Grinding in metallic materials Study the influence of cutting velocity

Transcription

1 Grinding in metallic materials Study the influence of cutting velocity Isabel Maria Abreu Madeira de Faria October 2007 Abstract. Metal cutting processes have an important role among other fabrication processes, due to capacity to generate almost any complex geometry with good tolerances and surface finishing. The machining processes such as turning, milling, drilling and others distinguish by their industrial and economic importance. This interest has motivated many research and development on these processes. Unfortunately, some of these processes have been paid less attention in teaching and investigation on universities, like grinding. The main goal of this investigation consists in analyze the influence of some parameters, such as cutting velocity in grinding forces. It was studied other parameters like depth of cut, cutting mode and the material influence in the process. The experimental work was carried out with a specially designed a numerical control grinder with forces instrumentation by the author. This equipment was developed and install on IMTT Lab (Industrial Machine tools Training Laboratory) of STM (Secção de Tecnologia Mecânica). It was developed a program based in LabView for control and monitoring of the process. The experimental tests allow identifying the principal impact of selected parameters. It is important the determination of velocity value which minimize grinding forces. This aspect has relative importance on grinding of slim components. And another issue, like the influence of the workpiece material on forces evolution and its related problems, was also cover by the present work. measurement Keywords. Grinding, Cutting velocity, Machine tool, Numerical control, Forces Introduction. Grinding is the most common word when we think in abrasive metal cutting. Nowadays, we could live on Stone Age if the pre-historic men didn t find a way to sharpen their tools, scratching on the stone. Four thousand years ago, Egyptians discovered the grinding process in metals, and it was when metallurgy begun. Since that, many developments have been done on machine tool design and its components. On XIX century appear the first machine tools powered by steam. Later, steam was replaced by electric energy. Though, it was during the two World Wars that occurs a considerably progress on machine tools. At the present time, grinding is responsible for 20 to 25% of expenses in machining operations on industrialize countries. One of the main problems with industrial application of metal cutting is related with the difficulty to optimize the principal parameters. It is usually the

2 manufacture quality change with the material, quantity mechanic and geometric characteristics of cutting tools, but the most important, with some parameters such as cutting velocity or chip thickness. The aim of this work is to evaluate the influence of the cutting velocity in grinding processes. It is an objective too develop an equipment to experiments, that allow realize cutting experiments and monitorship grinding forces. Theoretical background. Grinding processes have the objective of reducing the roughness or improving the surface quality by removing the marks left by other operations such as drilling, turning or milling, using the abrasive grains. Forces measurement and grinding power is an efficient way to identify the interaction wheel-material. Forces developed during the process, can be separated on tangential and normal components (refer to figure 1). The force tangent to the wheel-work contact, when multiplied by wheel velocity and a constant, determines the power consumed by the operation. The tangential and normal forces are related by the friction coefficient; therefore, in production grinding, in which the normal force is almost unknown (without a dynamometer of some type), the tangential force can be calculated from power measurements and used as an alternative for the normal force. F Q F P Figure 1 Forces diagram There are two types of velocity in this process: work velocity v (gives by the table move velocity in plain grinding) and grinding wheel velocity V. When both are on the same way, the process is called as down-cutting and the surface quality is improved relatively to up-cutting, lowering tool wear and improving grinding rates. Grinding forces are lower and consequently the specific energy requested [1]. For the same conditions, the increase of grinding wheel velocity leads to a reduction of mechanic strength on grains. At the same time, the temperature becomes higher, because more heat is generated. Experimental background. This section, presents the experimental develop that were necessary to study cutting velocity influence on grinding forces.

3 Materials and cutting tool. The cutting tool used in this process is a grinding wheel. These wheels are cutting tools whose body has a revolution form, and are composed by abrasive grains of hard and brittle substances, connected by another substance called bond. Gaps between grains and without bond are pores. Grinding wheel selected to conduct the experiments, had the following characteristics code 4A46Q4VB7 and had 13 mm of diameter. The mean of this code is the following: 4 Fabricant symbol A Abrasive type (aluminum oxide) 46 Grain size (medium) Q Hardness (relatively hard) 4 Structure (compact) VB Bond type (V vitreous; B resinous) 7 Wheel identification for manufacturer The materials of specimens were AISI 1045 CD steel and AA6009-T6 aluminum alloy. The main characteristics are represented on the Table 1 and Table 2: AISI 1045 CD Chemical Components Main Properties Component % Density (g/cc) 7.85 C Vickers Hardness 188 Fe Yield Strength (MPa) 530 Mn Elongation (%) 12 P Max 0.04 Elasticity modulus (GPa) 205 S Max 0.05 Poisson coefficient 0.29 Table 1 Steel properties AA 6009-T6 Aluminum Alloy Chemical Components Main Properties Component % Density (g/cc) 2.71 Al Vickers Hardness 102 Cu Yield Strength (MPa) 320 Mg Ultimate Strength (MPa) 340 Si Elongation (%) 12 Fe Max 0.5 Elasticity modulus (GPa) 69 Others Max 0.15 Poisson coefficient 0.33 Table 2 Aluminum properties Experimental apparatus. The experimental apparatus of this work consists on a plain grinder with numerical control. This machine tool is also designed to measure the grinding forces, allowing the assessment cutting velocity influence, and also to assure the necessary

4 accuracy of the different experimental grinding parameters. The apparatus is structured on three different groups: the machine tool, the numerical control software, and the measuring system. The initial idea to the structure of the plain grinder was building two modulus: a vertical component and a horizontal component, a machine with two defined axis. The horizontal component should allow the movement in xx axis. For that purpose it was used step motor which was fundamental to provide the horizontal movement. This step motor was tested using a computer program based on programming language LabView 8.0. The next step was building the body of the structure. The main body of the machine-tool in C structure has been building in heavy material to confer more stability. It was added some servo-mechanisms, ventilators, the bi-dimensional charge sensor. The result of this development is represented in figure 2. Rotation velocity 6000 to rpm X axis course 100mm Y axis course 150mm X movement accuracy 0.01 mm Y movement accuracy 0.01 mm X axis velocity 50 to 1200 mm/min Y axis velocity 50 to 1200 mm/min Evaluate forces capability Maxim charge on X axis 100kg Maxim charge on Y axis 150kg Sensibility x and y on charge sensor 10mV/V Step motors actuation Power 12W Feeding Monophase Control Open Loop Control software LabView Figure 2 Machine-tool for plain grinding To set the machine tool in motion, it was necessary to create a digital circuit that reproduces the right sequence to boost the internal coils of step motors. For this proposal, it was necessary test them with a bread board and create a software to control them. However, it is necessary a circuit that receives the control logic from the computer and turn on or off each coil. To execute this function, it was chosen an octal high voltage, high current Darlington transistor array, the ULN 2803 (refer to figure 3). The eight NPN Darlington connected transistors in this family of arrays are ideally suited for interfacing between low logic level digital circuitry, such as TTL (transistor-transistor logic) and the higher current/voltage requirements of lamps, relays, printer hammers or other similar loads for a broad range of computer, industrial, and consumer applications. All devices feature open collector outputs and free wheeling clamp diodes for transient suppression.

5 Figure 3 ULN 2803 Darlington transistor The device that was use to data acquisition was NI-6023E PCI from National Instruments manufacturer. This is ideal for applications ranging from continuous high-speed data logging to control applications to high-voltage signal or sensor measurements when used with NI signal conditioning. This DAQ (data acquisition) device includes the following features and technologies: 16 analog inputs at up to 200 ks/s, 12 or 16-bit resolution Up to 2 analog outputs at 10 ks/s, 12 or 16-bit resolution 8 digital I/O lines (TTL); Two 24-bit counter/timers Digital triggering 4 analog input signal ranges Creation phases and final setting of this digital circuit are represented on figure 4. a) b) c) Figure 4 a) Printed circuit; b) Digital circuit; c) Final setting of digital circuit Measure of the process consists essentially on observation of grinding forces with software help. For this purpose, the bi-dimensional charge sensor is fundamental and is used in mechanical strength measure. This sensor gives normal and tangential forces. Its function uses two Wheatstone bridge made of four strain gauges each one. The strain gauge is a transducer, which converts measure size through a physical effect of a change in conditions. One of the Wheatstone bridge measures normal force, and the other bridge measures tangential

6 component of grinding forces. This sensor has been designed for the expected range of grinding forces. The final scheme of data acquisition is depicted on figure 5. Figure 5 Representative scheme of measuring Experimental plan. The experimental work was planned to accomplish the main objectives of this work. It consists in make six experiments for each grinding mode in each cutting velocity. The vertical advance is executed for each two passages. Data acquired by the equipment correspond to grinding forces (cutting and penetration forces). Table 3 traduces the experimental plan. Case Material Cutting velocity (m/min) Passages Lubrification Penetration between passages (µm) Horizontal table velocity (mm/min) Up Aluminum 264 Grinding (CCA) AA Down Grinding (CSA) - Steel AISI Don t Table 3 Experimental plan Results and Discussion. Cutting velocity has direct influence on cutting power, on thermal losses in chips, in cutting tool and in grinding forces. The main goal of this work is evaluate how cutting velocity influences grinding forces. It is also study, how cutting mode has influence in grinding process, and a comparison between different materials, by evaluating the grinding forces.

7 Influence of cutting velocity. The figures 6 and 7 represent the evolution of grinding forces in function of cutting velocity. The present test in steel does not distinguish cutting mode A B C 1000 D 1100 Figure 6 Evolution of cutting force in function of cutting velocity A B C 1000 D 1100 Figure 7 Evolution of normal force in function of cutting velocity As noticed, normal forces are higher than tangential forces. On the first stage, occurs a reduction of forces, as predicted in literature, with the increase of velocity. At the contrary of tangential, normal force continue to decrease until a cutting velocity about 800 m/min. After this minim normal force suffer a sudden increase, which is explained by the generated heat and by

8 radial impact, influencing normal forces and less the tangential forces, because radial impact occurs in radial direction. Influence of the grinding mode. The following figures represent the evolution of grinding forces as a function of cutting velocity, for each cutting mode. Down-grinding Up-grinding A B C D Figure 8 Evolution of cutting force in function of cutting velocity in both grinding modes Down-grinding Up-grinding A B C D Figure 9 Evolution of normal force in function of cutting velocity in both grinding modes

9 As it is predicted, grinding mode doesn t have much influence in grinding forces. However, grinding forces are a little bit smaller in down-grinding than up-grinding. The similarity of both forces can be explained by the difference between cutting velocity and working velocity. Actually working velocity is almost insignificant compared with cutting velocity ( V >> ). vw Influence of the workpiece material. The following figures represent the evolution of grinding forces in function of cutting velocity, for two different workpiece materials. Aluminum Steel A B C D Figure 10 Evolution of cutting force in function of cutting velocity for both materials Aluminum Steel A B C D Figure 11 Evolution of normal force in function of cutting velocity for both materials

10 It was expected that grinding forces in aluminium will be smaller, however, they are much higher than grinding steel. In spite of comportment are similar, the magnitude of aluminium tangential force are above of steel tangential force. What can justify so drastically increase is the tendency to gather in pores of grinding wheel, enlarged with mentioned radial impact. The same factors increase normal forces. Conclusions. The experimental results agree with theoretical research, however with some justified variants. It was noticed that grinding forces changes with a complex behaviour due to interdependence of grinding parameters. Even so, it is possible to find an optimal cutting velocity that minimizes drastically the grinding forces. The material of specimens has also an important role. For a harder material, higher is the tendency to gather in pores of grinding wheel. This, traduces on a drastically increase of normal forces. Variations found on theoretically predicted and experimental results can be justified not only by vibration on the developed machine tool, but also by radial impact of the wheel as it contact with the specimen, and the temperature increase, result from the heating of cutting zone for absence of a cutting fluid. References. [1] Tawakoli Taghi, High efficiency deep grinding, VDI Verlag GmbH, London (1993); [2] Shaw M. C., Principles of Abrasive Processing, Clarendon Press, Oxford, (1996); [3] Sebenta de Tecnologia Mecânica I Associação de estudantes do Instituto Superior Técnico, Secção de Folhas 1993/1994; [4] Malkin, S., B.S., M.S., Sc.D., Grinding Technology Theory and Applications of Machining with Abrasives, Elli Horwood Limited, Chichester, (1989).