Title of Invention | MODULAR CUTTING TOOL ASSEMBLY |
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Abstract | A modular cutting tool assembly includes a tool holder having at least one insert receiving pocket, and first and second cutting inserts alternately receivable within the pocket. The fIrst cutting insert has a peripheral flank surface configured to provide abutment features with mxn-fold rotational symmetry about the insert's central axis so that it can be restrained against the lateral support surfaces in any of mxn angular positions, for at least one value of each of n and m where n ~ 3 and m 2.. 2. The second cutting insert has a peripheral flank surface configured to provide reduced-symmetry abutment features with only n-fold rotational symmetry so that the second cutting insert can be restrained against the lateral support surfaces in only n angular positions. The peripheral flank surface of at least the fIrst cutting insert is configured such that a lower cross-section taken through the lower part of the fIrst cutting insert perpendicular to the central axis has a recessed form. |
Full Text | The present invention relates to cutting tools and, in particular, it concerns a tool holder and a modular cutting tool assembly in which cutting inserts with different orders of rotational symmetry can be used alternatively with a single tool holder. It is known to employ cutting inserts made of various hard materials which have round, polygonal, or otherwise rotationally symmetric cutting edges mounted in a receiving pocket of a tool holder of a cutting tool. During a machining operation (turning, milling etc.), only a portion of the available cutting edge typically actually cuts the workpiece. The extent of this portion depends on the depth of cut. When a portion of an indexable cutting edge of a regular (e.g., polygonal) insert has been worked, the insert is indexed to present a whole new cutting edge. In the case of cutting inserts with polygonal cutting edges, proper operation can only be achieved by rotationally securing the insert within the pocket so as to withstand the cutting forces generated during use. Conventional teaching has relied on abutment surfaces resulting from the inherent polygonal shape of the cutting insert. However, especially in high torque applications, or in polygons with large numbers of sides, the inherent abutment geometry may prove inadequate to withstand torques which may result from cutting forces. Cutting inserts with round cutting edges exhibit rotational symmetry and could theoretically be continuously indexable in any angular position to accommodate actually observed wear. In practice, however, round inserts have also become restricted to use with a discrete number of indexing stations, angularly spaced by an equal shift angle. The angular shift between the indexing stations is referred to as the "angular pitch1" of the insert. In order to make optimal use of a round cutting insert, it is desirable to prevent rotation of the insert from its indexed position during the cutting process, thereby limiting the wear to a defined portion of the cutting edge. This ensures that the portion of the cutting edge presented after repositioning is, in fact, un-used. A number of designs have been suggested for anchoring round inserts against rotation. These designs may be subdivided into two types, namely, ""seat-pinning" and "lateral abutment". Examples of a seat-pinning design may be found in U.S. Patent No. 5,296,288 to Flueckiger and European Patent Publication No. 300,172 to Stashko. The lateral abutment design is exemplified by U.S. Patent No. 3,346,336 to Rescigno. All of these designs suffer from limited lorque-resisting capability due to the poor size, location and orientation of the contact surfaces. As will be clear from the above discussion, the requirements for effective restraining of a cutting insert are typically highly specific to the geometry of the cutting edge. Thus each type of insert, triangular, rectangular, square, or round, is provided with a dedicated tool holder design with a corresponding pocket shape. As a result, the use of multiple insert types is accompanied by the expense of providing and storing multiple tool holders, and necessitates the additional labor of exchanging the entire tool between cutting operations. In an attempt to reduce these costs and labor, modular systems based on replaceable cartridges have been developed. Examples of such systems include the "Modulmill" system commercially available from SANDVIK Coromant and the "NOVEX F 2010" system commercially available from Montanwerke Walter GmbH. These systems employ replaceable adapter cartridges each of which provides a pocket with clamping geometry suited to a specific cutting insert. Although these systems do allow the primary tool holder to be used with different cutting insert types, the replacement of the cartridges still adds a wasteful additional dissembly/assembly step to the changeover. There is therefore a need for a modular cutting tool assembly which allows alternate use of multiple cutting inserts with differing numbers of indexing positions within a single tool holder pocket. There is also a need for cutting inserts and tool holders for use in such an assembly. SUMMARY OF THE INVENTION The present invention provides a modular cutting tool assembly in which cutting inserts with various differing numbers of indexing stations or differing cutting geometries can be used alternately in a generic tool holder pocket structure. According to one aspect of the present invention, the insert-receiving pocket is structured to provide circumscribing "three-point" locating and support features. According to a further aspect of the invention, a reduced-symmetry cutting insert is provided with protruding features which inhibit erroneous indexing of the insert in an improper angular position. The indexable inserts of the present invention may form part of a set of coordinated interchangeable inserts for use with a tool holder of a single size and shape. The inserts and receiving pockets of the invention are of great value in many applications including, but not limited to, milling cutters, broaches, turning tools and the like. In one embodiment, the present invention provides an indexable insert which has a plurality of discrete abutment surfaces, three of which are used at a time. According to the teachings of the present invention there is provided, a modular cutting tool assembly comprising: (a) a tool holder having at least one insert receiving pocket with a base and a plurality of lateral support surfaces; (b) a first cutting insert receivable within the pocket, the first cutting insert having an upper surface, a bottom surface and a peripheral flank surface, the peripheral flank surface being configured to provide abutment features with mxn-fold rotational symmetry about a central axis of the first cutting insert such that the first cutting insert can be restrained against the lateral support surfaces in any of mxn angular positions, for at least one value of each of n and m where n > 3 and m>.2\ and (c) a second cutting insert receivable within the pocket, the second cutting insert having an upper surface, a bottom surface and a peripheral flank surface, the peripheral flank surface being configured to provide reduced-symmetry abutment features with only n-fold rotational symmetry about a central axis of the second cutting insert such that the second cutting insert can be restrained against the lateral support surfaces in only n angular positions, wherein the peripheral flank surface of the first cutting insert is configured such that a lower cross-section taken through the lower part of the first cutting insert perpendicular to the central axis exhibits a recessed form. According to a further feature of the present invention, the peripheral flank surface of the first cutting insert is configured such that an upper cross-section through the first cutting insert perpendicular to the central axis and proximal to the upper surface exhibits a non-recessed form. According to a further feature of the present invention, the upper cross-section corresponds to a substantially regular polygon with mxn sides. According to a further feature of the present invention, the upper cross-section is substantially circular. According to a further feature of the present invention, the peripheral flank surface of the second cutting insert is shaped such that a lower cross-section taken through the lower part of the second cutting insert perpendicular to the central axis at a point closer to the bottom surface exhibits a recessed form. There is also provided according to the teachings of the present invention, a modular cutting tool assembly comprising: (a) a tool holder having at least one insert receiving pocket with a base and a plurality of lateral support surfaces, reference being made to a feed direction defined relative to the lateral support surfaces; (b) a first cutting insert receivable within the pocket, the first cutting insert having an upper surface bounded by a cutting edge, a bottom surface and a peripheral flank surface, the peripheral flank surface being configured to provide abutment features with «-fold rotational symmetry about a central axis of the first cutting insert such that the first cutting insert can be restrained against the lateral support surfaces in n angular positions, for at least one value of n where n > 3, the first cutting insert being configured such that, when mounted within the pocket, the first cutting insert presents a primary operative portion of the cutting edge at a first entering angle to the feed direction; and (c) a second cutting insert receivable within the pocket, the second cutting insert having an upper surface bounded by a cutting edge, a bottom surface and a peripheral flank surface, the peripheral flank surface being configured to provide abutment features with /7-fold rotational symmetry about a central axis of the second cutting insert such that the second cutting insert can be restrained against the lateral support surfaces in n angular positions, the second cutting insert being configured such that, when mounted within the pocket, the second cutting insert presents a primary operative portion of the cutting edge at a second entering angle to the feed direction, the second entering angle being different from the first entering angle. According to a further feature of the present invention, the first entering angle differs from the second entering angle by about 180°/n. There is also provided according to the teachings of the present invention, a too! holder for alternately receiving a first cutting insert indexable in exactly n rotational positions and a second cutting insert indexable in mxn rotational positions, for at least one value of each of n and m where n > 3 and m > 2, the tool holder comprising at least one insert receiving pocket defined with reference to an axis passing through the pocket with which a central axis of the cutting inserts is to be aligned, the pocket having: (a) a base for supporting the cutting inserts; (b) first, second and third lateral support surfaces angularly spaced around, and substantially equidistant from, the axis; (c) a first lateral hollow located between the first and second lateral support surfaces; and (d) a second lateral hollow located between the second and third lateral support surfaces, wherein the first and second lateral hollows are formed such that geometrical mappings of the first and second lateral support surfaces by rotation through 360°/(mx«) about the axis lie within the first and second lateral hollows, respectively. According to a further feature of the present invention, the first, second and third lateral support surfaces define, respectively, first, second and third planes, the first plane being related to the second plane by a rotation through 360°//j about the axis. According to a further feature of the present invention, the second and third planes of the pocket are substantially coplanar. According to a further feature of the present invention, n = 4 and m = 2. There is also provided according to the teachings of the present invention, a cutting insert having «-fold rotational symmetry for some value of n where n>3, the cutting insert being indexable exclusively at n indexing stations within an insert receiving pocket which is configured to receive alternately cutting inserts having both n-fold rotational symmetry and mxrc-fold rotational symmetry for some value of in where m > 2, the cutting insert comprising a unitary structure having an upper surface bounded by a cutting edge, a base, a peripheral flank surface and a central axis, wherein the peripheral flank surface is shaped such that an outline of a first cross-section taken through the lower part of the cutting insert perpendicular to the central axis exhibits a recessed form including: (a) n corner portions angularly spaced around the central axis, the corner portions corresponding to /i-fold rotationally symmetric lateral abutment features; and (b) support protrusions protruding outwards from the peripheral flank surface between the corner portions, wherein the support protrusions are shaped such that, under any geometrical mapping of an outline of the comer portions by rotation through an angle of less than 3607n about the central axis, the support protrusions extend beyond the outline. According to a further feature of the present invention, the peripheral flank surface is further shaped such that an outline of a second cross-section taken through the cutting insert perpendicular to the central axis and proximal to the upper surface exhibits a non-recessed form. According to a further feature of the present invention, the comer portions correspond substantially to comer regions of a regular polygon of n sides. Accordingly the present invention provides a tool holder for alternately receiving a first cutting insert indexable in exactly n rotational positions and a second cutting insert indexable in mxn rotational positions, for at least one value of each of n and m where n ^ 3 and m^.2, the tool holder comprising at least one insert receiving pocket defined with reference to an axis passing through said pocket with which a central axis of the cutting inserts is to be aligned, said pocket having a base for supporting the cutting inserts; first, second and third lateral support surfaces angularly spaced around, and substantially equidistant from, said axis; a first lateral hollow located between said first and second lateral support surfaces; and a second lateral hollow located between said second and third lateral support surfaces, wherein said first and second lateral hollows are formed such that geometrical mappings of said first and second lateral support surfaces by rotation through 360("/(mxn) about said axis lie within said first and second lateral hollows, respectively. BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1A is a perspective view of a first preferred cutting insert, constructed and operative according to the teachings of the present invention, having a round cutting edge; FIG. IB is a side view of the cutting insert of Figure 1 A; FIG. 1C is a cross-sectional upward view taken along the line 1-1 of 20 Figure IB; FIG. ID is a side cross-sectional view taken along the line 11-11 of Figure 1C; FIG. IE is a top view of the cutting insert of Figure 1 A; FIG. 2A is a schematic representation of a geometric generating segment according to the teachings of the present invention showing the definitions of certain angles to be referred to in the description; FIG. 2B is a schematic representation of the geometry of a cross-section through a cutting insert constructed and operative according to the teachings of i the present invention, the geometry being produced by rotational copying of the generating segment of Figure 2A; FIG. 3 is a partially dissembled perspective view of a machine tool cutter, constructed and operative according to the teachings of the present invention, including four of the cutting inserts of Figure 1A mounted in pockets of a tool holder; FIG. 4 is an enlarged perspective view of one of the pockets of the tool holder of Figure 3; FIG. 5 is a horizontal cross-sectional view showing the abutment geometry of the cutting insert of Figure 1A in the pocket of Figure 4; FIG. 6 is a partial vertical cross-sectional view taken along the line VI-VI of Figure 5 showing a preferred geometry of contact between the cutting insert of Figure 1A and one of the support surfaces of the pocket of Figure 4; FIGS. 7A-7E are views similar to those of Figures I A-IE, respectively, illustrating a second preferred cutting insert, constructed and operative according to the teachings of the present invention, having a polygonal cutting edge; FIG. 8A is a perspective view of a third preferred cutting insert, constructed and operative according to the teachings of the present invention, mountable within the pocket of Figure 4 in a reduced number of indexing positions; FIG. 8B is a side view of the cutting insert of Figure 8A; FIG. 8C is a side cross-sectional view of the cutting insert of Figure 8A; FIG. 8D is a top view of the cutting insert of Figure 8A; FIG. 8E is a bottom view of the cutting insert of Figure 8A with added construction lines to emphasize the geometry of lateral abutment surfaces of the insert; FIG. 8F is a view similar to Figure 8E with added construction lines to compare the geometry of lateral support protrusions of the insert with the high rotational symmetry features of the inserts of Figures 1 and 7; FIG. 9A is a perspective view of a fourth preferred cutting insert, constructed and operative according to the teachings of the present invention, mountable within the pocket of Figure 4 in a reduced number of indexing positions; FIG. 9B is a side view of the cutting insert of Figure 9A; FIG. 9C is a side cross-sectional view of the cutting insert of Figure 9A; FIG. 9D is a top view of the cutting insert of Figure_9A; FIG. 9E is a bottom view of the cutting insert of Figure 9A with added construction lines to emphasize the geometry of lateral abutment surfaces of the insert; FIG. 9F is a view similar to Figure 9E with added construction lines to compare the geometry of lateral support protrusions of the insert with the high rotational symmetry features of the inserts of Figures 1 and 7; FIGS. 10 and 11 are views similar to Figure 5 showing the abutment geometry and entering angle for the inserts of Figures 8A and 9A, respectively, when correctly mounted in the pocket of Figure 4; FIGS. 12 and 13 are views similar to Figures 10 and 11, respectively, showing the effect of attempts to position the inserts of Figures 8A and 9A incorrectly within the pocket of Figure 4; FIG. 14 is a bottom view of a variation of the cutting insert of Figure 8A having a recessed cutting geometry; FIG. 15 is a schematic horizontal cross-sectional view showing the abutment geometry of a set of cutting inserts with triangular and hexagonal symmetry; and FIG. 16 is a schematic horizontal cross-sectional view showing the abutment geometry of a set of cutting inserts with pentagonal and decagonal symmetry. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a modular cutting tool assembly employing round and polygonal cutting inserts with peripheral flank surfaces shaped to provide abutment surfaces for opposing high torques, and corresponding insert-receiving pockets of tool holders. The geometry of the abutment surfaces enables the use of sets of inserts with differing numbers of indexing positions and differing entering angles within a given pocket geometry. The principles and operation of cutting tool assemblies, and their component cutting inserts and tool holders according to the present invention may be better understood with reference to the drawings and the accompanying description. Referring now to the drawings, Figures 1A-1E show a first cutting insert, generally designated 10, constructed and operative according to the teachings of the present invention. In general terms, cutting insert 10 has an upper surface 12 bounded by a cutting edge 14, a base 16, and a peripheral flank surface 18. The shape of peripheral flank surface 18 varies along the height of cutting insert 10. Near the top of peripheral flank surface 18, it conforms to the round or polygonal geometry of the cutting edge so as to provide support for cutting edge 14. Lower down cutting insert 10, peripheral flank surface 18 is shaped to provide transverse abutment surfaces, preferably with recessed features such as channels, and in certain cases, as chevron-type channels. This latter possibility leads to a lower cross-sectional geometry which approximates to an n-pointed star. The phrase "w-pointed star" as used in the specification and claims refers to a rotationally symmetric shape with n outermost points each of which is joined to its nearest neighbors by an inward pointing chevron or "V-shape". An example of a 5-pointed star is shown in Figure 2B. It should be noted that the lower peripheral flank surfaces of certain preferred cutting inserts of the present invention are described as having "approximately" or "substantially" n-pointed star geometry since the points and/or the chevron bases may be flattened, as will be described in more detail below. It should be appreciated that the present invention is applicable to a wide range of cutting inserts with rotationally-symmetric cutting edges. The term "rotationally-symmetric" is used herein in the description and claims to refer to shapes which are invariant under rotation through an angle of 3607« where n is at least three. The term so defined includes regular polygons of three or more sides, whether straight-sided or made up of more complex combinations of straight or curved line segments. The term also includes circular shapes. Also included are variations on the above-mentioned shapes in which a repetitive pattern is superimposed over the basic shape. Examples of such patterns include serrations, and scalloped or wavy cutting edges. It will be noted that the symmetry of the inserts of the present invention, unless otherwise stated, is taken to refer to the symmetry of the abutment features. The cutting edge may exhibit equivalent or higher degrees of symmetry than the abutment surfaces. It should also be appreciated that neither rotational symmetry nor n-pointed star geometry imply symmetry under reflection. In cases in which a specific cutting application generates torque primarily in one direction, the structures of the present invention may be constructed asymmetrically with abutment surfaces for opposing the primary torque component. However, the invention will be illustrated by way of example in the context of symmetrical embodiments for opposing torques in both senses about the central axis. Reference will be made throughout the specification and claims to a central axis of an insert. The axis in question is the axis about which the cutting edge exhibits rotational symmetry. Reference is also made to the "top" and "bottom" of the insert. Wherever such references appear, the insert is assumed to be mounted with its base downwards so as to present its cutting edge upwards and with its axis vertical. The abutment surface geometry permits the use of these lower symmetry inserts within pockets also designed to receive higher symmetry cutting inserts, as will be described in more detail below. Reference is made in the description and claims to recessed and non-recessed shapes. It should be understood that the term "recess" is used in the description and claims to refer to a part of a shape or surface which is concave or otherwise generates a hollow. In more precise terms, a recess may be identified as any part of a shape or surface which lies interior to a virtual straight line connecting adjacent parts of the shape or surface. Conversely, a shape which at no point has a recess so defined is termed a "non-recessed" shape. Thus, the chevron of an /i-pointed star constitutes a "recess" as herein defined, whereas a regular polygon is classified as a non-recessed shape. Turning now to the features of cutting insert 10 in more detail, upper surface or rake 12 has, in this example, an inwardly slanted, peripheral annular portion 20, bordered at the outer perimeter thereof by cutting edge 14, and a flat, inner annular portion 22, perpendicular to the axis of cutting insert 10. Upper surface 12 may also have additional chip control features such as a pattern of indentations or ridges. One such possibility is shown in Figure IE. Again, it should be noted that the cutting insert is considered rotationally symmetric independent of the fact that the indices and various other features of the upper surface may not conform to this symmetry. As can be best seen in Figure ID, inner annular portion 22 intersects at its inner extreme with a cylindrical extension 24 of a central, standard so-called "partly cylindrical" bore 26, used for the passage of a standard counter-sunk clamping screw (not shown). It should be appreciated that the specific clamping technique employed to clamp cutting insert 10 within a tool holder does not, per se, constitute a part of the present invention, and that variations may be made in accordance with any other clamping technique without straying from the scope of the present invention. Turning now to peripheral flank surface 18, it is a particular feature of certain preferred cutting inserts of the present invention that peripheral flank surface 18 is shaped such that a first cross-section taken through the cutting insert perpendicular to the central axis and proximal to upper surface 12 is bounded by a non-recessed form, whereas a second cross-section taken through the cutting insert perpendicular to the central axis at a point closer to base 16 exhibits w-fold rotational symmetry and includes recessed features for providing abutment surfaces for opposing torque about the central axis. To this end, peripheral flank surface 18 may be sub-divided along the height of cutting insert 10 into at least two, and typically three, portions. A first portion adjacent to cutting edge 14 is denoted the relief flank surface 28 and has a non-recessed cross-sectional outline. The lower portion of peripheral flank surface 18 constitutes a ribbed flank surface 30. Ribbed flank surface 30 is preferably linked to relief flank surface 28 by a transition flank surface portion 32 which provides a gradual transition between the two forms, as will be described below. These portions provide features having abutment surfaces for opposing torque about the central axis. Relief flank surface 28 typically corresponds to the geometry of the cutting edge employed. Thus, in cutting insert 10, relief flank surface 28 has a frustro-conical form, having a circular cross-section corresponding to the circular cutting edge illustrated. Thus, relief flank surface 28 may be described as having substantially constant cross-sectional geometry perpendicular to the central axis at all heights, although the dimensions of this geometry may vary somewhat with height. Relief flank surface 28 preferably extends downwards from cutting edge 14 at least about a tenth of the height of cutting insert 10. Relief flank surface 28 is typically inclined at an acute angle i[/ relative to the central axis of cutting insert 10. Angle \\> is commonly denoted the insert"s "primary normal relief angle" and is preferably less than about 20° and typically about 7°. For a negative insert, \\i may be about 0° such that relief flank surface 28 is cylindrical. Turning now to the features of ribbed flank surface 30 in more detail, this is preferably implemented as a frustro-pyramidal ribbed surface with channels 34 formed between projecting ridges 36. Channels 34 preferably have a chevron-type cross-section. Ribbed flank surface 30 extends from base 16 to a plane perpendicular to the insert axis indicated in Figure IB as plane I-I. Ribbed flank surface 30 preferably extends for at least about an eighth, and typically for at least about a quarter, of the height of the cutting insert. Depending on the design of transition flank surface 32, ribbed flank surface 30 may extend along the majority of the height of peripheral flank surface 18. Ribbed flank surface 30 may be regarded as having "n" ridges 36 each having a crest which blends smoothly with a pair of planar, oppositely sloping, lateral abutment surfaces 38. At the junction of adjacent ridges, adjacent abutment surfaces 38 intersect at smoothed root regions 40. The number of ridges is equal to the number of available indexing positions of the insert. For a circular cutting edge, n is preferably at least 5, and typically 8 or more. Turning briefly to Figures 2A and 2B, these illustrate how the cross-section of ribbed flank surface 30, shown here with «=5 may be considered an ij-pointcd star (Figure 2B) defined by rotationally copying n-\ times a concave chevron-shaped "generating segment" (Figure 2A) having symmetric legs on either side of a bisecting radius. The generating segment subtends a central pitch angle p equal to 360°/n. As will discussed further below, it is a feature of certain preferred implementations of cutting insert 10 that two spaced-apart abutment surfaces 38 are coplanar. In a simple example where n = 8, this may be achieved by ensuring that the symmetrical legs of the generating segment form angles y relative to a bisecting radius equal to (90° - p72°). The corresponding angle a is equal to (90° - p°). Thus, in this eight corner example illustrated, a = 45°, p = 45°, and y = 67.5°. Where larger numbers of corners are employed, the coplanar surfaces may be two or more ridges apart. It is a further feature of certain preferred implementations of cutting insert 10 that the two spaced-apart abutment surfaces 38 have perpendicular intersections with a cross-section taken through cutting insert 10 perpendicular to its axis. The surfaces themselves may vary from perpendicular due to tapering of ribbed flank surface 30. However, for convenience of reference, such surfaces are referred to as "perpendicular abutment surfaces". This requirement may also be defined by simple geometrical conditions and, by way of example, is provided by the angles of the eight cornered insert listed above. Furthermore, the existence of at least some mutually perpendicular abutment surfaces follows immediately in any case in which n is a multiple of 4. In a preferred case, ribbed flank surface 30 is ideally frustro-pyramidal, meaning, among other things, that all the lateral abutment surfaces 38 form the same angle T) relative to the axis of cutting insert 10, as indicated in Figure IB. As a direct consequence, an imaginary straight line of intersection between pairs of lateral surfaces 38 associated with each ridge 36, passing through aligned corners of such ideally star-shaped polygonal contour sections as defined above, forms a corresponding angle (p with a normal to the base which is a function of both a and r|, given by: r\. |
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Patent Number | 208539 | ||||||||||||
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Indian Patent Application Number | 56/MAS/1998 | ||||||||||||
PG Journal Number | 35/2007 | ||||||||||||
Publication Date | 31-Aug-2007 | ||||||||||||
Grant Date | 02-Aug-2007 | ||||||||||||
Date of Filing | 08-Jan-1998 | ||||||||||||
Name of Patentee | M/S. ISCAR LTD. | ||||||||||||
Applicant Address | ISCAR CENTER, P O BOX 11, TEFEN 24959, ISRAEL | ||||||||||||
Inventors:
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PCT International Classification Number | B 23 C 05/22 | ||||||||||||
PCT International Application Number | N/A | ||||||||||||
PCT International Filing date | |||||||||||||
PCT Conventions:
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