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An indexing head (also called a dividing head) is a specialized tool that allows a workpiece to be indexed in the rotational sense—that is, easily and precisely rotated to any angle or circular division. 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 tool is similar to a rotary table except that it is designed to be adjustable through at least 90°. Most adjustable designs allow the head to be tilted from 10° below horizontal to 90° vertical, at which point the head is parallel with the machine table.
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).
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.
Indexing plates
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°, and 30°). 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.
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