The tool is similar to a rotary table except that it is designed to be adjustable through at least 90° (in fact it will travel to approximately 95°).
The workpiece can be held with a collet in the indexing head, or between centers with the help of an accompanying tailstock (sometimes called a footstock).
Indexing heads are usually used on the tables of milling machines. The milling of spirals (helixes) (such as in thread milling or in the milling of the flutes of milling cutters or reamers) requires the rotation of the workpiece in a known relation to the traversal of the cutter.
The development of CNC has helped to automate the mathematics involved in creating the toolpath for a particular helix. CNC indexing heads and rotary tables are wired to the control and are labeled with their own axis name, just as with the other axes of the machine. However, regardless of the extent to which computer software eases the calculations, the fundamental principles of indexing are the same as those in manual control.
Simple indexing consists of a series of preset holes in a backing plate, these divisions are provided for the most common angles (such as 90 °, 45 °, 30 °, etc). The remaining divisions of a circle are provided by manually rotating the dividing arm using index plates. Tables or calculations are required to use this method.