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 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 fig1 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.