Nowadays special modified tools are mostly used for rough or semi-finishing milling in the mass production of ground or shaved gears today. These modifications ensure the desired chamfer at the head or the undercut at the bottom of the gear tooth. Diameters of the beginning and the end of the operational involute (exact knowledge of them is necessary for the calculation of important meshing parameters) are found by using several techniques. The first one is the simulation of the generating action of a hob tooth using suitable graphic software with the subsequent measuring of these diameters from the envelope of hob tooth positions which was created. The second one is measuring directly on the gear manufactured using a measuring device. These simulations or measuring are often not performed and the tool with recommended parameters of the protuberance or the ramp is simply chosen by an educated guess [1]. But it is not an acceptable technique in a mass production (car industry). Standard DIN 3960 [2] gives a certain manual for the determination of these diameters. It suggests the iterative method for the calculation of the chamfer beginning circle diameter but without a reliable guideline. And as regards the protuberance, it refers to the correct calculation only in theory. This paper deals with the computing method to determine diameters of the beginning and the end of the function part of a tooth flank involute. It is designed for a specified tool with modifications for creating the chamfer or the protuberance undercut. The paper also takes into account the necessary shaving (grinding) stock or the backlash. Furthermore it refers to possible problems when the basic profile of the generating tool with the protuberance is designed from the basic rack tooth profile.

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