1.Gear hobbing process - Overview

Hobbing machines provide gear manufacturers a fast and accurate method for cutting parts. This is because of the generating nature of this particular cutting process. Gear hobbing is not a form cutting process, such as gashing or milling where the cutter is a conjugate form of the gear tooth. The hob generates a gear tooth profile by cutting several facets of each gear tooth profile through a synchronized rotation and feed of the work piece and cutter.

 

 

As the hob feeds across the face of the work piece at a fixed depth, gear teeth will gradually be generated by a series of cutting edges, each at a slightly different position. The number of cuts made to generate the gear tooth profile will correspond to the number of gashes of the hob. Simply put, more gashes produce a more accurate profile of the gear tooth.

The hobs several cutting edges will be working simultaneously, which provide significant potential for fast cutting speeds and/or short cycle times. With this realization, one can see the hobbing process’s advantage over other cutting processes.

All gear hobbing machines, whether mechanical or CNC, consist of five common elements, which are listed below and shown in Figure 2.

  1. A work spindle to rotate the work piece (shown in blue)
  2. A cutter spindle to rotate the cutting tool, the hob (shown in yellow)
  3. A means to rotate the work spindle and cutter spindle with an exact ratio, depending on the number of teeth of the gear and the number of threads of the hob (shown in red)
  4. A means to traverse the hob across the face of the work piece (shown in green)
  5. A means to adjust the center distance between the hob and work piece for different size work pieces and hobs

The following schematic diagram shows a horizontal mechanical hobbing machine:

Figure 2

While the hob and work piece are rotating, the hob normally feeds axially across the gear face at the gear’s tooth depth to cut and produce the gear. In conventional hobbing, the direction of feed matches the direction of the cutting motion. Alternatively, in climb feeding, the feed is opposite to the direction of the cutting motion. Generally, conventional hobbing produces a better finish, whereas climb hobbing yields better tool life. For either method, the cutting forces of the hob should be directed towards the work spindle and not the tailstock.

Figure 3

To cut a helical gear, a standard hob cutter can be used. Mechanical hobbing machines provide a differential motion through a series of change gears to generate a gear tooth helix. Today, CNC hobbing machines electronically provide this necessary differential to produce helical gears.

Source – www.asthegearturns.com

2.Types of Gear hobbing Machines
Different types of Gear Hobbing Machine

Gear Hobbing Machine can be classified into two types –

  1. Vertical Gear Hobbing Machine – Vertical gear hobbing is very versatile and the most commonly used. In this machine the axis of the job is perpendicular to the machine’s bed.
  2. Horizontal Gear Hobbing Machine – Horizontal Hobbing machines are usually used for cutting longer work pieces. In this machine the axis of the job is parallel to the machine’s bed.
  3. Hob Cutter

    The hob is a cutting tool used to cut the teeth into the workpiece. It is cylindrical in shape with helical cutting teeth. These teeth have grooves that run the length of the hob, which aid in cutting and chip removal. There are also special hobs designed for special gears such as the spline and sprocket gears.[1]
    The cross-sectional shape of the hob teeth are almost the same shape as teeth of a rack gear that would be used with the finished product. There are slight changes to the shape for generating purposes, such as extending the hob’s tooth length to create a clearance in the gear’s roots.[2]Each hob tooth is relieved on the back side to reduce friction.[3]

    Most hobs are single-thread hobs, but double-, and triple-thread hobs increase production rates. The downside is that they are not as accurate as single-thread hobs.[4] Depending on type of gear teeth to be cut, there are custom made hobs and general purpose hobs. Custom made hobs are different from other hobs as they are suited to make gears with modified tooth profile. The tooth profile is modified to add strength and reduce size and gear noise.

This list outlines types of hobs:

  • Roller chain sprocket hobs
  • Worm wheel hobs
  • Spline hobs
  • Chamfer hobs
  • Spur and helical gear hobs
  • Straight side spline hobs
  • Involute spline hobs
  • Serration hobs
  • Semitopping gear hobs

References:

  1. Todd, Robert H.; Allen, Dell K.; Alting, Leo (1994),Manufacturing Processes Reference Guide, Industrial Press Inc.,ISBN 0-8311-3049-0. Page 59-60
  2. Jones, Franklin D. (1964),Machine Shop Training Course(5th, Illustrated ed.), Industrial Press Inc., ISBN 978-0-8311-1040-6. Page 288
  3. Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003),Materials and Processes in Manufacturing(9th ed.), Wiley, ISBN 0-471-65653-4. Page 768
  4. Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003),Materials and Processes in Manufacturing(9th ed.), Wiley, ISBN 0-471-65653-4. Page 770

4. Interesting reads about Gear Hobbing Solutions

 

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