Ultrasonic Machining (USM), WORKING PRINCIPLE OF Ultrasonic Machining (USM), Applications, Magnetostrictive transducers, Transducer Amplifications, Feed Mechanism

Ultrasonic Machining (USM)

PROCESS OF Ultrasonic Machining (USM)

This is also an impact erosion process to machine materials where the work material is removed by repetitive impact of abrasive particles carried in a liquid medium in the form of slurry under the action of a ‘shaped’ vibrating tool attached to a vibrating mechanical system “Horn”. The word ‘shaped’ explains that the process in capable of producing 3D, profiles corresponding to the tool shape. Basic scheme is as shown in Fig

This is also an impact erosion process to machine materials where the work material is removed by repetitive impact of abrasive particles carried in a liquid medium in the form of slurry under the action of a ‘shaped’ vibrating tool attached to a vibrating mechanical system "Horn". The word ‘shaped’ explains that the process in capable of producing 3D, profiles corresponding to the tool shape.
Ultrasonic machining (USM)

WORKING PRINCIPLE OF Ultrasonic Machining (USM)

The working principle is schematically shown in Fig, where a shaped tool is given a mechanical vibration. This vibration causes the abrasive particles in the slurry to hammer against a stationary work piece to cause micro-indentations to initiate fracture in work material, observed as stock removal of the latter. The abrasives outside the domain of the tool remain inactive and so the removal process is an impact form machining process. The machined surface is approximately the inverse profile of the tool form. The machining rate is mainly dependent on the amplitude and frequency of impact. 

WORKING PRINCIPLE OF Ultrasonic Machining (USM) The working principle is schematically shown in Fig. 2.2, where a shaped tool is given a mechanical vibration. This vibration causes the abrasive particles in the slurry to hammer against a stationary work piece to cause micro-indentations to initiate fracture in work material, observed as stock removal of the latter. The abrasives outside the domain of the tool remain inactive and so the removal process is an impact form machining process. The machined surface is approximately the inverse profile of the tool form. The machining rate is mainly dependent on the amplitude and frequency of impact.

The range of particle size used in the process generally lies between 10 and 150 microns, the amplitude of vibration is selected between 5 and 75 microns. The removal rate of material is also dependent on the rate at which the abrasive particles are hammered, i.e. the frequency of operation. Higher the frequency of operation better is the material removal rate. Higher frequency has some adverse effect on human being in shop floor. However, higher frequency of about 430 KHz was tried to produce vigorous agitation in the liquid, which in turn affected stray cutting effects on the work piece, whereas lower frequency does not have this effect. But higher frequency is utilised effectively to grind small parts.

Ultrasonic machining of hard materials in classified into two groups:

1. Working with freely directed abrasives using high frequency (grinding).

2. Precision ultrasonic machining under the purview of present deliberation.

The much more safer zone of ultrasonic machining is between 19 kHz and 25 KHz. So, the tool is to be vibrated with an amplitude of vibration of 5 to 75 microns at a frequency between 19 and 25 kHz.

Abrasive in a slurry form is more effective compared to abrasives in loose form, since the liquid would help removal of material due to cavitation effect during return stroke of thetool. Moreover, the liquid will help evenly distribution abrasive particles into the working gap.

Vibration to the tool is imparted from a transducer which is energised by a electronic generator. The transducer converts the electrical energy to mechanical vibration whose frequency corresponds to the electrical supply frequency.

Since the vibration amplitude achievable from the transducer generally does not exceed 3 to 5 microns,, it needs to be amplified on the principle of mechanical resonance by the concentrator (velocity transformer), between. The transducer and tool. This connector also serves as the tool holder. It has amplitude of vibration at about 3 to 5 micron corresponding to the transducer at one end required vibration amplitude of about 5 to 75 microns at tool end. The transducer along with the concentrator and tool is known as

ACOUSTIC HEAD

Hence working principle of the machine tool will depend on:

Transducer : Type of material and its operational characteristics.

Concentrator : Shape, material property.

Tool : Material, shape and wear rate.

Abrasive slurry : Material, shape and size.

Operating parameters : Frequency, amplitude of vibration, static load.

Others : Mounting, feed mechanism, working temperature, etc.

The cutting process employs the mechanical vibration of the tool. The process cannot be of high efficiency unless most of the input electrical energy to the transducer and the resulting mechanical energy thus produced in the vibrator (horn) suffers minimum losses. And thereby the energy transmitted to the working zone and via the vibrator itself is dependent on the load to which it is coupled. The maximum efficiency is obtained by a proper matching of the two; the transducer and the vibrator.

Major working parts of this machine is the acoustic head. The main function of this is to produce and propagate vibration in the tool. Energy is being drawn from the generator in electrical form and is converted to mechanical form either by of Piezoelectric or Magnetostriction method.

Though the former is easier for design, yet due to its non-availability, latter has been tried. The periodical shortening and lengthening of the transducer in synchronism with the generator frequency initiates the vibration. The inadequacy of this amplitude of vibration necessitates the use of a concentrator (Horn). The concentrator is simply a convergent wave-guide for designed amplitude at its far end. The two together form the vibrating system.

ACOUSTIC HEAD Hence working principle of the machine tool will depend on:  Transducer : Type of material and its operational characteristics.  Concentrator : Shape, material property.  Tool : Material, shape and wear rate.  Abrasive slurry : Material, shape and size.  Operating parameters : Frequency, amplitude of vibration, static load.  Others : Mounting, feed mechanism, working temperature, etc.  The cutting process employs the mechanical vibration of the tool. The process cannot be of high efficiency unless most of the input electrical energy to the transducer and the resulting mechanical energy thus produced in the vibrator (horn) suffers minimum losses. And thereby the energy transmitted to the working zone and via the vibrator itself is dependent on the load to which it is coupled. The maximum efficiency is obtained by a proper matching of the two; the transducer and the vibrator.  Major working parts of this machine is the acoustic head. The main function of this is to produce and propagate vibration in the tool. Energy is being drawn from the generator in electrical form and is converted to mechanical form either by of Piezoelectric or Magnetostriction method.  Though the former is easier for design, yet due to its non-availability, latter has been tried. The periodical shortening and lengthening of the transducer in synchronism with the generator frequency initiates the vibration. The inadequacy of this amplitude of vibration necessitates the use of a concentrator (Horn). The concentrator is simply a convergent wave-guide for designed amplitude at its far end. The two together form the vibrating system.

vibrator is made resonating to obtain sufficient amplitude. The length of propagation is made half-wavelength (or occasionally a multiple of this). The concentrator becomes a volume resonator tuned to the same frequency which produces best condition for maximum power transfer.

There is always a second channel for the energy to dissipate as waste through the holder 6 and the fixed part 6 of the feed-mechanism. To minimise the loss it is advisable to fix the holder at any nodal point of the vibrating system. From the above considerations it is seen that proper care must be taken while designing the different accessories of the machine in order to achieve maximum attainable efficiency of the machine.

 Magnetostrictive transducers

There are some materials which exhibit change in dimension when they are magnetised. This property is known as magnetostriction or piezomagnetism. The change can be either positive or negative in a direction parallel to the magnetic field and is also independent of the direction of magnetic field.

Magnetostriction can be explained in terms of Domain theory. Domains are very small elements of material in the order of 10-8-10-9 cm3, called as dipoles. The forces cause the magnetic moments of the atoms to orient in a direction easier for magnetisation, coinciding with the directions of the crystallographic axes. In the cubic-lattice crystals (ferromagnetic) like iron and nickel there are six directions of easier magnetostriction. During unmagnetised condition all these axes remain in equal number causing the magnetic moment order less and unoriented domains, compensate one another. Under sufficient magnetic field, the magnetic dipoles (domains) orient themselves in the direction of magnetic field. This causes the material either to expand or contract till all the magnetic domains become parallel (magnetically saturated). When the material expands on the application magnetic field irrespective of its direction is called as positive magnetostriction and similarly for negative- magnetostriction when it contracts.

Thus the strain produced (positive or negative) i.e. l / l is very small of the order of 10-6 to 10-5. The magnetostrictive curves for few commonly used materials are shown in.

It is observed that most of the material exhibits positive-striction but for nickel it is negative. Some materials like iron and cast cobalt exhibit dual characters i.e. below about 350 Oersted of field intensity they behave positive and above that their properties reverse.

Practical materials, iron-cobalt, iron-aluminium (Alfer), and nickel have the highest magneto striction. An alloy of platinum (32%) and iron (68%) has exhibited highest magnetostriction of all ( l / l = 1.8 X 10-4) but of no use in practice for its high cost.

However, the property deteriorates at higher temperature and completely ceases above Curie-temperature of individual material. So consideration is to be provided while designing and developing the cooling system for magnetostrictive transducers, otherwise the natural frequency of the magnetostrictive core may change to detain the system from resonance.

Iron-aluminium alloy, Alfer displays similar magnetostriction as Nickel but owing to its higher resistivity its hysteresis loss is lower. Moreover, Alfter is stronger mechanically than nickel. Permendur has a special feature of high brittleness, cracks are to be avoided after stamping and should not be subjected to shocks or bending after annealing. It has got higher hysteresis loss than Alfer.

The magnetic properties of rolled sheets of magnetostrictive materials are anisotropic, particularly Permendur in the rolling direction and least in the perpendicular direction. Therefore, sheets cut at 45° to the rolling direction are best for making transducers.

Transducer Amplifications

Piezoelectric or Electrostrictive transducer developed so are for very high frequency application because of its growth size. If it is required to use this type of transducers with large radiating surface or at low frequency, it runs into difficulties since in the first case it is to obtain large enough crystals or manufacture large enough crystals units and in the second case similar considerations apply along with the fact that the radiation impedance becomes extremely high so that matching of the transducer to the generator becomes impossible. Both the difficulties may be overcome by using a sandwich transducers which consists of piece or pieces of transducer materials cemented between two plates of non-piezoelectric materials. Using several pieces of transducer materials side by side the radiating surface can also be increased.

Since the complete sandwich system now form the resonating system, the operating frequency comes down mismatching with discrete transducer materials resonant frequency. However, this type of transducers are preferred for low power applications of ultrasonic machining and good quality factors are not achievable by these types of transducer. For high power applications (intensity > 0.5 watt/cm2) the performance is not satisfactory. For very high power (above about 500 watt) the cost also becomes very high.

For better quality and cheaper transducers, magnetostrictive types are preferred.

Magnetostrictive transducers are used in nearly all good quality ultrasonic machine tools. Their advantages are their high efficiency, easier manufacturability and higher reliability in the range of 15-30 kHz. It also needs cooperatively very low voltage supply. Unlike previous type of transducers, this type of transducer requires a closed magnetic path, hence a window type transducer is commonly preferred.

Transducer Amplifications Piezoelectric or Electrostrictive transducer developed so are for very high frequency application because of its growth size. If it is required to use this type of transducers with large radiating surface or at low frequency, it runs into difficulties since in the first case it is to obtain large enough crystals or manufacture large enough crystals units and in the second case similar considerations apply along with the fact that the radiation impedance becomes extremely high so that matching of the transducer to the generator becomes impossible. Both the difficulties may be overcome by using a sandwich transducers which consists of piece or pieces of transducer materials cemented between two plates of non-piezoelectric materials. Using several pieces of transducer materials side by side the radiating surface can also be increased.  Since the complete sandwich system now form the resonating system, the operating frequency comes down mismatching with discrete transducer materials resonant frequency. However, this type of transducers are preferred for low power applications of ultrasonic machining and good quality factors are not achievable by these types of transducer. For high power applications (intensity > 0.5 watt/cm2) the performance is not satisfactory. For very high power (above about 500 watt) the cost also becomes very high.  For better quality and cheaper transducers, magnetostrictive types are preferred.  Magnetostrictive transducers are used in nearly all good quality ultrasonic machine tools. Their advantages are their high efficiency, easier manufacturability and higher reliability in the range of 15-30 kHz. It also needs cooperatively very low voltage supply. Unlike previous type of transducers, this type of transducer requires a closed magnetic path, hence a window type transducer is commonly preferred

A magnetostrictive material in an alternating field vibrates at twice the frequency of field (frequency doubling) as follows from the symmetry of the magnetostrictive curve. Besides this the amplitude of mechanical vibration is small since the slope near zero magnetic field is very small. The system becomes non-linear and increases loss. All these undesired qualities are removed by shifting the excitation to one side of the linear portion of the electrostriction curve, i.e. by providing a steady field or a biasing magnetization. So a steady field superimposed on the alternating field would provide a steady undistorted and matched frequency mechanical vibration is obtained from the transducer (but with 180° out of phase).

Whatever type of transducers may be used, the losses in them are kept to minimum when the amplitude of vibrations are limited not to exceed 3-5 microns.

Since the required level of vibration at the tool end is more, the transducer vibration needs to be amplified many fold. This is done by means of the concentrator or the resonator which is nothing but a mechanical amplifier.

Concentrator

The concentrator provides the link between the transducer and tool and its main function is to amplify the amplitude of vibration as per requirements. This is also achieved through the principle of resonance. But then the end which will be connected to the exciter, i.e. the transducer, will suffer from amplitude mismatch which does not allow the system to work smoothly.

If a taper rod is chosen in place then the nodal point will be shifted from point 0 to 0′ because change of center of mass, shifted towards the larger end. This provides the smaller and to vibrate at higher amplitude x while the larger and is at 0 x to match comfortably with the transducer. Thus a gain in amplification is achieved for amplitude of vibration and for this reason the concentrator, sometimes, is called as a mechanical amplifier or resonator and because of its taper shape and amplification, commonly known as a ‘HORN’.

Nodal Point Clamping

Method of fixing the vibratory system as discussed earlier (consisting of transducer, horn and tool) to form the acoustic head is of very importance. It should be done in such a way that it makes a rigid system without enough loss in the mounting, so it has to be damped at the nodal points. Otherwise, losses are increased and fatigue failure is inhabitable. The dampening at nodal point at the transducer is not permitted because of electrical supplies and hence clamped at other nodal points are maintained as shown in Fig.

 Feed Mechanism

Feed system is to apply the static load between the tool and work piece during machining operation. The precision and sensitivity are of high importance. Feed may either be given to the acoustic had or to the work-piece as per the designer’s choice, but in general, the feed motion is given the acoustic head so as to permit x-y positioning facility to the work piece. In the figure (a) and (b) are for gravity feed devices where counter weights are used to apply the load to the head though a pulley and lever device respectively. In (c), a spring loaded system is shown; for high feed rate conditions either pneumatic or hydraulic system as shown in (d) may be preferred.

Feed Mechanism Feed system is to apply the static load between the tool and work piece during machining operation. The precision and sensitivity are of high importance. Feed may either be given to the acoustic had or to the work-piece as per the designer’s choice, but in general, the feed motion is given the acoustic head so as to permit x-y positioning facility to the work piece. In the figure (a) and (b) are for gravity feed devices where counter weights are used to apply the load to the head though a pulley and lever device respectively. In (c), a spring loaded system is shown; for high feed rate conditions either pneumatic or hydraulic system as shown in (d) may be preferred.



APPLICATIONS

Ultrasonic machining finds its application in processing material that cannot be machined by conventional cutting tool. Generally, it is used for conducting or non-conducting brittle materials. In recent years the use of ultrasonic machining in industries (ultrasonic welding in electronics industries as well) is increasing day by day. Currently, principal fields of application are in the following areas:

• Manufacture of hard alloy wire drawing, punching and blanking dies, also making small complicated dies and punches of steel.

• Machining semi-conducting materials such as germanium and silicon.

• Machining ferrite and other special metallo-ceramic materials used in electrical installations.

• Making instruments and optical parts of glass, quartz, fluoride and barium titanate.

• Making components of porcelain and special ceramics.

• Cutting accurate shallow holes of rectangular or other section in cemented and nitrided steel.

• Cutting of industrial diamonds.

• Grinding glass, quartz and ceramics.

• Cutting holes with curved or spiral center line and cutting threads in glass and mineral or metallo-ceramic.

LIMITATIONS

The USM process does not compete with conventional material removal operations on the basis of stock removal. Non-metal because of poor electrical conductivity that cannot be machined by EDM and ECM can very well be machined by USM.

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