1. Chapter 19 Electronic Electrochemical Chemical and Thermal Machining Processes (Review) EIN Manufacturing Processes Summer A, 2011 1

2. 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

3. 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

4. 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

5. 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 – 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

6. 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.

7. Table 19-1 Summary of NTM Processes

9. 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.

10. 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.
If baths are not agitated properly, defects result

11. Advantages and Disadvantages of Chemical Machining
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

12. 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).

13. Table 19-3 Material Removal Rates for ECM of Alloys Assuming 100% Current Efficiency

14. Advantages and Disadvantages of Electrochemical Machining
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

15. 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

16. 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.

17. 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.

18. 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.

19. Effect of Current on-time and Discharge Current on Crater Size
MRR = (C I)/(Tm1.23),
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, )
= in.3/min.

20. Figure 19-25 The principles of metal removal for EDM.

21. 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.

22. 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

23. Table 19-5 Melting Temperatures for Selected EDM Workpiece Materials

24. Advantages and Disadvantages of EDM
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