A Study on the Process of Granite Belt Grinding

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland A Study on the Process of Granite Belt Grinding Chengyong Wang, Zhe Qin, Xin Wei and Yunxia Wu Guangdong University of Technology, Guangzhou , China Keywords: Granite, Belt Grinding, Surfactant Abstract. Based on the analysis of the action of surfactant in granite grinding with diamond disk, the effects of the grinding coolant with different surfactant and concentration were studied in belt grinding of granite. The influences of different factors on belt grinding were analyzed and the machining parameters are optimized by experiments. The results show that belt removal rate was increased greatly using grinding coolant with surfactants. The grinding speed and grit mesh of belt and the rotating speed of table will affect the functions of surfactant in belt grinding process. Introduction Because of excellent performances of granite, such as high hardness, low coefficient of expansion, non-conductivity, non-magnetism, granite has widely been used in precise measurements and precise parts of high precise machine, including body, slide way and spindle, etc. There exists low efficiency and poor quality in the traditional granite machining. Belt grinding was developed in the 1960s, and it is highly efficient, with steady speed, highly grinding precision and low cost. Because belt grinding is a kind of elastic grinding, it has the mixed functions of grinding, rubbing and polishing. The excellent grinding performance of belt grinding of stone has been proved [1,2]. In the stone drilling and cutting process, water or coolants with surfactant are often used to lubricate, and discharge chips. The reports focused mostly on the effects of surfactant on cutting force and tool wear in the cutting process, and on drilling speed in the drilling process [3-5]. The mechanism of surfactant in granite grinding has been studied [6], which indicates that high removal rate can be obtained through the actions of surfactant. Based on the previous research work, the application of surfactant has been also studied in the belt grinding of granite. Experiments Grinding tests were performed using a plane belt-grinding machine with loading board and rotating table. The main minerals of the granite are plagioclase (55%), augite (45%), magnetite (2-3%), and little quartz (1-2%). The granite samples were cut into 100mm 100mm 20mm plates, and were fixed on the grinding machine table. Carborundum belts, with grit mesh 120#, 240#, 400#, 600#, was used as grinding tools in the experiment. Three kinds of surfactants, cationic surfactant, non-ionic surfactant and anionic surfactant were chosen. Micro-hardness and Roughness of Granite Surface The micro-hardness of granite surface was measured with Vickel hardness test. In the measurement, it was difficult to find contact surfaces without breakages because the surface of augite has many initial cracks. The micro-hardness of plagioclase granite was looked as the surface micro-hardness of granite. Fig.1 shows the measurement results. Surface roughness of granite under the different grinding conditions is illustrated in Fig.2. The grinding conditions were grit mesh 240#; speed of belt 15m/s; rotating speed of table 50 rpm in Fig.1 and Fig.2. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-13/05/16,10:35:50)

2 112 Advances in Abrasive Technology V Surface Microhardness HV cationic anionic non-ionic water Surfactant Concentration (%) Fig. 1 Surface micro-hardness of granite acted different surfactants Effects of Surfactant on Removal Rate Fig.3 showed the relations among the material removal rate, types and concentrations of surfactants. In the experiment the concentration of surfactant in the coolants were %, 0.02%, 0.1%, 0.5% with the same grinding conditions as in Fig.2. The effects of three kinds of surfactants on removal rate were obviously larger than water, and with same concentration, removal rate with cationic surfactant is obviously higher than those with non-ionic surfactant and anionic surfactant. When the concentrations of three kinds of surfactants arrived at 0.02%, the removal rate of granite was the highest. The experiment results indicated that material removal rate of belt grinding gone up when the -potential on the granite surface changed towards positive value, which was the same as the results in grinding process presented in reference [6]. 0.6 Ra (µm) cationic anionic 0.3 non-ionic water Surfactant Concentration (%) Fig. 2 Surface roughness of granite ground with different surfactants Material Removal Rate (mm 3 /s) cationic non-ionic anionic water Surfactant Concentration (%) Fig. 3 Material removal rates with different surfactants

3 Key Engineering Materials Vols Effects of Grinding Parameters on Belt Grinding With water and coolant of cationic surfactant, the experiments of belt grinding were performed with various speed of belt and rotating speed of table respectively. Experimental results of material removal rates were shown in the Fig.4. When water was used, although the speed of belt and rotating speed of table were increased, the material removal rate was much lower than that using coolant with surfactant. In the experiment above, the removal rate of granite increased as speed of belt grown. The reason for this was that the amount of grains being cut in unit time rises, and the times of grinding surface to be impacted increase, fatigue strength of granite descended. Due to the effect of surfactant, the cracks on the granite surface increased and material removal rate of granite increased remarkably. But when speed of belt was higher, the contact time and contact interval time between belt and granite were reduced, the grinding coolant couldn t penetrate into grinding area effectively, and the action on the grinding surface of surfactant decreased, resulting in the decrease of the increment of material removal rate. The effects of rotating speed of table on the grinding process were similar to that of speed of belt. With the increase of rotating speed of table, the material removal rate grown, but the increased range reduced. Owing to elasticity of belt, when the speed increased, the load on the contact zone, the pressure and the elastic deformation of belt increased. The depth of grinding was difficult to keep, so the material removal rate reduced. The effect of speed of belt was more obvious than that of rotating speed of table. Material Removal Rates (mm 3 /s) Speed of Belt (m/s) Coolant Rotating speed of table cationic 70rpm cationic 60rpm cationic 50rpm water 70rpm Fig. 4 Effects of parameters to material removal rates (Grinding conditions: concentration of cationic surfactant 0.02%, grit mesh 240#) Effects of Grit Mesh on Grinding Process Using the belts with different abrasive grit mesh, the results of the material removal rate and surface roughness of granite with different grit belt were shown in Fig.5. The trend of the material removal rate and granite surface roughness was inversely proportional to grit mesh of belt, that mean, the higher grit mesh of belts, the lower the material removal rates and granite surface roughness. When the abrasive is coarse, the depth of cut and removal rate were larger and the brittle breakage develops deeper, as a result the higher efficiency and the larger roughness are obtained. With the decrease of the abrasive grit size, the depth of cut and removal volume are reduced, the effects of rubbing and ploughing produced on the granite surface during grinding process increase and the brittle breakage deduce, resulting in lower material removal rate of granite.

4 114 Advances in Abrasive Technology V Material Removal Rate (mm 3 /s) Ra (um) 120 # 240 # 400 # 600 # Grit No. of Belts Fig. 5 Effects of grit mesh of belts (Grinding conditions: concentration of cationic surfactant 0.1%, speed of belt 15m/s, rotating speed of table 50rpm) Optimization of Grinding Parameters Speed of belt, grit mesh, rotating speed of table and types of surfactants are selected as the parameters for orthogonal experiment, the belt grinding experiment were performed by four factors and three levels orthogonal design L 9 (3 4 ). The data were processed by variance analysis method. From the results by variance analysis, it was found that in the belt grinding of granite, the effect of abrasive grit mesh on the material removal rate was remarkable; the next factor was coolant, speed of belt and rotating speed of table. Using cationic surfactant for cooling, the removal rate was the highest. The improvement of removal rate occurred with the increase of speed of belt and rotating speed of table. When speed of belt 18m/s, belt grit 240#, rotating speed of table 70rpm and cationic surfactant were used, the optimal result can be obtained. Among the influential factors on surface roughness, the most important factor was grit mesh of belt, the subsequent was speed of belt, rotating speed of table and surfactant. The finer the grit is, the smaller the surface roughness. The increase of speed of belt will improve removal rate, while the increase of rotating speed of table brings about the effect adversely. The roughness was up to the biggest value when cationic surfactant was applied for cooling. It was noticed that surface roughness reduced by increasing the speed of belt. When the speed of belt increased, the increasing of the number of grits engaging cutting in unit time and the reduction of cut thickness of single grit, cause the elastic deformation of cutting trace of grits to decrease and the surface roughness to drop. While increasing the rotating speed of table, the number of grits cutting materials in unit time decreased, the cut depth and grinding force were increased so that the elastic deformation of workpiece surface became greater and the surface roughness was increased. Discussion Comparing the surface micro-hardness of granite belt grinding under three kinds of surfactants coolant (Fig.1) with the results of surface potential in reference [6], when surface -potential of cationic surfactant increased to maximum value, surface micro-hardness of granite will reduce to the lowest. When -potential change to positive because of changes of surfactant concentration, the electrostatic effects on the surface make it possible to reduce the micro-hardness, which is favor of the impaction of granite so that the material removal rate goes up. Anionic surfactant and non-ionic surfactant also accord with the rule.

5 Key Engineering Materials Vols (a) (c) (b) (a) Cationic surfactant (b) Non-ionic surfactant (c) Anionic surfactant (d) Water Fig. 6 SEM of belt-grinding surfactant with different surfactants Considering the penetration of surfactants at the same time, the surfactants can penetrate into the micro-cracks of granite under water pressure in the grinding process, accelerating micro-cracks to extend and make the material easy to be removed. Here degree of granite surface cracks was great, the high efficiency can be obtained, but the surface roughness was high relatively. The surface roughness of granite under different conditions accords with the rule basically (see Fig.2). Surface topography of grinding granite was observed by scanning electron microscopy (SEM). Fig. 6 shows SEM pictures of granite grinding with surfactants (0.02%) and water. In the pictures, plagioclase is offwhite, and its surface is bestrewed with concavities and lesser cracks. Augite is gray with the thicker surface cracks. It is difficult to find integrate plates with big area. Other minerals are tiny, filled in the plagioclase and difficult to be identified. SEM pictures indicate that after grinding with surfactant coolants, the micro-cracks and the number of craters on the granite surface increased. There were few plastic grooves on the granite surface. Surface topography of granite showed that the penetration ability of surfactants to granite caused the cracks on granite surface increasing. More cracks and fragments on the surface of augite are observed. Summary Using grinding coolant with surfactants, removal rate of granite was increased greatly. The effects of the grinding coolant with different surfactant and concentration are different for granite grinding. The grinding coolant with cationic surfactant obviously can increase efficiency of grinding granite, and enable the surface roughness to reach maximum. All of speed of belt, grit mesh and workpiece rotating speed will affect the functions of surfactant in belt grinding process. Acknowledgment The authors acknowledge the support from the National Natural Science Foundation of China (Project no ). References [1] J.J.Gagliardi: American Ceramic Society Bulletin 71 (1992), p (d)

6 116 Advances in Abrasive Technology V [2] H.K.Tönshoff, et al: Industrie Diamant Review (3) (1994), p [3] F.C.Appl, et al: Industrial Diamond Review (6) (1981), p [4] A.R.C.Westwood, et al: AIME 256 (1974), p [5] R.X.Ge and R.Perami: Geology and Reconnoitre 24 (1988), p. 53. (in Chinese) [6] Z.Qin, C.Y.Wang, F.L.Zhang and X.Wei: Key Engineering Materials (2001), p. 285.

7 Advances in Abrasive Technology V / A Study of the Process of Granite Belt Grinding /