Workholding Chucks

The chuck is the workholding of round work, and it is the pair to the vice that this wiki’s workholding group has already met: where the vice holds a prismatic part between flat jaws, the chuck grips cylindrical, hexagonal and bar work on a turning machine or turning centre, holding it while the tools of the machine turn it. A chuck bolts or draws on to the spindle nose of the machine, at the headstock, and it turns with the spindle, carrying the work; its jaws close on the work and hold it firm against the forces of the turning tools. The chuck family has three great members — the scroll chuck, the collet chuck and the power chuck — and this entry sets out each of them and the principles of gripping round work truly.

The scroll chuck

The scroll chuck is the everyday chuck, and its name comes from the mechanism that makes it self-centring. Behind its three jaws lies a scroll plate — a disc cut with a spiral groove — and the three jaws engage that groove; when the chuck key turns the scroll, the spiral drives all three jaws in or out together, so they converge on the centre at the same rate. A part pushed between the three jaws is thus self-centred: one key, a few turns, and a round, hexagonal or other regular section is held on the machine’s axis without the user setting each jaw. Its limits follow from the same mechanism: the scroll wears, the jaws grip with slightly different pressure, and a worn or dirty scroll chuck holds the work with a small runout that must be indicated and corrected when the part must run true. The scroll chuck is the quick, general chuck of the lathe and the turning centre, its hardened jaws often faced with soft top jaws for holding finished work.

The independent and four-jaw chuck

For the work the scroll cannot centre, the four-jaw independent chuck is the precision answer: four jaws, each moved by its own screw, so each is set separately. There is no scroll to centre the work — the user sets each jaw against the part, adjusting the four in turn with an indicator reading the work, until the part runs true on the machine’s axis; a slow, careful setting, but one that holds the work truly. Because the jaws are independent, the four-jaw chuck also grips what a scroll cannot: the rectangular or irregular blank with no round surface to centre on, and the part held offset to machine an eccentric feature.

The collet chuck

The collet chuck is the chuck of accuracy and of bar work, and it grips by a different principle. Instead of jaws, it carries a collet — a hardened, slotted sleeve, split along its length, whose bore is sized to grip a bar of one diameter. The collet is drawn into a matching taper in the chuck by a draw tube, and the taper squeezes the collet’s slits closed, so the collet collapses evenly on the bar and grips it over its whole circumference — not at three points, as a jaw chuck does, but all the way round. That full-circumference grip gives the collet its qualities: the work is centred accurately and consistently, bar after bar, and thin and delicate bars are held without the three-point distortion of jaws. Each collet fits one diameter, so the shop keeps a rack of them, and the change from one size to another is a matter of changing the collet. On the turning machine with a bar feeder, the collet chuck holds the bar while the machine pulls stock through as each part is made — the setup of the swiss-type machine and of every long run of small round parts from bar.

The power chuck

The power chuck is the scroll chuck made for production, and its jaws are moved by power rather than by a key. Built on the scroll principle — its three jaws, often replaceable, self-centring on the work — the power chuck is closed and opened by a hydraulic or pneumatic cylinder behind the spindle, acting through a drawbar on the chuck’s mechanism, so that the machine clamps and unclamps the work without the operator turning a key. The clamping force is set at the cylinder and is the same every time, the jaws open and close in a second, and the chuck clamps at a chosen point of the cycle as the bar feeds or the robot loads. That is why the power chuck is the chuck of the production turning centre and the automated cell, gripping each part the same way, thousands of times a shift, with a force set once and repeated.

Gripping round work truly

Whatever the chuck, the principles of holding round work are the same, and they follow the workholding rules this group sets out. The chuck must hold the part against the cutting forces: the jaws grip the work’s diameter firmly enough that the tool cannot turn the part in the chuck, which is why the gripped length of the part is kept as short as is practical and the work is held close to the jaws — an overhang acts as a lever and springs under the cut. Long work is supported at its outer end by the tailstock centre or a steady, taking the cutting force off the chuck. The clamping must not distort the work: a thin-walled ring or a delicate tube gripped too hard is crushed out of round, so its chucking force is set to hold without squeezing, and soft jaws or a full collet spread the grip instead of three hard points. And the work must run true: the part is indicated at the chuck, the chuck’s own runout is known, and jaws and scroll are kept clean and free of chips, for a chip under a jaw or a worn scroll is a part running out of true.

Choosing a chuck

The three chucks answer three needs, and the choice is set by the work. The scroll chuck is the general chuck: quick, self-centring, good for the round and hexagonal stock of everyday turning. The collet chuck is the accurate chuck: the bar work and the small precision parts that must be centred truly and held without marks, size after size by a rack of collets. The power chuck is the production chuck: the high-volume part loaded by feeder or robot, gripped with a repeatable force at speed. And for the awkward, the rectangular, the eccentric or the truly accurate setup, the four-jaw independent chuck is set by hand, jaw by jaw, to hold what the others cannot centre. Between them they hold the round work of the turning centre and the lathe, from the bar end to the finished part — the chucking family that carries the cylindrical work of the machine shop.