Research Collection

Conference Paper

Die-sink EDM in meso-micro machining

Author(s): Maradia, Umang; Boccadoro, Marco; Stirnimann, Josef; Beltrami, I.; Kuster, Friedrich; Wegener, Konrad

Publication Date: 2012

Permanent Link: https://doi.org/10.3929/ethz-a-007355601

Rights / License: In Copyright - Non-Commercial Use Permitted

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ETH Library

Procedia CIRP 00 (2011) 000–000

5th

CIRP Conference on High Performance Cutting 2012

Die-sink EDM in meso-micro machining

U. Maradiaa*, M. Boccadoro

b, J. Stirnimann

c, I. Beltrami

b, F. Kuster

a, K. Wegener

a,c

aInstitute of Machine Tools and Manufacturing, ETH Zurich, Zurich 8092, Switzerland bAgie Charmilles SA, Losone 6616, Switzerland

cinspire AG, ETH Zurich, Zurich 8092, Switzerland

* Corresponding author. Tel.: +41-44-632 91 36; fax: +41-44-632 11 25.E-mail address: maradia@iwf.mavt.ethz.ch.

Abstract

Micro EDM has been identified since more than a decade as suitable process for machining complex shaped structures with high

aspect ratio, though only through variations of EDM such as micro EDM milling, micro EDM drilling, coated electrodes etc. In the

current paper, we present the research focused on analysing the capability for implementation of die-sink EDM in meso - micro

scale machining (structures with surface area smaller than 10mm2 down to 0.05mm2) by concentrating on primary process

parameters to obtain high material removal rate, low tool electrode wear with high form accuracy and precision. Graphite electrodes

were mill machined in meso-micro scale with high precision and accuracy. Low tool wear technology was developed for using

graphite electrodes in meso-micro EDM offering economical and energy efficient solution in meso-micro scale machining.

© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of CIRP

Keywords: Micromachining; Electrical Discharge Machining (EDM); Precision manufacturing

1. Introduction

Ever increasing space, energy and efficiency

requirements are pushing products towards

miniaturisation in several areas such as biomedicine,

electronics, dies-moulds, optics, energy, micro

mechanics, micro fluidics, aerospace and aeronautics to

name a few [1]. The parts for such devices are

manufactured directly or using replication techniques for

large throughput, where manufacturing restrictions

considerably influence the design process and sometimes

overall performance of the product [2]. In the

manufacturing process chains for such products, various

machining techniques such as micro-milling, micro-

EDM, micro-ECM, IBM, Laser ablation, etc. are

required either to machine final parts or for the die and

mould machining for the replication techniques such as

micro-injection moulding, micro-casting, micro-

stamping [2][3]. Micro-milling is a promising technique

with advancements in tool coatings, tool geometry, high

precision machine structures and spindles, though high

aspect ratio (L/D>10) structures and small feature

dimensions (below 100µm) remain a challenge. Laser

ablation with advancements in optics and with ultra-short

pico- and femto- second pulses is finding increasing

applications especially in bio-medicine sector, but higher

costs limits its wide spread application. On the other

hand, Micro EDM has been identified as one of the

potential micro-machining techniques with obvious

advantages of machining complex structures with high

aspect ratios, high precision and accuracy irrespective of

work-piece material hardness and toughness.

2. Meso - Micro EDM

The main difference between conventional EDM and

micro-EDM can be drawn by smallest feature

dimensions to be machined by the process which must

also consider the contemporary levels of conventional

technologies. In this regard, paradigm shift in process

behaviour and thus need for process control allows

definition of meso- and micro- EDM, where machining

of features with projection area above 10mm2 is

considered conventional EDM, between 10mm2-1mm2 is

considered meso EDM and surface area 1mm2 and below

or smallest dimension below 1mm is considered micro-

2 Maradia et al. / Procedia CIRP 00 (2011) 000–000

FormRadii / angle

Frontal wear

Lateral wear

Φ 0.8 mm

EDM. In this regime of EDM, variants such as micro

EDM drilling, micro EDM milling have been developed

[4] [5]. In spite of its ability to machine complex cavities

with high accuracy, micro EDM milling is employed

mainly in niche applications whereas micro EDM

drilling is limited by shape of the tool electrode.

Even though large amount of research is dedicated in

the field, there is currently no existing alternative of

EDM which offers manufacturing industry with an

option to machine features in meso- and micro scale with

the same ease as conventional EDM due to involved high

tool wear, low material removal rate, surface –

subsurface damages and thus poor process efficiency in

this regime. The aim of current research is to facilitate an

industrial, economical and energy efficient solution for

meso-micro scale machining using die-sink EDM

through qualitative and quantitative analysis of the

process. The current paper introduces challenges for die-

sink EDM implementation in meso-micro scale followed

by the experimental setup for machining graphite

electrodes in meso-micro scale and details of equipment

used for the current research. Initial results are then

presented along with discussion followed by conclusions

and outlook.

2.1. Process outputs and challenges

Material removal rate, tool wear and surface quality

are the main process outputs of EDM. The challenge for

direct implementation of EDM in meso- micro scale

machining is not only overall lower process efficiency

but also high tool electrode wear. Various methods have

been suggested such as coating of tool electrode [6],

novel materials for tool material [7] or tool wear

compensation [8] in addition to conventional method of

using multiple electrodes. However, these methods

cannot be widely implemented in machining industry

due to involved costs of materials, equipment and

process sensitivity. Also, electrode materials such as

tungsten carbide are very difficult to machine and thus

are limited by its machinability in meso-micro scale. On

other hand, conventional electrode materials such as

copper and graphite are easy to machine by milling or

WEDM in meso-micro scale but incur extremely high

electrode wear in meso-micro scale EDM, which

requires multiple electrodes strategy. Especially for high

aspect ratio structures and precision manufacturing, the

number of required electrodes is quite high for meso-

micro EDM since it follows a vicious cycle of corner

wear as shown in fig. 1, which ultimately restricts the

form accuracy of the machined part. As shown in fig. 1,

first tool electrode being subjected to corner wear with

increasing machining depth ultimately wear out frontally

causing frontal and lateral wear (see, fig. 2).

Fig. 1. Effect of tool wear on final workpiece form accuracy for multi

electrode machining strategy.

The subsequent electrodes 2, 3, 4, 5 then start

machining the contour produced on workpiece where

electrode corners are initially engaged causing again

high wear of corners leading to shape deformation. Thus,

for example one achieves desired form precision,

accuracy and smallest corner radius on workpiece by

using 5 electrodes. Multi-electrode strategy thus results

in higher costs, higher energy - resource requirements

and lower productivity for meso-micro scale machining.

Also for precision machining, lower accuracy of

machined parts is expected due to positioning errors

involved while machining and changing tool electrodes.

Fig. 2. Characterization of tool electrode wear: frontal wear, lateral

wear and form distortion. Original electrode diameter in background is

0.8mm, length 5mm; machined from Graphite with average grain size

7µm. Applied maximum current per pulse is 20A, positive polarity.

Maradia et al. / Procedia CIRP 00 (2011) 000–00 3

3. Experimental setup and methods

3.1. Milling of graphite electrodes in meso-micro scale

With developments in EDM grade graphite with fine

grain sizes (<5µm), machining of extremely small

structures with high aspect ratios and complex structures

has become possible. It must be noted only few

publications [9][10][11] are available dedicated to

graphite electrode machining and no publications were

found studying the graphite machining in meso- micro

scale.

High precision 5-axes machining centre Willemin

W518MT was used to machine the electrodes where a

hand-written cam strategy was used and optimised to

achieve precise form of the electrode with high

accuracy. To prevent damage of machine elements and

maintain stable process, a graphite dust suction system

was devised (see, fig. 3) using industrial vacuum suction

pump and a regulated air flow was provided to keep the

machining region of electrode free from dust-clogging

which may distort the machined electrode geometry.

Diamond coated special end-mill for graphite machining

from Fraisa SA was used. For roughing, 2mm diameter

and 10mm long end-mill whereas 1mm diameter and

10mm long end-mill with 200µm corner radius was used

for finishing operations.

Fig. 3. Experimental setup for machining of graphite electrodes using

milling in meso- micro scale.

Machined graphite for tool electrode was POCO

EDM-3 grade with average particle size smaller than

5µm and Ringsdorf R8650 with average particle size

7µm according to data specification from the

manufacturer. The spindle speed above 85’000RPM

specified by end-mill manufacturer was not attainable

with specified machine and thus maximum spindle speed

used was limited to 20’000RPM. Depth of cut was kept

at 0.1mm whereas cutting velocity was set at

300mm/min. Additionally, turning operation was used to

machine cylindrical electrodes and Wire-EDM was used

to machine copper electrodes in meso-micro scale.

3.2. Equipment for EDM

High precision die-sink EDM apparatus Form 1000

from Agie Charmilles SA was used during current

research. The machine structure is capable of positioning

accuracy of less than 1µm and designed with optimal

strategy for cooling, keeping workspace at constant

temperature to achieve high precision and accuracy. No

additional changes were made in the equipment since

main focus of research lies in facilitating every

manufacturing company with meso-micro scale

machining at no added costs or skills. Special adaptive

process control algorithms were generated during the

work to achieve desired process outputs.

3.3. Experimental conditions

As mentioned earlier, electrode materials were

selected as graphite and copper since they are most often

used electrode materials in conventional EDM.

Maximum pulse current in range of 2-20A with positive

tool polarity and pulse duration in range of 5-180µs were

used. Oelheld IME110 hydrocarbon based dielectric was

used for all EDM experiments and hot work steel 1.2343

was mainly used as work-piece material.

4. Results and discussion

4.1. Meso-micro scale graphite electrodes

Fig. 4. Examples of machined graphite electrodes in meso-micro scale:

Left: Features with different surface areas and ribs in micro scale and

6mm depth. Centre: Lookup table (LUT) processed image by

contrasting different machined features to attest their form accuracy.

Right: 15x15 mm2 graphite electrode with machined features of

different sizes from surface area 1mm2 down to 0.075mm2.

As can be seen in fig. 4, high precision micro

structures can be machined with relative ease in a

conventional workshop environment. Since the CAM

software strategies could not bring desired form

accuracy in micro scale dimensions (see, fig. 6c), a

manual tool path was generated using loop around

strategy as shown in fig. 5.

4 Maradia et al. / Procedia CIRP 00 (2011) 000–000

#1

#1

#2

#2

Work-piece

Work-piece

Work-piece

Work-piece

Work-piece

Tool electrode

Tool electrodeat end of roughing,

semi-finishing

Tool electrodeat end of finishing

End desired form in

work-piece

(a) (b)

1 mm

0.150 x 0.150 x 7.5 mm3

(a)

(b) (c)

Fig. 5. Examples of various machining strategies to achieve high form

accuracy including manual tool path adopted during this research using

loop around method.

Fig. 6. (a) Example of machined graphite electrode having cross

section area (0.15x0.15) mm2 and length 7.5mm (L/D =50). (b) Cross

section of shown electrode depicting the form precision achievable

using current loop around strategy. (c) Cross section of tool electrode

machined using normal CAM strategy.

Current limits were found by machining a (0.15x.015)

mm2 cross-section electrode with length of 7.5mm thus

L/D aspect ratio of 50 as shown in fig. 6. It is clear from

the results that graphite electrodes are easily machined in

meso- micro scale and thus graphite can be esteemed as

ideal electrode material for meso-micro EDM. Removal

of the dust from the machined region is crucial to keep

machining stable. Also, better contour strategies are

required in existing CAM software for meso-micro scale

milling of graphite electrodes.

4.2. Meso-micro EDM

Recently several EDM equipment manufacturers have

offered zero wear technology which considerably

reduces tool wear in conventional die-sink EDM. In

current research, this technology has been implemented

successfully even in meso-micro EDM, solving one of

the biggest challenges for implementation of die-sink

EDM in this regime even with conventional machines

and electrode materials.

As shown in fig. 7, a turned graphite electrode having

diameter 0.62mm and length 20mm was used to machine

hot work steel 1.2343 in a hydrocarbon based dielectric

oil using maximum 5A pulse current and positive tool

polarity. The process technology was optimized such as

to have near zero frontal and lateral tool wear and thus

achieve better form accuracy at end of roughing step

with just one tool electrode. As shown in fig. 8, this

technology avoids vicious cycle of tool corner wear for

semi-finishing and finishing operations and thus

considerably reduces number of required tool electrodes.

Fig. 7. Example of graphite electrode before and after erosion having

diameter 0.62mm and length 20mm. Depth of erosion was 18mm with

frontal gap compensation of 0.065mm for finishing operations. Cavity

produced in workpiece is shown on right.

Fig. 8. (a) Effect of low or zero frontal and lateral tool wear technology

on final cavity and reduced number of required electrodes using multi-

electrode strategy. (b) Longitudinal cross section of machined cavity

after roughing step with single 0.7mmx0.7mm cross section graphite

electrode. Depth of erosion is 5mm and applied current is 3A.

Fig. 9. Left image is Secondary Electron scan highlighting topography

of carbon layer on graphite tool electrode. Right image is Back

Scattered Electron Diffraction showing bright sub-micron sized

metallic particles embedded in a dark region which is carbon.

Underlying mechanism of this low or zero wear

technology is carbon layer build-up mainly on the frontal

face of the electrode where most of the discharges take

place.

G09Exact stop check

G64Normal cutting mode

Manual tool pathLoop around

G09

Exact stop

check

G64

Normal

cutting mode

Manual tool path

Loop around

Tool electrode Work-piece

Before erosion After erosion

Ø 0.780mm x 17.770mm

20mm

20µm 20µm

Maradia et al. / Procedia CIRP 00 (2011) 000–00 5

ED

EW

EE

Pulse duration

% e

ne

rgy

Total Energy E = EE + EW + ED

= U · I · Ton

As shown in fig. 9, this discharge plasma formed

layer consists of carbon embedded with sub-micron

sized metal particles ejected during the process from

workpiece material [12] and was confirmed using

Energy-dispersive X-ray spectroscopy (EDX). The

carbon layer formation as mentioned by Mohri et al. [13]

mainly contains turbostratic structure where Iron,

Nickel, Chromium, etc. act as a catalyst for carbon

precipitation [14]. The carbon layer is a decomposition

product from hydrocarbon based dielectric through

pyrolysis occurring during the discharge plasma. It has

been observed that lateral wear resulting in conical shape

of electrode after certain depth of erosion is mainly

caused by the flushing cycles used to remove debris

from discharge region at every 0.2-1 second interval and

rapid movement of the electrode (about 10m/s) in the

eroded cavity building high pressure gradients. The

eroded particles gaining this momentum would cause

abrasion of the faces of the electrode as shown in fig. 10.

Fig. 10. Left: Rapid electrode movement in the eroded cavity and

eroded particles causing abrasion of electrode faces. Right:

Hydrodynamic effect during spark as a possible cause for wear.

Also, hi-speed imaging (20-50,000 fps) was

employed to observe the process in-mist (fig. 10 right)

and in-liquid dielectric which revealed cloud build-up

and gas bubble - shock waves respectively suggesting

strong hydrodynamic forces in discharge region which

may also be contributing to tool wear.

The process originated pyrolytic graphite layer on the

electrode is mostly harder than the original graphite

electrode material [15] whereas embedded metal

particles are measured to be 10 to 30 times harder which

sustains the difficult erosion conditions and prevents

wearing away of the base electrode material. The type

and structure of carbon depends mainly on involved

temperature and pressure [16], which can be controlled

by EDM process parameters [17]. In terms of modelling

this effect, pyrolysis of hydrocarbon is complex and still

not fully understood [18] since it involves a very large

number of possible reactions and chains of reactions

occurring in their breakdown such as ionization,

dissociation, dissociative ionization, dissociative

recombination, charge exchange reactions with hydrogen

[19]. Carbon deposits during EDM are observed to be of

a hemispherical nature, suggesting cathodic deposits

mentioned by Koprinarov et al. [20].

Fig. 11. Energy balance over pulse duration of spark from few

microseconds after dielectric breakdown till 150µs, qualitatively

inferred from the process outputs.

Since the power supplied in the discharge gap is

dependent on current, discharge voltage and pulse

duration; the supplied constant energy is mainly divided

into three main components of energy dissipation into:

electrode (EE), workpiece (EW) and dielectric (ED) as

shown in fig. 11. The energy dissipating into dielectric is

mainly used for plasma channel expansion through

hydrocarbon decomposition at boundary layer. The

energy balance information and involved mechanisms

can then be used to shape pulses to achieve optimum

results e.g. low initial current can be applied to reduce

EE and sudden increase in applied current a few micro-

seconds after discharge breakdown to benefit from high

EW, resulting in low tool wear and high material removal

rate (MRR). This energy balance is also affected by

material properties of anode-cathode and dielectric.

Melting temperature of workpiece material seems to

have high influence on material removal rates whereas

specific heat of workpiece material has been currently

identified as having high impact on carbon build-up

process. Difficult to machine materials such as Titanium

alloys (TiAl6V4), Nickel alloys (Inconel 718, Inconel

738LC), powder metallurgy stainless steel

(X170CrVMo18-3-1), hot work steel (1.2343, 1.4435),

etc. have been machined successfully with zero or low

wear technology using graphite electrodes in meso-

micro scale EDM.

5. Conclusion and Outlook

Considering significance of machining requirements

for miniaturized products, die-sink EDM technology was

successfully implemented in meso-micro machining

offering high productivity, form accuracy-precision,

lower energy-resource requirements and lower overall

6 Maradia et al. / Procedia CIRP 00 (2011) 000–000

costs; that too without any additional need of apparatus,

materials, processes or skills. Using conventional

electrode materials such as graphite and copper having

good machinability, various difficult to machine

materials such as titanium alloys, nickel alloys, hot work

steels, stainless steels and powder metallurgy steels can

be ED-Machined with high accuracy and precision in

meso-micro scale (meso: 10mm2-1mm2, micro: <1mm2 /

<1mm) having high aspect ratios (L/D>10) and complex

shapes (circular, polygonal, etc.). Clearly, outstanding

performance of meso-micro EDM is incomparable to

currently available alternative micro-machining

techniques.

As further steps, the lower limit of micro EDM will

be pushed further from current limit of surface area

0.1mm2 down to 0.05mm2 or smaller. Here, due to its

advantages to machine multiple features on single

graphite electrode, surface adaptive technology will be

generated [21] in order to offer ability to machine

macro-meso-micro scale structures on single electrode

with high material removal rate while incurring lower

tool electrode wear. Also, another open aspect is

Nanometrology of precision meso-micro machined parts,

dies and moulds; which is extremely difficult with

currently available instruments due to involved smaller

dimensions and high aspect ratios.

Acknowledgements

The Authors wish to acknowledge the financial

support by the Swiss Commission of Technology and

Innovation (CTI). The first author would also like to

thank personnel at Agie Charmilles SA for their support.

The author also acknowledges support by the Electron

Microscopy Center of the ETH Zurich (EMEZ),

especially Dr. K. Kunze and finally Mr. J. Boos (Inspire

AG) for machining electrodes.

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