Collet
A collet is a segmented sleeve, band or collar. One of the two radial surfaces of a collet is usually tapered 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, but collets have 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, the collet proper which is inserted into the receiving sleeve, and 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.
| - | Collet | Scroll chuck | Independent-jaw chuck |
| 1. Fast chucking | |||
| 2. Self centering | |||
| 3. Strong clamping | |||
| 4. Resistance against being jarred loose | |||
| 5. Precise centering |
Collets have a narrow clamping range and a large number of collets are required to hold a given range of tools 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 and 5C. 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. 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 or soft collets. 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 or drawn 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 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.
ER collets
The "ER" collet system, developed and patented by Swiss manufacturer Rego-Fix in 1972, and standardized as DIN 6499, is the most widely used tool clamping system in the world and today available from many producers worldwide. The standard series are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The "ER" name came from an existing "E" collet which Rego-Fix modified and appended "R" for "Rego-Fix". The series number is the opening diameter of the tapered receptacle, in millimetres. collapse to hold parts up to 1 mm smaller than the nominal collet internal size in most of the series and are available in 1 mm or 0.5 mm steps. Thus a given collet holds any diameter ranging from its nominal size to its 1-mm-smaller collapsed size, and a full set of in nominal 1 mm steps fits any possible cylindrical diameter within the capacity of the series. With an ER fixture chuck, may also serve as workholding fixtures for small parts, in addition to their usual application as toolholders with spindle chucks. Although a metric standard, ER collets with internal inch sizes are widely available for convenient use of imperial sized tooling. The spring geometry of the ER collet is well-suited only to cylindrical parts, and not typically applied to square or hexagonal forms like 5C collets.Autolock collets
"Autolock" collet chucks were designed to provide secure clamping of milling cutters with only hand tightening. They were developed in the 1940s by a now defunct UK company, Clarkson Limited, and are commonly known as Clarkson chucks. Autolock collets require cutters with threaded shank ends to screw into the collet itself. Any rotation of the cutter forces the collet against the collet cap taper which tightly clamps the cutter, the screw fitting also prevents any tendency of the cutter to pull out. Collets are only available in fixed sizes, imperial or metric, and the cutter shank must be an exact match.The tightening sequence of Autolock collets is widely misunderstood. The chuck cap itself does not tighten the collet at all, with the cap tight and no tool inserted the collet is loose in the chuck. Only when a cutter is inserted will the collet be pressed against the cap taper. The back of the cutter engages with a centering pin and further turning drives the collet against the chuck cap, tightening around the cutter shank, hence "Autolock".
The correct installation sequence as per the original specification is:
- Insert the collet and hand tighten the chuck cap
- Insert the tool and hand tighten
While threaded shank "Autolock" tools may be gripped by plain collets, such as ER, plain shank tools should never be used in an "Autolock" collet as they will not be properly clamped or aligned.
R8 collets
R8 collets were developed by Bridgeport Machines, Inc. for use in milling machines. Unusually, R8 collets fit into the machine taper itself and tools with integral R8 taper can also be directly fitted. R8 was developed to allow rapid tool changes and requires an exact match between collet and tool shank diameter.R8 collets have a keyway to prevent rotation when fitting or removing, but it is the compressed taper and not the keyway that provides the driving force. Collets are compressed by a drawbar from behind, they are self releasing and tool changes can be automated.
5C collets
Unlike most other machine collet systems, 5C collets were developed primarily for work holding. Superficially similar to R8 collets, 5C collets have an external thread at the rear for drawing the collet closed, and so work pieces may pass right through the collet and chuck. Collets are also available to hold square and hex stock. 5C collets have a limited closing range, and so shank and collet diameters must be a close match. A number of other C-series collets with different holding ranges also exist.A collet system with capabilities similar to the 5C is the 2J.
355E Collets
The SO Deckel tool grinders use these. Sometimes called U2 collets.Watchmaker collets
at Waltham, Massachusetts led to the invention of collets. Watchmakers' lathes all take collets which are sized by their external thread. The most popular size is 8 mm which came in several variations but all 8 mm collets are interchangeable. Lorch, a German Lathe maker, started with 6 mm collets and the first Boleys used a 6.5 mm collet. 6 mm collets will fit into a 6.5 mm lathe but it is a poor practice. Another popular size is the 10 mm collet used by Clement and Levin. For work holding, collets are sized in 0.1 mm increments with the number on the face being the diameter in tenths of a millimetre. Thus a 5 is a 0.5 mm collet.Watchmaker collets come in additional configurations. There are step collets which step inward to hold gear wheels by the outer perimeter. These typically were made in sets of five to accommodate a range of different size gear wheels. These, like straight rod-holding collets, close on the outer taper. Ring collets also come in sets of five and hold work from inside a hole. They open as they are tightened by an outside taper against the outer taper of the lathe headstock.
Watch collets also include taper adapters and wax or cement chucks. These collets take an insert, usually brass, to which small parts are cemented, usually with shellac.
The book The Modern Watchmaker's Lathe and How to Use it contains tables of makers and sizes; note that it refers to basic collets as split wire chucks.