A jig is a work-holding device that holds, supports, and locates the workpiece and guides the one or more tools to perform a specific operation.
A fixture is a work-holding device that only holds, supports, and locates the workpiece, but does not guide the tools to perform a specific operation.
The tumbler gear unit consists of three gears, having the same number of teeth and it connects the spindle gear to the fixed gear. Tumbler gear unit is also called the reversing gear unit as it is used to change the direction of feed of the tool for the same direction of rotation of the spindle. Tumbler gear unit can be engaged and disengaged with the fixed stud gear by the operation of the hand lever provided in the unit.
Faceplates are used to hold work that is too large or of such a shape that it cannot be held in a chuck or between centers. Faceplates are equipped with several slots to permit the use of bolts to secure the work, so that the axis of the work-piece may be aligned with the lathe centers. When work is mounted off – center, a counterbalance should be fastened to the faceplate to prevent imbalance and the resultant vibrations when the lathe is in operation.