In this article we will discuss about:- 1. Meaning of Ultrasonic Machining (USM) 2. Characteristics of Ultrasonic Machining (USM) Process 3. Rotary Ultrasonic Machining 4. Applications of Ultrasonic Machining (USM) 5. Advantages of Ultrasonic Machining (USM) 6. Limitations of Ultrasonic Machining (USM).
Meaning of Ultrasonic Machining (USM):
Ultrasonic waves are sound waves which propagate at frequency of 18 to 30 kc/sec and are thus beyond the human ear to respond. No heat is generated in case of ultrasonic machining and it provides better working performance. This process is best suited for machining circular and non-circular holes in hard to machine and brittle metals whether conducting or non-conducting. Ultrasonic waves can be generated by piezoelectric or magnetostrictive effects.
Piezoelectric transducers utilise crystals like quartz which undergo dimensional changes when subjected to electrostatic fields; the change being proportional to the applied voltage. Obviously application of alternating voltage results in vibration, whose amplitude depends on the resonant frequency of the crystal.
High amplitude of vibration can be obtained by matching the length of the crystal to the frequency of the generator, or making length of the crystal equal to half the wavelength of sound in the material to produce resonant conditions.
Thus a crystal of about 114 mm length will produce desired conditions. Since large crystals are difficult to be produced and used, sandwich type transducers are commonly used. In place of quartz, polycrystalline ceramics like barium titanate with length of 75-100 mm are usually used.
Fig. 10.46 shows a section through sandwich type piezoelectric transducer. In order to obtain high amplitude of vibration at the radiating face, low density material is put in front and high density material at the back as shown in Fig. 10.46 because as per conservation of momentum, the velocity (amplitude of vibration) in the high and low density materials are inversely proportional to the density.
Magnetostrictive transducers work on the principle that if a piece of ferro-magnetic material (like nickel) is magnetised, then a change in dimension occurs. Eddy current losses can be reduced by using ferro-magnetic material in the form of insulated laminations assembled into a pack.
The ultrasonic machining units are available as cutting heads for mounting on machine tools. The power rating of machine is about 0.2 to 2.5 kW.
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In ultrasonic machining processes, a formed tool made of ductile and tough material, having the shape of the cavity to be machined is made to vibrate against the workpiece surface, and between the two, continuous flow of slurry of abrasive particles is maintained. Vibration of the tool tip accelerates the abrasive particles at very high rates and imparts the force necessary for the cutting action.
The gap between the tool and workpiece is of the order of 0.02 to 0.1 mm. The abrasive particles at such a high frequency erode the workpiece and produce the desired shape on it. Vibration of the tool tip is obtained through a transducer which works as an ultrasonic generator to convert electrical energy into mechanical energy.
The main difficulty is that the amplitude of the vibration of transducer is of order of 0.001 mm or even less and in order to obtain the desired results it is essential that this be amplified further to 0.025 to 0.075 mm. The higher is amplitude of vibrations, the faster will be cutting operation. The medium between the tool and the workpiece consists of abrasive slurry containing 280 grit boron carbide and water.
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The material is removed from the workpiece in the form of small grains by shear deformation, brittle fracture of work material by impact, cavitation and chemical reaction. The tool material is also subjected to wear, but it can be minimised by proper work-tool material combination. For producing vibrations the magnetostriction transducer principle is commonly used.
The transducer has solenoid type winding of wire over a stack of nickel laminations (which has rapid dimensional change when placed in magnetic fields) and is fed with an A.C. supply with frequencies upto 25,000 c/s. A cone shaped tool holder is used which helps in magnifying the tool tip vibrations. The tool tip is made of brass or steel and bears exact shape as to be cut into the workpiece.
Other tool materials in common use are medium- tough malleable steel in cold rolled state or other tough material from which tool can be manufactured rapidly and economically and the tool wear is small and cutting rates are high. The abrasive particles are carried to machining zone by a slurry which also takes away the worn particles and provides cooling of the workpiece and the tool. The abrasive slurry circulated by pump is cooled by a refrigerated cooling system.
It should be able to remove the generated heat to prevent it from boiling in the gap and causing the undesirable cavitation effect caused by high temperature. For fine surface finish abrasive particles of finer grit are chosen. The proper selection of abrasive particles is dependent on the type of material to be machined, hardness of the material, metal removal rate desired and the surface finish required.
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Boron carbide, the hardest and most expensive abrasive particle is used for machining tungsten carbide, die steel and gems. Silicon carbide is difficult to keep in suspension. Al2O3 is the softest abrasive and is used for glass and ceramics. The abrasive is carried in a slurry of water with 30—60% by volume of the abrasives.
Feeding of the tool is provided by applying a working force (of about 1.8 kg) between the tool and the workpiece. The feeding mechanism may be gravity type or spring loaded type. Very low feed rates (maximum 0.1 mm/sec) are employed.
If d is the diameter of abrasive particle, it makes an indentation on workpiece of diameter D and penetration of depth ‘h’. Since h is extremely small, D = 2 √dh.
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Volume of material dislodged per impact (V) is proportional to D3 or V α (dh)3/2 Nf,
where N = number of particles making impact/cycle; f = frequency.
Usually the abrasive grain causes slight indentation in tool material also.
Let ht = indentation caused in tool and
hw = indentation in the work
The force of tool varies as it moves down.
Though theoretically MRR is proportional to d, but actually abrasive grains get crushed under heavy load and MRR falls after a particular feeding force. Fig. 10.48 shows these relationships.
Material removal rate and surface finish are greatly influenced by grit or grain size of the abrasive. Maximum rate in machining is attained when the grain size is comparable to the tool amplitude. Usually grit sizes of 200— 400 are used for roughing and 800—1000 for finishing. For effective machining the abrasives should be replaced periodically as dull abrasives stop cutting action.
Material removal rate is function of inverse of cutting tool area, tool vibrations, type of abrasive, its size and concentration of slurry. Tool ratio (volume of material removed from workpiece/volume removed by wear from tool) for stainless steel tools is 100 : 1 for glass and 40 : 1 for satellite and these values for brass tool are 40 : 1 and 13 : 1 respectively.
Accuracy of this process is ± 0.007 mm and surface finish upto 0.02 to 0.8 micron R.M.S. value can be achieved. Metal removal rate is very slow (of the order of 3 mm3/ sec.). Generators of 100 W output are suitable for small work. However, generators as large as 2000 Watts capable of driving tools with area of several cm2 have been developed.
Figs. 10.49 to 10.55 show how the material removal rate in ultrasonic machining process varies with other process parameters.
The surface finish in USM process is direct function of the size of the abrasive grains. Fig. 10.56 shows this relationship for glass and WC. It will be seen that WC being very hard, the size of the fragments dislodged through a brittle fracture is nearly independent of size of impacting grains.
In Fig. 10.57 is shown the process of ultrasonic machining in which the vibrations of transducer are transmitted to the tool through a mechanical focusing device known as velocity transformer which is rigidly attached to the radiating face of the transducer. These are made of materials like brass having high fatigue resistance and low energy loss.
These are usually exponentially tapered, the ratio of the increase in amplitude being inversely proportional to the ratio of the areas of the two ends. The tool performs hampering action on the abrasive particles which are continuously circulated between the tool and the workpiece.
Ultrasonic machines are available for various applications like machining profile holes, trepanning, slicing, drilling, engraving, etc. Machines of different grades of sensitivity of feed mechanisms are required for machining different kinds of materials. These can be classified by their size and capacity (i.e. table size, tool travel and feed force, type of generator, slurry circulation method, type of accounting head etc.
Tool holder must have adequate fatigue strength since it has to transfer high vibrations. The shape of the tool holder is cylindrical or conical, or a modified cone with the centre of mass of the tool on the centre line of the tool holder. In order to reduce the fatigue failures, it should be free from nicks, scratches and tool marks and polished smooth. Tool material should be tough and ductile.
Low carbon steel and stainless steels give good performance. As the ratio of hardness of workpiece and tool increases, metal removal rate decreases. Tools are usually 25 mm long; its size is equal to the hole size minus twice the size of the abrasives. Mass of tool should be minimum possible so that it does not absorb the ultrasonic energy.
The metal removal rate for this process according to Prof. Shaw is given by the relation:
Characteristics of Ultrasonic Machining (USM) Process:
i. Material removal mechanism – Complex mechanism involving both fracture and plastic deformation by impact of grains due to vibrating tool.
ii. Abrasive – B4C, SiC, Al2O3 (200—400 grit size for roughing and 800 — 1000 grit size for finishing).
iii. Medium – Slurry of water with 30—60% be volume of the abrasives
iv. Vibration frequency and amplitude – 15 to 30 kHz and 25 to 100 µm
v.Toll material – Soft steel
vi. Material removal rate / Tool wear rate – 1.5 : 1 for WC, 100 : 1 for glass, 50 : 1 for quartz, 1 : 1 for hardened tool steel, and 75 : 1 for ceramic.
vii. Materials machined – Economical for materials having hardness > 50 HRC like stainless steel, germanium, glass, ceramic, quartz.
viii. Shapes produced – Micro holes of upto 0.1 mm diameters round and irregular holes, coining.
ix. Overcut – Twice the size of abrasive particles.
x. Surface finish – 0.2—0.8 µm with finer sizes of abrasives
xi. Limitations – Low metal removal rate, high rate of tool wear, hole depth to diameter ratio of 10 : 1.
Rotary Ultrasonic Machining:
It is observed that if a rotating diamond tool is vibrated at an ultrasonic frequency when machining hard brittle materials, a significant improvement in cutting rate and tool life results.
Rotary ultrasonic machine tool comprises:
(i) A power supply unit to convert electrical energy to ultrasound energy,
(ii) Rotary head assembly consisting of a motor driven, bearing mounted spindle or sonic converter, and
(iii) A diamond cutting tool.
Rotary head assembly contains two lead ziroconate titanate discs which convert electrical energy to mechanical vibration. These discs processes piezo electric properties and physically expand and contract in accordance with the polarity of the voltage applied.
The resulting vibration is amplified by a titanium alloy horn or impedance transformer, accurately machined to have a longitudinal natural frequency of 20 kHz and to deliver the maximum amplitude of vibration at the tip.
Diamond core drills and mandrels, both plated and impregnated types are used as cutting tools. Diamond tool is attached to the horn tip tightly by brazing or epoxying to ensure good acoustic coupling. It may be remembered that tool should be light, as a heavy one will throw the frequency of the horn assembly beyond the resonant frequency of the transducer and power supply, and the tool will not vibrate.
About 35 gm weight of tool is found to be good practice. Weight is kept down by the use of titanium threaded adaptors and epoxy for attaching the tools to the adaptor.
Rotating diamond core drill is used for drilling. Ultrasonic results in accelerated cutting and effect is greater with lower tool pressure. It is thus possible to drill holes close to the edge or corner without breakdown of the edge of the piece. Light pressure on the tool enables the cutting action to be maintained to a greater depth before the push-out occurs.
Ultrasonically vibrating rotating diamond tool is also found to significantly improve the degree of accuracy of a perfectly vertical hole. It is possible to drill 1 mm hole drilled into the edge of a piece of glass and holes may be upto 60 mm deep and separated by 0.5 mm. Ultrasonic vibration of the tool reduces friction both at the cutting face and in the bore.
Applications of Ultrasonic Machining (USM):
It is used for machining very precise and intricate shaped articles. Holes as small as 0.1 mm can be drilled. Hole depth is limited by the tool wear, slenderness ratio of tool, and ease of supplying abrasive slurry to the working gap.
It is mainly used for:
(i) Drilling,
(ii) Grinding,
(iii) Profiling,
(iv) Coining,
(v) Threading,
(vi) Welding operations on all materials which can be treated suitably by abrasives.
Hard materials like stainless steel, glass, ceramic, carbide, quartz and semi-conductors are machined by this process.
It has been efficiently applied to machine glass, ceramics, precision mineral stones, tungsten. It has also been applied for piercing of dies and for parting off operations. This process can be adopted in conjunction with other new technological processes to achieve better efficiency.
Advantages of Ultrasonic Machining (USM):
(a) This process can be used for drilling of circular or non-circular holes in very hard materials like stones, carbides, ceramics and exceptionally brittle materials.
(b) The metal to be machined may be non-conducting of the electricity, such as glass, ceramics and semi-precious stones.
Limitations of Ultrasonic Machining (USM):
Under ideal conditions, penetration rates of 5 mm3/min can be obtained. Power units are usually 530—1000 watt output. Specific material removal rate on brittle materials is 0.018 mm3/Joule. Normal hole tolerances are 25 µm and surface finish is 0.5 to 0.7 µm.
Example 1:
What is the amplitude of vibration and frequency of tool, the gap between tool and workpiece, and time of contact between abrasive particle and workpiece in ultrasonic machining? What is the order of tolerance of size and finish possible?
Solution:
Amplitude of vibration—0.012 to 0.08 mm
Vibration frequency—20 kHz
Gap between tool and workpiece—0.1 mm
Time of contact between abrasive particle and surface of workpiece—10 to 100 µs
Tolerance—0.01 mm
Finish Ra—0.2 to 1.6 µm
Example 2:
If a spherical particle of diameter d causes a brittle fracture of diameter D at a depth of h in ultrasonic machining, what is the volume of material removed by this grain. If n particles impact workpiece every cycle at frequency ‘f then what is the material removal rate. What is the usual order of material removal rate and the size of hole it can drill?
Solution:
Refer Fig. 10.58.
If efficiency of operation is ƞ, then material removal rate = ƞ Vnf
Usually MRR is of the order of 0.5 to 1.0 cm3/min. Size of hole is around 0.3 mm.
Example 3:
For what type of materials, ultrasonic machining (USM) is most suited?
Solution:
USM is suited for hard materials (hardened steels), and brittle materials (ceramics, carbides, glass and precious stones).
Example 4:
A hole is to be drilled in 20 mm thick tungsten carbide sheet by ultrasonic method. The slurry is made of 1 part of 320 grit (15 micron radius) boron carbide mixed with 1 ¼ parts of water. The static stress is 1.4 kg/cm2 and the amplitude of tool oscillation is 0.025 mm. The machine operates at 25,000 cycles / sec. The compression fracture strength of WC is 225 kg/mm2. Calculate the time required to perform drilling. Assume that only one pulse out of 10 pulses are effective.
Solution:
The metal removal rate for ultrasonic machining is given by:
