In this paper, an optimal axial immersion is defined and it is calculated for the milling process. The analytical calculation is based on one degree of freedom model. A helical tool is considered during the calculation. Axial immersion values arc found where large material removal rate can be achieved while negligible and tolerable vibration occurs. We explain this with the help of the Fourier series of the time-periodic cutting force. To support our analytical results measurements were carried out. Instead of making numerous measurements with distinct spindle speeds, we used wedge-shaped work-pieces to speed-up the lengthy experiments. In this way, the axial immersion was increased continuously during the cutting process. The stability boundary and the optimal axial immersions could be found with a single test at cach spindle speed. We found that the vibration amplitudes were significantly reduced in case of the theoretically predicted optimal axial immersions.