Gear hobbing is an approach that is used to construct gear teeth configurations that are perfect for using in a large amount of machinery components. It is one of the most profitable and accurate processes for producing gear teeth. It is the process of inducing gear teeth by virtue of a spinning cutter which is usually called a hob. It is characterized by a constant movement, where cutting fool and work piece are spinning while a hob is involved into work. If to compare gear hobbing to other processes, it is less expensive, but remains being accurate. It is usually used for a big amount of pieces and quantities. However, hobbing is only used for creating spur and worn gears. Interior or shoulder gears cannot be utilized in hobbing process.
There are several types of hobbing. Here are the most common of them:
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The first type is called Arial hobbing. The aim of using it is to cut spur or helical gears. Here, the gear blank is moved in the direction to the hob to get the necessary teeth depth. When the table is clamped, the hob moves directly to the face of the blank in order to complete the job. Additionally, Arial hobbing that is used for cutting spur and helical gears can be gained by convential hobbing or climb noting.
The second one is a radial hobbing. It is basically used to cut worm wheels. In this technique the gear blank and the hob are situated next to each other. After that the gear blank keeps spinning on a high speed while the rotating hob is given a supply in a radial orientation. After necessary depth of teeth is cut, the motion of feed is stopped.
The last, but not the least method for cutting worn wheel is tangential hobbing. The worn wheel blank is spinning in a vertical plane over the horizontal axes. The hob keeps its blank or axis. Before starting cutting, the hob is located deep into the die tooth. Only after that it starts spinning. The spinning hob is fed forward axially. The front piece is cut to an appropriate length. The cut is made in a tangential way, which is why the name of this method is tangential hobbing (Franklin D. Jones, 1924).
There several materials that are appropriate to use in gear hobbing processes. They are:
- Cast iron. It is one of the most widespread and accessible materials because of its good swearing properties, manufacturability and simplicity in producing intricate shapes.
- Steel. It is notable for its strength and resistance.
- Cast steel. It is utilized when it is hard to produce the gears.
- Carbon steel. It is mostly used when the high strength combined with the toughness.
- Alloy steel. It is usually appropriate when low teeth wear and high teeth strength are necessary.
- Aluminum. It is good for low inertia of rotating mass (Kombogiannis S., 2002).
- Non-metalic materials. They are perfect for soundless operations at high speed.
Hobbing utilizes a hobbing machine with the two skew spindles, where the first one is installed with a blank workpiece and the second one with the hob. The gradient amidst the hob's and workpiece's spindle differs, hanging on the sort of produced product.
Contemporary hobbing machines are called hobbers now. They are completely automatized and differ in size, while they are about to create any detail, ranging from little instrument gear to 3 m diameter marine gears. Every gear hobbing machine is formed of a chuck and tailstock. These elements are made for holding the spindle, where the hob is situated, the workpiece and the drive motor. There is a restriction that only five or less teeth can be cut into the spindle at the same time. Nevertheless, it often happens that various gears are cut at the same time. If the gear is big, the blank is usually notched to the rough space in order to make hobbing easier (Edgar J., 1914).
Hobbing machines are distinguished by the bigger diameter of pitch or module it can generate. Mostly hobbing machines are vertical hobblers that make it clear that the blank is situated vertically. Unlike vertical hobblers, horizontal hobbing machines are used to cut longer workpieces.
The gear hobbing machines use the modular conception which is divided into four segments of different size. These are:
- Platform 1: up to 180 mm
- Platform 2: up to 380 mm
- Platform 3: up to 1600 mm
- Platform 4: up to 6000 mm
Gear hobbing machines use a number of modified bits that enable to create the particular kinds of cutting and shaping. A modified bit is used for a certain size and sort of gear hobbing, which ensures the smoothness of the cuts.
Custom gear hobbing is followed in many countries all over the world. Primarily, it included tools and bits in order to construct the gear form. Only after that, it started thoroughly form every teeth on the gear. Mostly this requires creator to have permission to use the machine which houses the gear. It gives an opportunity to test the fit of teeth from time to time. This type of gear hobbing may include such materials as hardwood. Basically, this process of gear hobbing creates gears which originated for uncomplicated no motor driven types of machines.
As every process, the gear hobbing has its advantages and disadvantages. Here are some advantages of using gear hobbing process:
- High rate of productivity;
- Low-cost and cost-effective functioning;
- Multifunctionality of operations.
As for the disadvantages, they are as follows:
- Insufficient surface finish;
- The requirements of good servicing using the minimum play in moving parts and feed mechanism (Wermeister G., 2011).
All in all, gear hobbing is quite a profitable craft that has been taken place for thousands of years. With the development of technology, new methods of gear manufacturing will move forward the technique of gear hobbing that may result in more accurate gear configurations. It has become one of the most productive among all gear productive processes. Furthermore, it can only be utilized for making spur and worn gears.
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