Fine module hobs for processing cylindrical involute gears

Aspects of formation of a hobs cutting part for processing of gear wheels are considered. In the traditional manufacture of hobs for processing the rear surfaces of cutting teeth, the process of radial or oblique relieving is used, which due to its dynamic nature associated with reciprocating motions of a grinding head with a grinding wheel installed thereon, makes it difficult to ensure high accuracy of teeth pitch and profile, as well as main hob parameters. These shortcomings are effectively eliminated by using the process of screw relieving to process the lateral rear surfaces of the hob teeth. Execution of a hob teeth profi le in the form of a gear rack enables to obtain a hob for processing gear wheels. A number of formulas are given, by means of which it is reasonably proved that hobs made using the considered principle of formation of teeth back surfaces can have a high class of accuracy — AA or AAA. Calculations showed that the displacement of the hob teeth at two revolutions of the main worm screw with a probability of 0.9973 does not exceed 0.13 μm, which is signifi cantly higher than the requirements of GOST 10331-81 for hobs with AAA accuracy class (not more than 4 μm for hobs with a pitch over 0.5 to 0.9 mm). The considered method of forming the hob rear surfaces in the form of counter right and left multi-turn screw surfaces (screw relieving) allows to obtain hobs not only for machining involute gears, but also other types of cutters, e.g. for machining threads, ratchet wheels and other parts having a regular profile.