Headstock & Turret
Headstock & turret are the two working units at the ends of every turning machine, and between them they do the whole job of turning. The headstock is the unit that holds and rotates the work: it houses the main spindle, the chuck at its nose grips the part, and the machine’s drive turns it at the commanded speed. The turret is the unit that holds the tools: a block of stations arranged round its face, each carrying a cutting tool, that indexes the next tool into the cut as the program calls for it. Where the headstock makes the work spin and the turret brings the tools to it, the two units work across the machine’s structure — the tool cutting the rotating work — and the accuracy of the part is decided by how truly the headstock spins and how exactly the turret positions. On the manual lathe the headstock has been there since the beginning and the turret was an early labour-saver; on the turning centre this wiki treats, both are servo-driven, program-controlled units doing their work automatically. This entry sets out what each unit is, what it does, and how the pair make a turning machine.
The headstock
The headstock is the business end that makes the work turn, and everything about a lathe’s accuracy begins there. It is the casting at the machine’s left, fixed to the bed, that carries the main spindle in its bearings; the spindle’s nose holds the chuck or collet that grips the work. What the machinist feels as the machine’s quality is largely the headstock’s: the spindle bearings must hold the spindle rigidly and run true, so the work rotates without wobble and the turned diameters are true to the axis; the spindle’s nose and its taper must seat the chuck concentrically, so what is gripped runs true to what is turned; and the drive must deliver the speed and power the cut needs, smoothly enough not to mark the surface. On the older machine the headstock hid a gearbox and a clutch, the operator changing speed by levers; on the CNC lathe the drive is a servomotor turning the spindle directly, its speed commanded by program and held precisely through the cut, with the spindle’s angular position known to the control so the machine can orient it and drive it as the C axis when the work must be milled as well as turned.
The turret
The turret is the unit that answers the oldest question of the lathe — what to do when one tool is not enough. On the earliest lathes the operator changed the tool by hand for every operation: a tool for facing, one for turning, one for the groove, one for the thread. The turret ends that by carrying many tools at once, mounted round its working face, and indexing — rotating to bring the station holding the required tool into the cutting position — as the program demands. The turret sits on the machine’s cross-slide, so it carries the whole movement of the tool: it feeds the tool into the work for the cut, moves it along the work for turning, and then indexes the next tool for the next operation, tool after tool, until the part is complete. Each station on the turret holds a toolholder clamped in its bore — the turning tool, the boring bar, the drill, the tap — set to a known position, and the accuracy of every operation rests on the turret stopping at the same place every time: a turret that repeats its index to a few micrometres holds the part’s diameters and faces true, and one that drifts writes its own error into every tool it presents.
Turrets, static and driven
The turret comes in the forms that suit the work, and the distinction that matters on modern machines is between static and driven tooling. A static turret simply holds cutting tools that cut against the rotating work — the turning, facing, grooving and threading tools that need no power of their own. A driven or live turret carries stations that can themselves rotate, each driven by a small motor through the turret, so the turret can present a rotating tool — a drill, an end mill, a tap — against the work. A driven turret turns the turning centre into a machine that mills as well as turns: with the spindle holding the work as the C axis, the driven tool can mill flats, drill cross holes and cut slots while the part is still in the chuck, completing in one setup what would otherwise need a second machine. The two turrets that some large turning centres carry, one above and one below, or the twin turrets of the mill-turn machines this wiki treats under the mill-turn machine, push the same idea further — more tools at the work, cutting both ends and both sides of the part at once.
The pair at work
Put together, the headstock and the turret are the whole of turning, and their coordinated work is what the program commands. The part is gripped in the headstock’s chuck; the spindle turns it at the commanded speed; the turret feeds its tool in, cuts the diameter or the face, withdraws, indexes the next tool, and cuts again; and through it all the headstock holds the work spinning true and the turret holds each tool exactly on centre. The job cycle this wiki describes runs on exactly this pair: the part loaded, the tools presented by the turret one after another, the spindle turning and stopping and orienting as the operations demand, until the finished part is parted off and the next begins. And as with the automatic tool changer on the mill, the turret’s value is that the machine changes its own tools — working under the control this wiki treats under CNC machining, the turning machine runs operation after operation with the same automatic rhythm that defines the modern machine shop.
The units and the machine
The headstock and the turret are the parts that most define a turning machine, and the rest of the machine exists to serve them. The bed and the structure carry them rigidly; the tailstock, where the work is long, supports the far end; the coolant and the chip management wash the cut away; and the control commands the pair through the whole turning cycle. Every specification that a shop reads in a turning machine — the maximum turning diameter, the spindle speed and power, the number of turret stations, the repeatability of the index — is a specification of these two units, and the quality of the parts the machine makes is decided by how the headstock spins and how the turret repeats. They are the classic machine-tool pair, the one making the work turn and the other bringing the tools to it, and between them they have turned round parts since the lathe began — now driven by servos and program to the accuracies that this wiki’s turning entries describe.