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The term ultrasonic is generally used for describing a frequency vibration wave that is above the upper frequency limit of a human ear. It refers to all frequencies that are above 16 kc/s. In 1927, Loomis and Wood noted that the use of high frequency sound waves had a certain influence on machining. High frequency waves are required in the production of brittle materials which include ceramic, diamond and glass (Maluf & Williams, 2004). The production of ultrasonic machines started way back in 1950s. Ultrasonic impact grinding is also used to refer to ultrasonic machining. There are specifically two types of waves, longitudinal and shear waves. Since longitudinal waves are easy to generate and propagate them in liquids, gases and solids, they are used in most of the ultrasonic applications. Longitudinal waves have a short wavelength and, therefore, travel at a high velocity and are preferred (Hinduja & Fan, 2007).
Describing the Process of Ultrasonic Machining
Ultrasonic machining refers to a process in which materials are able to be removed because of the abrasive grains action. It starts by driving the abrasive particles into the work surface using a tool that oscillates at high frequency and it is normal to the work surface. The tool is then subjected to vibrations that have specific frequency, intensity and direction. Vibration is produced by a transducer which is then transmitted to the tool by use of a vibration system and in most times there are changes in the amplitude and direction.
The tool is usually made of materials that are soft. It then oscillated at frequencies of about 25 kc/s and has equivalent amplitude of 0.19 mm. a load of a few kilograms is used to press the tool against the work piece moving it downwards while cavity is cut in the work. It is clear that the machining rate do not actually depend on the load used. There is no relationship that lies in the rate of machining and the physical, electrical and chemical attributes of the material of work.
In general ultrasonic machining (USM) process is more useful in machining brittle and hard materials into composite shapes that are designed with a reasonable finish and with good accuracy. Hard alloys which offer high resistance to ultrasonic machining may not actually be applicable. Results have shown that the process can be used in order to make holes of any shape that curved to the axis. Ultrasonic machining process has a low MRR which introduces a lot of limitations. The metal cutting rates can thus be improved with the work piece and not affected. Therefore, ultrasonic machining causes little damage on the surface of the work material and with its low MRR, it can be used in nonconductive and conductive electrical material machining (Astashev & Babitskii, 2007).
Materials That Are Utilized In the Process
There are four main materials or elements that are used in ultrasonic machining process. They include the work material, abrasive slurry, tool tip and cone and the ultrasonic machine (Pandey & Shan, 1980).
The material is removed from the process by brittle failure hence brittle materials are the only ones that the machine can process. The results have shown that there is no limitation to the range of the usable materials. The only property that makes the material to be used is that it should not dissolve into the slurry media. The materials should also not react with the slurry material. Work materials that are ductile and soft are more economical to cut.
Tool Tip and the Horn
The tool cone is also referred to as the horn. It only focuses and intensifies on the mechanical energy that the transducer produces and ensures that the utilization of energy is very optimal. Usually it takes short time and studies have shown that it is smaller than half the wavelength and can act as a velocity transformer. The tool cone gives the necessary force amplitude ratio through modifying the vibratory energy. If a horn has a low gain it yields to low amplitude that is characterized by high force capability, and, in order to operate more efficiently, it must be tuned with a frequency that has a minimal kc/s. the material that is used for the cone should have enough strength in order to be able to sustain the stresses that could occur during its operation. In most cases the tool cone or horn is made of titanium which is considered as the best material. On the other hand, the tool tip is usually attached by a silver brazing to the tool cone base. It requires the area for the tip and it should not be larger than that of the cone. Higher penetration rate is caused by a small area of contact which also leads to a good abrasive flow.
Diamond dust, silicon carbide, alumina and boron carbide are the common types of abrasives that are used. Although boron carbide is very expensive it is the most suitable for cutting precious stones and steel tools. Alumina is used for cutting ceramics, germanium and glass, but it wears out quickly. Diamond dust ensures good surface finish, cutting rates and better accuracy and is used in cutting rubies and diamond.
The main parts of the ultrasonic machine generator are abrasive feed system, tool feed mechanism and the acoustic head. Most transducers that are of the magnetostriction type use the longitudinal type of waves and are usually made of nickel, because it has high strength and better insulation properties. The ultrasonic machine’s total feed mechanism should provide cutting force that is adequate, bring the tool slowly to the work piece, decrease force and is able to return the tool. The abrasive slurry is hand supplied in case of a small machine and should be stored in a bath at the cutting zone which helps to prevent the tool from scattering. The generator in the ultrasonic machine should be cheap, efficient, reliable and simple in design
Ultrasonic can be used in other engineering applications apart from machining, (Pandey& Shan, 1980). These areas of application include:
- Forming of plastics.
- To measure the velocity of fluids which are in motion.
- Ultrasonic is used in welding and casting metals.
- To determine the grain size and hardness of metals.
- It also can be used in detecting flaws and leaks.
- Ultrasonic is applicable in the measuring constants such as elastic, viscosity and density.
- It is used in determining nondestructive residual stress.
- It is used in treating and diagnosing certain diseases in the medical field.
The ultrasonic machine quality will highly depend on the size of grit of the abrasive that is used and its power rating (Hocheng & Tsai, 2012). Tool wear leads to poor holes which in turn make the materials of low quality. In summary quality problems in the process are brought about by wearing of the tool and the size of the grit.
Ultrasonic machining is one of the modernized machining processes used in machining of fragile and brittle materials. These materials have poor machinability and others are very hard to deal with. The critical need is to investigate ultrasonic machining application which cost is effective for the materials that are tough such as alloys and nickel. The obtained surface roughness actually increases with the ultrasonic oscillations energy, abrasive concentration and the size of the grain.
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