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].
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.
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]:
1 2 1 2 2 1 2 1 W MRR or , T W W H H H T T T T T Δ − Δ − = = = Δ − Δ − (1) where:
W1 – initial weight of sample,
W2 – final weight of sample,
T1 – time at onset of lapping,
T2 – time at the end of lapping,
H1 – initial thickness of sample,
H2 – 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]: (Average initial Average final ) 100
Average initial a a a a R R KR R − × = (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.
Fig. 1. One-plate lapping machine ABRALAP 380
Fig. 2. Samples location in the conditioning ring
Abrasive concentration which is defined as: Mass of the abrasive , Mass of the lapping liquid
m= (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.
v = 27 m/min v = 38 m/min v = 49 m/min 0,20 0,15 0,10 0,05 0,00 15 min 20 min ΔW [g] 0, 010 0,008 0,006 0,004 0,002 0,000 15 min 20 min MRR [g/min] [g]
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)
v = 27 m/min v = 38 m/min v = 49 m/min 0,40 0,30 0,20 0,10 0,00 15 min 20 min ΔH [mm] 15 min 20 min MRR [mm/min] 0,020 0,015 0,010 0,005 0,000
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.
v = 27 m/min v = 38 m/min v = 49 m/min 0,8 0,6 0,4 0,2 0,0 15 min 20 min Ra [μm] 15 min 20 min KRa [%] 10 8 6 4 2 0
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)
p = 0,025 MPa p = 0,038 MPa p = 0,051 MPa 0,30 0,20 0,10 0,00 15 min 20 min ΔW [g] 15 min 20 min MRR [g/min] 0,012 0,009 0,006 0,003 0,000
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.
v = 0,025 MPa v = 0,038 MPa v = 0,051 MPa 0,80 0,60 0,40 0,20 0,00 15 min 20 min ΔH [mm] 15 min 20 min MRR [mm/min] 0,04 0,03 0,02 0,01 0,00
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)
v = 0,025 MPa v = 0,038 MPa v = 0,051 MPa 0,8 0,6 0,4 0,2 0,0 15 min 20 min Ra [μm] 15 min 20 min KRa [%] 15 12 9 6 3 0
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.
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.
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 wymiarowo-kształtowej 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 wyko-nawczym, 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.
