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  • 标题:Detection of CWEDM process irregularity with discharge pulses monitoring.
  • 作者:Gjeldum, Nikola ; Veza, Ivica ; Bilic, Bozenko
  • 期刊名称:Annals of DAAAM & Proceedings
  • 印刷版ISSN:1726-9679
  • 出版年度:2009
  • 期号:January
  • 语种:English
  • 出版社:DAAAM International Vienna
  • 摘要: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).
  • 关键词:Electric discharges;Electric discharges through gases;Machining;Production management;Wire

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
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