chapter 19 electronic electrochemical chemical and thermal machining processes ein 3390 ...
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Chapter 19 Electronic Electrochemical Chemical and Thermal Machining Processes EIN 3390 Manufacturing Processes Fall, 2011. Non-traditional machining (NTM) processes have several advantages Complex geometries are possible Extreme surface finish Tight tolerances Delicate components - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 19Chapter 19
Electronic Electrochemical Electronic Electrochemical ChemicalChemical
and Thermal Machining and Thermal Machining ProcessesProcesses
EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing Processes
Fall, 2011Fall, 2011
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19.1 Introduction19.1 IntroductionNon-traditional machining (NTM) processes
have several advantages◦Complex geometries are possible◦Extreme surface finish◦Tight tolerances◦Delicate components◦Little or no burring or residual stresses◦Brittle materials with high hardness can be
machined◦Microelectronic or integrated circuits (IC) are
possible to mass produce
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NTM ProcessesNTM ProcessesFour basic groups of material removal using NTM processes
◦Chemical: Chemical reaction between a liquid reagent and workpiece
results in etching◦Electrochemical
An electrolytic reaction at workpiece surface for removal of material
◦Thermal High temperature in very localized regions evaporate
materials, for example, EDM◦Mechanical
High-velocity abrasives or liquids remove materials
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Limitations of Conventional Limitations of Conventional Machining ProcessesMachining ProcessesMachining processes that involve chip
formation have a number of limitations◦Large amounts of energy◦Unwanted distortion◦Residual stresses◦Burrs ◦Delicate or complex geometries may be difficult or impossible
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Conventional End Milling vs. NTMConventional End Milling vs. NTMTypical machining parameters
◦Feed rate (5 – 200 in./min.)◦Surface finish (60 – 150 in) AA – Arithmetic
Average◦Dimensional accuracy (0.001 – 0.002 in.)◦Workpiece/feature size (25 x 24 in.); 1 in. deep
NTM processes typically have lower feed rates and require more power consumption
The feed rate in NTM is independent of the material being processed
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Table 19-1 Summary of NTM ProcessesTable 19-1 Summary of NTM Processes
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19.2 Chemical Machining 19.2 Chemical Machining ProcessesProcessesTypically involves metals, but ceramics
and glasses may be etchedMaterial is removed from a workpiece by
selectively exposing it to a chemical reagent or etchant◦Gel 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.
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MaskingMaskingSeveral different
methods◦Cut-and-peel◦Scribe-and-peel◦Screen printing
Etch rates are slow in comparison to other NTM processes
Figure 19-1 Steps required to produce a stepped contour by chemical machining.
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Defects in EtchingDefects 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.
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Advantages and Disadvantages Advantages and Disadvantages of Chemical Machiningof Chemical MachiningAdvantages
◦Process is relatively simple
◦Does not require highly skilled labor
◦Induces no stress or cold working in the metal
◦Can be applied to almost any metal
◦Large areas◦Virtually unlimited
shape◦Thin sections
Disadvantages◦Requires the handling
of dangerous chemicals
◦Disposal of potentially harmful byproducts
◦Metal removal rate is slow
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Design Factors in Chemical Design Factors in Chemical MachiningMachiningIf artwork is used, dimensional variations can occur
through size changes in the artwork of phototool film due to temperature and humidity changes
Etch factor (E)- describes the undercutting of the maskant◦Areas that are exposed longer will have more metal
removed from them◦E=U/d, where d- depth, U- undercutting
Anisotropy (A)- directionality of the cut, A=d/U, and Wf = Wm + (E d), or
Wm = Wf - (E d)where Wf is final desired width of cut
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19.3 Electrochemical Machining 19.3 Electrochemical Machining ProcessProcess
Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyte
The tool is the cathode and the workpiece is the electrolyte
Figure 19-17 Schematic diagram of electrochemical machining process (ECM).
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19.3 Electrochemical Machining 19.3 Electrochemical Machining ProcessProcess
Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyte
The tool is the cathode and the workpiece is the electrolyte
Figure 19-17 Schematic diagram of electrochemical machining process (ECM).
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Table 19-3 Material Removal Rates for ECM Alloys Table 19-3 Material Removal Rates for ECM Alloys Assuming 100% Current EfficiencyAssuming 100% Current Efficiency
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Electrochemical ProcessingElectrochemical ProcessingPulsed-current ECM (PECM)
◦Pulsed on and off for durations of approximately 1ms
Pulsed currents are also used in electrochemical machining (EMM)
Electrochemical polishing is a modification of the ECM process◦Much slower penetration rate
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Other Electrochemical ProcessingOther Electrochemical ProcessingElectrochemical hole machining
◦Used to drill small holes with high aspect ratiosElectrostream drilling
High velocity stream of charged acidic, electrolyteShaped-tube elecrolytic machining (STEM)
◦Capable of drilling small holes in difficult to machine materials
Electrochemical grinding (ECG) ◦Low voltage, high-current variant of ECM
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Figure 19-19 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.
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Figure 19-20 Equipment setup and electrical circuit for electrochemical grinding.
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Other Electrochemical ProcessesOther Electrochemical ProcessesElectrochemical deburring
◦Electrolysis is accelerated in areas with small interelectrode gaps and prevented in areas with insulation between electrodes
Design factors in electrochemical machining◦Current densities tend to concentrate at sharp
edges or features◦Control of electrolyte flow can be difficult◦Parts may have lower fatigue resistance
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Table 19-4 Metal Removal Rates for ECG for Various Table 19-4 Metal Removal Rates for ECG for Various Metals (Electrochemical Grinding – ECG)Metals (Electrochemical Grinding – ECG)
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Advantages and Disadvantages Advantages and Disadvantages of Electrochemical Machiningof Electrochemical Machining
Advantages◦ECM is well suited for the
machining of complex two-dimensional shapes
◦Delicate parts may be made
◦Difficult-to machine geometries
◦Poorly machinable materials may be processed
◦Little or no tool wear
Disadvantages◦ Initial tooling can
be timely and costly
◦Environmentally harmful by-products
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19.4 Electrical Discharge 19.4 Electrical Discharge MachiningMachiningElectrical 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 ionized
Two different types of EDM exist based on the shape of the tool electrode◦Ram EDM/ sinker EDM◦Wire EDM
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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.
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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.
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EDM ProcessesEDM Processes
Slow compared to conventional machining
Produce a matte surface
Complex geometries are possible
Often used in tool and die making
Figure 19-22 Schematic diagram of equipment for wire EDM using a moving wire electrode.
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EDM ProcessesEDM Processes
Figure 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).
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Effect of Current on-time and Effect of Current on-time and Discharge Current on Crater SizeDischarge Current on Crater SizeMRR = (C I)/(Tm
1.23),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.
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Figure 19-25 The principles of
metal removal for EDM.
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Effect of Current on-time and Effect of Current on-time and Discharge Current on Crater SizeDischarge 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.
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Considerations for EDMConsiderations 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 wear
The dielectric fluid has four main functions◦Electrical insulation◦Spark conductor◦Flushing medium◦Coolant
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Table 19-5 Melting Temperatures for Selected EDM Table 19-5 Melting Temperatures for Selected EDM Workpiece MaterialsWorkpiece Materials
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Advantages and Disadvantages Advantages and Disadvantages of EDMof EDM
AdvantagesApplicable to all
materials that are fairly good electrical conductors
Hardness, toughness, or brittleness of the material imposes no limitations
Fragile and delicate parts
DisadvantagesProduces a hard
recast surfaceSurface may
contain fine cracks caused by thermal stress
Fumes can be toxic
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Electron and Ion MachiningElectron 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 metal
Ion 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)
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Laser-Beam MachiningLaser-Beam Machining
Laser-beam machining (LBM) uses an intensely focused coherent stream of light to vaporize or chemically ablate materials
Figure 19-27 Schematic diagram of a laser-beam machine, a thermal NTM process that can micromachine any material.
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Plasma Arc Cutting (PAC)Plasma Arc Cutting (PAC)Uses a superheated
stream of electrically ionized gas to melt and remove material
The process can be used on almost any conductive material
PAC can be used on exotic materials at high rates
Figure 19-29 Plasma arc machining or cutting.
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Thermal DeburringThermal DeburringUsed to remove
burrs and fins by exposing the workpiece to hot corrosive gases for a short period of time
Thermal deburring can remove burrs or fins from almost any material but is especially effective with materials of low thermal conductivity
Figure 19-31 Thermochemical machining process for the removal of burrs and fins.
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HW for Chapter 19HW for Chapter 19Review Questions:7, 17(page 521)