During gear hobbing, the surface quality at the gear root is determined by the fillet radius and rake angle of the top edge of the hob. If the fillet radius r of the top edge of the hob is too large, it may cause instability in the transmission of the gear, aggravate the wear of the gear surface, and cause tooth breakage in serious cases. The fillet radius r of the top edge of the hob is too small, which can improve the surface quality at the tooth root, but it is easy to produce stress concentration at the tooth root, reduce the bending fatigue strength of the gear, and cause the gear to get stuck or damaged. For manufacturing convenience, zero rake angle hobs are generally used. The fillet of the top edge is on the tangent line between the top edge and the edges on both sides. The calculation formula of the fillet radius r of the top edge is:
Where: SA is the tooth thickness of hob indexing circle; Ha is the tooth top height of hob; α A is the pressure angle of hob. The top edge fillet radius of the hob used in the experiment is obtained by calculation. In order to unify the processing parameters, two groups of 6 20CrMnTi gear specimens are selected. The hobs with top edge fillet radius of 1.882 and 1.617 mm are selected for gear hobbing experiments respectively, and the tooth surface roughness is measured.
According to the analysis data, the fillet radius of hob top edge is an important factor affecting the tooth surface roughness of gear hobbing. Under the condition of no transition interference in gear hobbing, a large fillet radius of hob top edge can obtain higher gear surface quality.