Thread tension regulating apparatus for an over-lock sewing machine

A thread tension regulating apparatus for an over-lock sewing machine, comprising:

• a batch conversion device including
• (i) thread tension regulation cams, fixed on a dial cam shaft, for setting thread tension pressure adapted to a stitching method,
• (ii) swing pieces exhibiting a swinging motion according to a set lift of said thread tension regulation cams, respectively,
• (iii) spring bearings provided on a dial tension shaft so as to exhibit a swinging motion via engagement with ends of said swing pieces, respectively, and
• (iv) thread tension discs for forming set thread tension pressures, respectively, under spring forces supported by said spring bearings, respectively;
• fine adjustment devices for finely adjusting, via respective adjusting knobs, the set thread tension pressures formed by said thread tension discs;
• a home position returning member;
• a first interlock device for displacing said home position returning member in a first direction in response to rotation of a respective one of the adjusting knobs from a home position in a pressurizing direction;
• and
• a second interlock device for displacing said home position returning member in the first direction in response to rotation of the respective one of the adjusting knobs from a home position in a depressurizing direction,
• wherein
• (i) said first interlock device includes a first interlock obviating device for causing said second interlock device to take a non-operating condition when said first interlock device operates, and
• (ii) said second interlock device includes a second interlock obviating device for causing said first interlock device to take a non-operating condition when said second interlock device operates.