Rotary Table, Trunnion & Indexer

Rotary table, trunnion & indexer are the rotating work-positioning devices that add a fourth and fifth axis to a machining centre, and they exist because the three linear axes cannot reach every side of a part. A machining centre with only X, Y and Z cuts the top of the work, and reaches other faces only if the part is reset in the fixture; every reset costs time and accuracy, because the part must be re-located each move. The rotary devices of this entry answer by holding the work on a table that itself rotates: the workholding is mounted on a rotary table that spins about an axis, so the machine can present a different face of the part to the spindle without an operator ever touching it. This entry treats the family together, because the rotary table, the indexer and the trunnion are the same idea at different degrees of power. What they share is rotation: the ability to turn the work under the spindle, under program, and so machine more of the part in one setup.

What rotation adds

Rotation adds the dimensions the linear axes do not have. A cylindrical part — a hub, a manifold, a valve body — has features on faces all round it: a flat on one side, a drilled hole at ninety degrees to it, a slot on the far side. Machined on a plain three-axis machine, each face needs its own setup; machined on a rotary, the part is held once and rotated to bring each face to the spindle in turn, the machine machining all round the part without a single re-clamp. Rotation also changes what can be cut at all. With the rotary axis driven in step with the linear axes — continuous four-axis machining — the machine can cut features that wrap around the part: the groove that spirals round a cylinder, the profile cut round a flange, the port drilled at an angle that only a rotated part presents square to the drill. And rotation positions the part for features on faces that a plain machine could not reach at all, or only with angle plates and improvisation. The rotary table turns the machining centre from a top-cutting machine into one that can present any face of the part to the tool.

The rotary table and the indexer

The family has two grades, distinguished by how the rotation is used. The indexer — the dividing head of the older shop — rotates the work to discrete, precise angular positions and stops there: index to zero degrees and machine this face, index to ninety and machine the next, each stop accurate and repeatable to seconds of arc. It is the device for parts with features on regularly spaced faces — the flats of a hexagon, the holes round a flange, the teeth and slots round a component — machined face by face with the table locked at each stop. The rotary table proper does everything the indexer does and more: it is a full rotary axis under the machine’s control, able not only to index to stops but to rotate continuously in step with the linear axes, so the machine can cut the spiral groove and the wrapped profile as well as the indexed flat. On the modern machine the two are one device — an NC rotary table can index when indexing is wanted and rotate when rotation is wanted — the distinction surviving mainly in the manual indexer still used for simple dividing work.

The trunnion and the fifth axis

The trunnion carries the idea one stage further, and it is the rotary table that tilts. A trunnion table is a rotary axis mounted on a cradle that itself rotates about a perpendicular axis — typically the table rotates (the C or A axis) and the whole cradle tilts (the A or B axis) — so the work can be not only turned about one axis but tipped, any face of a cube-shaped part angled to the tool. The trunnion is the workhorse of the five-sided part: mounted on it, a component is machined on its top, its four sides and often its bottom in one setup, each face and angle brought square to the spindle by the cradle’s tilt. And when the trunnion’s two rotary axes are driven continuously in step with the three linear axes, the machine becomes a true five-axis machining centre of the kind this wiki treats under five-axis machining: the tool angled to the work to reach undercuts and contours, the part turned and tipped under a fixed tool point, cutting the complex sculptured forms that only five simultaneous axes can produce. The trunnion is how many shops get five-axis capability — a rotary-and-tilt unit bolted to the table of a capable three-axis machine.

Workholding on the rotary

A rotary table is only as useful as what is mounted on it, and the workholding it carries is the same family this wiki treats under workholding, set on a rotating face. The simplest mount is a fixture plate bolted to the table’s face, carrying the part or parts in their fixtures, rotated to present each to the spindle. For round work the table often carries a chuck or collet, so a cylindrical blank is gripped on the table’s axis and machined all round. For long parts a rotary table pairs with a tailstock on the far end, the part held between the two like work between centres, rotating about its long axis while the machine cuts its length. And for families of parts, a tombstone — the four-sided carrier this wiki describes — stands on the rotary table, its faces each carrying work, the table indexing the tombstone round so the machine reaches every face, multiplying the parts in one setup. The rotary table, in every case, is the platform that turns the workholding — and the work — to the spindle.

Rotation and the modern machine

The rotary table, trunnion and indexer have followed the rest of the machine into full numerical control, and on the modern machining centre they are driven axes like any other. The rotary axis has its own servo and its own encoder, its position commanded by the program and its coordinate known to the control as part of the machine’s rotary coordinate system, so the machine indexes the table to a face, machines it, rotates to the next, and machines on — the whole sequence under the same numerical control that moves X, Y and Z. Mounted on a machine tool that can drive them, the rotary devices turn the machining centre into the four-axis and five-axis machine this wiki’s machining entries describe, machining round, under and between the faces of a part in one setup. In the modern shop the rotary table is not an accessory but an axis — machining more of the part in fewer setups, with fewer hands and greater accuracy than the three linear axes alone could ever manage.