention is made so as to eliminate drawbacks of a conventional endmill. This object is achieved by an endmill according to Claim 1 and a method according to Claim 12. Further developments of the invention are given in the dependent claims. Other features and advantages will be well understood by referring to the embodiments one to eight, which will be explained in more detail below as examples of an endmill useful for understanding the present invention, of which the sixth embodiment is an embodiment of the present invention. The invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a front view, partly in cross-section, showing a first embodiment; FIG. 2 is a side view showing the first embodiment; FIGS. 3A to 3C, each, show a practical form of a head of the embodiment; FIG. 4 is an explanative view showing a state in which the embodiment of the present invention is used; FIGS. 5A and 5B, each, show a practical form of a head in a second embodiment; FIG. 6 is a front view showing a third embodiment; FIG. 7 is a side view showing a head in a fourth embodiment ; FIG. 8 is a cross-sectional view showing a head in a fifth embodiment; FIG. 9 is a cross-sectional view showing a head in a sixth embodiment showing an embodiment of the present invention; FIG. 10 is a front view, partly in cross-section, showing a seventh embodiment; FIG. 11 is a front view, partly in section, showing an eighth embodiment; and FIG. 12 is a front view showing a variant of the head. A first embodiment will be described below. A ball endmill as shown in FIG. 1 comprises a cylindrical shank 1 detachably mounted on a main spindle of a machining center for example and a milling head 6 mounted on a forward end of the shank 1. FIGS. 1 and 2, each, show a combined state of the head 6 and shank 1 and FIGS. 3A to 3C show the head 6 only. As shown in FIGS. 1 and 2, the shank 1 has an axially penetrating coolant passageway 2 at proper two places in its diameter direction. Further, a cotter seat 3 and cylindrical recess 4 are provided in a forward end face of the shank 1 so as to align with the head 6 which is mounted on the shank 1. As indicated by dotted lines in FIG. 2, the cotter seat 3 is provided in the diameter direction of the shank member 1 and has a predetermined width. By the cotter seat 3, the shank member 1 and head 6 are prevented from being rotated relative to each other. The cylindrical recess 4 is provided coaxial with the center axis of the shank 1 and defines a circular recess smaller in diameter than the cotter seat 3 and deeper than the cotter seat 3. The cylindrical recess 4 aligns the axis of the head 6 with that of the shank 1. A screw hole 5 is opened in a bottom surface wall of the cylindrical recess 4 and extends along the rotation center axis of the shank 1. The shank 1 is made of, for example, a quenchable steel series alloy such as a SCM440 (according to the Japanese Industrial standard) steel. The head 6 is comprised of an integral component part of an ultra-hard tool material such as a cemented carbide alloy. As shown in FIGS. 3A and 3B, the head 6 has a plurality of ball teeth 7 provided at a predetermined pitch along a circumferential direction. The ball teeth 7 are of a straight type, not of a twisted type and, as shown in FIG. 3C, such that they describe a circular arc with a predetermined radius around a center point A on a rotation center axis. Further, as shown in FIG. 3C, a stepped bolt hole 10 is formed on the rotation center axis of the head 6 with its large-diameter section 10a side opened at its tip end face of the ball teeth 7 side. The diameter of the large-diameter section 10a is so determined that an angle a between an edge B of the large-diameter section 10a and the center point A on the rotation center axis of the ball teeth is 10° to 90°, more preferably 20° to 45°. In the present embodiment, the angle α is so formed as to define an angle of 41°. Further, a cotter 8 fitted in the cotter seat 3 of the shank 1 and a projection 9 of the head 6 fitted in the cylindrical recess 4 of the shank 1 are provided on a back side of the head 6 to align the head 6 with the shank 1. Coolant holes 16 are provided in the head 6 to communicate with the coolant passageways 2 in the shank 1. The coolant holes 16 are opened in tip pockets 12 defined between the ball teeth 7. When the head 6 is to be mounted to the shank 1, the projection 9 of the head 6 is aligned with the recess 4 in the forward end portion of the shank 1, and the head 6 and shank 1 are relatively rotated to position the cotter 8 relative to the cotter seat 3. And, in this state, the head 6 is fitted to the shank 1. The projection 9 and recess 4 may have a short taper. After, in this way, the head 6 is exactly aligned with, and fitted to, the shank 1, a hexagonal head-equipped bolt 13 is inserted via the large-diameter section 10a of the bolt hole 10 in the head 6 into the screw hole 5 in the shank 1 as shown in FIG. 1. In a state in which the head 6 has been completely threaded into the shank 1, the head 13a of the bolt 13 is placed in a retracted state at the bolt hole 10 of the head 6 as shown in FIG. 1, that is, in a state in which the head 13a of the bolt 13 is completely sunken within the head 6 of the ball endmill. It is to be noted that the bolt 13 is threaded in a direction to be tightened by the rotation of the ball endmill. In the case where, for example, the ball endmill is rotated in a right-handed direction, then use is made of a right-handed screw. In use, the ball endmill thus structured has a rear end portion of its shank 1 fixed to the main spindle of, for example, a machining center (3-, 4- or 5-axis controlled machining center). FIG. 4 shows a state in which cutting is made using this ball endmill. In the case where cutting is made on the machining center, programming is so made as to have the center axis of the ball endmill tilted at an angle of β relative to a cutting face including a cutting point Q. In this Figure, by utilizing an imaginary apex P on the tip end of the ball endmill from the standpoint of a convenience, that angle β between a normal line at the apex P and the center axis of the ball endmill is set at an angle of above 1/2 the above-mentioned angle α, that is, at an angle of about 5° ≦ β ≦ 90°, desirably an angle of 10° ≦ β ≦ 90°. In this embodiment, that tilt angle used is 10°. It is to be noted that a NC program may be run using the cutting point Q in place of the crosspoint P. The difference between these points is slight and there is no substantial inconvenience in using either one of these. One of the advantages of the ball endmill thus constructed lies in that a dent (10a) is provided near the central area of a tip end of the head 6 which involves a less cutting power and the bolt 13 can be fixed using that area. That is, the shank member 1 and head 6 can be made as separate component parts and can be fixed to each other by a single bolt 13. Since, by doing so, only the head 1 can be made of an expensive material, a yield in material is improved. Further, the head 6 can be fixed to the shank 1 by the single bolt 13 provided at the rotation center axis and thus it is possible to obtain a ball endmill of a smaller diameter. This ensures a better balance of the tool and hence a high-speed cutting. In a conventional ball endmill, it is most difficult to manufacture a ball teeth near a rotation center. In the present invention, a space is created at that control area of the tool and it is easier to form ball teeth 7. Moreover, many ball teeth 7 can be provided in a denser, mutually closer way, thus improving a cutting performance. Further, by providing the coolant passageways 2 and 16 in the ball endmill, it is possible to directly supply a coolant from the inside of the tool to the cutting edges and, in comparison with an ordinary structure supplying a coolant from an outside, a prominent advantage can be exhibited from the ball endmill of the present invention. Although, in the above-mentioned embodiment, six straight edges are provided as the ball teeth 7, twisted ones may be used instead and the number of teeth used is not restricted thereto. Although, in the above-mentioned embodiment, as the ultra-hard tool material use is made of the cemented carbide alloy, use may be made of a cement, ceramics and covered ultra-hard alloy. FIGS. 5A and 5B show a second embodiment with the same reference numerals employed to designate parts or elements corresponding to those in FIGS. 3A to 3C and an explanation made only on those different parts or elements. In the second embodiment, a head 6 made of an ultra-hard tool material is of such a type that a stepped cut section is provided at those areas corresponding to ball teeth 7 and, in this case, an ultra-high pressure sintered tip 14 such as a sintered diamond is joined to the stepped cut section to provide ball teeth 11. If the ball teeth is of such a structure, it is possible to highly efficiently use them on a high-speed cutting area where the characteristic of the ultra-high sintered tip end 14 is utilized. FIG. 6 shows a third embodiment with the same reference numerals employed to designate parts or elements corresponding to those in FIG. 1 and an explanation made only on those different parts or elements. In the third embodiment, a diameter d of a shank 1 is so formed as to be smaller than a diameter D of a head 6 as shown in FIG. 6. This structure ensures less risk of interference between a work and a shank 1 and provides an easier-to-use ball endmill. FIG. 7 shows a fourth embodiment with the same reference numerals employed to designate parts or elements corresponding to those in FIG. 2 and an explanation made only on those different parts or elements. In the fourth embodiment, a ball endmill is of such a type that a coolant passageway 17 penetrates a head 6, hexagonal head-equipped bolt 13 and axial center of a shank 1. This structure ensures a better rotation balance of a resultant tool and hence a suitable application for use at high-speed rotations. FIG. 8 shows a fifth embodiment with the same reference numerals employed to designate parts or elements corresponding to those shown in FIG. 3C and an explanation given only on those different parts or elements. In the fifth embodiment, a ball endmill is of such a type that the projection 9 of the head 6 and recess 4 of the shank member 1 in the first embodiment are replaced, respectively, by a recess 9' of a head and projection (not shown) of a shank 1. Further, the fifth embodiment is not equipped with a cotter and cotter seat. The head member 6, being so structured, becomes a compact, lightweight unit and a rare material of which the head is made can be more saved than otherwise. FIG. 9 shows a sixth embodiment, which is an embodiment of the present invention. In FIG. 9, the same reference numerals are employed to designate parts or elements corresponding to those shown in FIG. 3C and, for brevity in explanation, any further explanation is omitted except for different parts or elements. A head of the sixth embodiment is so formed as to have a substantially spherical shape. The head has mutually continuous ball teeth 7a and 7b at opposite ends. A bolt hole 10 has large-diameter sections 10a and 10b at its opposite ends. One of these large-diameter sections 10b serves the same function as that of the recess 9' of FIG. 8. According to such a structure, the mutually continuous ball teeth 7a and 7b can both be mounted to the shank member 1 and cutting can be made at either side. Further, it is desired that the diameter of the shank 1 with the head 6 mounted thereon be made smaller than the outer diameter of the head 6 to allow the ball teeth 7a (7b) to be used in a broader range. FIG. 10 shows a seventh embodiment. In FIG. 10, the same reference numerals are employed to designate parts or elements corresponding to those shown in FIG. 1 and any further explanation is omitted, for brevity's sake, except for different parts or elements. In the seventh embodiment, use is made of a bolt 13' having a length corresponding to a substantially whole length of a shank member 1. Further, the shank 1 has a through hole 5 over its full length and an internal thread section 15 is provided in the other end portion of the through hole 5 with the bolt 13' threaded therein. And the bolt 13' is inserted from the forward end of the head 6 and threaded into the internal thread section 15 in the other end portion of the shank 1 to fix the head 6 to the shank 1. A coolant passageway 2' is provided in the bolt 13', and through the coolant passageway 2', a coolant is supplied to a cutting face. According to this structure, by fastening the bolt 13' into the internal thread section, the forward end of the head 6 and other end portion (15) of the shank 1 are pulled toward each other in a mutually tensioned direction whereby a compression force acts on the ball endmill along an axial direction to provide improved rigidity to the ball endmill. Note that, even where the endmill is a type having an integral unit of a shank 1' and head 6' (as shown in FIG. 11), the use of the bolt 13' provides improved rigidity to the ball endmill. In such structure, a coolant can be supplied to a head 6 in the same way as in the fourth embodiment and it is possible to obtain a ball endmill assuring a well-balanced rotation. FIG. 11 shows an eighth embodiment. A ball endmill of this embodiment has an integral unit of a shank 1' and head 6' having a coolant passageway 2" extending therethrough along its rotation axis. The present invention is not restricted to the above-mentioned sixth embodiment and various changes or modifications can be made without departing from the scope of the present invention. For example, cutting teeth 7 of the head 6 need not be so defined as to have a circular arc with a point A as an origin as shown in FIG. 3C and may be of such a type as to have circular arcs one with a point A1 as an origin and one with a point A2 as an origin as shown in FIG. 12. This type of endmill having cutting teeth 7' as shown in FIG. 12 is referred to as a radius endmill.

Claims: An endmill having a shank (1) and a milling head (6) having cutting teeth (7a, 7b), the endmill having a through hole (2, 2", 10) extending from the forward tip face of the head (6) to the shank (1) past the head (6) along a rotation axis of the endmill, the head (6) and shank (1) comprising separate component parts and the head (6) being mounted on the shank (1) by inserting a screw member (13) from the tip end face into the hole (10) and fixing the screw member (13) to the shank (1), wherein a stepped bolt hole (10) is formed on the rotation center axis of the head (6), wherein the cutting teeth (7a) are continuous from a peripheral surface along a circular arc toward a forward tip face and the cutting teeth (7b) are continuous from the outer peripheral surface to a rear end face, characterized in that the head (6) has the forward end face side and rear end face side formed in a symmetrical way to allow either of cutting teeth (7a) on the forward end face side and cutting teeth (7b) on the rear end face side to be used. An endmill according to claim 1, characterized in that the screw member (13) is completely buried in the head. An endmill according to claim 1 or 2, characterized in that the head (6) has a means (8, 9) for preventing any displacement from the shank (1). An endmill according to one of claims 1 to 3, characterized in that the shank (1) has a diameter smaller than that of the head (6). An endmill according to one of claims 1 to 4, characterized in that the hole (2', 2") extends through the shank (1). An endmill according to claim 5, characterized in that the hole has a coolant passageway (2) for discharging a coolant An endmill according to claim 6, characterized in that a coolant discharge passageway (2) is provided in the head (6) and shank (1) to allow a coolant to be discharged from between the cutting teeth. An endmill according to claim 6, characterized in that the screw member (13) has a discharge hole (16, 2') for discharging a coolant. An endmill according to one of claims 1 to 8, characterized in that the cutting teeth (7a, 7b) are so shaped as to define the circular arc of a predetermined curvature radius about any given center point (A, Al, A2) in the head (6). An endmill according to claim 9, characterized in that the point (A) is located on the rotation axis of the endmill and a sector angle a connecting an edge (B) of the through hole (10) at the forward end side of the head (1) to the center point (A) of the cutting teeth (7a, 7b) is 10° ≤ α ≤ 90°. An endmill according to one of claims 1 to 10, characterized in that the cutting teeth (7a, 7b) have a tip of an ultra-hard sintered body. A method for cutting a work with the use of an endmill according to one of claims 1 to 11, by cutting a work surface in a manner to tilt an axis of the endmill at a predetermined angle (B) to the work surface.

Inventors: KISHITA SHOICHI, SHIKATA HIROSHI, UJIIE NOBUHISA, KISHITA, SHOICHI, SHIKATA, HIROSHI, UJIIE, NOBUHISA
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1999
Main classification: B23C5
IPC codes: B23C5