§1Holding work and tool
Every machine tool must hold two things rigidly and concentrically: the workpiece and the cutter. The devices that do so — chucks, collets, arbors — all seat onto the spindle, which turns and drives them.
Three qualities matter in any hold. It must be concentric, so the work or tool runs on the spindle axis with minimal runout (§4); it must be firm, gripping hard enough to resist the cutting force and not slip, even as speed tries to loosen it (§5); and it must be convenient, quick to load and repeatable. No single device maximises all three, so the shop keeps several — a self-centring chuck for speed, an independent chuck or collet for accuracy — and chooses by the job. The spindle taper beneath them (its own page) is what makes each interchangeable on the same machine.
Contents§2Three-jaw and four-jaw chucks
The two workhorse lathe chucks trade convenience against precision, and the difference is in how their jaws move.
A three-jaw self-centring chuck moves all three jaws together through a single scroll, so a round or hexagonal bar is centred automatically in one turn of the key — fast and convenient, and the usual choice for round stock, but its concentricity is limited by the scroll’s accuracy and wear. A four-jaw independent chuck has four jaws each adjusted separately; it takes longer to set, since each jaw is wound in while the work is dialled true with an indicator, but it can hold the work to a few micrometres of runout, grip square and irregular shapes, and deliberately set a workpiece off-centre. Three-jaw for speed on round work; four-jaw for accuracy, odd shapes and offset work. The hero contrasts their jaw arrangements and typical runout.
Contents§3Collets
For the truest running and the fastest repeatable hold on smaller work, a collet beats a chuck — a split sleeve that grips all round the work at once.
A collet is a slotted, hardened sleeve that closes evenly around the workpiece as it is drawn into a matching taper, gripping over its whole circumference rather than at three or four points. Because the grip is uniform and the collet is precisely matched to one diameter, runout is very low and loading is quick and repeatable — ideal for production and for delicate or small parts. The limitation is range: each collet suits a narrow band of sizes, so a set is needed, and large work is beyond collet capacity. Where a chuck grips at points, a collet grips all round, which is why it runs truer and marks the work less.
Contents§4Runout
Runout — how far the held work or tool wobbles off the true axis as it turns — is the single number that measures a hold’s concentricity, read as total indicator reading (TIR).
Runout is measured by resting a dial indicator against the rotating surface and reading the total swing, hence total indicator reading. Typical figures tell the story of the three holds: a collet runs at roughly 0.01 mm TIR, a three-jaw chuck around 0.05 mm (worse as the scroll wears), and a four-jaw chuck can be dialled to 0.005 mm or better with patience. Runout matters because it becomes error in the part — an off-running drill cuts oversize, an off-running workpiece turns eccentric — and because it throws the assembly out of balance at speed. When accuracy is the aim, the hold is chosen, and checked, by its runout, and this is exactly why a reamer or a boring bar, which cannot correct their own position, demand a low-runout hold.
Contents§5Grip and speed
A crucial and sometimes dangerous fact: a chuck grips less firmly the faster it spins, because centrifugal force flings the jaws outward, relieving their inward clamp.
Each jaw has mass, and spinning it at radius r throws it out with force F = m ω², where ω = 2πN/60. A 0.5 kg jaw at 50 mm radius spinning at 3000 rev/min sees ω = 314 rad/s and F = 0.5 × 314² × 0.05 = 2467 N — nearly a quarter-tonne of outward pull, directly subtracting from the clamp holding the work. Double the speed to 6000 rev/min and, since force grows with the square of speed, it quadruples to 9870 N. This is why every chuck carries a maximum rated speed, why heavy jaws are dangerous at high rpm, and why high-speed work uses collets or purpose-built high-grip chucks: beyond the rated speed, centrifugal force can relax the jaws until the work flies out.
§6Arbors and spindles
Milling cutters and grinding wheels are carried on arbors, and everything — chuck, collet or arbor — is ultimately driven by the spindle.
An arbor is a precision shaft that carries a cutter: a milling arbor mounts slab and side-and-face cutters between spacers and is supported at both ends against the heavy interrupted cutting force, while a stub arbor carries a single face mill or a grinding wheel. The spindle is the machine’s driven axis — a stiff, accurately-bearinged shaft with a standard taper socket (Morse, 7:24 or HSK, per the tapers page) into which chucks, collets and arbors all fit. Spindle accuracy sets the floor for everything: its bearings determine the runout every hold inherits, and its rigidity determines how heavy a cut the machine can take without chatter. Good workholding on a poor spindle is wasted — the whole chain from spindle bearing to cutting edge is only as true as its least accurate link.
Contents§7Quick reference
The working core of the page on one card rack.
Three-jaw
self-centring, fast
TIR ~0.05 mm
Four-jaw
independent, precise, odd shapes
dialled ~0.005 mm
Collet
grips all round, truest
TIR ~0.01 mm
Grip vs speed
F = m ω² flings jaws out
rises with speed²
Spindle
drives all · sets the runout floor