Indexing (motion)

Indexing in reference to motion is moving (or being moved) into a new position or location quickly and easily but also precisely. After a machine part has been indexed, its location is known to within a few hundredths of a millimeter (thousandths of an inch), or often even to within a few thousandths of a millimeter (ten-thousandths of an inch), despite the fact that no elaborate measuring or layout was needed to establish that location. Indexing is a necessary kind of motion in many areas of mechanical engineering and machining. A part that indexes, or can be indexed, is said to be indexable.

Usually when the word "indexing" is used, it refers specifically to rotation. That is, indexing is most often the quick and easy but precise rotation of a machine part through a certain known number of degrees. However, the swapping of one part for another, or other controlled movements, are also sometimes referred to as "indexing", even if rotation is not the focus.__TOC__

Examples from everyday life

There are various examples of indexing that laypersons (non-engineers and non-machinists) can find in everyday life. These motions are not always called by the name "indexing", but the idea is essentially similar:
* The motion of pins inside a pin tumbler lock, which the correct key can move quickly and easily but also rather precisely into the correct position to allow the lock's cylinder to turn
* Indexable driver bits for screwdrivers
* The motion of a retractable utility knife blade, which often will have well-defined discrete positions (fully retracted, ¼-exposed, ½-exposed, ¾-exposed, fully exposed)
* The indexing of a revolver's cylinder with each shot

Manufacturing applications

Indexing is vital in manufacturing, especially mass production, where a well-defined cycle of motions must be repeated quickly and easily—but precisely—for each interchangeable part that is made. Without indexing capability, all manufacturing would have to be done on a craft basis, and interchangeable parts would have very high unit cost because of the time and skill needed to produce each unit. In fact, the evolution of modern technologies depended on the shift in methods from crafts (in which toolpath is controlled via operator skill) to indexing-capable toolpath control.

How indexing is achieved in manufacturing

Indexing capability is provided in two fundamental ways: with or without IT.

Non-IT-assisted physical guidance

Non-IT-assisted physical guidance was the first means of providing indexing capability, via purely mechanical means. It allowed the Industrial Revolution to progress into the Machine Age. It is achieved by jigs, fixtures, and machine tool parts and accessories, which control toolpath by the very nature of their shape, physically limiting the path for motion. Some archetypal examples, developed to perfection before the advent of the IT era, are , the turrets on manual turret lathes, and indexing heads for manual milling machines. Purely mechanical indexing is still a vital part of current technology, even as it has been extended to newer uses, such as the indexing of CNC milling machine toolholders or of indexable cutter inserts, whose precisely controlled size and shape allows them to be rotated or replaced quickly and easily without changing overall tool geometry.

IT-assisted physical guidance

IT-assisted physical guidance (for example, via NC, CNC, or robotics) has been developed since the World War II era and uses electromechanical and electrohydraulic servomechanisms to translate digital information into position control. These systems also ultimately physically limit the path for motion, as jigs and other purely mechanical means do; but they do it not simply through their own shape, but rather using changeable information.