The geometric tolerances of cylindrical workpieces are highly influenced by clamping forces. This relation is of special importance in slender workpieces such as thin rings. Better tolerances are achieved with lower clamping forces, but the disadvantage is that friction is reduced and the risk of slipping increases. Thus, in order to control the process, a key factor is achieving the lowest possible clamping force while still ensuring safety. Cylindrical parts are usually machined in lathes that have concentric plate chucks that are fixed either mechanically with wrenches or hydraulically by controlling pressure with valves. This paper proposes a method for measuring clamping forces in lathes during the turning process. The method allows the clamping force to be calculated from the torque applied by a dynamometric wrench or from valve controlled hydraulic pressure.