Gear Hobbing Process
HOBBING
Hobbing is a machining process for gear cutting, cutting splines, and cutting sprockets on a hobbing machine, which is a special type of milling machine. The teeth or splines of the gear are progressively cut into the material (a flat, cylindrical piece of metal) by a series of cuts made by a cutting tool called a hob.
Hobbing is relatively fast and inexpensive compared to most other gear-forming processes and is used for a broad range of parts and quantities.]Hobbing is especially common for machining spur and helical gears. A type of skiving that is analogous to the hobbing of external gears can be applied to the cutting of internal gears, which are skived with a rotary cutter (rather than shaped or broached).
History of Gear Hobbing:
Gears themselves are simple machines that date back to the 27th Century BCE. For many centuries, gears were used in basic applications like water wheels and clocks, but it wasn’t until the 1600s, with the discovery and utilization of the involute curve, that gear functionality was able to expand. In the 1800s, gear manufacturers started using form cutters and rotating cutters to shape gears more precisely.
The first gear hobbing process was patented in 1835; hobbing machines use hobs, which are specialized spiral-shaped cutting tools to generate gear teeth. Throughout the 1800s, manufacturers worked on making semi-automated processes with these hobs, with the first hobbing machine capable of cutting both spur and helical gears emerging in 1897.
It wasn’t until the late-20th century that hobbing became fully automated, capable of producing thousands of gears in an hour; in 1975, the Pfauter Company introduced the first NC hobbing machine, followed by the full 6 axis machine in 1982. Hobbing machines can now be used to produce cycloid gears, helical gears, splines, sprockets, worm gears, and of course, spur gears.
GEAR HOBBING
For high or low volume production of external cylindrical gears, the industry frequently uses the continuous gear creation technique known as hobbing. One of the main manufacturing techniques used in the sector is gear hobbing. The requirements for additional gearing of both old and new varieties with a better and higher degree of accuracy are continuously rising in modern engineering practice.
Gear Hobbing is the process of generating gear teeth by means of a rotating cutter referred to as a ‘hob’. A hob resembles a worm gear; it has a number of flutes (also referred to as a gash) around its periphery, parallel to the axis, to form cutting edges.
CONSTRUCTION OF GEAR HOBBING MACHINE
· 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.
WORKING OF GEAR HOBBING
· By simultaneously spinning the gear blank and the cutter, known as a hob, with a fixed gearing ratio between the two, gears are cut using the gear hobbing process. The gear blank is fed into the rotating hob in this method until the necessary depth is obtained. Until all of the teeth are formed, the hob is fed across the blank’s face.
- 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.
1. Convectional Hobbing
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
2. Climb hobbing.
In conventional hobbing, the hob is fed into the work, moving toward the table or spindle nose, parallel to the blank axis. In climb hobbing, the hob is fed moving away from the table or spindle nose, parallel to the blank axis
TYPES OF HOBBING PROCESS
1. Hobbing with Axial Feed
In the process of hobbing with axial feed, the gear hob is fed against the gear blank work-piece along the face of the gear blank and parallel to its axis of rotation. This method is generally employed in order to manufacture spur and helical gears.
2. Hobbing with Radial Feed
In this method of hobbing with radial feed, the hob or the cutting tool and gear blank work-pieces are set with their axis at an angle perpendicular to each other. The rotating hob or the cutting tool is slowly fed against the gear blank in a radial direction or a direction normal to the axis of gear blank workpiece. This method is suitable for the manufacturing of worm wheels.
3. Hobbing with Tangential Feed
This type of hobbing process is generally utilized for cutting teeth on the worm wheel. In this case, the hob is being held with its axis horizontal but at an angle perpendicular to the axis of the gear blank. The hob or cutting tool is fixed at full depth of the cutting tooth and then fed constantly forward axially. The hob in this operation is fed tangentially to the face of the gear blank.
ADVANTAGES OF GEAR HOBBING PROCESS
1. Gear hobbing is a fast and continuous process so it is realized as an economical process as compared to other gear generation processes.
2. Lower production cycle time, i.e. faster production
3. 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.
4. Several gear blanks, mounted on the same arbor, can be processed simultaneously.
5. 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 PROCESS
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.
Gear Hobbing Applications
1. Gear hobbing machine can be used to produce the following types of finished gears:
· Sprockets
· Spur gears
· Cycloid gears
· Helical gears
· Splines
· Worm gears
· Involute gears
· Ratchets
2. It can also be used for producing internal gears for which the machine should have a facility for fitting a special head.