Topics covered in this article are Ultrasonic Machining Process, Working Principle, Advantages, disadvantages, and how it works in detail.
What is Ultrasonic Machining Process ?
Ultrasonic machining process (USM) is the process of removal of hard and brittle materials using an axially oscillating tool at ultrasonic frequencies [18–20 kilo-hertz (kHz)].
During that oscillation, the abrasive slurry of B4C or SiCis continuously fed into the machining zone between a soft tool (brassor steel) and the workpiece. The abrasive particles are, therefore, hammered into the workpiece surface and cause chipping of fine particles from it.
The oscillating tool, at amplitudes ranging from 10 to 40 µm, imposes a static pressure on the abrasive grains and feeds down as the material is removed to form the required tool shape. Balamuth first discovered USM in 1945 during ultrasonic grinding of abrasive powders.
The industrial applications began in the 1950s when the new machine tools appeared. USM is characterized by the absence of any deleterious effect on the metallic structure of the workpiece material.
Working Principle of Ultrasonic Machining Process
It operates on the same principles as ultrasonic welding.
This method of machining employs ultrasonic waves to generate high frequency, low amplitude forces that act as abrasive driving forces. The ultrasonic machine produces a high frequency vibrating wave with a frequency of about 20000 to 30000 Hz and an amplitude of about 25-50 micron.
This high frequency vibration is transferred to the abrasive particles contained in the abrasive slurry. This causes the abrasive particle to indent the brittle work piece and removes metal from the contact surface.
Parts of Ultrasonic Machining
- Power supply
- High frequency generator
- Ultrasonic amplitude transformers or tool horn
- Cooling mechanism
- Tool holder
1. Power supply
This machining process typically necessitates a current power supply ranging from 50 to 60 Hz. As a result, an alternating current power supply is available to begin the process.
2. Transducers ( Magnetostrictor )
This transducer is magnetostrictive in nature. This transducer, once converted into a magnet, will change the frequency of mechanical vibrations by acting on the basis of magnetostrictive action. This transducer will vibrate in both up and down directions.
3. High Frequency Generator
A high-frequency generator is also known as an ultrasonic power supply or an electronic oscillator. It is commonly used to convert conventional power supplies operating at 50 or 60 hertz to high-frequency electrical energy. The most commonly used frequencies are 20 to 40 kHz. These frequencies are then fed into the electrical transducer.
4. Ultrasonic Amplitude Transformers or Tool Horn
This unit, as the name suggests, connects the tool to the transducer. It transmits amplified vibration from the booster to the tool. It should have a high endurance limit.
It is also referred to as a tool concentrator. The vibration amplitude generated by the transducer, with a range of approximately 0.025 millimeters, is insufficient for machining. It’s used to boost the amplitudes of vibrations.
The vibration is also directed and concentrated towards the tooltip. The tool is attached to the lower end of the tool horn and will aid in the removal of the material. Welding, screwing, brazing, or soldering are used to connect the tool to the horn of the tool.
5. Cooling Mechanism
A cooling system is installed on top of the transducer. Cold water enters through an entrance gate, receiving heat from the transducer and avoiding exit. A casing surrounds the transducer for cooling purposes, and water flows inside this casing.
Ultrasonic machining equipment is typically made of a strong hardening and brittle material that does not fail under brittle fracture and is ductile, such as tungsten carbide, stainless steel, titanium, copper, and so on.
Materials are removed from the workpiece using the tools. The devices are made in the same shape as the cavity that must be formed on the workpiece’s surface.
7. Tool Holder
It is used for holding the tool.
The nozzle is made of tungsten. Tungsten is used to make a nozzle because the slurry from the pump can damage the nozzle if it is made of a soft material; it is made of a hard material, such as tungsten.
In the machining area, silicon carbide, boron carbide, and mixed alumina solutions containing hard abrasive particles in water or oil are typically provided on a continuous basis.
The pump is used for supplying the abrasive solutions on the nozzle.
Working of Ultrasonic Machining
The machining system, shown in Figs, is composed mainly from the magnetostrictor, concentrator, tool, and slurry feeding arrange-ment.
This high-frequency input is fed to the electromechanical transducer i.e ( Magnetostrictor ) which is energized at the ultrasonic frequency and
produces small-amplitude vibrations.
The horn lie between the transducers and the tool holder. The horn is used to increase the amplitude of the transducer’s vibration, which is then focused and directed at the instrument. When an alternating current supply is connected to a high-frequency generator, the frequency of the input supply rises from 20 to 40 kHz. As the device vibrates, the tool holder grabs it.
The abrasive slurry is fed at a constant rate with the help of pump between the tool surface and the workpiece as the equipment vibrates. The tool is then lightly pressed against the workpiece, leaving enough space for the slurries to flow between the tool and the workpieces. The size generated in the workpiece as a result of material removal will be the same as the size of the tool.
As the vibrating device is pressed against the workpiece, the high kinetic energy of the vibration is transmitted to these abrasive particles, and these abrasive particles are applied to the workpiece’s surface, removing material due to microscopic friction.
Materials Removal Process Using Ultrasonic Machining ( USM )
Figure shows the complete material removal mechanism of USM,
which involves three distinct actions:
1. Mechanical abrasion by localized direct hammering of the abrasive grains stuck between the vibrating tool and adjacent work surface.
2. The microchipping by free impacts of particles that fly across the machining gap and strike the workpiece at random locations.
3. The work surface erosion by cavitation in the slurry stream.
The relative contribution of the cavitation effect is reported to be less than 5 percent of the total material removed. The dominant mecha- nism involved in USM of all materials is direct hammering. Soft and
elastic materials like mild steel are often plastically deformed first and
are later removed at a lower rate.
In case of hard and brittle materials such as glass, the machining rate is high and the role played by free impact can also be noticed.
Ultrasonic Machining Process Cutting Rate Depends On
Cutting rate : Cutting rate by using USM varies on certain factors. These are :
1. Grain size of abrasive.
2. Abrasive materials.
3. Concentration of slurry.
4. Amplitude of vibration.
Accuracy : The maximum speed of penetration in soft and brittle materials such as soft ceramics are of the order of 20 mm/min, but for hard and tough materials, the penetration rate is lower. Dimensional accuracy upto + 0.005 mm is possible and surface finishes down to an R, value of 0.1-0.125u can be obtained. A minimum comer radius of 0.10 mm is possible in finish machining. The range of sizes of USM machines varies from a light portable type having an input of about 20W to heavy machines taking an input of upto 2kW.
Applications of Ultrasonic Machining
Application : The simplicity of the process makes it economical for a wide range of applications such as :
1. Introducing round holes and holes of any shape for which a tool can be made. The range of obtainable shapes can be increased by moving the workpiece during cutting.
2. In performing/machining operations likes drilling, grinding, profiling and milling operations on all materials both conducting and non-conducting.
3. In machining glass, ceramic, tungsten and other hard carbides, gem, stones such as synthetic ruby.
4. In cutting threads in components made of hard metals and alloys by approximately rotating and translating either the workpiece or the tool.
5. In making tungsten carbide and diamond wire drawing dies and dies for forging and extrusion processes.
6. Enabling a dentist to drill a hole of any shape on teeth without creating any pain.
Advantages and Disadvantages of Ultrasonic Machining
1. Brittle, non-conductive, hard, and fragile materials can all be machined using ultrasonic machining.
2. Because no heat is generated during this machining process, there is very little or no physical change in the workpiece.
3. Nonmetal that cannot be machined by EDM or ECM due to poor electrical conductivity, but can be machined very well by Ultrasonic Machining.
4. It is a burr-free and distortion-free process.
5. It can be used in conjunction with other emerging technologies such as EDM, ECG, and ECM.
6. There is no noise during operation.
7. Both skilled and unskilled operators can use the equipment used in this machining.
8. It is possible to achieve a high level of accuracy while maintaining a high level of surface finish.
9. Regardless of its conductivity, any material can be machined.
1. Because of the micro chipping or erosion mechanism, metal removal is slow.
2. The sonotrode tip wears out more quickly.
3. Deep hole machining is difficult with this method due to the inability of abrasive slurry to flow at the bottom of the hole (Except rotary ultrasonic machining).
4. Only materials with a hardness value of at least 45 HRC can be machined using ultrasonic vibration machining (HRC: Rockwell Scale to measure hardness of a material).
Limitations of the Ultrasonic Machining
Limitations of the process : The major limitation of the process is its comparatively low metal cutting rates. The maximum metal removal rate is 3 mm/s and the power consumption is high. The depth of cylindrical holes is presently limited to 2.5 times the diameter of the tool. Wear of the tool increases the angle of hole, while sharp corners become rounded. This implies that tool replacement is essential in the production of accurate blind holes. Also, the process is limited, in its present form to machine on surfaces of comparatively small size.
The tool material employed in USM should be tough and ductile. The difficulties with very ductile metals like Aluminium can be traced due to its short tool life. This difficulty can be eliminated by using low carbon steel and stainless steel as tool materials.
Experimental verification has shown that Metal Removal Rate decreases with the ratio of workpiece hardness and tool hardness. Thus if the workpiece hardness increases, it is expected that the tool hardness is also increased.
The choice of tool metal is one of the most important decision making for optimization of metal removing and tool cost. The mass length of the tool also pose difficulty as the tool materials absorbs much of the ultrasonic energy, reducing the efficiency. Longer tool causes overstressing. The grains size and abrasive slurry also of the correct dimension. It has been observed that if grain size is more or less than the amplitude of the vibration, machining rate decreases.
Choosing a grain for finish machining should not overlap with the specified grains of rough machining while cutting deep holes special techniques are needed for supplying the slurry through the tool holder else accumulation of grain particles, inside the hole will abstract further machining.
Forced circulation, mixing alternatively higher and lower sized grains, suctioning are some of the many effective methods followed to remove this deep hole machining problem.
Recently Development in Ultrasonic Machining
Recent development : Recently a new development in ultrasonic machining has taken place in which a tool impregnated with diamond dust is used and no slurry is used. The tool is oscillated at ultrasonic frequencies as well as rotated. If it is not possible to rotate the tool the workpiece may be rotated.
This innovation has removed of the drawbacks of conventional process in drilling deep holes. For instance the hole dimensions can be kept within + 0.125 mm. Holes upto 75 mm, depth have been drilled in ceramics without any fall in the rate of machining as is experienced in the conventional process.
Frequently Asked Questions
In ultrasonic machining the material is removed by
A. Using abrasive slurry between the tool and work
B. Direct contact of tool with the work
C. Maintaining an electrolyte between the work and tool in a very small gap between the two
D. Erosion caused by rapidly recurring spark discharges between the tool and work
Answer: Option A
Ultrasonic machining (USM) is the removal of material by the abrading action of grit-loaded liquid slurry circulating between the workpiece and a tool vibrating perpendicular to the workpiece at a frequency above the audible range.
In ultrasonic machining the function of transducers is to
A. convert mechanical energy into heat
B. convert electrical energy into heat
C. convert electrical energy into mechanical vibrations
D. convert mechanical energy into electrical energy
The transducer converts the oscillating current to a mechanical vibration. Two types of transducers have been used in ultrasonic machining; either piezoelectric or magnetostrictive: … Magnetostriction is an effect which causes a material to change shape slightly when a magnetic field through it changes.
In Ultrasonic machining, the tool moves
A. moves in transverse direction
B. moves in longitudinal direction
C. vibrates in transverse direction
D. vibrates in longitudinal direction
These were all the information regarding Ultrasonic Machining Process, How it works, on what principle it works, what are the advantages and disadvantages of Ultrasonic Machining Process.
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