What are a tailstock and a steady rest?

The tailstock is the sliding unit on the bed opposite the main spindle of a lathe. With a centre in its quill it supports the end of the workpiece in a centre hole; a live centre turns with the workpiece, a dead centre does not. A steady rest is a support with rollers or pads that holds a long shaft between its ends. A fixed steady sits on the bed, a follow rest is mounted on the carriage and travels with the tool. Both support the workpiece without driving it.

When do you need a tailstock or steady rest?

Whether support is needed depends on the length-to-diameter ratio, the cutting force, the material and the tolerance on straightness, roundness and diameter. In the simple beam model, deflection under the same force rises with the cube of the free length and falls with the fourth power of the diameter; a shaft twice as long then deflects about eight times as far. Deflection produces size variation along the length, between centres often a barrel shape and on an overhanging shaft a taper, and it can start chatter. As the shaft warms up it expands against the centre, so the tailstock thrust needs attention.

How do tailstock and steady rest work in practice?

Support takes preparation: a centre hole at the end, a round running band for the steady that you turn first, setting the steady and often gentler cutting data with extra finishing passes. Sometimes you have to move the steady to machine the full length. So machining time can rise faster than the length of the shaft; by how much depends on the length-to-diameter ratio, the material and the tolerances. Whether a centre hole may remain in the finished part belongs on the drawing; if it may not, an extra step is needed to remove it.

What are tailstock and steady rest often confused with?

A tailstock is not a sub-spindle: the sub-spindle clamps and drives the workpiece itself, the tailstock only supports it. Nor is a steady rest the guide bush of a Swiss-type lathe: the guide bush supports the bar right next to the tool while the bar slides through it, the steady supports a shaft at a fixed point or travels with the carriage.