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ULTRA SONIC MACHINING USM

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ULTRA SONIC MACHINING USM Powered By Docstoc
					Chapter Ⅰ USM
Definition Of USM

   Material Removing Process:
  USM is used to erode holes and cavities in hard or
brittle workpieces by using shaped tools high-frequency
mechanical motion and an abrasive slurry.



  USM is able to ef-fectively machine all hard materials
whether they are electrically conductive or not.
  Principle Of USM
The process and cutting tool
• The process is performed by a cutting tool, which 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.
• The high-speed reciprocations of the tool drive the abrasive
  grains across a small gap against the workpiece .
• The tool is gradually fed with a uniform force.
• 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.
Principle Of USM
Ultrasonic Machining
     Principle Of USM

   Elements of ultrasonic machining


• The tool is oscillated by a
  longitudinal magnetostriction
• A magnetic field variation at
  ultrasonic frequencies
• The length of a ferromagnetic
  object changes
 Principle Of USM
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.
 Principle Of USM
Material removal
Principle Of USM

The basic components to the cutting
action are believed to be

 1

 2

3
3

 4
    USM System

• Small, tabletop-sized units to large-capacity machine
  tools,
• Bench units, and as self-contained machine tools.
• Power range from about 40 W to 2.5 kW.
• The power rating strongly influences the material
  removal rate.
   USM System
Subsystems of USM System


                  B



          A                    C




              E            D
     USM System

    A


• 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.
    USM System

   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
     USM System

   B




• Magnetostrictive transducers are usually constructed from a
  laminated stack of nickel or nickel alloy sheets.
• Magnetostriction is explained in terms of domain theory .
       USM System

      B



• Domains are very small regions, of the order of l0-8 ~ l0-9 cm3,
• In which there are forces that cause the magnetic moments of the
  atoms to be oriented in a single direction.
• In each domain the atomic magnetic moments are oriented in one
  of the directions of easy magnetization
    USM System

  B



• In the cubic-lattice crystals of iron and nickel there are
  six directions of easy magnetization.
• In unmagnetized material all these directions are
  present in equal numbers, the magnetic moments of the
  orderless, unorientated domains compensate one
  another
    USM System

  B



• 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.
  USM System

B



• As the temperature is raised, the amount of
  magnetostrictive strain diminishes .
• Magnetostrictive transducers require cooling by fans or
  water.
   USM System

  B



• Such as quartz or lead,zirconate,titanate, 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.
    USM System

   B



• Piezoelectric materials are composed of small particles bound
  together by sintering.
• The material undergoes polarization by heating it above the
  Curie point.
• Such transducers exhibit a high electromechanical conversion
  efficiency that eliminates the need for cooling.
  USM System

B



• The magnitude of the length change is limited by the
  strength of the particular transducer material.
• The limit is approximately 0.025 mm.
    USM System

C

• 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.
       USM System

   C

• Monel and titanium have good acoustic properties and are often
  used together with stainless steel, which is cheaper.
• However, stainless steel has acoustical and fatigue properties
  that are inferior to those of Monel and titanium, limiting it to
  low-amplitude applications.
• Nonamplifying holders are cylindrical and result in the same
  stroke amplitude at the output end as at the input end.
• Amplifying toolholders have a cross section that diminishes
  toward the tool, often following an exponential function.
• An amplifying toolholder is also called a concentrator.
     USM System

 C



• Amplifying holders remove material up to 10 times faster
  than the nonamplifying type.
• The disadvantages of amplifying toolholders include
  increased cost to fabricate, a reduction in surface finish
  quality, and the requirement of much more frequent running
  to maintain resonance.
     USM System

 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.
    USM System

D



• 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.
       USM System

   E

• The criteria for selection of an abrasive for a particular
  application include hardness, usable life, cost, and particle size.
• Diamond is the fastest abrasive, but is not practical because of
  its cost.
• Boron carbide is economical and yields good machining rates.
• Silicon carbide and aluminum oxide are also widely used.
       USM System

   E



• Coarse grits exhibit the highest removal rates,when the grain
  size becomes comparable with the tool amplitude, cut more
  slowly.
• The larger the grit size, the rougher the machined surface.
     USM System

 E



• With an abrasive concentration of about 50% by weight
  in water,but thinner mixtures are used to promote
  efficient flow when drilling deep holes or when forming
  complex cavities.
    USM System

E
     USM System
 Example


• Find the machining time for a hole 5mm in diameter in a
  tungsten carbide plate 1cm thick.
• The grains are 0.01mm in diameter, the feed force is 3N, and
  the amplitude of oscillation is 20 micro m at a frequency of
  25KHz.
• The fracture hardness is approximately 6900N/mm2.
• The slurry is mixed in equal parts water and abrasive.
       USM System

    Example
    - Basic machine layout


 The acoustic head is the
  most complicated part of
  the machine.
 It must provide a static
  force, as well as the high
  frequency vibration
     USM System

Example
- Basic machine layout




   Magnetostrictive materials should have a good coupling of
    magnetic and mechanical energy
  USM System

Example   Basic machine layout
USM System
       USM System
• If a tool is designed to increase flow, better cutting speeds will
  occur.
• Tools
  - hard but ductile metal
  - stainless steel and low carbon
  - aluminum and brass tools wear near 5 to 10 times faster
•    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)
        USM System
    •   liquid
          - water most common
          - benzene
          - glycerol
          - oils
    •   high viscosity decreases mrr
    •   typical grit size is 100 to 800

 Little production of heat and stress, but may chip at exit
  side of hole.
 Sometimes glass is used on the back side for brittle
  materials.
       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
• Material/tool wear = 1.5 for WC workpiece, 100 for glass
• Gap 25-40 micro m
• 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.
     General Questions

1. A cylindrical impression with a diameter of 10mm and a depth
   of 1mm has to be made on a tungsten carbide surface. The
   feed force is constant and equal to 5N. The average diameter
   of the grains in the abrasive slurry is 0.01mm. The tool
   oscillates with an amplitude of 30 micro m at 20 KHz. The
   slurry contains 1 part of abrasive to about 1 part of water. The
   fracture hardness of tungsten carbide workpiece may be
   taken as 7000 N/mm2. Estimate the machining time.
     General Questions

2. A square through hole of 5mm by 5mm has to be drilled in a
   5mm thick tungsten carbide sheet. The slurry is made of 1
   part of 10 micro m radius boron carbide grains mixed with 1.5
   parts of water. The feed force is 4N. The tool oscillates with
   an amplitude of 0.015mm at 25KHz. Assuming that only 20%
   of the pulses are effective, calculate the time required to
   complete the job.


3. In an ECM operation, a pure copper block is being machined.
   If a current of 5000A is used, determine the volume rate of
   material removal from the copper block.
    General Questions

4. The composition of a Nimonic alloy turbine blade is 18%
   cobalt, 62% Ni, and 20% chromium. It is being machined
   electrochemically with a current of 1500A. Find out the volume
   removal rate if the density of the alloy is 8.3g/cm3. The
   dissolution valency of chromium is 6, whereas that for both
   nickel and cobalt is 2.

5. The composition of a monel alloy workpiece undergoing
   electrochemical machining is as given here:
   63% Ni, 31.7% Cu, 2.5% Fe, 2% Mn, 0.5% Si, 0.3% C
   if the machining current is 1000A, estimate the volume
   removal rate.
     General Questions

6. The equilibrium gap when machining (electrochemically) iron,
   using NaCl solution in water as the electrolyte, is found to be
   0.2mm. The current density is 200A/cm2, the operating
   voltage being 12V. Iron dissolves at a valency 2, the density
   of iron is 7.8 g/cm3, and the specific resistance of the
   electrolyte is 2.8 ohm cm. Calculate the metal removal
   rate/unit work surface area. The overvoltage may be taken as
   1.5V.
    General Questions

7. In an electrochemical trepanning operation on a flat iron
   surface, an electrode in the form of a tube (with an outer
   diameter of 1cm). A laser beam with a power intensity of
    2 * 105 W/mm2 is used to drill a 0.2mm diameter hole in a
   tungsten sheet of 0.4mm thickness. If the efficiency of the
   operation is only 10%, estimate the time required.

8. TRUE / FALSE - Water is the main cutting tool in Ultra
   Sonic machining.

 9. Why are the vibrations in USM so small?
    General Questions


10. USM will be used to add the following pattern to an object,
   If the tool is Tungsten carbide, and the work is Cu, with an
   amplitude of oscillation of 10 μm, at 30KHz, how long will the
   operation take? (Note: the grain diameter is 20μm, and the
   head has a static force of 6N)
   General Questions

11. When is the abrasive added into the flow for the various
    abrasive jet machining processes?

12. Why is the depth of material removed by abrasive jet
    machining so variable?

13. Describe the ability of the abrasive processes to
    produce sharp corners.