The process of enlarging the end of a hole to an even depth for housing the socket screw head is called as counter boring.
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.