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Nontraditional Machining and

Thermal Cutting Processes� Nontraditional machining refers to a group a processes

which removes excess material by various techniques

involving mechanical, thermal, electrical or chemical

energy� These processes do not use a sharp cutting tool in the

conventional sense



� Nontraditional processes have been developed in

response to new and unusual machining requirements,

including

± The need to machine newly developed materials with special

properties (high strength, high hardness, high toughness)

±

The need for unusual and/or complex geometries ± The need to avoid surface damage



� Classification of nontraditional manufacturing

processes by principle form of energy

± Mechanical - mechanical energy in some form different from

the action of a conventional cutting tool; erosion of the

workpiece material is typical

± Electrical - electrochemical energy to remove material



± Thermal - thermal energy generally applied to a small portion

of the work surface, causing removal by fusion and/or

vaporization; thermal energy is generated by conversion of

electrical energy

± Chemical - most materials are susceptible to chemical attack

by certain acids or other etchants; chemicals selectively remove

material from portions of the workpiece, while other portionsof the surface are protected



� Available nontraditional material removal processes

± Mechanical

� AFM - abrasive flow machining

� AJM - abrasive jet machining

� HDM - hydrodynamic machining

�

LSG - low stress grinding� RUM - rotary ultrasonic machining

� TAM - thermally assisted machining

� TFM - total form machining

� USM - ultrasonic machining

� WJM - water jet machining



± Electrical

� ECD - electrochemical deburring

� ECDG - electrochemical discharge grinding

� ECG - electrochemical grinding

� ECH - electrochemical honing

� ECM - electrochemical machining

� ECP - electrochemical polishing

� ECS - electrochemical sharpening

� ECT - electrochemical turning

� ES - electro-stream

� STEM - shaped tube electrolytic machining



± Thermal

� EBM - electron beam machining

� EDG - electrical discharge grinding

� EDM - electrical discharge machining

� EDS - electrical discharge sawing

� EDWC - electrical discharge wire cutting

� LBM - laser beam machining

� LBT - laser beam torch

� PBM - plasma beam machining



± Chemical

� CHM - chemical machining

� ELP - electropolish

� PCM - photochemical machining

� TCM - thermochemical machining

� TEM - thermal energy machining

� While many processes are available, only the most

commercially important processes are discussed here



Mechanical Energy Processes

� Ultrasonic machining (USM)

± Abrasives contained in a slurry are driven at high velocity

against the work by a tool vibrating at low amplitude (.003in)

and high frequency (20-100khz)

± The tool oscillates in a direction perpendicular to the

workpiece surface and is fed slowly into the workpiece so that

the shape of the tool is formed in the part



± The action of the abrasives impinging against the work surface

performs the cutting ± Tool materials - soft steel, stainless steel

± Abrasive materials - boron nitride, boron carbide, aluminum

oxide, silicon carbide and diamond

± The vibration amplitude should be set approximately equal to

the grit size, and the gap size should be maintained at abouttwo times the grit size





± The ratio of work material to tool material removed during the

cutting process ranges from ~100:1 for cutting glass down to

~1:1 for cutting tool steel

± Workpiece materials: hard and brittle such as ceramics, glass

and carbides; successfully used on certain metals such as

stainless steel and titanium

± Shapes obtained by USM include nonround holes, holes along

a curved axis and coining operation, in which an image pattern

on the tool is imparted to a flat work surface



� Water jet cutting (WJC)

± Nozzle diameter: 0.004-0.016 in

± Pressure: up to 60,000psi

± Jet velocity: up to 3000 ft.Sec

± Nozzle made of sapphire, ruby or diamond

± Cutting fluids: polymer solutions; preferred because of their

tendency to produce a coherent stream



± Important process parameters: standoff distance, nozzle

operating diameter, water pressure and cutting feed rate

± Typical feed rates: 12 in/min to well over 1200 in/min

± The water jet cutting process is usually automated using CNC

robots to manipulate the nozzle unit along the desired

trajectory

± Materials cut by water jet: plastic, textile, composites, tiles,

carpet, leather and cardboard



± Advantages: no crushing or burning of the work surface,

minimum material loss because of the narrow cut slit, noenvironmental pollution, and easy automating the process

± Limitation: not suitable to cut brittle material because of their

tendency to crack during cutting





� Abrasive water jet cutting (AWJC)

± Introduction of abrasive particles into the stream adds to the

number of parameters that must be controlled; among these

are: abrasive type, grit size and flow rate

± Type of abrasive materials: aluminum, oxide, silicon dioxide

and garnet (a silicate mineral)

± Grit size: ranges between 60 and 120

± Flow rate: approximately 0.5 lb/min

± Nozzle orifice diameter: 0.010 - 0.025in; somewhat larger that

in water jet cutting to permit higher flow rates and more

energy to be contained in the stream prior to the infection of

abrasives



� Abrasive jet machining (AJM)

± A high velocity stream of gas containing small abrasive

particles

± Pressure: 25 - 200 psi

± Nozzle orifice diameter: 0.003 - 0.040 in

± Velocities: 500 - 1000 ft/min

± Gases: dry air, nitrogen, carbon dioxide and helium

±

The process is usually carried out manually by an operator ± AJM is normally used as a finishing process

± Applications: deburring, trimming and deflashing, cleaning

and polishing

± Applied on hard, brittle materials (glass, silicon, mica and

ceramics) that are in the form of thin flat stock ± Typical abrasives: aluminum oxide (for aluminum and brass),

silicon carbide (for stainless steel and ceramics), and glass

beads (for polishing)



± Grit sizes are small, 15-40Qm in diameter and must be very

uniform in size for a given application

± No recycling of abrasives; abrasive grains are fractured, worn

and contaminated



Electrochemical Machining Processes

� Electrochemical machining (ECM)

± It removes metal from an electrically conductive workpiece by

anodic dissolution, in which the shape of the workpiece is

obtained by a formed electrode tool in close proximity to, butseparated from the work by a rapidly flowing electrolyte

± Underlying principle: material is deplated from the anode and

deposited onto the cathode in the presence of an electrolyte

bath







± The difference in ECM is that the electrolyte bath flows

rapidly between the two poles to carry off the deplatedmaterial

± The electrode tool, usually made of copper, brass or stainless

steel, is designed to posses approximately the inverse of the

desired final shape of the part

±

Gap distance: usually from 0.003 - 0.030 in ± A water solution of sodium chloride is commonly used as the

electrolyte



± Electrolyte serves for:

� Carrying off the material that has been removed from theworkpiece

� Removing hear and hydrogen bubbles created in the

chemical reactions of the process

± Removed material in the form of microscopic particles must be

separated from the electrolyte through centrifuge,sedimentation or other means

± Large amount of electrical power is required to perform ECM



± Voltage is kept relatively low to minimize arcing across the gap

± Use when:

� The material is very hard or difficult to machine or

� Where the workpiece geometry is difficult or impossible

to accomplish by conventional machining methods

± Typical ECM applications

� Die sinking

� Multiple hole drilling

� Holes that are not round

� Deburring



± Advantages:

� Little surface damage to the work part

� No burrs as in conventional machining

� Low tool wear

� Relatively high metal removal rates for hard and

difficult to machine metals

± Disadvantages

� Significant cost of electrical power to drive the

operation

� Problems of disposing of the electrolyte sludge



� Electrochemical deburring (ECD)

± An adaptation of ECM designed to remove burrs or round

sharp corners

± The same ECM principles of operation apply to ECD

± Much less material is removed in ECD, thus cycle times are

much shorter



� Electrochemical grinding (ECG)

± Special form of ECM

± A rotating grinding wheel with a conductive bond material is

used to augment the anodic dissolution of the metal workpart

surface

± Bond material: metallic (diamond abrasives) or resin bond

impregnated with metal particles (aluminum oxide)

± Most of the machining is accomplished by electrochemical

action, therefore the grinding wheel lasts much longer



± Applications:

� Sharpening of cemented carbide tools

� Grinding of surgical needles, other thin wall tubes and

fragile parts







Thermal Energy Processes

� Electric discharge machining (EDM)

± One of the most widely used nontraditional processes

± Shape of the finished work surface is produced by a formed

electrode tool

± EDM process must take place in the presence of a dielectric

fluid





± Discharge region heated to extremely high temperature so that

a small portion of the work surface is melted and removed ± Individual discharges occur hundreds or thousands of times

per second to give a gradual erosion of the entire surface

± Process variables:

� Discharge current

� Frequency of discharges





± The high spark temperature causes the tool to melt, resulting

in a small cavity opposite the cavity produced in the work ± Wear ratio:

� Work material removed/tool material removed

� Ranges from 1.0 - 100 depending on the combination of

work and electrode materials

± Electrode materials: graphite, copper, brass, copper tungsten,silver tungsten, etc.

± Metal removal rate:

� MRR = K I/Tm1.23



± Dielectric fluids used: hydrocarbon oils, kerosene and distilled

or deionized water ± Applications:

� tool fabrication and parts production

� delicate parts

� hole drilling with hole axis at an acute angle to the surface

� production machining of hard and exotic metals



� Electric Discharge Wire Cutting (EDWC or wire EDM)

± special form of EDM using a wire as the electrode

± cutting action achieved by thermal energy from electric

discharges between the electrode wire and the workpiece



± Workpiece fed continuously and slowly past the wire to achieve

cutting path ± NC used to control workpart motions

± Wire EDM must be carried out in the presence of a dielectric

± Wire diameters: 0.003 - 0.012 in.

± Wire materials: brass, copper, tungsten and molybdenum

± Dielectric fluids: deionized water or oil



± Overcut ranges from 0.0008 - 0.002in.And remains fairly

constant and predictable once cutting conditions areestablished



� Electron Beam Machining (EBM)

± A high velocity stream of electrons is focused on the workpiecesurface to remove material by melting and vaporization

± Electron beam gun accelerates a stream of electrons to ~3/4 c

and focused through an electromagnetic lens

± Kinetic energy of beam converted to thermal energy of

extremely high density, melting or vaporizing material in avery localized area

± EBM must be carried out in a vacuum



± Can be used on any known material

± Applications:� drilling of extremely small diameter holes - down to 0.002

in

� drilling holes with high depth/diameter ratios, greater than

100:1

± Limitations:

� need of a vacuum

� high energy required

� expensive equipment



� Laser Beam Machining (LBM)

± Uses light energy from a laser to remove materials byvaporization and ablation

± Types of lasers:

� CO2

� solid-state

± Energy is concentrated optically and in terms of time

± Light beam pulsed so that the released energy results in an

impulse against the work surface, producing evaporation and

melting



± Used for:

� drilling - down to 0.001 in� slitting

� slotting

� scribing

� marking

± Not considered a mass production process; generally used on

thin stock

± Range of work materials virtually unlimited





� Plasma Arc Cutting (PAC)

± Plasma - a superheated, electrically ionized gas

± PAC uses a plasma stream operating at temperatures in the

range from 18,000o - 25,000o F to cut metal

± The high-velocity plasma stream is directed at the workpiece,

melting it and blowing the molten metal through the kerf



± Plasma arc generated between an electrode inside the torch

± Plasma flows through a water-cooled nozzle, which constrictsand directs the stream

± Hot enough to cut through metal 6 in thick

± Gases used:

� nitrogen, argon-hydrogen or a mixture (primary gases)

� secondary gases or water directed to surround the plasma

jet to confine the arc and clean the kerf



± Most applications consist of cutting flat metal sheets and plates

± Can be used to cut nearly any electrically conductive metal ± Feed rates:

� as high as 430 in/min for 1/4 in. aluminum

� 200 in/min for 1/4 in. steel

� 20 in/min for 4 in. aluminum



± Advantage: high productivity

± Disadvantages: rough cut surface, metallurgical damage



� Air Carbon Arc Cutting

± arc generated between a carbon electrode and the metallicwork

± High-velocity air jet used to blow away the melted portion of

the metal

± Used to form a kerf for severing the piece or to gouge a cavity

in the pat ± Used on a variety of metals, including cast iron, carbon steel,

low alloy and stainless steels

± Sputtering of molten metal is a hazard



� Other Arc Cutting Processes

± Gas metal arc cutting

± Shielded metal arc cutting

± Gas tungsten arc cutting

± Carbon arc cutting



Chemical Machining

� Mechanics and Chemistry of Chemical Machining

± Differences in applications and the ways in which the steps are

implemented account for the different forms of CHM; the

steps are

� Cleaning - to ensure that material will be removed

uniformly from the surfaces to be etched



� Masking - maskant, chemically resistant to the etchant,

applied to portions of the work surface not to be etched� Etching - the material removal step; part immersed in an

etchant that chemically attacks unmasked portions; part

removed and washed when desired amount of material has

been removed

�

Demasking - maskant removed from the part ± Masking and etching involve significant variations in methods,

materials and process parameters



± Maskant materials: neoprene, polyvinyl chloride, polyethylene

and other polymers ± Masking methods

� Cut and peel - performed by hand, used for large

workparts, low production quantities and where accuracy

is not a critical factor

�

Photographic resist - normally applied where small partsare produced in high quantities and close tolerances are

required

� Screen resist - used in applications that are between the

other two masking methods in terms of accuracy, part size

and production quantity



± Etchant selection - depends on work material, desired depth

and rate of etch, and surface finish requirements



� Chemical Milling

± First CHM process to be commercialized

± Used largely in the aircraft industry

± Applicable to large parts where substantial amounts of metal

are removed

± Cut and peel maskant method employed

± As depth increases, surface finish becomes worse

± Metallurgical damage very small





� Chemical Blanking

± Uses chemical erosion to cut very thin sheet-metal parts, downto 0.001 in. and/or for intricate cutting patterns

± Produces burr free parts

± Photoresist or screen resist method applied

± Maximum stock thickness ~0.030 in.

± Hardened or brittle materials can be processed





� Chemical Engraving

± A

chemical machining process used for making flat panels thathave lettering and/or artwork on one side

± Can be used to make raised or recessed lettering by reversing

the portions of the panel to be etched

± Masking done by either photoresist or screen resist methods

± Filling operation to apply paint or other coating followsetching



� Photochemical Machining (PCM)

±

Chemical machining in which the photoresist masking methodis used

± Employed in metalworking when close tolerances and/or

intricate patterns are required

± Used extensively in electronics industry (makes VLSI possible)

±

Photoresist materials in current use are sensitive to UV light,but not other wavelengths



± No need to carry out process in a darkroom

± Anisotropy: depth of cut d divided by undercut u; reciprocal of the etch factor

� A=1/Fe = d/u

� A: degree of anisotropy

� Fe: etch factor





Application Considerations

� Workpart Geometry Features

± Very small holes - (below 0.005 in. in diameter) use LBM

± Holes with large depth/diameter ratios - (d/D > 20) use

ECM and EDM

± Nonround holes - use EDM and ECM

± Narrow slots that are not straight - use EBM, LBM, wire

EDM, WJC and AWJC



± Micromachining - use PCM, LBM and EBM

± Shallow pockets and surface details in flat parts - use CHMand its variations

± Creation of special contoured shapes for mold and die

applications - (die sinking) use EDM and ECM



� Work Materials



� Performance of Nontraditional Processes

± Nontraditional processes are generally used when conventionalmethods are not practical or economical

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