Principally the Y axis. Driven tooling alone allows cutting on or through the centreline, which covers cross-holes and end-face work. A Y axis moves the tool off centre, making flats, pockets, off-centre holes and slots practical anywhere on the part. Combined with a sub-spindle, that is what allows a component to be finished completely rather than merely having its secondary operations reduced.

Machining a component complete in a single cycle, so it leaves the machine finished rather than going on to another operation. Beyond the obvious saving in handling and work in progress, it removes the location error introduced every time a part is re-gripped. Where a tolerance is specified between features that would otherwise be cut in different setups, single-setup machining is often the only practical way to hold it.

It approaches the main spindle, grips the part, and takes it as it is parted off, then presents the second end to the tooling. The transfer happens under program control with the part still rotating, and synchronisation between the spindles is what makes it possible without marking or losing the part. The second end is then machined in the same cycle, completing the component.

Yes, appreciably. Multiple axes, two spindles, several tool carriers and synchronised operations mean the program controls things happening at once rather than in sequence, and the potential for collision is much greater than on a lathe. CAM with accurate machine kinematics and full simulation is standard practice, and a correctly configured post-processor for the specific machine is essential rather than optional.

The machine becomes a single point of failure for the whole component rather than for one operation, so a breakdown stops everything rather than delaying a stage. Setup errors affect the complete part. Cycle times are longer per part, so a machine down is more disruptive. Shops usually mitigate with tool life management, in-process probing and by keeping the capability to split the job if needed.

When parts are complex, tolerances between turned and milled features are tight, batch sizes are moderate, and labour or work in progress is a significant cost. If parts are simple, if the milled features are trivial, or if the shop already has spare capacity on a mill, separate operations may be cheaper and more flexible. The case usually rests on accuracy and lead time as much as on labour.

Driven tool holders are specific to the turret and considerably more expensive than static holders, and their speed, torque and runout determine what milling is actually possible. Check how many positions can accept driven tools and whether they can be positioned without interference. Tool length and clearance are more constrained than on a machining centre, particularly around the sub-spindle and tailstock.