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A Holistic Approach for Zero-Defect Micro-EDM Milling

Jun Qian1, Jun Wang,Eleonora Ferraris,DominiekReynaerts 1Afd. PMA, Leuven University, Celestijnenlaan 300b - bus 2420, 3001 Leuven, Belgium

jun.qian@kuleuven.be

Abstract—Micro-EDM (µEDM) milling is a well-established micro-manufacturing technique, which offers three-dimensional and flexible machining capabilities for structuring electrically conductive difficult-to-machine materials. There has been considerable progress in micro-manufacturing capabilities by µEDM in recent years. Nevertheless there still exist a few challenges in terms of process accuracy and efficiency, and the achievable shape accuracy in micro-EDM milling is limited to 3-4 microns in vertical direction and 2-3 microns in plane. In order to reach zero-defect manufacturing with this micro-sparking technique, a holistic approach for precision micro-EDM milling is pursued at Leuven University. To improve the overall performance of the micro-EDM milling process, various upgrading has been carried out on a SARIX® machine, which includes monitoring and controlling of the stability of the sparking process (gap variation and energy distribution etc.), wear compensation of the tool-electrode, and on-machine metrology. Preliminary experiments have been carried out with promising results, and further system integration and application on industrial demonstrators are in progress.

Keywords—Micro-EDMmilling,tool-wear compensation, process monitoring

I. INTRODUCTION

Electrical Discharge Machining, more commonly known as Spark Erosion, is one of the popular non-traditional material removal processes in today’s manufacturing practice. Micro-EDM milling is one of the derivatives of the EDM process, which applies the same electro-thermal material removal mechanism but with much lower sparking energy. To realize low sparking energy, usually a relaxation (RC) type generator is used in micro-EDM machining. Meanwhile a small (diameter in the range of 1mm to 20µm) rotating cylindrical electrode travels under servo-control, and follows a similar predefined CAM route as in normal mechanical milling process. Materialisremovedlayerbylayer, and the layer thickness varies from a few µm to even below 1 µm. Fig 1

illustrates the basic principle of EDM and the schematics of

micro-EDM milling process.

Defects in micro-EDM in a broad definition include both undesired change of surface integrity and geometrical deviation compared to the technical specifications on the workpiece. The fundamentals of the EDM process imply that material removal is inevitable on the tool electrode. Therefore, without compensation of this tool-wear there will be geometrical defects generated on the workpiece. To reduce the influence of tool-wear in micro-EDM milling, researchers have been investigating the possible ways of predicting and compensating the tool-wear. With current technology (e.g. the combination of in-process discharge detection and anticipating tool-wear compensation) it is able to accurately predict the tool-wear for long prismatic tool electrodes in stable sparking conditions [1, 2, 3].

Althoughit is possible to quantitatively reduce the tool-wear by applying negative polarity on the tool and with very short pulses, accurate tool-wear compensation is still not available in cases such as changes offeature geometry, toolsof different dimensions, tool path variation or extended machining duration. Leuven University has a long tradition in EDM research and industrial cooperation including machine tool manufacturers. In recent years, research on micro-EDM milling technology for ceramic materials [4, 5] and precision mold-making has been a research priority at Leuven University and a holistic approach has emerged for zero-defect micro-EDM processing. This includes on-machine sparking monitoring, special wear compensation mechanism and upgrade of machine tool accuracy.

II. IN-SITU SPARKING MONITORING AND DISCRIMINATION Tool-wear on the electrode in the micro-EDM milling

process turns out to be one of the critical factors which affecs the dimensional and shape accuracy of produced components. In order to reduce these errors and process uncertainty, the applied energy in micro milling EDMis substantially low to the order of a few µJ, even in case of roughing process. A common approach for tool-wear compensation is through slot test machining. Fig.2 shows a measurement of the cross section profile of a machined test slot with nominal depth of 30 µm without wear compensation. The depth deviation in the slot is measured first; then the tool ratio is calculated and finally it is compensated in the following machining with an additional in-feed rate in the electrode’s axial direction. Although usually 5 test slots are machined to average out the random deviations, this approach still cannot represent the various sparking conditions, especially under unstable

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Fig. 1. EDM principle and schematics of micro-EDM milling [5]

machining situations such as at corners and small electrode.

An in-situ sparking monitoring and pumethod has been developed at Leuven approach primarily aims at establishing between sparking conditions and the welectrode, whereby the variable for wear comachine’s servo controller can be in-situ upFig. 3 depicts the scheme of the sparkinglayout of the measurement system. This icarried out on a SARIX® SX-100 machineacquisition is realized in LabVIEW® enviranalysis is carried out with MatLab®.

It is found in experiments that mosdischarge pulses consist of a positive dischnegative charge pulse. Positivepulse durationenergy setting (E), while negative pulse duboth energy setting (E) and open voltage (UE and U, both the sparking duration and negative pulses become comparable to ththus their influence on machining perfsignificant. In the lower energy regime (Figenergy distribution is close to a normal d

Fig. 2.Measured slot profile in test micro-ED

Fig. 3.Experimental setup for pulse monitoring a

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deep cavities with

ulse discrimination University. The the relationship

wear on the tool ompensation in the pdatedperiodically. g monitoring and implementation is . High-speed data ronment and data

st of the normal harge pulse and a n depends only on

uration depends on U). With increased the energy of the e positive pulses,

formance can be .4-a), the sparking distribution; while

with high energy setting (Fig. drastically and there are more t

Experimentresults also shobe established between the mpulse pattern and the tool-windicated in Fig.5, the Tdiscrimination is close to tTWD. This complies with prediction stated in [6], which a whole regardless of the differ

III. WIRE-EDMThe second component i

method of tool-wear compfeeding of fresh electrode mateEDM milling. The schematic the left in Fig. 6. A small diammoves around a small tool tip of the wire under this tip is efirst prototype of this experimthe middle of Fig.6. One pull

DM milling

and discrimination

(a)E

(b)E

Fig. 4.Sparking energy distribution h

Fig. 5.Tool-wear versus

4-b) the sparking energy varies than one energy concentration.

ow that a linear relationship can mean discharge energy of each ear per discharge (TWD). As WD calculated using pulse

the experimentally determined the conclusion on tool-wear is based on pulse population as

rence in pulse patterns.

M MICRO-MILLING n this holistic approach is a

pensation through continuous erial, which is realized by wire-of this method is illustrated on meter (ø30µm – ø100µm) wire during the process and the part

engaged in micro-sparking. The mental mechanism is depicted in

ley drives the ø100µm wire to

E100

E365

histogram at different energy settings

average sparking energy

feed around a small groove on the tool tip tungsten carbide which is shown on the left o

Fig.7 shows the results of preliminary this prototype. The effect of continuous feobvious and it enables the production of micro-fluid applications with constant depththe WC tool tip is also conductive, sparkinbetween this wire-holding tip and workpieceto the tool tip. A second generation tool tip under construction to overcome this problem

Fig. 6. Schematics and setup of wire-EDM

Fig. 7. Small grooves and pocket produced wire-E

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(ø1mm) made of of Fig.6.

experiments with feeding of wire is

small groove for h. However, since ng may also occur e, causing damage with Ruby ring is

m.

IV. IMPROVEMENT OF O

The third component of upgrading of the on-machine mto improve the absolute dimecomponents, the positioning ushould be as well. Therefore, carried out on the SARIX® HEIDENHAIN® linear scale haxis. The machine is originallyon the lead screw for the Z movthe backlash errors in the threinstallation of the linear scalelong-stroke measurement uncein the case of short stroke momeasured machine movement addition to the linear scale, a installed on the machine todiameter and length in a more erepeatability can be better than

V. CO

To realize zero-defectmicroapproach has been pursued to iprocess. Three of the key combeen introduced, namely in-siwear compensation mechanismmetrology. Preliminary experhave been carried out and fapplication on industrial demon

TABLE I. COMPARISONO

M milling

EDM milling

ON-MACHINE METROLOGY

the holistic approach is the

measurement hardware. In order ensional accuracy on machined uncertainty of the tool electrode

there have been two upgrades micro-EDM machine. First, a

has been installed along the Z-y equipped with a rotary encoder vement. Therefore, it is prone to ead of the lead screw. With the e, both the backlash errors and rtainty are improved, especially ovement. A comparison of the errors is given in Table 1. In BLUM® sensor has also been

o measure the tool electrode efficient way. The measurement 0.5µm.

ONCLUSION o-sparking machining, a holistic improve the performance of this mponents in this approach have itu process monitoring, unique m and upgrade of on-machine riments with promising results further system integration and nstrators are in progress.

OF ERRORSIN Z-AXIS MOVEMENT

229

ACKNOWLEDGMENT Part of this research has been carried out within the

framework of EC projectsMIDEMMA (Grant No. 285614) and Hi-Micro (Grant No. 314055).

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Manufacturing Technology, vol.49(2),pp. 473-488, 2000.. [2] Z.Y.Yu,T. Masuzawa,M. Fujiino,“Micro-EDM for

threedimensionalcavities - development of uniform wear method,”CIRP Annals - Manufacturing Technology,vol.47(1),pp.169–172, 1998.

[3] M. Kunieda, B. Lauwers,K. Rajurkar,B. Schumacher,“Advancing EDM through fundamental insight into the Process,”CIRP Annals - Manufacturing Technology,vol.54(2),pp.599–622,2005.

[4] K. Brans,Electrical Discharge Machining of Advanced Ceramic Composites, PhD Thesis, KU Leuven.

[5] K. Liu,B. Lauwers,D. Reynaerts ,“Process capabilities of micro-EDM and its applications,” in the international Journal of Advanced Manufacturing Technology,vol.47,pp.11-19,2010.

[6] G.Bissacco,H.N. Hansen, G.Tristo, J.Valentincic, “Feasibility of wear compensation in micro EDM milling based on discharge counting and discharge population characterization,”CIRP Annals-Manufacturing Technology, vol.60 (1),pp. 231–234,2011.