Author Topic: Parametric influence on cutting parameters characteristics in precision machinin  (Read 2939 times)

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Author : Prof. Mohammed Yunus, Dr.J. Fazlur Rahman
International Journal of Scientific & Engineering Research Volume 3, Issue 1, January-2012
ISSN 2229-5518
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Abstract— Ceramic coated components have the advantage features of both metal and ceramics, i.e. good toughness, high hardness and wear resistance. Despite their outstanding characteristics, ceramic materials are not used in many cases due to high cost of machining. A major drawback to engineering applications of ceramic is due to their brittleness and fracture toughness, which makes them difficult and costly to machine.
 In order to study the precision machining processes of ceramics like grinding and lapping, experiments were conducted to find out the influence of various cutting parameters on surface quality production as well as grinding and lapping performance.
 Surface grinding of ceramic coating materials was done on samples specimens coated with Alumina (Al2O3), Alumina- Titania (Al2O3-TiO2)  and Partially stabilized zirconia (PSZ )using diamond and CBN (cubic boron nitride) grinding wheels. Based on the experimental results, the influence of cutting parameters, namely, cutting force, surface roughness and bearing area characteristics were evaluated and optimum machining conditions have been suggested for better performance in precision machining of ceramic coating materials.

Index Terms— Bearing Area Characteristics, Cutting force, Diamond and CBN grinding wheel, Grinding and Lapping, Optimizations of Cutting parameters, Precision Machining Processes Surface roughness. 

1   INTRODUCTION                                                                      
WITH  the projected wide spread applications of ceramic coating materials, it is necessary to develop an appropriate technology for their efficient and cost effective machining processes[5],[7], [9] and  [11 ]. Grinding of ceramics is a difficult task, as it is generally associated with cracking, splintering and delamination of surfaces. Conventional processes and tools are not generally suited for the ma-chining of ceramics. Standard machining tools can be used with optimization of machining parameters in operating conditions [ 13-14 ]. Various precision machining techniques that could be adopted for efficient machining of ceramic materials, namely grinding and lapping processes are studied in detail for the parametric influence of various cutting parameters of precision machining of ceramic coating materials.
     Ceramic coated components used in industrial applications, generally require post treatments like heat treatment and surface finishing by precision machining [1-8]. Good surface finish and high efficiency in machining to meet the demands of tight tolerances are generally achieved by grinding, lapping and polishing like precision machining processes [3].

Grinding of ceramics is a difficult task, as it is generally associated with cracking of surface. In order to study the effect of precision measuring processes [15], experiments were conducted to check the machining parameters like sur-face quality, grinding forces etc. on ceramic coated compo-nents for different machining conditions.
   The main object of this study is to evaluate the behavior of A, AT, PSZ, Super-Z alloy and ZTA ceramic coating materials subjected to different grinding conditions. The performance was evaluated by machining grinding force [20-22], surface finish [17] and bearing area characteristics [18] and also oil film retainability characteristics [15].

Three different commercially available ceramic coating powder materials namely, Alumina (Al2O3), Alumina-Titania (Al2O3-TiO2), Partially Stabilized Zirconia (PSZ) were used for the preparation of coatings [10-14]. A 40 KW Sulzer, Metco plasma spray system with 7MB gun is used for this plasma spraying of coatings. Mild steel plates of 50x50x6 mm were used as substrate to spray the ceramic oxides. They were grit blasted, degreased and spray coated with a 50 to 100 microns Ni Cr Al bond coat. The above ceramic materials were then plasma sprayed using optimum spray parameters.
2.1 Precision Machining of Ceramic Coating
Grinding: Using diamond and CBN grinding wheels [20] with the surface grinding trials were conducted with the grinding conditions mentioned below in table 1. Machining trials were conducted on different ceramic coated specimens (A, AT and PSZ ceramic coatings).

The main object of this study is to evaluate the behavior of
A, AT and PSZ ceramic coatings subjected to different grind-ing conditions. The performance [18] was evaluated by mea-suring
1.   Grinding force (Normal and Tangential force)
2.   Surface finish produced which also includes the bearing area characteristics
3.   Oil retainability characteristics.

2.2  Force Measurement
The normal grinding force (Fn) and the tangential force (Ft) were measured using grinding dynamometer [17] and [21].The ground samples were measured for different surface finish parameters such as Ra, Rt, and tp using Taylor Hobson’s stylus tracing profilometer.
2.3 Lapping
     A circular disc of 200 mm in diameter made of bright steel and a pin of 6mm diameter were coated with NiCrAlumel bond coat of thickness 75μm and subsequently coated with different coaing materials namely, Alumina (A), Alumina-Titania (AT), Partial Stabilized Zirconia (PSZ), Super-Z alloy and ZTA [19-20].
     Samples were initially ground to achieve pre lapping finish and then further lapped under the conditions mentioned above in table 2. The process variables were lapping time (ranging 5 to 25 minutes) and size of the diamond abrasives in lapping [18].
     The lapped discs were thoroughly cleaned and measurement in respect of surface finish was made using Taylor Hobson’s surface finish profilometer.

2.4 Oil retainability test
The oil retainability of ceramic coated surfaces was estimated using SAE 120 lubricating oil. In order to explain the oil retain ability of ceramic coated surfaces, the coated plates (specimens) were ground and lapped to half the area of the plate and the rest half left as it is. Oil droplets were then put on these two parts of the specimen and left untouched for 3 hours. The specimens with the oil droplets were then observed further increase in diameter using a travelling microscope and oil spreadability on ground and lapped surfaces were studied.

3.1   Results of Grinding
It has been noticed that, during the grinding of ceramic coat-ings, grinding forces were found to be varying considerably with increasing grinding speed. Also, it is observed that, with CBN wheels grinding force components (Ft and Fn) were found to be higher when compared to diamond grinding wheel as shown in figures1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 30, 31, 32 and 33. Besides, it is noticed that, the increase in the depth of grinding, generally improved the surface finish. During trials on grinding of ceramic coatings, grinding velocity, a range of 10 – 15 m/sec and depth of grinding 30µm were assessed to be more critical. It is also observed grinding of Alumina-Titania (AT) and Partially stabilized zirconia (PSZ) ceramic coatings with diamond wheel gave better surface finish.

3.2   Results of Lapping
It is concluded that, surface finish of lapped ceramic coat-ings, improved with lapping time and remains constant after 15 minutes of lapping time in case of AT and PSZ, whereas in case of Alumina (A), it attains saturation after 20 minutes of lapping time. It is also seen among the coatings that, AT could be lapped better than the other two.
     Bearing area characteristics of sprayed and ceramic coatings with diamond wheels are shown in figures 36, 37, 38, 39, 40, 41, 42, 43, 44 and 45. It is observed there is no much variation in the bearing area characteristics of coated ceramic surfaces subjected to diamond wheel grinding, but AT exhibits faster tendency to attain cent percent tp area. With CBN grinding, rapid improvement in bearing area characteristics of Alumina is observed due to improved grinding of brittle materials (like Alumina). This is due to grinding forces associated with CBN grinding wheels, CBN grains, cuts the ceramics relatively cooler (CBN is thermally more conductive)  compared to diamond wheel. It is also seen that, diamond wheel is more sensitive to grinding condition and with CBN wheel it is possible to go for higher depth of grinding , because of better thermal properties of CBN.

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