Provides geometrical and other design data and formulae for worms and worm-wheels with thread forms, and applies both to cylindrical involute helicoid and cylindrical milled helicoid threads. Four classes of gears A, B, C, D are provided, relating to function and precision. Modules range from 0.052 to 1, gear ratios range up to 100:1, and rational series of centre distances and diameter quotients are listed. Typical drawings show information to be given when ordering and appendices cover determination of load capacity, design calculations for typical gears, design procedure based on axial pitch of the worm, and design of worms to suit standard diametral pitch cutting tools corresponding to the normal pitch of the worm.