Journal of Materials Processing Technology 89 – 90 (1999) 440 – 446

Electropolishing and electrobrightening of holes using different

feeding electrodes
H. Hocheng *, P.S. Pa
Department of Power Mechanical Engineering, National Tsing Hua Uni6ersity, Hsinchu, Taiwan, ROC

Received 23 September 1998

Abstract

The current study discusses the surface finish of several common die materials, of which the inner holes are electropolished and
electrobrightened by different types of feeding electrodes. In the experiment, two types of electrode are used with the application
of continuous direct current and axial electrode feed. The controlled factors include the dimension of the electrode as well as the
chemical composition and the concentration of the electrolyte. The parameters are current rating, electrode geometry, die material,
electrode rotational speed and feed rate. It was found that electrobrightening after reaming requires quite a short working time,
whilst the electropolishing avoids the need for reaming, thus makes the total cycle time less than electrobrightening. The electrode
of boring cutter type performs better in the current investigation. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Electropolishing; Electrobrightening

1. Introduction components, plastic or press dies, wire-drawing dies,

optical units, and electric components (silicon chips,
Amongst traditional techniques of hole machining, VLSI/ULSI chips) can apply this technique as well.
drilling is often followed by reaming for precision. Electrochemical machining was introduced by Faraday
However, this technique is limited by the attainable in the eighteenth century, who made use of elastic
surface roughness. Subsequent conventional techniques energy and chemical energy to remove materials [2].
such as polishing by hand or machine boring are often More industrial applications have been realized
employed. Polishing by hand is heavily dependent on throughout the subsequent decades, such as electro-
experience, and both hand polishing and machine bor- chemical drilling, electrochemical grinding, electro-
ing will result in non-uniform residual stress due to the chemical deburring, and electropolishing [3]. However,
contact between the tool and the workpiece. Surface the potential for internal hole polishing is yet to be
cracks and depressions are often induced and reduce explored. The main difficulty lies in the design of the
the service life of the part. When an internal hole is tool electrode in consideration of the complicated pro-
produced by electric discharge machining, on the other cess of metal removal. In addition, hole machining by
hand, the brittle surface layer due to the carburation the electrochemical process can affect the precision of
and quenching in the process creates additional the hole due to improper control of the machining
difficulty for the following conventional polishing. The conditions or the electrode geometry [4]. Gusseff first
application of electropolishing can overcome the above- filed a patent on ECM. He found that ECM is suitable
mentioned shortcomings and will produce workpieces for the machining of an alloy with high strength and a
efficiency without residual stress or burr [1]. high melting point: this encouraged the application of
The application of electropolishing is suitable for ECM [5]. Kascheev et al. advised that high-density
such as difficult-to-machine materials, ceramic and cer- pulse direct current improves the precision of the work-
mit. Also, those concerned with the production of die piece [6]. Laboda used a water solution of NaC1O3 as
an electrolyte to replace the conventional salt water in
* Corresponding author. Fax: +886-25-722840. order to increase precision, but NaC1O3 is easy to burn

0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 0 2 0 - 5
 H. Hocheng, P.S. Pa / Journal of Materials Processing Technology 89–90 (1999) 440–446 441

Fig. 3. Electropolishing at different gap widths between the electrode

and the internal hole (Type A, non-rotational, 2 mm/mm, NaNO3,
25wt%, 4 1/min, continuous DC, 10A).
Fig. 1. Design of feeding electrode: (A) Cylinder with lap/non-rota-
tional and rotational; (B) Boer type (rotational).
trode gap width in the electrolytic machining, suggest-
and becomes a concern. NaClO3 was then replaced by ing a controlling technique on the workpiece geometry
NaNO3 as electrolyte [7]. Opitz et al. studied the elec- in electrolytic machining [8]. Bannard brought forth the

Fig. 2. Experimental set-up.

Table 1
Chemical composition of the workpieces (wt%)

Fe C Si Mn P S Cr Mo Al V Cu Ni

SKD61 91 0.4 1 0.4 0.3 0 5.3 1.1 — 0.8 — —

SKD11 89 1.4 0.4 0.3 0 0 8.2 0.8 — 0.2 — —
NAK80 92 0.1 0.6 1.5 — — — 0.3 1.1 — 1.2 3.1
SNCM8 96 0.4 0.3 0.9 0 0 0.8 0.3 — — 0 2
442 H. Hocheng, P.S. Pa / Journal of Materials Processing Technology 89–90 (1999) 440–446

maximum efficiency varies with the type of electrolyte

[9]. Datta showed that the gap width between the
electrode and the workpiece directly influences the cur-
rent condition and the discharge dregs of the electrolyte
[10]. The experiment result of Mileham et al. estab-
lished that in electrochemical machining, the quality of
the machined surface will be influenced by the main
factors including current density, and the flow rate of
the electrolyte as well as the gap width [11]. Phillips
noted that in electrochemical grinding, the major fac-
tors affecting the metal removal rate are the conductiv-
ity of the workpiece, the rate of decomposition, the
current capacity of the power supply, and the ingredi-
ents, concentration, and temperature of electrolyte.
Fig. 4. Electropolishing at different flow rate of the electrolyte (Type
Thanks to these continual efforts, electrochemical
A, non-rotational, 2 mm/mm, NaNO3, 25wt%, 4 1/min, continuous
DC, 10A).
grinding is able to be applied to difficult-to-machine
alloys [1]. Acharya et al. compared ECM with the
traditional machining system. The investment cost of
ECM is higher, whilst greater profit and higher quality
can be obtained when operated under optimal condi-
tions [12]. Bejar used NaNO3 as electrolyte and
changed the machining gap width as well as the concen-
tration of the electrolyte to investigate in the influence
upon current efficiency. They found that the electrical
current efficiency is increased with increases in the
current density and electrolytic concentration [13]. Shen
used NaNO3 as the electrolyte in the electropolishing of
a die surface. The result showed that the surface rough-
ness of workpieces decreases with increased current
density, flow rate and concentration of electrolyte.
Moreover, polishing with pulse direct current is found
to be better than continuous direct current [14]. Ra-
Fig. 5. Electropolishing with different current ratings (Type A, non-
rotational, 2 mm/mm, NaNO3, 25wt%, 4 1/min, continuous DC,
jurkar et al. obtained the minimum gap width based on
10A). Ohm’s Law, Faraday’s Law, and the principle of con-
servation of energy, beyond which the electrolyte will
be boiled in electrochemical machining. An on-line
monitoring system was proposed [15].
In the current study, the authors conducted electro-
polishing with two types of inserting electrodes with
axial feed for several die materials, after drilling and
reaming. Amongst various factors affecting the electro-
polishing, the design of the tool electrode is discussed.
An efficient polishing process with a low-cost electrode
design is presented.

2. Experimental

The feeding electrodes discussed in this experiment

are shown in Fig. 1. The equipment of electrochemical
machining includes a DC power supply, pump, pipe,
Fig. 6. Electropolishing at different rotational speeds of the electrode flow meter, electrolytic tank, and filter. The experimen-
(Type A, rotational, 2 mm/mm, NaNO3, 25wt%, 4 1/min, continuous
DC, 10A).
tal set-up is illustrated in Fig. 2. The materials of the
workpiece are SKD11; SKD61; NAK80; and SNCM8.
theory that correlates the current efficiency with the The chemical compositions are shown in Table 1. The
current density and the flow rate of the electrolyte. The dimensions of the polishing area are f10× 20 mm2.
 H. Hocheng, P.S. Pa / Journal of Materials Processing Technology 89–90 (1999) 440–446 443

The electrolyte is NaNO3 of 25%wt. The temperature

of the machining is maintained at 259 5°C. The side

Fig. 7. Electropolishing with different feed rates of the electrode

(Type A, rotational, 2 mm/mm, NaNO3, 25wt%, 4 1/min, continuous
DC, 10A).
Fig. 10. Electropolishing with different feed rates of the electrode at
different current ratings (Type A, rotational, 2 mm/mm, NaNO3,
25wt%, 4 1/min, continuous DC, 10A).

Fig. 8. Electrobrightening with different feed rates of the electrode

(Type A, rotational, 2 mm/mm, NaNO3, 25wt%, 4 1/min, continuous
DC, 10A). Fig. 11. Electrobrightening with different feed rates of the electrode at
different current ratings (Type A, rotational, 2 mm/mm, NaNO3,
25wt%, 4 1/min, continuous DC, 10A).

Fig. 9. Electropolishing with different types of electrode at the same

amount of material removed (4 l/min, continuous DC, 10A).

The hole surface is drilled to f7.8 or reamed to f8.0. Fig. 12. Electropolishing with different current ratings for different
All workpieces are cleaned for 5 min by ultrasonics types of feeding electrode (2 mm/mm, NaNO3, 25wt%, 4 1/min,
before electropolishing. continuous DC).
444 H. Hocheng, P.S. Pa / Journal of Materials Processing Technology 89–90 (1999) 440–446

electrode is 100, 200, 400, 600, 800, 1000, 1200 rpm,

except that type A is also operated non-rotational. The
flow rate of the electrolyte is 1, 2, 3, 4 l/min. The
current rating is 5, 10, 15, 20 A. The experiment is
divided into two parts. The first part proceeds with
electropolishing after the workpiece has been drilled to
f7.8, whilst the second part proceeds with electro-
brightening after the workpiece has been further
reamed to f8.0. The main parameters also include the
current rating and feed rate.

3. Results and discussions

3.1. First-stage experiment on basic parameters

Fig. 13. Electrobrightening with different current ratings with differ- Fig. 3 shows that a smaller side gap width between
ent types of feeding electrode (2 mm/mm, NaNO3, 25wt%, 4 1/min, the electrode and the hole wall produces a smoother
continuous DC, 10A).
surface. However, gap width down to 0.2mm tends
cause to short circuit. The electrolyte flushing also
becomes more difficult, as the time of ECM is longer.
Thus the side gap width of 0.3 mm is suggested for the
next-stage test. Under the same machining conditions,
the polishing effect of SKD61 is the best, followed by
that of SKD11, NAK80, and SNCM8.
As to the effect of electrolytic flow rate, Fig. 4 shows
that the larger the flow rate, the more rapidly can the
electrolytic depositions and heat be brought away, and
the surface roughness of workpiece is improved. As a
result, the use of large electrolytic flow rate is advanta-
geous, provided that the nozzle can bear the injection
pressure. From Fig. 5, it can be seen that the polishing
effect is better at a current rating between 10A and
15A. As to the stable operation of the electrical equip-
ment, 10A is better than 15A. On the other hand, as far
Fig. 14. Electropolishing with different types of feeding electrode (2 as the material removal rate is concerned, 15A is more
mm/mm, NaNO3, 25wt%, 4 1/min, continuous DC, 10A).
effective. The electrochemical response to 5A is very
mild and the electrical equipment can be easily con-
trolled, although the polishing effect is limited and
takes a longer time. 20A takes the least time for the
same amount of material removal, but the discharge of
electrolytic depositions from the gap is difficult, so that
the polishing effect is reduced. The effect of electrode
rotational speed is shown in Fig. 6. The range between
400–800rpm produces a better polishing effect. Below
200 rpm, the centrifugal force is insufficient for effective
flushing, whilst too a strong centrifugal force may cause
the run-out of the electrode, which will affect the
stability of the gap width and further the electrolytic
homogeneity, thus worsening the polishing effect.
Therefore, it is helpful for the electrolytic depositions
discharge and the surface smoothness of workpiece
Fig. 15. Electrobrightening with different types of feeding electrode (2
when the electrode rotates at proper speed. To sum up,
mm/mm, NaNO3, 25wt%, 4 1/min, continuous DC, 10A).
a side gap width between the electrode and the internal
holes of 0.3 mm, an electrode rotational speed 600 rpm,
gap width between electrode and the hole varies at 0.2, an electrolytic flow rate of 41/min, and a current rating
0.3, 0.4, 0.5, 0.6 mm. The rotational speed of the of 10A, are adopted for the second-stage experiment.
 H. Hocheng, P.S. Pa / Journal of Materials Processing Technology 89–90 (1999) 440–446 445

3.2. Second-stage experiment on electrode design 15A and 5 mm/min for brightening. The electropolish-
ing and electrobrightening of SKD61 at different cur-
In the second-stage experiment, the result of differ- rent rating using different types of feeding electrode
ent materials in electropolishing and electrobrighten- (see Figs. 12 and 13) are further studied. The result
ing with electrode type A (600 rpm) at different feed shows that between the two types of electrode, a cur-
rates is shown as Figs. 7 and 8. If the feeding rate is rent rating of 10A with a feed rate of 2 mm/min
too slow, the electrochemical reaction is intense and produces the best surface quality. As seen in Figs. 14
make the deposition discharge difficult, leading to a and 15, type B performs the best between the differ-
reduced polishing effect. When the feed rate is high, ent types of electrode, and is obviously better than
the polishing effect can not be completely developed. type A. Comparing the design of the two types of
The results show that a feed rate kept at 2.0 mm/min electrode, type A has only a circular lap on the lead-
for electropolishing and 4.0 mm/min for electrobright- ing edge of the cylinder, whilst type B obviously has
ening of SKD61 produces the best effect. much more space for dreg discharge, so that the pol-
According to the equation of theoretical removal ishing effect of type B is better. The polishing effect
rate on alloy from the Faraday Law [2]: of type A with rotation is better than that with non-
rotational, because the rotational speed helps to dis-
 
hIt
W= charge dregs by means of centrifugal force.
nA n
F aA + B aB +…
MA MB
where h is the efficiency of current, I is the current, t
4. Conclusion
is time, F is the Faraday constant, ni is the atomic
number, ai is the proportion of the composition, Mi is
A side gap width of 0.3mm, an electrode rotational
the atomic mass.
speed of 400–800 rpm, and a current rating of 10A
Let w= W/At:
are found to be optimal in the current study for elec-

 
hI tropolishing. The flow rate of electrolyte can be
w=
nA n higher. The surface roughness of workpieces obtained
FA aA + B aB +… either from electropolishing after drilling or electro-
MA MB
brightening after reaming is similar. Although the
and fm =w/r:
time of the electropolishing is longer, the process cy-

 
hI cle time is shorter because the preceeding reaming
fm =
nA n operation is saved. On the other hand, electrobright-
FAr aA + B aB +… ening with reaming simplifies the electrochemical ma-
MA MB
chining. The polishing and brightening effect using
where A is the electrochemical machining area, r is
borer electrode is improved significantly compared to
the density of the workpiece, and fm is the feed veloc-
the electrode design of a simple cycle lap on the cylin-
ity of the electrode. Under the same machining condi-
der of the electrode.
tions, the theoretical feed velocity of the electrode of
different materials for the same material removal rate
is calculated, where h, I, F, and A are regarded as
constant for the four materials. The results of the References
removal rate are SNCM8 \ SKD61 \SKD11\
NAK80. Under the same conditions, the polishing re- [1] R.E. Phillips, What is electrochemical grinding and how does it
sults of SKD61 are the best, followed by SKD11, work, Carbide and Tool J. 18 (6) (1986) 12 – 14.
[2] J.A. McGeough, Principles of Electrochemical Machining, 1974,
NAK80 and SNCM8, as shown in Fig. 9. Compared pp. 1 – 10.
with the results from the experiment, this agrees well [3] J. Wilson, Practice and Theory of Electrochemical Machining,
except for SNCM8. 1971, pp.79 – 161.
The electropolishing and electrobrightening of [4] K. Jain V., V.N. Nanda, Analysis of taper produced in size zone
SKD61 with different feed rates of the electrode at during ECD, Precis. Eng. 8(1) (1986) 27 – 33.
[5] J.A. McGeough, Advanced Methods of Machining, Chapman
different current ratings are considered (see Figs. 10 and Hall, 1988.
and 11), the result showing that a lower current rat- [6] V.D. Kashcheev, N.S. Merkulova, A.D. Davydov, Applied Elec-
ing with a lower electrode feed rate produces a good trical Phenomena 5 (1996).
polishing effect. In order to produce the best effect, [7] M.A. LaBada, L.M. Mc Millan, A new electrolyte for electro-
the following combination of parameter values is sug- chemical machining, Electr. Technol. 5 (7-8) (1967) 340–345.
[8] H. Opitz, H. Heitmann, V. Becker-Barbrock, Ann. C.I.R.P. 15
gested: 5A and 1 mm/min, 10A and 2.0 mm/min, 15A (1967).
and 3.0 mm/min, and 20A and 3.5 mm/min for pol- [9] J. Bannard, Effect of flow on the dissolution efficiency of mild
ishing; and 5A and 2 mm/min, 10A 4mm/min and . steel during ECM, J. Appl. Electr. 7 (1977) 267 – 270.
446 H. Hocheng, P.S. Pa / Journal of Materials Processing Technology 89–90 (1999) 440–446

[10] M. Datta, D. Landolt, Electrochemical machining under pulsed electrochemical machining with a variable gap, J. Mater. Proc.
current conditions, Electr. Acta 26 (7) (1981) 899–907. Technol. 37 (1993) 691 – 699.
[11] A.R. Mileham, S.J. Harrey, K.J. Stout, The characterization of [14] W.M. Shen, The Study of Polishing of Electric Discharge-Ma-
electrochemically machined surfaces, Wear 109 (1986) 207 – chined Mold With ECM, M.Sc. Thesis, National Yunlin Insti-
214. tute of Techndogy, Taiwan, 1995.
[12] B.G. Acharya, V.K. Jain, J.L. Batra, Multi-objective optimi- [15] K.R. Rajurkar, Modeling and monitoring interelectrode gap in
zation of the ECM process, Precis. Eng. 8 (2) (1986) 88– 96. pulse electrochemical machining, Ann. C.I.R.P. 44 (1995) 177–
[13] M.A. Bejar, et al., On the determination of current efficiency in 180.