The envelope worm indexing cam mechanisms was a new type of indexing mechanism. The indexing cams that were enveloped with the different surface of indexing plate showed different characteristics. Considering the screw gear had many good qualities, we attempted to envelope the indexing cam with involute screw surface and hope to get good results. Using the differential geometry theories, we built the surface formulas of indexing plate. Using the meshing theories, we built the space coordinate system of the indexing plate and cam, and derived the formulas of contact lines. The distribution rule of the contact lines was analyzed. Compared with cylindrical surface and involute columnar surface, the contact lines on indexing plate of involute screw surface distributed over more of the indexing plate area, the distribution rule have much benefit.%利用微分几何、啮合原理，建立了渐开线螺旋面包络蜗杆分度凸轮机构分度盘轮齿的廓面方程，推导了接触线方程，分析了分度盘轮齿廓面上接触线分布规律与圆柱面及渐开线柱面相比，渐开线螺旋面作为分度盘轮齿廓面具有很多优点。
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