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 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 as shown in fig1 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.