The cross piece connecting centering frames are called a lag or bolst. The structure would be built round a post acting as a datum, and each course of stonework would be set at a distance from the datum as measured by a stick or string. Centering in construction pdf simple arch centre suitable for single skin brickwork. A centre for a flat segmental arch.
The centring for a semicircular arch. Centering 2, Centring 2″ def. Whitney, William Dwight, and Benjamin E. Center 2, Centre 2″ def. New York: Van Nostrand Reinhold. This page was last edited on 23 July 2017, at 00:54.
On a lathe the chuck is mounted on the spindle which rotates within the headstock. The jaws are tightened up to hold the tool or workpiece. A few chuck designs are more complex yet, and they involve specially shaped jaws, higher numbers of jaws, quick-release mechanisms, or other special features. Some chucks, such as magnetic chucks and vacuum chucks, are of a different sort from the radially symmetrical mechanical clamps mentioned above. Self-centering 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. Sometimes this type of chuck has 4 or 6 jaws instead of 3.
More jaws grip the workpiece more securely if it is truly cylindrical, and thin-walled work will deform less. Four jaws are also useful for square bar work. 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. The most commonly used name for this type is a brand name, Set-Tru. Top: an assembled keyless chuck. This type of chuck is tightened by twisting the body using firm hand pressure only. Bottom: the widely used keyed type of drill chuck with its key.
These chucks require a toothed key to provide the necessary torque to tighten and loosen the jaws. When the key is turned its teeth mate with teeth on the chuck, turning an internal screw which in turn moves the threaded jaws in or out along a tapered surface. The end view shows the three small jaws that slide within the body. The top one is assembled, the lower one shows the body and nose cap assembled with the collet piece below it. Some high-precision chucks use ball thrust bearings to reduce friction in the closing mechanism and maximize drilling torque.