Nontraditional Manufacturing Processes, MF 30604

Principle of Ultrasonic Machining

In Ultrasonic Machining process, material is removed by micro-chipping or erosion with abrasive particles. Between the tool and workpiece, the machining zone is flooded with hard abrasive particles generally in the form of a water based slurry.

Tool is oscillated at a frequency of about 20 kHz with an amplitude of about 10-50m. (Vmax = 2.5m/s, Acceleration = 3x105m/s2) Tool forces the abrasive grits to impact normally and successively on the work surface, thereby machining the work surface.

All hard materials- electrically conductive or not can be effectively machined by USM

A Cutting tool oscillates at high frequency, typically 20-40 kHz, in abrasive slurry.

The shape of the tool corresponds to the shape to be produced in the workpiece.

Reciprocating motion of tool drive the abrasive grains across a small gap against the workpiece .

Tool is gradually fed with a uniform force.

The impact energy of the abrasive is principally responsible for material removal in the form of small wear particles that are carried away by the abrasive slurry.

The tool material, being tough and ductile, wears out at a much slower rate.

The USM process:Longitudinal motion of tool at Ultrasonic frequency:Piezoelectric TransducersMagnetostrictive Transducers

Generation of Ultrasonic waves: By transducer- a device which converts energy from one form to another

Piezoelectric Transducers: Employ the inverse piezoelectric effect(piezoelectric effect- production of electrical voltage when compressed with the application of mechanical force) using natural or synthetic single crystals (such as quartz) or ceramics (such as barium titanate) which have strong piezoelectric behavior.

Inverse piezoelectric effect: Conversion of Electrical voltage oscillation to Mechanical Vibration

Electromechanical conversion efficiency up to 96% Usually no need of water coolingTransducers available with power capabilities up to 900W

Magnetostrictive Transducers : Uses the inverse magnetostrictive effect to convert magnetic energy into ultrasonic energy.

Ultrasonic waves are generated by applying a strong Alternating magnetic field to certain metals, alloys and ferrites : Laminated stacks of Nickel or Nickel alloy sheets

Electromechanical conversion efficiency ~ 20-35% Water Cooling

Magnetostrictive Transducers available with power capabilities up to 2400W

Magnitude of Length change in both types of transducer limited by the strength of material to ~ 25m

Ultrasonic MachineSub-systems: * Transducer- generates ultrasonic vibration

Horn or concentrator / Tool holder- mechanically amplifies the vibration to required amplitude of 15 50 m and holds tool at its tip. * Slurry delivery and return system * Feed mechanism to provide a downward feed to tool during machining

Transducer: Magnetostrictive transducers are most popular and robust amongst all.Driven by Electronic generator Creates Sine waves at 19.5 - 20.5 kHz, range and automatically adjusts the frequency, f to match the resonant frequency of the tool, which depends on the horn shape and materialTransducer converts the electrical pulses into vertical stroke. Vertical stroke is transferred to the horn, which amplifies the stroke amount in 20-50 m range and is then relayed to the tool . Vibration amplitude Diameter of the abrasive grit used.Vibration propagation & AmplificationLength of horn, l = 3/2 = Vs/f

The horn or concentrator can be of different shape like Tapered or conical Stepped ExponentialMachining of tapered or stepped horn much easier as compared to the exponential oneMechanical Amplification ~600% Transducer Vib. Amplitude 3-25m Tool Vibration Amplitude 5-75mMaterials for horn: Monel, Titanium, Stainless steel Good acoustic property Highly resistance to fatigue crackingResonance Frequency, f = Vs/, Sonic Velocity Vs = {(E/)((1-)/(1+)(1-2))}1/2 E= Youngs modulus, = Density, = Poissons ratio Length of horn is so chosen than node ( max. amplitude) is formed at the endFor Steel, E ~2x1011Pa, ~0.3, = 8000kg/m3 Vs = 5x103m/s & ~30cmIncrease in amplitude Function of shape and ratio of end diameters

Abrasive Slurry 1. Aluminum oxide: Best for glass, ceramics & germanium2. Boron carbide: Expensive but best suited for cutting WC, tool steel & precious stones3. Silicon Carbide: Finds maximum applications due to its hardness and life4. Diamond dust: machining diamond, Ruby etc.Abrasive size: 200-2000grit. ( ~ 125- 12.5 micron)Surface roughness: 280 grit- > Ra=0.5micron 800 grit -> Ra = 0.2 micronWater based slurry mostly used.

Grit concentration ~ 10-40%Typical~ 30%

Process Parameters and their Effects: Process parameters which govern the ultrasonic machining process: Amplitude of vibration (ao) : 15 50 m Frequency of vibration (f) : 19 25 kHz Feed force (F) related to tool dimensions Feed pressure (p) F=A.p Abrasive size, diameter, dg = 15 m 50 m Abrasive material Al2O3 ( tends to wear out fast)- SiC - B4C - Boron silicarbide - Diamond Flow strength of work material (w) Flow strength of the tool material (t) Contact area of the tool A Volume concentration of abrasive in water slurry C

dg/opt~ 100mc/opt 30%

Process Mechanism: During one strike,

When the grit size is close to the mean position, the tool hits the grit with its full speed.

Smaller the grit size, the lesser the momentum it receives from the tool.

Therefore, there is an effective speed zone for the tool and, correspondingly there is an effective size range for the grits. WorkToolaV=0V=VmaxV=0a0=a/2USM is mainly used for machining brittle materials {which are poor conductors of electricity and cannot be processed by Electrochemical and Electro-discharge machining} In brittle material machining is due to crack initiation, propagation and brittle fracture.

In the machining process, Tool impacts on the largest grits,

Grits forced into tool and workpiece.

Cracks initiate just below contact site,

As indentation progresses cracks propagate due to increase in stress WorkToolBrittle fractureUltimately lead to brittle fracture of the work material under each individual interaction site

As the tool moves further down, more grits with smaller sizes come in contact with the tool.

Eventually, the tool comes to the end of its strike, the number of grits under impact force from both tool and workpiece becomes maximum.

Tool material should be such that indentation by the abrasive grits does not lead to brittle failure. Therefore, tools are made of tough, strong and ductile materials like steel, stainless steel and other ductile metallic alloys.

Process Model:

Identical grits of average grit diameter = dgWith local spherical bulges of diameter db = .d2gIn brittle fracture the volume of material removed per indentationb = 2/3.x3 = 2/3 .(db.w)3/2 as x2 = db.wMRR = b .n.f = 2/3 .(db.w)3/2.n.f -----(1)

n = Ave. no. of grits & f = indentation frequency

Given: Volumetric Concentration of grits in slurry = CArea of Tool = ASpacing between tool & work when they are pressing against grits dgVolume of Grits = A.dg.C This will be equal to n. (/6)dg3

n = 6AC/.dg2 ------ (2)Tool & Work pressing each other against grits will deform inversely proportional to their strength t/W = w/t = -----(3)And, total depth of indentation. = t + W --------- (4)During machining the impulse of force on the tool & work would be balanced.

= t +wDuring tool oscillation, it engages & presses grits only during a time , a part of one-fourth of the cycle (T/4) when it is moving from its mid-point towards workpiece/a0 / (T/4) = T(t +w)/4.a0 -------(5)Total impulse per second by Tool on workpiece, It = n.f.Fmax., ---(6) where Fmax is the maximum indentation force per abrasive.Now, the tool is fed with an average force F.Thus, F = n.f.Fmax. = n.f.Fmax T(t +w)/4.a0 -----(7)tw

The strength of work material = w Fmax = .x2. w ---(8)F = .x2. w.n.f. T(t +w)/4.a0 ---- (9)Substituting values of db, n, & t /w = F = (3AC/ 4a0 ). ,w.w2(1+ ){as f.T = 1, db =d2g } w = {(4a0.F)/ (3A.C. ,w.(1+ ))}1/2 --------------(10)

MRR = bn.f = 2/3 .(db.w)3/2 .n.f= 4A.C.dg.3/2.f. { (4a0F)/(3.A.C.w(1+)}3/4

MRR dg. f (C1/4.A1/4.F3/4.a03/4. 3/4)/ {w3/4(1+)3/4} For tool pressure p, F =A.p MRR dg. f (C1/4.A.p3/4.a03/4. 3/4)/ {w3/4(1+)3/4}

Attributes & Applications * Normal hole tolerances are0.007 mm and a surface finish of 0.02 to 0.7 micro meters.* Specific material removal rate on brittle materials is 0.018 mm 3/Joule. * Penetration rates of 5 mm/min

Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics, carbides etc. Used for machining round, square, irregular shaped holes and surface impressions. Machining, wire drawing, punching or small blanking dies.

Limitations Low MRR High tool wear Low hole depth