Material Removal Processes:
Abrasive, Chemical, Electrical, and
• 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
Types of Abrasives
• Conventional abrasives:
• Super abrasives
Cubic boron nitride
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.
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
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
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
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
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
VC = vd
And because l =
the un-deformed chip thickness in the surface grinding is
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
The formula for un-deformed length and thickness
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
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
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.
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
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
712000 = (Fc) (10/2)(2π)(4000) = Fc = 57 lb
The thrust force is obtained from technical data that is 30% higher than the cutting
Fn = (1.3)(57) = 74 lb
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
• 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)
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
The different types are: Surface, Cylindrical, internal and center-
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
• 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.
• A small grinding wheel is used to grind inside
diameter of parts, such as brushings and
FIGURE 9.15 Schematic illustrations of internal-grinding operations.
• 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
FIGURE 9.16 (a-c) Schematic illustrations of centerless-grinding operations. (d) A
computer-numerical-control centerless grinding machine. Source: Cincinnati
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.
• Coated abrasives: typical example is sandpaper and
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)
• 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
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.
• 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
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.
• 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.
• 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
• 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.
• 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
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
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.
From equations 9.13 and 9.14, feed rate can be expressed as:
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.
• 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 =
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
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
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
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
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
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.
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.
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.