Abstract:
A sewing machine frame having reinforced structure for use in a sewing machine is disclosed. The sewing machine frame has a frame member formed of a synthetic resin and having a bed portion, a tower portion upstanding from the bed portion, and an arm portion extending from the tower portion at a position above the bed portion, the bed portion, the tower portion and the arm portion being formed integrally and providing a concaved peripheral wall defining a stitch working space. The sewing machine frame is characterized by a peripheral wall reinforcing rib protruding from the frame member, the peripheral wall reinforcing rib extending along the peripheral wall and ranging at least from a boundary between the bed portion and the tower portion to a boundary between the tower portion and the arm portion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a sewing machine frame made from a synthetic resin in which an arm portion, a tower portion and a bed portion are provided integrally. The present invention also relates to a sewing machine having the sewing machine frame.  
           [0002]    In the sewing machine frame, a horizontally extending arm portion supports a reciprocation mechanism for a needle carrying a needle thread, and the tower portion vertically extends from the bed portion for supporting the arm portion in a cantilevered fashion. In the bed portion, a loop taker is supported for trapping a loop of the needle thread carried on the vertically reciprocating needle in order to form a stitch.  
           [0003]    In the sewing machine, a smooth stitching operation is required To this effect, vibration and displacement of a needle tip due to the vertically reciprocating motion of the needle must be reduced or minimized, otherwise a loop seizing beak of the loop taker disposed in the bed portion cannot trap the needle thread loop formed by vertical reciprocation of the sewing needle. Thus, the stitching may be degraded.  
           [0004]    In order to avoid this problem, the needle &amp; rotary hook timing must be adequately provided. To this effect, the sewing machine frame must provide high rigidity capable of avoiding deformation or displacement thereof due to reaction force occurring when the needle penetrates a workpiece fabric. Therefore, in the conventional sewing machine, a metallic frame having high rigidity is provided in an interior of a sewing machine cover, and a stitch forming mechanism including a needle vertical reciprocating mechanism and the loop taker is attached to the metallic frame.  
           [0005]    However, such a conventional arrangement is costly, bulky and heavy. More specifically, the sewing machine frame has a rigid box shape arrangement in order to provide high rigidity. Further, the frame is made from a metal such as a cast iron or aluminum, which in turn increase weight and size. Further, high skill and elaboration is required for assembling the sewing machine because the stitch forming mechanism must be installed into the metallic frame through a small area opening thereof. This increases assembly cost.  
           [0006]    Laid open Japanese Patent Application Kokai No.Hei-11-137880 discloses a sewing machine frame made from a synthetic resin to reduce production cost and to provide a light weight frame. As shown in FIG. 16, the frame  300  has an open end arrangement in a U-shape cross-section in which a bed portion  304 , a tower portion  303  and an arm portion  302  are provided integrally, and a reinforcing plate  301  is fixed between upper and lower portions at the open end of the bed portion  304 .  
           [0007]    However, the disclosed sewing machine frame  300  provides a rigidity still lesser than that of the metallic frame. More specifically, as shown in FIG. 16, vertical vibration occurs in the arm portion  302  due to a load exerted along a vertical line containing the needle, the load being caused by the reciprocating motion of the needle during stitching operation. Further, a horizontal swing also occurs at an upper portion of the tower portion  303  during stitching.  
           [0008]    Such vibration and swing occur due to the cantilevered support structure of the arm portion  302  with respect to the tower  303 . That is, a combination of the arm portion  303 , the tower portion  303  and the bed portion  304  provides an arcuate recessed wall  305 , and a stress generated by the vertically reciprocating motion of the needle will be concentrated on the wall  305 . However, the wall  305  does not have a sufficient rigidity, and therefore, such unwanted vibration and swing occur to lower stitching quality in comparison with the conventional sewing machine provided with the metallic frame.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to overcome the above-described problems and to provide a sewing machine frame having a bed portion, a tower portion and an arm portion those integrally with each other and formed of a synthetic resin, yet having high rigidity, and to provide a sewing machine having such an improved sewing machine frame.  
           [0010]    This and other objects of the present invention will be attained by a sewing machine frame for use in a sewing machine including a frame member, and a peripheral wall reinforcing rib. The frame member is formed of a synthetic resin and has a bed portion, a tower portion upstanding from the bed portion, and an arm portion extending from the tower portion at a position above the bed portion. The bed portion, the tower portion and the arm portion are formed integrally and provide a concaved peripheral wall defining a stitch working space. The peripheral wall reinforcing rib protrudes from the frame member. The peripheral wall reinforcing rib extends along the peripheral wall and ranges at least from a boundary between the bed portion and the tower portion to a boundary between the tower portion and the arm portion.  
           [0011]    In another aspect of the invention, there is provided a sewing machine frame for use in a sewing machine including an outer panel wall, a side wall, a peripheral wall reinforcing rib, and an outer panel wall reinforcing rib. The outer panel wall constitutes a front wall and a rear wall and has a peripheral edge. The side wall protrudes from the peripheral edge to provide a closed space with the outer panel wall and is formed integrally with the outer panel wall with a synthetic resin. A combination of the outer panel wall and the side wall provides a bed portion, a tower portion upstanding from the bed portion, and an arm portion extending from the tower portion and positioned above the bed portion. The side wall has a part providing a concaved peripheral wall which defines a stitch working space surrounded by the bed portion, the tower portion and the arm portion. The peripheral wall reinforcing rib protrudes from the outer panel wall and extends along the peripheral wall. The peripheral wall reinforcing rib ranges at least from a boundary between the bed portion and the tower portion to a boundary between the tower portion and the arm portion. The outer panel wall reinforcing rib protrudes from the outer panel wall for reinforcing the same.  
           [0012]    In still another aspect of the invention, there is provided a sewing machine frame including a bed portion, a tower portion upstanding from the bed portion, and an arm portion extending from the tower portion in a cantilevered fashion, a stitch forming mechanism of the sewing machine being assembled in the sewing machine frame. The sewing machine frame includes an integral main frame body, an integral frame cover and a concave wall reinforcing rib. The integral main frame body is made from a synthetic resin and to which the stitch forming mechanism is assembled. The integral main frame body includes a back panel wall having a first peripheral edge, and a first side wall integrally protruding from the first peripheral edge. The integral main frame body provides an arm section, a tower section and a bed section. The integral frame cover is made from a synthetic resin and is attached to the main frame body. The integral frame cover includes a front panel wall having a second peripheral edge, and a second side wall integrally protruding from the second peripheral edge for providing a complementary bed section to form the bed portion with the bed section, a complementary tower section to form the tower portion with the bed section, and a complementary arm section to form the arm portion with the arm section. The first side wall and the second side wall have parts defining a concave wall surroundingly provided by the combination of the arm portion, the tower portion, and the bed portion. The concave wall reinforcing rib extends along the concave wall and ranges at least from a boundary between the bed portion and the tower portion to a boundary between the tower portion and the arm portion.  
           [0013]    In still another aspect of the invention, there is provided a sewing machine frame for use in a sewing machine including an outer panel wall, a side wall, and a reinforcing member. The outer panel wall constitutes a front wall and a rear wall. The side wall protrudes from a peripheral edge of the outer panel wall to provide a closed space with the outer panel wall and is formed integrally with the outer panel wall with a synthetic resin. A combination of the outer panel wall and the side wall provides a bed portion extending in its longitudinal direction, a tower portion upstanding from the bed portion, and an arm portion extending in its longitudinal direction from the tower portion and positioned above the bed portion. A congregated area among the bed portion, the tower portion and the arm portion provides a concaved peripheral wall defining a stitch working space of the sewing machine. The reinforcing member is formed integrally with the outer panel wall and has a generally semi-circular hollow cross-section. The reinforcing member is positioned along the peripheral wall and has one end portion positioned in the arm portion and extending in the longitudinal direction thereof, and has another end portion positioned in the bed portion and extending in the longitudinal direction thereof.  
           [0014]    In still another aspect of the invention, there is provided a sewing machine frame including a bed portion, a tower portion upstanding from the bed portion, and an arm portion extending from the tower portion in a cantilevered fashion, a stitch forming mechanism of a sewing machine being assembled in the sewing machine frame. The sewing machine frame includes an integral main frame body, an integral frame cover, and a reinforcing member. The integral main frame body is made from a synthetic resin and to which the stitch forming mechanism is assembled. The integral main frame body includes a back panel wall having a peripheral edge, and a side wall integrally protruding from the peripheral edge. The integral main frame body provides an arm section, a tower section and a bed section. The side wall has a part defining a peripheral wall surroundingly provided by the combination of the arm section, the tower section and the bed section. The integral frame cover serves as a front panel wall and is made from a synthetic resin and is attached to the main frame body for providing a complementary bed section to form the bed portion with the bed section, a complementary tower section to form the tower portion with the tower section, and a complementary arm section to form the arm portion with the arm section. The reinforcing member is formed integrally with the main frame body and has a generally semi-circular hollow cross-section. The reinforcing member is positioned along the peripheral wall and has one end portion positioned in the arm section and extending in the longitudinal direction thereof, and has another end portion positioned in the bed section and extending in the longitudinal direction thereof.  
           [0015]    In still another aspect of the invention, there is provided a sewing machine including a stitch forming mechanism and any one of the above-described sewing machine frames. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein:  
         [0017]    [0017]FIG. 1 is a front view showing the overall construction of a sewing machine comprising a frame according to the preferred embodiment;  
         [0018]    [0018]FIG. 2 is a side view showing the overall construction of the sewing machine in FIG. 1;  
         [0019]    [0019]FIG. 3 is a perspective view showing the external appearance of a main frame;  
         [0020]    [0020]FIG. 4 is a perspective view showing the internal construction of the main frame;  
         [0021]    [0021]FIG. 5 is a plan view showing the internal construction of the main frame;  
         [0022]    [0022]FIG. 6(A) is a cross-sectional view along the plane of the main frame indicated by the arrows A in FIG. 5;  
         [0023]    [0023]FIG. 6(B) is a cross-sectional view along the plane of the main frame indicated by the arrows B in FIG. 5;  
         [0024]    [0024]FIG. 7(A) is a cross-sectional view along the plane of the main frame indicated by the arrows C in FIG. 5;  
         [0025]    [0025]FIG. 7(B) is an enlarged view showing the lower end of the main frame;  
         [0026]    [0026]FIG. 7(C) is a cross-sectional view along the plane of the main frame indicated by the arrows D in FIG. 5;  
         [0027]    [0027]FIG. 8(A) is a cross-sectional view along the plane of the main frame indicated by the arrows E in FIG. 5;  
         [0028]    [0028]FIG. 8(B) is a cross-sectional view along the plane of the main frame indicated by the arrows F in FIG. 5;  
         [0029]    [0029]FIG. 8(C) is an enlarge view of a protrusion;  
         [0030]    [0030]FIG. 8(D) is a cross-sectional view along the plane of the main frame Indicated by the arrows M in FIG. 5;  
         [0031]    [0031]FIG. 9(A) is an enlarged plan view showing the main frame from the perspective of the line G in FIG. 5;  
         [0032]    [0032]FIG. 9(B) is an enlarged plan view showing the main frame from the perspective of the line H in FIG. 5;  
         [0033]    [0033]FIG. 10 is a perspective view showing the external appearance of the frame cover;  
         [0034]    [0034]FIG. 11 is a perspective view showing the internal construction of the frame cover;  
         [0035]    [0035]FIG. 12 is a plan view showing the internal construction of the frame cover;  
         [0036]    [0036]FIG. 13 is a cross-sectional view along the plane of the frame cover indicated by the arrows I in FIG. 12;  
         [0037]    [0037]FIG. 14(A) is a cross-sectional view along the plane of the frame cover indicated by the arrows J in FIG. 12;  
         [0038]    [0038]FIG. 14(B) is an enlarged view showing the lower end of the frame cover;  
         [0039]    [0039]FIG. 15(A) is an enlarged plan view along the plane of the frame cover indicated by the arrows K in FIG. 12;  
         [0040]    [0040]FIG. 15(B) is an enlarged plan view along the plane of the frame cover indicated by the arrows L in FIG. 12; and  
         [0041]    [0041]FIG. 16 is a perspective view showing a conventional sewing machine frame. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    Structure of a Sewing Machine  
         [0043]    A sewing machine frame according to a preferred embodiment of the present invention will be described while referring to the accompanying drawings. First the overall construction of a sewing machine comprising a frame according to the preferred embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a front view, and FIG. 2 is a side view showing the overall construction of the sewing machine comprising a frame  1  according to the preferred embodiment.  
         [0044]    As shown in FIG. 1, the frame  1  substantially comprises a bed  8 , a cantilever support  7  provided vertically on the bed  8 , an arm  6 , and an arm  6  cantilevered from the cantilever support  7  above the bed  8 . The bed  8 , the cantilever support  7 , and the arm  6  are integrally formed of a synthetic resin in a substantially C shape.  
         [0045]    The frame  1  supports a stitch forming mechanism including a loop taker and a mechanism for driving a needle  16  reciprocally up and down, and constitutes a shell of the sewing machine. In other words, the frame  1  does not need any metallic frame for mounting the stitch forming mechanism. Accordingly, it is possible to manufacture a lighter frame  1  having simplified structure, compared with a conventional metal frame to mount a stitch forming mechanism, covering with a resin cover. The frame  1  may be formed of a synthetic resin material by using a well-known injection molding method.  
         [0046]    The synthetic resin material for the frame  1  may be a noncrystalline thermoplastic resin, such as a styrene resin. More specifically, the material may be one or mixture of acrylonitrile-butadiene-styrene copolymer, polystyrene, acrylonitrile-styrene, acrylonitrile-acrylate-styrene, acrylonitrile-ethylene-styrene, chlorinated acrylonitrile-polyethylene-styrene. Of these materials, a resinous matter having acrylonitrile-butadiene-styrene copolymer as the primary component with an inorganic additive of talc or glass bead has good rigidity and a good thermal expansion coefficient. The usage of the above material may eliminate frame coating in the later step due to a good appearance of the frame.  
         [0047]    The arm  6  supports a top mechanism  3  for reciprocally driving the needle  16  up and down, the needle  16  retaining needle thread. A motor  2  provided in the cantilever support  7  generates rotational motion. The top mechanism  3  converts this rotational motion to reciprocal motion by means of a crank mechanism to transfer the reciprocal motion to the needle  16 . The top mechanism  3  comprises a spindle  12 , a thread take-up crank  13 , a needle bar holder  14 , a needle bar  15 , and a thread take-up lever link hinge pin  17  mounted in a metal top frame  11  The top frame  11  is directly attached to the frame  1  by several screws.  
         [0048]    Next, the operations of the top mechanism  3  will be described. A rotational driving force generated by the motor  2  is transferred to a large pulley  35  via a motor belt  36 . The rotational driving force transferred to the large pulley  35  is further transferred to the thread take-up crank  13  via an arm shaft  31  and the spindle  12 . The arm shaft  31  is rotatably supported by two bearings  32 ,  32 . The spindle  12  is linked to the arm shaft  31  via a coupler. Through the movement of a needle bar crank rod, rotational motion transferred to the thread take-up crank  13  is converted to reciprocal motion of the needle bar  15  that is supported rotatably on the needle bar holder  14 . The needle bar  15  is capable of moving vertically in the needle bar holder  14 . This reciprocal motion is transferred to the needle  16 .  
         [0049]    The arm  6  is supported on the top end of the cantilever support  7 , while the bed  8  is connected to the bottom end of the cantilever support  7 . A drive transferring mechanism  5  is disposed in the cantilever support  7  for transferring rotational driving force generated by the motor  2  to the top mechanism  3  housed in the arm  6  and a lower mechanism  4  housed in the bed  8 . The drive transferring mechanism  5  comprises the motor  2 , the large pulley  35 , the motor belt  36 , a pulley  38 , a pulley  39 , and a timing belt. The drive transferring mechanism  5  is directly attached to the frame  1 . The motor  2  is supported by motor supporting brackets  33  that are fixed near the bottom end of the cantilever support  7 .  
         [0050]    Next, the operations of the drive transferring mechanism  5  will be described. The rotational driving force provided by the motor  2  is transferred to the large pulley  35  via the motor belt  36 . The rotational driving force transferred to the large pulley  35  is then transferred to the arm shaft  31  rotatably supported by the two bearings  32 ,  32 . As described above, this rotational motion is transferred to the top mechanism  3  via the spindle  12 , while this movement is also transferred to the lower mechanism  4 . That is, the pulley  39  is fixed at approximately the center point of the arm shaft  31 . Rotational motion transferred to the pulley  39  is further transferred to the pulley  38  disposed in the bed  8  via the timing belt  41 . A rotary hook shaft  37  is rotatably supported by a bearing  32 . Since the rotary hook shaft  37  is linked to the pulley  38 , the rotary hook shaft  37  rotates in synchronization with the rotations of the arm shaft  31  due to the rotational motion of the pulley  38 .  
         [0051]    The cantilever support  7  is formed on one end of the bed  8 . The bed  8  supports a rotary hook  23  constituting a loop taker for catching a thread loop of the needle thread as the needle moves up and down and forming a stitch. The lower mechanism  4  is provided inside the bed  8  for rotating the rotary hook  23  in synchonization with the reciprocal motion of the needle  16 . The lower mechanism  4  comprises a rotary hook shaft  21 , a helical gear  22 , the rotary hook  23 , a helical gear  24 , and the rotary hook shaft  37  mounted on a metal lower frame  20 . The lower frame  20  is mounted directly on the frame  1  by a plurality of screws.  
         [0052]    Next, the operations of the lower mechanism  4  will be described. The rotational motion transferred via the timing belt  41  to the pulley  38  is transferred to the helical gear  22  via the rotary hook shaft  37  rotatably supported by the bearing  32  and the rotary hook shaft  21  rotatably supported by two bearings  25 ,  25  and linked to the rotary hook shaft  37  via a coupler. As shown in FIG. 2, the helical gear  22  is fixed on the rotary hook shaft  21 . A rotary hook shaft on which the rotary hook  23  is fixed is rotatably supported on the lower frame  20  for rotating beneath the top surface of the bed  8 . The helical gear  24  engaged with the helical gear  22  is fixed to the rotary hook shaft. Accordingly, when the rotary hook shaft  21  rotates, the rotary hook  23  rotates via the helical gear  22  and helical gear  24 . At the same time, A loop seizing beak of the loop taker moves in synchronization with the tip of the needle  16 , and catches the thread loop of the needle thread supported on the needle  16  as the needle  16  moves vertically.  
         [0053]    Sewing Machine Frame  
         [0054]    In order to execute smooth sewing operations with a sewing machine having the construction described above, it is necessary to minimize vibration caused by the vertical movement of the needle  16 . Simultaneously, displacement of the needle tip caused by deformation of the frame  1  due to the vertical movement of the needle  16  is required to be minimized. This is because large amount of the displacement and the vibration of the needle tip can prevent the loop seizing beak of the loop taker provided in the bed  8  from catching the thread loop, resulting in the formation of an inappropriate stitch. To avoid this, it is necessary to maintain at all times an appropriate needle and rotary hook timing between the loop seizing beak of the rotating rotary hook  23  and the needle  16  that is moved reciprocally up and down. Accordingly, the frame  1  must have high rigidity in order to prevent deformation (displacement) due to a reaction force generated when the needle penetrates a working piece cloth. However, since it is difficult to maintain sufficient rigidity in a frame formed of synthetic resin, the frame  1  of the present embodiment employs various constructions to achieve sufficient rigidity.  
         [0055]    As shown in FIG. 2, the frame  1  is formed of a main frame  1 A and a frame cover  1 B along a dividing plane  52  formed in approximately the center of the periphery of the frame  1  when viewed from the end (the dotted line in FIG. 2). The main frame  1 A is provided with the stitch forming mechanism including the top mechanism  3  for driving the needle  16  reciprocally up and down and the lower mechanism  4  for rotating the rotary hook  23  is mounted. The frame cover  1 B is coupled to the main frame  1 A to cover the stitch forming mechanism.  
         [0056]    The insides of the main frame  1 A and frame cover  1 B are configured to accommodate the top mechanism  3  and the lower mechanism, as shown when the main frame  1 A and frame cover  1 B are in an open state divided along the dividing plane  52  (refer to FIGS. 4 and 11) When assembling the sewing machine, the top mechanism  3  and the lower mechanism are first mounted in the main frame  1 A while the main frame  1 A is rendered in an open state. The main frame  1 A and frame cover  1 B are then joined together by inserting screws through couplings  90 ,  190  provided in the main frame  1 A and the frame cover  1 B (see FIGS. 4 and 11). By simplifying the process for assembling the sewing machine in this way, it is possible to reduce the assembly costs. Since the open area of the frame is closed after assembly, the frame retains sufficient rigidity, and the arm  2  is not easily subject to torsional deformation due to reciprocal motion of the needle  16 .  
         [0057]    Main Frame  
         [0058]    Next, the main frame  1 A of the frame  1  will be described with reference to FIGS. 3 through 9. FIG. 3 is a perspective view showing the external appearance of the main frame  1 A. FIG. 4 is a perspective view showing the internal construction of the main frame  1 A. FIG. 5 is a plan view showing the internal construction of the main frame  1 A. FIG. 6(A) is a cross-sectional view along the plane of the main frame  1 A indicated by the arrows A in FIG. 5. FIG. 6(B) is a cross-sectional view along the plane of the main frame  1 A indicated by the arrows B in FIG. 5. FIG. 7(A) is a cross-sectional view along the plane of the main frame  1 A indicated by the arrows C in FIG. 5. FIG. 7(B) is an enlarged view showing the lower end of the main frame  1 A. FIG. 7(C) is a cross-sectional view along the plane of the main frame  1 A indicated by the arrows D in FIG. 5. FIG. 8(A) is a cross-sectional view along the plane of the main frame  1 A indicated by the arrows E in FIG. 5. FIG. 8(B) is a cross-sectional view along the plane of the main frame  1 A indicated by the arrows F in FIG. 5. FIG. 8(C) is an enlarge view of a protrusion shown in FIG. 8(B). FIG. 8(D) is a cross sectional view along the plane of the main frame  1 A indicated by the arrows M. FIG. 9(A) is an enlarged plan view showing the main frame  1 A from the perspective of the line G in FIG. 5. FIG. 9(B) is an enlarged plan view showing the main frame  1 A from the perspective of the line H in FIG. 5.  
         [0059]    As shown in FIG. 3, the main frame  1 A substantially comprises the arm  6 , the cantilever support,  7 , and the bed  8  formed integrally. The semicircular space surrounded by the arm  6 , cantilever support  7 , and bed  8  is a space  9 .  
         [0060]    In addition, the main frame  1 A comprises a back panel wall  250  constituting a back side of the sewing machine, and side wall  251  extending from a peripheral edge  250   a  of the back panel wall  250 . Especially, the surface of the main frame  1 A facing the space  9  is designated as an inner surface wall  51 . The inner surface wall  51  has a rectangular opening  53  that a cloth-pressing lever for fabric (not shown) is passed through.  
         [0061]    As shown in FIGS. 1, 4 and  5 , the main frame  1 A is provided with an arrangement for mounting stitch forming mechanism. More specifically, the interior of the arm  6  is provided with a pair of thread take-up shaft supports  140 ,  140  for rotatably supporting the thread take-up lever link hinge pin (not shown); a needle bar holder mount  141  on which the needle bar holder  14  is mounted; an upper frame mount  142  on which the top frame  11  is mounted; and a pair of arm shaft supports  144 ,  144  for rotatably supporting the arm shaft  31  that transfers the rotational drive force from the motor  2  to the top mechanism  3 . Motor support bracket mounts  146  are mounted in the cantilever support  7  for attaching the motor supporting brackets  33  that fixedly support the motor  2 . Further, the interior of the bed  8  is provided with a pair of lower conducting shaft supports  147 ,  147  for rotatably supporting the rotary hook shaft  37  that transfer the rotational drive force from the motor  2  to the lower mechanism  4 , and a lower frame mount  148  on which the lower frame  20  is mounted.  
         [0062]    Reinforcing Member  
         [0063]    Referring to FIGS. 4 and 5, a reinforcing member  60  is provided around the inner surface wall  51  of the main frame  1 A facing the space  9  surrounded the arm  6 , cantilever support  7 , and bed  8 . The reinforcing member  60  is formed integrally with the back panel wall  250 . One end of the reinforcing member  60  extends along the longitudinal direction of the arm  6  to the point adjacent to the side wall  251  at one end of the arm  6  opposing the cantilever support  7 . The other end of the reinforcing member  60  extends along the longitudinal direction of the bed  8  to the point adjacent to the side wall  251  at one end of the bed  8  opposing the bed  8 . As described above, the reinforcing member  60  comprises three parts: one part placed around the inner surface wall  51  in a semicircle shape, another part placed in a linear manner as if it crosses the arm  6 , and the other part placed in a linear manner as if it crosses the bed  8 . Accordingly, the reinforcing member  60  is placed in a continuous manner to form a U-shape as a whole. The above structure of the reinforcing member  60  reinforces projecting portions of the arm  6  and the bed  8  which extend from the cantilever support  7 .  
         [0064]    Referring to FIG. 8(D), the reinforcing member  60  has a tubular shape with a hollow circular cross-section. This reinforcing member  60  is formed with the back panel wall  250  integrally to project from the inner surface of the back panel wall  250 . The reinforcing member  60  is formed in a tubular shape for the following reasons. As described above, the main frame  1 A is formed according to an injection molding method. In this method, after injecting a molten resinous material in a cavity die shell, the resinous material is cooled. At this time, thicker portions of the molded product harden slower than thinner portions. Since contraction is greater at the thicker portions, shrinkage occurs in those portions. In order to prevent such shrinkage, it is necessary to maintain a uniform thickness in the molded product. For this reason, the reinforcing member  60  is formed in a hollow tubular shape. When forming the frame  1 , the tubular shape of the reinforcing member  60  is formed by injecting an inert fluid, such as argon gas or nitrogen gas, through an injection hole  61  formed at one end of the reinforcing member  60  adjacent to the side wall  251 , and subsequently cooling the reinforcing member  60 .  
         [0065]    The above structure of the reinforcing member  60  ensures the rigidity of the inner surface wall  51  facing the space  9  surrounded by the arm  6 , the cantilever support  7 , and the bed  8  on which stress caused by the reciprocating motion of the needle  16  is concentrated. The above structure of the reinforcing member  60  also ensures the rigidity of the back panel wall  250  and the side wall  251  of the arm  6 , cantilever support  7 , and bed  8  adjacent to the inner surface wall  51 . Accordingly, a sewing machine including the main frame  1 A prevents horizontal and vertical vibrations of the main frame  1 A caused by the reciprocating motion of the needle  16 , thereby performing a smooth stitch forming action.  
         [0066]    In addition, the reinforcing member  60  has a semicircle hollow section to achieve a light weight and provide sufficient rigidity. The reinforcing member  60  is formed integrally with the back panel wall  250 . Accordingly, process for manufacturing the main frame  1 A is simplified.  
         [0067]    In the embodiment described above, the reinforcing member  60  has one end extending to the point adjacent to the side wall  251  placed at the tip of the arm  6 , and the other end extending to the point adjacent to the side wall  251  placed at the tip of the bed  8 . In another embodiment, the reinforcing member  60  may extend to a certain point between the arm  6  and the bed  8 . It is preferable that the reinforcing member  60  is provided around at least the space  9 . In this case, the arrangement of the reinforcing member  60  may have a J-shape, C-shape, or a rectangular shape with one open side.  
         [0068]    Auxiliary Reinforcing Member  
         [0069]    Referring to FIGS. 4 and 5, the back panel wall  250  of the main frame  1 A has an auxiliary reinforcing member  66  formed integrally therewith. The auxiliary reinforcing member  66  is placed substantially parallel to the reinforcing member  60  outside thereof at a predetermined interval. The auxiliary reinforcing member  66  is placed in a continuous manner described as follows: The auxiliary reinforcing member  66  extends from a certain point between the cantilever support  7  and the side wall  251  at the arm  6  along the longitudinal direction of the arm  6  within the arm  6  to one end of the cantilever support  7 . The auxiliary reinforcing member  66  is then curved in a semicircle shape within the cantilever support  7  to extend to one end of the bed  8 . The auxiliary reinforcing member  66  further extends from the one end of the cantilever support  7  along the bed  8  with in the bed  8  to the point adjacent to the side wall  251  opposing to the cantilever support  7 . As describe above, the parallel arrangement of the reinforcing member  60  and the auxiliary reinforcing member  66  leads to a uniform filling to the interior of the back panel wall  250  between the reinforcing member  60  and the auxiliary reinforcing member  66  with synthetic resin, thereby preventing weld line and shrinkage appearing on the back panel wall  250 . As a result, the main frame  1 A can obtain a good appearance.  
         [0070]    Referring to FIG. 7( c ), the auxiliary reinforcing member  66  has the substantially semicircle cross section similar to that of the reinforcing member  60 . The auxiliary reinforcing member  66  has a hollow tubular shape having a hollow space  68  within the auxiliary reinforcing member  66 . The auxiliary reinforcing member  66  is formed integrally with the back panel wall  250  in a manner to project from the interior of the back panel wall  250  of the main frame  1 A. The reason why the auxiliary reinforcing member  66  has a tubular shape is the same as that of the reinforcing member  60 . Additionally, a method to form the auxiliary reinforcing member  66  is the same as that of the reinforcing member  60 .  
         [0071]    The above arrangement of the auxiliary reinforcing member  66  ensures the rigidity of the back panel wall  250 . Therefore, a sewing machine including the above main frame  1 A can advantageously prevent horizontal and vertical vibrations of the main frame  1 A caused by the reciprocating motion of the needle  16 , thereby performing smooth stitch forming action  
         [0072]    In the above embodiment, the main frame  1 A is provided with the reinforcing member  60  and the auxiliary reinforcing member  66 , while the frame cover  1 B does not has any reinforcing member and auxiliary reinforcing member (See FIG. 11). The reason why frame cover  1 B has no reinforcing member is as follows: the main frame  1 A accommodates the stitch forming mechanism including the tope mechanism  3  for reciprocating the needle  16  and the lower mechanism  4  for rotating the rotary hook  23 . Therefore, vibrations or displacement are more easily induced to the main frame  1 A than the frame cover  1 B. However, the frame cover  1 B may be provided with a reinforcing member or an auxiliary reinforcing member, if necessary. In that case, the frame cover  1 B obtains stronger rigidity.  
         [0073]    Inside Wall Reinforcing Rib  
         [0074]    As shown in FIGS. 4 and 5, an inside wall reinforcing rib  70  for reinforcing the inner surface wall  51  of the main frame  1 A facing the space  9  is provided on the inside of the back panel wall  250  around the periphery of the space  9 . A lot of inside wall reinforcing ribs  70  are provided around the periphery of the space  9  from the joint of the arm  6  and the cantilever support  7  to the joint of the cantilever support  7  and the bed  8 .  
         [0075]    The inside wall reinforcing rib  70  comprises a partitioning rib  71  spaced from the inner surface  51  and a plurality of intermediate ribs  72  intersecting with the inner surface  51  and partitioning rib  71 . The partitioning rib  71  extends from the inside of the back panel wall  250  and parallel and perpendicularly to the inner surface wall  51  in a continuous manner. The intermediate rib  72  extends from the inside of the back panel wall  250  between the inner surface wall  51  and the partitioning rib  71  at a constant intervals perpendicularly to the back panel wall  250 . The intermediate rib  72  connects the inner surface wall  51  to the partitioning rib  71 , and connects the inner surface wall  51  and the partitioning rib  71  to the back panel wall  250 . The above arrangement of the inner surface wall  51 , the partitioning rib  71 , and the intermediate ribs  72  provides a plurality of cells  73  in the space between the inner surface  51  and partitioning rib  71 . The intermediate ribs  72  are arranged radially from a center point located in the space  9 , because the inner surface wall  51  surrounding the space  9  has a semicircle shape. Accordingly, each intermediate rib  72  intersects the inner surface  51  and partitioning rib  71  at a perpendicular angle. Thus, the arrangement of the ribs is optimized, thereby reinforcing the inner surface wall  51  advantageously.  
         [0076]    The above structure of the inside wall reinforcing ribs  70  provides the rigidity equal to that of the inner surface wall  51  having a considerable thickness. In other words, the above structure of the inside wall reinforcing ribs  70  ensures the rigidity over the back panel wall  250  from the area adjacent to the joint of the arm  6  and the cantilever support  7 , through the cantilever support  7 , to the area adjacent to the joint of the cantilever support  7  and the bed  8 . A sewing machine having the main frame  1 A can prevent horizontal and vertical vibrations of the main frame  1 A caused by the reciprocating motion of the needle  16 , thereby performing a smooth stitch forming action.  
         [0077]    In the above embodiment, the inside wall reinforcing ribs  70  are provided on the back panel wall  250  from the joint of the arm  6  and the cantilever support  7  through the  7  through the  7  to the joint of the cantilever support  7  and the bed  8 . In another embodiment, the inside wall reinforcing rib  70  may be formed over the whole of the inner surface wall  51 . In the above embodiment, a lot of intermediate ribs  72  are provided. However, in another embodiment, the number of the intermediate ribs  72  may be only one or a few. Each of the intermediate ribs  72  may be coupled or crossed to each other, so that the resultant arrangement of the intermediate ribs  72  may have honeycomb or diagram shape.  
         [0078]    As described above, the hollow reinforcing member  60  having a substantially semicircle shape is formed integrally with the back panel wall  250  around the inner surface wall  51 . In other words, both the reinforcing member  60  and the inside wall reinforcing rib  70  are formed at the substantially same positions on the inner surface wall  51 . Especially, the reinforcing member  60  is located near the back panel wall  250  inside of the inside wall reinforcing rib  70 . The inside wall reinforcing rib  70  projects from the surface of the reinforcing member  60 . The above structure is necessary to obtain considerable reinforcement, because stress induced by the reciprocating motion of the needle  16  is concentrated on the inner surface wall  51 . In addition, the space around the inner surface wall  51  has sufficient spare room because the stitch forming mechanism is not mounted. Therefore, the inside wall reinforcing rib  70  having a considerable height can be formed.  
         [0079]    Outside Wall Reinforcing Rib  
         [0080]    As shown in FIGS. 4 and 5, outside wall reinforcing ribs  80  are formed in a matrix shape over nearly the entire inside of the back panel wall  250 . The outside wall reinforcing rib  80  projects from the inside of the back panel wall  250 . The outside wall reinforcing rib  80  is formed of vertical ribs  81  vertically oriented when the sewing machine is placed on a working surface, and horizontal ribs  82  oriented horizontally when the sewing machine is in the same position. As shown in FIGS.  6 (A) and  6 (B), these vertical ribs  81  and horizontal ribs  82  are approximately perpendicular to the back panel wall  250 . The ends of the vertical ribs  81  and horizontal ribs  82  are joined with the side wall  251  on the side portions of the main frame  1 A. The spaces surrounded by pairs of intersecting vertical ribs  81 ,  81  and horizontal ribs  82 ,  82  form approximately square or rectangular shaped cells  83 . Hence, a plurality of cells  83  are formed on the back side of the back panel wall  250 .  
         [0081]    Among the cells  83 , the outside wall reinforcing rib  80  defining a cell  83  having a wider area is formed to have a higher height from the back panel wall  250 , compared to a cell  83  having a narrower area. The above structure of the cell  83  will be explained with respect to a wider cell  83 A located on the right side of the arm conducting shaft supports  144  in the cantilever support  7  (see FIGS. 4 and 5), and a narrower cell  83 B located on the lower-right side of the needle bar holder mount  141  in the arm  6  (see FIGS. 4 and 5).  
         [0082]    As shown in FIG. 5, the vertical length X of the wider cell  83 A is identical to the vertical length U of the narrower cell  83 B. On the other hand, the horizontal length Y of the wider cell  83 A is longer more than two times of the horizontal length V of the narrower cell  83 B. Thus, the area of the wider cell  83 A is wider than that of the narrower cell  83 B.  
         [0083]    Referring to FIG. 6(A), the height Z from the  250  of the outside wall reinforcing rib  80  constituting the wider cell  83 A (horizontal rib  82 ) is higher than the height W from the back panel wall  250  of the outside wall reinforcing rib  80  constituting the narrower cell  83 B (vertical rib  81 ). In the case where the outside wall reinforcing ribs  80  have different height from each other due to requirements for a design of the main frame  1 A, the wider area of the higher outside wall reinforcing rib  80  and the narrower area of the narrower outside wall reinforcing rib  80  lead to the uniform rigidity over the whole of the back panel wall  250 . Accordingly, the action of stress on the particular point on the back panel wall  250  can be avoided. Thus, the main frame  1 A ensures considerable rigidity as a whole.  
         [0084]    The outside wall reinforcing rib  80  on the accommodating part for the stitch forming mechanism in the arm  6  or the bed  8  has a lower height from the back panel wall  250  than those of the outside wall reinforcing ribs  80  on the inside of the back panel wall  250  other than the accommodating part. In other words, as described above, the narrower cell  83 B is located on the right-lower side of the needle bar holder mount  141  for mounting the needle bar holder  14  constituting the tope mechanism  3 , thereby corresponding to the part accommodating the stitch forming mechanism. Therefore, the outside wall reinforcing rib  80  (vertical rib  81 ) has a relatively lower height W from the back panel wall  250  so as to face the stitch forming mechanism at a closer distance. On the other hand, the wider cell  83 A is not a part for accommodating the stitch forming mechanism. Accordingly, as described above, the outside wall reinforcing rib  80  (horizontal rib  82 ) has a relatively higher height Z form the back panel wall  250 . However, the above structure may lead to insufficient rigidity over the part for accommodating the stitch forming mechanism. To overcome the above problem, the narrower area of the cell  83 , that is, the formation of the narrower cell  83 B, results in the increase of the rigidity thereof. The resultant rigidity is substantially the same as that of the wider cell  83 A. Accordingly, the concentration of stress to a certain point of the back panel wall  250  can be prevented, so that the main frame  1 A can obtain sufficient rigidity.  
         [0085]    The above arrangement of the outside wall reinforcing rib  80  ensures the sufficient rigidity of the back panel wall  250 , thereby minimizing or restricting distortion appearing on the back panel wall  250  of the arm  6  due to the reciprocating motion of the needle  16 . The above arrangement of the outside wall reinforcing rib  80  also minimizes distortion appearing on the back panel wall  250  of the cantilever support  7  and the bed  8  due to the distortion of the arm  6 . In this embodiment, the outside wall reinforcing ribs  80  extend in vertical and horizontal directions on the back panel wall  250  to define the cells  83 . This arrangement results in the sufficient rigidity of the back panel wall  250  in the case where the outside wall reinforcing rib  80  is not allowed to have a higher height in order that the main frame  1 A accommodates the stitch forming mechanism. Accordingly, a sewing machine having the above main frame  1 A can prevent vertical and horizontal vibrations of the main frame  1 A caused by the reciprocating motion of the needle  16 , thereby performing a smooth stitch forming action.  
         [0086]    In another embodiment, the outside wall reinforcing rib  80  may not be formed over the whole back panel wall  250 , but be formed over only the part of the back panel wall  250  which needs sufficient rigidity of the back panel wall  250  for accommodating the stitch forming mechanism. In another embodiment, the outside wall reinforcing ribs  80  may be arranged in order that the cells  83  have hexagonal or octagonal shapes.  
         [0087]    It should be noted that the inside wall reinforcing rib  70  has a higher height from the back panel wall  250  than that of the outside wall reinforcing rib  80 . More specifically, as shown in FIG. 8(A), at the base end of the arm  6 , the inside wall reinforcing rib  70  is formed at a height from the back panel wall  250  reaching the dividing plane  52 . In contrast, the vertical ribs  81  reach approximately halfway to the dividing plane  52  from the back panel wall  250 . As shown in FIG. 8(B), in the center portion of the cantilever support  7 , the intermediate ribs  72  have a height from the sidewall  50  reaching the dividing plane  52 . In contrast, the horizontal ribs  82  reach less than half the height of the dividing plane  52  from the sidewall  50 . A high rigidity is necessary for the inner surface wall  51  since stress generated by the vertical movement of the needle  16  is concentrated in this area. On the other hand, these height differences are necessary to maintain space at the inside of the back panel wall  250  for accommodating the stitch forming mechanism including the top mechanism  3  and the lower mechanism  4 .  
         [0088]    Couplings  
         [0089]    As shown in FIGS. 4 and 5, a plurality of couplings  90 ,  92 ,  94 , and  96  are provided in the back panel wall  250  of the main frame  1 A for joining the main frame  1 A to the frame cover  1 B. The coupling  90  is formed near the inner surface wall  51  in the area adjacent to the joint of the bed  8  and the cantilever support  7 . More specially, the coupling  90  is placed in the vicinity of the inside wall reinforcing rib  70  and the reinforcing member  60 . The above arrangement of the coupling  90  is aimed at preventing distortion of the arm  6  and the cantilever support  7  which causes swings of the top portion of the cantilever support  7  during the reciprocating motion of the needle  16 . The coupling  92  is formed near the inner surface wall  51  at the joint area of the arm  6  and the cantilever support  7 . More particularly, the coupling  92  is placed in the vicinity of the inside wall reinforcing rib  70  and the reinforcing member  60 . The coupling  94  is formed near the inner surface wall  51  in the vicinity of the end of the inside wall reinforcing rib  70  near the arm  6 . The couplings  92 ,  94  are placed on the circumference of the semicircle of the space  9  at constant intervals with respect to the coupling  90 . A plurality of couplings  96  are formed on the sides and the corners of the inside of the back panel wall  250  in order to couple the main frame  1 A and the frame cover  1 B by a uniform pressure.  
         [0090]    Screw holes  91 ,  93 ,  95 , and  97  are formed inside the couplings  90 ,  92 ,  94 , and  96 . The main frame  1 A and frame cover  1 B can be detachably joined together by inserting screws (not shown) in the screw holes  91 ,  93 ,  95 , and  97  when the couplings  90 ,  92 ,  94 , and  96  are aligned with couplings  190 ,  192 ,  194 , and  196  (see FIG. 11) provided in corresponding positions on the frame cover  1 B. Accordingly, the sewing machine is easily assembled by mounting the stitch forming mechanism to the main frame  1 A, and then screwing the frame cover  1 B to the main frame  1 A, thereby enabling cost reductions. In the case of maintenance, only undoing the screws leads to remove of the frame cover  1 B from the main frame  1 A, so that all the stitch forming mechanism is exposed. Therefore, the maintenance work is facilitated. In the present embodiment, screws are used to join the main frame  1 A to the frame cover  1 B, but bolts and nuts may also be used in place of the screws.  
         [0091]    When stress induced by the reciprocating motion of the needle  16  forces the inner surface wall  51  of the main frame  1 A and an inner surface wall  161  of the frame cover  1 B to relatively move in a vertical or horizontal directions, relative movement of the main frame  1 A and the frame cover  1 B is restricted because a plurality of couplings  190 ,  192 , and  194  (see FIG. 11) are arranged around the inner surface walls  51 ,  161 . Therefore, the inner surface wall  51  of the main frame  1 A remains contact with the inner surface wall  161  of the frame cover  1 B. A appropriate coupling between the main frame  1 A and the frame cover  1 B is maintained. Stress is transmitted from the main frame  1 A including the stitch forming mechanism which generates vibrations to the frame cover  1 B through the inner surface walls  51 ,  161  which are contact to each other, thereby dispersing over the whole frame  1 . The stress dispersion ensures the sufficient rigidity of the frame  1 . As a result, a sewing machine including the frame  1  can prevent vertical vibrations and horizontal swings of the frame  1  induced by the reciprocating motion of the needle  16 , thereby performing a smooth stitch forming action.  
         [0092]    In another embodiment, two or more than four couplings may be formed around the inner surface wall  51  of the main frame  1 A.  
         [0093]    Protrusions  
         [0094]    As shown in FIG. 4, protrusions  100 ,  101 ,  102 , and  103  are formed on the main frame  1 A at the dividing plane  52 . These protrusions  100 ,  101 ,  102 , and  103  engage with engaging units  111 ,  112 ,  113 , and  114  provided on the frame cover  1 B at the dividing plane  52  (see FIG. 11) when the main frame  1 A is joined with the frame cover  1 B. The protrusions  100 ,  101 ,  102 , and  103  are aimed at limiting the relative movement of the main frame  1 A and frame cover  1 B in the horizontal direction.  
         [0095]    Next, the reason that the sewing machine frame of the present invention is configured in this way will be described. As mentioned earlier, a swing effect occurs in the horizontal direction in the top portion of the cantilever support  7  due to the vertical movement of the needle  16 . When this happens, the main frame  1 A and frame cover  1 B can move relative to one another in the horizontal direction, shifting their relative positions. When this positional shifting occurs, a reliable joined state cannot be maintained, resulting in insufficient rigidity, thereby promoting vibrations and displacement in the frame  1 . Moreover, the main frame  1 A and frame cover  1 B are joined by screws through considerable pressure, causing a large frictional coefficient. As a result, when the relative position of the main frame  1 A and frame cover  1 B shifts, they do not easily return to their original positions. The above construction is employed because it is necessary to prevent such shifting in the relative position of the main frame  1 A and frame cover  1 B from occurring. With this construction, it is possible to maintain sufficient rigidity in the frame  1 .  
         [0096]    As shown in FIG. 9(A), the protrusion  100  protrudes from the bottom of the arm  6  at the dividing plane  52  substantially perpendicular to the frame cover  1 B and near the border between the horizontal portion on which the mechanism for reciprocally driving the needle  16  is supported and the semicircular portion by which the space  9  is formed. An opening  143  is formed in the front end of the arm  6  from which the reciprocally driving mechanism protrudes downward. The protrusion  100  is positioned to one side of the opening  143 . The protrusion  100  fits in the engaging unit  111  provided on the arm  6  of the frame cover  1 B (see FIG. 11). This configuration prevents relative movement of the main frame  1 A and frame cover  1 B generated by vibrations and displacement at the dividing plane  52  of arm  6 .  
         [0097]    As shown in FIG. 9(B), the protrusions  101  and  102  protrude from the top of the bed  8  at the dividing plane  52 , that is, at both ends of an opening  149  approximately perpendicular to the frame cover  1 B. The opening  149  is aimed for exposing rotary hook  23 . The protrusions  101 ,  102  are fitted into engaging units  112 ,  113  provided in the bed  8  of the frame cover  1 B (see FIG. 11). The above arrangement can prevent relative movement of both the main frame  1 A and the frame cover  1 B caused by vibrations and displacement at the dividing plane  52  of the bed  8  in the main frame  1 A and the frame cover  1 B.  
         [0098]    Referring to FIGS.  8 (B),  8 (C), the protrusion  103  protrudes to the frame cover  1 B being coupled at a predetermined point on the dividing plane  52  around the space  9 . The predetermined point is placed on the intermediate rib  72  constituting the inside wall reinforcing rib  70  in the vicinity of a cross point with the inner surface wall  51  around the space  9 . The protrusion  103  fits a channel-shaped engaging unit  114  (see FIG. 11) provided the periphery of the frame cover  1 B facing the space  9 . The above structure prevents vibrations and displacement at the dividing plane  52  around space  9 , thereby restricting relative movement of the coupled main frame  1 A and frame cover  1 B.  
         [0099]    Referring to FIG. 9(A), an engaging unit  110  for receiving the protrusion  104  (see FIG. 11) protruding from the dividing plane  52  below the arm  6  of the frame cover  1 B. The place of the engaging unit  110  is on the dividing plane  52  below the arm  6  of the main frame  1 A. The above arrangement prevents vibrations and displacement at the dividing plane  52  of the arm  6  of the coupled main frame  1 A and frame cover  1 B, thereby restricting relative movement of the main frame  1 A and frame cover  1 B.  
         [0100]    Top Edge  
         [0101]    As shown in FIGS. 4 and 7(A), a top edge  120  is formed across the top of the main frame  1 A for contacting the frame cover  1 B. A raised step  121  is formed across nearly the entire top edge  120 , the bottom of raised step  121  protruding toward the frame cover  1 B. The protruding portion of the raised step  121  fits into a recessed step  126  formed in a top edge  125  of the frame cover  1 B for contacting the main frame  1 A (see FIG. 11). By engaging the raised step  121  with the recessed step  126  from above, this construction can limit the relative movement of the main frame  1 A in the upward direction.  
         [0102]    Next, the reason that the sewing machine frame of the present invention is configured in this way will be described. As mentioned earlier, the portion of the main frame  1 A near the arm  6  vibrates in the vertical direction due to the vertical movement of the needle  16 . In particular, the main frame  1 A on which the top mechanism  3  is mounted for supporting the needle  16  tends to move in the upward direction. When this happens, the main frame  1 A and frame cover  1 B can move relative to one another in the vertical direction, shifting their relative positions. When this positional shifting occurs, a reliable joined state cannot be maintained, resulting in insufficient rigidity, thereby promoting vibrations and displacement in the frame  1 . Moreover, the main frame  1 A and frame cover  1 B are joined by screws through considerable pressure, causing a large frictional coefficient. As a result, when the relative position of the main frame  1 A and frame cover  1 B shifts, they do not easily return to their original positions. The above construction is employed because it is necessary to prevent such shifting in the relative position of the main frame  1 A and frame cover  1 B from occurring. With this construction, it is possible to maintain sufficient rigidity in the frame  1 .  
         [0103]    While the raised step  121  in the present embodiment is formed across nearly the entire length of the top edge  120  of the main frame  1 A that contacts the frame cover  1 B, it is not necessary for the raised step  121  to span the entire length of the top edge  120 . In view of the reason described above for forming the raised step  121 , however, it is desirable that the raised step  121  be formed on the top edge  120  at least at portions of the main frame  1 A corresponding to the arm  6 . Similarly, the recessed step  126  (see FIG. 11) should be formed on the top edge  125  at least on portions of the frame cover  1 B that correspond to the arm  6 . With this construction, it is possible to achieve sufficient rigidity for the arm  6 .  
         [0104]    A bottom edge  130  is formed across the bottom of the main frame  1 A for contacting the frame cover  1 B. A raised step  131  is formed across nearly the entire length of the bottom edge  130 , the top of the raised step  131  protruding toward the frame cover  1 B. As shown in FIG. 7(B), the raised step  131  comprises an insertion part  132  for inserting into a recessed step  136  (see FIG. 11) formed on a bottom edge  135  of the frame cover  1 B for contacting the main frame  1 A; a sliding surface  133  for guiding the raised step  131  into the recessed step  136 ; and an engaging wall  134  for engaging in the recessed step  136  after the recessed step  136  has been slid to a prescribed position. By inserting the insertion part  132  in the recessed step  136  of the frame cover  1 B and engaging the sliding surface  133  with the bottom of the recessed step  136 , it is possible to limit relative movement of the main frame  1 A in the downward direction.  
         [0105]    Next, the reason that the sewing machine frame of the present invention is configured in this way will be described. As mentioned earlier, the portion of the main frame  1 A tends to move upward due to the vertical movement of the needle  16 . When this happens, the bed  8  of the frame cover  1 B engaged with the main frame  1 A attempts to move downward relative to the main frame  1 A. As a result, the frame cover  1 B shifts vertically from the main frame  1 A, promoting the generation of vibrations and displacement in the frame  1 . Hence, it is necessary to prevent such shifting in the relative position of the main frame  1 A and frame cover  1 B from occurring. With this construction, it is possible to maintain sufficient rigidity in the frame  1 .  
         [0106]    While the raised step  131  in the present embodiment is formed across nearly the entire length of the bottom edge  130  of the main frame  1 A that contacts the frame cover  1 B, it is not necessary for the raised step  131  to span the entire length of the bottom edge  130 . In view of the reason described above for forming the raised step  131 , however, it is desirable that the raised step  131  be formed on the bottom edge  130  at least at portions of the main frame  1 A corresponding to the bed  8 . Similarly, the recessed step  136  (see FIG. 11) should be formed on the bottom edge  135  at least on portions of the frame cover  1 B that correspond to the bed  8 . With this construction, it is possible to achieve sufficient rigidity for the bed  8 .  
         [0107]    Here, the sliding surface  133  of the raised step  131  is retracted further internally than the back panel wall  250  of the main frame  1 A. When the recessed step  136  of the frame cover  1 B overlaps this portion, the sidewall of the main frame  1 A and frame cover  1 B become the same height, Accordingly, by engaging the main frame  1 A with the frame cover  1 B, the sidewall of the main frame  1 A and frame cover  1 B forms a continuous surface at this point, improving the appearance of the frame  1 .  
         [0108]    While a detailed construction of the raised step  121  described above is not shown in the drawings, this construction is similar to the raised step  131  of the bottom edge  130  shown in FIG. 7(B). However, the raised step  121  is vertically symmetrical to the raised step  131 .  
         [0109]    Flame Cover  
         [0110]    Next, the frame cover  1 B of the frame  1  will be described with reference to FIGS.  10  through needle bar  15 . FIG. 10 is a perspective view showing the external appearance of the frame cover  1 B. FIG. 11 is a perspective view showing the internal construction of the frame cover  1 B. FIG. 12 is a plan view showing the internal construction of the frame cover  1 B. FIG. 13 is a cross-sectional view along the plane of the frame cover  1 B indicated by the arrows I in FIG. 12. FIG. 14(A) is a cross-sectional view along the plane of the frame cover  1 B indicated by the arrows J in FIG. 12. FIG. 14(B) is an enlarged view showing the lower end of the frame cover  1 B. FIG. 15(A) is an enlarged plan view along the plane of the frame cover  1 B indicated by the arrows K in FIG. 12. FIG. 15(B) is an enlarged plan view along the plane of the frame cover  1 B indicated by the arrows L in FIG. 12.  
         [0111]    As shown in FIG. 10, the frame cover  1 B comprises the arm  6 , cantilever support  7 , and bed  8 , and is integrally formed of a synthetic resin with the arm  6 , cantilever support  7 , and bed  8 . The semicircular area surrounded by the arm  6 , cantilever support  7 , and bed  8  is the space  9  the main frame  1 A substantially comprises the arm  6 , the cantilever support  7 , and the bed  8  formed integrally. The semicircular space surrounded by the arm  6 , cantilever support  7 , and bed  8  is a space  9 .  
         [0112]    In addition, the frame cover  1 B comprises a front panel wall  252  constituting a front side of the sewing machine, and side wall  253  extending from a peripheral edge  252   a  of the front panel wall  252 . Especially, the surface of the frame cover  1 B facing the space  9  is designated as an inner surface wall  161 . A side portion of the arm  6  is provided with a thread cassette mount  203  in which a thread cassette including different kinds of thread.  
         [0113]    Inside Wall Reinforcing Rib  
         [0114]    As shown in FIGS. 11 and 12, an inside wall reinforcing rib  170  for reinforcing the inner surface wall  161  of the frame cover  1 B facing the space  9  is provided on the inside of the front panel wall  252  around the periphery of the space  9 . A lot of inside wall reinforcing ribs  170  are provided around the periphery of the space  9  from the joint of the arm  6  and the cantilever support  7  to the joint of the cantilever support  7  and the bed  8  in order to surround the inner surface wall  161 .  
         [0115]    The inside wall reinforcing rib  170  comprises a partitioning rib  171  spaced from the inner surface  161  and a plurality of intermediate ribs  172  intersecting with the inner surface  161  and partitioning rib  171 . The partitioning rib  171  extends from the inside of the front panel wall  252  and parallel and perpendicularly to the inner surface wall  161  in a continuous manner. The intermediate rib  172  extends from the inside of the front panel wall  252  between the inner surface wall  161  and the partitioning rib  171  at a constant intervals perpendicularly to the front panel wall  252 . The intermediate rib  172  connects the inner surface wall  161  to the partitioning rib  171 , and connects the inner surface wall  161  and the partitioning rib  171  to the front panel wall  252 . The above arrangement of the inner surface wall  161 , the partitioning rib  171 , and the intermediate ribs  172  provides a plurality of cells  173  in the space between the inner surface  161  and partitioning rib  171 . The intermediate ribs  172  are arranged radially from a center point located in the space  9 , because the inner surface wall  161  surrounding the space  9  has a semicircle shape. Accordingly, each intermediate rib  172  intersects the inner surface  161  and partitioning rib  171  at a perpendicular angle. Thus, the arrangement of the ribs is optimized, thereby reinforcing the inner surface wall  161  advantageously.  
         [0116]    The above structure of the inside wall reinforcing ribs  170  provides the rigidity equal to that of the inner surface wall  161  having a considerable thickness. In other words, the above structure of the inside wall reinforcing ribs  170  ensures the rigidity over the front panel wall  252  from the area adjacent to the joint of the arm  6  and the cantilever support  7 , through the cantilever support  7 , to the area adjacent to the joint of the cantilever support  7  and the bed  8 . A sewing machine having the frame cover  1 B can prevent horizontal vibrations and swings of the frame cover  1 B caused by the reciprocating motion of the needle  16 , thereby performing a smooth stitch forming action.  
         [0117]    In the above embodiment, the inside wall reinforcing ribs  170  are provided on the front panel wall  252  from the joint of the arm  6  and the cantilever support  7  through the  7  through the  7  to the joint of the cantilever support  7  and the bed  8 . In another embodiment, the inside wall reinforcing rib  170  may be formed over the whole of the inner surface wall  161 . In the above embodiment, a lot of intermediate ribs  172  are provided. However, in another embodiment, the number of the intermediate ribs  172  may be only one or a few. Each of the intermediate ribs  172  may be coupled or crossed to each other, so that the resultant arrangement of the intermediate ribs  172  may have a honeycomb or diagram shape.  
         [0118]    In order to further support the partitioning rib  171  of the inside wall reinforcing ribs  170 , a supplemental concave wall reinforcing rib  177  is provided outside of the inside wall reinforcing ribs  170 . The supplemental concave wall reinforcing rib  177  comprises an auxiliary partitioning rib  174  and a plurality of auxiliary intermediate ribs  175 . The auxiliary partitioning rib  174  is provided in a continuous manner along the partitioning rib  171 , while being spaced from the partitioning rib  171 . The auxiliary intermediate ribs  175  intersect the partitioning rib  171  and partitioning rib  174  at predetermined intervals, and form a plurality of cells or compartments  176  between the partitioning rib  171  and partitioning rib  174 . This construction attains further rigidity of the inner surface  161  of the space  9 . In another embodiment, supplemental concave wall reinforcing ribs may be provided outside of the inside wall reinforcing rib  70  of the main frame  1 A, if the main frame  1 A has sufficient spare space.  
         [0119]    Outside Wall Reinforcing Rib  
         [0120]    As shown in FIGS. 11 and 12, outside wall reinforcing ribs  180  are formed in a matrix shape over nearly the entire inside of the front panel wall  252 . The outside wall reinforcing rib  180  projects from the inside of the front panel wall  252 . The outside wall reinforcing rib  180  is formed of vertical ribs  181  vertically oriented when the sewing machine is placed on a working surface, and horizontal ribs  182  oriented horizontally when the sewing machine is in the same position. As shown in FIGS. 13 and 14(A), these vertical ribs  181  and horizontal ribs  182  are approximately perpendicular to the front panel wall  252 . The ends of the vertical ribs  181  and horizontal ribs  182  are joined with the side wall  253  on the side portions of the frame cover  1 B. The upper ends of the vertical ribs  181  are not coupled to the side wall  253 . This is because the upper portion of the frame cover  1 B needs sufficient space to accommodate thread cassettes and an LED display substrate. The spaces surrounded by pairs of intersecting vertical ribs  181 ,  181  and horizontal ribs  182 ,  182  form approximately square or rectangular shaped cells  183 . Hence, a plurality of cells  183  are formed on the back side of the front panel wall  252 .  
         [0121]    Among the cells  183 , the outside wall reinforcing rib  180  defining a cell  183  having a wider area is formed to have a higher height from the front panel wall  252 , compared to a cell  183  having a narrower area. The outside wall reinforcing rib  180  on the accommodating part for the stitch forming mechanism in the arm  6  or the bed  8  has a lower height from the front panel wall  252  than those of the outside wall reinforcing ribs  180  on the inside of the front panel wall  252  other than the accommodating part. The cells  183  in the vicinity of the accommodating part for the stitch forming mechanism have narrower areas than those of the cells  183  provided on the area other than the accommodating part. The reason the above arrangement has been adopted is the same as that of the main frame  1 A, so that detailed explanation will be omitted.  
         [0122]    The above arrangement of the outside wall reinforcing rib  180  ensures the sufficient rigidity of the front panel wall  252 , thereby minimizing or restricting distortion appearing on the front panel wall  252  of the arm  6  due to the reciprocating motion of the needle  16 . The above arrangement of the outside wall reinforcing rib  180  also minimizes distortion appearing on the front panel wall  252  of the cantilever support  7  and the bed  8  due to the distortion of the arm  6 . In this embodiment, the outside wall reinforcing ribs  180  extend in vertical and horizontal directions on the front panel wall  252  to define the cells  183 . This arrangement results in the sufficient rigidity of the front panel wall  252  in the case where the outside wall reinforcing rib  180  is not allowed to have a higher height in order that the frame cover  1 B accommodates the stitch forming mechanism. Accordingly, a sewing machine having the above frame cover  1 B can prevent vertical and horizontal vibrations of the frame cover  1 B caused by the reciprocating motion of the needle  16 , thereby performing a smooth stitch forming action.  
         [0123]    It should be noted that the inside wall reinforcing rib  170  has a higher height from the front panel wall  252  than that of the outside wall reinforcing rib  180 . More specifically, as shown in FIG. 14(A), at the base end of the arm  6 , the inside wall reinforcing rib  170  is formed at a height from the front panel wall  252  reaching the dividing plane  52 . In contrast, the vertical ribs  181  reach approximately halfway to the dividing plane  52  from the front panel wall  252 . The reason is as follows: the inner surface wall  161  needs sufficient rigidity, because stress induced by the reciprocating motion of the needle  16  generally tends to concentrate on the inner surface wall  161 .  
         [0124]    In another embodiment, the outside wall reinforcing rib  180  may be provided on the only part of the frame cover  1 B. Alternatively, the frame cover  1 B may have no outside wall reinforcing rib  180 . The frame cover  1 B does not need so high rigidity as that of the main frame  1 A.  
         [0125]    Couplings  
         [0126]    As shown in FIGS. 11 and 12, a plurality of couplings  190 ,  192 ,  194 , and  196  are provided in the front panel wall  252  of the main frame  1 A for joining the main frame  1 A to the frame cover  1 B. The coupling  190 ,  192 ,  194 , and  196  are placed at positions corresponding to the positions of the couplings  90 ,  92 ,  94 , and  94  of the main frame  1 A. The coupling  190  is formed near the inner surface wall  161  in the area adjacent to the joint of the bed  8  and the cantilever support  7 . More specially, the coupling  190  is placed in the vicinity of the inside wall reinforcing rib  170  formed outside of the inner surface wall  161 . The above arrangement of the coupling  190  is aimed at preventing distortion of the arm  6  and the cantilever support  7  which causes swings of the top portion of the cantilever support  7  during the reciprocating motion of the needle  16 . The coupling  192  is formed near the inner surface wall  161  at the joint area of the arm  6  and the cantilever support  7 . More particularly, the coupling  192  is placed in the vicinity of the inside wall reinforcing rib  170  outside of the inner surface wall  161 . The coupling  194  is formed near the inner surface wall  161  in the vicinity of the end of the inside wall reinforcing rib  170  near the arm  6 . The couplings  192 ,  194  are placed on the circumference of the semicircle of the space  9  at constant intervals with respect to the coupling  190 . A plurality of couplings  196  are formed on the sides and the corners of the inside of the back panel wall  250  in order to couple the main frame  1 A and the frame cover  1 B by a uniform pressure.  
         [0127]    Screw holes  191 ,  193 ,  195 , and  197  are formed inside the couplings  190 ,  192 ,  194 , and  196 . The main frame  1 A and frame cover  1 B can be detachably joined together by inserting screws (not shown) in the screw holes  191 ,  193 ,  195 , and  197  when the couplings  190 ,  192 ,  194 , and  196  are aligned with couplings  90 ,  92 ,  94 , and  96  provided in corresponding positions on the main frame  1 A.  
         [0128]    Engaging Unit  
         [0129]    As shown in FIG. 11, engaging units  111 ,  112 ,  113 , and  114  are formed in the frame cover  1 B at the dividing plane  52 . These engaging units  111 ,  112 ,  113 , and  114  engage with protrusions  100 ,  101 ,  102 , and  103  provided on the main frame  1 A at the dividing plane  52  (see FIG. 4) when the main frame  1 A is joined with the frame cover  1 B and function to limit the relative movement of the main frame  1 A and frame cover  1 B in the horizontal direction.  
         [0130]    As shown in FIG. 15(A), the engaging unit  111  is recessed in the bottom of the arm  6  on the frame cover  1 B at the dividing plane  52  and on one side of an opening  200  through which the mechanism for reciprocally driving the needle  16  protrudes downward. The engaging unit  111  engages with the protrusion  100  (see FIG. 4) formed on the arm  6  of the main frame  1 A. This construction limits relative movement of the main frame  1 A and frame cover  1 B generated by vibrations and displacement at the dividing plane  52  of the arm  6 .  
         [0131]    As shown in FIG. 15(B), the engaging units  112  and  113  are recessed in the top of the bed  8  at the dividing plane  52  and on both sides of an opening  202  for exposing the rotary hook  23 . The engaging units  112  and  113  engage with the protrusions  101  and  102  formed on the bed  8  of the main frame  1 A (see FIG. 4). This construction restricts relative movement of the main frame  1 A and frame cover  1 B caused by vibrations and displacement at the dividing plane  52  of the bed  8 .  
         [0132]    As shown in FIG. 11, the engaging unit  114  is formed in a continuous channel on the inner surface  161  of the space  9 . The protrusions  103  provided on the main frame  1 A (see FIG. 4) engage with this channel portion. This construction restricts relative movement of the main frame  1 A and frame cover  1 B caused by vibrations and displacement at the dividing plane  52  of the space  9 .  
         [0133]    Protrusion  
         [0134]    As shown in FIG. 15(A), the protrusion  104  is formed on the bottom of the arm  6  of the frame cover  1 B at the dividing plane  52  and on the opposite side of the opening  200  as that in which the engaging unit  111  is formed. The protrusion  104  protrudes substantially perpendicularly to the frame cover  1 B. The protrusion  104  fits in the engaging unit  110  provided on the arm  6  of the main frame  1 A (see FIG. 4). This construction restricts relative movement of the main frame  1 A and frame cover  1 B caused by vibrations and displacement at the dividing plane  52  of the arm  6 .  
         [0135]    Recessed Top Edge  
         [0136]    As shown in FIG. 14(A), the recessed step  126  is formed across nearly the entire top edge  125  on the frame cover  1 B that contacts the main frame  1 A for accommodating the raised step  121  formed on the top edge  120  of the main frame  1 A and engaging the raised step  121  from the top. As shown in FIG. 14(B), the recessed step  126  comprises an engaging wall  127  protruding toward the main frame  1 A for engaging the raised step  121  of the main frame  1 A when the raised step  121  is guided to a prescribed position; a sliding surface  128  for guiding the raised step  121 ; and an accommodating portion  129  for accommodating the insertion part of the raised step  121 . By accommodating the insertion part of the raised step  121  in the accommodating portion  129  and when the sliding surface of the raised step  121  engages with the sliding surface  128  from above, it is possible to limit relative movement of the main frame  1 A in the upward direction.  
         [0137]    The recessed step  136  is formed across nearly the entire bottom edge  135  of the frame cover  1 B that contacts the main frame  1 A for accommodating the raised step  131  formed on the bottom edge  130  of the main frame  1 A and engaging the raised step  131  from below. While a detailed construction of the recessed step  136  is not shown in the drawings, this construction is basically the same as the recessed step  126  of the top edge  125  shown in FIG. 14(B). However, the recessed step  136  is vertically symmetrical to the recessed step  126 . By engaging the raised step  131  with the recessed step  136 , it is possible to limit the relative movement of the main frame  1 A in the downward direction.  
         [0138]    It is understood that the foregoing description and accompanying drawings set forth the preferred embodiments of the invention at the present time. Various modifications, additions and alternative designs will, of course, become apparent to those skilled in the art in light of the foregoing teachings without departing from the spirit and scope of the disclosed invention. Thus, it should be appreciated that the invention is not limited to the disclosed embodiments but may be practiced within the full scope of the appended claims.