What are gear hobs?

Structural Features: A gear hob externally resembles a worm. It has a large helical angle β0 and a small number of thread heads (usually 1 - 3 teeth). The teeth are long and can wind around the hob's dividing cylinder multiple times. To form the cutting edges, several chip - holding grooves are milled along the axis on the worm's end face, creating the front face and the rake angle. After relieving and grinding, the back face and the clearance angle are formed.

Working Principle: During the gear - cutting process, the gear hob and the gear blank interact in a way similar to the meshing of a pair of helical gears. As the hob rotates, it simultaneously moves axially along the length of the gear blank, and the gear blank rotates at a synchronized speed. For a single - head hob, when it rotates one full turn, the gear blank rotates one tooth around its own axis. For a multi - head hob, the number of teeth the gear blank rotates is equal to the number of heads of the hob. This coordinated motion gradually cuts the teeth of the gear around the circumference of the blank.

By Processing Nature:

Rough - cutting Hobs: Used for the initial stage of gear processing, they remove a large amount of material quickly to form the basic shape of the gear teeth.

Finish - cutting Hobs: Employed in the final processing stage to achieve high - precision tooth profiles, ensuring the gear meets the required accuracy and surface finish standards.

Pre - shaving Hobs: These hobs are designed to create a specific tooth shape in the pre - shaving process. Their tooth shape is customized according to the requirements of the shaving allowance, often with features like a convex - shaped tooth tip and a chamfered tooth root, which result in a uniform shaving allowance in the middle part of the gear tooth.

By Structure:

Integral Hobs: Usually made of high - speed steel for small and medium - modulus gears (modulus generally less than 10mm). They are a single - piece structure, offering high rigidity and accuracy during the cutting process.

Welded Hobs: The cutting part and the shank are welded together. This structure can save on high - cost tool materials to some extent while maintaining good cutting performance.

Assembly Hobs: The gear hob consists of multiple parts. For example, the tooth - cutting part is made into a rack - like shape with high - speed steel, and after heat treatment, it is fastened to the tool body. This structure is suitable for large - modulus gears, reducing material waste and manufacturing difficulties.

High - speed Steel: Widely used in the manufacturing of gear hobs due to its good cutting performance, high toughness, and relatively low cost. It can meet the processing requirements of most general - purpose gears, such as those in ordinary machinery and automotive transmissions.

Carbide: Carbide - made gear hobs have high hardness, excellent wear resistance, and can withstand high - speed cutting. They are often used for processing hard - to - machine materials, such as hardened steel gears or gears made of high - strength alloys. Although the cost is relatively high, they can significantly improve cutting efficiency and the service life of the hob.

Different countries and industries have their own standards for the accuracy grades of gear hobs. For example, in some international standards, the accuracy grades are divided into AAA, AA, A, and B levels.

AAA - grade Hobs: Used for manufacturing extremely high - precision gears, such as those in aerospace and precision instrument industries, where the requirements for gear accuracy are extremely strict.

AA - grade Hobs: Commonly used in high - precision gear manufacturing, such as in the production of high - end automotive transmissions and precision machine tools.

A - grade Hobs: Suitable for general - precision gear processing, which can meet the requirements of most industrial applications.

B - grade Hobs: Usually used for gears with relatively low accuracy requirements, such as some simple mechanical equipment or large - scale transmission gears where the cost - effectiveness ratio is more important.