Research on a method for designing land surfaces of a dual-cone double enveloping hourglass worm wheel hob

摘要

Cutting teeth of a dual-cone double enveloping hourglass worm wheel hob retain land surfaces with a certain width to increase the service life of the hob because this kind of hob cannot be re-sharpened. The land widths from the top to the root of all of the cutting teeth should be uniform so that all of them have the same cutting performance. However, the accuracy and the uniformity of the land width of the land surface of hob with spiral rake faces have not been studied till now. To solve this problem, this paper puts forward a method for designing the land surfaces of the hob with spiral rake faces and studies the accuracy and the uniformity. The spiral rake faces of the teeth of the hob are generated by a cylindrical surface. The offset surfaces of the spiral rake faces are obtained by offsetting the spiral rake faces along their normal vector directions with the offsetting distance equal to the land width. The rake faces and the offsetting surfaces intersect with the helical surfaces so the cutting edges and the land edges can be gotten, respectively. The land width of each cutting tooth is the distance between its cutting edge and its land edge. The results of a numerical example show that the calculated land widths on the pitch torus decrease firstly and then increase from one end to another end along the axial direction. The difference of the calculated land widths between the top and the root is larger at side teeth than that at middle teeth. The calculated land widths are slightly larger than the given one and all the calculated land widths change within a small range that can be ignored. Therefore, the method can keep the accuracy and the uniformity.