For efficient and flexible production modern milling machining centres are increasingly equipped with an integrated turning option. Therefore the importance of safe workpiece clamping already increases during milling if a workpiece is to be turned subsequently. If an insufficient clamping effect is already present during milling which leads to a change of the clamping situation there is an increased risk that the workpiece will come loose and be released uncontrollably during subsequent turning. Finally the high kinetic energy of the workpiece results in a high risk of injury for machine operators. The clamping effect of a three-jaw chuck is essentially determined by the standstill clamping force that is applied to the workpiece before machining when the chuck is stationary. The standstill clamping force is usually determined by the analytical calculation model according to VDI guideline 3106. This calculation model mainly takes horizontal turning operations into account and cannot simply be transferred to vertical workpiece clamping in mill-turn-centers due to different load conditions and machine structure. This paper therefore determines the required standstill clamping force for milling in mill-turn-centers theoretically and experimentally.