index

To index is to move a workpiece or tool to a predetermined position with precision, then lock it there. In manufacturing, indexing is the controlled advance of material through a sequence of work stations, each stopping at an exact location for the next operation. The most common application is the indexing table on a milling machine or rotary transfer machine, where a part rotates to align with successive cutters or stations.

Indexing mechanisms fall into two broad types. Mechanical indexing uses a fixed gear ratio, cam, or detent system: a ratchet and pawl, for example, can advance a circular table by one tooth per cycle. Hydraulic or pneumatic indexing relies on actuators and solenoid valves to position the table and hold it against cutting forces. CNC machines use servo motors or stepper motors to index continuously to any point within a range, rather than stepping through fixed stops.

Where indexing fails and why

Wear in the lock mechanism, backlash in gears, or loss of hydraulic pressure allows the workpiece to drift during cutting. Dead-length tolerances stack up across multiple indexed stations if the machine lacks rigidity or if parts are loaded without repeatable location. Dust and coolant clogging detent pins or solenoid valves are common causes of missed or erratic indexing on older transfer machines.

The term comes from the Latin index, meaning a pointer or indicator. In shop language, indexing and "stepping" are often used interchangeably, though indexing emphasizes the precision location whereas stepping may imply any incremental movement. In gear cutting, an index plate or dividing head holds a blank at specific angular positions to cut the correct number of teeth. On assembly lines and transfer machines, indexing time is part of the machine cycle and is separate from cutting or processing time.

Repeatability and resolution define the quality of an indexing system. A mechanical ratchet table might repeat to within 0.01 inch. A servo-indexed CNC table might hold 0.001 inch. For high-speed production, indexing speed matters: a cam-driven index on a transfer machine may cycle in 2 to 5 seconds, while a slow hydraulic index can take 10 seconds or more. The choice of indexing method reflects the trade-off between cost, speed, and accuracy demanded by the job.