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TitleAdvanced Machining Processes
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Total Pages369
Document Text Contents
Page 1

ADVAIVCKD
M A C I I I N I N C
PROCESSES

V IJA Y K. JA IN

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usually have closed loop control arrangement for work table and wire guide posi­
tions. This facility has been used to make, in one stroke, a draft or relief in the
mould, and constant radius comers along the tapered walls [Albert, 1983] of
molds and dies.

Stratified Wire
Properties of the wire used in this process have an impact on MRR and quality

of the cut surface. Nowadays, stratified wires are used as electrodes. These wires
are made of copper core with a thin layer of zinc over it (Fig. 7.24). Such a wire
can carry more current hence gives higher MRR. A stratified wire for wire EDM
has been developed by Charmiles Corporation of Switzerland. This wire has a
layer of zinc / zinc alloy on the outside and a core of drawn copper wire on the
inside just like electrical wire is clad with vinyl plastic for insulation. The wire is
used only once and then scrapped because it is not very expensive, and also
because it gets eroded during the process.

copper

zinc / zinc alloy

Fig. 7.24 Stratified wire used in wire EDM.

A wire can carry heavier load if it can absorb more amount of heat without
breaking. A heavier load also means a spark with more energy hence higher MRR
resulting in higher cutting speed. Zinc melts and even evaporates at a temperature
lower than the melting temperature of copper. In other words, core of the stratified
wire gets no hotter than the temperature at which the outer layer of the zinc will
vaporize. Hence, more powerful sparks can be discharged. MRR is related to
current density which, in turn, is related to the surface area of the wire engaged in
the cut. 168

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In the tiny space (0.025 mm or so), workpiece is eroded by a violent, extremely
localised and momentary action of the spark. Gas bubbles are rapidly formed
during this process; and on the way to escape out, these bubbles press against wire
in the gap. This pressure results in bowing of wire which tends to drag on bends
and sharp comers as it advances. As a remedy, wire guiding and feeding mecha­
nism should be designed such that the wire can withstand higher tension. There­
fore, the effect of gap condition on the quality of the product will be minimized.
Huge sized spools are used which can supply wire for machining for a long period
of time, say 60 hr or so [Albert, 1981].

PRO CESS VARIABLES
Most of the variables that control the process are common in case of EDM die

sinking as well as wire EDM. The linear cutting rate in wire EDM is dependent on
the thickness of the workpiece but not on the complexity of the cut. Wire speed
may be as high as 40 mm/s.

PRO CESS C H A R A C T ER ISTIC S

This process produces accurate m atte finish. Thousands of tiny craters on the
machined surface help in retaining the lubricating oil and result in increased die
life. Surface finish of the order of 0.1 jam can be achieved in finish pass. Accura­
cies of the magnitude of ±7 |im or even lower, in some cases, can be achieved.
However, uniformity of wire diameter, temperature as well as resistivity of the
dielectric should be closely controlled. With today’s systems, machining rate for
definite materials has gone up from 12.50 cm2/hr to about 40 cm2/hr.

A P P L IC A T IO N S

Wire EDM has been employed for making dies of various types. It is possible
to control tolerances very effectively. The process is also used for fabrication of
press tools and electrodes for use in other areas of EDM.

PROBLEM S
Problem 1

EDM is used to machine a metallic sheet. Calculate surface finish value if C = 15
|iF, Vb = 130 V, K6 = 4.0. Use the equation based on experimental results.

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EMPIRICAL APPROACH
DISCREPANCY BETWEEN THE ANALYTICAL AND EXPERIMENTAL RESULTS
EMPIRICAL EQS. DERIVED TO EVALUATE IEG IN THE TRANSITION & SIDE ZONES

NOMOGRAPHIC APPROACH
WITHOUT CALCULATIONS, IT PROVIDES DESIRED UNKNOWN VALUES OF PARAME­
TERS LIKE SIDE GAP, ELECTROLYTE TEMPERATURE, ETC, USEFUL IN PLANNING AN
ECM OPERATION

NUMERICAL METHODS
USED TO SOLVE LAPLACE EQN. FOR DETERMINING POTENTIAL OR TEMPERATURE
DISTRIBUTION IN IEG. THEN EQUILIBRIUM SHAPE IS DETERMINED AS DISCUSSED
EARLIER

CATHODE DESIGN FOR ECM PROCESS
COS ‘0’ METHOD

• 0 ANGLE BETWEEN FEED DIRN. AND NORMAL TO THE TOOL SURFACE
• NOT FOR SHARP CORNERS
• DETERMINE TOOL SHAPE FOR THE GIVEN WORKPIECE SHAPE

CORRECTION FACTOR METHOD
MODIFY ASSUMED TOOL SHAPE IN DIFFERENT STAGES UNTIL THE DIFFERENCE
BETWEEN THE PREDICTED W/P SHAPE AND DESIRED W/P SHAPE IS WITHIN SPECI­
FIED TOLERANCES

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