Which one moves. A turning centre rotates the workpiece against a stationary tool, so it excels at anything concentric with an axis: diameters, faces, bores, grooves and threads. A machining centre holds the workpiece still and rotates the tool, which suits prismatic parts and surfaces. Modern turn-mill machines blur the line by adding driven tooling, but the underlying distinction still determines what each is efficient at.

It follows the part and the volume. Chuck and collet work suits individual blanks, castings, forgings and larger parts, loaded one at a time. Bar work feeds continuous stock through the spindle and produces part after part unattended, which is how small turned components are made economically in quantity. Bar capacity is limited by the spindle bore, so part diameter effectively decides whether bar work is possible.

The ability to mill, drill and tap features that are not concentric with the turning axis, without moving the part to another machine. The C axis positions the spindle as a rotary axis so features can be placed at any angular position, and driven tooling in the turret provides the rotation to cut them. Together they eliminate a large share of the secondary operations that plain turning would require.

Because without one, driven tooling can only cut on the centreline or on a radius through it. A Y axis moves the tool off centre, which is what makes real milling possible on a lathe: flats, pockets, off-centre holes and slots away from the axis. It is the difference between a lathe that can drill a cross-hole and one that can genuinely complete a milled feature.

To take the part from the main spindle and present its back face for machining. The component is parted off into the sub-spindle and the second end is finished in the same cycle, so the part leaves the machine complete. That removes a second operation, its setup and the concentricity error that comes from re-gripping a part, which is why sub-spindle machines dominate small precision turned work.

By whether the machine can hold every tool a complete part needs, including duplicate and sister tools for unattended running. Count the positions, but also check how many can carry driven tooling, since that is usually a smaller number, and whether tools interfere with each other or the tailstock at the required positions. A machine that cannot hold the whole job forces a tooling change mid-run.

A bed inclined behind the spindle rather than horizontal. Chips fall clear into a conveyor by gravity instead of accumulating on the ways, the operator has better access to the chuck, and the structure is stiffer in the direction of the main cutting force. Virtually all CNC turning centres use it, which is one of the clearest differences from a conventional manual lathe.