Which purpose?

If you mean the auxiliary spindle - this is only mounted when i need it, that was the whole idea really - its a normal Bridgeport plus a high-speed engraving/milling machine when needed.

The back of the ram was looked at, but adding a full Z-axis of any rigidity would be a considerable task, the aux idea is less cost and means not swivelling the ram.

I have even seen a full linear rail Z axis mounted between the ram and the existing mill head - the user locked the quill up all the time and moved the whole head up and down. Sounds nice but to get it working would not be cheap or easy i feel.