Chapter 19

Electronic Electrochemical Chemicaland Thermal Machining Processes(Review) EIN 3390 Manufacturing ProcessesSpring, 2012

19.1 IntroductionNon-traditional machining (NTM) processes have several advantagesComplex geometries are possibleExtreme surface finishTight tolerancesDelicate componentsLittle or no burring or residual stressesBrittle materials with high hardness can be machinedMicroelectronic or integrated circuits (IC) are possible to mass produce

NTM ProcessesFour basic groups of material removal using NTM processesChemical: Chemical reaction between a liquid reagent and workpiece results in etchingElectrochemicalAn electrolytic reaction at workpiece surface for removal of materialThermal High temperature in very localized regions evaporate materials, for example, EDMMechanicalHigh-velocity abrasives or liquids remove materials

Limitations of Conventional Machining ProcessesMachining processes that involve chip formation have a number of limitationsLarge amounts of energyUnwanted distortionResidual stressesBurrs Delicate or complex geometries may be difficult or impossible

Conventional End Milling vs. NTMTypical machining parametersFeed rate (5 200 in./min.)Surface finish (60 150 min) AA Arithmetic AverageDimensional accuracy (0.001 0.002 in.)Workpiece/feature size (25 x 24 in.); 1 in. deepNTM processes typically have lower feed rates and require more power consumptionThe feed rate in NTM is independent of the material being processed

Table 19-1 Summary of NTM Processes

19.2 Chemical Machining ProcessesTypically involves metals, but ceramics and glasses may be etchedMaterial is removed from a workpiece by selectively exposing it to a chemical reagent or etchantGel milling- gel is applied to the workpiece in gel form.Maskant- selected areas are covered and the remaining surfaces are exposed to the etchant. This is the most common method of CHM.

MaskingSeveral different methodsCut-and-peelScribe-and-peelScreen printingEtch rates are slow in comparison to other NTM processes

Figure 19-1 Steps required to produce a stepped contour by chemical machining.

Defects in EtchingIf baths are not agitated properly, defects resultFigure 19-2 Typical chemical milling defects: (a) overhang: deep cuts with improper agitation; (b) islands: isolated high spots from dirt, residual maskant, or work material inhomogeneity; (c) dishing: thinning in center due to improper agitation or stacking of parts in tank.

Advantages and Disadvantages of Chemical MachiningAdvantagesProcess is relatively simpleDoes not require highly skilled laborInduces no stress or cold working in the metalCan be applied to almost any metalLarge areasVirtually unlimited shapeThin sectionsDisadvantagesRequires the handling of dangerous chemicalsDisposal of potentially harmful byproductsMetal removal rate is slow

19.3 Electrochemical Machining ProcessElectrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyteThe tool is the cathode and the workpiece is the anode

Figure 19-17 Schematic diagram of electrochemical machining process (ECM).

19.3 Electrochemical Machining ProcessElectrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyteThe tool is the cathode and the workpiece is the electrolyte

Figure 19-17 Schematic diagram of electrochemical machining process (ECM).

Advantages and Disadvantages of Electrochemical MachiningAdvantagesECM is well suited for the machining of complex two-dimensional shapes Delicate parts may be madeDifficult-to machine geometriesPoorly machinable materials may be processedLittle or no tool wearDisadvantagesInitial tooling can be timely and costlyEnvironmentally harmful by-products

19.4 Electrical Discharge MachiningElectrical discharge machining (EDM) removes metal by discharging electric current from a pulsating DC power supply across a thin interelectrode gap The gap is filled by a dielectric fluid, which becomes locally ionizedTwo different types of EDM exist based on the shape of the tool electrodeRam EDM/ sinker EDMWire EDM

Figure 19-21 EDM or spark erosion machining of metal, using high-frequency spark discharges in a dielectric, between the shaped tool (cathode) and the work (anode). The table can make X-Y movements.

Figure 19-21 EDM or spark erosion machining of metal, using high-frequency spark discharges in a dielectric, between the shaped tool (cathode) and the work (anode). The table can make X-Y movements.

EDM ProcessesSlow compared to conventional machiningProduce a matte surfaceComplex geometries are possibleOften used in tool and die makingFigure 19-22 Schematic diagram of equipment for wire EDM using a moving wire electrode.

EDM ProcessesFigure 19-24 (above) SEM micrograph of EDM surface (right) on top of a ground surface in steel. The spherical nature of debris on the surface is in evidence around the craters (300 x). Figure 19-23 (left) Examples of wire EDM workpieces made on NC machine (Hatachi).

Effect of Current on-time and Discharge Current on Crater SizeMRR = (C I)/(Tm1.23),Where MRR material removal rate in in.3/min.; C constant of proportionality equal to 5.08 in US customary units; I discharge current in amps; Tm melting temperature of workpiece material, 0F.

Example:A certain alloy whose melting point = 2,000 0F is to be machined in EDM. If a discharge current = 25A, what is the expected metal removal rate?

MRR = (C I)/(Tm1.23) = (5.08 x 25)/(2,0001.23) = 0.011 in.3/min.

Figure 19-25 The principles of metal removal for EDM.

Effect of Current on-time and Discharge Current on Crater SizeFrom Fig 19 25: we have the conclusions:Generally higher duty cycles with higher currents and lower frequencies are used to maximize MRR.Higher frequencies and lower discharge currents are used to improve surface finish while reducing MRR.Higher frequencies generally cause increased tool wear.

Considerations for EDMGraphite is the most widely used tool electrodeThe choice of electrode material depends on its machinability and coast as well as the desired MRR, surface finish, and tool wearFour main functions of dielectric fluid:Electrical insulationSpark conductorFlushing mediumCoolant

Advantages and Disadvantages of EDMAdvantagesApplicable to all materials that are fairly good electrical conductorsHardness, toughness, or brittleness of the material imposes no limitationsFragile and delicate partsDisadvantagesProduces a hard recast surfaceSurface may contain fine cracks caused by thermal stressFumes can be toxic

Electron and Ion MachiningElectron beam machining (EBM) is a thermal process that uses a beam of high-energy electrons focused on the workpiece to melt and vaporize a metalIon beam machining (IBM) is a nano-scale machining technology used in the microelectronics industry to cleave defective wafers for characterization and failure analysis

Figure 19-26 Electron-beam machining uses a high-energy electron beam (109 W/in.2)

Laser-Beam MachiningLaser-beam machining (LBM) uses an intensely focused coherent stream of light to vaporize or chemically ablate materialsFigure 19-27 Schematic diagram of a laser-beam machine, a thermal NTM process that can micromachine any material.

Plasma Arc Cutting (PAC)Uses a superheated stream of electrically ionized gas to melt and remove materialThe process can be used on almost any conductive materialPAC can be used on exotic materials at high rates

Figure 19-29 Plasma arc machining or cutting.

HW for Chapter 19Review Questions:17, 19, 20 (page 521)

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