Reasons why turning eccentric parts should use ball centres, radised centre drills or a combination of both.

[Parlo]

This demonstration highlights the use of a radiused form centre drill and a ball centre. A radiused form centre drill must be used when eccentric turning to allow the workpiece to pivot correctly on a standard centre. A ball centre does the same but in a tapered recess, any angle can be used to locate the ball and the workpiece can pivot on the ball. The reason these two principles are important is that when using a standard centre drill with standard centre the workpiece will not run concentrically on a centre when turning eccentrically. When a workpiece is held eccentrically between centres the clamping distance between standard centres inside standard tapered centre drilled holes can vary especially if the centre recesses are different diameters.