A bar of the type described may be secured in any convenient 3 or
4 jaw chuck.The chucking errors simply add to any errors in diameter and eccentricity which the bar already possesses at the truly circular machined measurement locations. The sum of the errors is precisely cancelled out by the average clock reading indication method.
It's important to understand that the clock must be mounted horizontal and rigidly fixed to the lathe carriage so that there can be no chance of the clock zero shifting as the carriage is traversed between the measurement locations.
With a perfect bar slowly rotating in a perfect lathe the clock will remain at its abitrary reading (conveniently zero) as the carriage is traversed between measurement locations and this defines the spindle axis to be parallel to the bed in the horizontal plane. Vertical clock mounting would establish parallelism in the vertical plane.
In our imperfect bar there will be some resultant eccentricity at each of the measurement locations but. on truly circular measurement locations the average of the minimum and maximum clock readings still precisely defines the centre about which the bar is rotating.
This measurement result is the axis of spindle rotation and is not affected by errors in the spindle nose fitting.
Jim