Chapter 19 Electronic Electrochemical Chemical and Thermal Machining Processes EIN 3390 Manufacturing Processes Spring, 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 in) AA – Arithmetic Average ◦Dimensional accuracy (0.001 – 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 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.

Table 19-1 Summary of NTM Processes

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 rate is slow

Photochemical Machining Figure 19-4 Basic steps in photochemical machining (PCM).

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 W f = W m + (E D), or W m = W f - (E D) where W f 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 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 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 Equipment setup and electrical circuit for electrochemical grinding.

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 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 Schematic diagram of equipment for wire EDM using a moving wire electrode.

EDM Processes 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). Figure (left) Examples of wire EDM workpieces made on NC machine (Hatachi).

Effect of Current on-time and Discharge Current on Crater Size MRR = (C I)/(T m 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, 0 F. Example: A certain alloy whose melting point = 2,000 0 F is to be machined in EDM. If a discharge current = 25A, what is the expected metal removal rate? MRR = (C I)/(T m 1.23 ) = (5.08 x 25)/(2, ) = in. 3 /min.

Figure 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