Ball centre is sometimes used for tapering by the "set-over tail stock" method. The standard centres cannot sit squarely in the center hole when the tail stock is set over.
Lathe mandrels are devices used to hold the job for machining on lathes. They are mainly used for machining outside diameters with reference to bores which have been duly finished by either reaming or boring on a lathe.
Revolving Centres are Frictionless. Used for supporting heavy jobs and jobs revolving with high speeds. A high-speed steel inserted centre is supported by two bearings housed in a body. It is also called the revolving dead centre.
The centre, which is accommodated in the main spindle sleeve, is known as a ‘live centre’ and the centre fixed in the tailstock spindle is known as a dead centre. In construction, both centres are identical, made as one unit that consists of a conical point of 60° included angle, a body provided with a Morse taper shank and a tang.
Though it is termed as half centre, little less than half is relieved at the tip portion. Used while facing the job without disturbing the setting.