Ultrasonic Machining

Content Introduction of USM Schematic diagram Principle & Working Mechanism Mathematic Design for MMR Process parameter & effect USM system & subsystem Application Advantages & disadvantages Summary

Introduction Ultrasonic machining is a non-traditional mechanical means of uniform stock material removal process It is applicable to both conductive and nonconductive materials. Particularly suited for very hard and/or brittle materials such as graphite, glass, carbide, and ceramics.

Various work samples machined by USM                                                                                      1- The first picture on the left is a plastic sample that has inner grooves that are machined using USM. 2- The Second picture (in the middle is a plastic sample that has complex details on the surface 3- The third picture is a coin with the grooving done by USM

Systematic diagram Force feed motion Transducer Acoustichead/toolholder Workpiece Slurry to machining zone Slurry pump Return slurry Slurry tank

Principal and working Force, F The process is performed by a cutting tool, which oscillates at high frequency, typically 20-40 kHz, in abrasive slurry. Horn The tool is gradually fed with a uniform force. The high-speed reciprocations of the tool drive the abrasive grains across a small gap against the workpiece . Slurry of abrasive and water The impact of the abrasive is the energy 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 shape of the tool corresponds to the shape to be produced in the workpiece.

Mechanism The basic components to the cutting action are believed to be The direct hammering of the abrasive into the work by the tool (major factor) 1 2 The impact of the abrasive on the work 3 Cavitation induced erosion 4 Chemical erosion caused by slurry

Mechanism Material removal Occurs when the abrasive particles, suspended in the slurry between the tool and workpiece, are struck by the downstroke of the vibration tool. The impact propels the particles across the cutting gap, hammering them into the surface of both tool and workpiece. Collapse of the cavitation bubbles in the abrasive suspension results in very high local pressures. Under the action of the associated shock waves on the abrasive particles, microcracks are generated at the interface of the workpiece. The effects of successive shock waves lead to chipping of particles from the workpiece.

Subsystems of USM System C D A Power supply Transducer Toolholder Abrasive Tools

The power supply is a sine-wave generator The user can control over both the frequency and power of the generated signal. It converts low-frequency (50/60 Hz) power to high-frequency (10-15 kHz) power Supply to the transducer for conversion into mechanical motion.

Transducer - Magnetostriction B Two types of transducers are used in USM to convert the supplied energy to mechanical motion. They are based on two different principles of operation - Magnetostriction - Piezoelectricity - Magnetostriction When the material is placed in a sufficiently strong magnetic field, the magnetic moments of the domains rotate into the direction of the applied magnetic field and become parallel to it. During this process the material expands or contracts, until all the domains have become parallel to one another. very common magnetostrictive composite is the amorphous alloy Fe81Si3.5B13.5C2 with its trade name Metglas 2605SC Electostriction: when the material is placed in strong electric field the dipole of the molecules rotate into the direction of applied field. During this process the material expand or contract .

- Piezoelectric transducers Such as quartz or lead zirconate titanate (PZT), generate a small electric current when compressed. Conversely, when an electric current is applied, the material increases minutely in size. When the current is removed, the material instantly returns to its original shape.

Toolholder/ Acoustic head Its function is to increase the tool vibration amplitude and to match the vibrator to the acoustic load. It must be constructed of a material with good acoustic properties and be highly resistant to fatigue cracking. Monel and titanium have good acoustic properties and are often used together with stainless steel, which is cheaper. Monel is a trademark of Special Metals Corporation for a series of nickel alloys, primarily composed of nickel (up to 67%) and copper, with some iron and other trace elements. Monel was created by David H. Browne, chief metallurgist for International Nickel Co

Different Horns used in USM The acoustic head can be of different shape like • Tapered or conical • Exponential • Stepped The acoustic head is the most complicated part of the machine. It must provide a static force, as well as the high frequency vibration   Machining of tapered or stepped horn is much easier as compared to the exponential one. Exponential tapered stepped Different Horns used in USM

- The geometry of the tool Tools D Tools should be constructed from relatively ductile materials. The harder the tool material, the faster its wear rate will be. It is important to realize that finishing or polishing operations on the tools are sometimes necessary because their surface finish will be reproduced in the workpiece. - The geometry of the tool The geometry of the tool generally corresponds to the geometry of the cut to be made, Because of the overcut, tools are slightly smaller than the desired hole or cavity Tool and toolholder are often attached by silver brazing.

Abrasives E Abrasive Slurry - common types of abrasive - boron carbide (B4C) good in general, but expensive - silicon carbide (SiC) glass, germanium, ceramics - corundum (Al2O3) - diamond (used for rubies , etc) - boron silicon-carbide (10% more abrasive than B4C) liquid - water most common - benzene - glycerol - oils - high viscosity decreases MMR proper selection of abrasive particles is dependent on the type of material being machined, hardness of the material, the desired removal rate of material, and the surface finish needed. The larger grit is used for a rougher cut on the piece, and the smaller grit is used to create a finer finished surface.

Applications It is mainly used for (1) drilling (2) grinding, (3) Profiling (4) coining (5) piercing of dies (6) welding operations on all materials which can be treated suitably by abrasives. (7) Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics, carbides etc. (8) Used for machining round, square, irregular shaped holes and surface impressions.

Advantages of USM Machining any materials regardless of their conductivity USM apply to machining semi-conductor such as silicon, germanium etc. USM is suitable to precise machining brittle material. USM does not produce electric, thermal, chemical abnormal surface. Can drill circular or non-circular holes in very hard materials Less stress because of its non-thermal characteristics

Disadvantages of USM USM has low material removal rate. (3-15mm3/min) Tool wears fast in USM. Machining area and depth is restraint in USM.

Process Parameters and their Effects The process parameters which govern the ultrasonic machining process have been identified and the same are listed below along with material parameters Amplitude of vibration (a) - 15- 50 um Frequency of vibration (f)-19-25kHz Flow Strength of the tool and work piece Feed force (F) Abrasive material – Al2O3 - SiC - B4C - Diamond Feed pressure (p) Abrasive size-15-150um Contact area of the tool - A Volume concentration of abrasive in slurry - C

Mathematical design for MRR Hemispherical material removed due to brittle Bulges on abrasive grits  material removal per abrasive grit is given as indentation depth in the work material is characterised by If at any moment of time, there are an average ‘n’ of grits and the tool is vibrating at a frequency ‘f’ then material removal rate can be expressed as (1)

Again, if the flow strength of work material is taken as the impulse of force on the tool and work would be balanced. Thus total impulse on the tool can be expressed as or as the tool and workpiece would be pressing against each other, contact being established via the abrasive grit, both of them would deform or wear out. Out of this quarter cycle, some part is used to engage the tool with abrasive particle a Thus the time of indentation t can be roughly estimated as where Fmax is the maximum indentation force per abrasive. Now in the USM, the tool is fed with an average force F Again, if the flow strength of work material is taken as (2)

3 If n is the number of grits then the total volume of n grits is Now, Thus the concentration of abrasive grits in the slurry is related as follows: 3

Is a ratio of w and t Now it is expected that indentation would be inversely proportional to the flow strength then,

the effect of parameters on MRR.

Summary of USM Mechanics of material removal - brittle fracture caused by impact of abrasive grains due to vibrating at high frequency Medium - slurry Abrasives: B4C; SiC; Al2O3; diamond; 100-800 grit size Vibration freq. 15-30 KHz, amplitude 25-100 micro m Tool material soft steel , ductile Critical parameters - frequency, amplitude, tool material, grit size, abrasive material, feed force, slurry concentration, slurry viscosity Material application - metals and alloys (particularly hard and brittle), semiconductors, nonmetals, e.g., glass and ceramics Shape application - round and irregular holes, impressions Limitations - very low mrr, tool wear, depth of holes, and cavities small.