About machining

1. MODERN MACHINING PROCESS
CHAPTER NO. 02
MECHANICAL PROCESSES

2. ULTRASONIC MACHINING
 The term ultrasonic is used to describe vibrational waves having a
frequency above the hearing range of normal human ear i.e. beyond 18kHz.
 Over the years, ultrasonics have found wide applications in industries.
 Predominant areas of its application being three.
 Area of Application:
 Non destructive testing of metals and non metals,
 Measurement and control, and
 Material processessing: metal cutting, ultra-sonic cleaning etc.
 There are two types of waves, namely shear waves and longitudinal waves.
 Longitudinal waves are mostly used in ultrasonic applications since they are
easily generated.
 They can be propagated in solids, liquids and gases and can travel at a high
velocity.
 The device for converting any type of energy into ultrasonic waves is called
ultrasonic transducer.

3. CONTI…..
 Most industrial applications employ transducers energized by electrical
power at the required frequency.
 The electrical energy is converted into mechanical vibrations.
 In addition to machining or cutting, ultra-sonics find many other engineering
applications. Some of these are:
 Measurement of velocity of moving fluids.
 Measurement of hardness and grain size determination of metals.
 Non destructive residual stress determination.
 Flaw detection and leak detection.
 The use of ultrasonics in the medical field for the diagnosis and treatment
of certain diseases has also been reported.

4. PRINCIPLE OF USM
 In USM (also known as ultra-sonic grinding or impact grinding), material is
removed due to action of abrasive grains which are hammered into the work surface
by a tool, oscillating at high frequency normal to the work surface.
 The tool oscillation frequency for this purpose is in the range of 20kHz.
 Ultra-sonic are generated by feeding high frequency electrical current to a
transducer which converts it to high frequency mechanical vibrations.
 The tool is pressed against the work piece with a load of few kilograms and fed
downwards continuously as the cavity is cut in the work piece.
 The tool is shaped as the approximate mirror image of the configuration of the
cavity desired in the work.

5. CONTI….
 Ultra sonic are generated by feeding high frequency electrical current to a
transducer which converts it to high frequency mechanical vibrations.
 The vibrations thus, produced are focused to the cutting point by means of
horn or a tuned vibration concentrator.
 Abrasive used in this process is supplied in the form of slurry suspended in
the carrier fluid.
 Ultrasonic machining is a copying operation where the shape of the work
produced is a mirror image of the tool itself.
 Thus, accuracy of the tool itself depends upon the manufacturing accuracy
of the tool itself.

6. ELEMENTS OF THE PROCESS
 Ultrasonic machine
 Abrasive slurry
 Work material
 Tool cone and tool tip

7. SCHEMATIC REPRESENTATION OF USM

9. CONTI….
 The work piece is mounted on a vice, which can be located at the desired position
under the tool using a 2 axis table.
 The table can be further lowered or raised to accommodate work of different
thickness.
The typical elements of an USM are:
 Slurry delivery and return mechanism.
 Feed mechanism to provide a downward feed force on the tool during machining.
 The transducer which generates the ultrasonic vibration.
 The horn concentrator, which mechanically amplifies the vibration to the required
amplitude and accommodates the tool at its tip.

10. HIGH FREQUENCY OSCILLATING CURRENT GENERATOR
 Its purpose is to produce high frequency oscillation current.
 The generator is when electrically tuned generates high frequency electrical
impulses.
 The high frequency electrical impulses are fed to a transducer which converts them
into mechanical vibrations.
 The frequency of electrical impulses produced is about 22000 Hz, which is in
ultrasonic range.
The transducer
 The transducer converts high frequency electrical impulses fed from the oscillator
into mechanical vibrations.
 Some common types of ultrasonic electro-mechanical transducers are piezoelectric
crystal, piezoelectric ceramic etc.
 All these transducers have the general property of undergoing mechanical vibrations
in desired direction is an alternating high frequency electromotive force is applied in
one particular direction.

11. WORK MATERIAL
 Earlier , it was assumed that material is removed in this process by brittle failure,
and so only brittle materials were thought to be machinable by this process.
 But, now it has been confirmed that chips can form in this process, that is, ductile
failure can also take place.
 There appears to be no limitation to the range of materials that can be machined,
except that they should not dissolve in the slurry media or react with it.
 Soft and ductile materials , however , are usually cut more economically by other
methods.
Tool Cone and Tool Tip
 The tool cone (also called ‘horn’) amplifies and focuses the mechanical energy
produced by the transducer and imparts this to the work piece in such a way that
energy utilization is optimum.

12. DIFFERENT TYPES OF HORNS WHICH ARE USED IN USM ARE GIVEN BELOW
Horns are usually made of brass, steel, titanium, or aluminum

13. CONTI….
 Titanium is a good material for tool cone.
 The tool tip is attached to the base of the cone by soft soldering or by means of
screws.
 The area of the tool should not exceed the area of the small section of the cone by
more than 10-15 percent.
 The tool geometry governs the shape of the impression or cavity to be produced.
 A 11.98 mm diameter tool tip may produce a 12.00+- 0.005mm hole, when a
600grit (0.01mm particle size) is used.
 The area of the tip influences the rate of penetration. The smaller the contact area the
better the abrasive flow under the tool and the higher penetration rate.

14. CONTI…
 The choice of material for the tool is very vital because the cost of making the tool
and the time required to change tools are critical factors in the economics of
ultrasonic machining.
 Also, the tool tip has to stand vibrations and it should not fail or wear out quickly.
 Most of the wear occurs at the end; wear at the sides is ten times less.
 The use of tungsten carbide as a tool material presents many problems in shaping the
tool; also the cost of such a tool will be high, though malleable materials, such as
alloy steel and stainless steel prove satisfactory.

15. ABRASIVE SLURRY
 Some of the main types of abrasives in use are:
Aluminium oxide
Boron carbide
Silicon carbide
Diamond dust
 The size of abrasives varies between 200 and 2000 grit.
 Coarse grades are good for roughing, whereas finer grades (say 1000 grit) are used
for finishing.
 The extremely fine grades of 1200 to 2000 grit are used only for a finishing pass
over jobs of extreme accuracy.
Liquid Media
The abrasive is suspended in liquid. The liquid performs many functions:
 Acts as coolant

16. CONTI….
 Provides medium to carry the abrasive to the cutting zone.
 Helps efficient energy transfer between the work piece and the tool.
 Helps to carry away the worn abrasive.
The characteristics of a good suspension media (the liquid) are:
 Good wetting properties to wet the tool, work and abrasive.
 High thermal conductivity for efficient removal of heat from cutting zone.
 Low viscosity to carry the abrasive down the sides of the hole between the tool and
the workpiece.
 Non corrosive properties to avoid corrosion of the workpiece and tool.
Water is frequently used as the liquid carrier since it satisfies most of the requirements.

17. PROCESS PARAMETERS
Performance of USM process depends on several parameters which are as follows:
Work piece characteristics –
Material type, hardness and impact brittleness of the material.
 Slurry Characteristics –
Abrasive type, grain size, liquid component of slurry, method of feeding the slurry.
 Tool characteristics -
Tool material, size and shape of the tool, hollow tool or solid tool, finish and
accuracy of the tool, the method of mounting the tool to the shank.
 Horn characteristics –
material of horn, profile of the horn used, amplification provided by the horn and the
arrangements provided for cooling the horn.
 Machine settings –
Frequency and amplitude of the vibrations.

18. EFFECT OF PARAMETERS
 Effect of Amplitude and frequency of vibration
 Different researchers have different findings.
 Shaw and Goetze predicted linear relationship between
frequency and metal removal rate.
 Other researchers predicted non-linear relationship between
frequency and penetration rate.
 Hence the frequency used for the machining process must
be the resonant frequency of acoustic system in order to
obtain the greatest amplitude and to achieve maximum
utilization of acoustic system.

19. EFFECT OF PARAMETERS
 Effect of Grain Diameter

20. EFFECT OF PARAMETERS
 Effect of Slurry, Tool and work material.
Increase in slurry concentration increases cutting
rates.
Improving slurry circulation doubles the material
removal rate in US drilling.
Increasing slurry pressure increases MRR by ten
times.
Cutting rate is increased by Conical cone shaped
tool rather than Cylindrical.
Brittle non-metallic materials can be cut at higher
rates than ductile materials.


21. ECONOMIC CONSIDERATIONS
 The process has an advantage of machining hard and brittle materials to
complex shapes with good accuracy and reasonable surface finish.
 Considerable economy results from the ultrasonic machining of hard alloy
press tools, dies and wire drawing equipment on account of high wear
resistance of tools made of these alloys.
 The machines have no high speed moving parts. Working on machines is not
hazardous, provided care is to be taken to shield ultrasonic radiations from
falling on the body.
 The cost of manufacture and use of the tools, particularly if they have
complicated contours, is very high.
 Another item adding to cost of ultrasonic machining is abrasive.

22. CONTI…
 The abrasive slurry has to be periodically replaced because during use the particles
are eventually broken and blunted.
 Ultrasonic machines are not yet completely reliable, it is probable that with more
research in the near future on techniques and machines, the process will have more
economic advantage.
Applications
 This method of machining is not limited by the electrical or chemical characteristics
of work materials, which makes it suitable for both conductive and non conductive
materials.
 Tungsten and other hard carbides and gem stones, such as synthetic ruby are being
successfully machined by this method.

23. CONTI…..
 The process is particularly suited to make holes with a curved axis of any shape that
can be made on tool.
 The range of shapes can be increased by moving the workpiece during cutting.
 The smallest hole that can currently be cut by the ultrasonic machining method is
0.050 mm in diameter, the hole size being limited by the strength of the tool and the
clearance required for the flow of the abrasive.
 The largest diameter solid tool reported to have been employed in some application
s is 100mm in diameter.
Limitation
 The major limitation of the process is its comparatively low metal cutting rates.
 The depth of the cylindrical holes is presently limited to 2.5 times the diameter of
the tool.

24. ULTRASONIC MACHINE IS USEFUL FOR HARD MATERIAL
 This method is the best choice for working with hard materials
such as ceramic matrix composites, ruby, piezo-ceramics, glass,
ceramics, Quartz, ferrite, diamonds, alumina, sapphire, CVD
silicon carbide.