Chapter 19Chapter 19

Electronic Electrochemical Electronic Electrochemical ChemicalChemical

and Thermal Machining and Thermal Machining ProcessesProcesses

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing Processes

Spring, 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 Processes

Machining 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.

Table 19-1 Summary of NTM ProcessesTable 19-1 Summary of NTM Processes

MaskingMasking

Several 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

Photochemical MachiningPhotochemical MachiningFigure 19-4 Basic steps in photochemical machining (PCM).

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

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).

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

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

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

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.

Figure 19-20 Equipment setup and electrical circuit for electrochemical grinding.

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)

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.

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 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.

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).

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.

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

Table 19-5 Melting Temperatures for Selected EDM Table 19-5 Melting Temperatures for Selected EDM Workpiece MaterialsWorkpiece Materials

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