High quality finishing of bevel gears by electrochemical honing.
Shaikh, J.H. ; Jain, N.K.
1. Introduction
Bevel gears are used to transmit motion and/or power between two
intersecting shafts. They are mainly used in automobiles, aerospace and
marine applications, machine tools, process industry equipments,
construction equipments, etc. The operating performance, service life,
transmission efficiency, and noise and vibration characteristics of
these gears depends on surface finish and accuracy of the gear tooth.
Usually more than one finishing operations are required after
manufacturing of gear, in order to achieve the required surface finish
and geometric accuracy. Improving surface finish of the gear tooth helps
in preventing different modes of gear failure such as micro-pitting,
pitting, adhesive wear, abrasive wear and scuffing (Davis, 2005). The
two conventional bevel gear finishing processes are gear grinding and
gear lapping. But, both these processes suffer from some inherent
limitations. Gear grinding is a complicated and expensive process.
Moreover, it can cause undesirable effects such as transverse grind
lines which affect the noise and vibration characteristics of the gears
and grinding burns which damage the surface integrity of the ground
gears (Karpuschewski et al., 2008). Gear lapping is very slow operation
and it can correct only minute deviations from the desired gear tooth
profiles. Due to longer finishing time, this process does not have any
control over the geometric accuracy. Moreover, the members of the lapped
gear pair cannot be interchanged with the members of any other similar
pair (Karpuschewski et al., 2008). Electrolytic dissolution based
finishing processes can overcome most of these limitations due to their
unique capabilities such as finishing performance being independent of
material hardness, better surface finish without any mechanical and
thermal damage and higher productivity as mentioned by Rajurkar et al.
(1999).
Electrochemical honing (ECH) is a hybrid finishing process
combining capabilities of electrochemical machining (ECM) to give faster
material removal rate (MRR) and stress-free surfaces and capabilities of
mechanical honing to correct geometric errors and give controlled
functional surface, in a single operation. At the same time, ECH also
overcomes their individual limitations. It has many advantages over the
conventional gear finishing processes such as its applicability
regardless of material hardness, high material removal rate, better
surface finish, better geometric accuracy and absence of tool wear.
2. Review of Past Work
Capello and Bertoglio (1979) were probably the first researchers to
use ECH for finishing the hardened helical gear of involute profile.
They used a specially designed cathodic helical gear tool, NaNO3 as
electrolyte, inter-electrode gap (IEG) of 0.4 mm, and voltage in the
range of 10-15V. Their results were not acceptable in terms of the helix
angle and involute profiles of the finished gear. Chen et al. (1981)
developed ECH setup for high accuracy finishing of spur gears and
reported an improvement in the accuracy of profile as well as in the
surface finish of the spur gear teeth and reduction in noise level. Yi
et al. (2000) used ECM-based process for tooth profile-modification of
carburized hypoid gears and investigated current density distribution in
the gear teeth. They reported that both the current and processing
periods affect the volume of crown and the amount of modification. Yi et
al. (2002) used electrochemical tooth-profile modification process on
real-time control basis and used artificial neural network for its
mathematical modeling. Naik et al. (2008) investigated on finishing of
spur gears by ECH and reported percentage improvement up to 80% and 67%
in average surface roughness ([R.sub.a]) and maximum surface roughness
([R.sub.tm]) respectively. Misra et al. (2010) used ECH for finishing
the helical gears made of EN8 and investigated the effects of voltage,
electrolyte concentration and rotary speed of the workpiece gear on
surface finish using an aqueous solution of NaCl and NaN[O.sub.3] in a
ratio of 3:1 as electrolyte. They reported that electrolyte
concentration and voltage have more significant effects on ECH process
performance than the rotary speed. Ning et al. (2011) used pulse
electrochemical finishing (PECF) for the spiral bevel gears in which
only one gear tooth was finished at a time. They used a cathode cutter
which rotates and passes through the tooth space of the workpiece gear.
After reaching the full depth, the cutter withdraws and the gear is
indexed for the finishing of the next tooth. They reported the
improvements in surface roughness and geometric accuracy. They also
developed a mathematical model for total thickness of the material
removed and surface roughness produced, and validated it with the
experimental results. Misra et al. (2012) used pulsed-ECH (PECH) for
finishing the spur gears and studied the effects of composition and
temperature of electrolyte on the surface finish. They also found a
ratio of 3:1 of NaCl and NaN[O.sub.3] as optimum electrolyte composition
and 30[degrees]C as optimum electrolyte temperature.
3. Research Objectives and Methodology
From the review of the past work it is evident that as of now no
research has been reported on development of ECH for finishing the
straight bevel gears. The present work bridges this gap through
development of the ECH process for improving surface finish and surface
integrity of the straight bevel gears and experimental investigations on
the process performance. Following were the objectives indentified on
the basis of the review of the past work:
1. To conceptualize the arrangement of workpiece, cathode and
honing gears so as to finish the complex conical geometry of the bevel
gears by ECH process.
2. To design and fabricate an experimental setup based on the
conceived working principle for finishing the bevel gears by ECH.
3. To study the effects of ECH process parameters on the surface
finish, geometric accuracy and MRR of the finished gears through
systematic planning and designing of the experiments and thorough
analysis of the experimental results.
4. To study the changes in surface characteristics as surface
integrity, bearing area curve and microstructure of the gears finished
by ECH.
5. To study the mechanism of material removal and the contribution
of mechanical honing and ECM in MRR and surface roughness generation.
6. To develop theoretical models of material removal rate (MRR) and
surface roughness and validate them experimentally. The theoretical
models will have wide applicability than the empirical or semi-empirical
models.
7. Multi-objective optimization of the ECH process parameters
considering the conflicting objectives such as MRR, surface roughness,
and geometric accuracy. The use of optimum ECH parameters will optimize
the process performance with high productivity.
Following research methodology was adopted to meet the
above-mentioned research objectives. A novel concept of using twin
complementary cathode gears was conceived to (i) ensure finish of the
entire face width of the bevel gear teeth with reciprocating motion as
required in the finishing of the cylindrical gears by ECH, and (ii)
ensure the inter electrode gap (IEG) between the workpiece and cathode
gears so that short-circuiting between them is avoided. An innovative
experimental setup was designed and developed based on this concept for
high-quality finishing of the bevel gears by ECH. The most commonly used
material in production of commercial bevel gears namely 20MnCr5 alloy
steel was chosen as the workpiece material. An aqueous mixture of NaCl
and Na[No.sub.3] was used as the electrolyte. Experimental
investigations were conducted in the four different stages namely: (i)
trial experiments, (ii) pilot experiments, (iii) main experiments, and
(iv) confirmation experiments. Table 1 presents the summary of
objectives, details of fixed and variable ECH parameters, measures of
process performance or responses, and methodology of design of
experiments for each stage of the experimental investigations. The
approach for the design of experiments for each stage was decided
keeping in view its objectives, number of variable parameters involved
and their levels, and the constraints on the experimental resources.
Four trial experiments were conducted using full factorial approach
to check the working of the setup and for initial bracketing of
operating ranges for the ECH parameters for the pilot experiments.
Twelve pilot experiments were conducted to fix the electrolyte
composition and finishing time for the main experiments by studying
their effects on the surface finish and geometric accuracy. The effects
of electrolyte composition and finishing time on enhancing the geometry
accuracy are reported by Shaikh & Jain (2013a). Haisch et al. (2001)
have reported that the choice of electrolyte strongly influences the
surface finish produced in the electrochemical dissolution based
processes. Therefore, this chapter reports about the effects of the
electrolyte composition and finishing time on the surface finish and
microstructure of bevel gears. The analysis has shown strong influence
of electrolyte composition and finishing time on process performance.
The experimental results have shown significant percentage improvement
in the surface finish.
The main experiments were designed and performed according to
[L.sub.27] ([3.sup.13]) orthogonal array of Taguchi's experimental
design using fixed and variable parameters as mentioned in Table 1. In
these experiments five ECH parameters namely concentration, temperature
and flow rate of the electrolyte, voltage applied across IEG, and rotary
speed of the workpiece gear. were varied at three levels each to study
their effects on the average and maximum surface roughness ([R.sub.a]
and [R.sub.max]) values and MRR. Shaikh et al. (2013) have reported the
investigations on [R.sub.a] and [R.sub.max]. Theoretical models for MRR
and surface roughness for ECH were developed based on the proposed
mechanism of material removal by Shaikh & Jain (2013 b).
Confirmation experiments were conducted to validate the theoretical
models and results of multi-objective optimization.
4. Experimental Setup
4.1 Principle of Finishing the Bevel Gears by ECH
Fig. 1 depicts the proposed working principle for finishing the
bevel gears by ECH. The anodic workpiece gear '1' is mounted
on the spindle of a drilling machine. Since, for the ECM, the cathode
gears have to be in constant mesh with the anodic workpiece gear
therefore, to avoid the short circuiting by providing an IEG and to
ensure finishing of the entire face width of bevel gear tooth, a novel
concept of using twin complementary cathode gears was conceived. For
this, in one of the complimentary cathode gear '3', an
insulating layer of metalon is sandwiched between two conducting layers
of copper while, in the other complimentary cathode gear '4',
a conducting layer of copper is sandwiched between two insulating layers
of metalon. Fig. 2 depicts the photographs of these complementary
cathode gears. To ensure the IEG between the cathode and anode gears,
the conducting layer is undercut by 1 mm as compared to the insulating
layers. A honing gear '2' (a bevel pinion with hardness
greater than that of the workpiece bevel gear) at the backside is in
mesh with the workpiece gear. Both the cathode and honing gears have the
same involute profile as the workpiece gears. The axes of the shafts of
workpiece gear, cathode gears and honing gear are perpendicular to each
other. A full stream of electrolyte '5' is supplied to the
IEG, and a DC current is passed through the gap. During the
electrochemical process of material removal from the tooth flank, the
electrolysis action forms a metal oxide passivating layer on the tooth
surface of the workpiece gear which inhibits the further material
removal by ECM. This passivating layer is scraped by the honing gear. A
tight meshing between the honing and the workpiece gears ensures the
dual flank contact and the pressure required to remove the passivating
layer. The honing gear scraps the passivating layer from the high spots.
This results in relatively more material removal from the protruding
high spots by the electrochemical action in the next cycle. This cyclic
sequence of ECM and mechanical honing leads to improvement surface
finish and surface characteristics of all the teeth of the workpiece
gear simultaneously.
Based upon the above-mentioned principle, an innovative
experimental setup for finishing the bevel gears by ECH was designed and
developed whose schematic is shown in Fig. 3(a) and its photograph in
the Fig. 3(b). This setup has four subsystems namely (i) power supply
system; (ii) electrolyte supply, cleaning and recirculating system;
(iii) machining chamber housing workpiece, cathode and honing gears; and
(iv) A machine frame to support the machining chamber and to provide
motion to the workpiece gear. The cathode gears and honing gear rotate
due to meshing with the workpiece gear. A DC power supply system capable
of supplying an output voltage in the range of 0-100 V, current in the
range of 10-110 A and with a programmable pulse-on time and pulse-off
time in the range of 5-999 [micro]s through space-mark controller was
used. The power supply can be operated either as a constant current
source or as a constant voltage source. The positive terminal of the
power supply is connected to the stainless steel shaft supporting the
workpiece gear while, the negative terminal is connected to two cathode
gears through carbon brush and slip ring assembly. The electrolyte
supply, cleaning and re-circulating system has been designed to supply
the filtered electrolyte to the machining chamber and recirculate it
back to the storage tank. A rotary pump made of stainless steel and
capable of developing a wide range of pressures and flow rates was used
to supply the electrolyte which is an aqueous mixture of NaCl and
Na[No.sub.3]. Filtration was achieved by using two double-staged
magnetic and stainless steel mesh filters provided in the electrolyte
flow path. Electrolyte pressure and flow rate measuring devices and flow
control valve were employed at the pump outlet. The electrolyte
temperature was maintained by a heating element fitted with a precise
temperature controller. Rotary motion to the workpiece gear is provided
by a DC motor fixed on the frame of a drilling machine of 38-mm drilling
capacity. This motor has a controller to vary the rotary speed
continuously in the range of 30-1500 rpm. The machining chamber has been
fabricated using Perspex sheets to provide better visualization of the
ECH process and better strength-to-weight ratio
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Pedestal type ball bearings were used to mount and support the
stainless shafts on which honing and two cathode gears were mounted.
Metalon blocks were used to support and mount the bearings due to its
corrosion resistance, electrical insulation and higher
strength-to-weight ratio. The table of the drilling machine having
dimensions of 400 mm x 400 mm was used to mount and support the
machining chamber.
[FIGURE 3 OMITTED]
The workpiece, cathode and honing gears were straight bevel gears
having module as 4.83. The workpiece gear was having 16 teeth while
cathode and honing gears were having 10 teeth each. The workpiece bevel
gears were cut on bevel gear generator based on the Gleason method in a
reputed gear manufacturing company. Case hardened 20MnCr5 alloy steel
(with the composition shown in the Table 2) having surface hardness in
the range of 50-54 HRC was used as workpiece material because it is the
most commonly used material for the production of commercial bevel gears
for typical industrial applications. Honing gear was also made of the
same material but of higher surface hardness (i.e. 58-62 HRC).
5. Experimentation
The trial experiments were performed to check the working of the
developed experimental setup and to decide the operating range of the
selected variable parameters. Based on these experiments, the value of
the DC voltage and range of the finishing time were decided. The voltage
was kept at 24 V and ECH was done for a finishing time of duration 2, 4,
6 and 8 minutes. Values of the other parameters used were electrolyte
composition (25% Na[No.sub.3] and 75% NaCl by weight), electrolyte
concentration (7.5% by weight), electrolyte flow rate (20 lpm), rotary
speed of workpiece gear (60 rpm) and electrolyte temperature
(32[degrees]C). The levels of these parameters were selected on the
basis of the design constraints and review of the past work. After
finishing, the surface roughness of the finished gear was checked. It
was found that the higher values of the voltage and finishing time
deteriorated the surface finish. Therefore, value of the DC voltage was
decided to be 12 V and finishing time in the range of 2-6 minutes for
the further experiments. The electrolyte composition was varied at 4
levels and finishing time at 3 levels to study their effects on surface
finish as mentioned in the Table 2. The other ECH parameters such as
voltage (V), electrolyte concentration (C), electrolyte temperature (T),
electrolyte flow rate (F) and rotary speed of the workpiece gear (R)
were fixed at their central values. The 12 experiments were conducted
using full factorial approach.
Surface roughness was measured before and after ECH on a
contracer-cumsurface roughness tester of KOSAKA make. For analysis of
surface roughness parameters two gear teeth were selected. For each
tooth, two measurements one on left hand profile and other on the right
hand profile were performed and the average value of the concerned
parameter was used to calculate the average percentage improvement in
that parameter. The percentage improvement in average surface roughness
'[R.sub.a]' values (PI[R.sub.a]) was calculated using the
Eq.(1), similarly maximum surface roughness '[R.sub.max]'
values (PI[R.sub.max]) were calculated. Higher values of PI[R.sub.a]/
PI[R.sub.max] indicate the smaller value of final [R.sub.a]/[R.sub.max].
PI[R.sub.a] = Initial [R.sub.a] value--Final [R.sub.a]
value/Initial [R.sub.a] value (1)
6. Results and Discussion
Fig. 4 and 5 depict the variation in the percentage improvements
average surface roughness value 'PI[R.sub.a]' and maximum
surface roughness value 'PI[R.sub.max]' respectively with
finishing time for different compositions of the electrolyte. It is
clear from Fig. 4 and Fig. 5 that, increase in amount of NaCl causes
deterioration in the surface finish. This is due to relatively more
material removal rate (MRR) due to corrosive nature of NaCl, while the
combination of 50% NaN[O.sub.3] + 50% NaCl gives the highest
'PI[R.sub.a]' and PI[R.sub.max]' due to the passivating
nature of NaN[O.sub.3]. On safe side we can select 75% NaN[O.sub.3] +
25% NaCl as the optimum electrolyte composition, as it is the second
candidate giving highest 'PI[R.sub.a]' and
'PI[R.sub.max]' values. It is evident from the Fig. 4 and Fig.
5 that, the electrolyte composition of 75% Na[No.sub.3] + 25% NaCl with
2 minutes finishing time yields the highest improvements in the surface
finish, Hence, this combination is identified as optimum combination for
the ECH of bevel gears.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Figures 6(a) and 6(b) illustrate the profile of the surface
roughness before and after the ECH for the optimum parameters. It is
clear from these figures that after ECH for 2 minutes the maximum
surface roughness [R.sub.max] value has improved significantly from
20.23 [micro]m to 11.4 [micro]m while, average surface roughness
[R.sub.a] value has improved from 2.30 [micro]m to 1.99 [micro]m.
[FIGURE 6 OMITTED]
Figures 7(a) and 7(b) depict the bearing area curve (BAC) for the
depth of 0.5 [im before and after ECH. The improvement in percentage
material in BAC after ECH results in larger contact area and hence less
noise and vibration during the operation and less wear.
[FIGURE 7 OMITTED]
Figures 8(a) and 8(b) show the SEM micrographs for an unfinished
gear and ECH finished gear respectively. From the SEM micrograph of
unfinished gear the micro-pits on the tooth flank surface are clearly
visible, which may lead to the fatigue failure in operating life of the
gears. These pits are smoothened by the ECH as shown in Fig. 8(b).
[FIGURE 8 OMITTED]
7. Conclusion and Future Scope
This paper reported about the experimental investigations on effect
of electrolyte parameters on the enhancement of surface finish and
surface characteristics of the straight bevel gears using an
innovatively developed ECH setup. Following are the conclusions drawn
based on this study:
1. The composition of electrolyte should be chosen very carefully.
Use of more amount of corrosive electrolyte may deteriorate the surface
finish.
2. ECH for longer duration deteriorates the surface finish rather
than improving it.
3. ECH significantly improves the surface integrity of the gears.
4. The study confirms ECH being an economical and highly productive
alternative finishing process for the bevel gears because surface finish
improved significantly within the finishing time of 2 minutes.
5. Performance of ECH is independent of the gear material hardness
therefore it can improve the surface finish and surface integrity of any
gear material which consequently improves the operating performance and
service life of the gears.
Since, the present work is the first attempt to establish ECH as an
alternative productive finishing process for the straight bevel gears
therefore, there is lot of scope for the future work in this area.
Following are the some future research directions:
1. improving the geometric accuracy of bevel gears by,
* Using the workpiece gears with the finishing stock, as estimated
from the experimental data and/or analytical models.
* Modifying the profile of the cathode gears from the analysis of
simulation of meshing dynamics of gears in the ECH process.
* Using cathode gears of high geometric accuracy and surface
finish.
2. Automation of the ECH process using noise and vibration sensors
which can stop the process whenever desired level of performance is
reached.
3. Use of ECH to finish of other gears of conical geometry such as
spiral bevel gears and hypoid gears using the concept of complimentary
cathode gears.
4. Use of ECH for gear tooth profile modifications such as tooth
profile crowing, root relief, tip relief etc. by modifying the profile
of cathode gears.
5. Use of pulsed-power supply to further enhance the process
performance of ECH for finishing the conical gears.
Further research is under progress at IIT Indore on all the
above-mentioned aspects under the guidance of the second author of this
Chapter.
8. Acknowledgements
The authors gratefully acknowledge (i) CSIR, New Delhi (India) for
the financial support received under the Project No. 22/
(0468)/09/EMR-II, (ii) SnH Gears, Dewas, MP (India) for providing their
facilities for fabrication of the bevel gears, and (iii) VE Commercial
Vehicles, Pithampur, MP (India) for allowing to use their facilities for
surface roughness measurements.
9. References
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Authors' data: Shaikh, J[aved] H[abib]; Jain, N[eelesh]
K[umar] *, Discipline of Mechanical Engineering, Indian Institute of
Technology Indore, 453 446, MP, INDIA, shaikhjaved1@gmail.com,
nkjain@iiti.ac.in
This Publication has to be referred as: Shaikh, J[aved] H[abib]
& Jain, N[eelesh] K[umar] (2013) High Quality Finishing of Bevel
Gears by Electrochemical Honing, Chapter 41 in DAAAM International
Scientific Book 2013, pp. 697-710, B. Katalinic & Z. Tekic (Eds.),
Published by DAAAM International, ISBN 978-3-901509-94-0, ISSN
1726-9687, Vienna, Austria
DOI: 10.2507/daaam.scibook.2013.41
Tab. 1. Details of objectives, fixed and variable ECH
parameters, responses, and approach of design of experiments
for each stage of the experimental investigations
Composition (by % Wt.) of the workpiece material
(i.e. 20MnCr5 alloy steel): Cr (0.8-1.1); Mn (1-1.3);
C (0.14-0.19); P (0.035 max.); S (0.035 max.); Si (0.15-0.40);
and balance Fe.
Stage of Objectives Process parameters
experimentation
Trial To check the Variable parameter:
working of the 1.Finishing Time: 4 levels
experimental (2, 4, 6, 8 min.)
setup.
Fixed parameters:
To decide the 1.Electrolyte composition (E):
range of the 75%NaN[O.sub.3] + 25% NaCl.
selected ECH
parameters 2. Electrolyte concentration
for pilot (C): 7.5% (by wt.)
experiments.
3. Electrolyte temperature
(T): 32[degrees]C
4.Voltage: 24 V
5. Electrolyte flow rate
(F): 20 lpm
6. Rotary speed of workpiece
gear (R): 60 rpm
Pilot To find the Variable parameters:
optimum levels
of ECH 1. Electrolyte composition
parameters (E): 4 levels (100% NaN[O.sub.3];
which are 50% NaN[O.sub.3] + 50% NaCl; 25%
difficult to NaN[O.sub.3] + 75% NaCl; 75%
change in the NaN[O.sub.3]+25% NaCl)
main
experiments. 2. Finishing time (t): 3 levels
(2 min., 4 min., 6 min.)
Fixed parameters:
1. Electrolyte concentration
(C): 7.5% (by wt.)
2. Electrolyte temperature
(T): 32[degrees]C
3. Voltage (V): 12 V
4. Electrolyte flow rate
(F): 20 lpm
5. Rotary speed of workpiece
gear (R): 60 rpm
6. Inter electrode gap: 1 mm
Main To study the Variable parameters:
effect of
variable 1. Electrolyte concentration
parameters and (C): 3 levels (5% 7.5%; 10%)
their (by weight)
interactions
on the response 2. Electrolyte temperature
and to optimize (T):3 levels (27[degrees]C
their values. 32[degrees]C; 37[degrees]C)
3. Voltage (V): 3 levels
(8 V;12 V; 16 V)
4. Electrolyte flow rate
(F): 3 levels (10 lpm; 2 lpm;
30 lpm)
5. Rotary speed of workpiece
gear (R): 3 level (40 rpm; 60
rpm; 80 rpm)
Fixed parameters:
1. Electrolyte composition
(E): 75% NaN[O.sub.3]+25% NaCl
2. Finishing time (t): 2 min.
3. Inter electrode gap: 1 mm
Confirmation To confirm the Optimum parameters given by
results of different models
the main
experiments
Stage of Responses Approach for DOE
experimentation and no. of
experiments
Trial Surface quality Full factorial
through
visual 4 Experiments
examination
Pilot [PIR.sub.a], Full factorial
[PIR.sub.max],
[PIf.sub.u], 12 Experiments
[PIF.sub.p],
[PIF.sub.r]
Bearing area
curve
SEM micrographs
Main [PIR.sub.a], Taguchi's
[PIR.sub.max] method using [L.sub.27]
([3.sup.13]) orthogonal
array
Bearing area 27 Experiments
curve
SEM micrographs
Confirmation [PIR.sub.a], 4 Experiments
[PIR.sub.max]
Tab. 2. Details of the input parameters and responses used in
the experimentation on finishing of the bevel gears by ECH
Composition (by % Wt.) of the workpiece material (i.e. 20MnCr5
alloy steel): Cr(0.8-1.1); Mn (1-1.3); C (0.14-0.19);
P (0.035 max.); S (0.035 max.); Si (0.15-0.40); and balance Fe.
Approach of design of experiments: Full Factorial Design
Fixed input 1. Voltage (V): 12 V
parameters 2. Electrolyte concentration (C) : 7.5% (by weight)
3. Electrolyte flow rate (F): 20 lpm
4. Rotary speed of workpiece gear (R): 60 rpm
5. Electrolyte temperature (T): 32[degrees]C
6. Inter electrode gap: 1 mm
Variable input 1. Electrolyte Composition (C): 4 levels
parameters (100% NaN[O.sub.3]; 75% NaN[O.sub.3] + 25% NaCl;
50% NaN[O.sub.3] + 50% NaCl; 25% NaN[O.sub.3]
+ 75% NaCl)
2. Finishing time (t): 3 levels (2 min.; 4 min.;
6 min.)
