What is Gear Hobbing? (How is Gear Hobbing Done?)

Gear Hobbing

What is Gear Hobbing?
Gear hobbing is a method of making gear teeth by the help of a rotating cutter called a hob.

By using gear hobbing operation, most of the involute gears are manufactured.

Gear hobbing is a faster and continuous method of gear manufacturing, hence it shows high production rate in less time.

Gear hobbing is used to manufacture spur gears, splines, helical gears, worms, worm wheels, sprockets, etc.

Well, this was just a small introduction about gear hobbing. But further I have mentioned the entire working of gear hobbing as well as its advantages, disadvantages and applications.

So let’s dive right into it!

How is Gear Hobbing done?

how is Gear Hobbing done

In gear hobbing, the cutting tool known as hob is used to cut the gear teeth on the blank.

The gear hob looks like a worm, but it has straight flutes and cuts on the periphery which acts like separate cutting edges.

Gear Hobbing operation

During the gear hobbing operation, the hob is rotated at some suitable speed and then it is fed into a gear blank.

At the same time, the gear blank is also rotated on its axis.

The gear hob and the gear blank are connected with a help of a change gear. 

The interesting thing here is that the change gear and hob are perfectly synchronised with each other in such a way that the one rotation of gear hob leads to the movement of blank by one pitch.

The hobs used in gear hobbing are generally made of high speed steels (HSS).

Gear Hob

Sometimes hobs with cemented carbide tips are also used in gear hobbers.

The hobs may have one, two or more starts.

Out of these, the single start hob is widely used.

But if there is a need for high accuracy, a rough cut is made by double or triple start hobs and then finishing cut is given with a single start hob.

A single start hob cutting a gear having T number of teeth is geared to the gear spindle so that in the time in which the gear blank males one rotation, the hob makes T rotation.

Similarly a double start hob makes T/2 rotation and triple start hob makes T/3 rotation for a single rotation of the blank. 

In the beginning of the gear hobbing operation, the gear blank (or workpiece) is moved towards the rotating hob until it reaches the desired depth.

This action is similar to the meshing of gear with the rack. 

After this, the hob cutter is fed across the face of the gear and in this way, all the teeth are made on the gear blank.

In the hobbing of spur gears, the hob is set in such a way that its teeth are parallel to the axis of gear blank.

The feed is given to the hob during the rotation of gear blank and hob, which results in cutting action on the blank.

gear hobbing of helical gears

In machining of helical gears, the axis of hob is set at the required angle to get the proper helix.

By using gear hobbing, the worm gears can also be cut by keeping the axis of hob at the right angle to the gear and then the hob is fed tangentially as the gear rotates.

Gear hobbing is generally done on single purpose gear hobbers.

According to the axis of rotation of spindle, the gear hobbers are of two types.

  1. Horizontal work spindle
  2. Vertical work spindle

Gear hobbers with vertical work spindle are widely used nowadays. 

Hobbers with multiple spindles are also preferred where the production requirements are high.

Having said that, let’s now see the advantages and disadvantages of gear hobbing.

Advantages of gear hobbing

The advantages of gear hobbing are mentioned below.

  • Gear hobbing is faster and continuous operation, that makes it quite economical.
  • The gear manufacturing rate by hobbing operation is high.
  • Gear hobbing is used to make spur gears, helical gears, splines, worm wheels, sprockets, etc.
  • The accuracy of the gear teeth is high.
  • Gear hobbers have simple settings and they are simple to operate.

Disadvantages of gear hobbing

The disadvantages of gear hobbing are mentioned below.

  • Gear hobbing is not suitable for generating internal teeth on a gear blank.
  • Gear hobbers are not useful for cutting teeth on gears which are very close to the shoulders.

Applications of gear hobbing

Gears made by hobbing are used in various applications like;

  • Automobiles
  • Machine tools
  • Instruments
  • Clocks
  • And many other equipment.

How is Gear Hobbing different from Gear Shaping?

Gear hobbing:
Gear hobbing operation is an accurate operation as it involves continuous cutting, plus it has non-reciprocating motion of hob.
The heat generated during the hobbing operation is equally distributed on the hob cutter and the workpiece. Because of this, the teeth does not show any defects related to excess heat.
Hobbing is a simple operation and it involves less settings.

Gear shaping:
Gear shaping can be used for making internal gears, herringbone gears, as well as gear clusters.
By gear shaping, it is possible to cut gears that have flanges and shoulders.
It is difficult to make very precise and identical teeth on the gear shaper.

External links:
Gear hob: Image by Dirk Gräfe, CC BY-SA 3.0, via Wikimedia Commons

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