CIRP Annals - Manufacturing Technology 58 (2009) 185–188

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CIRP Annals - Manufacturing Technology

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Shaped tube electrochemical drilling of good quality holes

S. Ali a, S. Hinduja (1)b,*, J. Atkinson b, M. Pandya a

a ELE Advanced Technologies Ltd, Colne, UKb School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester M60 1QD, UK

A R T I C L E I N F O

Keywords:

ECM

Deep hole drilling

STEM

A B S T R A C T

This paper is concerned with the shaped tube electrochemical drilling (STED) of small diameter cooling

holes in turbine blades. Following an initial identification of the common defects occurring in STED, a

partial factorial design of experiments (DOE) was carried out to identify the more important process

parameters that affect hole quality. For these process parameters, a full factorial design of experiments

was conducted to study their effects not only on hole diameter but also on hole taper. Guidelines and

equations for determining appropriate operating conditions to drill a good quality hole are presented for

one drill diameter.

� 2009 CIRP.

1. Introduction

Turbine blades and nozzle guide vanes usually have severalsmall diameter cooling channel holes, with diameters as small as0.8 mm and aspect ratios as large as 250. Because of these largeaspect ratios, processes such as laser beam machining, electronbeam machining and conventional drilling are unsuitable forproducing these holes. The most suitable process is shaped tubeelectrolytic machining (STEM) as there are no residual stresses inthe workpiece and no cutting forces acting on the slender tool.

The earliest reference to STEM is due to Jackson and Olson [1]who used an acid electrolyte to dissolve the removed anodic metalrather than it forming sludge. Bellows [2] drew on experiences atthe General Electric Company to disseminate knowledge of theSTEM process by citing practical examples, metal removal ratesand achievable tolerances. In addition to comparing the capabil-ities of STEM, electrojet, electrostream and capillary drillingprocesses, Jorg [3] provided an overview of the practical difficultiesin shaped tube electrochemical drilling (STED). Cox [4] improvedour understanding of STEM by carrying out experimentalinvestigations which led him to conclude that (i) the nitric acidelectrolyte strength should be maintained between 12–15% (v/v)which is in agreement with the results of Jackson and Olson, and(ii) the surface finish of the drilled hole is affected by the durationof the forward and reverse voltage pulses. Janssen demonstratedthat STED could also be used for turbulated and micro holes [5].More recently, Sharma et al. [6], and Bilgi et al. [7] studied theSTEM process using pulsed electrochemical machining, the earlyanalysis of which was investigated by Kozak et al. [8], Rajurkaret al. [9] and Yu et al. [10]. Because these researchers did not usereverse polarity, a clean tool surface could not be assured asundissolved metal by-products may have adhered to the toolsurface. However, with reverse voltage, the tool must beperiodically redressed.

* Corresponding author.

0007-8506/$ – see front matter � 2009 CIRP.

doi:10.1016/j.cirp.2009.03.070

In order to drill good quality holes, Sharma et al. [6] tried todetermine the optimum values of the process parametersexperimentally but they limited their investigations to only twoprocess parameters i.e. voltage and feedrate. In addition, theydefined hole quality in terms of overcut variation. In a subsequentwork, Bilgi et al. [7] included pulse on-time, duty cycle and thelength of un-insulated tool. However, they employed a non-standard tool and a pulsed DC voltage. The main objectives of thiswork are: identify the different types of defects in STED; determinethe effects of the process parameters on hole quality (diameter andtaper); and determine the values of the process parameters for agiven hole quality.

2. Types of defects

From the authors’ industrial experience, the more importantdefects have been identified as follows:

(i) D

rill wander. Occasionally the drill deviates from the desiredpath and in extreme circumstances even breaks out into theblade surface or adjacent hole. Cox [4] suggests that the drillderives its stiffness from the electrolyte pressure and hence, atlow pressures, it is prone to wander (see Fig. 1(a)).

(ii) T

hreading. Sometimes the hole may exhibit a repetitive patternof diameter variation which is reminiscent of a threaded hole(see Fig. 1(b)). It is seen to occur at high electrolyte pressures.

(iii) H

ole inaccuracy. The two main types of inaccuracies are holetaper and deviationsfromthe required holesize. Fig. 1(c) showsaconvergent hole. Another type of defect is longitudinal striationswhich are associated with hydrodynamic flow patterns.

(iv) D

rill bending. There are two possible causes for its occurrence.In the first case, due to a combination of high feedrates and lowvoltages, short circuits can occur, leading to drill bendingwhich can be corrected by replacing or straightening the drill.In the second case, the use of reverse voltage causes the drill towear. If this wear is excessive, the overcut is progressivelyreduced leading to drill clamping and bending of the drill.

Fig. 1. Common defects in STED.Fig. 2. Tip-end views of new and worn drills.

S. Ali et al. / CIRP Annals - Manufacturing Technology 58 (2009) 185–188186

(v) B

TablValu

1

2

3

4

5

6

7

8

ulrushing. This defect appears as a sudden local increase inthe hole diameter (Fig. 1(d)). The reason for its occurrence isnot fully known but it is believed to be linked with highpressures and low feedrates.

Fig. 3. Current and flow signals for tests 5 and 8.

3. Experimental set-up

The experimental set-up consisted of a lab-scale ECM machine,the tool feed axis of which was numerically controlled. Themachine was equipped with sensors to record the voltage andcurrent, and electrolyte flow, pressure and temperature at the inletof the tool. Commercial STEM drills were used and these weretitanium tubes. Experiments were carried out with three drill sizes(0.84, 1.73 and 2.62 mm) but only the results obtained with the1.73 mm drill are reported herein as they are more typical of theholes produced. These drills had a bore diameter of 1.17 mm andwere insulated along their entire working length, the thickness ofthe insulation being 0.076 mm. The electrolyte was nitric acid(HNO3) diluted to 20% by weight in water. The applied voltage wasobtained from a pulsating power supply with the polarity beingreversed for a very small fraction of the cycle time.

The experiments were conducted in two stages: preliminarytests to determine the more important parameters; and two-levelfull factorial tests.

In stage 1, the workpiece was an Inconel block of height100 mm into which through holes were drilled. The resultsindicated that a block height of 35 mm would be sufficient for theexperiments in stage 2.

4. Results and discussion

4.1. Stage 1

Since it is a shop-floor belief that metal-ion concentration (Me,g/l) is an important parameter, it was not considered in the firststage but directly in the second. The main reason for doing this wasto keep the number of experiments to a more manageable level inthe first stage. The other input variables were feedrate (f, mm/min), forward voltage (Vf, volts), reverse voltage (Vr, volts), forwardvoltage time (tf, s), reverse voltage time (tr, s) and pressure (p, bar).The output parameters were the hole diameter at entry (Dent, mm)and taper, which is given by Dexit � Dent. Table 1 shows the valuesof the input and output variables.

This STEM drill was expected to produce a nominal hole ofdiameter 2.05 mm with a tolerance of �0.1 mm, thus most of theresults shown are acceptable.

e 1es of the variables in stage 1 (hole depth = 100 mm).

f Vf Vr tf

1.5 14 1.5 10

1.5 14 3.5 5

1.5 10 1.5 5

1.5 10 3.5 10

2.5 14 1.5 10

2.5 14 3.5 5

2.5 10 1.5 5

2.5 10 3.5 10

In test 1, the combination of low feedrate, high forward voltageand low reverse voltage resulted in the maximum hole size. On theother hand, the combination of high feedrate, low forward and highreverse voltages resulted in the smallest hole diameter (test 8).Unlike the other tests, 7 and 8 resulted in a converging taper, thetaper in test 8 being considerably greater. This is understandablesince the high value of reverse voltage would have caused the toolto wear at a faster rate. The right half of Fig. 2 shows a worn tool,the chamfer on the end face of which has become concave. Thisconcavity would have reduced the intensity of the electric fieldnear the cylindrical surface of the hole. Progressive tool wear,combined with low voltage, would have caused the overcut to keepdiminishing resulting in a converging taper. A diminishing overcutwould have an adverse effect on the flowrate. Consequently, theelectrolyte in front of the tool would get heated up and evenpossibly boil. Fig. 3 shows that the flowrate in test 8 decreasedrapidly from its initial value of 320 ml/min to below 20.Measurements of flowrate below this value were outside therange of the flowmeter used. The output values shown in Table 1are the averages of three runs but in the case of test 8, only one runwas successful and the other two resulted in drill bending,indicating that the radial gap had reduced even further.

In tests 1–6, holes were produced with diverging tapers. Even inthese cases, there was a reduction in the flow rate after the initialrapid fall e.g. test 5 (Fig. 3). A consequence of this would be a rise inelectrolyte temperature and conductivity, accompanied by asteady increase in current (test 5, Fig. 3), resulting in acontinuously increasing overcut.

tr p Dent Taper

0.2 2.5 2.10 +0.03

1 1.5 2.05 +0.08

0.2 1.5 2.01 +0.09

1 2.5 1.96 +0.03

1 1.5 1.94 +0.19

0.2 2.5 1.96 +0.04

1 2.5 1.87 �0.05

0.2 1.5 1.80 �0.10

Table 2Main effects of the input variables on hole diameter.

f Vf Vr tf tr p

�0.14 +0.10 �0.04 �0.02 �0.01 +0.02

Fig. 4. Main effects of process parameters on (a) Dent and (b) taper.

S. Ali et al. / CIRP Annals - Manufacturing Technology 58 (2009) 185–188 187

The main purpose of the stage 1 experiments was to prioritisethe parameters in terms of their effect on the hole quality. Theeffect of the feedrate was determined by calculating the averagevalues of entry hole diameters (Dent) at feedrates of 1.5 and2.5 mm/min. The average diameter values at these feedrates were2.03 and 1.89 mm respectively, i.e. the hole diameter decreased by0.14 mm when the feedrate was increased from 1.5 to 2.5 mm/min.The effects of the other parameters were similarly determined andthe results are summarised in Table 2 from which it is clear that themore important parameters are forward and reverse voltages andfeedrate. In practice, the reverse voltage is limited to a maximum of2 V as higher values only tend to accelerate tool wear. Had thevalue of reverse voltage in stage 1 been limited to 2 V, its effect onthe hole diameter would have been negligible, i.e. �0.01 ratherthan �0.04 mm. Hence the parameters that were considered instage 2 were f, Vf, p and Me.

4.2. Stage 2

The results obtained from the second stage are shown inTable 3. As before, only the results obtained from a 1.73 mm drillare shown. Note that the taper values shown in Table 3 relate to alength of 35 mm and not 100.

In all but two of the tests, conical holes were produced. All theholes, except four, were within tolerance. The unacceptable holeswere produced in tests 6, 8, 14 and 16 due to bulrushing. For these,the values shown in column 7 indicate diameter growth, i.e. thediameters increased to 2.75, 2.41, 2.69 and 2.46 mm respectively.As before, each test was conducted twice and Table 3 shows theaverage values of the output; bulrushing was repeatable for thefour above-mentioned tests. Cox [4] held that bulrushing occurs asa result of vibration transmitted to the drill from the electrolytepump. No such vibrations were observed in our case and this wasfurther confirmed by the smooth pressure and flow curves. In allbulrushing cases, except test 8, there was no appreciable drop inthe flowrate (�DQ). In all the non-bulrushing tests, flowrate didfall because, with progressive hole depth, the resistance to flowincreases. But with bulrushing, the sudden local increase indiameter has a much diminished effect on flowrate. Although itwas difficult to pin-point the conditions which triggered the onsetof bulrushing, it is certain that bulrushing occurred at highpressures and low feedrates, accompanied by a very slight decreasein flowrate.

Table 3Values of the variables in stage 2 (hole depth = 35 mm).

Me Vf f p Dent Taper �DQ

1 1 12 2.5 2 2.01 0 51

2 1 12 2.5 4 1.96 0.08 51

3 1 18 2.5 2 2.08 0.03 59

4 1 18 2.5 4 2.06 0.03 69

5 1 12 1.5 2 2.06 0.03 68

6 1 12 1.5 4 2.11 0.64a 9

7 1 18 1.5 2 2.08 0.05 46

8 1 18 1.5 4 2.13 0.28a 54

9 1.5 12 2.5 2 1.98 0.03 58

10 1.5 12 2.5 4 1.98 0 93

11 1.5 18 2.5 2 2.06 0.03 62

12 1.5 18 2.5 4 2.06 0.03 86

13 1.5 12 1.5 2 2.11 �0.03 55

14 1.5 12 1.5 4 2.08 0.61a 13

15 1.5 18 1.5 2 2.10 0.05 45

16 1.5 18 1.5 4 2.21 0.25a 9

a Bulrushing occurred; value indicates growth in diameter.

The effects of feedrate, forward voltage, metal-ion concentra-tion and pressure on the entry hole diameter as well as the holetaper are shown in Fig. 4. It is obvious from Fig. 4(a) that, for therange considered, metal-ion concentration has only a marginaleffect on hole size. The associated curve has a positive slope whichis to be expected because the increased metal ions in theelectrolyte would effectively increase its conductivity.

In the case of the entrance hole diameter, as expected, thetwo dominant variables are feedrate and applied voltage. But inthe case of hole taper (Fig. 4(b)), all the variables are dominantexcept feedrate. Consider the effect of pressure on the holetaper. With a 2-bar pressure, the average flowrate dropped from309 to 253 ml/min, whereas with a 4-bar pressure, the averagedrop was from 401 to 326 ml/min (excluding the bulrush cases).Hence, in the latter case, the bigger drop would have resulted ina reduced heat transport and hence a bigger change in holediameter.

Similarly, the voltage also increases the hole taper. Increasingthe applied voltage results in a proportionate increase in thecurrent, which for the same flowrate, would result in a higherheating effect at greater depths.

Increasing the metal-ion concentration has a negative effect onhole taper. For example, in test 2, with a concentration of 1 g/l, thecurrent increased from 5.8 A at the start of the drilling process to6.2 A at exit. However, with a concentration of 1.5 g/l (test 10), thecurrent changed from 5.5 to 5.6 A. The smaller change in the lattercase means that there is a smaller increase in the process heat,resulting in reduced heating of the electrolyte and hence a reducedtaper.

The interacting effects of the different variables on the holediameter and taper are shown in Fig. 5, the degree of interactionbetween any two variables being indicated by the angle includedbetween the corresponding curves. Hence, the strongest interac-tion in Fig. 5(b) is between the applied voltage and feedrate(curves in row 2, column 3 i.e. cell (2, 3)) as the correspondingcurves intersect and include an angle of approximately 908. Thesecurves show the effect of varying the feedrate whilst keeping thevoltage constant. The curves in cell (3, 2) contain the sameinformation as those in cell (2, 3) except that in the former, the

Fig. 5. Effects of the interactions between process parameters on (a) Dent and (b)

taper.

S. Ali et al. / CIRP Annals - Manufacturing Technology 58 (2009) 185–188188

curves were obtained at constant feedrates. Referring to thecurves in cell (1, 3), the interaction between metal-ion concen-tration and feedrate is weak because the curves include arelatively small angle. The selection of values for the machiningparameters is done in two steps. In the first step, the curves shownin Fig. 5(a) and (b) are used as guidelines to deduce a set of valuesfor the process parameters to achieve a required hole quality.Referring again to the curves in cell (2, 3), Fig. 5(b), the 12 V curvecorresponds to smaller values of taper than the 18 V curve. Hence,to minimise hole taper, one should operate at 12 V and at theminimum feedrate i.e. 1.5 mm/min. If all the curves in Fig. 5(b) areanalysed, the guidelines for minimising hole taper are: use lowvalues of feedrate, voltage and pressure and a high value of metal-ion concentration.

Similarly, study of the curves in Fig. 5(a) reveals guidelines forobtaining a hole of a particular size. Thus in cell (2, 3), if a holediameter of 2.05 mm is required, either a mid-range voltage anda high feedrate, or a mid-range feedrate and low voltage shouldbe selected. The high feedrate, in the first combination, wouldresult in a higher production rate but is in conflict with therequirements for producing a small taper. Hence the secondcombination is to be recommended. Of course, these valuesshould be used in conjunction with low pressure and high metal-ion concentration.

Another advantage of using a full factorial design of experi-ments technique is that the results can be used to develop a process

model in the form of equations relating hole quality in terms of theprocess parameters. Thus, for the 1.73 mm drill:

Dent ¼ 2:191þ 0:048Me � 0:022V f � 0:005 f þ 0:001 pþ 0:006Me

� V f � 0:075Me � f þ 0:013Me � pþ 0:007V f � f

þ 0:004V f � p� 0:031 f � p

taper ¼ 0:033� 0:268Me þ 0:006V f � 0:028 f þ 0:106 p

þ 0:012Me � V f þ 0:090Me � f � 0:055Me � p� 0:006V f

� f � 0:002V f � p

In the second step, these equations are used to check if the user-selected values will indeed result in the required values of Dent andtaper. If, for example, Vf = 16 V, f = 1.6 mm/min, p = 2 bar andMe = 1.5 g/l, then Dent and the taper (over a length of 35 mm) arepredicted to be 2.10 and 0.02 mm respectively.

Alternatively, step 1 can be bypassed and the values of theprocess parameters obtained from the equations by trial and error.

5. Conclusions

(i) T

he more important parameters that influence hole size andhole taper in STED are feedrate, forward voltage, electrolytepressure and metal-ion concentration.

(ii) T

aper holes are explainable in terms of electrolyte flow rateand tool wear. In a few tests, bulrushing was evident at highpressure and low feedrate.

(iii) A

full factorial approach has given greater insight into theprocess, yielded guidelines for selecting process conditions andled to the development of a process model which establishes thevalues of the process parameters for a desired hole quality.

Acknowledgments

The authors would like to acknowledge the funding receivedfrom Knowledge Transfer Partnership (No. 1131) EPSRC/DTI andI*PROMS, an EU FP6 Network of Excellence.

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