Presentation on theme: "Chapter 19 Electronic Electrochemical Chemical and Thermal Machining Processes EIN 3390 Manufacturing Processes Fall, 2011 1."— Presentation transcript:
1 Chapter 19 Electronic Electrochemical Chemical and Thermal Machining Processes EIN Manufacturing Processes Fall, 20111
2 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
3 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 materialThermalHigh temperature in very localized regions evaporate materials, for example, EDMMechanicalHigh-velocity abrasives or liquids remove materials
4 Limitations of Conventional Machining Processes Machining processes that involve chip formation have a number of limitationsLarge amounts of energyUnwanted distortionResidual stressesBurrsDelicate or complex geometries may be difficult or impossible
5 Conventional End Milling vs. NTM Typical machining parametersFeed rate (5 – 200 in./min.)Surface finish (60 – 150 min) AA – Arithmetic AverageDimensional accuracy (0.001 – 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
8 19.2 Chemical Machining Processes Typically 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.
9 Masking Several different methods Cut-and-peelScribe-and-peelScreen printingEtch rates are slow in comparison to other NTM processesFigure 19-1 Steps required to produce a stepped contour by chemical machining.
10 Defects in Etching If baths are not agitated properly, defects result Figure 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.If baths are not agitated properly, defects result
11 Advantages and Disadvantages of Chemical Machining Process 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
12 Design Factors in Chemical Machining If artwork is used, dimensional variations can occur through size changes in the artwork of phototool film due to temperature and humidity changesEtch factor (E)- describes the undercutting of the maskantAreas that are exposed longer will have more metal removed from themE=U/d, where d- depth, U- undercuttingAnisotropy (A)- directionality of the cut, A=d/U, and Wf = Wm + (E d), orWm = Wf - (E d)where Wf is final desired width of cut
14 19.3 Electrochemical Machining Process Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyteThe tool is the cathode and the workpiece is the electrolyteFigure Schematic diagram of electrochemical machining process (ECM).
15 19.3 Electrochemical Machining Process Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyteThe tool is the cathode and the workpiece is the electrolyteFigure Schematic diagram of electrochemical machining process (ECM).
16 Table 19-3 Material Removal Rates for ECM Alloys Assuming 100% Current Efficiency
18 Electrochemical Processing Pulsed-current ECM (PECM)Pulsed on and off for durations of approximately 1msPulsed currents are also used in electrochemical machining (EMM)Electrochemical polishing is a modification of the ECM processMuch slower penetration rate
19 Other Electrochemical Processing Electrochemical hole machiningUsed to drill small holes with high aspect ratiosElectrostream drillingHigh velocity stream of charged acidic, electrolyteShaped-tube elecrolytic machining (STEM)Capable of drilling small holes in difficult to machine materialsElectrochemical grinding (ECG)Low voltage, high-current variant of ECM
20 Figure The shaped-tube electrolytic machining (STEM) cell process is a specialized ECM technique for drilling small holes using a metal tube electrode or metal tube electrode with dielectric coating.
21 Figure 19-20 Equipment setup and electrical circuit for electrochemical grinding.
22 Other Electrochemical Processes Electrochemical deburringElectrolysis is accelerated in areas with small interelectrode gaps and prevented in areas with insulation between electrodesDesign factors in electrochemical machiningCurrent densities tend to concentrate at sharp edges or featuresControl of electrolyte flow can be difficultParts may have lower fatigue resistance
23 Table 19-4 Metal Removal Rates for ECG for Various Metals (Electrochemical Grinding – ECG)
24 Advantages and Disadvantages of Electrochemical Machining ECM is well suited for the machining of complex two- dimensional shapesDelicate 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
25 19.4 Electrical Discharge Machining Electrical discharge machining (EDM) removes metal by discharging electric current from a pulsating DC power supply across a thin interelectrode gapThe 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
26 Figure 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.
27 Figure 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.
28 EDM Processes Slow compared to conventional machining Produce a matte surfaceComplex geometries are possibleOften used in tool and die makingFigure Schematic diagram of equipment for wire EDM using a moving wire electrode.
29 EDM ProcessesFigure (left) Examples of wire EDM workpieces made on NC machine (Hatachi).Figure (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).
30 Effect of Current on-time and Discharge Current on Crater Size MRR = (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, )= in.3/min.
31 Figure 19-25 The principles of metal removal for EDM.
32 Effect of Current on-time and Discharge Current on Crater Size From 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.
33 Considerations for EDM Graphite 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 wearThe dielectric fluid has four main functionsElectrical insulationSpark conductorFlushing mediumCoolant
34 Table 19-5 Melting Temperatures for Selected EDM Workpiece Materials
35 Advantages and Disadvantages of EDM Applicable 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
36 Electron and Ion Machining Electron 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 analysisFigure Electron-beam machining uses a high-energy electron beam (109 W/in.2)
37 Laser-Beam MachiningLaser-beam machining (LBM) uses an intensely focused coherent stream of light to vaporize or chemically ablate materialsFigure Schematic diagram of a laser-beam machine, a thermal NTM process that can micromachine any material.
39 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 ratesFigure Plasma arc machining or cutting.
40 Thermal DeburringUsed to remove burrs and fins by exposing the workpiece to hot corrosive gases for a short period of timeThermal deburring can remove burrs or fins from almost any material but is especially effective with materials of low thermal conductivityFigure Thermochemical machining process for the removal of burrs and fins.
41 HW for Chapter 19Review Questions:7, 17(page 521)