Ultrasonic Machining by Ms Shikha Kashyap

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Ultrasonic Machining
Ms Shikha Kashyap
Assistant Professor Dept
of Mechanical Engineering
The NorthCap University, Gurgaon
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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.

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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.

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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.

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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.

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Elements of the Process

• Ultrasonic machine
• Abrasive slurry
• Work material
• Tool cone and tool tip

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Schematic representation of USM
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Conti.

• The basic mechanical structure of a USM is very
  similar to drill press.
• 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.

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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.

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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.

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Different types of horns which are used in USM
are given below
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Conti..

• The amplitude of the vibratory motion of the
  transducer is small and is usually inadequate for
  material removal purposes.
• The horn/trunk amplifies and focuses the
  vibrations of the transducer to an intensity
  necessary to drive the tool to do its work.
• The increase in amplitude of the vibrations at
  the tool end is obtained by reducing the cross
  section of the horn at the tool end.
• The trunks are specially shaped to provide a
  reduction in cross section at the tool end.
• The trunk provides an increased amplitude in the
  order of 30 to 120 microns at the tool face.

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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 too 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.

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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 o 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.

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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

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Conti.

• Provides medium to carry the abrasive to the
  cutting zone.
• Helps efficient en 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.

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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.

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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.

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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.

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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.

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Recent Developments

• Mullard Research Laboratories, USA, have
  developed a process that combines
  electrochemical reaction with ultrasonic
  abrasion.
• Using a 60W ultrasonic drill and abrasive
  suspended in an alkaline electrolyte, Mullard
  researchers have reported that tool steel can be
  machined nine times facter than by ultrasonics
  alone..

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Thanks