On appying power feed to tailstocks
(This is regarding manual lathes. On cnc lathes the tailstock can be powered.)
As is shown else, a tailstock can be used to hold either a workpiece or a piece of tooling. In either case it might seem that there are times when it would be useful if the tailstock could be given some sort of power-feed.
Suppose a workpiece was being held in a chuck mounted on the headstock spindle. In the tailstock is a drill in a drill chuck. It might seem that it would be useful if the tailstock could be feed under power towards the workpiece.
It might appear that this could be done by linking the tailstock to the saddle and then use the power feed to the saddle to move the tailstock. But..
The way the tailstock works means that its position up/down and in/out is only fixed when the tailstock is clamped to the bed of the lathe. When it is not clamped it is probably in the right position if nothing is happening. But it is easy to see that any force in/out or up/down or rotational can move it. This can be tested simply by trying to move it by hand when it is not clamped.
In general, it does not appear that the mechanism for clamping the tailstock can be adjusted to prevent this from happening.
The solution to this is to mount the tooling/workpiece in some way onto the cross slide.