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The primary use of a center in Metalworking is to ensure Concentric work is produced, this allows the workpiece to be transferred between operations without any loss of accuracy. A part may be ''turned'' in a Lathe , sent off for Hardening And Tempering and then ground ''between centers'' in a Cylindrical Grinder . The preservation of concentricity between the turning and grinding operations is crucial for quality work. A center is also used to support longer work pieces where the Cutting forces would deflect the work excessively, reducing the finish and accuracy of the workpiece, or creating a hazardous situation. A center has applications anywhere that a ''centered'' workpiece may be used, this is not limited to lathe usage but may include setups in Dividing Head s, Cylindrical Grinder s, Tool And Cutter Grinder s or other related equpiment. The term ''between centers'' refers to any machining operation where the job needs to be performed using centers. A center is inserted into a matching hole drilled by a Center Drill . DEAD CENTER A dead center (one that does not turn freely, ie:- ''dead'') may be used to support the workpiece at either the fixed or rotating end of the machine. When used in the fixed position, a dead center produces friction between the workpiece and center, due to the rotation of the workpiece. Lubrication is therefore required between the center and workpiece to prevent friction welding from occurring. Additionally the tip of the center may have an insert of Carbide which will reduce the friction slightly and allow for faster speeds. Dead centers may also be fully hardened to prevent damage to the important mating surfaces Taper of the taper and to preserve the 60 ° nose taper. SOFT CENTER Soft centers are identical to dead centers except the nose is deliberately left soft (unhardened) so that it may be readily machined to the correct angle prior to usage. While this operation may appear unnecessary it ensures that the required angle is truly concentric to the axis of rotation. It is performed on the headstock center, not on the tailstock. LIVE CENTER A live center (one where the 60 ° center runs in its own bearings) is used at the non driven or Tailstock end of a machine. It allows higher speeds without the need for separate lubrication, and also greater clamping pressures. They are used almost exclusively in CNC Lathes as well as for general machining operations. PIPE CENTER A pipe center has a larger diameter at the 60 ° taper end. This allows the center to be used in the bore of a pipe (or similar workpiece). While a pipe center ensures the workpiece remains concentric, its main advatage is that it supports the workpiece securely. Thin walled material such as pipes easily collapse if excessive pressures are used at the Chuck end, CUP CENTER The cup center is a variation of the live center and is used in woodworking to support the softer material around the actual center and prevent the material splitting. DRIVE CENTER A drive center is used in the driving end of the machine (headstock). It consists of a dead center surrounded by hardened teeth. These teeth ''bite'' into the softer workpiece allowing the workpiece to be driven directly by the center. This allows the full diameter of the workpiece to be machined in a single operation, this contrasts with the usual requirement where a Carrier is attached to the workpiece at the driven end. They are often used in woodworking or where softer materials are machined. |
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