I n d I a n I n s t I t u t e o f t e c h n o l o g y

R o p a R [ I n d I a ]

Manufacturing with Metallic Materials MEL202

E L E C T R O J E T D R I L L I N G

By Manish Anand

CONTENTS Introduction Working Experimental Results and Discussions MRR Radial Overcut Hole Taper

Desirable Conditions of hole Advantages and Disadvantages Conclusions References

Introduction

Demands of small size machines have directed our attention to nontraditional techniques

EJD is Non-traditional method Micro level hole drilling Use in = cooling holes in jet turbine blades,

printed circuit board, inkjet printer head, surgical implants,

Working

a negatively charged stream of acid electrolyte is impinged on the workpiece to form a hole. The acid electrolyte (10–25% concentration) is passed under pressure (0.3–1.0 MPa) through a finely drawn glass tube nozzle. The electrolyte jet acts as a cathode when a platinum wire, inserted into the glass tube well above the fine capillary is connected to the negative terminal of a DC power supply. The workpiece acts as an anode. When a suitable electric potential is applied across the two electrodes, the material removal takes place through electrolytic dissolution as the electrolyte stream strikes the workpiece. The metal ions thus removed are carried away with the flow of the electrolyte. A much longer and thinner electrolyte flow path requires much higher voltage (150–750 V) so as to effect sufficient current flow.

Lack of Knowledge Available literature mainly deals with qualitative

description Relation between influencing parameters and effects are

not completely known. Experiments

Central composite Design[CCD] Response Surface Method[RSM]

INPUT OUTPUT

Applied Voltage, Electrolyte concentration, Feed Rate

Overcut, Taper, material Removal

Material Removal Rate(MRR) MRR=(W1-W2)/time

W1,W2 are initial and final weight of work piece. Radial Overcut The difference between the size of the electrode and the size

of the cavity created during machining. Overcut =[d(entry)-d(glass capillary)]/2

d(entry),d(glass capillary are diameter of entry to work piece and capillary

Hole Taper An angled, gradually narrowing feature on a part. Tapper(Ø)=Tan-1[(dentry-dexit)/2t]

dentry-dexit=difference between diameters of jet interring to piece and exiting piece. t= thickness of work piece.

Experimental results and discussion Using CCD and RSD techniques experiment and data

collection was performance. Graphs were plotted and discussions were noted.

Experiment setup

MRR

• Applied Voltage – As voltage increases-current increases-MRR

increases(Faraday’s Law) – Increases rapidly above 350 V

• Electrolyte Conc. – Increase in electrolytes conc. –increases MRR

–because it increases conductivity –more amount of current flow

• Feed rate – Increase in FR-reduces inter electrode gap-

leads to smaller ohmic resistance-inc electrolyzing current

3D surface of MRR model

Main Effect of Input parameters on MRR

Radial Overcut

• Decides the quality of EJD holes • Applied Voltage

– Increases in applied voltage- greater overcut • Electrolyte Conc.

– Increase in electrolytic conc.-greater overcut • Feed Rate

– Higher feed rate-less radial overcut-because less interaction time

• Current – Inc. in current-increases overcut

3D surfaces of radial overcut

Main Effect of parameter over radial overcut

Hole Taper

• Hole taper –depends on diff. between hole entrance diameter and hole exit diameter.

• Increasing applied voltage and electrolyte concentration- results in greater hole taper - reasons for this is that the electro jet remains in contact with the entry side of the workpiece for a maximum period of time resulting in a larger hole entrance diameter than the hole exit diameter

3D surface of hole taper model

Main effect of input parameters on hole taper

Desirable Condition for hole making

• Low overcut and reduced hole taper

– In favoured condition

• V=325 DC Volts • 17.5 % electrolyte conc. • Feed rate 0.5 mm/min

Advantages and Disadvantages • Advantages

– Micro-level holes can be made. – Applied on hard and brittle material – Material are removed easily – Less costly than traditional drilling

• Disadvantages – Set up should not vibrate otherwise hole will get large

or deform. – More maintenance

Conclusions • Applied voltage, electrolyte concentration, feed rate and interaction between the

applied voltage and electrolyte concentration are the significant parameters in all the three models.

• The increase in feed rate not only enhances the material removal but it also enables to control the radial overcut as well as hole taper. Since the fine glass nozzle, used to direct the electrolyte jet on the workpiece in the EJD process, moves inside the workpiece therefore a judicious selection of feed rate is very important for the success of the process in view of the fragile nature of the glass. The selected feed rates should be compatible with the dissolution efficiency of the work material at the applied potential level.

• Within the overall range of test conditions employed, the optimum results, that is maximum MRR with least radial overcut and small hole taper were achieved in the following ranges of the parameters.

– Voltage: 190−325 V – Electrolyte concentration: 13-17.5% – Feed rate: 0.2–0.5 mm/min.

• Higher applied voltage, though it results in greater MRR, tends to produce holes of poor quality in terms of large radial overcut and hole taper.

References

• Iit roorkee sites • Ahmed MS, Duffield A (1990) The drilling of

small deep holes by acid ECM. Proc Advanced Machining Technology III Conference, Chicago, IL, 4–6 Sept 1990, MR90-243, pp1–13

• Design-Expert Version 6.0.8, Stat-Ease Inc., Minneapolis

• Montgomery DC, Peck EA (1992) Introduction to linear regression analysis. Wiley, New York