frequency signals it is implemented using fft which is known and is omitted in the following for simplicity additionally fft comprises drawbacks as described above thus the invention provides a second determination condition for enhanced accuracy in the following describes the process of determining whether signal points generated by oscillation displacement based on rotational rate or acceleration during a milling process are located in a stable area derived using a sliding function a milling process generates relative displacement between a milling tool and a workpiece enabling plastic deformation and separate milling the milling tool generates milling resistance i e milling force the milling force degree changes according to processing methods milling tool quality milling conditions such as rotational rate of a spindle feed rate axial milling depth radial depth and so forth workpiece materials and uniformity a milling processing system is a complicated and non linear dynamic system as a result a system model must be simplified and linearised and a milling system models is thus obtained according to non deformed milling thickness if dynamic milling force is ignored the relation between milling force and non deformed milling thickness may be a direct proportion represented as where f t represents milling force b represents axial milling depth k c represents milling resistance coefficient and u t represents non deformed milling thickness if average milling depth is twice larger than.