The Production of Small Worm Gears for Experimental Work

Small worm gear drives are a common feature in the design of many types of apparatus, and the following description of the methods used for producing them in an experimental establishment may be of interest. Quantities are small, one or two to each pattern being the general rule, but there is certainly no lack of variety. The worms and wheels most often called for range in size up to 1½ inches and 6 inches diameter respectively, while pitches vary from 10 to 60 d.p. (diametral pitch). Addendum and dedendum proportions of 1/ PN and 1·25/ PN have been standardized, and a pressure angle of 20 deg. is adopted throughout. The gears are designed as hollow-faced helical (spiral) gears, and all calculations are based on the normal pitch. This is to enable standard hobs and cutters to be used for the worms. The shaft angle is usually 90 deg., but the angle of crossing may be varied up to 10 deg. either way on the particular machine employed for cutting the wheels. For many applications, backlash must be reduced to the very minimum consistent with smooth running; and to avoid the extreme accuracy of workmanship which an exact centre distance would necessitate, provision is usually made for adjustment of the worm. Although the Reinecker tangential feed method of worm wheel generation by a single-point tool —representing one tooth of a hob—is generally known, very little information on cutter forming is available. The method outlined was developed at the Admiralty Research Laboratory. Given the use of a modern worm grinder (not available), it should be possible to profile-relief grind these cutters after hardening.