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Chapter 19Chapter 19
Electronic Electrochemical Electronic Electrochemical Chemical and ThermalChemical and Thermal Machining Processes Machining Processes
(Review) (Review)
EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSpring, 2011Spring, 2011
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
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
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
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
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.
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.
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.
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
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
d/3
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).
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
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
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
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.
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.
Figure 19-25 The principles of
metal removal for EDM.
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.
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