An internal thread forming apparatus comprising:
- a housing body adapted to be fixed on a machine tool;
- a driving spindle journalled in a top portion of said housing body and adapted to be connected securely to a rotatable main shaft of said machine tool, so as to rotate with said rotatable main shaft;
- a speed reduction gearing, installed in said housing body, connected to said driving spindle, including a rotating output member rotatable at a speed less than that of said driving spindle;
- a feeding device including a movable body movable vertically within a lower portion of said housing body, a rotary lever installed on said housing body and actuatable to move said movable body downward, and a first return spring biasing said rotary lever to return to a static position;
- a positive and reverse revolution gearing mounted in said movable body and including a positive-revolution face gear, a reverse-revolution face gear, and at least one idle face gear meshing with said positive-revolution face gear and said reverse-revolution face gear, so as to rotate said positive-revolution and reverse-revolution face gears in opposite directions, said positive and reverse revolution gearing being movable synchronously with said movable body in said housing body, said positive-revolution and reverse-revolution face gears being rotatable by said rotating output member of said speed reduction gearing;
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- an automatic clutch including a positive-revolution driving body mounted rotatably in said housing body and connected securely to said positive-revolution face gear so as to rotate said positive-revolution driving body synchronously with said positive-revolution face gear, a reverse-revolution driving body mounted rotatably in said housing body and connected securely to said reverse-revolution face gear so as to rotate said reverse-revolution driving body synchronously with said reverse-revolution face gear, an output spindle mounted slidably within said movable body, a driving block secured to said output spindle and carried on said output spindle to move between an uppermost position, where said driving block engages said positive-revolution driving body, and a lowermost position, where said driving block engages said reverse-revolution driving body, and a second return spring installed in said housing body so as to bias said positive-revolution driving body to engage with said driving block, said output spindle being adapted to mount a tap on a lower end thereof, said positive and reverse revolution gearing impelling said output spindle to move downward while keeping said driving block to engage with said positive-revolution driving body when said rotary lever is actuated, engagement of said driving block with said positive-revolution driving body rotating said tap in one direction, while engagement of said driving block with said reverse-revolution driving body rotates said tap in the opposite direction;
- whereby, when said rotary lever is released after feeding said tap with a predetermined depth of cut, said first return spring biases said movable body to move upward relative to said output spindle, due to the fact that said tap is stuck in a workpiece to be machined, so as to engage said reverse-revolution driving body with said driving block, thereby removing said tap from said workpiece with assistance of said first return spring.