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 (less than in diameter) 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 (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. 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 (and which is not free of spiral or 'wind') 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. Spiders A spider is a simple, relatively inexpensive, limited-capability version of an independent-jaw chuck. It typically consists