Collet

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A collet is a holding device that forms a collar around the object to be held and exerts a strong clamping force on the object when it is tightened. It may be used to hold a workpiece or a tool.
Collets generally have a narrow clamping range which means a large number are required to hold a given range of materials, unlike a chuck that will generally cover the full range that the machine is designed for. The collet's advantage over the chuck is that it offers much higher precision when reclamping a single object to work it in a series of steps.

1 Woodwork
2 Metalwork
3 Semiconductor work
4 Internal combustion engines

[edit] Woodwork

On a wood router (a hand-held or table-mounted power tool used in woodworking), the collet is what holds the bit in place. In the U.S. it is generally for 1/4 or 1/2 inch bits, while in Europe bits are most commonly 6 mm or 8 mm. The collet is hexagonal on the outside so it can be tightened or loosened with a standard wrench, and has threads on the inside so it can be screwed on to the motor arbor.

[edit] Metalwork

There are literally hundreds of designs of collet used in the metalworking industry. There are some common designs like the 5C or the R8 collet and then there are many proprietary designs that only fit one specific manufacturer's equipment. Collets can range in holding capacity from zero to several inches in diameter. The most common collet grips a round bar or tool but there are collets for square, hexagonal and other shapes. In addition to the outside holding collets that are so common, there are collets that are used for holding a part on its inside surface so that it can be machined on its outside surface. Furthermore, it is not uncommon to make a collet that can hold irregularly shaped parts, these are variously called soft collets or emergency collets.

The two top left collets are the side and end view of one style of collet, the two right collets are from a smaller range.
The lower image is of a disassembled pin chuck that is used to hold small drills or work pieces. The left most part is the body, the collet is shown protruding from the body and the nose cap is separate, on the right.
In use, the part to be held is

inserted into the collet
the collet is pushed fully into the body ( which has a taper to match the collet machined into the front)
the nose cap is screwed onto the body ( which also has a matching taper machined into it).

When the nose cap is tightened properly, enough force is applied to the collet that the taper at the front and back of the collet are pushed into their matching tapers, causing the collet to constrict in diameter. This constriction exerts considerable force onto the shank of the workpiece (drill) that is being held in the collet. It is then securely held against any forces it may encounter during normal operation.

[edit] Semiconductor work

In semiconductor industry, die collet is used for picking a die up from a wafer after die cutting process has finished, and bonding it into a package. Some of them are made with rubber, and use vacuum for picking.