ULTRA SONIC MACHINING (USM)-
The term Ultrasonic refers to the waves of very high frequency that is above the audible range i.e., above 22 kHz. The Ultrasonic Machining method was first proposed by L. Balamuth in the year 1945. This Ultrasonic Machining method was developed for finishing operations by electro spark machines. This development of electro discharge machining made less importance for the use of this Ultra Sonic Machining.
As the machining of the hard and brittle materials which are either electrically conducting materials or also non conducting became more important in present trends. Thus, for machining these components, several types of Ultra Sonic Machining have been developed gradually.
The Ultra Sonic Machining process consists of a tool which is completely made up of ductile and a very tough material. This cutting tool oscillates with high frequency and a continuous flow of abrasive slurry is made to pass into the gap between the tool and the work piece. This tool has the same shape as required in the work piece. The impact of hard materials abrasives fractures the work piece, resulting in metal removal in the form of very small and fine particles. The wear particles are removed gradually by using the abrasive slurry.
In the ultrasonic machining process the metal removal is facilitated through abrasive action of each grain. These grains are hammered and forced into the work material by high frequency oscillating tool, due to which this is also named as Ultrasonic Grinding.
This machining method comprises more number of components. During the operation, an ultra sonic oscillator passes a high frequency electric current to the ultra sonic transducer. The ultra sonic transducer’s main function is converting the electrical energy into mechanical vibrations. A transducer is made of magnetostrictive material for the generation of mechanical vibration which usually ranges from 20 kHz to 30 kHz. These vibrations are transmitted through intermediate connecting parts- Transducer cone, Connecting body and Tool holder. As a result, the tool vibrates in a longitudinal direction. The cutting tool is attached at the end of the focusing unit. A slurry of small abrasive particles in fed into machining location by means of a pump. In addition to this, a cooling system can also be equipped for keeping the work piece at lower temperature.
Function of Transducer-
The main function of the transducer present in the Ultra Sonic Machining equipment is converting the whole electrical energy into vibratory motion by utilizing the principle of piezoelectric effect. Most transducers work on the principle of this because of high efficiency, high reliability, low voltage supply and simple arrangement for cooling. These transducers are magnetized rods which are constructed from materials like nickel and aluminium. When an electric current is applied, a strong magnetic field is induced due to which the size of the material is increased. On removing electric current, the material returns back to its original size.
Function of Tool Cone-
The tool cone is also known as Horn. They are generally made up of titanium, monel and stainless steels. The function of tool cone is to amplify the mechanical energy which is produced by the transducer. This horn mechanically modifies the vibratory energy to obtain the required force-amplification ratio. High gain horn has high amplitude with a low force capability and vice versa for other type.
In Ultra Sonic Machining process, large variety of abrasive slurry are used namely Boron Carbide, Silicon Carbide, Aluminium Oxide and Diamond dust.
Advantages of Ultra Sonic Machining-
1. The cost of metal removal is low.
2. As there is no heat generation in this process, there is no change in physical properties of the job during machining.
3. Machining of hard and brittle material can be performed very easily.
4. There are no residual stresses in the machined work piece.
5. This machining method produces high accurate profiles and good surface finishes.
6. The operation of the equipment is quite safe and noiseless.
Disadvantages of Ultra Sonic Machining-
2. Initial machining and tooling costs are very high.
3. This process is not suitable for heavy metal removal operations.
4. Softer materials give much difficulty in machining.
5. Periodic replacement of slurry is necessary for efficient cutting operations.
6. Replacement of tool is required for accurate machining as high ware rate of the tool results in poor dimensional accuracy.
7. Power consumption is very high.
8. Limited sized cavities are produced by this machining process.
Applications of Ultra Sonic Machining-
2. This method is most preferred in dentistry work for fine drilling of required shapes in length.
3. This method is suited to make round holes and holes of any shape that can be made on the tool.
4. It is used to perform coining operations for materials such as glass, ceramics, etc.
5. It has also been employed for multi step processing for fabrication silicon nitride turbine blades.
Limitations of Ultra Sonic Machining-
2. The power consumption is also very high when compared to other similar processes.
3. This method is limited for machining of small work pieces only.
4. Machining of deep holes is difficult, as the slurry movement is restricted.
5. The tool wear is relatively high.
The tool mechanism must perform all these functions for the proper working-
1. The tool must have sufficient cutting force and it must also withstand this cutting force during the operation.
2. The cutting force must be decreased when the required length is achieved.
3. The tool must be moved very slowly towards the work piece.
4. The tool must over run a small distance to ensure accurate and required hole size.
After Machining operation, this tool must be returned to its original position.
5. All the types of feed mechanisms that can be used for Ultra Sonic Machining process namely Counter Weight Type, Spring Type, Hydraulic and Pneumatic Type and motor type.