Nonabrasive Finishing Methods,Nonabrasive Finishing Methods,Nonabrasive Finishing Methods
NONABRASIVE FINISHING METHODS are processes in which the surface generation occurs with a very little or
insignificant amount of mechanical interaction between the processing tool and the workpiece surfaces. These processes
are classified in general as the nontraditional or unconventional machining processes. In these processes, chemical,
electrical, or thermal actions, or a combination, are used for metal removal. Nontraditional processes include
electrochemical machining (ECM), electrodischarge machining (EDM), and laser beam machining (LBM). The inherent
nature of ECM and EDM makes them ideal for stock removal as well as finishing operations. The same is not true of
LBM at the present time. Therefore, this article provides a brief review of ECM and EDM and their role in finishing
operations.
Electrochemical Machining
ECM consists basically of the electrochemical dissolution of the surface metal of a workpiece by conversion of metal to
its ions by means of an electric current. The whole process is accomplished in an electrolytic cell by applying a positive
(anodic) potential to the workpiece and a negative (cathodic) potential to the tool used to shape the workpiece. ECM can
be used for shaping, finishing for improving the quality of the surface, deburring, and radiusing. One kind of ECM is
electropolishing. Figure 1 shows the various schematics for machining different geometries using ECM

The rate of material removal in ECM is governed by Faraday's law, since it is a function of current. The primary variables
that affect the current density and the material removal rate are:
· Voltage
· Feed rate
· Electrolyte conductivity
· Electrolyte composition
· Electrolyte flow
· Workpiece material

The voltage across the gap influences the current and the material removal rate and is controlled in most ECM
operations. However, for a constant voltage, the current also depends on the electrical resistance of the cutting gap.
Resistance is much more difficult to control because it depends on the conductivity of the electrolyte and the distance
across the gap.
The feed rate, or penetration rate, is also controlled in most ECM operations. At a constant voltage, the gap is inversely
proportional to the feed rate. The distance across the frontal gap is a function of feed rate because, as the cathode is fed
into the workpiece at a higher rate, the gap closes, causing resistance to drop. As resistance drops, amperage increases;
therefore, machining rate also increases until an equilibrium is reached. At slower feed rates, the material removal rate
decreases as the gap increases because the cathode is not keeping up with the workpiece surface. As the gap increases, the
resistance rises and amperage drops. Frontal gaps are usually between 0.1 to 0.8 mm (0.005 to 0.030 in.), and side gaps, in
the case of drilling, are about 0.5 to 1.3 mm (0.020 to 0.050 in.).
The feed rate also varies directly with the current. For example, a hole machined at 2.5 mm/min (0.100 in./min) at 10 V
and 1000 A would require 2000 A if the feed were increased to 5.0 mm/min (0.200 in./min). This would also require a
potential of about 20 V and would increase power consumption (V · I) from 10 to 40 kW.
The feed rate also depends on the application. Typical feed rates for different ECM operations on Inconel 718 are:

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