A collet is a segmented sleeve, band or collar. One of the two radial surfaces of a collet is usually tapered (i.e a truncated cone) and the other is cylindrical. The term collet commonly refers to a type of chuck that uses collets to hold either a workpiece or a tool (such as a drill) but has other mechanical applications.
An external collet is a sleeve with a cylindrical inner surface and a conical outer surface. The collet can be squeezed against a matching taper such that its inner surface contracts to a slightly smaller diameter, squeezing the tool or workpiece to hold it securely. Most often the collet is made of spring steel, with one or more kerf cuts along its length to allow it to expand and contract. This type of collet holds the external surface of the tool or workpiece being clamped. This is the most usual type of collet chuck. An external collet clamps against the internal surface or bore of a hollow cylinder. The collet's taper is internal and the collet expands when a corresponding taper is drawn or forced into the collet's internal taper.
As a clamping device, collets are capable of producing a high clamping force and accurate alignment. While the clamping surface of a collet is normally cylindrical, it can be made to accept any defined shape.
Collet chucks for machine tools
Generally, a collet chuck, considered as a unit, consists of a tapered receiving sleeve (sometimes integral with the machine spindle), the collet proper (usually made of spring steel) which is inserted into the receiving sleeve, and (often) a cap that screws over the collet, clamping it via another taper.
For machining operations, such as turning, chucks are commonly used to hold the workpiece. The table below gives a functional comparison of the three most common types of chuck used for holding workpieces.
Collets have a narrow clamping range and a large number of collets are required to hold a given range of tools (such as drills) or stock material. This gives the disadvantage of higher capital cost and makes them unsuitable for general usage in electric drills, etc. However, the collet's advantage over other types of chuck is that it combines all of the following traits into one chuck; making it highly useful for repetitive work.
Metalworking
There are many types of collet used in the metalworking industry. Common industry-standard designs are R8 (internally threaded for mills) and 5C (usually externally threaded for lathes). There are also proprietary designs which only fit one manufacturer's equipment. Collets can range in holding capacity from zero to several inches in diameter. The most common type of collet grips a round bar or tool, but there are collets for square, hexagonal, and other shapes. In addition to the outside-holding collets, there are collets used for holding a part on its inside surface so that it can be machined on the outside surface (similar to an expanding mandrel). Furthermore, it is not uncommon for machinists to make a custom collet to hold any unusual size or shape of part. These are often called emergency collets (e-collets) or soft collets (from the fact that they are bought in a soft (unhardened) state and machined as needed). Yet another type of collet is a step collet which steps up to a larger diameter from the spindle and allows holding of larger workpieces.
In use, the part to be held is inserted into the collet and then the collet is pressed (using a threaded nose cap) or drawn (using a threaded drawbar) into the body which has a conjugate taper form. The taper geometry serves to translate a portion of the axial drawing force into a radial clamping force. When properly tightened, enough force is applied to securely clamp the workpiece or tool. The cap or drawbar threads act as a screw lever, and this leverage is compounded by the taper, such that a modest torque on the screw produces an enormous clamping force.
The precise, symmetric form and rigid material of the collet provide precise, repeatable radial centering and axial concentricity. The basic mechanism fixes four of the six degrees of kinematic freedom, two locations and two angles. Collets may also be fitted to precisely align parts in the axial direction (a fifth degree of freedom) with an adjustable internal stop or by a shoulder stop machined into the internal form. The remaining sixth degree of freedom, namely the rotation of the part in the collet, may be fixed by using square, hexagonal, or other non-circular part geometry.