Once the unsupported length of a workpiece exceeds roughly three times its diameter, deflection under cutting pressure becomes significant and grows quickly beyond that. The signs are taper along the length, chatter, poor surface finish and dimensions that differ between the ends. Long shafts, spindles and bars, and any job requiring boring or facing at the free end of a long part, are the standard cases.

A steady rest is clamped to the bed and supports the work at one fixed point while the work rotates within it, so the tool can operate beyond that point. A follow rest is mounted on the saddle and moves with the tool, supporting the workpiece directly opposite the cutting force wherever the tool happens to be. Steady rests support at a location; follow rests support at the cut.

Pads, usually bronze or phenolic, give the most rigid support and suit lower speeds with proper lubrication, but they generate heat and can mark or pick up on the workpiece if run dry. Rollers tolerate higher speeds, run cooler and cleaner and need less attention, at some cost in rigidity. For general work rollers are the easier choice; for maximum stability at low speed, pads.

Yes, almost always. The bearing surface must be round, concentric and smooth, so the usual practice is to turn a short journal at the support position before setting the rest up. Running fingers on a rough, out-of-round or interrupted surface transfers that error into the part, damages the contact faces and can produce vibration worse than the deflection the rest was fitted to cure.

Self-centring rests move all fingers together, so they close concentrically on the work and set up quickly, which suits repetitive production. Individually adjustable fingers are set one at a time, which is slower but allows the supported axis to be positioned deliberately. That matters when the support journal is not concentric with the axis being machined, as in repair work or eccentric components.

Yes. Hydraulic and pneumatic self-centring steadies open and close under program control, so a long part can be supported and released within an automatic cycle without operator intervention. They are standard on CNC turning of shafts and are usually specified with the machine, since they require a hydraulic or pneumatic supply and control integration that is awkward to retrofit.

Keep the contact faces clean and lubricated, since swarf trapped between finger and workpiece scores both. Check pads and rollers for wear and flat spots, and replace them as a set so the support stays concentric. Confirm the clamping to the bed is tight and that the base and bed ways are clean, because a rest that shifts under load is worse than no rest, and check adjustment as the part heats up during a long cut.