Hobbing is one of the most used methods in gear manufacturing, in particular for spur and helical gears. It is a generating process in so far as during the working phase both the hob and the workpiece rotate according to a specific synchronism, simulating a worm screw/worm gear coupling. In order to make this process utmost productive, the hob should be manufactured following specific steps and features.

Hob manufacturing is composed by two main phases: design and working phase, which are analyzed below.

Design:

The design phase is a pivotal phase in which the geometric definition of the hob is set. Engineers analyze the data of the gear in order to define the rack pitch line. Thanks to the use of the rack pitch line, engineers can delimit the geometries in movement. In this phase you can easily see the similarity between the rack pitch line and enveloping of the different hob’s position. At the end of this phase, the hob’s geometry is well defined.

Hobs manufacturing

Working Phase:

The second phase is composed by the working phase, which is mainly divided in:

  • Material choice

The technical characteristics of different types of steel largely depend on the so-called alloying components. In fact, a high speed steel is made from an iron based (Fe), to which carbon (C) and other precious metals are joined, such as vanadium (V), tungsten (W), chromium (Cr), molybdenum (Mo), cobalt (Co), etc.

These elements, when combined with carbon, create the so-called carbides, which give super-fast steel its characteristics of hardness, wear resistance and toughness. The different percentage compositions of alloy components give rise to the various types of high speed steels.

  • “Soft” machining

The “soft” machining is quite a central part, at the end of which the shape of the hob is made. It starts with the turning process, which can be external or performed by drilling machines following by threading. After an analysis of the slots geometry, we pass to the milling operation through insert cutters. So now our hob is threaded and milled. The following process is tip relieving, which is characterized by two main elements:

  • Rack angle
  • Tip relief angle
  • Heat Treatment

The heat treatment is carried out trough furnaces, which are essentially chambers in which the vacuum is practice. Temperature is maintained at the desired value by a series of electrodes through an electronic control system, which receives information from a series of thermocouples placed inside the furnace. Generally, the air inside the chamber is sucked to a pressure of about 5×10-3 bar, but then the nitrogen (N) is inserted to a pressure of about 1.5 bar. In the cooling phase, nitrogen is still inserted to reach a pressure of 8 bar.

  • Grinding

The heat treatment phase is followed by the grinding of the central hole of side faces and collars. Grinding can be carried out with ceramic wheels or CBN wheels.

  • Check

Before the final quality control phase, there are two important checks:

  • Optical extension
  • Tolerance control

The final quality check, carried out with a probe, is used to check indexing errors, lead and runout. At the end of this stage a report is issued.

Thereafter, the hob is passing through the sand-blasting phase, where a thin material layer is removed in order to eliminate any abrasions and grinding burns. The cutting edges are made rounded for 10uM in order to avoid peeling on the coating.

 

  • Coating

Coating represents the last phase and is carried out through electrodeposition in a controlled atmosphere. There are different types of coatings available on the market, from the newest Altensa to Alcrona, including TiN.

At the end of these phases, the final hob is ready. However, it is extremely important to make the right choice in terms of material, coating and dimensions. The choice you make will determine the future performance and quality of your tool.

In this scenario, it is important to deal with gear cutting tool manufacturing companies that can assist you with technical expertise and capabilities. Samputensili Cutting Tools has a solid presence in the market and is able to design your tools based on your needs and industry requirements in order to optimize:

  • Performance
  • Quality
  • Productivity
  • Reliability

Samputensili Cutting Tools supports you in the choice of the proper tool, taking into account the size of your batch and the features of the material you have to cut. In addition, our advice concerning the right cutting parameters to be set on machine tools will be a further aid to support gear manufacturers in increasing of your profitability.

To sum up, hobbing in gear manufacturing is a widespread activity which greatly depends on the quality of the cutting tool. The initial choices will determine the quality and productivity of your tool. Hobs need to be designed and manufactured having in mind the gear to be produced and its application.