Influence of lapping velocity, pressure and time on ceramic

Influence of lapping velocity, pressure and time on ceramic

Justyna Molenda
Akademia Morska w Gdyni
INFLUENCE OF LAPPING VELOCITY, PRESSURE AND TIME
ON CERAMIC ELEMENTS MACHINING RESULTS
Ceramics in recent years have been sought in many applications due to their improved properties like
low density, high fracture toughness, high hardness and wear resistance, good high temperature
strength and others. On the negative side, they are far less ductile than metals and tend to fracture
immediately when any attempt is made to deform them by mechanical work. This is why machining of
ceramic materials is a big challenge and quite expensive affair. Primarily they are finished by
abrasive machining processes such as grinding, lapping and polishing.
Lapping is used for achieving ultra-high finishes and close tolerances between mating pieces. It has
been found very useful in the manufacture of optical mirrors and lenses, ceramics, hard disk drive,
semiconductor wafers, valve seats, ball bearings, and many more parts. Lapping process on ceramics
usually produces the surface finish as about 1÷0.01 µm of Ra.
Aluminium oxide is one of the hardest materials known. Its high hardness promotes a series of
applications in mechanical engineering, such as bearings and seals. During research Al2O3 sealing
elements were lapping. The main goal was to check the results of machining for different process
parameters. The experiments were conduct during flat lapping with use of ABRALAP 380 lapping
machine. The lapping machine executory system consists of three conditioning rings. The process
results were surface roughness Ra and material removal rate.
Keywords: one side lapping, Al2O3 lapping, lapping process results, material removal rate.
INTRODUCTION
The finishing processes are an important perspective to be considered today to
meet the goals like parallelism, tolerances, flatness, and smooth surface of
workpieces. These processes are high-precision abrasive processes used to generate
surfaces of desired characteristic such as geometry, form, tolerances, surface
integrity, and roughness characteristics. A leading importance in this perspective
has the lapping process. It leads to a surface with low roughness and high
precision. The topographical structure resulting from lapping is very advantageous
in sliding joints, because of the high ability of lubricant retention, as well as in
nonsliding joints because of the high load-carrying ability. Lapping process is used
in a wide range of applications and industries. Typical examples of the processed
components are pump parts, transmission equipment, cutting tools, hydraulic and
pneumatic, aerospace parts, inspections equipment, stamping and forging [3, 4, 6].
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The most extensively used type of lapping process is flat lapping. Its goal is to
achieve extremely high flatness of the workpiece and/or close parallelism of
double-lapped faces. The other applications include removal of damaged surface
and sub-surface layers and, enhancement of the surface finish on workpieces [2, 4, 8].
1. CERAMIC MATERIALS LAPPING
Modern-day products are characterised by high-precision components.
Ceramic materials have been widely adapted as functional materials as well as
structural materials in various industrial fields and their application to precision
parts also increased. However, the high dimensional accuracy and good surface
quality required for precision parts are not necessarily obtained by the conventional
forming and sintering process of ceramic powders. Thus finishing processes of the
ceramics after forming and sintering are an important perspective to be considered
to meet the goals like parallelism, tolerances, flatness, and smooth surface. These
processes are high-precision abrasive processes used to generate surfaces of desired
characteristics such as geometry, form, tolerances, surface integrity, and roughness
characteristics. Abrasive finishing processes are used in a wide range of material
applications and industries. Grinding, lapping, and polishing have a leading
importance in these perspective [4, 5, 7].
To obtain closer tolerances, ceramic materials demand a very highly
sophisticated equipment and skilled labor, which will obviously lead to high
manufacturing costs. Surface and subsurface damage (after grinding process) is one
of the problems that is seriously affecting the performance of ceramic components.
Hence to obtain all necessary machining qualities without much investment, design
engineers have suggested the lapping process, used especially after grinding. The
relative speed in lapping is much lower than in grinding. Consequently, the
concentration of energy in the contact area is much lower.
Polishing usually is used after lapping. Lapping tends to decrease the original
surface roughness but it’s main purpose is to remove material and modify the
shape, whereas polishing implies better finish with little attention for form
accuracy [4, 5, 7].
2. EXPERIMENTS PROCEDURE
Aluminium oxide is one of the hardest materials known. Its high hardness
promotes a series of applications in mechanical engineering, such as bearings and
seals. A slurry composed of diamond grains mixed with liquid or paste carrier is
generally used for that material machining. Due to diamond price this is the
expensive solution, especially when considering continuous supplying.
J. Molenda, Influence of lapping velocity, pressure and time on ceramic elements machining results
87
This paper reports the observations of Al2O3 lapping process results received
with use of different abrasive material, cheaper than diamond – boron carbide.
Specifically, the material removal rate (MRR) and surface characteristic are studied
in the light of varying lapping parameters, like lapping velocity, pressure, and time.
Each workpiece was weighed before and after lapping using a precision weighing
scale precise to within 1×10–4 g to determine the material removal rate in gram per
minute. In addition, the initial thickness of each sample was determined with
a digital micrometer precise to within 1×10–3 mm. The difference between the
initial thickness and final thickness was used to obtain the material removal rate in
mm per minute. Equation (1) was used to calculate the MRR [1]:
MRR =
where:
W1
W2
T1
T2
H1
H2
–
–
–
–
–
–
ΔW W1 − W2
ΔH H 1 − H 2
=
or
=
,
ΔT
T2 − T1
ΔT
T2 − T1
(1)
initial weight of sample,
final weight of sample,
time at onset of lapping,
time at the end of lapping,
initial thickness of sample,
final thickness of sample.
A Hommeltester T8000-R60 profilometer with a resolution of 0.01 µm was
used to determine the surface roughness before and after lapping. The radius of the
stylus used was 2 µm.
Percentage Ra improvement was determined using [1]:
KRa =
(Average initial Ra − Average final Ra ) × 100
Average initial Ra
(2)
The experiments were carried out on a one-plate lapping machine ABRALAP
380 with a grooved cast-iron lapping plate and three conditioning rings (Fig. 1).
The machine kinematics allows for direct adjusting wheel velocity in range up to
65 rev/min. It is also equipped with a four-channel tachometer built with optical
reflectance sensors SCOO-1002P, and a programmable tachometer 7760 Trumeter
Company, which enables to read the value of rings and plate rotational speed.
During experiments three values of lapping speed: 49, 38, and 27 m/min were
executed.
Workpieces were commercially available valve sealing elements placed in the
conditioning rings with use of workholdings (Fig. 2). Samples were lapped during
15 and 20 minutes.
ABRALAP 380 is also equipped with liquid slurry dispensing system,
enabling constant supplying of fresh abrasive grains into the work zone. The
supply of the slurry was maintained at 19·10–8 m3/s. It was composed of boron
carbide grains mixed with kerosene and machine oil. Abrasive grains size used was
F400/17.
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Fig. 1. One-plate lapping machine ABRALAP 380
Fig. 2. Samples location in the conditioning ring
Abrasive concentration which is defined as:
m=
Mass of the abrasive
,
Mass of the lapping liquid
(3)
was m = 0.25.
The lapping pressure was provided by dead weights. During experiments three
values were executed: 0.025 and 0.038 and 0.051 MPa.
3. TESTS RESULTS
Figures 3–8 presents some results obtained during the tests. There are
presented dependencies of ΔW, ΔH, MRR, in mg/min and mm/min, and surface
roughness parameter Ra and KRa on lapping velocity, pressure and time.
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J. Molenda, Influence of lapping velocity, pressure and time on ceramic elements machining results
ΔW [g]
MRR [g/min] [g]
0,20
0, 010
0,15
0,008
0,006
0,10
0,004
0,05
0,00
15 min
20 min
v = 27 m/min
v = 38 m/min
v = 49 m/min
0,002
0,000
15 min
20 min
Fig. 3. Test results of ΔW and MRR depending on lapping time and velocity obtained
for Al2O3 elements (BC-F400/17, p = 0.038 MPa)
MRR [mm/min]
ΔH [mm]
0,40
0,020
0,30
0,015
0,20
0,010
v = 27 m/min
0,10
v = 38 m/min
v = 49 m/min
0,00
15 min
0,005
0,000
20 min
15 min
20 min
Fig. 4. Test results of ΔH and MRR depending on lapping time and velocity obtained
for Al2O3 elements (BC-F400/17, p = 0.038 MPa)
It can be seen that both ΔW and ΔH are dependent on lapping time and
velocity. The smallest values were obtained for v = 27 m/min and for 15 minutes of
lapping. Achieved values are consistent with others published works. Fig. 5 shows
that also Ra parameter varies with lapping time and velocity. The smallest value of
Ra was achieved after lapping with maximum plate speed and time, but the
differences as can be seen on Fig. 5, are slight. The best surface improvement (KRa
about 10%) was obtained during lapping with highest value of v = 49 m/min and
lasting 20 minutes. This value was about 3 times bigger than after 15 minutes.
Ra [μm]
KRa [%]
10
0,8
8
0,6
6
0,4
4
v = 27 m/min
0,2
v = 38 m/min
v = 49 m/min
0,0
15 min
20 min
2
0
15 min
20 min
Fig. 5. Test results of Ra and KRa depending on lapping time and velocity obtained
for Al2O3 elements (BC-F400/17, p = 0.038 MPa)
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MRR [g/min]
ΔW [g]
0,012
0,30
0,009
0,20
0,006
p = 0,025 MPa
0,10
p = 0,038 MPa
0,00
15 min
20 min
p = 0,051 MPa
0,003
0,000
15 min
20 min
Fig. 6. Test results of ΔW and MRR depending on lapping time and pressure obtained
for Al2O3 elements (BC-F400/17, v = 49 m/min)
Fig. 6–8 presents the changes of examined parameters with lapping time and
pressure. It can be seen that both ΔW and ΔH are dependent on those two
parameters. The biggest values were obtained for p = 0.051 MPa and for 20
minutes of lapping. As can be seen also MRR varies with lapping time and
pressure. Its changes are similar to ΔW and ΔH.
ΔH [mm]
MRR [mm/min]
0,80
0,04
0,60
0,03
0,40
v = 0,025 MPa
0,20
v = 0,038 MPa
0,02
0,01
v = 0,051 MPa
0,00
15 min
0,00
20 min
15 min
20 min
Fig. 7. Test results of ΔH and MRR depending on lapping time and pressure obtained
for Al2O3 elements (BC-F400/17, v = 49 m/min)
KRa [%]
Ra [μm]
0,8
15
0,6
12
9
0,4
0,2
v = 0,025 MPa
6
v = 0,038 MPa
3
v = 0,051 MPa
0,0
15 min
20 min
0
15 min
20 min
Fig. 8. Test results of Ra and KRa depending on lapping time and pressure obtained
for Al2O3 elements (BC-F400/17, v = 49 m/min)
As Fig. 8 presents KRa values significantly varies most of all with lapping
time. The influence of lapping pressure is smaller.
J. Molenda, Influence of lapping velocity, pressure and time on ceramic elements machining results
91
CONCLUSIONS
Lapping process is commonly used for ultra-precision machining of various
materials, especially when they are difficult to machine. This group includes
ceramic materials. Among them widely used is Al2O3. Because of its applications
requiring extreme dimensional accuracy, straightness and concentricity, lapping
process is used. Because of the lack of its complex model there is a need to
empirically find optimal parameters. The results are partially presented in this
paper. The material removal rate and specimens surface characteristic are studied
in the light of used lapping parameters, like pressure, velocity, and time. Achieved
values of total ΔW, ΔH, and MRR per minute are similar to those presented in
others authors works what can prove their correctness. Here were presented results
for rough lapping conditions (F400/17). Others parameters, like abrasive grains
size influence will be studied and the results will be presented in future works.
REFERENCES
1. Agbaraji C., Raman S., Basic observations in the flat lapping of aluminum and steels using
standard abrasives, International Journal of Advanced Manufacturing Technology, 2009, No. 44.
2. Crichigno Filho J.M., Teixeira C.R., Valentina L.V.O.D., An investigation of acoustic emission to
monitoring flat lapping with non-replenished slurry, International Journal of Advanced
Manufacturing Technology, 2007, No. 33.
3. Horng J.H., Jeng Y.R., Chen C.L., A model for temperature rise of polishing process considering
effects of polishing pad and abrasive, Transactions of ASME, Vol. 126, 2004.
4. Marinescu I.D., Uhlmann E., Doi T.K., Handbook of lapping and polishing, CRC Press Taylor &
Francis Group, Boca Raton 2007.
5. Molenda J., Barylski A., Al2O3 sealing elements lapping, Journal of KONES Powertrain and
Transport, Vol. 19, 2012, No. 3.
6. Molenda J., Barylski A., Analysis of mathematical model describing a problem of temperature
rise during one-sided surface lapping, Journal of KONES Powertrain and Transport, Vol. 16,
2009, No. 4.
7. Sreejith P.S., Ngoi B.K.A., Material removal mechanism in precision machining of new
materials, International Journal of Machine Tools & Manufacture, 2001, No. 41.
8. www.engis.com.
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WPŁYW NACISKU JEDNOSTKOWEGO, PRĘDKOŚCI
I CZASU DOCIERANIA NA EFEKTY OBRÓBKI ELEMENTÓW
Z CERAMIKI TECHNICZNEJ
Streszczenie
Zakres zastosowań ceramiki technicznej obejmuje współcześnie prawie wszystkie dziedziny techniki.
Tak szerokie wykorzystywanie wynika z jej licznych zalet, jak korzystny stosunek masy do objętości,
wysoka twardość, odporność na ścieranie, odporność na korozję, mechaniczną wytrzymałość
w wysokiej temperaturze, trwałość kształtu i inne. Istotną cechą prawie wszystkich materiałów
ceramicznych jest ponadto ich kruchość. Te szczególne cechy ceramiki, w połączeniu z wysokimi
wymaganiami pod względem jakości powierzchni obrobionej oraz dokładności wymiarowokształtowej wyrobu, sprawiają, że należy ona do grupy najtrudniej obrabianych materiałów
konstrukcyjnych i należy przywiązywać szczególną uwagę do wyboru metody obróbki i doboru jej
parametrów. Zastosowanie znajdują tylko niektóre metody wytwarzania. Szeroko wykorzystywane są
przede wszystkim szlifowanie, docieranie i polerowanie.
Docieranie stosuje się najczęściej wtedy, gdy wymagana jest jednocześnie wysoka dokładność
kształtu, dokładność wymiarowa oraz określona mikrostereometria powierzchni obrobionej. Jako
rodzaj obróbki wykończeniowej docieranie ma obecnie wiele zastosowań, między innymi w przemyśle
kosmicznym, samochodowym, narzędziowym, medycznym, elektrooptyce, wytwarzaniu elementów
urządzeń do archiwizacji danych, elementów pomp i zaworów. Pozwala ono na uzyskanie
chropowatości powierzchni elementów ceramicznych Ra = 1–0.01 µm.
Tlenek glinu jest jednym z najtwardszych materiałów konstrukcyjnych, co umożliwia jego szerokie
zastosowanie w budowie maszyn, między innymi na elementy łożysk i uszczelnień. W pracy
przedstawiono wyniki docierania elementów wykonanych z tego materiału. Głównym celem było
sprawdzenie efektów obróbki przy zastosowaniu różnych parametrów procesu. Badania prowadzono
w czasie docierania jednotarczowego na docierarce ABRALAP 380 o podstawowym układzie wykonawczym, składającym się z trzech pierścieni prowadzących. Analizowano chropowatość powierzchni
opisaną parametrem Ra i ubytek materiałowy, liniowy i masowy.
Słowa kluczowe: docieranie jednotarczowe, docieranie ceramiki Al2O3, efekty docierania, ubytek
materiałowy.