Chapter 27
Advanced Machining Processes
Parts Made by Advanced Machining
Processes
(a)
(b)
Figure 27.1 Examples of parts produced by advanced machining processes. (a)
Samples of parts produced from waterjet cutting. (b) Turbine blade, produced by
plunge EDM, in a fixture to produce the holes by EDM. Source: (a) Courtesy of
Omax Corporation. (b) Courtesy of Hi-TEK Mfg., Inc.
Chapter 27: Advanced machining
processes
• There are situations where conventional machining
processes are not satisfactory, economical, or impossible
for the following reasons:
–
–
–
–
–
Material is very hard and strong, or too brittle.
Workpiece is too flexible, delicate, or difficult to fixture.
Complex shapes.
Surface finish and dimensional accuracy requirements.
Temperature rise and residual stresses are not desirable.
27.2 Chemical machining (CM)
• Carried out by chemical dissolution using reagents or
etchants, such as acids and alkaline solutions.
• engraving metals and hard stones, in deburring, and in the
production of printed-circuit boards (PCB) and
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microelectronic devices
Chemical Milling
• In chemical milling, shallow cavities are produced on plates,sheets,
forgings, and extrusions, generally for the overall reduction of weight
• The procedure for chemical milling consists of the following:
1. if part has residual stresses from previous operations, stresses are
first relieved.
2. degrease & clean surfaces to ensure adhesion of masking material
and uniform material removal.
3. apply masking material. The maskant material should not react
with the chemical reagent.
4. peel off masking that covers various regions that require etching.
5. exposed surfaces are etched with etchants such as NaOH for Al,
solutions of HCl & HNO3 acids for steels, or FeCl3 for stainless
steel.
6. after machining, parts should be washed to prevent further
reactions with any etchant residues.
7. rest of masking material is removed and part is cleaned and
inspected.
8. finishing operations
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9. This sequence of operations can be repeated to produce stepped
cavities and various contours
Chemical Milling
Figure 27.3 (a) Schematic illustration of the chemical-machining process. Note that no
forces or machine tools are involved in this process. (b) Stages in producing a profiled
cavity by chemical machining; note the undercut.
Chemical blanking and Photochemical
blanking (PCB)
• typical applications: PCB, decorative panels, thin sheet metal stampings,
production of complex or small shapes.
• Photochemical blanking (PCB) (Photoetching)
• Modification of Chemical Milling
• PCB Steps are:
1. design of part to be blanked is prepared at a magnification of up to
100X.
2. a photographic -ve is made & reduced to the size of the finished
part.
3. sheet blank is coated with a photosensitive material and is then
dried in an oven.
4. the –ve is placed over the coated blank and exposed to UVL, which
hardens the exposed areas.
5. the blank is developed, which dissolves the unexposed areas.
6. the blank is then immersed into a bath of reagent (or sprayed over
it) which etches away the exposed areas.
7. the masking material is removed and the part is washed.
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Photochemical blanking (PCB)
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Photochemical blanking (PCB)
• Process capabilities:
– typical applications: fine screens, PCB, electric
motor laminations, flat springs, masks for colored TV.
– Skilled labor required, but tooling costs are low.
– Can be automated
– Economical for medium to high production volume.
– Very small parts can be produced.
– Effective for blanking fragile workpiece and material.
– Because etchant attacks material in both vertical and
horizontal directions, undercuts may develop.
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Parts Made by Chemical Blanking
Figure 27.5 Various parts made by chemical blanking. Note the fine detail.
Source: Courtesy of Buckbee-Mears, St. Paul.
27.3 Electrochemical machining
• An electrolyte acts as a current carrier, and the high rate of electrolyte
movement in the tool-work piece gap washes metal ions away from the
work piece.
• Modifications of this process are used for turning, facing, slotting,
trepanning, and profiling operations in which the electrode becomes the
cutting tool.
• Tool: brass, Cu, bronze and stainless steel.
• a highly conductive fluid, such as an aqueous solution of sodium nitrate
NaNO3.
• A dc power supply in the range of 5-25V maintains current densities (20200 A/mm2) of active machined surface. MRR 1.5 - 4 mm3 per A-min
• Penetration rate of tool is proportional to current density.
Figure 27.6 Schematic
illustration of the
electrochemical
machining process.
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Electrochemical machining
Process capabilities:
– Used to machine complex cavities in high strength
materials.
– Used to machine forging die cavities.
– Used to produce small holes.
– Burr free surface.
– No thermal damage to part.
– Lack of tool forces prevents distortion of part
– No tool wear.
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Parts Made by Electrochemical Machining
Figure 27.7 Typical parts made by electrochemical machining. (a) Turbine blade made of nickel
alloy of 360 HB. Note the shape of the electrode on the right. (b) Thin slots on a 4340-steel rollerbearing cage. (c) Integral airfoils on a compressor disk.
27.4 Electrochemical grinding
• Combines ECM with conventional grinding.
• Wheel is metal bonded with diamond or AL2O3 abrasives, and
rotates at a surface speed of 1200-2000m/min.
• The abrasive has 2 functions:
1. Insulator between wheel and work piece.
2. Mechanically remove electrolytic products from the working
area.
• Majority of metal removal in ECG is by electrolytic action, and
less than 5% is removed by abrasive action of wheel. So wheel
wear is minimum and the work piece remains cool.
• ECG process is suitable for applications similar to those for
milling, grinding, and sawing.
• Not adaptable to cavity sinking operations.
• Successfully applied to carbides and high strength alloys.
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Electrochemical-Grinding Process
Figure 27.9 (a) Schematic illustration of the electrochemical-grinding process.
(b) Thin slot produced on a round nickel-alloy tube by this process.
27.5 Electrical Discharge Machining (EDM)
• A shaped tool (electrode) and work piece connected to a dc
power supply and placed in a dielectric fluid.
• When potential difference between tool and workpiece is
sufficiently high, a transient spark discharges through the fluid,
removing a very small amount of metal from workpiece surface.
• The capacitor discharge is repeated at rates between 50kHz &
500kHz, with voltages 50-380V and currents 0.1-500A.
• The functions of the dielectric fluid:
1. Act as an insulator until the potential is sufficiently high
2. Act as a flushing medium and carry away the debris in the
gap
3. A cooling medium.
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Electrical-Discharge Machining Process
Figure 27.10 (a) Schematic illustration of the electrical-discharge machining process. This is one of the
most widely used machining processes, particularly for die-sinking applications. (b) Examples of cavities
produced by the electrical-discharge machining process, using shaped electrodes. Two round parts (rear)
are the set of dies for extruding the aluminum piece shown in front (see also Fig. 19.9b). (c) A spiral cavity
produced by EDM using a slowly rotating electrode similar to a screw thread. (d) Holes in a fuel-injection
nozzle made by EDM; the material is heat-treated steel. Source: (b) Courtesy of AGIE USA Ltd.
27.5 Electrical discharge machining
• The gap between tool and workpiece is critical, thus downward feed of
tool is controlled by a servomechanism.
• Dielectric fluid: mineral oils, kerosene, distilled and deionized water.
• EDM can be used on any material that is an electrical conductor.
• As the melting point and the latent heat of melting increase, rate of
material removal decreases.
• Volume of material removed per discharge: 10-6 to 10-4 mm3.
• Removal rate and surface roughness increase with increasing current
density & decreasing frequency of sparks.
• Electrodes: graphite, brass, Cu, Cu-tungsten alloy.
• Electrodes as small as 0.1mm in diameter, and depth to hole diameter
ratio of 400.
• Tool wear is related to the melting points of the materials involved.
• The lower the melting point, the higher the wear rate. Graphite has the
highest wear resistance.
Process capabilities:
• Internal cavities can be produced by using a rotating electrode with a
movable tip.
• Metal removal rates: 2-400mm3/min.
• Because of the molten and re-solidified surface structure, high rates
produce a very rough surface finish with poor surface integrity and low
fatigue properties.
Stepped Cavities Produced by EDM
Process
Figure 27.11 Stepped cavities produced with a square electrode by the EDM
process. The workpiece moves in the two principle horizontal directions (x – y), 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.
Wire EDM
• Used to cut plates as thick as 300mm, making punches,
tools, and dies from hard metals.
• Wire is usually made of brass, Cu, or, W.
• Wire diameter: 0.3mm for rough cuts and 0.2mm for finish
cuts.
• Wire is used only once.
• It travels at a constant velocity of 0.15-9m/min.
• Cutting speed is generally given in terms of cross
sectional area cut per unit time.
• Examples: 18000mm2/hr for 50mm thick D2 tool steel, and
45000mm2/hr for 150mm thick Al.
• these removal rates indicate a linear cutting speed of
360mm/hr, and 300mm/hr respectively.
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The Wire EDM Process
Metal removal rate :
MRR  4 10 4 ITw1.23
where
I  current in amperes
Tw  melting temperature of workpiece, C
Figure 27.12 Schematic illustration of the

wire EDM process. As many as 50 hours of
machining can be performed with one reel of
wire, which is then discarded.
Electrical discharge Grinding (EDG)
• Grinding wheel is made of graphite or brass and
contains no abrasives.
• Material is removed from workpiece surface by
repetitive spark discharges between the rotating wheel
and the workpiece.
• In sawing with EDM, a setup similar to a band or
circular saw (but without teeth) is used with the same
electrical circuit for EDM. Narrow cuts can be made at
high rates of metal removal.
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27.6 Laser Beam Machining (LBM)
• Source of energy is a laser which focuses optical energy on
surface of workpiece.
• The highly focused, high density energy melts and evaporates
portions of workpiece in a controlled manner.
• No vacuum involved.
• Used to machine a variety of metallic and nonmetallic
materials.
• The lower the reflectivity and thermal conductivity of workpiece
surface and its specific heat and latent heats of melting and
evaporation, the more efficient the process.
• The surface produced by LBM is usually rough and has a heataffected zone.
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Laser-Beam
Machining
(LBM)
Figure 27.14 (a) Schematic
illustration of the laser-beam
machining process. (b) and (c)
Examples of holes produced in
nonmetallic parts by LBM. (d)
Cutting sheet metal with a laser
beam. Source: (d) Courtesy of
Rofin-Sinar, Inc.
27.6 Laser Beam Machining (LBM)
Process capabilities:
• Widely used for drilling and cutting metals, nonmetallic
materials, ceramics, and composite materials.
• Holes as small as 0.005mm, with hole depth to diameter
ratios of 50.
• Steel plates as thick as 32mm can be cut.
• Typical applications: bleeder holes for fuel pump
covers and lubricant holes in transmission hubs.
• The inherent flexibility of laser cutting process, with its
fiber-optic beam delivery, simple fixturing, and low setup
times, and the availability of multi-kW machines and 2D
and 3D computer controlled laser cutting systems are
attractive features.
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27.7 Electron beam machining (EBM)
• Source of energy in EBM is high velocity electrons, which
strike surface of workpiece and generate heat.
• The machines utilize voltages in the range of 50-200KV to
accelerate electrons to speed of 50-80% of the speed of light.
• EBM requires vacuum. Consequently, it is used much less
than laser-beam machining.
• EBM can be used for very accurate cutting of a wide variety of
metals.
• Surface finish is better and kerf width is narrower than that for
other thermal cutting processes.
• The interaction of the electron beam with workpiece surface
produces hazardous x-rays.
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Electron-Beam Machining Process
Figure 27.15 Schematic illustration of the electron-beam machining
process. Unlike LBM, this process requires a vacuum, so workpiece size
is limited to the size of the vacuum chamber.
Plasma Arc Cutting (PAC)
• Plasma beams (ionized gas)
are used to rapidly cut ferrous
and nonferrous sheets and
plates.
• Temperatures generated are
very high (9400oC in the torch
for oxygen as plasma gas).
• Process is fast
• Kerf width is small
• Surface finish is good.
• Parts as thick as 150mm can
be cut.
• Material removal rates are
much higher than those
associated with EDM and LBM
processes.
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Plasma Arc Cutting (PAC)
Illustrated Guide to Plasma Gas Selection:
Plasma Gas / Mild Steel
Shield
Stainless
Aluminum
Air / Air
Good cut
quality/speed
Economical
Good cut quality/speed
Economical
Not recommended
Not recommended
Good cut quality/speed.
Economical
Oxygen (O2) / Excellent cut
Air
quality/speed. Very little
dross
Nitrogen (N2) Fair cut quality, some
Good cut quality
/ CO2
dross. Excellent parts life Excellent parts life
Excellent cut quality.
Excellent parts life
Nitrogen (N2) Fair cut quality, some
Good cut quality
/ Air
dross. Excellent parts life Excellent parts life
Good cut quality
Excellent parts life
Nitrogen (N2) Fair cut quality, some
Excellent cut quality. Excellent cut quality.
/ H20
dross. Excellent parts life Excellent parts life
Excellent parts life
Argon
Hydrogen /
N2
Not recommended
Excellent on thick
>1/2"
Excellent on thick >1/2"
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27.8 Water jet machining
• Water jet acts like a saw and cuts a narrow groove in the
material.
• Pressure level of about 400MPa is generally used for efficient
operation. May reach 1400 MPa.
• Jet nozzle diameters: 0.05-1mm.
• Materials cut: plastics, fabrics, rubber, wood, paper, leather,
brick, and composite materials.
• Thickness can range up to 25mm and higher.
• Advantages:
– cuts can be started at any location without the need of
predrilled holes.
– no heat is produced, and no deflection of the rest of the
workpiece takes place
– little wetting of workpiece, and minimum burr
– environmentally safe.
http://www.omax.com/learn/how-does-waterjet-work
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WaterJet
Cutting
Process
Figure 27.16 (a) Schematic illustration of the water-jet machining process. (b) A
computer-controlled water-jet cutting machine cutting a granite plate. (c) Examples of
various nonmetallic parts produced by the water-jet cutting process. (Enlarged on next
slide). Source: Courtesy of Possis Corporation
Abrasive water jet machining (AWJM)
• the water jet contains abrasive particles (silicon
carbide or Al2O3) which increase the material
removal rate above that of water jet machining.
• Suitable for heat sensitive materials that can not
be machined by processes in which heat is
produced.
• Cutting speeds: as high as 7.5m/min for reinforced
plastics, but much lower for metals.
• Min hole size = 3mm
• Max hole depth = 25mm
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27.9 Abrasive Jet Machining (AJM)
• High velocity jet of dry air, N2, CO2 containing abrasive
particles is aimed at the workpiece surface under controlled
conditions.
• Typical operations:
– cutting small holes, slots in very hard or brittle metallic and
nonmetallic materials.
– De-burring or removing small flash from parts
– trimming and beveling
– removing oxides and other surface films
– general cleaning of components with irregular surfaces.
• Gas supply pressure: 850kPa
• Abrasive jet velocity can be as high as 300m/s.
• Abrasive size: 10-50 µm.
• Some hazard involved because of airborne particulates.
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Abrasive-Jet
Machining
(b)
Figure 27.17 (a) Schematic illustration of the abrasive-jet machining process. (b) Examples of parts
produced through abrasive-jet machining, produced in 50-mm (2-in.) thick 304 stainless steel.
Source: Courtesy of OMAX Corporation.
27.10 Hybrid Machining Systems
• Two or more machining processes are combined into one
system to take advantage of the capabilities of each process
increasing production speed and thus improving the efficiency of
the operation.
• Examples of such systems include combinations and
integration of the following processes:
A.
B.
C.
D.
Abrasive machining and electrochemical machining
Abrasive machining and electrical discharge machining
Abrasive machining and electrochemical finishing
Water-jet cutting and Wire EDM
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27.10 Hybrid Machining Systems
E. High-speed milling, laser ablation, as an example of
integrated processes.
F. Machining and blasting
G. Electrochemical and electrical discharge machining
(ECDM), also called electrochemical spark machining
(ECSM).
H. Machining and forming processes, such as laser cutting
and punching of sheet metal.
I. Combinations of various forming, machining, and joining
processes.
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27.10 Hybrid Machining Systems
• The implementation of these concepts and the development of
machinery and control systems present significant challenges.
• Important considerations include factors such as:
A. The workpiece material and its manufacturing characteristics
B. Compatibility of processing parameters, such as speeds, sizes, forces,
energies, and temperature, among the two or more processes to be
integrated
C. Cycle times of each individual operation involved.
D. Possible adverse effects of the presence of various elements such as
abrasives, chemicals, wear particles, chips, and contaminants on the
overall operation.
E. Consequence of a failure in one of the stages in the system, since the
operation involves sequential processes
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27.11 Economics of Advanced Machining
Processes
• The economic production run for a particular process depends on
the costs of tooling and equipment, the operating costs, the
material-removal rate required, and the level of operator skill
required, as well as on secondary and finishing operations.
• In chemical machining (slow process), an important factor is the
cost of reagents, maskants, and disposal-together with the cost of
cleaning the parts.
• In electrical-discharge machining (slow process), the cost of
electrodes and the need to periodically replace them can be
significant.
• The rate of material removal and the production rate can vary
significantly in these processes.
• The high capital investment for machines (electrical and highenergy-beam machining has to be justified in terms of the
production runs and the feasibility of manufacturing the same part
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by other means if at all possible.