Tech Talk
What is the difference between a "Live" and "Dead" Centre?
In the case of the centre held workpieces there are two types of support, live centres and dead centres. Live centres are centres which rotate with the workpiece. No movement occurs between the centre and the centre hole in the workpiece. The live centre is held in the tailstock, spring loaded into the centre hole in the workpiece, and set to a nominal pressure. Little heat is generated with a live centre and no wear takes places between the centre and the centre hole in the workpiece. This method provides good concentricity and minimal run-out.
These types of centres are not prescribed for extremely accurate or heavy work as the run-out of the workpiece will bear a direct relationship to the accuracy and clearances in the bearing of the live centre. Typically, live centres will produce concentricity better than 0.0004'' (0.01mm) and dead centres better than 0.00008'' (0.002 mm).
Where very high concentricity is required, and particularly where a part might be loaded more than once for different operations, dead centres are best used. The centre taper hole should be accurately drilled and honed to a fine finish for both low friction and high precision. Cooling and lubrication of dead centres is essential. Lard oils, soaps, or heavy grease is best used for this purpose.
