Section 13. qxd

electrochemical grinding (ECG)
process (Fig. 13.4.23) is a combi-
nation of electrochemical machining and abrasive cutting where most
of the metal removal results from the electrolytic action. The process
consists of a rotating cathode, a neutral electrolyte, and abrasive parti-
cles in contact with the workpiece. The equipment is similar to a
conventional grinding machine except for the electrical accessories.
The cathode usually consists of a metal-bonded diamond or aluminum
oxide wheel. An important function of the abrasive grains is to maintain
a space for the electrolyte between the wheel and workpiece.
Surface finish, precision, and metal-removal rate are influenced by
the composition of the electrolyte. Aqueous solutions of sodium sili-
cate, borax, sodium nitrate, and sodium nitrite are commonly used as
electrolytes. The process is primarily used for tool and cutter sharpen-
ing and for machining of high-strength materials.
A combination of the electric-discharge and electrochemical meth-
ods of material removal is known as 
electrochemical discharge grinding
(ECDG).
The electrode is a pure graphite rotating wheel which electro-
chemically grinds the workpiece. The intermittent spark discharges
remove oxide films that form as a result of electrolytic action. The
equipment is similar to that for electrochemical grinding. Typical appli-
cations include machining of fragile parts and resharpening or form
grinding of carbides and tools such as milling cutters.
In 
chemical machining (CM)
material is removed by chemical or electro-
chemical dissolution of preferentially exposed surfaces of the workpiece.
Selective attack on different areas is controlled by masking or by partial
immersion. There are two processes involved: chemical 
milling
and chem-
ical 
blanking.
Milling applications produce shallow cavities for overall
weight reduction, and are also used to make tapered sheets, plates, or
extrusions. Masking with paint or tapes is common. Masking materials
may be elastomers (such as butyl rubber, neoprene, and styrene-butadiene)
or plastics (such as polyvinyl chloride, polystyrene, and polyethylene).
Typical blanking applications are decorative panels, printed-circuit etch-
ing, and thin stampings. Etchants are solutions of sodium hydroxide for
aluminum, and solutions of hydrochloric and nitric acids for steel.