Optical components with freeform surface can achieve excellent performance. It is used more and more widely in the modern optical system. Slow Tool Servo (STS) is a new technique to machining freeform surface. Due to the complexity of freeform surface and the particularity of STS, the relative position relationship between tool and work piece must be deeply researched. In this research, STS machining technique was introduced. Two tool geometry selection methods were proposed and researched on. One of the two methods was based on section curves and the other was based on whole surface using NURBS.