If a cylindrical work piece has two separate axis of rotation one being out of centre to the other, the work piece is termed eccentric and turning of different surfaces of the wok piece is known as eccentric turning. The shaft is first mounted on its true centre and the part forming the journal is turned. The job is then remounted on the offset centre and the eccentric surfaces are machined.
CHAMFERING
Chamfering is the operation of beveling the extreme end of a work piece this done to remove the burrs, to protect the end of the work piece from being damaged and to have a better look. The operation may be preformed after knurling, rough turning, boring, drilling or thread cutting. Chamfering is an essential operation after thread cutting so that the nut may pass freely on the threaded work piece.
THREAD CUTTING:
Thread cutting is the most important operation performed in lathe.
The principle of thread cutting is to produce a helical groove on a cylindrical or conical surface by feeding the tool longitudinally when job is revolved between centers or by a chuck. The longitudinal feed should be equal to the pitch of thread to be cut per revolution of the work piece. The lead screw of the lathe, through which the saddle receives its traversing motion, has a definite pitch. A definite ratio between the longitudinal feed and rotation of the head stock spindle should therefore be found out so that the relative speeds of rotation of the work and the lead screw will result in the cutting of a screw of the desired pitch. This is affected by change gears arranged between the spindle and lead screw or by the change gear mechanism or feed box used in a modern lathe where it provides a wider range of feed and the speed ratio can be easily and quickly changed.
fr. NTTF ( NETTUR TECHNICAL TRAINING FOUNDATION)
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