Controls tool position along the workpiece and across it, which is enough to produce every feature concentric with the turning axis: diameters, faces, tapers, radii, grooves, undercuts, bores and threads. Anything off-axis, such as a cross-hole or a flat, requires either driven tooling on the same machine or a secondary operation elsewhere. For much general turned work, two axes is all that is needed.

Cross-drilling, tapping and slotting on the centreline or on a radius passing through it, positioned angularly by the C axis. That covers a large proportion of the secondary features on turned parts, such as cross-holes, keyways and end-face drilling, and removes the setup and handling of a separate operation. What it cannot do is mill features away from the centreline, which is what a Y axis adds.

Swing over the bed and over the cross-slide, which limit part diameter; maximum turning length or distance between centres; spindle bore, which governs the largest bar that can pass through for bar work; and spindle power and torque at the speeds you will use. Check swing over the cross-slide in particular, since it is smaller than swing over the bed and is what actually constrains many parts.

Considerably. A standard three-jaw chuck is quick but repeats and centres only moderately well. Collets grip ground bar accurately and repeatably and suit smaller diameters. Soft jaws bored in place give very good runout and are the normal answer for accurate second operations. Where a part must run true to an existing feature, workholding choice matters more than machine specification.

Once the part extends much beyond about three diameters from the chuck, deflection under cutting pressure becomes significant and produces taper, chatter and poor finish. A tailstock supports the free end for work that can accept a centre; a steady rest supports along the length where the end must remain accessible. Long shaft work generally requires one or the other as a matter of course.

A bar feeder to supply stock, a parts catcher or conveyor to collect finished components, a chip conveyor to clear swarf, and reliable tool life management including sister tooling. Beyond hardware, it needs stable processes and in-process checking, since an unattended machine will happily produce a full magazine of parts out of tolerance if a tool wears or breaks unnoticed.

By part size and volume. Bar work suits parts small enough to pass through the spindle bore, made in quantity, where the machine can run continuously with no loading. Chuck work suits larger parts, castings, forgings and second operations, and accepts manual or robotic loading. The spindle bore is the practical dividing line, and it is fixed at purchase, so it deserves attention early.