Analysis of a High-Precision Grinding Machine

Transcription

1 ISSN International Journal of Emerging Trends in Engineering Research (IJETER), Vol. No.1, Pages : 1 16 (015) Analysis of a High-Precision Grinding Machine Seon-Yeol Oh 1, Joon Jang, Woo Chun Choi 1 Graduate School, School of Mechanical Engineering, Korea University, 15dh15@korea.ac.kr Graduate School, School of Mechanical Engineering, Korea University, gojeeeak@korea.ac.kr School of Mechanical Engineering, Korea University, wcchoi@korea.ac.kr Abstract : A finite element model of an ultra-precision grinding machine that can have high precision and high stiffness is constructed and structural analysis is done with equivalent stiffnesses of linear motion guides by after structural design and the deformation of the grinding machine is obtained. In order to reduce the deformation of the grinding machine that causes bad influence on precision, structural complement is conducted by adding ribs at the lower part of the column. Also, the straightness of the grinding machine is improved by lifting the base side of the column. Modal analysis is done to find the natural frequencies. And thermal analysis is done to obtain temperature distributions and thermal deformation due to heat sources. Experiments were also done to measure the straightness of the machine and the temperature on LM guide. The deformation of the grinding machine is reduced by adding ribs and the grinding machine is stable for vibration because the machine has high natural frequency. It is confirmed that the ultra-precision grinding machine with linear motion guide applied is very stable for thermal deformation. Key words : Ultra-precision grinding machine, Structural analysis, Thermal analysis, Precision INTRODUCTION Research and development of high-stiffness and high-precision grinding machines for finishing in various production lines requiring high precision is very important. Those kinds of grinding machines should have sufficient stiffness and structural stability to enable precision positioning. Since heat generation from various heat sources has a harmful effect on machining precision, machines should have sufficient thermal stability. In previous researches on structural analysis of grinding machines, analysis and evaluation of the effect of weight and cutting force on the structural deformation of high-precision grinding machines was done, based on a finite element model constructed from design data. Thermal analysis of grinding machines was also done including convection, and temperature rise and thermal deformation was obtained. [1-5]. In this study, finite element analysis is done to develop a high-precision grinding machine for machining highprecision mirror surfaces and micro-patterns for optics, aeronautical, semiconductor, display, electronicscommunication, and so on. Before manufacturing a machine, analysis result is used to improve the structure and to reduce To achieve ultra-precision grinding, a machine tool should have sufficient stiffness and precision positioning. In this study, in order to develop a grinding machine with linear motion guides, AutoCad and Inventor of Autodesk are used to construct a finite element model, ANSYS is used to perform structural and thermal analysis, and experiments are done. The machine should have an allowable column deformation of less than 1μm. STRUCTURAL ANALYSIS OF A HIGH-PRECISION GRIDING MACHINE D structural design of a high-precision grinding machine The grinding machine is a 4 axis system, having three linear motion axes (x, y, and z) and one rotational axis, as shown in Table 1. The table work space is 400 x 400 mm. The strokes of the x-, y-, and z axis are 500mm, 600mm and 00mm, respectively, and the c axis has a maximum rotational speed of 10rpm. High speed linear motors are employed in x and y axis for high speed feeding, and a high-frequency air- bearing-supported GW spindle of 0~60,000rpm and an LM guides are used. In order to analyze structural deformation of the machine using FEM, D models of essential elements were constructed, as shown in Fig. 1. The column of the system is a portal type and y-axis table moves along an LM guide and c axis rotates on the y axis. X axis moves along a guide in front of the column, z axis with a spindle loaded moves up and down. The material of major components such as bed, column, y axis table, c axis table, and z axis table is FC00, and the material of y axis sub components is SS400, and the material of the c axis shaft is S45C. Fig. shows the finite element model composed of 188,758 nodes and 7,545 elements. Table shows the properties of major structural components. Table 1 Specification of high-precision grinding machine cost and development period. 1

2 International Journal of Emerging Trends in Engineering Research (IJETER), Vol. No.1, Pages : 1 16 (015) design revision were 5.19 and 4.47 m, respectively, as shown in Fig. 5. This reinforcement shows a good result. The structure was constructed according to the design revision, as shown in Fig. 6. Fig. 1: Main structure modules of the grinding machine Structural analysis and design revision of grinding machine column The column of the grinding machine is expected to have the largest deflection. Before combining x and z tables to the column, finite element analysis was done to find deflection due to its own weight, and structural design revision was done. (a) (b) (c) Fig. 4: Deformation of the modified columns by adding ribs Fig. : Finite element of the grinding machine Table : Properties of the materials Material Density Elasticity Poisson s Ratio GC00 708kg/m 90GPa 0.5 SUS0 SS400 S5C S45C 7900kg/m 7861kg/m 7858kg/m 7860kg/m 19GPa GPa GPa GPa 0.8 Ribs shown in Fig. (a)-(c) were added to the structure and structural analysis was done. Fig. 4 shows deflections for three ribs. It was found that rib (b) results in the smallest maximum deflection of 0.94 m compared with 1.16 m of the original design. The structure was revised with rib (b) added. (a) Before adding ribs (b) After adding ribs Fig. 5: Deformation of columns assembled Fig. 6: Actual machine with ribs Structural analysis of the grinding machine (a) (b) (c) Fig. : Ribs added to the column With x and z axis tables and spindle combined to the column, the deformation of the column before and after Linear motion guide feeding system In the grinding machine, LM guides and linear motors were used for x-, y- and z-axis table feeding. In this study, NRS5LR LM guide was used for y- and x-axis, and NRS0LR was used for z axis. Those guides are good for 1

3 International Journal of Emerging Trends in Engineering Research (IJETER), Vol. No.1, Pages : 1 16 (015) high load, high stiffness, high positioning accuracy and high speed. In order to do structural analysis, the stiffness of the LM guide should be known. Many researches predicted linear stiffness of balls between LM block and rail (Fig. 7), through Hertz contact theory and nonlinear equation of motion. [6] Structural analysis of the grinding machine Eleven support points were selected at the bed bottom surface, 100N of weight was applied at the c axis table top surface, 15N of cutting force on spindle was applied. Fig. 10 shows the deformation of the machine. The maximum deformation was found to be 6.6 m at the top surface of z axis table. (a)the cross-section Fig. 7: LM guide and block (b)actual LM guide Fig. 8 shows the relation between force and displacement of LM guides. Fig. 9 shows the equivalent spring constant k from the relation curves. The stiffness in the vertical direction k r and in the horizontal direction k h were obtained at a mid preload condition from the following equation. [7] (a)nrs5lr (b)nrs0lr Fig. 8: LM guide relation curve between load and displacement Fig. 9: Equivalent stiffness of LM guide Fig. 10: Deformation of the grinding machine THERMAL ANALYSIS OF THE GRINDING MACHINE Since heat generation due to heat sources has a harmful effect on precision, structural deformation as well as thermal deformation is important. In this study, heat sources were investigated, and thermal deformation was obtained using finite element analysis. Heat sources in grinding machine There are several heat sources in the grinding machine: a linear motor for x- and y- axis feeding, a ball screw for z-axis motion, a rotary motor for c axis rotation, and linear guides. Heat generation from x and y linear guides of 18m/min maximum speed is about 1.9W. Since z axis speed is m/min, heat generation can be neglected. [8] Table shows data of a 410-6P linear motor.[9] Equation () represents the power of the motor. The first term represents heat and the second term mechanical consumption. [10] Ptotal I phase R phase F () where I phase =phase current= Iline current /, R phase = phase resistance, v= velocity, and F=force. Table : Data of the linear motor Axis Motor Mass [kg] Force[N] Current[A] X 410-6P Y 410-6P Power[W] F k () where k is stiffness, m) is displacement and F(N) is the design load. The k r and k h values of NRS5LR are 0.71kN/ m and 0.56kN/ m, respectively. Those values of NRS0LR are 0.57kN/ m and 0.47kN/ m, respectively. NRS5LR has higher stiffness. These values are used in the analysis. 14 For a delta wound motor, R phase =6ohm, and Iphase Iline current /. Based on the data, heat generations of the x- and y-axis linear motors are calculated to be 1.4 and 0.7W, respectively. Friction heat in a ball screw is generated between ball screw and nut. When a AKM 4E servo motor is in a peak condition, the torque is 11. Nm, the maximum feedrate is m/min and the corresponding rotation speed(n actuater ) is 19.1 rpm. As shown in Fig. 11, the shaft radius(r shaft ) is

4 International Journal of Emerging Trends in Engineering Research (IJETER), Vol. No.1, Pages : 1 16 (015) 1.5x10 - m and the length of the nut(l nut ) is 0.086m. From the data, heat generation can be calculated from equation (4).[11] T nactuater q rshaft Lnut (4) Fig. 11: Ball-screw-nut system Temperature distribution due to heat sources The temperature distribution was obtained under the condition that the ambient temperature is C and convection cooling is considered. The temperature distributions are shown in Fig. 1. When the ball screw is at a peak condition, temperature rises by 5 C. For the x- and y axis linear motors, temperature increased by 1.5 and 0.8 C, respectively. For y axis linear guide, temperature increased by 0.9 C. For linear motors, WinTips program was used to calculate the temperature. As a result, the maximum temperature was found to be increased by 9 C for x axis and 4 C for y axis. From the result, temperature rise due to heat sources is tolerable. Thermal-structural analysis In this study, thermal-structural analysis was also done. Heat generation of x and y axis linear motor was 0.7 and 1.W, respectively. Heat generation of the ball screw was qpeak 50. W / m, and heat generation of x and y axis linear guides is 1.9W. Heat generation of z axis linear guide is neglected. Fig. 1: Thermal-structural Analysis The boundary conditions are: 11 supports on the bed bottom surface, 100N of weight at c axis table top surface, 15N cutting force at the spindle. The maximum deformation of 6.69 m was found at the top surface of z axis table, as shown in Fig. 1. The deformation without heat sources was 6.6 m. The deformations with and without considering heat sources are very close. This means that the grinding machine is very stable for thermal load. CONCLUSION In this study, the effect of weight and cutting force in a grinding machine on deformation was investigated, using finite element analysis. In order to reduce deformation, design was revised by adding ribs. Heat sources were investigated, and thermal-structural analysis was done. The result was used to build a high precision grinding machine. The following conclusions are drawn: - By adding ribs on the column, deformation was reduced. - Equivalent stiffness of LM guide was obtained from the ex perimental result. - Thermal structural analysis showed that the deformations with and without considering heat sources were very similar. This means that the grinding machine is thermally stable. REFERENCES Fig. 1: Temperature distribution with heat sources considered [1] S. E. Choi, Analysis of Structural and Vibration for UT- 80 Maching Center, KSMPE autums conference(009), pp [] D. G. Lee, Structual Design and Evaluation of the Three Axis Ultra-precision CNC Grinding Machine, Journal of me chanical science and technology, Vol., NO. (1995), pp [] S. I. Kim, Thermal Characteristics Analysis of a High-S peed HMC Spindle System, KSMTE spring conference(00 1), pp [4] S. I. Kim, Structural Characteristic Analysis of a High-P recision Centerless Grinding Machine with Concrete-Filled Bed, KSPE autums conference, Vol., NO. (005), pp

5 International Journal of Emerging Trends in Engineering Research (IJETER), Vol. No.1, Pages : 1 16 (015) [5] S. I. Kim, Structural Characteristic Analysis on the Hydr ostatic Guide Way and Feeding System of a High-Precision Centerless Grinder for Machining Ferrules, KSPE autums c onference, Vol., NO. (004) pp [6] K. H. Kim, The Accuracy Design of LM Guide System i n Machine Tools, KSPE autums conference (000), pp [7] Information on Samick Th k Technical Support [8] Seok-Il Kim, Ha-Kyoung Seong Thermal Characteristics Analysis of a High -Speed HMC, KSPE spring conference( 00), pp.1~6 [9] Information on daekhon corp oration [10] Information on m, Motor parameters application note [11] Mahmmod Aziz Muhammed, the effect of heat generat ed by friction in the ball-screw-nut system on the precision of high speed machine,