The back spindle is a second spindle positioned opposite the main spindle.
It receives a part whose outside diameter and threads have been machined on the main spindle and machines the opposite end face, drills holes, and so on.
It is also called the "sub-spindle."
A Closer Look
With a back spindle, the front and back of a part can be machined in succession on a single machine.
The workpiece no longer has to be moved to another machine, and it comes off the lathe as a finished part — process consolidation that raises productivity and stabilizes precision.
Some models can machine on the main spindle and the back spindle simultaneously, further shortening cycle times.
The back spindle is a large part of the reason a CNC automatic lathe can go "from bar stock to finished part in one continuous operation."