A Chuck is a specialised type of clamp used to hold rotating tools or materials.
A collet is a sleeve with a (normally) cylindrical inside and a conical outside. The collet has kerf cuts along its length to allow it to expand and contract. A threaded section at the rear of the collet is used to pull it into a matching conical socket. As the collet is pulled into the socket, the collet will contract - gripping the contents of the inner cylinder.Collets are most commonly found on milling machines, lathes, wood routers, and precision grinders. There are many different systems, common examples being the ER and R8 systems. Collets can also be obtained to fit Morse or Brown and Sharpe taper sockets.
Typically collets offer far higher levels of precision and accuracy than self-centering chucks, and have a far shorter setting up time than independent-jaw chucks. The penalty is that most collets can only accommodate a single size of workpiece. An exception are ER collets which typically have a working range of 1 mm (about 0.04 inches).
Collets usually are made to hold cylindrical work, but are available to hold square, hexagonal or octagonal workpieces.
Keyless and keyed drill chucks
A drill chuck is a specialised three-jaw chuck used to hold drill bits or related tools.
The image at right shows an assembled keyless chuck at the top. The tightening action of this chuck style is performed by twisting the body using firm hand pressure only.
The lower images show the traditional keyed style of drill chuck with its key. The arbor is shown separately to the right. These chucks require a key to provide the necessary torque to tighten and loosen the jaws. The rotary action of the key turns the outer body which acts on an internal screw; this in turn moves the threaded jaws in or out along a tapered surface. The taper allows the jaws to encompass various sizes of drill shanks. The end view shows the three small jaws that slide within the body.
Some high precision chucks use ball thrust bearings to reduce friction in the closing mechanism and maximizing drilling torque. These chucks are sometimes referred to as "super chucks".
Special Direct System (SDS)
Developed by Bosch in 1975 for hammer drills, the SDS uses a cylindrical shank on the tool, with indents to be held by the chuck. A tool is inserted into the chuck by pressing in, and is locked in place until a separate lock release is used – no tightening required. The rotary force is supplied through wedges that fit into two or three open grooves. The hammer action actually moves the bit up and down within the chuck since the bit is free to move a short distance. Two sprung balls fit into closed grooves, allowing movement whilst retaining the bit. SDS relies on a tool having the same shank diameter as the chuck - there are three standard sizes:
- SDS-Plus – a 10 mm shank with two open grooves held by the driving wedges and two closed grooves held by locking balls. This is the most common size and takes a hammer up to 4 kg. The wedges grip an area of 75 mm2 and the shank is inserted 40 mm into the chuck.
- SDS-top a 14 mm shank similar to SDS-plus, designed for hammers from 2 to 5 kg. The grip area is increased to 212 mm2 and the shank is inserted 70 mm. This size is not common.
- SDS-max – an 18 mm shank with three open grooves and locking segments rather than balls. It is designed for hammers over 5 kg. The wedges grip an area of 389 mm2 and the shank is inserted 90 mm.
Many SDS drills have a "rotation off" setting, which allows the drill to be used for chiselling. The name SDS comes from the German "Steck – Dreh – Sitz" (Insert – Twist – Stay). German-speaking countries may use "Spannen durch System" (Clamping System), though Bosch uses "Special Direct System" for international purposes.
[. (German language)]
A three-jaw chuck is a rotating clamp which uses three interconnected dogs or 'jaws' to hold on a tool or work piece. Three-jaw chucks are usually self-centering and are best suited to grip circular cross sections, though independent versions can be obtained.
The image shows a three-jaw chuck and key with one jaw removed and inverted showing the teeth that engage in the scroll plate. The scroll plate is rotated within the chuck body by the key, the scroll engages the teeth on the underside of the jaws which moves the three jaws in unison, to tighten or release the workpiece.
The Griptru™ style of self-centering chuck from Pratt Burnerd Intl., Ltd. has further adjustment screws which can be used to further improve the accuracy of the chuck at any chosen diameter of workpiece.
Three-jaw chucks can be found on lathes and indexing heads.
A four-jaw chuck is similar to a three-jaw chuck, but with four jaws, each of which can be moved independently. This makes them ideal for gripping non-circular cross sections, but difficult to centre precisely. Four-jaw chucks are almost never used for tool holding. Four-jaw chucks can be found on lathes and indexing heads.
The image shows a four-jaw chuck with the jaws independently set. The key is used to adjust each jaw separately.
For special purposes, and also the holding of fragile materials, chucks are available with six or eight jaws. These are invariably of the self-centering design, and are built to very high standards of accuracy.
Self-centering four jaw
A four jaw chuck with a mechanism for centering the work piece. Sometimes used to refer to chucks where the jaws are moved in interconnected pairs.
Used only for holding ferro-metallic work pieces, a magnetic chuck consists of an accurately centred permanent magnet face. Electro Magnets or permanent magnets are brought into contact with fixed ferrous plates, or 'pole pieces', contained within a housing. These pole pieces are usuall flush with the housing surface. The part or 'work piece' to be held forms the closing of the magnetic loop or path, onto those fixed plates, providing a secure anchor for the work piece