What are Gear Hobs?

Gear processing tools whose teeth are arranged in a spiral line along a cylinder or cone. They are used to process cylindrical gears, worm gears and other cylindrical workpieces with teeth (see Gear Processing, Gears). According to different uses, hobs are divided into gear hobs, worm hobs, non-involute stretch hobs and fixed hobs.

Commonly used for machining external meshing spur and helical gears

Commonly used worm machining tools. Worm hob basic

The gear hob is a tool that cuts gears by the forming method in accordance with the helical gear pair meshing principle. The gear hob is equivalent to a pinion and the gear is cut.

Hob

Equivalent to a large gear, as shown in Figure 9-24. A gear hob is a helical gear with a large helix angle 0 and a small number of threads (1 to 3 teeth). The teeth are long and can be indexed by a cylinder with a large number of turns. A small worm. In order to form the cutting edge, several chip flutes are milled along the axis of the worm end face to form the front and rake angles; after the teeth and shovel are ground, the rake face and rake angle are formed, as shown in Figure 9-25. The blades on both sides of the gear hob are the intersection of the front and the surface of the side shovel. It should be distributed on the spiral surface of the worm. This worm is called the basic worm of the hob. There are three basic worms: ^[1]

Hob involute worm

The involute worm's thread tooth flanks are involute helical surfaces. The intersection of any plane tangent to the base cylinder and the involute helical surface is a straight line, and its end section is an involute. The intersection of the axial section of the involute worm with the involute spiral is a curve. The hob manufactured with this basic screw has no tooth shape design error and high precision for cutting gears. However, manufacturing hobs is difficult. ^[1]

Hob Archimedes Worm

The side of the spiral tooth of the Archimedes worm is the Archimedes spiral. Through the worm axis profile with Archimedes worm

Hob

The intersection of the planes of rotation is a straight line, the other sections are curved, and the end sections are Archimedes spirals. The hob made with this basic worm is simpler and more convenient to manufacture and inspect the hob tooth profile than the involute worm. However, there is a slight tooth shape error. However, this error is within the allowable range. For this reason, the basic worm of most finishing hobs in production uses Archimedes worms instead of involute worms. ^[1]

Hob normal straight profile worm

The tooth profile in the normal profile of the straight straight worm is straight and the end profile is an extended involute. This basic worm is used instead of the involute basic worm as a hob. The tooth shape design error is large, so it is generally used as a large modulus, multi-head and roughing hob. ^[1]

How the hob works

The knife rotates once, the gear rotates around its own axis through a gear, and the multi-head hob rotates once. The number of teeth that the gear rotates is equal to the number of the hob head. The worm gear hob's hob is often made into a straight chip flute when the helix rise angle is less than 5 °. It is easy to manufacture and sharpen. The disadvantage of cutting with a large negative rake angle (see tool). Small and medium-modulus gear hobs made of high-speed steel generally adopt an integrated structure. For hobs with a modulus of more than 10 mm, in order to save high-speed steel, avoid forging difficulties and improve the metallographic structure, the insert structure is often used (Figure 1 gear hob). The structure of the insert hob has many forms. The commonly used structure is the rack structure, that is, the tooth part is made of high-speed steel and rack-shaped. The use of cemented carbide hobs can significantly improve cutting speed and cutting efficiency. Solid carbide hobs have been widely used in the watch and instrument manufacturing industry to process various small modulus gears. Medium modulus monolithic and insert carbide hobs have been used to machine cast iron and bakelite gears. Carbide hobs with a modulus of less than 3 mm are also used for machining steel gears. Carbide hobs can also process hardened gears (hardness HRC50 ~ 62). This hob often uses a single-tooth welding structure with a negative rake angle of 30 °, which scrapes off a layer of tooth surface during cutting. Manufacturers of hobs and trading markets are located all over the country, such as state-owned enterprises specializing in the production of gear cutters: Chongqing Tool Factory, Hanjiang Tool Factory, Harbin First Tool Factory. Wenling and other places in Zhejiang are mostly private enterprises, such as "Wenling Kaiyuan Tool Factory", "Zhejiang Work Volume Cutting Tool Trading Market" and "Chongqing Xingwang Tool Manufacturing Factory". . The country classifies hob accuracy grades as AA, A and B. Generally the national standard hob uses AA grade. Ministry standard and enterprise standard adopt A grade. The sprocket hob requires low accuracy, and generally adopts B grade. In order to process extremely precise gears, some countries also have AAA hobs. During the tooth cutting process, the manufacturing error of the hob mainly affects the tooth profile error and base deviation of the gear. The hob used for gear profile before shaving is called the hob before shaving. The tooth shape of the hob before shaving must be made into a special shape according to the requirements of the remaining amount of shaving teeth (Figure 2 Tooth shape of the hob before shaving). The hob tooth profile with crown at the top and chamfer at the root is a more commonly used form of shaving. It causes some roots to be dug at the tooth root of the processed gear, some chamfer at the top of the tooth, and shaving in the middle The balance is even. (See the color drawing of the hardened surface gear cutting tools-carbide scraping hob, gear cutting tools-hob, gear shaper, shaver) ^[1]