One of the simplest devices for partially automating an assembly process is a rotary indexing table.

This device consists of a circular steel plate and a drive for turning it. Parts rest in nests spaced equally along the table’s circumference. Self-contained machines and tooling for specific tasks are arrayed around the perimeter. As the table rotates, it stops intermittently to “dwell” for a fixed period of time, so that an assembly operation can be performed at each station.

The operation is analagous to that of a rotary dial assembly machine, except that in the latter, the operations at every station are fully automated. However, with the proliferation of sophisticated motion control systems, the practical distinction between the two approaches is mainly cost. Rotary indexing tables are typically smaller and more economical. Rotary indexing tables work well with machines that perform simple, fast, vertical operations, such as presses, screwdrivers, riveters, dispensers, pad printers, pick-and-place units, and ultrasonic or resistance welders. Parts can be loaded and unloaded manually or automatically.

Besides boosting output, indexing tables improve safety. Hazardous operations can be placed on the opposite side of the table from the operator. A light curtain or safety glass installed above the table, along its diameter, will ensure that the hazardous operation and the operator remain separated.

Indexing tables can be pneumatically or electrically powered. Which to use depends on indexing speed, load size, cost and accuracy requirements.

Pneumatic tables are best-suited for small and medium loads, says Werner R. Stutz, vice president of Starion Products Corp. (Patterson, NY). This type of table is powered by one or two pneumatic cylinders, depending on the load. Each cylinder actuation represents one index. A pawl mechanism locks the table in place during the cylinder’s return stroke. In some models, this mechanism can be adjusted to change the number of indexes—a money-saving feature if stations must be added later.

Electric tables are driven by an electric motor and a worm gear or camshaft. They are generally faster and more responsive than pneumatic tables, and they can handle heavier loads, says Bob Antic, sales manager at Fibro Inc. (Rockford, IL). However, they are also more expensive.

With a camshaft, the cam controls the number of indexes, as well as the acceleration, deceleration and velocity of each index. Usually, one rotation of the cam provides one dwell and one index. If additional indexing positions are needed, the cam must be changed, which can be a significant expense.

With a worm gear, the index characteristics are controlled directly by the motor. Worm gears are available in gear ratios ranging from 10-to-1 to 360-to-1.

“The advantage of [a worm-gear drive] is that you can go to any location within the rotation; you just tell the motor to go to that location,” says Gary Wester, vice president of sales and marketing at Lintech Inc. (Monrovia, CA). “However, a worm gear tends to be more expensive [than a cam drive].”

The bearings that support rotary indexing tables determine their load capacity and accuracy, and should not be overlooked, says Wester. Recirculating ball bearings are the least expensive option, but they don’t provide as much axial stiffness or load capacity. Angular contact bearings have better load capacity and axial stiffness and are slightly more expensive. Cross-roller bearings offer the highest load capacity and the greatest axial stiffness, but they are also the most expensive.

When specifying a rotary indexing table, engineers should provide the table’s diameter, the number of stations, dwell time, accuracy requirements, and the size and weight of both the parts and fixtures. They should also identify the axial, radial and tangential forces that will be exerted on the table. Tables with unequal indexes can be made.