22-09-2012, 01:14 PM
MACHlNES FOR NON-CONVENTIONAL TECHNOLOGIES
1Electrodischarge machines.pdf (Size: 2.61 MB / Downloads: 83)
Classification and comparisons
Machines and equipments which do not apply machining technology based on cutting process
can be classified as non-conventional machines. The marking as “non-conventional” express
the special principle of technology such as electro-chemical, mechanical - so called “cool”
processes or electro-thermal processes.
Figure 6.1 illustrates the position occupied by erosion machines in the total complex of metalseparation
machines. According to the erosion principle involved, differentiation is made
between erosion through chemical and electrochemical action, erosion by discharge of an
electric spark, and erosion through high energy beams as in laser and electron-beam
techniques.
Electrochemical machines (ECM)
The principle underlying electrochemical machining (ECM) rests on an exchange of charge and
material between a positively charged anodic workpiece and a negatively charged cathodic tool
in an electrolyte. In such conditions, the anode dissolves whilst the cathode (tool) is not
affected. The volume of metal removal may be calculated according to Faraday's Law:
v= C I t
where C is a constant dependent on work material, I is the current flowing between the tool and
the work, and t is the time of erosion. The current is dependent on the gap between tool and
work, the are a of erosion and the conductivity of the electrolyte, as well as the supply voltage.
The working gap maintained between the electrode and work allows machining to' take place
without physical contact. The electrolyte (e.g. NaCl or NaNO3 in water solution) is pumped into
the working gap and also serves as a coolant which is necessary due to the high energy
density. The material which has been eroded from the work forms a sludge, and must be
separated from the electrolyte by filters or centrifuges.
Electrochemical die-sinking machines
Figure 6.3 presents, in schematic form, the main components of an electrochemical diesinking
machine. A feeding device advances the tool towards the work in accordance with the
rate of metal removal. When producing internal forms, a design problem arises in relation to
the shape and size of the too1. The gap is not constant, but is a function of the state of the
surface to be eroded and the rate of tool advance. If, for example, a cylindrical bore is to be
sunk, a simple cylindrical tool (as shown in Fig. 6.4) is not suitable. This would result in a
constantly increasing gap size and the current density would decrease in proportion (Fig. 6.4,
left). With a tool suitably insulated on its sides (Fig. 6.4, right), the offending excessive erosion
of cylindrical sides will be suppressed.
Electrochemical de-burring machines
In contrast to electrochemical die-sinking, when ECM is used for de-burring there is no
advance of the tool. All other machine elements are identical. The work is mostly carried out in
a fixture as is shown by the example in Fig. 6.8 of a machine for de-burring gear wheels. The
fixture contains the tool electrode and holds the work. The working current is transmitted
through spring contacts. The cathodic areas consist of those points from which the material is
to be removed (in the example, the front and back of the tooth flank). The electrolyte is fed to
the same points. The extent to which the sharp edges and burrs are removed is dependent
upon the magnitude of the electric field.
Electrodischarge machines (EDM) (spark erosion)
When applying the EDM process, the material is eroded as a result of an electrical discharge
between tool and work. Due to the resultant short-lived, but very high, temperature rises, metal
particles at the point of discharge are molten, partially vapourized and removed from the melt
by mechanical and electromagnetic forces. The working medium is a dielectric, which washes
the eroded material away and simultaneously acts as a coolant.
As in the case of ECM, EDM is a copying process where there is no contact between tool
and work. Contrary to ECM, however, there is some erosion of the tool in EDM, which must be
allowed for in the tool design to ensure accuracy of machining.