When it comes to manufacturing any type of gear, the engineer is faced with a number of decisions as to what are the processes needed to produce the gears. One of the most common metal removal processes in producing the right tooth space for either spur or helical gears and even splines is known as gear hobbing. It has been used for more than a century already and has been proven to be an effective gear manufacturing process. It’s basically a form-generating process wherein the gear rotates and the hobbing machine also rotates axially across the face of the gear.
Gear hobbing process involves some complex cutting kinematics in which every tooth of the hob cuts into the different position in the space between each tooth, producing varying chip-forming characteristics. There are fewer hob teeth engaged during the cutting process as the tooth space gets bigger and the diameter of the gear decreases. In this case, the chip thickness also increases for every gear revolution. When such a gap becomes larger, the tooth of the hob also takes a bigger bite on the gear and the chip area further increases. It means that those pinions with bigger gaps and fewer teeth than their mating gear can become more challenging to hob.
Another important thing to consider in gear hobbing is the required undercut and root configuration. This is usually necessary when grinding or another additional finishing process is needed to produce the gear. These undercuts can be controlled with the use of lead hob designs, which are important when hobbing fewer number of gear teeth.
In order to successfully and effectively control the excessive amount of undercuts, special off lead hob designs are used along with custom tools needed. The major drawback with this situation is that it may take longer to complete and can also cost much higher than the conventional gear hobbing process. Yet, the results are worth it if it’s what the gear application requires.