Detection of CWEDM process irregularity with discharge pulses monitoring.
Gjeldum, Nikola ; Veza, Ivica ; Bilic, Bozenko 等
1. INTRODUCTION
The trend in many industries, such as medical and aerospace, is to
minimize size components. At the same time the used materials, and
geometry demand are to some extent very difficult or not possible to
produce by convectional methods. Wire Electrical Discharge Machining
(WEDM) is a widely accepted non-traditional material removal process
used to manufacture components with complicated shapes and profiles.
WEDM utilizes a continuously travelling wire electrode made of copper,
brass or tungsten of diameter 0.020.3 mm which is capable of achieving
very small corner radii (Ho et al., 2004). Cylindrical Wire Electrical
Discharge Machining (CWEDM) is combination of WEDM machine and submerged
rotation spindle as a clamping system. Results of applying the CWEDM
process to generate precise cylindrical forms on hard materials which
are difficult to machine, are also presented (Qu et al., 2002).
Unlike traditional cutting and grinding processes which rely on a
much harder tool or abrasive material to remove the softer work material
the CWEDM process utilizes thermal energy to erode the workpiece material and generate the desired shape. Any materials that conduct
electricity can be machined by CWEDM. With CWEDM cylindrical parts with
complex geometry and very high L/D ratio can be produced in one process
step. In Fig. 1. are shown two main L/D geometry aspect ratios, which
value, by CWEDM technology, can be easily exceeded beyond the
conventional machining methods limits. In Fig. 2. two main cutting
approach are shown.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
The main application disadvantage of any EDM processes is amount of
time necessary for part production in comparison to conventional
machining. On the other side, the quality of produced components,
expressed by required product specifications such as used material,
dimension accuracy and surface properties, can be achieved easily by EDM
process. Machining speed depends on machining parameters like discharge
current, pulse duration, pulse frequency, wire speed and flushing
(Mahapatra & Patnaik, 2007). The internal corner radius produced in
CWEDM operations is limited by wire diameter and sparking gap. Produced
cylindrical electrodes can be used for EDM drilling of small holes,
micro deep holes, micro milling, and manufacturing of micro-nozzles.
Surface roughness Ra, roundness and MRR study on the CWEDM has been
carried out (Haddada & Tehranib, 2008). The material chosen in this
case was AISI D3 tool steel due to its growing range of applications in
the field of manufacturing tools, dies and moulds as punch, tapping,
reaming in cylindrical forms. The surface integrity of CWEDT parts is
investigated through a mathematical model. The production of gear wheels
with integrated shafts for easy gear assembly as one of possible
application is presented (Masuzawa et al., 2002).
2. ANALYSIS OF THE PULSES USED IN THE EDM PROCESS
Depending upon the situation in the gap which separates wire
electrode and workpiece, principally four different electrical pulses
may be distinguished: effective discharges or real sparks, arcs, short
circuits and open circuit or open voltage. Different pulse types are
shown in Fig. 3.
[FIGURE 3 OMITTED]
The effects of different pulse types on material removal and
surface quality differ quite significantly. Open voltages occur when the
distance between both electrodes is too large. When contact between tool
and workpiece takes place, a short circuit occurs. Sparks and arcs
contribute to material removal. Arcs reoccur in the same plasma channel
and can therefore severely damage the workpiece surface.
3. INFLUENCE OF WIRE FEED RATE
The overall process speed can be set by the wire feed rate along
numeric defined path. As the height of machined workpiece is not
constant it is not possible to use constant feed rate for every
combination of axial and radial paths. On the other side, every WEDM
machine is equipped with automatic servo feed which uses pulses
monitoring for closed loop feed rate control. The percent of efficiency
can be set as closed loop feedback information. Process irregularity shown in Fig. 6. can occur mainly during radial cut, because of very
small contact surface between workpiece and wire, in several conditions:
* The feed rate is set to higher value than can be achieved by
material removal rate.
* The geometry feature like crease or notch is big enough to
initialize process instability.
* The servo feed rate control loop failed to maintain process
stability, and it is unable to repair developed problem.
In Fig. 4. is shown voltage signal of CWEDM process during 90[ms]
period. Experiment had been done on AgieCUT 270 SF+F machine with System
3R spindle and Bercocut 0.25 wire. Despite impossibility of
distinguishing every discharge due to wide range of captured time, it is
possible to notice long periods of short circuits, which occur
periodically. Rotation of workpiece was set at rate n=1200[rpm]. The
time for one revolution [t.sub.r] can be therefore calculated:
[t.sub.r] = 60/n = 0.050 [s] = 50 [ms] (1)
In Fig. 4. failure in material removal process is obvious in
intervals of 50[m. ]. As the process is submerged, with high rotation
speed, it is mostly not possible to notice process irregularity. Servo
feed is normally not fast enough to reverse direction and repair the
problem, and process often continues until finished working path,
producing scrap workpiece.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Additionally, with constant feed rate it is possible to repair
developed irregular shape. Pulse graph of repair is shown in Fig. 5.
Cross section of machined workpiece during irregularity is shown in Fig.
6. After normal process up to radius [r.sub.n], the problem occurs, and
while wire can proceed further to the radius [r.sub.w], volume of
material A remains not removed. The only indicator of irregularity is
decreasing of feed rate and slightly changed process sound. In a case of
constant feed rate, process continues until full time short circuit
automatically stops the machine.
4. CONCLUSION
For WEDM the most important performance measures are material
removal rate, surface finish, dimensional accuracy and geometry
features. For CWEDM, workpiece rotation causes instability of material
removal process due to change of cutting height along machined path and
time. Process errors can be unrecognized by machine feed rate control
loop, what leads to final product geometry failure, or to necessary
rework. In case of constant feed rate, leak of parameter technology
tables contribute to feed rate selection failure.
Gap voltage monitoring can detect initialization of process
irregularity in early stage, which enables operator to slow down the
feed rate immediately and repair the problem. The rework procedure can
also be controlled with this monitoring approach. As this approach is
time consuming for operator, the future work will be in direction of
creating frame for CWEDM parameter technology and setting the cutting
strategies which will avoid possibility of this type of process errors.
Research described in this paper will result in one solution field
border establishment for given CWEDM technology setting.
5. REFERENCES
Haddada, M. J. & Tehranib A. F. (2008). Investigation of
cylindrical wire electrical discharge turning (CWEDT) of AISI D3 tool
steel based on statistical analysis, Journal of Material Processing
Technology, Vol. 198, page numbers 77-85. ISSN 0924-0136
Ho, K. H.; Newman, S. T.; Rahimifard, S. & Allen, R.D. (2004).
State of the art in wire electrical discharge machining (WEDM),
International Journal of Machine Tools and Manufacture, Vol. 44, page
numbers 1247-1259, ISSN 0890-6955
Mahapatra, S. S. & Patnaik, A. (2007). Optimization of wire
electrical discharge machining (WEDM) process parameters using Taguchi
method, International Journal of Advanced Manufacturing Technology, Vol.
34, page numbers 911-925, ISSN 0268-3768
Masuzawa, T.; Okajima, K.; Taguchi, T. & Fujino, M., (2002).
EDM-lathe for micro machining, CIRP Annals, Vol. 51, page numbers
355-358, ISSN 0007-8506
Qu, J.; Shih, A.J. & Scattergood, R.O. (2002). Development of
the cylindrical wire electrical discharge machining process, Part 1:
concept, design, and material removal rate, Journal of Manufacturing
Science and Engineering, Vol. 124, page numbers 705-712, ISSN 1087-1357
