UNIT-I ULTRASONIC MACHINING

UNIT-I ULTRASONIC MACHINING  USM is a mechanical type Unconventional Machining Process.  Ultrasonic means a vibratory wave having frequency larger t...
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UNIT-I ULTRASONIC MACHINING  USM is a mechanical type Unconventional Machining Process.  Ultrasonic means a vibratory wave having frequency larger than the upper frequency limit of human ear i.e. greater than 16 KHz.  USM machines are available in the range of 40W to 2.4KW.  USM is a good process for machine very thin and fragile components.

# Schematic Representation of USM

WORKING PROCESS OF USM: AC Power Supply



Vibrating Abrasives



US Waves Generator



US

  Transducer

W/P





Tool Connector 

Throwing and/or Hammering  Action Errodes W/P

Tool

Finished Component

High power sine wave generator converts low frequency (60HZ) electrical power to high frequency (Greater than 16KHZ) electrical power. This high frequency electrical signal is transmitted to the ultrasonic transducer. This Ultrasonic transducer converts high frequency electrical signal into high frequency linear mechanical vibration. These high frequency vibrations are transmitted to the tool via tool holder. The Tool holder holds and connects the tool to the transducer. It actually transmits the energy to the tool and in some cases, amplifies the amplitude of vibration. The Tool vibrates in its downward stroke, it strikes the abrasive particles. This impact from the tool propels the abrasive particles across the gap between the tool and the work piece. These vibrating Abrasives attain K.E. and strike the work piece surface with a force much higher than their own weight. That is each down stroke of the tool accelerates numerous abrasive particles resulting in the formation of thousands of tiny chips per second. So USM gives low Material removal rate but it is capable to machine intricate cavities in single pass in fragile or /and hard materials.

ELEMENTS OF USM PROCESS:  High Power Sine Wave generator (Ultrasonic Wave Generator)  Acoustic Head (Ultrasonic Transducer)  Tool Connector (Tool holder) (Horn)  Tool  Abrasive Slurry (Vibrating Abrasives)  Work Material

Ultrasonic Wave Generator: It converts low frequency (60Hz) electrical power to high frequency (Greater than 16 KHz) electrical power. The main requirements of a generator are reliability, efficiency, simplicity in design and low cost. USM is usually employed with Vacuum tube generators.

Ultrasonic Transducer: It converts high frequency electrical signal into high frequency linear mechanical vibration. In USM either of the two types of transducers are used, i.e. piezoelectric transducers or magnetostrictive transducers. Piezoelectric transducer:

When an electric current is passed through the piezoelectric

crystal (quartz) it expands, when the current is removed the crystal attains its original size. This effect is known as piezoelectric effect. These are available up to 900W power supply & 95% efficiency. Magneto-strictive transducer: When an object made of ferromagnetic materials (Nickel & Nickel alloy sheets) is placed in the continuously changing magnetic field, a change in its length takes place. These are available up to 2.4KW power supply & 20% - 30% efficiency. For this transducer cooling is essential to remove the waste heat. The coefficient of magnetostrictive elongation is equal to the ratio of change in length to the length of the magnetostrictive coil.

Tool Holder: The Tool holder holds and connects the tool to the transducer. It actually transmits the energy to the tool and in some cases, amplifies the amplitude of vibration. It is a velocity transformer. These are usually made of high resistance to fatigue cracking materials like Monel, Titanium & Stainless Steel.

Tool: Tool shape is made converse to the desired cavity. Each down stroke of the tool accelerates numerous abrasive particles resulting in the formation of thousands of tiny chips per second. These are usually made of relatively ductile materials (High wear resistance) like Brass, Stainless Steel & Mild Steel.

Abrasive Slurry: Commonly used abrasives are Al2O3, Sic & B4C (Boron Carbide). Vibrating Abrasives attain K.E. and strike the work piece surface with a force much higher than their own weight. That is each down stroke of the tool accelerates numerous abrasive particles resulting in the formation of thousands of tiny chips per second.

Work Material: USM usually is employed to machine hard and/or brittle materials but there is no limitation to the range of materials that can be machined, except that they should not dissolve in the slurry media.

MECHANICS OF CUTTING: The mechanics of material removal from a surface by forcing sharp grains into it have been studied by many researchers they are: 1. Theory of Miller 2. Theory of Shaw 3. Theory of Kazantsev

1. THEORY OF MILLER: Assumptions: Cutting involves plastic flow of materials. Abrasive particles are cubes of side d. All particles under the tool cut effectively.

Volume Rate of Metal Removal (V) = f {PD, TN, WHR, VC, CR, T}

PD = Volume of plastic deformation of grains TN = Total number of abrasive particles striking per unit time WHR = Work hardening per unit of plastic deformation VC = Volume of work material chipped by each grain in each blow CR = Frequency of vibration of tool tip T = Rate of flow of abrasive particles under tool tip = V = K (F d a p f C) / (q G b r ρ V1 (C+1)) F = Average force on all particles, d = Size of abrasive particle, a = Amplitude of vibration, p = Atmospheric pressure, f = Frequency of vibration, C = Ratio of mass of abrasive to the mass of carrier, q = Work hardening capacity, G = Shear modulus, b = Burger’s vector, r = Radius of the tool, ρ = Density of abrasive, V1 = Volume of the slurry. Drawbacks: Miller considered the use of plastic materials only, whereas most materials worked by this process are brittle in nature, and so the rate cannot be dependent on plastic deformation. The estimation of the number of particles under the tool is also based on many hypothetical considerations.

2. THEORY OF SHAW: Shaw considered the material removal of USM to take place in four possible actions: 1. The abrasive grains are thrown onto the work surface causes material removal. 2. The abrasive grains are hammered onto the work surface causes material removal. 3. Cavitation occurs under the tool causes material removal. 4. Chemical corrosion associated with abrasive carrying media causes material removal. Actions 3 & 4 are not of much importance, because the machining rates are observed to be extremely low. Abrasive particles are assumed to be spherical in shape having a uniform diameter of d. It is assumed that a particle after hitting the work surface generates a crater of height h and radius r.

The volume of material (Vg) removed due to fracture per grit per cycle =

From the geometry of the figure:

From above equations,

Number of impacts on the work piece by the grits in each cycle =

Volume of the material removal per cycle =

Then volume (Vs) of material removed per second will be equal to the frequency (f) times the amount of material removed per cycle (Vc)

For calculating the depth of penetration (h) of an abrasive particle, Shaw proposed two models.

Model 1 (Grain Throwing Model): If the size of the particle is small enough as compared to the gap between the tool and work surface, the particle will be thrown by the tool, to hit the work surface (throwing model). It is assumed that an abrasive particle on being hit by the vibrating tool accelerates towards the work surface, it can be considered as thrown by the tool onto the work piece surface. Assuming sinusoidal vibration, the displacement (Y) of the tool is

Where,’t’ is time period & a/2 is amplitude of oscillation and ρa is density of the abrasive particles.

Therefore, Substitute F in hth,

Model 2 (Grain Hammering Model):

Large size abrasive grains on the other hand are likely to get entrapped between the work surface and the vibrating tool. Such particles would eventually be hammered down on to the work surface by the tool-hammering action. This would lead to partial penetration of the entrapped grain in the tool (htl) as well as the work piece (hw).

From the computational results obtained, it is observed that Vh >> Vth.

EFFECT OF PROCESS PARAMETERS: Performance (MRR, accuracy and surface finish) of the USM process depends upon the following parameters: Abrasives: (material size, shape, and concentration), •

Abrasive size – 15 μm – 150 μm



Abrasive material – Al O , SiC, B C, Boronsilicarbide, Diamond



Volume concentration of abrasive in water slurry – C

2

3

4

Tool and tool holder: (material, frequency of vibration and amplitude of vibration), •

Amplitude of vibration (a ) – 15 – 50 μm



Frequency of vibration (f) – 19 – 25 kHz



Feed force (F) – related to tool dimensions

o

Work piece: (hardness). •

Flow strength of work material



Flow strength of the tool material

ECONOMIC CONSIDERATIONS:  The USM process has the advantage of machining hard and brittle materials to complex shapes with good accuracy and reasonable surface finish. Considerable economy results from the USM of hard alloy press tools, dies and wire drawing equipment.  The power consumption of USM is 0.1 W-h / mm3 for glass and about 5 W-h / mm3 for hard alloys. APPLICATIONS  Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics, carbides.  Used for processing of silicon nitride turbine blades.  Used for machining round, square, irregular shaped holes and surface impressions.  Machining, wire drawing, punching or small blanking dies.  USM also used in conjunction with ECM, EDM. LIMITATIONS:  Low MRR (The major limitation of the process is its comparatively low metal cutting rates).  Low Depth of hole (The depth of the cylindrical hole is presently limited to 2.5 times the diameter of the tool).  Rather high tool wear (Tool wear increases the angle of the hole, while sharp corners become rounded. This implies that tool replacement is essential for producing accurate blind holes). RECENT DEVELOPMENTS:  Mullard Research Laboratories have developed a process that combines electrochemical reaction with ultrasonic abrasion. A 60W ultrasonic drill and abrasive suspended in an alkaline electrolyte can be machined nine times faster than by ultrasonic alone.

 Engis Limited of England has developed Diesonic Die Ripper in which the diamond-plated tool oscillates at ultrasonic speed as well as rotates at high speed in a liquid to rapidly remove material from tungsten carbide dies. The use of the diamond-plated tool eliminates the need of abrasive slurry and frequent re-grinding of steel tools. The oscillating cum rotational system is claimed to increase the material removal rate several times.

PROBLEM: Find the approximate time required to drill a hole of 6 mm diameter in a tungsten carbide plate 2

9

2

(fracture hardness = 6900 N/mm = 6.9 × 10 N/m ) of thickness equal to one and half times the hole diameter. The mean abrasive grain diameter is 0.015 mm. The feed force is equal to 3.5 N. The amplitude of tool oscillation is 25 μm and the frequency is equal to 25 kHz. The tool material used is 2

copper having fracture hardness equal to 1500 N/mm . The slurry contains one part abrasive to one 2

part of water. Take the values of different constants as K1 = 0.3, K2 = 1.8 mm , K3 = 0.6, K1 = 1.8 2

3

mm , C = 1, and abrasive density = 3.8 g/cm . Also calculate the ratio of volume removed from the work piece by hammering mechanism to the volume removed by throwing mechanism.

SOLUTION: Given Data: 23

23

Hole diam. = 6 × 10 m, plate thickness = 1.5 × hole diam. = 9× 10 m, mean abrasive grain size (d) 25

26

= 1.5 × 10 m, feed force (Favg) = 3.5 N, amplitude of tool oscillation (a/2) = 25 × 10 m, frequency 9

2

of oscillation (f) = 25000 cps, fracture hardness of work piece material σw (=H ) = 6.9 × 10 N/m , w

9

2

3

fracture hardness of tool material (H ) = 1.5 × 10 N/ m , abrasive grain density (ρ ) = 3.8 × 10 tl

3

a

2

26

2

1

26

2

kg/m j = H / H = 4.6, K1 = 0.3, K2 = 1.8 mm = 1.8 × 10 m , K3 = 0.6, K = 1.8 × 10 m , C = 1. w

tl

Step 1: Calculate the value of “h” which is different for throwing model (h ) and for hammering model (h ). th

w

Step 2: After computing the values of h and h , calculate V and V . Find total volume of material removed th

w

th

h

per unit time (V ) by adding V and V . s

th

h

Step 3: Calculate the total amount of material to be removed to make a hole. Divide it by V to find the total s

time required to make the hole.

Step 4: Find the ratio of V / V th

h.

Note: Therefore the material removed in hammering is much more than throwing (approximately 43 times), hence, for approximate calculations, Vth can be ignored compared to Vh.