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To improve the cutting performance of an hourglass worm gear hob and the accuracy of the resulting worm gear, the rake angles of the teeth on the pitch circle should be reduced. A method of forming the spiral flutes by using a variable transmission ratio is developed in this study. To ensure that the rake angles on the indexing torus of each tooth are approximately 0°. Based on the gear meshing theory and the hourglass worm forming method, the discretization mathematical model of the rake face of a planar double-enveloping worm gear hob was established by using cylindrical generating surface, and the feed parameters for machining the rake face for a variable transmission ratio were obtained. The spiral flute was simulated in the virtual software and the hob was machined on a four-axis CNC milling machine. A contourgraph was used to measure the rake angle. The measurement results show that this method can reduce the absolute value of the rake angle on both sides of the cutting teeth, which can be used for machining the spiral flute rake face of an hourglass worm gear hob.