One of the significant features of CNC technological superiority is in the practical applications, according to the necessary movement of processed parts required, we can use generating method to transform CNC machine tools and receive the desired results. This article discussed under the premise that lack of special equipment, transforming ordinary horizontal CNC milling to machine worm gear. The advantage of this numerical transformation approach is using the processing functions of thread and tapered thread instruction, skillfully translating into control of the CNC machining worm, expanding the CNC machining capabilities and the scope of practical application. Also apply the advantages of CNC technology, reduce or eliminate the accumulation of worm in the processing of pitch error which makes the worm processing precision and smoothness of worm drive better. This paper discussed the principles of numerical control transformation, transformation of milling machine structure and the conversion calculation of parameters in programming, analyzed the causes and processing effects of worm gear processing errors control and accuracy improving.