Chuck (engineering)

A chuck is a specialized type of clamp used to hold an object with radial symmetry, especially a cylinder. In a drill, a mill and a transmission, a chuck holds the rotating tool; in a lathe, it holds the rotating workpiece.
Chucks commonly use jaws to hold the tool or workpiece. The jaws are typically arranged in a radially symmetrical pattern like the points of a star. Jawed chucks may require a wrench-like device called a chuck key to be tightened or loosened, but other jawed chucks may be tightened or loosened by hand force alone, offering convenience at the expense of gripping force. Chucks on some lathes have jaws that move independently, allowing them to hold irregularly shaped objects. More complex designs might include specially shaped jaws, greater numbers of jaws, or quick-release mechanisms.
Instead of jaws, a chuck may use magnetism, vacuum, or collets, which are flexible collars or sleeves that fit closely around the tool or workpiece and grip it when squeezed.

Types

Jawed chucks

Self-centering

A self-centering chuck, also known as a , uses jaws, interconnected via a scroll gear, to hold onto a tool or workpiece. Because they most often have three jaws, the term three-jaw chuck without other qualification is understood by machinists to mean a self-centering three-jaw chuck. The term universal chuck also refers to this type. These chucks are best suited to grip circular or hexagonal cross-sections when very fast, reasonably accurate centering is desired.
Sometimes this type of chuck has four or six jaws instead of three. Four-jawed chucks are primarily useful for gripping square or octagon material, while six-jawed chucks hold thin-walled tubing and plastic materials with minimum distortion.
There are also independent-jaw chucks with three jaws, but they offer few advantages and are very rare.
There are hybrid self-centering chucks that have adjustment screws that can be used to further improve the concentricity after the workpiece has been gripped by the scroll jaws. This feature is meant to combine the speed and ease of the scroll plate's self-centering with the run-out eliminating controllability of an independent-jaw chuck. The most commonly used name for this type is a brand name, GripTru. To avoid undue genericization of that brand name, suggestions for a generic name have included "exact-adjust".
Three-jaw chucks are often used on lathes and indexing heads.

Drill chuck

A drill chuck is a specialised self-centering, three-jaw chuck, usually with capacity of or less, and rarely greater than, used to hold drill bits or other rotary tools. This type of chuck is used on tools ranging from professional equipment to inexpensive hand and power drills for domestic use.
Some high-precision chucks use ball thrust bearings to reduce friction in the closing mechanism and maximize drilling torque. One brand name for this type of chuck, which is often genericized in colloquial use although not in catalogs, is Super Chuck.
A pin chuck is a specialized chuck designed to hold small drills that could not be held securely in a normal drill chuck. The drill is inserted into the pin chuck and tightened; the pin chuck has a shaft which is then inserted into the larger drill chuck to hold the drill securely. Pin chucks are also used with high-speed rotary tools other than drills, such as die grinders and jig grinders.

Independent-jaw

On an , each jaw can be moved independently. Because they most often have four jaws, the term four-jaw chuck without other qualification is understood by machinists to mean a chuck with four independent jaws. The independence of the jaws makes these chucks ideal for gripping non-circular cross sections and gripping circular cross sections with extreme precision. The non-self-centering action of the independent jaws makes centering highly controllable, but at the expense of speed and ease. Four-jaw chucks are almost never used for tool holding. Four-jaw chucks can be found on lathes and indexing heads.
Self-centering chucks with four jaws also can be obtained. Although these are often said to suffer from two disadvantages: inability to hold hex stock, and poor gripping on stock which is oval, only the latter is true. Even with three jaw self centering chucks, work which is not of uniform section along the work should not be gripped, as the jaws can be strained and the accuracy permanently impaired.
Four-jaw chucks can easily hold a workpiece eccentrically if eccentric features need to be machined.

Cat's Paw

A "cat's paw" is a simple, relatively inexpensive, limited-capability version of an independent-jaw chuck. It typically consists of a ring of metal with screw threads tapped radially into it, in which screws serve as independent jaws. Cat's paws can serve various purposes:

As auxiliary features that complement the main lathe chuck:
* To hold the bar or workpiece at the back end of the spindle bore and support it concentrically, so that it resists wobbling or whipping while the spindle is turning. Gun barrels and oil pipes are examples of workpieces that benefit.
* To hold the bar or workpiece at the tailstock end or following the tool.
In place of the main lathe chuck
Specialty jawed types (two-, six-, eight-jaw; other)

For special purposes, chucks are available with six or eight jaws. These are usually of the self-centering design, and may be built to very high standards of accuracy. However, it is a misconception that such chucks necessarily offer more precision in holding solid workpieces than conventional three-jawed self-centering chucks. Indeed, hot-rolled or other imperfectly round workpieces may "teeter" insecurely between opposing jaws of scroll chucks having even numbers of jaws, in the same manner that a four-legged stool teeters on a rough floor while a three-legged stool never does. The primary purpose of six- and eight-jawed chucks is to hold thin-walled tubing with minimum deformation. By having twice as many clamping points, a six-jaw chuck induces less than half as much clamping distortion in a thin-walled workpiece, compared to a three-jawed chuck.
Two-jaw chucks are available and can be used with soft jaws that can be machined to conform to a particular workpiece. It is a short conceptual leap from these to faceplates holding custom fixtures, wherein the part is located against fixed stops and held there with toggle clamps or toe clamps.

Jaw construction

Many chucks have removable jaws, which allows the user to replace them with new jaws, specialised jaws, or soft jaws. Soft jaws are made of soft materials such as soft metal, plastic, or wood. They can be machined as needed for particular setups. The typical interface between the master jaw and the removable jaw is a matching pair of serrated surfaces, which, once clamped by the mounting screws, cannot allow relative slipping between the two parts.

Collet

A collet, one type of chuck, 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 whose secure holding is desired. Most often this is achieved with a spring collet, made of spring steel, with one or more kerf cuts along its length to allow it to expand and contract. An alternative collet design is one that has several tapered steel blocks held in circular position by a flexible binding medium. The Jacobs Rubber-Flex brand is a name that most machinists would recognize for this type of collet chuck system.
Regardless of the collet design, the operating principle is the same: squeeze the collet radially against the tool or workpiece to be held, resulting in high static friction. Under correct conditions, it holds quite securely. Almost all collet chucks achieve the radial squeezing motion via moving one or more male-female pairs of tapered surfaces axially, which produces the radial squeezing in a highly concentric manner. Depending on the collet design, it can be either pulled or pushed into a matching conical socket to achieve the clamping action. As the collet is forced into the tapered socket, the collet will contract, gripping the contents of the inner cylinder.
One of the corollaries of the conical action is that collets may draw the work axially a slight amount as they close. Collet chuck systems that make no provision to prevent this draw-in are often called draw-in collet chucks, in contrast to systems which circumvent this movement, usually by pushing the tapered closing ring toward the collet rather than pulling the collet into the ring. Such non-draw-in types are often called "dead-length" or "non-draw-in" collet chucks. Draw-in is not always a problem, but avoiding it can be helpful on some work where failing to account for it might result in inaccuracy on part overall length, shoulder lengths, etc.
Collets are most commonly found on milling machines, lathes, wood routers, precision grinders, and certain handheld power tools such as die grinders and rotary tools. There are many different systems, common examples being the ER, 5C, and R8 systems. Collets can also be obtained to fit Morse or Brown and Sharpe taper sockets.
Typically collets offer higher levels of precision and accuracy than self-centering chucks, and have a shorter setting up time than independent-jaw chucks. The penalty is that most collets can only accommodate a single size of workpiece. An exception is the ER collet which typically has a working range of 1 mm.
Collets usually are made to hold cylindrical work, but are available to hold square, hexagonal or octagonal workpieces. While most collets are hardened, "emergency" collets are available that can be machined to special sizes or shapes by the user. These collets can be obtained in steel, brass, or nylon. Step collets are available that are machinable to allow holding of short workpieces that are larger than the capacity of normal collets.