kalpakjian 15

AdvancedMachiningProcesses

ERPTACH

° It is often necessary to machine or finish products that are made of especiallyhard or strong materials. In that case, the conventional machining and grindingstrategies described thus far become impractical.

° This chapter describes advanced machining processes that are based on nonme-chanical means of material removal. The chapter begins by examining chemicalmachining and blanking and photochemical blanking processes, in whichmaterial is removed through the corrosive action of fluid.

° Electrochemical machining and grinding are then described, where material is

removed by the action of an electrical power source and ion transfer inside anelectrolytic fluid.

° Electrical-discharge machining removes material by melting small portions ofthe workpiece by a spark.

° High-energy beams in processes such as laser and electron beam machininghave been used extensively, and these are also described, as are water-jet andabrasive-jet machining operations with unique applications.

° The chapter ends with an examination of trends in hybrid machining and theeconomics of advanced machining operations.

Typical parts made: Skin panels for missiles and aircraft, turbine blades, nozzles,parts with complex cavities and small-diameter deep holes, dies, laser cutting ofsheet metals, cutting of thick metallic and nonmetallic parts.

Alternative methods: Abrasive machining, ultrasonic machining, and precisionmachining.

27.l Introduction

The machining processes described in the preceding chapters involved materialremoval by mechanical means: chip formation, abrasion, or microchipping. However,there are situations in which mechanical methods are not satisfactory, economical, oreven possible, for the following reasons:

° The strength and hardness of the workpiece material are very high, typicallyabove 400 HB (see Fig. 2.15).

27.l Introduction 75927.2 Chemical Machining 76|27.3 Electrochemical

Machining 76527.4 Electrochemical

Grinding 76827.5 Electrical-discharge

Machining 76927.6 Laser-beam

Machining 77427.7 Electron-beam

Machining 77727.8 Water-jet

Machining 77827.9 Abrasive-jet

Machining 77927.|0 Hybrid Machining

Systems 78027.l l Economics of Advanced

Machining Processes 78|

EXAMPLE:

27.l Combining Laser Cuttingand Punching of SheetMetal 776

CASE STUDIES:

27.l ElectrochemicalMachining of a BiomedicalImplant 767

27.2 Manufacture of SmallSatellites 78|

159

Chapter 27 Advanced Machining Processes

Turbine blade

electrode

(3) (D)

FIGURE 27.I Examples of parts made by advanced machining processes. (a) Samples of partsproduced by water-jet cutting. (b) Turbine blade, produced by plunge electrical-dischargemachining, in a fixture to produce the holes also by electrical-discharge machining. Source:(a) Courtesy of OMAX Corporation; (b) Courtesy of HI-TEK Mfg., Inc.

° The workpiece material is too brittle to be machined without damage to theworkpiece. This is typically the case with highly heat treated alloys, glass,ceramics, and powder-metallurgy parts.

° The workpiece is too flexible or too slender to withstand forces in machiningor grinding, or the parts are difficult to clamp in fixtures and work-holdingdevices.

° The shape of the part is complex (Fig. 27.1), including such features as inter-nal and external profiles or holes with high length-to-diameter ratios in veryhard materials.

° Special surface finish and dimensional tolerance requirements exist thatcannot be obtained by other manufacturing processes or are uneconomicalthrough alternative processes.

° The temperature rise during processing and residual stresses developed in theworkpiece are not desirable or acceptable.

The preceding difficulties led to the development of chemical, electrical,laser, and high-energy beams as energy sources for removing material frommetallic or nonmetallic workpieces, as outlined in Table 27.1. These advancedmethods, which in the past have been called nontraditional or unconventionalmachining, began to be introduced in the 1940s. As shall be seen, such processesremove material not by producing chips as in machining and grinding, but by

means such as chemical dissolution, etching, melting, evaporation, and hydrody-namic action-sometimes with the assistance of fine abrasive particles. A majoradvantage of these processes is that their efficiency is independent of workpiecehardness. When selected and applied properly, advanced machining processesoffer major technical and economic advantages over more traditional machiningmethods. This chapter describes those processes, including their characteristics,typical applications, limitations, product quality, dimensional accuracy, surfacefinish, and economics.

TABLE 21.l

Section 27.2 Chemical Machining 76|

General Characteristics of Advanced Machining Processes

Process parameters andtypical material-removal

Process Characteristics rate or cutting speed

Chemical machining Shallow removal on large flat or curved surfaces; 0.0025-0.1 mm/min.(CM) blanking of thin sheets; low tooling and equipment

cost; suitable for low-production runs

Electrochemical Complex shapes with deep cavities; highest rate of V; 5-25 DC; A; 1_5 -8 A/nnn2;machining (ECM) material removal among other nontraditional processes; 2_5-12 mm/niin, depending gn

expensive tooling and equipment; high power Current densityconsumption; medium-to-high production quantity

Electrochemical Cutting off and sharpening hard materials, such as A; 1-3 A/nnnz; typicallygrinding (ECG) tungsten-carbide tools; also used as a honing process; 25 inni3/5 per 1000 A

higher removal rate than grinding

Electrical-discharge Shaping and cutting complex parts made of hard V: 50-380; A: 0.1-500; typicallymachining (EDM)

Wire electrical-dischargemachining

Laser-beam machining(LBM)

Electron-beam machining(EBM)

Water-jet machining(WJM)

Abrasive water-jetmachining (AWJM)

Abrasive-jet machining

materials; some surface damage may result; also usedas a grinding and cutting process; expensive toolingand equipment

Contour cutting of flat or curved surfaces; expensiveequipment

Cutting and hole making on thin materials; heat-affected zone; does not require a vacuum; expensiveequipment; consumes much energy

Cutting and hole making on thin materials; very smallholes and slots; heat-affected zone; requires a vacuum;expensive equipment

Cutting all types of nonmetallic materials; suitablefor contour cutting of flexible materials; no thermaldamage; noisy

Single~layer or multilayer cutting of metallic andnonmetallic materials

Cutting, slotting, deburring, etching, and cleaning of

300 mm3/min

Varies with material andthickness

0.50-7.5 m/min

1-2 mm3/min

Varies considerably withmaterial

Up to 7.5 m/min

Varies considerably with(AJM) metallic and nonmetallic materials; tends to round off material

sharp edges; can be hazardous

27.2 Chemical Machining

Chemical machining (CM) was developed from the observation that chemicalsattack and etch most metals, stones, and some ceramics, thereby removing smallamounts of material from the surface. The CM process is carried out by chemicaldissolution using reagents or etchants, such as acids and alkaline solutions.Chemical machining is the oldest of the advanced machining processes and has beenused in engraving metals and hard stones, in deburring, and in the production ofprinted-circuit boards and microelectronic devices (see Chapters 28 and 29).

Chemical Milling. In chemical milling, shallow cavities are produced on plates,sheets, forgings, and extrusions, generally for the overall reduction of weight(Fig. 27.2). The process has been used on a wide variety of metals, with depths of

2 Chapter 27 Advanced Machining Processes

4 mm ,

sr 2 o ....f~ 'e"‘1* >

\\ 1 machining) 1 se\~~ s Chemically Secuon machined area fi nj

(3) (D)

FIGURE 27.2 (a) Missile skin-panel section contoured by chemical milling to improve thestiffness-to-Weight ratio of the part. (b) Weight reduction of space-launch vehicles by thechemical milling of aluminum-alloy plates. The plates are chemically milled after they have beenformed into shape by a process such as roll forming or stretch forming. The design of thechemically machined rib patterns can be modified readily at minimal cost.

Workpiece support 3rd Zniteps 1st Edge of maskant

Tank ,L

_ Unde t N* 'L Material removedQ rou

W°'k'°'e°e i;%%_-_*_-_L-J iChemica* : iciy §""’"“YGHQGHY ,,..,,- ,, V- .. 1. 7' Heating/Cooung ".' Jfiéli

Cons Workpiece

(H) (D)

FIGURE 27.3 (a) Schematic illustration of the chemical-machining process. Note that noforces or machine tools are involved in this process. (b) Stages in producing a profiled cavityby chemical machining; note the undercut.

removal as large as 12 mm. Selective attack by the chemical reagent on differentareas of the Workpiece surfaces is controlled by removable layers of material (calledmasking, Fig. 27.3a) or by partial immersion in the reagent.

The procedure for chemical milling consists of the following:

If the part to be machined has residual stresses from prior processing, thestresses first should be relieved in order to prevent warping after chemicalmilling.

2. The surfaces are degreased and cleaned thoroughly to ensure both good adhe-sion of the masking material and uniform material removal. Scale from heattreatment also should be removed.

3. The masking material is applied. Masking with tapes or paints (maskants) is a

common practice, although elastomers (rubber and neoprene) and plastics

Section 27.2

(polyvinyl chloride, polyethylene, and polystyrene) also are used. The maskantmaterial should not react with the chemical reagent.

4. If required, the maskant covering various regions that require etching is peeledoff by the scribe-and-peel technique.

5. The exposed surfaces are machined chemically with etchants, such as sodiumhydroxide (for aluminum), solutions of hydrochloric and nitric acids (forsteels), and iron chloride (for stainless steels). Temperature control and agitation(stirring) during chemical milling is important in order to remove a uniformdepth of material.

6. After machining, the parts should be washed thoroughly to prevent furtherreactions with, or exposure to, any etchant residues.

7. The rest of the masking material is removed and the part is cleaned andinspected. Note that the masking material is unaffected by the reagent, butusually is dissolved easily by a different type of solvent.

8. Additional finishing operations may be performed on chemically milled parts.

9. This sequence of operations can be repeated to produce stepped cavities andvarious contours (Fig. 27.3b).

Chemical milling is used in the aerospace industry to remove shallow layersof material from large aircraft components, missile skin panels (Fig. 27.2), andextruded parts for airframes. Tank capacities for reagents are as large as 3.7 ><

15 m. The process is also used to fabricate microelectronic devices and often is re-ferred to as Wet etching for these products (see Section 28.8.1). The ranges of sur-face finish and tolerance obtained by chemical machining and other machiningprocesses are shown in Fig. 27.4.

Some surface damage may result from chemical milling because of preferentialetching and intergmnular attack, which adversely affect surface properties. Thechemical milling of welded and brazed structures may result in uneven materialremoval. The chemical milling of castings may result in uneven surfaces caused byporosity in, and nonuniformity of, the material.

Chemical Blanking. Chemical blanking is similar to the blanking of sheet metals inthat it is used to produce features which penetrate through the thickness of the mate-rial (as shown in Fig. 16.4), with the exception that the material is removed by chem-ical dissolution rather than by shearing. Typical applications for chemical blankingare the burr-free etching of printed-circuit boards, decorative panels, and thin sheet-metal stampings, as well as the production of complex or small shapes.

Photochemical Blanking. P/ootoclveniical blanking (also called plvotoetclving) is amodification of chemical milling. Material is removed (usually from a flat, thin sheet)by photographic techniques. Complex, burr-free shapes can be blanked (Fig. 27.5 ) onmetals as thin as 0.0025 mm. Sometimes called photochemical machining, thisprocess is also used for etching.

The procedure in photochemical blanking consists of the following steps:

I. The design of the part to be blanked is prepared at a magnification of up to100 ><. A photographic negative is then made and reduced to the size of thefinished part. The reduced negative of the design is called artwork. The origi-nal (enlarged) drawing allows inherent design errors to be reduced by theamount of reduction (such as 100 ><) for the final artwork image.

2. The sheet blank is coated with a photosensitive material (photoresist) bydipping, spraying, spin casting, or roller coating and is then dried in an oven.This coating is often called the emulsion.

Chemical Machining


MECHANICAL1 Abrasive-flow machining 1ward Low-stress grinding = 1 Ultrasonic machining sal

ELECTRICAL(b) Electrochemical deburring _,aa iii Electrochemical grinding

Electrochemical milling (frontal)1111Hi Electrochemical milling (side wall)(C) 'R

(D) (dl 3( )

Ii? H”"~ I I il*

~~L »» ngall l1 1*iii

1(3) Electrochemical polishing

b Shaped tube electrolytic machining THERMAL Electron-beam machining i= ri ? I-1 Electrical-discharge grinding -1

1 Electrical-discharge machining (finishing) Er! S Electrical-discharge machining (roughing) 1 Laser-beam machining _ lil3 1 Plasma-beam machining

CHEMICAL

(a) Chemical machining ai 1 Photochemical machining in QQ(a) (b) n -1» ..f~ Eiecrropoiishing -1 . ;

CONVENTIONAL MACHINING

ilQ1llQi “Q Surface grinding 1 25 6.3 1.60 0.4 0.1 0.025 2500 500 125 25 5 1.25

50 12.5 3.12 0.8 0.2 0.05 0.012 1250 250 50 12.5 2.5

SurfaCer0u9hf1eSS,F<a(mm) Tolerance (1 mm >< 1053)

Note: (a) Depends on state of starting surface. 1 Average application (normally anticipated values)

Eb; I|'lar§"Um a"?l;i5 a'§;9e"e'a"Y fougher than “'°ke| a"°VS- IZ Less frequent application (unusual or precision conditions)c ig -curren- ensiyareas. ._

(d) LOW_Current_denSity areas Flare (special operating conditions)

FIGURE 21.4 Surface roughness and tolerances obtained in various machining processes.Note the Wide range Within each process (see also Fig. 2313). Source: Machining DataHandbook, 3rd ed. Copyright © 1980. Used by permission of Metcut Research Associates, Inc.

3. The negative is placed over the coated blank and exposed to ultraviolet light,which hardens the exposed areas.

4. The blank is developed, dissolving the unexposed areas.

5. The blank is then immersed into a bath of reagent (as in chemical milling) or

sprayed with the reagent, which etches away the exposed areas.

6. The masking material is removed, and the part is Washed thoroughly to removeall chemical residues.

Typical applications for photochemical blanking are fine screens, printed-circuitcards, electric-motor laminations, flat springs, and assorted components for minia-turized systems. Although skilled labor is required, tooling costs are low, the processcan be automated, and it is economical for medium- to high-production volume.

Photochemical blanking is capable of making very small partswhen traditional blanking dies (Section 16.2) are difficult to pro-duce. The process is also effective for blanking fragile workpiecesand materials.

The handling of chemical reagents requires precautions andspecial safety considerations to protect the workers against expo-sure to both liquid chemicals and volatile chemicals. Furthermore,the disposal of chemical by-products from this process is a majordrawback, although some by-products can be recycled.

Design Considerations for Chemical Machining. Design guide-lines for chemical machining are as follows:

Section 27.3 Electrochemical Machining 765

° Because the etchant attacks all exposed surfaces continu-ously, designs involving sharp corners, deep and narrowcavities, severe tapers, folded seams, or porous workpiecematerials should be avoided.

blankFIGURE 21.5 Various parts made by chemical

ing. Note the fine detail. Source: Courtesyof Buckbee-Mears, St. Paul, Minnesota.

° Because the etchant attacks the material in both vertical andhorizontal directions, undercuts may develop (as shown by the areas under theedges of the maskant in Fig. 27.3). Typically, tolerances of ;’:10% of the mate-rial thickness can be maintained in chemical blanking.

° In order to improve the production rate, the bulk of the workpiece should beshaped by other processes (such as machining) prior to chemical machining.

° Dimensional variations can occur because of size changes in deposited maskpattern (artwork) due to humidity and temperature. These variations can beminimized by properly selecting artwork media and by controlling both theenvironment in which the artwork is generated and the production area inthe plant.

° Many product designs are now made with computer-aided design systems(Chapter 38). However, product drawings must be translated into a protocolthat is compatible with the equipment that photochemically generates artwork.

o n o

27.3 Electrochemical Machining

Electrochemical machining (ECM) is basically the reverse of electroplating (Sec-tion 34.9). An electrolyte acts as current carrier (Fig. 27.6), and the high rate of elec-

trolyte movement in the tool-workpiece gap (typically 0.1 to 0.6 mm) washes metal

(T) |

ions away from the workpiece (anode) before theyhave a chance to plate onto the tool (cathode). Notethat the cavity produced is the female mating imageof the tool shape

The shaped tool either a solid or tubularform, is generally made of brass, copper, bronze, orstainless steel. The electrolyte is a highly conductiveinorganic fluid, such as an aqueous solution ofsodium nitrate. It is pumped through the passagesin the tool at rates of 10 to 16 m/s. A DC power sup-ply in the range from 10 to 25 V maintains currentdensities, which, for most applications, are 20 to200 A/cmz of active machined surface.

powerSUDDW

Insulatlngcoatrng

Workpiece Pump forcirculatingelectrolyte

4-1

Tool

Tank

Electrolyte

FIGURE 21.6 Schemmachining process.

atic illustration of the electrochemical

I f65 mmi`\X Electrolyte


The material-removal mte (MRR) in electrochemical machining for a currentefficiency of 100% may be estimated from the equation

MRR = CI (27.1)

where MRR is in mm3/min, I is the current in amperes, and C is a material constant,in mm3/A-min. For pure metals, C depends on the valence: The higher the valence,

the lower is its value of C.Machines having current capacities as high as 40,000 A and as low as 5 A are

available. The penetration rate of the tool is proportional to the current density. Thematerial removal rate typically ranges between 1.5 and 4 mm3 per A-min. Because

the metal-removal rate is a function only of the ion exchange rate, it is not affected

by the strength, hardness, or toughness of the workpiece.

Process Capabilities. The basic concept of electrochemical machining was patentedin 1929 and developed rapidly during the 1950-19605, whereupon it became an

important manufacturing process, although not as widely used as other processesdescribed in this chapter. It is generally used to machine complex cavities and shapesin high-strength materials, particularly in the aerospace industry for the mass produc-tion of turbine blades, jet-engine parts, and nozzles (Fig. 27.7), as well as in the auto-motive (engines castings and gears) and medical industries.

Electrochemical machining is also used for machining and finishing forging-diecavities (die sinking) and to produce small holes. Versions of this process are used forturning, facing, milling, slotting, drilling, trepanning, and profiling, as well as in the

production of continuous metal strips and webs. More recent applications of ECMinclude niicronmchining (Chapters 28 and 29) for the electronics industry.

A modification of ECM is shaped-tube electrolytic machining (STEM), used

for drilling small-diameter deep holes, as in turbine blades. The tool is a titaniumtube, coated with an electrically insulating resin. Holes as small as 0.5 mm can be

drilled, at depth-to-diameter ratios of as high as 30011.

T75 mm 140 mm e escopmg cover Ram

lnsulatlng ___A/-M ayer

\\ V Q Copper electrode I\/lachlned Electrodeworkpiece carrier

(H)

i 14 holes 86 mm

"`e§f;; M "" "<" 1 ` T: T is 112 mm ccc l iiiii (D) (C)

FIGURE 27.1 Typical parts made by electrochemical machining. (a) Turbine blade made of a

nickel alloy of 360 HB. Note the shape of the electrode on the right. (b) Thin slots on a 4340-steel roller-bearing cage. (c) Integral airfoils on a compressor disk.

Design Considerations for Electrochemical Machining. The following are general

Section 27.3 Electrochemical Machining 767

The ECM process leaves a burr-free, bright surface; in fact, it can also be usedas a deburring process. It does not cause any thermal damage to the part, and theabsence of tool forces prevents part distortion. Furthermore, there is no tool wear(since only hydrogen is generated at the cathode), and the process is capable of pro-ducing complex shapes. However, the mechanical properties of components made bythe ECM process should be compared carefully with those of components made byother material-removal methods.

Electrochemical-machining systems now are available as numerically con-trolled machining centers with the capability of high production rates, high flexibil-ity, and the maintenance of close dimensional tolerances. The ECM process also canbe combined with electrical-discharge machining (EDM) on the same machine(hybrid machining).

design guidelines for electrochemical machining:

° Because of the tendency for the electrolyte to erode away sharp profiles, elec-trochemical machining is not suited for producing sharp square corners or flatbottoms.

° Controlling the electrolyte flow may be difficult, so irregular cavities may notbe produced to the desired shape with acceptable dimensional accuracy.

° Designs should make provision for a small taper for holes and cavities to bemachined.

¢ o u

CASE STUDY 21.I Electrochemical Machining of a Biomedical Implant

A total knee-replacement system consists of a femoraland tibial implant combined with an ultrahigh-molecular-weight polyethylene (UHMWPE) insert, asshown in Fig. 27.8a. The polyethylene has superiorwear resistance and low friction against the cobalt-chrome alloy femoral implant. The UHMWPE insert

is compression molded (Section 19.7), and the metalimplant is cast and ground on its external matingsurfaces.

Designers of implants, manufacturing engi-neers, and clinicians have been concerned particu-larly with the contact surface in the cavity of the

Sodium-nitrate

electrode l,...

Cobalt-chromeimplant

(8) (bl

FIGURE 21.8 (a) Two total knee-replacement systems, showing metal implants (top pieces) with anultrahigh-molecular-weight polyethylene insert (bottom pieces). (b) Cross section of the ECM process asapplied to the metal implant. Source: Courtesy of Biomet, Inc.


metal implant that mates with a protrusion on thepolyethylene insert. As the knee articulates duringnormal motion, the polyethylene slides against themetal part, becoming a potentially serious wear site.This geometry is necessary to ensure lateral stabilityof the knee (that is, to prevent the knee from buck-ling sideways).

In order to produce a smooth surface, the grind-ing of the bearing surfaces of the metal implant usingboth hand-held and cam-mounted grinders was a

procedure that had been followed for many years.However, grinding produced marginal repeatabilityand quality. The interior surfaces of this part areextremely difficult to access for grinding, and thecobalt-chrome alloy is difficult to grind. Consequently,advanced machining processes (particularly electro-chemical machining) were ideal candidates for this

operation.

As shown in Fig. 27.8b, the current procedure con-sists of placing the metal implant in a fixture and bring-ing a tungsten electrode of the desired final contour in

close proximity to the implant. The electrolyte is a sodi-um nitrate and water mixture and is pumped throughthe tool, filling the gap between the tool and implant. A

power source (typically 10 V and 225 A) is applied, caus-ing local electrochemical machining of the high spots onthe implant surface and producing a polished surface.

The electrolyte flow volume can be controlledto maximize surface quality. When the flow rate is

too low, defects appear on the machined surface aslocalized dimples;'if the rate is too high, machiningtimes become longer. Typical machining times forthis part are four to six minutes.

Source: Courtesy of T. Hershberger and R. Redman,Biomet, Inc., Warsaw, Indiana.

27.3.l Pulsed Electrochemical Machining

The pulsed electrochemical machining (PECM) process is a refinement of ECM; it

uses very high current densities (on the order of 1 A/mmz), but the current is pulsedrather than direct current. The purpose of pulsing is to eliminate the need for highelectrolyte flow rates, which limit the usefulness of ECM in die and mold making(die sinking). Investigations have shown that PECM improves fatigue life, and theprocess has been proposed as a possible method for eliminating the recast layer lefton die and mold surfaces by electrical-discharge machining. The tolerances obtainedtypically are in the range from 20 to 100 /dm.

Machines can perform a combination of both EDM and PECM; thus, the needto move the tool and workpiece between the two processes is eliminated. However,it is difficult to maintain precise alignment when moving the workpiece from theEDM to the PECM. If misaligned significantly, all polishing will occur where thegap is smallest, and passivation will occur where the gap is largest. Also, the processleaves metal residues suspended in the aqueous solution, which is harmful to the en-vironment if discarded without treatment.

The ECM process can be effective for micromachining as well. The completeabsence of tool wear implies that the process can be used for precision-electroniccomponents, although the erosion problem due to stray current has to be overcome.ECM machines now have increasing flexibility by the implementation of numericalcontrols.

27.4 Electrochemical Grinding

Electrochemical grinding (ECG) combines electrochemical machining with conven-tional grinding. The equipment used is similar to a conventional grinder, except thatthe wheel is a rotating cathode embedded with abrasive particles (Fig. 27.9a). Thewheel is metal bonded with diamond or aluminum-oxide abrasives and rotates at a

surface speed from 1,200 to 2,000 m/min.

Section 27.5 Electrical discharge Machining

Electrolyte from pump

Electrode (grinding wheel)

Spindle

Electrical connection

Insulatingabrasive \ i gi icig Nparticles T

Workpiege ||'1SU|afll’1g 0.4 mm

(H) (bl

FIGURE 21.9 (a) Schematic illustration of the electrochemical-grinding process. (b) Thinslot produced on a round nickel-alloy (Inconel) tube by this process.

The abrasives have two functions: (1) to serve as insulators between the wheeland the workpiece and (2) to mechanically remove electrolytic products from theworking area. A flow of electrolyte solution (usually sodium nitrate) is provided forthe electrochemical machining phase of the operation. Current densities range from1 to 3 A/mmz. The majority of metal removal in ECG is by electrolytic action, andtypically, less than 5% of the metal is removed by the abrasive action of the wheel.Therefore, wheel wear is very low, and the workpiece remains cool. Finishing cutsusually are made by the grinding action, but only to produce a surface with goodfinish and dimensional accuracy.

The ECG process is suitable for applications similar to those for milling, grind-ing, and sawing (Fig. 27.9b), but it is not adaptable to cavity-sinking operations.EGC can be applied successfully to carbides and high-strength alloys. It offers adistinct advantage over traditional diamond-wheel grinding in processing very hardmaterials where wheel wear can be high. ECG machines are available withnumerical controls, which improve dimensional accuracy and repeatability andincrease productivity.

Electrochemical honing combines the fine abrasive action of honing with elec-trochemical action. Although the equipment is costly, this process is as much as 5 timesfaster than conventional honing, and the tool lasts as much as 10 times longer.Electrochemical honing is used primarily for finishing internal cylindrical surfaces.

Design Considerations for Electrochemical Grinding. In addition to those designconsiderations already listed for electrochemical machining, ECG requires twomore:

° Designs should avoid sharp inside radii.° If flat surfaces are to be produced, the electrochemically ground surface should

be narrower than the width of the grinding wheel.

27.5 Electrical-discharge Machining

The principle of electrical-discharge machining (EDM) (also called electrodischargeor spark-erosion machining) is based on the erosion of metals by spark discharges.We know that when two current-conducting wires are allowed to touch each other,


an arc is produced. If we look closely at the point of contact between the two wires,we note that a small portion of the metal has been eroded away, leaving a smallcrater.

Although this phenomenon has been known since the discovery of electricity,it was not until the 1940s that a machining process based on that principle was de-veloped. The EDM process has become one of the most important and widely used

production technologies in manufacturing.

Principle of Operation. The basic EDM system consists of a shaped tool(electrode) and the workpiece, connected to a DC power supply and placed in a

dielectric (electrically nonconducting) fluid (Fig. 27.10a). When the potential differ-ence between the tool and the workpiece is sufficiently high, the dielectric breaksdown and a transient spark discharges through the fluid, removing a very smallamount of metal from the workpiece surface. The capacitor discharge is repeatedat rates between 200 and 500 kHz, with voltages usually ranging between 50 and380 V and currents from 0.1 to 500 A. The volume of material removed per sparkdischarge is typically in the range from 10% to 10`4 mm3.

The EDM process can be used on any material that is an electrical conductor.The melting point and the latent heat of melting are important physical propertiesthat determine the volume of metal removed per discharge. As these quantities

CurrentRectifier control Servo control

lV|0\/able Worn electrode

Power supply E ;;f‘*“ ~“i‘1 Sparkj ii , i _..._r, Tank wglggece

(H)

5 3.5 mm

T( dia.

Workmece 8 holes, 0.17 mm ‘Wi

75%, 1.1-mm wall/iii fi I. ,1

Heat-treated steelElectrode

(bl (C) (dl

FIGURE 21.l0 (a) Schematic illustration of the electrical-discharge machining process. Thisis 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 shapedelectrodes. The two round parts (rear) are the set of dies used in extruding the aluminumpiece shown in front (see also Fig. 15.9b). (c) A spiral cavity produced by EDM using a slowlyrotating 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.

Section 27.5 Electrical discharge Machlmng

increase, the rate of material removal decreases. The material-removal fate can beestimated from the approximate empirical formula

MRR = 4 >< 104 IT,;1'23, (27.2)

where MRR is in mm3/min, I is the current in amperes, and Tw is the melting pointof the workpiece in °C.

The workpiece is fixtured within the tank containing the dielectric fluid,and its movements are controlled by numerically controlled systems. The gap be-tween the tool and the workpiece (oz/ercut) is critical. Thus, the downward feedof the tool is controlled by a servomechanism, which automatically maintains a

constant gap.Because the process doesn’t involve mechanical energy, the hardness, strength,

and toughness of the workpiece material do not necessarily influence the removalrate. The frequency of discharge or the energy per discharge, the voltage, and the

current usually are varied to control the removal rate. The removal rate and surfaceroughness increase with la) increasing current density and (b) decreasing frequencyof sparks.

Dielectric Fluids. The functions of the dielectric fluid are to

I. Act as an insulator until the potential is sufficiently high.

2. Provide a cooling medium.

3. Act as a flushing medium and carry away the debris in the gap.

The most common dielectric fluids are mineral oils, although kerosene anddistilled and deionized water also are used in specialized applications. Clear, low-viscosity fluids also are available; although more expensive, these fluids make clean-ing easier. The machines are equipped with a pump and filtering system for thedielectric fluid.

Electrodes. Electrodes for EDM usually are made of graphite, although brass,copper, or copper-tungsten alloys also are used. The tools can be shaped by form-ing, casting, powder metallurgy, or CNC machining techniques. Tungtsen-wire elec-

trodes as small as 0.1 mm in diameter have been used to produce holes withdepth-to-hole diameter ratios of up to 400:1.

The sparks in this process also erode away the electrode, changing its geom-etry and adversely affecting the shape produced and its dimensional accuracy.Tool (electrode) wear is thus an important factor. Wear ratio is defined as the ratioof the volume of workpiece material removed to the volume of tool wear. Thisratio ranges from about 3:1 for metallic electrodes to as high as 100:1 for graphiteelectrodes.

It has been shown that tool wear is related to the melting points of the materi-als involved: The lower the melting point of the electrode, the higher is the wearrate. Also, the higher the current, the higher is the wear. Consequently, graphite elec-

trodes have the highest wear resistance. Tool wear can be minimized by reversingthe polarity and using copper tools-a process called no-wear EDM.

Process Capabilities. Electrical-discharge machining has numerous applications,such as the production of dies for forging, extrusion, die casting, injection molding,and large sheet-metal automotive-body components (produced in die-sinkingmachining centers with computer numerical control). Other applications includedeep, small-diameter holes with tungsten wire used as the electrode, narrow slots in

parts, cooling holes in superalloy turbine blades, and various intricate shapes (see


FIGURE 21.Il Stepped cavities producedwith a square electrode by the EDM process.The workpiece moves in the two principalhorizontal directions, and its motion is

synchronized with the downward movementof the electrode to produce these cavities.Also shown is a round electrode capableof producing round or elliptical cavities.

Figs. 27.10b and c). Stepped cavities can be produced by controllingthe relative movements of the workpiece in relation to the electrode(Fig. 27.11 >.

Because of the molten and resolidified (recast) surface struc-ture, high rates of material removal produce a very rough surface fin-ish with poor surface integrity and low fatigue properties. Therefore,finishing cuts are made at low removal rates, or the recast layer is re-moved subsequently by finishing operations. It has been shown thatsurface finish can be improved by oscillating the electrode in a plane-tary motion at amplitudes of 10 to 100 /.rm.

Design Considerations for EDM. The general design guidelines forelectrical-discharge machining are as follows:

° Parts should be designed so that the required electrodes can beshaped properly and economically.

° Deep slots and narrow openings should be avoided.° For economic production, the surface finish specified should

not be too fine.

Source: Courtesy of AGIE USA, Ltd.° In order to achieve a high production rate, the bulk of material re-

moval should be done by conventional processes (roughing out).

27.5.l Wire EDM

A variation of EDM is wire EDM (Fig. 27.12), or electrical-discharge wire cutting.In this process, which is similar to contour cutting with a band saw (see Fig. 24.25),a slowly moving wire travels along a prescribed path, cutting the workpiece. Thisprocess is used to cut plates as thick as 300 mm and to make punches, tools, anddies from hard metals. It also can cut intricate components for the electronics indus-try. Figure 27.13a shows a thick plate being cut by this process, and a wire EDMmachine is shown in Fig. 27.13b.

The wire is usually made of brass, copper, tungsten, or molybdenum; zinc- orbrass-coated and multicoated wires also are used. The wire diameter is typically

Wire

-~'"f*Dieiecrric diameter - .. . . ..». a Spark gap

Workpie _i_

* Sim (ken)

ire .432 wMW_,,,...¢»<»" .-~ / guides LQ _,y ff Hee'

FIGURE 21.12 Schematic illustration of the wire EDM process. As many as

50 hours of machining can be performed with one reel of wire, which is thendiscarded.

about 0.30 mm for roughing cuts and0.20 mm for finishing cuts. The wireshould have high electrical conductivityand tensile strength, as the tension on itis typically 60% of its tensile strength.

The wire is usually used only once,as it is relatively inexpensive comparedwith the type of operation it performs. Ittravels at a constant velocity in the rangefrom 0.15 to 9 m/min, and a constantgap (kerf) is maintained during the cut.The trend in the use of dielectric fluids is

toward clear, low-viscosity fluids.The cutting speed generally is

given in terms of the cross-sectionalarea cut per unit time. Typical exam-ples are 18,000 mmf/hr for 50-mm thick

Section 27.5 Electrical discharge Machining

Workpiece

Wire -

Wire spool

(H) (D)

FIGURE 21.|3 (a) Cutting a thick plate with wire EDM. (b) A computer-controlled wireEDM machine. Source: Courtesy of AGIE USA, Ltd.

D2 tool steel and 45,000 mm;/hr for 150-mm thick aluminum. These removal ratesindicate a linear cutting speed of 18,000/50 2 360 mm/hr = 6mm/min and45,000/150 = 300 mm/hr = 5 mm/min, respectively.

Multiaxis EDM wire-cutting machining centers are capable of producingthree-dimensional shapes and are equipped with such features as

° Computer controls for controlling the cutting path of the wire (Fig. 27.13b)and its angle with respect to the workpiece plane.

° Multiheads for cutting two parts at the same time.° Features such as controls for preventing wire breakage.° Automatic self-threading features in case of wire breakage.° Programmed machining strategies to optimize the operation.

Two-axis computer-controlled machines can produce cylindrical shapes in amanner similar to a turning operation or cylindrical grinding. Many modern wireEDM machines allow the control of the feed and take-up ends of the wire to traverseindependently in two principal directions, so that tapered parts can be made.

21.5.2 Electrical-discharge Grinding

The grinding wheel in electrical-discharge grinding (EDC) is made of graphite orbrass and contains no abrasives. Material is removed from the workpiece surface byspark discharges between the rotating wheel and the workpiece. This process is usedprimarily for grinding carbide tools and dies but can also be used with fragile parts,such as surgical needles, thin-walled tubes, and honeycomb structures. The ECDGprocess is faster than EDG, but power consumption is higher. The EDG processcan be combined with electrochemical grinding. The process is then calledelectrochemical-discharge grinding (ECDG).

Material is removed by chemical action (with the electrical discharges from thegraphite wheel breaking up the oxide film) and is washed away by the electrolyteflow. The material-removal rate in EDG can be estimated from the equation

MRR = KI, (27.3)

where MRR is in mm3/min, I is the current in amperes, and K is a workpiecematerial factor in mm3/A-min. For example, K = 4 for tungsten carbide andK = 16 for steel.

Chapter Z7 Advanced Machining Processes

In sawing with EDM, a setup similar to a band or circular saw (but withoutany teeth) is used with the same electrical circuit for EDM. Narrow cuts can bemade at high rates of metal removal. Because cutting forces are negligible, theprocess can be used on thin and slender components as well.

27.6 Laser-beam Machining

In laser-beam machining (LBM), the source of energy is a laser (an acronym for lightamplification by stimulated emission of radiation), which focuses optical energy onthe surface of the workpiece (Fig. 27.14a). The highly focused, high-density energysource melts and evaporates portions of the workpiece in a controlled manner. Thisprocess (which does not require a vacuum) is used to machine a variety of metallicand nonmetallic materials.

There are several types of lasers used in manufacturing operations (Table 27.2):

CO2 (pulsed or continuous u/uve)Nd:YAG (neodymium: yttrium-aluminum-garnet)

Flash lampReflective end

Laser crystal _Partially reflective end M

° N , Lens lf”

(21)

3.4-mm dia.

_Tl F- ‘l`311 mmi tRubber

(bl

L T1 mm

Plastic

(C) (Cl)

FIGURE 21.|4 (a) Schematic illustration of the laser-beam machining process. (b) and(c) Examples of holes produced in nonmetallic parts by LBM. (dl Cutting sheet metal with a

laser beam. Source: (d) Courtesy of Rofin-Sinar, Inc.

Section 27.6 Laser beam Machmmg

TABLE 21.2

General Applications nf Lasers in Manufacturing

Application Laser type

CuttingMetals PCO2, CWCO2, Nd:YAG, rubyPlastics CWCO2Ceramics PCO;

DrillingMetals PCO2, Nd:YAG, Ndxglass, rubyPlastics Excimer

MarkingMetals PCO2, Nd:YAGPlastics ExcimerCeramics Excimer

Surface treatment CWCO2Welding

Metals PCO2, CWCO2, Nd:YAG, Ndaglass, ruby, DiodePlastics Diode, Nd:YAG

Note: P = pulsed, CW = continuous wave,

Ndzglass, rubyDiode lasers

Nd:YAG = neodymiumzyttrium-aluminum-garnet.

Excimer lasers (from the words excited and dimer, meaning two mers, or twomolecules of the same chemical composition).

Important physical parameters in LBM are the reflectivity and thermal con-ductivity of the workpiece surface and its specific heat and latent heats of meltingand evaporation. The lower these quantities, the more efficient is the process. Thecutting depth may be expressed as

CPif _ E, (27.4)

where t is the depth, C is a constant for the process, P is the power input, 1/ is thecutting speed, and d is the laser-spot diameter. Peak energy densities of laser beamsare in the range from 5 to 200 kW/mmz.

The surface produced by LBM is usually rough and has a heat-affected zone(as described in Section 30.9), which, in critical applications, may have to beremoved or heat treated. Kerf width is an important consideration, as it is in othercutting processes, such as sawing, wire EDM, and electron-beam machining.

Laser beams may be used in combination with a gas stream (such as oxygen)to increase energy absorption (laser-beam torch) for cutting sheet metals. High-pressure, inert-gas (nitrogen or argon) assisted laser cutting is used for stainless steeland aluminum; it leaves an oxide-free edge that can improve weldability. Gasstreams also have the important function of blowing away molten and vaporizedmaterial from the workpiece surface.

Process Capabilities. Laser-beam machining is used widely for drilling, trepan-ning, and cutting metals, nonmetallic materials, ceramics, and composite materials(Figs. 27.14b and c). The abrasive nature of composite materials and the cleanlinessof the operation have made laser-beam machining an attractive alternative to tradi-tional machining methods. Holes as small as 0.005 mm with depth-to-diameter


ratios of 50:1 have been produced in various materials, although a more practicalminimum is 0.025 mm. Steel plates as thick as 32 mm can be cut with laser beams.

Laser-beam machining is being used increasingly in the electronics and auto-motive industries. Bleeder holes for fuel-pump covers and lubrication holes in trans-mission hubs are being drilled with lasers, for example. The cooling holes in the firststage of producing vanes for the Boeing 747 jet engines also are produced by lasers.Significant cost savings have been achieved by laser-beam machining-a processthat is competing with electrical-discharge machining.

Laser beams are also used for the following applications:

° Welding° Small-scale and localized heat treating of metals and ceramics to modify their

surface mechanical and tribological properties° Marking of parts, such as letters, numbers, codes, etc. Marking also can be

done (a) with ink, (b) with mechanical devices such as punches, pins, styluses,and scroll rolls, (c) by stamping, and (d) by etching. Although the equipment is

more expensive than that used in other methods, marking and engraving withlasers has become increasingly common due to its accuracy, reproducibility,flexibility, ease of automation, and online application in manufacturing.

The inherent flexibility of the laser-cutting process-including its fiber-opticbeam delivery, simple fixturing, and low setup times, as well as the availability ofmulti-kW machines and two- and three-dimensional computer-controlled roboticlaser-cutting systems-is an attractive feature. Therefore, laser cutting for sheets cancompete successfully with the traditional punching processes described in Chapter 16.

There are efforts now to combine the two processes for improved overall efficiency.(See Section 27.10 and Example 27.1.)

Extreme caution should be exercised with lasers. Even low-power lasers cancause damage to the retina of the eye if proper safety precautions are not observed.

Design Considerations for LBM. General design guidelines for laser-beam machin-ing are as follows:

° Designs with sharp corners should be avoided, since they can be difficult toproduce.

° Deep cuts will produce tapered walls.° Reflectivity of the workpiece surface is an important consideration in laser-beam

machining; because they reflect less, dull and unpolished surfaces are preferable.° Any adverse effects on the properties of the machined materials caused by the

high local temperatures and heat-affected zone should be investigated.

EXAMPLE 27.| Combining Laser Cutting and Punching of Sheet Metal

As has been seen, laser cutting and punching processeshave their respective advantages and limitationsregarding both technical and economic aspects. (Seehybrid machining, Section 27.10.) The advantages oflaser cutting are generally (a) smaller batches, (b) theflexibility of the operation, (c) a wide range of thick-nesses, (d) prototyping capability, (e) materials andcomposites that otherwise might be cut with difficulty,and (f) complex geometries that can be programmed.

Advantages and drawbacks of punching include(a) required large lot sizes in order to economicallyjustify purchasing tooling, (b) relatively simple parts,(c) a small range of part thicknesses, (d) fixed andlimited punch geometries (even when using turrets),(e) rapid production, and (f) integration with subse-quent processing after punching.

It is evident that the two processes coverdifferent, but complementary, ranges. It is not difficult

to visualize parts with some features that can beproduced best by one process and other features thatare best produced by the other process.

Machines have been designed and built in such a

manner that the processes and fixturing can be utilized

jointly to their full extent, but without interfering witheach other’s operational boundaries. The purpose ofcombining them is to increase the overall efficiencyand productivity of the manufacturing process forparts that are within the capabilities of each of the twoprocesses-similar to the concept of the machiningcenters described in Section 25.2. For example, turret-punch presses have been equipped with an integrated

Section 27.7 Electron-beam Machining 777

laser head; the machine can punch or laser cut, but itcannot do both simultaneously.

Several factors have to be taken into account in

such a combination with respect to the characteristicsof each operation: (1) the ranges of sizes, thicknesses,and shapes to be produced and how they are to benested; (2) processing and setup times, including theloading, fixturing, and unloading of parts; (3) pro-gramming for cutting; and (4) the process capabilitiesof each method, including dynamic characteristics,vibrations, and shock from punching (and isolation)that may disturb adjustments and alignments of thelaser components.

27.7 Electron-beam Machining

The energy source in electron-beam machining (EBM) is high-velocity electrons,which strike the workpiece surface and generate heat (Fig. 27.15 ). The machines uti-lize voltages in the range from 50 to 200 kV to accelerate the electrons to speeds of

50 to 80% of the speed of light. Applications of this process are similar to those of

laser-beam machining, except that EBM requires a vacuum. Consequently, it is usedmuch less than laser-beam machining.

Electron-beam machining can be used for very accurate cutting of a wide varietyof metals. Surface finish is better and kerf width is narrower than in other thermalcutting processes. (See also Section 30.6 on electron-beam welding.) The interactionof the electron beam with the workpiece surface produces hazardous X-rays.Therefore, the equipment should be used only by highly trained personnel.

High-voltage cable (30 kV, Dc)

ai

Cathode gridL i I l 3 Anode

Optical viewing Electron streamsystem §

H Magnetic lensIZI 1

Viewing x a Deflection coilsport »_.. _.._ .._.....,...., _Y ...»~..1»,. ~..» )w...W,.1aa..- , “mamma

Vacuum chamber Workpie-Ce ." . . .. -> ' ..,, ,_;jf ‘... High'

§,.. ::=._.:n »»;.~.w»¢ena=.=2=a“=-, ir.>z::e=.~.':zi¢¢fmam=m~sf pu m p

FIGURE 21.l5 Schematic illustration of the electron-beam machining process. Unlike LBM,this process requires a vacuum, so the workpiece size is limited to the size of the vacuumchamber.


Design Considerations for EBM. The guidelines for EBM generally are similar tothose for LBM; additional considerations are as follows:

° Because vacuum chambers have limited capacity, individual parts or batchesshould closely match the size of the vacuum chamber for a high productionrate per cycle.

° If a part requires electron-beam machining on only a small portion of its vol-ume, consideration should be given to manufacturing it as a number of smallercomponents and assembling them after electron-beam machining.

Plasma-arc Cutting. In plasma-arc cutting (PAC), plasma beams (ionized gas) areused to rapidly cut ferrous and nonferrous sheets and plates. (See also Section 30.3.)The temperatures generated are very high (9400°C in the torch for oxygen as a plas-ma gas). Consequently, the process is fast, the kerf width is small, and the surfacefinish is good. Parts as thick as 150 mm can be cut. Material-removal rates are muchhigher than those associated with the EDM and LBM processes, and parts can bemachined with good reproducibility. Plasma-arc cutting is highly automated today,using programmable controllers.

27.8 Water-jet Machining

When we put our hand across a jet of water or air, we feel a considerable concen-trated force acting on it. This force results from the momentum change of the streamand, in fact, is the principle on which the operation of water or gas turbines is based.In water-jet machining (WJM) (also called hydrodynamic machining), this force is

utilized in cutting and deburring operations (Fig. 27.16 ). (See also water-jet peening,Section 34.2.)

The water jet acts like a saw and cuts a narrow groove in the material. A pres-sure level of about 400 MPa is generally used for efficient operation, although pres-sures as high as 1400 MPa can be generated. ]et-nozzle diameters range between 0.05and 1 mm. A water-jet cutting machine and its operation are shown in Fig. 27.16b. A

variety of materials can be cut, including plastics, fabrics, rubber, wood products,paper, leather, insulating materials, brick, and composite materials (Fig. 27.16c).

Depending on the materials, thicknesses can range up to 25 mm and higher.Vinyl and foam coverings for automobile dashboards (as well as some body panels)are being cut by multiple-axis, robot-guided water-jet machining equipment.Because it is an efficient and clean operation compared with other cutting process-es, it is also used in the food-processing industry for cutting and slicing food prod-ucts.

The advantages of this process are as follows:

° Cuts can be started at any location without the need for predrilled holes.° No heat is produced.° No deflection of the rest of the workpiece takes place; thus, the process is suit-

able for flexible materials.° Little wetting of the workpiece takes place.° The burr produced is minimal.° It is an environmentally safe manufacturing process.

Abrasive Water-jet Machining. In abrasive water-jet machining (AWJM), thewater jet contains abrasive particles (such as silicon carbide or aluminum oxide),which increase the material-removal rate above that of water-jet machining.

Section 27.9 Abrasive jet Machining 77

Accumulator Comms _ I ValveFluid supply f

Mixer and filter 0 _ .. , Sapphire nozzle

pum p i” tr iv- Water et---- Hydraulic .;. 1

lntensifier _ unit Workpiece Drain(H)

X-axis controlControl panel y-axis control ~ 3

~».,.»~~<fT;v fix. .f

Abrasive-jethead

"\Collection tank % fl

(bl (C)

FIGURE 27.I6 (a) Schematic illustration of the vvater~jet machining process. (b) A computer-controlled water-jet cutting machine. (c) Examples of various nonmetallic parts produced bythe Water-jet cutting process. Source: Courtesy of OMAX Corporation.

Metallic, nonmetallic, and advanced composite materials of various thicknesses canbe cut in single or multilayers.

AWJM is suitable particularly for heat-sensitive materials that cannot bemachined by processes in which heat is produced. Cutting speeds can be as high as7.5 m/min for reinforced plastics, but much lower for metals. Consequently, the processmay not be acceptable for situations requiring high production rates.

The minimum hole size that can be produced satisfactorily to date is about3 mm; maximum hole depth is on the order of 25 mm. With multiple-axis and robot-controlled machines, complex three-dimensional parts can be machined to finish di-mensions. The optimum level of abrasives in the jet stream is controlled automaticallyin modern AWJM systems. Nozzle life has been improved by making nozzles from ru-bies, sapphires, and carbide-based composite materials (Fig. 27.16a).

27.9 Abrasive-jet Machining

In abrasive-jet machining (AJM), a high-velocity jet of dry air, nitrogen, or carbondioxide containing abrasive particles is aimed at the workpiece surface under con-trolled conditions (Fig. 27.17). The impact of the particles develops a sufficientlyconcentrated force (see also Section 26.6) to perform operations such as (a) cutting

'~ T ww

1 7 We/..<, .5 %


PowderFilters Supply -ul... .,,=.. _ ..§. rnixer

Exhaust/ 0 Y¢.,,. “ A Hood 6 Pressure `"`; “;-‘- " ~--, NOZZ|e

r9QU|at°f :Sgr @` //, `\ '21,

Ga , " _Suppw \/'brator Foot controlvalve

(3) (b)

FIGURE 27.l 1 (a) Schematic illustration of the abrasive-jet machining process. (b) Examples

of parts made by abrasive-jet machining, produced in 50-mm thick 304 stainless steel.

Source: Courtesy of OMAX Corporation.

small holes, slots, or intricate patterns in very hard or brittle metallic and nonmetal-lic materials, (b) deburring or removing small flash from parts, (C) trimming andbeveling, (d) removing oxides and other surface films, and (e) generally cleaningcomponents with irregular surfaces.

The gas-supply pressure is on the order of 850 kPa, and the abrasive-jet veloc-ity can be as high as 300 m/s and is controlled by a valve. The nozzles are usuallymade of tungsten carbide or sapphire, both of which have abrasive Wear resistance.The abrasive size is in the range from 10 to 50 um. Because the flow of the free

abrasives tends to round off corners, designs for abrasive-jet machining shouldavoid sharp corners. Also, holes made in metal parts tend to be tapered. There is

some hazard involved in using this process, because of airborne particulates. Theproblem can be avoided by using the abrasive Water-jet machining process.

21.I0 Hybrid Machining Systems

A more recent development in manufacturing is the concept of hybrid machiningsystems. Two or more machining processes are combined into one system to takeadvanta e of the ca abilities of each rocess increasin roduction s eed and thusS P P a S P Pimproving the efficiency of the operation. The system is able to handle a variety of

materials, including metals, ceramics, polymers, and composites. Examples of such

systems include combinations and integration of the following processes:

a. Abrasive machining and electrochemical machining

b. Abrasive machining and electrical discharge machining

c. Abrasive machining and electrochemical finishing

d. Water-jet cutting and Wire EDM

e. High-speed milling, laser ablation, and blasting, as an example of three inte-grated processes.

f. Machining and blasting

g. Electrochemical and electrical discharge machining (ECDM), also called elec-

trochemical spark machining (ECSM).

Section 27.11 Economics of Advanced Machining Processes 78|

h. Machining and forming processes, such as laser cutting and punching of sheetmetal, described in Example 27.1.

i. Combinations of various forming, machining, and joining processes.

The implementation of these concepts and the development of machinery andcontrol systems present significant challenges. Important considerations include fac-tors such as (a) the workpiece material and its manufacturing characteristics; see, forexample, Table 1.3 in the General Introduction, (b) compatibility of processingparameters, such as speeds, sizes, forces, energies, and temperature, among the twoor more processes to be integrated, (c) cycle times of each individual operationinvolved, (d) possible adverse effects of the presence of various elements such as

abrasives, chemicals, wear particles, chips, and contaminants on the overall opera-tion, and (e) consequence of a failure in one of the stages in the system, since theoperation involves sequential processes.

21.ll Economics of Advanced Machining Processes

Advanced machining processes have unique applications and are useful particularlyfor difficult-to-machine materials and for parts with complex internal and externalprofiles. The economic production run for a particular process depends on the costsof 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 opera-tions that subsequently may be necessary.

In chemical machining (which is inherently a slow process), an important fac-tor is the cost of reagents, maskants, and disposal-together with the cost of clean-ing the parts. In electrical-discharge machining, the cost of electrodes and the needto periodically replace them can be significant.

The rate of material removal and the production rate can vary significantly inthese processes, as can be seen in Table 27.1. The cost of tooling and equipment alsovaries considerably, as does the operator skill required. The high capital investment formachines (such as those for electrical and high-energy-beam machining, especiallywhen equipped with robotic controls) has to be justified in terms of the production runsand the feasibility of manufacturing the same part by other means if at all possible.

CASE STUDY 21.2 Manufacture of Small Satellites

Satellites constructed in the early days of the SpaceAge (1960s) were very large, and those smaller than1000 kg were very rare. Table 27.3 shows the classi-fication of modern satellites by their mass. This casestudy describes the manufacture of propulsion sys-tems for micro- and nanosatellites.

There are several compelling' reasons to reducethe size of satellites, none greater than the cost ofputting the satellite into orbit. One of the mainsources of weight in a satellite is its propulsion sys-tem, which is essential to changing its orbit or cor-recting for drift. Figure 27.18a shows the propulsionsystem for a microsatellite, incorporating a number

of cold gas microthrusters, a propellant storage tank,filters, and temperature and pressure sensors.

Selected components of the propulsion system areshown in Fig. 27.18b. Note that the production of theseminiature parts would be extremely difficult throughconventional forming, casting, or machining technol-ogy. Furthermore, connecting the plumbing for all ofthe components would be very difficult even with largercomponents, and almost impossible to perform inside aclean room environment. An attractive alternative is theproduction of an integrated system, with fluid connec-tions made internally through a photochemically etchedand diffusion-bonded support on which components


TABLE 21.3

Satellite Classification

Mass

Group name kg of 3

Large satellite >1000 kgMedium satellite 500-1000Minisatellite 1 00-500Small satellites

Microsatellite 1 0-100Nanosatellite 1-10Picosatellite 0.1-1Femtosatellite < 1 00 g

(H) (D)

FIGURE 27.l8 Propulsion system for a small satellite. (a) Miniaturized system suitable for a micro- or nanosatellite,and (b) selected propulsion system components. Source: Courtesy of R. Hoppe, VACCO Industries, Inc.

(21) (D) (C)

FIGURE 21.19 Photochemically etched and blanked components for micro- and nanosatellites. (a) Mounting boardincorporating fluid flow channels in an integrated package; (b) microscale valve spring placed next to a U.S. penny; and

(C) fuel filter. Source: Courtesy of R. Hoppe, VACCO Industries, Inc.

are Welded or mechanically fastened. Such a support is Figure 27.20 depicts the manufacturing sequenceshown in Fig. 27.19, along with valve springs and filters followed. Titanium is commonly used for propulsionthat are produced through photochemical blanking system components, because it has a high strength-to-combined With diffusion-bonding processes. weight ratio, thus making possible lightweight designs.

Section 27.11 Economics of Advanced Machining Processes 78

'rr

(H) (b)

'~ 41:51. \:""”‘.W::..2,.,“W§"'£;§;§¢-,¢»;»,¢¢;;2$§§f', »:f§2Z2T”‘!i‘Ef:E3C. (C) (Ci)

(9) (T)

FIGURE 21.20 Processing sequence for photochemical etching of microsatellite components: (a) Clean theraw material; (b) coat with photosensitive material; (c) expose with photographic tool; (d) develop a resistimage; (e) etch; and (f) remove the resist. Source: Courtesy of R. Hoppe, VACCO Industries, Inc.

A mask is prepared as described in Section 27.2, and Such fully integrated systems have resulted inthe titanium is etched or blanked in a solution of satellite propulsion systems that are less complex, morehydrofluoric and nitric acid. Multiple layers of tita- robust, and less massive than those in previous designs.nium are then diffusion bonded (see Section 3l.7) tocreate internal features such as flow channels. Source: Courtesy of R. Hoppe, VACCO Industries, Inc.


KEY TERMS

Abrasive-jet machiningAbrasive water-jet

machiningChemical blankingChemical machiningChemical millingDielectricDie sinkingElectrical-discharge grindingElectrical-discharge

machining

BIBLIOGRAPHY

SUMMARY

Advanced machining processes have unique capabilities and utilize chemical, elec-

trochemical, electrical, and high-energy-beam sources of energy. The mechanicalproperties of the workpiece material are not significant, because these processes rely

on mechanisms that do not involve the strength, hardness, ductility, or toughness of

the material. Rather, they involve physical, chemical, and electrical properties.

Chemical and electrical methods of machining are suitable particularly for hardmaterials and complex shapes. They do not produce forces (and therefore can be

used for thin, slender, and flexible workpieces), significant temperatures, or resid-ual stresses. However, the effects of these processes on surface integrity must be

investigated, as they can damage surfaces considerably, thus reducing the fatiguelife of the product.

High-energy-beam machining processes basically utilize laser beams, electronbeams, and plasma beams. They have important industrial applications, possess a

high flexibility of operation with robotic controls, and are economically compet-itive with various other processes.

Water-jet machining, abrasive water-jet machining, and abrasive-jet machiningprocesses can be used for cutting as well as deburring operations. Because they donot utilize hard tooling, they have an inherent flexibility of operation.

Hybrid machining processes offer possibilities for more efficient production ofcomplex parts by increasing speed and reducing, production times.

Electrochemical-discharge Laser Reagentgrinding Laser-beam machining Shaped-tube electrolytic

Electrochemical grinding No-wear EDM machiningElectrochemical honing Photochemical blanking UndercutElectrochemical machining Photochemical machining Water-jet machiningElectrode Photoetching Wire EDMElectrolyte PhotoresistElectron-beam machiningEtchantHybrid machiningHydrodynamic machining

Plasma-arc cuttingPlasma beamsPulsed electrochemical

machining

ASM Handbook, Vol. 16: Machining, ASM International,1989.

Brown, ]., Advanced Machining Technology Handbook,McGraw-Hill, 1998

Chryssolouris, G., and Sheng, P., Laser Machining, Theoryand Practice, Springer, 1991.

Crafer, R.C., and Oakley, P.]., Laser Processing inManufacturing, Chapman 86 Hall, 1993.

Dahotre, N.B., and Harimkar, S.P., Laser Fabrication andMachining of Materials, Springer, 2007.

El-Hofy, H.A.-G., Advanced Machining Processes, McGraw-Hill, 2005.

Gillespie, L., Deburring and Edge Finishing HandbookSociety of Manufacturing Engineers, 1999.

Grzesik, W, Advanced Machining Processes of MetallicMaterials: Theory, Modelling and ApplicationsElsevier, 2008.

Guitrau, E.B., The EDM Handbook, Hanser-Gardner, 1997]ain, V.K., and Pandey, P.C., Theory and Practice of

Electrochemical Machining, Wiley, 1993.jameson, E.C., Electrical Discharge Machining, Society of

Manufacturing Engineers, 2001.Lange, K. (ed.), Die Manufacture, Chapter 32 in Handbook

ofMetal Forming, McGraw-Hill, 1985.

McGeough, ].A., Advanced Methods of Machining,Chapman Sc Hall, 1988.

Migliore, L., Laser Materials Processing, Marcel Dekker,1996.

Momber, A.W, and Kovacevic, R., Principles of AbrasiveWater _let Machining, Springer, 1998.

Powell, ]., CO2 Laser Cutting, 2nd ed., Springer, 1998.

REVIEW QUESTIONS

27.l. Describe the similarities and differences betweenchemical blanking and conventional blanking using dies.

27.2. Name the processes involved in chemical machining.Describe briefly their principles.

27.3. Explain the difference between chemical machiningand electrochemical machining.

27.4. What is the underlying principle of electrochemicalgrinding?

27.5. Explain how the EDM process is capable of producingcomplex shapes.

QUALITATIVE PROBLEMS

27.12. Give technical and economic reasons that theprocesses described in this chapter might be preferred overthose described in the preceding chapters.

27. I 3. Why is the preshaping or premachining of parts some-times desirable in the processes described in this chapter?

27.14. Explain why the mechanical properties of workpiecematerials are not significant in most of the processes describedin this chapter.

27.I5. Why has electrical-discharge machining become sowidely used in industry?

27.16. Describe the types of parts that are suitable for wireEDM.

27.|7. Which of the advanced machining processes wouldcause thermal damage? What is the consequence of suchdamage to workpieces?

27.l8. Which of the processes described in this chapterrequire a vacuum? Explain why?

Quantitative Problems 785

Sommer, C., Non-Traditional Machining Handbook,Advance Publishing, 1999.

Sommer, C., and Sommer, S., Wire EDM Handbook,Technical Advanced Publishing Co., 1997.

Steen, WM., Laser Material Processing, Springer, 1991.Taniguchi, N., Energy-Beam Processing of Materials,

Oxford, 1989.

27.6. What are the capabilities of wire EDM? Could thisprocess be used to make tapered parts? Explain.

27.7. Describe the advantages of water-jet machining.27.8. What is the difference between photochemical blank-ing and chemical blanking?

27.9. What type of workpiece is not suitable for laser-beammachining?

27.10. What is an undercut? Why must it be considered inchemical machining?

27.l I. Explain the principle of hybrid machining.

27.|9. Describe your thoughts regarding the laser-beammachining of nonmetallic materials. Give several possibleapplications, including their advantages compared with otherprocesses.

27.20. Are deburring operations still necessary for someparts made by advanced machining processes? Explain andgive several specific examples.

27.2l. List and explain factors that contribute to a poorsurface finish in the processes described in this chapter.

27.22. What is the purpose of the abrasives in electrochemi-cal grinding?

27.23. Which of the processes described in this chapter aresuitable for producing very small and deep holes? Explain.

27.24. Is kerf width important in Wire EDM? Explain.

27.25. Comment on your observations regarding Fig. 27.4.27.26. Why may different advanced machining processesaffect the fatigue strength of materials to different degrees?

QUANTITATIVE PROBLEMS

|]27.27. A 200-mm-deep hole that is 30 mm in diameter isbeing produced by electrochemical machining. A high produc-tion rate is more important than machined surface quality.Estimate the maximum current and the time required to per-form this operation.n27.28. If the operation in Problem 27.27 were performedon an electrical-discharge machine, what would be the esti-mated machining time?

27.29. A cutting-off operation is being performed with a

laser beam. The workpiece being cut is 12 mm thick and380 mm long. If the kerf is 2.4 mm wide, estimate the time re-quired to perform this operation.|]27.30. A 20-mm-thick copper plate is being machined bywire EDM. The wire moves at a speed of 1.2 m/min and thekerf width is 1.6 mm. What is the required power? Note thatit takes 1550 ] to melt one gram of copper.


SYNTHESIS, DESIGN, AND PROIECTS

27.3l. Explain why it is difficult to produce sharp profilesand corners with some of the processes described in thischapter.

27.32. Make a list of the processes described in this chapterin which the following properties are relevant: (a) mechani-cal, (b) chemical, (c) thermal, and (d) electrical. Are thereprocesses in which two or more of these properties are impor-tant? Explain.

27.33. Would the processes described in this chapter be dif-

ficult to perform on various nonmetallic or rubber-like mate-rials? Explain your thoughts, commenting on the influence of

various physical and mechanical properties of workpiecematerials, part geometries, etc.

27.34. Describe the types of parts that would be suitable forhybrid machining. Consider one such part and make a pre-liminary sketch for a hybrid machine to produce that part.

27.35. Describe your thoughts as to whether the processesdescribed in (a) Chapters 13 through 16, and (b) Chapters 23

and 24 can be suitable for a hybrid system of making parts.Give a preliminary sketch of a machine for the two groups of

processes listed.

27.36. Make a list of machining processes that may be suit-able for each of the following materials: (a) ceramics, (b) cast

iron, (c) thermoplastics, (d) thermosets, (e) diamond, and(f) annealed copper.

27.37. At what stage is the abrasive in abrasive water-jetmachining introduced into the water jet? Survey the availableliterature, and then prepare a schematic illustration of theequipment involved.

27.38. How would you manufacture a large-diameter, coni-cal, round metal disk with a thickness that decreases from thecenter outward? Make appropriate sketches.

27.39. Describe the similarities and differences among thevarious design guidelines for the processes described in thischapter.

27.40. Describe any workpiece size limitations in advancedmachining processes. Give examples.

27.4|. Suggest several design applications for the types of

parts shown in Fig. 27.5. (See also Fig. 27.16c.)

27.42. Based on the topics covered in Parts III and IV, make

a comprehensive table of hole-making processes. Describe theadvantages and limitations of each method, and comment onthe quality and surface integrity of the holes produced.

27.43. Review Example 27.1 and explain the relevantparameters involved; then design a system whereby bothprocesses can be used in combination to produce parts fromsheet metal.

27.44. Marking surfaces with numbers and letters for part-identification purposes can be done with a variety of mechan-ical and nonmechanical methods. Based on the processesdescribed throughout this book thus far, make a list of thesemethods, explaining their advantages, limitations, and typicalapplications.27.45. Precision engineering is a term that is used to de-scribe manufacturing high-quality parts with close dimen-

sional tolerances and good surface finish. Based on theirprocess capabilities, make a list of advanced machiningprocesses with decreasing order of the quality of parts pro-duced. Comment on your observations.

27.46. With appropriate sketches, describe the principles of

various work-holding methods and work-holding devices thatcan be used for the processes described in this chapter.

27.47. Make a table of the process capabilities of theadvanced machining processes described in this chapter. Useseveral columns and describe the machines involved, the typeof tools and tool materials used, the shapes of blanks andparts produced, the typical maximum and minimum sizes,surface finish, tolerances, and production rates.

27.48. One of the general concerns regarding advancedmachining processes is that, in spite of their many advan-tages, they generally are slower than conventional machiningoperations. Conduct a survey of the speeds, machining times,and production rates involved, and prepare a table compar-ing their respective process capabilities.

27.49. It can be seen that several of the processes describedin Part IV of this book can be employed, either singly or in

combination, to make or finish dies for metalworking opera-tions. Write a brief technical paper on these methods, describ-ing their advantages, limitations, and typical applications.

Micromanufacturingand Fabrication ofMicroelectronicDevices

RTPA

In order to appreciate the importance of the topics covered in the two chapters inthis part of the book, consider the manufacture of a simple spur gear made of metal.If the gear is 100 mm in diameter, it can be produced by traditional means, such asstarting with a cast or forged blank and machining it, as described in various chap-ters. A gear that is 2 mm in diameter, however, can be difficult to produce to thedesired dimensional accuracy. If sufficiently thin, the gear could be made from sheetmetal by very fine blanking or chemical etching, or by electroforming. If the gear is

only a few micrometers in size, it can be produced with techniques involving opticallithography, wet and dry chemical etching, and related processes described in thenext two chapters. A gear that is only a few nanometers in diameter would beextremely difficult to produce; indeed, such a gear would, at most, have only a fewtens of atoms across its surface. The challenges faced in producing gears of increas-ingly smaller sizes is highly informative and can be put into proper perspective byreferring to the illustration of length scales shown in Fig. V1.

For most of the engineering profession’s history, engineers have emphasizedthe design and manufacture of relatively large components. Conventional manufac-turing processes, described in Chapters 11 through 27, typically produce parts thatare larger than a millimeter or so and can be described as visible to the naked eye.The size of such parts generally are referred to as macroscale, the word “macro”being derived from the Greek ma/eros, meaning “long”. The processing of such partsis known as macromanufacturing. Numerous examples can be given, ranging fromproducts found in a hardware store, to castings and forgings used in machinery, andto products as large as automobiles, aircraft, and ships. Macroscale is the mostdeveloped and best understood size range from a design and manufacturing stand-point, with a wide variety of processes available for producing components of thatsize. Note that all of the examples and case studies given thus far in this book havebeen examples of macromanufacturing.

The gear shown in Fig. V.1 is the size of a few tens of micrometers across andfits into the realm of micromanufacture. Micromanufacturing, which by definitionrefers to manufacturing on a microscopic scale (that is, not visible to the naked eye),has been developed mostly for electronic devices of all kinds, including computerprocessors and memory chips, sensors, and magnetic storage devices. For the mostpart, this type of manufacturing relies heavily on lithography approaches, wet anddry etching, and coating operations. In addition, the micromanufacturing of semi-conductors exploits the unique ability of silicon to form oxides.

8

Part V Micromanufacturing and Fabrication of Microelectronic Devices

‘|Om

1m

0.1 m

0.01 m= 1 cm

1mm

0.1 mmCD =100 /.Lm

.NU)

10p.m

1/sum

0.1/Jsm= 100 nm

10 nm

1nrp=1OA

1A

I> I,MiernmanufacturingIlMacromanufacturmgl§`.Q-

0 1i tween-1. 3_5

8"o11 _

Q

EE oISlCD355é:5.3Z=*-59.c<3gU,"O6'3r:'E°%"’39°g'52m3233§<1‘D3

y 13E1l5- iw.,‘MM "wi

Qlisié§Tz» \i1i1111g5,-

EEg Sff,2<8>gcnQ-5''U3

Qsr2. <1°°27°O'1

.2“QS;5 \||\1|1l|1|‘l\1~ 1.-` l|,,.-7%ll ll! 11; 11~ ~11111A is%§=;;,,a~a&.1ggy ,Qg~'P1

2-2'asQS32-S3a5Seea<f>Q9-ea'G23*2

»-e-S1-13»-o-E3OQ3Q3fbF5~29;§

5'Qrv

Lf

FIGURE V.| Illustration of the regimes of macro-, meso-, micro-, and nanomanufacturing,the range of common sizes of parts, and the capabilities of manufacturing processes in

producing those parts.

Examples of products that rely upon micromanufacturing approaches are a

wide variety of sensors and probes (see Fig. VZ), ink-jet printing heads, microactua-tors and associated devices, magnetic hard-drive heads, and microelectronic devicessuch as computer processors and memory chips. Micromanufacturing methodsallow the production of a Wide variety of features at these length scales, but mostexperience is With electronic devices. Microscale mechanical devices are still a rela-

tively new technology, but one that has developed with surprising speed.Mesomanufacturing overlaps macro- and micromanufacturing, as seen by the

illustrations given in Fig. V.1. Examples of mesomanufacturing are extremely smallmotors, bearings, and components for miniature devices such as hearing aids; med-ical devices such as stents and valves; and mechanical watches, with componentsexactly the same as the gear shown in Fig. V.1.

Part V Micromanufacturing and Fabrication of Microelectronic Devices

Engine management system Stereo Air-conditioningAirflow sensor Temperature sensorTemperature sensor Humidity sensorPosition sensor Light sensor

Drive-traintorque sensor

Air bag sensor

Antilock brakescontrolAccelerometerPosition sensor

Seat Tire-pressure Oxygencontrol sensor sensor

FIGURE M2 Microelectronic and microelectromechanical devices and parts used in a typicalautomobile.

In nanomanufacturing, parts are produced at nanometer length scales, that is,

one billionth of a meter and typically between 1076 and 10” m in length. Many ofthe features in integrated circuits are at this length scale, but very little else; molecu-larly engineered medicines and other forms of biomanufacturing are the only com-mercial examples. However, it is now recognized that many physical and biologicalprocesses act at the nanoscale and that this approach holds much promise for futureinnovations.

This part of the book emphasizes micro- and nanomanufacturing. Althoughthese subdisciplines within the broad range of manufacturing engineering are nowover five decades old, they have developed rapidly in the past two decades or so.Products made by micro- or nanomanufacturing have become very pervasive inmodern society. Computers, communications, video, and control hardware of alltypes depend upon these manufacturing and material approaches.

In Chapter 28, we describe the manufacture of silicon wafers and microelec-tronic devices, which include a wide variety of computer processors, memory devices,and integrated circuits. Communication, entertainment, control, transportation,engineering design and manufacturing, and medicine have been changed greatly by

the ready availability of metal-oxide-semiconductor (MOS) devices, usually based onsingle-crystal silicon. Microelectronics is the best known and commercially impor-tant example of micromanufacturing, with some aspects of the applications exempli-fying nanomanufacturing. The techniques used in packaging and assemblingintegrated circuits onto printed circuit boards also are presented.

The production of microscale devices that are mechanical and electrical in

nature is described in Chapter 29. Depending on their level of integration, thesedevices are called micromechanical devices or microelectromechanical systems(MEMSS). While the historical origins of MEMS manufacture stem from the sameprocesses used for microelectronic systems and from identical processes and produc-tion sequences that are still used, several unique approaches have been developed forthe manufacture of microscale electromechanical devices and systems.

Documents

kalpakjian 15