Chapter 19

Electronic Electrochemical Chemical

and Thermal Machining Processes

EIN 3390 Manufacturing ProcessesFall, 2011

19.1 Introduction

Non-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 Processes Four 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 Machining 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. NTM

Typical machining parameters◦ Feed rate (5 – 200 in./min.)

◦ Surface finish (60 – 150 min) 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

Table 19-1 Summary of NTM Processes

19.2 Chemical Machining Processes Typically involves metals, but ceramics

and glasses may be etched

Material 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.

Masking

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 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 of Chemical Machining Advantages◦ 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 rateis slow

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 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 Process

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).

19.3 Electrochemical Machining Process

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 Assuming 100% Current Efficiency

Electrochemical Processing

Pulsed-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 Processing

Electrochemical hole machining◦ Used to drill small holes with high aspect ratios

Electrostream drilling High velocity stream of charged acidic, electrolyte

Shaped-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.

Other Electrochemical Processes

Electrochemical 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

Table 19-4 Metal Removal Rates for ECG for Various Metals (Electrochemical Grinding – ECG)

Advantages and Disadvantages of 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 Machining

Electrical 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 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 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 Discharge 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 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.

Considerations for EDM

Graphite is the most widely used tool electrode

The 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 Workpiece Materials

Advantages and Disadvantages of EDM

Advantages Applicable to all

materials that are fairly good electrical conductors

Hardness, toughness, or brittleness of the material imposes no limitations

Fragile and delicate parts

Disadvantages Produces a hard

recast surface Surface may

contain fine cracks caused by thermal stress

Fumes can be toxic

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 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)

Laser-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.

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.

Thermal Deburring Used to remove

burrs and fins by exposing the workpiece to hot corrosive gasesfor 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.

HW for Chapter 19

Review Questions:7, 17(page 521)