Gear Hobbing – Parts, Working, Diagram, Advantages, Disadvantages

Gear Cutting :

A gear is a rotating machine part having cut teeth, which mesh with another toothed part to transmit torque. Gear is cut from round blank carrying teeth along its periphery. Gear cutting is a specialized job. Gear cutting is any machining process for creating a gear. The most common gear-cutting processes include hobbing, broaching, milling, and grinding. Such cutting operations may occur either after or instead of forming processes such as forging, extruding, investment casting, or sand casting.

Gear Hobbing Process – Introduction 

Hobbing is a continuous gear generation process widely used in the industry for high or low volume production of external cylindrical
gears. Gear hobbing is one of the major manufacturing processes in the industry. Modern engineering practice is continually increasing its demands for more gearing of old and new types with a higher and higher degree of accuracy.

Construction Of Gear Hobbing Machine 

  • Hob– In this process, the gear blank is rolled with a rotating cutter called a hob. Gear hobbing is done by using a multipoint cutting tool called gear hob. It looks like a worm gear having a number of straight flutes all around its periphery parallel to its axis. These flutes are so shaped by giving proper angles to them so that these work as cutting edges.
  • In gear hobbing operation, the hob is rotated at a suitable rpm and simultaneously fed to the gear blank.
  • The gear blank is also kept as revolving. Rpm of both, gear blank and gear hob are so synchronized that for each revolution of gear bob the gear blank rotates by a distance equal to one pitch distance of the gear to be cut.
  • The motion of both gear blank and hob is maintained continuously and steady. A gear hob is shown in Figure and the process of gear hobbing is illustrated in Figure
  • The hob teeth behave like screw threads, having a definite helix angle. During operation the hob is tilted to helix angle so that its cutting edges remain square with the gear blank.
  • Gear hobbing is used for making a wide variety of gears like spur gear, helical, hearing-bone, splines and gear sprockets, etc.
Process Of Gear Hobbing 
Process Of Gear Hobbing

Working Of Gear Hobbing :

  • Gear hobbing is a process in which gear is cut by a generating process by rotating the gear blank and the cutter called a hob at the same time with a fixed gearing ratio between hob and gear blank. In this process the gear blank is fed towards the rotating hob until the required depth is reached. The hob is fed across the face of the blank until all the teeth are complete.
  • In hobbing spur gear the hob teeth are set parallel to the axis of rotation of the blank. For helical teeth bobbing, the axis of the hob is set over an angle to produce the proper helix. While worm gears are produced with the axis of the hob set the right angle to the gear blank.
  • Generally two techniques are used in the hobbing process.

i) Conventional hobbing
ii) Climb hobbing.

  • The rate of production is quite high as compared to the other gear generating processes. The process can be used for the production of spur, helical, worm gears, sprockets, splines, etc. But due to the rotary cutter cutting up to the shoulder is not possible.

Gear Hobbing Parameters : 

Three important parameters are to be controlled in the process of gear hobbing indexing movement, feed rate, and the angle between the axis of gear blank and gear hobbing tool (gear hob).


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Types Of Hobbing Process : 

The process of gear hobbing is classified into different types according to the directions of feeding the hob for gear cutting. The classification is described as given below.
Hobbing with Axial Feed
In this process the gear hob is fed against the gear blank along the face of the blank and parallel to its axis. This is used to make spur and helical gears.
Hobbing with Radial Feed
In this method the hob and gear blanks are set with their axis normal to each other. The rotating hob is fed against the gear blank in radial direction or perpendicular to the axis of gear blank. This method is used to make the worm wheels.
Hobbing with Tangential Feed
This is also used for cutting teeth on the worm wheel. In this case, the hob is held with its axis horizontal but at the right angle to the axis of the blank. The hob is set at full depth of the tooth and then fed forward axially. The hob is fed tangentially to the face of the gear blank.

Advantages and Limitations of Gear Hobbing Process

Advantages of gear hobbing process are described below :
(a) Gear hobbing is a fast and continuous process so it is realized as an economical process as compared to other gear generation processes.
(b) Lower production cycle time, i.e. faster production rate.
(c) The process has a larger variability’s in the following of sense as compared to other gear machining processes.
(i) Capable to make a wide variety of gears like spur gear, helical gears, worms, splines, sprockets, etc.
(ii) The process of required indexing (named so) is quite simplified and capable to make any number of teeth with consistent accuracy of the module.
(iii) A special type of gear named herringbone gear cam be generated by gear hobbing exclusively.
(iv) A wide variety of batch sizes (small to large volume) can be accommodated by this process.
(d) Several gear blanks, mounted on the same arbor, can be processed simultaneously.
(e) Hob is a multipoint cutting tool having multi cutting teeth or edges at a time few number of cutting edges work so lots of time is available to dissipate the generated heat. There is no overheating and cutting tool.

Disadvantages of Gear Hobbing machine : 

1. Not adopted to generate internal gears.
2. Restricted adjacent shoulders larger than the root diameter of the gear.
3. Splines and serrations are not suitable for hobbing.

Applications of Gear hobbing :

1. It is widely used to produce spur, helical gears, worms, and worm wheels.
2. It can also be used for producing internal gears for which the machine should have a facility for fitting a special head.

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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