Collets and chucks are two common types of mechanism used to hold spindle tooling or workpieces. Technically speaking, a collet is a type of chuck, but the two terms are usually used exclusively. A 'chuck' usually refers to any type of work holder which is not a collet.
Collets and chucks have distinct advantages and intended applications, but which is better for your application?
One of the most important considerations is reducing runout. Runout is the cumulative inaccuracy of a rotating tool. Runout is inherent within any system, and cannot be compensated, but can be minimised by the use of high-accuracy rotary tooling. Lower runout means even wear and a longer tool life, as well as a better finish. A lower runout usually goes hand-in-hand with higher accuracy, longer tool life, and higher speed capabilities.
A collet is a collar-like tool holder, usually manufactured from an elastic material such as spring steel, and tightened by way of a collet nut - this nut has an eliptical hole so as it is turned the collet's inside diameter increases or decreases depending on the direction. The most commonly-used collet in modern machining is an ER collet.
The main advantage of a collet is that its cylindrical construction gives a greater number of contact points when compared to a three-jaw chuck. More points of contact mean greater stability, which makes collets particularly useful for gripping extrusions. More points of contact has several additional benefits within a CNC machine, such as reduced runout, even tool wear, and generally increasing tool life. Generally, collets have lower runout compared to jaw chucks.
The number of segmentations on a collet can vary from type to type, which increases versatility compared to 3 jaw constructions. More segments means greater flexibility and a larger gripping range, at the cost of some of the concentricity and stability of more rigid collets.
Collets are also generally better suited to very small tooling when compared to jaw-style chucks. When choosing a collet with the correct gripping range, it is important to remember that all collets collapse onto the chosen tooling - they are not designed to expand. If a size is written on the collet itself, this is usually the maximum tooling diameter which can be accommodated.
The downside of collets is that they are often less suitable for general use due to the smaller clamping ange - several collets are required to achieve the same range as a standard jaw chuck. Collets are most useful in repetitive work where clamping requirement does not vary significantly.
When compared to collets, jaw-style chucks represent an older, but nonetheless reliable method of toolholding, which relies on several (usually three) mechanical jaws to hold a piece of tooling.
Jaws offer a larger gripping range - collets are a solid construction, usually manufactured from spring steel, but with a very limited clamping range (usually no more than 1mm per collet). Jaws, on the other hand, are opened and closed by a mechanism with a much larger range, and are intended to accommodate a full range of tool sizes, with no need for supplementary devices such as alternatively sized collets.
This more versatile mechanism makes chucks suitable for larger sizes, and may be a preferable option when a wide range of drill sizes is being used.
Jaw chucks generally utilise a longer internal grip length when compared to chucks, which provides a greater amount of contact with the enhanced gripping ability. A greater grip strength dampens vibration and reducing chatter, allowing the chuck to operate at higher speeds.
Chucks also allow for the use of soft jaws and hard jaws. While collets are a relatively restrictive in their construction. Hard jaws are usually manufactured from hardened steel, and are not designed to be modified, but soft jaws are generally manufactured from machinable steel, and are designed to be modifiable to suit the shape of a custom application. The advantage of jaw-style chucks is that their increased design simplicity allows for greater freedom in modification. Jaw chucks are also available in several different pre-made variants, including jaws which are made for certain tapers and locking methods.
The video below by Stuart de Haro shows a typical ER collet & it's matching nut and holders:
If you ever wondered what the 'ER' stands for in ER Collet - it comes from the name of the town where the company that pioneered this design originated - Reigoldswil, Switzerland. The company was called Rego-Fix founded by Fritz Weber - he noticed that a regular E Collet posed challenges to the machinist when removing it from the mill and simply introduced a groove to facilitate removal.
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