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Material Removal Processes Abrasive Chemical Electrical and

VIEWS: 129 PAGES: 57

									  Material Removal Processes:
Abrasive, Chemical, Electrical, and
       High-Energy Beams
                  Abrasives
• When the material is either too hard or brittle,
  or the shape is difficult to produce, with
  dimensional accuracy by the one of the
  machining processes as described in previous
  chapter, abrasives are used.
• Abrasive machining processes are generally
  among the last operations performed on
  manufactured products.
               Types of Abrasives
• Conventional abrasives:
       Aluminium oxide
       Silicon carbide
• Super abrasives
       Cubic boron nitride
       Diamond
In addition to hardness, an important characteristic of an
   abrasive is friability.
Friability is the ability of an abrasive grain to fracture into
   smaller pieces. Friability gives abrasives self sharpening
   characteristics which are important in maintaining the
   sharpness of abrasives during use. The shape and size
   of abrasive grain also affect its friability.
       Typical Grinding wheel



 FIGURE 9.1 Schematic illustration of a physical model of a grinding
 wheel, showing its structure and grain wear and fracture patterns.

TABLE 9.1 Knoop hardness range for various materials and abrasives.
Grinding Wheel Types, super abrasives
        and Marking systems
• Grinding Wheel Types




 FIGURE 9.2 Some common types of grinding wheels made with conventional
 abrasives (aluminum oxide and silicon carbide). Note that each wheel has a specific
 grinding face; grinding on other surfaces is improper and unsafe.
                                  Super abrasives




FIGURE 9.3 Examples of superabrasive wheel configurations. The rim consists of superabrasives and
the wheel itself (core) is generally made of metal or composites. Note that the basic numbering of
wheel types (such as 1, 2, and 11) is the same as that shown in Fig. 9.2. The bonding materials for the
superabrasives are: (a), (d), and (e) resinoid, metal, or vitrified; (b) metal; (c) vitrified; and (f) resinoid.
      Grinding wheel Marking system




FIGURE 9.4 Standard marking system for aluminum-oxide and silicon-carbide bonded abrasives.
Diamond and cBN marking system




FIGURE 9.5 Standard marking system for diamond and cubic-boron-nitride bonded abrasives.
           Mechanics of Grinding
• Grinding is basically a chip removal process in
  which the cutting too is an individual abrasive
  grain. It differs from single point cutting tool as:
  - The individual grain has irregular geometry
  - The average rake angle of the grains is highly
  negative i.e. -60 degree or even lower
  - The grains on the periphery of a grinding wheel
  has different radial positions
  - The cutting speed of the grinding wheels are
  very high i.e. 30m/s (6000 ft/min)
An example of chip formation by an abrasive
  grain is shown in figure below: From the figure
  it is noted that a variety of metals chips are
  obtained.




   FIGURE 9.7 (a) Grinding chip being produced by a single abrasive grain. Note
   the large negative rake angle of the grain. Source: After M.E. Merchant. (b)
   Schematic illustration of chip formation by an abrasive grain. Note the
   negative rake angle, the small shear angle, and the wear flat on the grain.
The mechanics of grinding and variables an best be studied by
  analyzing the surface grinding operations as shown below:




FIGURE 9.8 Basic variables in surface grinding. In actual grinding operations, the
wheel depth of cut, d, and contact length, l, are much smaller than the wheel
diameter, D. The dimension t is called the grain depth of cut.
In figure, a grinding wheel of diameter D is removing a layer of
   metal of depth d, known as wheel depth of cut.
An individual grain on the periphery of the wheel is moving at a
   tangential velocity V and work piece is moving with velocity
   v.
The grain is removing a chip of an un-deformed thickness (grain
   depth of cut), t, and un-deformed length l.
For the condition of v<<V, the un-deformed chip length is
   approximately
                        l~
For external (cylindrical) grinding
For internal grinding


Where Dw is the diameter of the work piece.
The relationship between t and other process variables can be derived
    as follows:
Let C = Number of cutting points per unit area of wheel surface
V and v = Surface speeds of the wheel and work piece
w = width of work piece to be unity
        Number of grinding chips produced per unit time is VC
        Volume of material removed per unit time = vd
 letting also that r be the ratio of chip width, w, to the average chip
    thickness, then volume of chip with rectangular cross sectional area
    and constant width along its length is
                  Vol chip =       =
The volume of material removed per unit time is the product of the
  volume of each chip and the number of chip produced per unit
  time.
Thus
                  VC       = vd

And because l =


the un-deformed chip thickness in the surface grinding is
Example 9.1
Estimate the un-deformed chip length and the un-deformed
   chip thickness for a typical surface grinding operation. Let
D = 200 mm, d = 0.05 mm, C = 2 per mm2, and r = 15
Solution
The formula for un-deformed length and thickness
   respectively are
 l=          and t =
From table 9.2, the following values are selected:
              v = 0.5 m/s and V = 30m/s
Therefore, l =                  = 3.2 mm = 0.126 in
And
t=                                  = 0.006 mm = 2.3x10-4 in
Note that due to plastic deformation, the actual length of the
   chip is shorter and the thickness greater than these values
                Grinding forces
The knowledge of force is essential for the
  deflections that the work piece and machine will
  undergo. If we assume that force on the grain is
  proportional to the cross-sectional area of the
  un-deformed chip, it can be shown that the
  relative grain force is given by

Relative grain force

The specific energy consumed in producing a
  grinding chip consists of three components.
      u = uchip+uplowing+usliding
                       FIGURE 9.9 Chip formation and plowing (plastic
                       deformation without chip removal) of the
                       workpiece surface by an abrasive grain.



TABLE 9.2 Typical ranges of speeds and feeds for abrasive processes.
Example 9.2
Assume that you are performing a surface grinding operation on a low carbon steel
    work piece using a wheel of diameter D = 10 in that rotates at N = 4000 rpm. The
    width of cut w = 1 in., depth of cut d = 0.002 in and the feed rate of the work
    piece is v = 60 in/min. Calculate the cutting force, Fc, and the thrust force, Fn
Use Table 9.3 for this example
Solution:
The material removal rate is as follows:
          MRR = dwv = (0.002)(1)(60) = 0.12 in3/min
The power consumed is given by
           Power = (u) (MRR) where u is the specific energy as obtained from table 9.3.
    for low carbon steel, let’s estimate u to be 15 hp-min/in3. Hence
           Power = (15) (0.12) = 1.8 hp
By noting that 1 hp = 33,000 ft-lb/min = 396000 in-lb/min, we obtain
           Power = (1.8) (396000) = 712000 in-lb/min
Since, power is defined as
           Power = Tω, where T = Torque and is equal to (Fc) (D/2) and ω is the
    rotational speed of the wheel in radians per minute, we also have ω = 2πN
Thus
           712000 = (Fc) (10/2)(2π)(4000) = Fc = 57 lb
The thrust force is obtained from technical data that is 30% higher than the cutting
    force, Fc
                   Fn = (1.3)(57) = 74 lb
                         Temperature
The surface temperature rise, ∆T, has been found to be a function of the ratio
   of the total energy input to the surface area ground. Thus in surface
   grinding, if w is the width and L is the length of the surface ground, then
                           ∆T ∝          ∝ud

If we introduce size affect and assume that u varies inversely with the un-
    deformed chip thickness t, then the temperature rise is
          Effects of Temperature
• Tempering (temper and soften surface of steel
  components)
• Burning (If tempt is high, it will burn the surface)
• Heat checking (high tempt lead to thermal stresses
  and cause thermal cracking known as heat checking)
• Residual stresses (Tempt change and gradients are
  responsible for residual stresses in grinding)
                   Residual Stresses




FIGURE 9.10 Residual stresses developed on the workpiece surface in grinding
tungsten: (a) effect of wheel speed and (b) effect of type of grinding fluid. Tensile
residual stresses on a surface are detrimental to the fatigue life of ground
components. The variables in grinding can be controlled to minimize residual
stresses, a process known as low-stress grinding. Source: After N. Zlatin.
             Grinding wheel wear
• Attritious Wear (the cutting edge become dull by
  attrition, developing a wear flat)
• Grain Fracture (Since, abrasive grains are brittle, so their
   fracture characteristics is important)
In order to avoid such situations, following combinations are used
1. Aluminium oxide (steels, ferrous alloys and alloy steels)
2. Silicon Carbide (cast iron, non ferrous metals etc)
3. Diamond (for ceramics and some hardened steels)
4. Cubic boron nitride (for steels and cast irons at 50 HRC and
    for high temperatures)
    Grinding ratio = Volume of material removed/volume of wheel wear
      Dressing/truing and shaping
• Dressing is the process of conditioning worn grains on the
  surface of a grinding wheel in order to produce a sharp new
  grains and for truing an out of round wheel.
       Grinding Operations/ Machines
  The grinding operations are carried out in variety of ways. The
    selection of a grinding process for a particular application
    depends on part shape, size, ease of fixturing and production
    rates required.
  The different types are: Surface, Cylindrical, internal and center-
    less grinding




FIGURE 9.12 Schematic illustrations of surface-grinding operations. (a) Traverse
grinding with a horizontal-spindle surface grinder. (b) Plunge grinding with a
horizontal-spindle surface grinder, producing a groove in the workpiece. (c)
Vertical-spindle rotary-table grinder (also known as the Blanchard-type grinder).
   Surface grinding Machine




FIGURE 9.12 Schematic illustration of a horizontal-spindle surface
grinder.
           Cylindrical Grinding
• In this type the work piece external cylindrical
  surfaces and shoulders are ground such as
  crankshaft bearings, spindle pins, rolls etc




            FIGURE 9.14 Threads produced by (a)
            traverse and (b) plunge grinding.
                  Internal Grinding
• A small grinding wheel is used to grind inside
  diameter of parts, such as brushings and
  bearing races.




  FIGURE 9.15 Schematic illustrations of internal-grinding operations.
                  Centerless Grinding
• It is high production process for continuously grinding cylindrical surfaces
  in which the work piece is supported not by centers or chucks but by a
  blade




FIGURE 9.16 (a-c) Schematic illustrations of centerless-grinding operations. (d) A
computer-numerical-control centerless grinding machine. Source: Cincinnati
Milacron, Inc.
                    Creep Feed Grinding
   • In this type, the wheel depth of cut d is as
     much as 6 mm and work piece speed is low




FIGURE 9.17 (a) Schematic illustration of the creep-feed grinding process. Note the
large wheel depth of cut. (b) A groove produced on a flat surface in one pass by creep-
feed grinding using a shaped wheel. Groove depth can be on the order of a few mm.
(c) An example of creep-feed grinding with a shaped wheel. Source: Courtesy of
Blohm, Inc. and Society of Manufacturing Engineers.
           Finishing Operations
• Coated abrasives: typical example is sandpaper and
  emery cloth
                                            FIGURE 9.18 Schematic illustration of the
                                            structure of a coated abrasive. Sandpaper,
                                            developed in the 16th century, and emery cloth
                                            are common examples of coated abrasives.


• Wire Brushing: The work piece is held against a circular
  wire brush that rotates at high speed.




              FIGURE 9.19 Schematic illustration of a honing tool to improve the
              surface finish of bored or ground holes.
• Honing: It is an operation used primarily to
  give holes a fine surface finish.




         FIGURE 9.20 Schematic illustration of the honing process
         for a cylindrical part: (a) cylindrical microhoning; (b)
         centerless microhoning.
                            Lapping
• Lapping is finishing process on flat or
  cylindrical surfaces. The lap is made of cast
  iron, copper, leather or cloth




 FIGURE 9.21 (a) Schematic illustration of the lapping process. (b)
 Production lapping on flat surfaces. (c) Production lapping on cylindrical
 surfaces.
    Chemical Mechanical Polishing
• It is important in semi conductor industry. The
  process removes material from work piece surfaces
  through combined actions of abrasion and corrosion.




FIGURE 9.22 Schematic illustration of the chemical-mechanical polishing
process. This process is widely used in the manufacture of silicon wafers and
integrated circuits, where it is known as chemical-mechanical planarization.
Additional carriers and more disks per carrier also are possible.
    Polishing using Magnetic fields
• Two types are: Magnetic float polishing and
  magnetic field assisted polishing




FIGURE 9.23 Schematic illustration of the use of magnetic fields to polish
balls and rollers: (a) magnetic float polishing of ceramic balls and (b)
magnetic-field-assisted polishing of rollers. Source: After R. Komanduri, M.
Doc, and M. Fox.
               Ultrasonic Machining
• The material is removed from a work piece surface by the
  mechanism of micro-chipping or erosion with abrasive
  particles. The tip of the tool called a sonotrode, vibrates at
  amplitude of 0.05 to 0.125 mm (0.002 to 0.005 in) and at a
  frequency of 20 kHz. This vibration in turn transmits a high
  velocity to fine abrasive grains between tools and the surface
  of the work piece.




FIGURE 9.24 (a) Schematic illustration of the ultrasonic-machining process;
material is removed through micro-chipping and erosion. (b) and (c) Typical
examples of cavities produced by ultrasonic machining. Note the dimensions of
cut and the types of work piece materials.
• Best suited for hard and brittle materials, such as ceramics,
  carbides, glass etc.
• The tip is made of low carbon steel and attached to a
  transducer through the tool holder. With fine abrasives,
  dimensional tolerances of 0.0125 mm can be achieved.
Micro-chipping in ultrasonic machining is possible of the high
  stresses produced by particles striking a solid surface. The
  contact time between the particle and the surface is very short
  and area of the contact is very small. The contact time to can
  be expressed as




Where r is the radius of spherical particle, co is the elastic wave
 velocity in work piece co =
And v is the velocity with which the particle
  strikes the surface.
The force of the particle on the surface is
  obtained from the rate of change of
  momentum, that is
                  F=
Where m is the mass of the particle. The
  average force of the particle striking the
  surface and re-bouncing is
Advance Machining Processes




   TABLE 9.4 General characteristics of advanced machining processes.
                              Chemical Milling
  • In chemical milling, shallow cavities are produced on sheets,
    plates, forgings, and extrusions. It is used for wide variety of
    metals, with depth of material removal to as much as 12 mm.
  • Used in aerospace industry. The process is either used to
    fabricate microelectronic devices termed as wet etching.




FIGURE 9.25 (a) Missile skin-panel section contoured by chemical milling to improve the stiffness-to-weight
ratio of the part. (b) Weight reduction of space launch vehicles by chemical milling of aluminum-alloy plates.
These panels are chemically milled after the plates have first been formed into shape, such as by roll
forming or stretch forming. Source: ASM International.
                       Chemical Machining
 • This is the oldest non-traditional machining process and
   removing material from surface by chemical dissolution using
   reagents or etchants such as acids and alkaline solutions.




FIGURE 9.26 (a) Schematic illustration of the chemical machining process. Note that no forces are involved
in this process. (b) Stages in producing a profiled cavity by chemical machining.
Roughness and Tolerance capabilities
                    Chemical Blanking
• It is similar to blanking of sheet metal. It is used to produce
  features that penetrate through the thickness of the material,
  with the exception that material is removed by chemical
  dissolution rather than shearing.




 FIGURE 9.28 Typical parts made by chemical blanking; note the fine detail. Source: Courtesy of
 Buckabee-Mears St. Paul.
       Electrochemical Machining
• It is reverse of electroplating process. An electrolyte acts as
    current carrier, and the high rate of electrolyte movement in
    the tool-work piece gap washes metal ions away from the
    work piece before they have a chance to plate onto the tool.
    The material removal rate can be calculated by
                        MRR = CIη
Where MRR = Material Removal rate, I = Current in amperes and
    η = current efficiency, C = material constant.
If a cavity of uniform cross-sectional area Ao is being
    electrochemically machined, the feed rate f, in mm/min
    would be                     f = MRR/ Ao
It is used for complex cavities, high strength materials in
    aerospace industry.
                                                      FIGURE 9.30 Typical parts made by electrochemical
FIGURE 9.29        Schematic illustration of the      machining. (a) Turbine blade made of a nickel alloy,
electrochemical-machining process. This process is    360 HB; the part on the right is the shaped electrode.
the reverse of electroplating, described in Section   Source: ASM International. (b) Thin slots on a 4340-
4.5.1.                                                steel roller-bearing cage. (c) Integral airfoils on a
                                                      compressor disk.
Example 9.5 A round hole 12.5 mm (0.5in) diameter is being produced in a
   titanium-alloy block by electrochemical machining. Using a current density
   of 6 A/Sq. mm, estimate the time required for machining a 20-mm-deep
   hole. Assume that the efficiency is 90%. Compare this time with that
   required for ordinary drilling.
Solution
From equations 9.13 and 9.14, feed rate can be expressed as:
         f=
Letting C = 1.6 sq.mm/A-min, I/ Ao = 6 A/sq.mm, the feed rate
         f = (1.6)(6)(0.9) = 8.64 mm/min. Since the hole is 20 mm deep,
         Machining time = 20/8.64 = 2.3 min
To determine the drilling time, refer to table 8.12 and note the data for
   titanium alloys. Selecting the following values for 12.5 mm drill-rpm = 300
   and feed = 0.15 mm/rev, it can be seen that the feed rate is (300
   rev/min)(0.15 mm/rev) = 45 mm/min. Since the hole is 20 mm deep,
         Drilling time = 20/45 = 0.45 min which is about 1/5 of the time
   required for ECM.
         Electrochemical Grinding
• The wheel is metal bonded, with diamond or aluminium-
  oxide. The abrasives serve as insulators between the grinding
  wheel and the work piece and mechanically remove
  electrolyte products from the working area.
The material removal rate can be calculated as
               MRR =      , where, G = mass in grams, I = current
  in amperes, ρ = density in g/sq.mm and F = Faraday’s
  constant. The speed of the penetration Vs of the grinding
  wheel into the work piece is given by
               Vs =            K

Where Vs is in sq.mm/min, E = cell voltage, g = wheel work piece
 gap in mm, Kp co-efficient of loss, ranges b/w 1.5 to 3 and K =
 electrolyte conductivity
FIGURE 9.31 (a) Schematic illustration of the electrochemical grinding process. (b)
Thin slot produced on a round nickel-alloy tube by this process.
     Electrical Discharge Machining
• It is based on erosion of metals by spark discharge. When two current
  conducting wires are allowed to touch each other, an arc is produced.
  When closed examined, it is noted that portion of metals has been eroded
  away leaving a small crater.
• The EDM system consists of a shaped tool (electrode) and the work piece,
  which are connected to a DC power supply and placed in a dielectric fluid.
  When voltage is applied an intense electrical arc is generated causing
  sufficient heating to melt a portion of the work piece .




    FIGURE 9.32 Schematic illustration of the electrical-discharge-machining proces
                          EDM examples



FIGURE 9.33 (a) Examples of shapes produced by the electrical-discharge machining
process, using shaped electrodes. The two round parts in the rear are a set of dies for
extruding the aluminum piece shown in front; see also Section 6.4. Source: Courtesy of
AGIE USA Ltd. (b) A spiral cavity produced using a shaped rotating electrode. Source:
American Machinist. (c) Holes in a fuel-injection nozzle produced by electrical-discharge
machining.

                                            FIGURE 9.34 Stepped cavities produced
                                            with a square electrode by EDM. In this
                                            operation, the workpiece moves in the two
                                            principal horizontal directions, and its motion
                                            is synchronized with the downward
                                            movement of the electrode to produce these
                                            cavities. Also shown is a round electrode
                                            capable of producing round or elliptical
                                            cavities. Source: Courtesy of AGIE USA Ltd.
                 Analysis of EDM
The material removal is the function of current and melting point
    of the work piece material. The following approx. Empirical
    relationship can be used to estimate the metal removal
                 MRR = 4x104 I
I = Current in amperes, Tw is the melting point of the work piece
    in degree C.
The wear rate of the electrode Wt can be as
                 Wt = 1.1x1011 I
The wear ratio of the work piece to electrode R, can be
    estimated from
                 R = 2.25
Where Tr is the ratio of work piece to electrode melting points.
Example 9.6. Calculate the machining time for producing the hole in example
   9.5 by EDM and compare the time with that for drilling and for EDM.
   Assume that the titanium alloy has a melting point of 1600 degree C (see
   table 3.3) and that the current is 100A.calculate the wear rate of the
   electrode, assuming that the melting point of the electrode is 1100 degree
   C.
Solution
1.       MRR = (4X104) (100) (1600-1.23)
The volume of the hole is
         V=π           (20) = 2452 mm3.

Hence the machining time for EDM is 2454/458 = 5.4 min, this time is 2.35
    times that for ECM and 11.3 times that for drilling. If the current is
    increased to 300A, the machining time for EDM will be only 1.8 min, which
    is less than the time for ECM
2. The wear rate of the electrode is calculated using 9.19, thus
           Wt = (11x103) (100) (1100-2.38) = 0.064mm3/min
                               Wire EDM
The process is similar to contour cutting with a band saw. It is
  used to cut plates as thick as 300 mm. The wire is made of
  brass and have enough tensile strength. The MRR is
                        MRR = Vf hb




FIGURE 9.35 Schematic illustration of the wire EDM process. As much as 50 hours of
machining can be performed with one reel of wire, which is then recycled.
Laser Machining
         FIGURE 9.36 (a) Schematic illustration of
         the laser-beam machining process. (b)
         Cutting sheet metal with a laser beam.
         Source: (b) Courtesy of Rofin-Sinat, Inc.
             Electron Beam Machining




FIGURE 9.37 Schematic illustration of the electron-beam machining process. Unlike LBM, this
process requires a vacuum, and hence work piece size is limited by the chamber size.
                      Water-Jet Machining




FIGURE 9.38 (a) Schematic illustration of water-jet machining. (b) A computer-controlled water-jet
cutting machine. (c) Examples of various nonmetallic parts machined by the water-jet cutting
process. Source: Courtesy of OMAX Corporation.
             Abrasive Jet Machining




FIGURE 9.39 (a) Schematic illustration of the abrasive-jet machining process. (b) Examples of
parts produced by abrasive-jet machining; the parts are 50 mm (2 in.) thick and are made of
304 stainless steel. Source: Courtesy of OMAX Corporation.
Thank you

								
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