BS 721-1:1963

Cylindrical involute helicoid worms with a normal pressure angle of 20 in a range of axial modules from 0.052 in to 1 in. Design, formulae for calculating worm and wormwheel dimensions; four gear classes, with tolerances for pitch, profile and worm thread thickness, and five grades of backlash for assembly. Drawings show information to be given when ordering. Appendices: determination of load capacity, design calculations for typical gears, design procedure based on axial pitch of the worm, design of worms to suit standard diametral pitch cutting tools corresponding to the normal pitch of the worm.