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Chuck (engineering)
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A chuck is a specialized type of clamp used to hold rotating tools or materials.
Main article: Collet
A collet, one type of chuck, is a sleeve with a (normally) cylindrical inner surface and a conical outer surface. The collet has kerf cuts along its length to allow it to expand and contract. Depending on the collet design, it can be either pulled (via a threaded section at the rear of the collet) or pushed (via a threaded cap with a second taper) into a matching conical socket to achieve the clamping action.
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A chuck is a specialized type of clamp used to hold rotating tools or materials.
Collet
- Main article: Collet
A collet, one type of chuck, is a sleeve with a (normally) cylindrical inner surface and a conical outer surface. The collet has kerf cuts along its length to allow it to expand and contract. Depending on the collet design, it can be either pulled (via a threaded section at the rear of the collet) or pushed (via a threaded cap with a second taper) 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.
Collets are most commonly found on milling machines, lathes, wood routers, and precision grinders. 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 (about 0.04 inches).
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.
For more information check the catalogues...
Drill
A drill chuck is a specialised three-jaw chuck, usually with capacity of =0.5" (=13 mm) and rarely >1" (>25 mm), used to hold drill bits or other rotary 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. While convenient, this feature can cause the chuck to loosen if too much torque is applied.
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 – 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 mm˛ 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 mm˛ and the shank is inserted 70 mm. This size is uncommon.
- 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 mm˛ 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 – Sitzt" (Insert – Twist – Fits). German-speaking countries may use "Spannen durch System" (Clamping System), though Bosch uses "Special Direct System" for international purposes.
Three-jaw
A three-jaw chuck is a rotating clamp which uses three dogs or 'jaws', usually interconnected via a scroll gear (scroll plate), to hold onto a tool or work piece. Three-jaw chucks are usually self-centering (as a result of the jaws' meshing with the scroll plate) and are best suited to grip circular or hexagonal cross sections when very fast, reasonably accurate (±.005" TIR) centering is desired. Independent-jaw (non self centering) versions also 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.
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 runout-eliminating controllability of an independent-jaw chuck.
Three-jaw chucks can often be found on lathes and indexing heads.
Four-jaw
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 (a) gripping non-circular cross sections and (b) gripping circular cross sections with extreme precision (when the last few hundredths of a millimeter [or thousandths of an inch] of runout must be manually eliminated). The non-self-centering action of the independent jaws makes centering highly controllable (for an experienced user), 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.
The image shows a four-jaw chuck with the jaws independently set. The key is used to adjust each jaw separately. The second image shows an older and larger 4 jaw chuck. Note how it is able to grip an irregularly cut piece of used metal. Though not found on small chucks it is common for larger chucks (the one in the second photo was made around 1900 and is 24" in diameter) to have many of the features of a Lathe faceplate. The jaws are stepped on one side and full height for gripping on the other and are reversible. Generally the jaws are usable for holding either outside as shown here, or inside as in gripping the inside of a pipe.
Four-jaw chucks can easily hold a workpiece eccentrically if eccentric features need to be machined.
Multi-jaw
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.
Two jaw chucks are available and can be used with soft jaws (typically an aluminum alloy) that can be machined to conform to a particular workpiece. Many chucks have removable jaws, which allows the user to replace them with new jaws, specialized jaws, or soft jaws.
Self-centering four-jaw
A four-jaw chuck with an integrated mechanism for centering the work piece. Sometimes used to refer to chucks where the jaws are moved in interconnected pairs.
Magnetic
Used for holding ferromagnetic work pieces, a magnetic chuck consists of an accurately centered permanent magnet face. Electromagnets or permanent magnets are brought into contact with fixed ferrous plates, or pole pieces, contained within a housing. These pole pieces are usually 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.
Electrostatic
Commonly used for holding silicon wafers during lithography processes, an electrostatic chuck comprises a metal base-plate and a thin dielectric layer; the metal base-plate is maintained at a high voltage relative to the wafer, and so an electrostatic force clamps the wafer to it. Electrostatic chucks may have pins, or mesas, the height of which is included in the reported dielectric thickness; a design by Sandia National Laboratory uses a patterned silicon-dioxide dielectric to form the pins .
See also
External links
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