| 摘要 |
A positive feed spindle with automatic retraction for driving a thread cutting tool is provided by using a fixed feed nut threaded on the spindle. A motor drives the spindle through a gear train that includes a first coupling gear with upper side teeth for engaging lower side teeth of a second coupling gear that also has upper side teeth engaging lower side teeth of a third coupling gear. The first and third coupling gears are continually driven by the motor in opposite directions through radial teeth of each. The second coupling gear on the same shaft as the first and third coupling gear is automatically raised when the spindle has been driven a predetermined extent by raising the shaft to disengage from the first and engage the third to reverse direction of the spindle. Radial teeth of the second coupling gear remain meshed with teeth of an idle gear in the drive train. When the spindle has been withdrawn the same extent, the shaft is automatically lowered just enough to disengage the second coupling gear from the third without re-engaging the first coupling gear until another thread tapping cycle is manually started.
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