The main problem is that the cause and effect of certain faults are still unclear.
More research is required to find out which sensors are more adequate for moni-
toring specific faults so that redundant information can be minimized. Further-
more, more work has to be done to reduce the size and cost of the sensors so that
a ‘complete sensing’ approach can be seriously taken into account.
277
4.5.5
References
1
Barthel, K., Trunzer, W., Laser-Praxis
(1994) 10.
2
Anon., Opto Laser Eur. (1997) 3.
3
Schwede, H., Kramer, R., in: Proceedings
of ICALEO 1998, Orlando, FL;
www.primes.de.
4
Eg, Prometec UFC60; www.prometec.com.
5
Www.precitec.com.
6
Friedel, R:, EuroLaser (2000) 1.
7
Tönshoff, H. K., Schumacher, J., in:
Proceedings of ICALEO 1996, Detroit, MI.
8
Ikeda, T., Kojima, T., Tu, J. F., Ohmura,
E., Miyamoto, I., Nagashima, T., Tsubo-
ta, S., Ishide, T., in: Proceedings of ICA-
LEO 1999, San Diego, CA.

Thus the machining force is a minor or, rather, a negligible parameter in the
control of this process. On the other hand, precise, dynamic control of the spark
gap is essential for stable repetition of the discharge. These features make the
control system unique and, consequently, the sensing strategy must be different
from that in the case of conventional machining processes.
Sensors in Manufacturing. Edited by H.K. Tönshoff, I. Inasaki
Copyright © 2001 Wiley-VCH Verlag GmbH
ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic)
4.6.2
Principle of EDM
From the viewpoint of shape specification of the workpiece, EDM is included in
the same group as cutting where the shape of the tool is copied on to the work-
piece or further modified according to the trajectory of the tool movement (rela-
tive to the workpiece). The tool is referred to as the electrode.
The main difference between EDM and conventional machining processes such
as cutting is in the mechanism of material removal. The material removal process
in EDM is illustrated in Figure 4.6-1.
A voltage pulse from a pulse generator initiates and maintains a short arc dis-
charge through the dielectric fluid in the gap between the electrode and the work-
piece. The discharge heats the surface layer of the workpiece locally and melts
(and also partially vaporizes) a small part of the workpiece. The simultaneous va-
porization of the fluid produces a high pressure and the molten material disinte-
grates into microsize particles, forming a shallow crater on the workpiece surface.
The process leading to this crater formation is repeated over the entire work-
piece surface facing the electrode, producing a thin cavity layer on the workpiece.
By continuing this process with the feed of the electrode facing downward, the en-
tire electrode shape is replicated on to the workpiece.
4 Sensors for Process Monitoring278
Fig. 4.6-1
Principle of EDM

Fortunately, as the removal mechanism of EDM is based on an electrical phe-
nomenon, useful information is obtainable from the electrical parameters of the
circuit for discharge. The most frequently used parameters are the gap voltage
and the current through the gap, as shown in Figure 4.6-3.
Some other parameters can also provide information on the machining status.
The more promising ones are the electromagnetic radiation from the arc and the
acoustic radiation caused by discharges.
In the detection of these parameter values, sensors may play a useful role. To
date, sensors have played a minor role in EDM control, because the more useful
parameters, such as the voltage and the current, can be detected without special
sensors. However, in some cases, sensors offer merits from the viewpoints of im-
proved technology and economics.
4.6 Electrical Discharge Machining 279
The following is an overview of the major technologies used in obtaining gap
information.
4.6.4.1
Gap Voltage
In EDM, the gap voltage contains numerous information. The status of the gap is
indicated by the gap voltage as shown in Figure 4.6-4.
The figure shows only typical examples but there also occur various transient
and combined types of voltage waveforms. These voltage pulses are input into the
control system and statistical calculation is performed which provides the neces-
sary values that can be compared with the set values for proceeding according to
the flow chart shown in Figure 4.6-3.
In this detection of gap voltage, no special sensor is usually required. A conven-
tional high-impedance probe commonly used for measuring equipment such as
an oscilloscope can be used without any problems. The voltage range is usually
from 50 to 200 V.
4 Sensors for Process Monitoring280
Fig. 4.6-2

according to the status of gap