Description [1] A ROTARY SHUTTLE DEVICE WITH A PART OPENED BOBBIN CASE BODY FOR SEWING MACHINE [2] TECHNICAL FIELD [3] The present invention relates to a rotary shuttle device for a sewing machine, and more particularly, to a rotary shuttle device with a part opened bobbin case body for a sewing machine, in which the bobbin case body has an expanded diameter and is partially opened so as to minimize a main wall within an extent that an under thread withdrawing region is not invaded, that is, the main wall of the bobbin case base is partially cut to form a recess portion in which an under thread withdrawing portion is inserted, so that an under thread receiving space can be considerably increased without exerting an influence on tension of the under thread, thereby remarkably improving productivity of a sewing operation and a sewing quality of a high-class sewn product. [4] BACKGROUND ART [5] Generally, in the sewing machine, as shown in FIG. 1, there is provided a first con¬ ventional rotary shuttle device 100 for supplying the under thread. The rotary shuttle device 100 is disposed at a driving shaft 110 which is rotatably connected to a driving apparatus, and comprised of a hook body 120, a bobbin case BC accommodated in the hook body 120 and a bobbin 140 on which the under thread is wound. The bobbin case BC comprises a bobbin case base 130 accommodated in the hook body 120 and a bobbin case body 150 for receiving the bobbin 40 from an outside of the bobbin 140. [6] The bobbin case body 150 functions to guide the under thread, and includes a butt end portion 152 provided with a latch 151, and a main wall 153. A withdrawing opening 154 for withdrawing the under thread reeled off from the bobbin 140 is formed at a side of the main wall 153. The bobbin case body 150 further includes a guide member 155 formed into a leaf spring passed so as to guide the under thread passing through the withdrawing opening 154 and prevent deviation of the under thread. [7] At a bottom of the bobbin case base 130 in which the bobbin case body 150 is inserted, there is upright disposed a stud 131 onto which the bobbin 140 is rotatably inserted. A stepped portion 133 is formed at a part of the main wall so as to be cor¬ respondent to the guide member 155 of the bobbin case body 150. [8] According to the above-mentioned construction of the bobbin case BC, the under thread reeled from the bobbin 140 passes through the withdrawing opening 154 of the main wall 153 and then passes between an outer surface of the main wall 153 and the guide member 155. [9] Now, an operation of the rotary shuttle device as described above will be described below. The hook body 20 is rotated by rotation of the driving shaft 110 so as to hook upper thread and form a loop of the upper thread for enclosing the bobbin case BC. The under thread withdrawn through the withdrawing opening 154 of the bobbin case body 150 and the guide member 155 is hooked up by the loop of upper thread, thereby performing the sewing operation. [10] In the sewing operation, firstly, the upper thread can have to smoothly hook up the under thread, and the tension of the hooked under thread has to be balanced with that of the upper thread. If the tension of the under thread is lower or higher than the preset tension, the tension balance between the upper thread and the under thread is destroyed. Therefore, the thread is broken or the thread is dangled down and fluttered roughly, thereby deteriorating the sewing quality. [11] Meanwhile, if an amount of under thread mounted in the sewing machine is increased, the number of replacing the bobbin is reduced and the productivity is thus increased. Therefore, numerous efforts have been devoted to the development of techniques for increasing the amount of under thread. The applicant also has proposed various relevant techniques. [12] In order to increase the amount of under thread, the bobbin and the bobbin case BC receiving the bobbin should have an expanded inner space. As one of such techniques, there has been known a second conventional rotary shuttle device 200 including a bobbin case body 250 as shown in FIGs 2 and 3 (the guide member 255 is not shown). In the conventional rotary shuttle device 200, a bobbin case body 250 has an opened main wall 253 of which only a part adjacent to a butt end portion 252 is remained and other part which is inserted into the bobbin case base 230 is removed, so that the main wall 253 of the bobbin case base 150 is entered into the bobbin case base 230. Ac¬ cordingly, the inner space of the bobbin case base 130 occupied by the main wall 153 of the bobbin case body 150 in the first conventional rotary shuttle device 100 can be used as a receiving space of the under thread wound on the bobbin. According to the construction of the second conventional rotary shuttle device as described above, the amount of under thread wound on the bobbin can be remarkably increased. [13] However, to this end, since the withdrawing opening 254 and the guide member 255 should be disposed maximally adjacent to the butt end portion 252, the withdrawing opening 254 and the guide member 255 are disposed to be leaned to the side of the butt end portion 252 in comparison with the bobbin case body 150 of the first conventional rotary shuttle device 100 in which the withdrawing opening 254 and the guide member 255 are disposed at the center portion thereof. Since the withdrawing opening 254 and the guide member 255 is leaned to the side of the butt end portion 252, as shown in FIG. 3, a withdrawing angle Θ2 of the under thread becomes narrow, and collision is occurred between the adjacent under threads wound on the bobbin side by side, when the under thread is withdrawn. Therefore, there are some problems that the unbalance of tension is brought about and the under thread is broken or dangled down and fluttered roughly. [14] That is, in the second conventional rotary shuttle device 200, the amount of under thread wound on the bobbin can be increased, but it is difficult to actually apply the technology to a rotary shuttle device. [15] DISCLOSURE OF THE INVENTION [16] Therefore, an object of the present invention is to provide a rotary shuttle device in which a bobbin case body has an expanded diameter and is partially opened so as to minimize a main wall of a bobbin case base within an extent that an under thread withdrawing region is not invaded, that is, the main wall is partially cut to form a recess portion in which an under thread withdrawing portion is inserted, so that an under thread receiving space can be considerably increased without exerting an influence on tension of the under thread, thereby remarkably improving productivity of a sewing operation and a sewing quality of a high-class sewn product. [17] According to the present invention, a rotary shuttle device with a part opened bobbin case body for a sewing machine, comprising, a hook body 20 disposed at a driving shaft; a bobbin case BC provided with a bobbin case base accommodated in the hook body and having a stud rod at a bottom thereof, and a bobbin case body for holding a bobbin at an outside; and a bobbin disposed in the bobbin case and on which lower thread is wound, wherein a stepped portion is formed at a part of a main wall of the bobbin case base, and the bobbin case body is formed with a butt end portion having a latch, and a main wall having a leaf spring guide member, is characterized in that only an under thread withdrawing portion of the main wall, in which a withdrawing opening and the guide member are disposed, is remained, and other part is opened, and the bobbin case base is formed with a recess portion at a part of an inner circumferential surface of a main wall so as to accommodate a distal end of the under thread withdrawing portion of the bobbin case body. [18] BRIEF DESCRIPTION OF DRAWINGS [19] FIG. 1 is an exploded perspective view of a first conventional rotary shuttle device for a sewing machine; [20] FIG. 2 is a perspective view showing a bobbin case body of a second conventional rotary shuttle device; [21] FIG. 3 is a cross-sectional view showing an operating state of FIG. 2; [22] FIG. 4 is an exploded perspective view of a rotary shuttle device according to the present invention; [23] FIG. 5 is a perspective view of a bobbin case body of FIG. 4 when viewing at another angle; and [24] FIG. 6 is an enlarged cross-sectional view showing an operating state of FIG. 4. [25] BEST MODE FOR CARRYING OUT THE INVENTION [26] The present invention will now be described in further detail by examples. It would be obvious to those skilled in the art that these examples are intended to be more concretely illustrative and the scope of the present invention as set forth in the appended claims is not limited to or by the examples. [27] FIG. 4 is an exploded perspective view of a rotary shuttle device according to the present invention, FIG. 5 is a perspective view of a bobbin case body of FIG. 4 when viewing at another angle, and FIG. 6 is an enlarged cross-sectional view showing an operating state of FIG. 4. [28] A rotary shuttle device with a part opened bobbin case body for a sewing machine according to an embodiment of the present invention, as shown in FIGs. 4 to 6, comprises a hook body 20 disposed at a driving shaft 10; a bobbin case BC provided with a bobbin case base 30 accommodated in the hook body 20 and having a stud rod 31 at a bottom thereof, and a bobbin case body 50 for holding a bobbin at an outside; and a bobbin 40 disposed in the bobbin case BC and on which lower thread is wound. [29] A stepped portion 33 is formed at a part of a main wall of the bobbin case base 30. The bobbin case body 50 is formed with a butt end portion 52 having a latch 51, and a main wall having a leaf spring guide member 55. [30] The main wall of the bobbin case body 50 has an expanded diameter. Only an under thread withdrawing portion 53A of the main wall, in which a withdrawing opening 54 and the guide member 55 are disposed, is remained, and other part is opened. The bobbin case base 30 is formed with a recess portion 35 at a part of an inner circum¬ ferential surface of a main wall 32 so as to accommodate a distal end 53a of the under thread withdrawing portion 53A of the bobbin case body 50. [31] Herein, the recess portion 35 of the bobbin case is formed at an inner side of the stepped portion 33, and a distal end 35a of the recess portion 35 is formed between an inner surface 32a and the stepped portion 33. [32] According to the present invention, a size of the expanded diameter of the main wall of the bobbin case body 50 is correspondent to a thickness of the main wall, and a recessed size of the recess portion 35 of the bobbin case base 30 is also correspondent to the thickness of the main wall. [33] The recess portion 35 of the bobbin case base 30 is formed by cutting away a part of the thickness of the main wall 32 from the inner surface 32a. The under thread withdrawing portion 53A of the bobbin case body 50, which is disposed in the recess portion 35, is formed to be minimized under a status that the guide member 55 and the withdrawing opening 54 does not deviate from their original position, i.e., a position of the guide member 155 and the withdrawing opening 154 in the first conventional rotary shuttle device 100. Therefore, a width of the under thread withdrawing portion 53A is correspondent to the stepped portion 33 of the bobbin case base 30. [34] In the above-mentioned construction, the under thread withdrawing portion 53A as the main wall of the bobbin case base 30 is not positioned in the inner space of the bobbin case base 30 but in the main wall 32. As a result, a diameter of the inner space of the bobbin case base 30 is expanded, thereby remarkably increasing a receiving amount of the under thread. [35] According to the present invention as described above, since the guide member 55 and the withdrawing opening 54 is positioned at a center portion of the bobbin case body 50 even while the receiving amount of the under thread in the bobbin case BC is increased, the withdrawing angle θl of the under thread T is the same as that in the bobbin case body 150 of first conventional rotary shuttle device. [36] Therefore, regardless of an increase in the amount of the under thread, the tension of the under thread T can be exactly maintained, and the problems occurred in the second conventional rotary shuttle device are prevented. [37] While the present invention has been described in detail, it should be understood that various changes, substitutions and alterations could be made hereto without departing from the spirit and scope of the invention as defined by the appended claims. [38] INDUSTRIAL APPLICABILITY [39] In the rotary shuttle device with a part opened bobbin case body for a sewing machine according to the present invention, as described above, the bobbin case body has an expanded diameter and is partially opened so as to minimize a main wall within an extent that an under thread withdrawing region is not invaded, that is, the main wall of the bobbin case base is partially cut to form a recess portion in which an under thread withdrawing portion is inserted, so that an under thread receiving space can be considerably increased without exerting an influence on tension of the under thread, thereby remarkably improving productivity of a sewing operation and a sewing quality of a high-class sewn product.