Temperature Field Simulation of Ballscrew Whirlwind Milling Yan Feng Li 1,3,a,Jian Song 2,b,Shao Hui Liu 3,c, Xian Chun Song 3,d

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Temperature Field Simulation of Ballscrew Whirlwind Milling Yan Feng Li 1,3,a,Jian Song 2,b,Shao Hui Liu 3,c, Xian Chun Song 3,d 1 School of Mechanical Engineering,Shandong University,Jinan ,China 2 School of Mathematics,Shandong University,Jinan ,China 3 School of Mechanical and Electronic Engineering,Shandong jianzhu University,Jinan ,China a liayanafenga@163. com, b songjianxiaoxiao@126.com, c shhll_1983@126.com, d songxch@sdu.edu.cn Keywords: Ballscrew, Whirlwind Milling, Temperature Field, ANSYS Abstract. Ball screw is the key parts of NC machine, precise instrument and many kinds of mechanical devices. Whirlwind milling is a new and effective thread manufacture technology. This article studies the internal heat conduction equation of whirlwind milling ball screw based on Heat Transfer, calculates ANSYS simulation result. The simulation results show that the surface temperature distribution rule and internal temperature distribution rule of a ballscrew is consistent. The article analyzes the influence factors of the thermal elongation. It provides a basis for compensating the thermal deformation error of whirlwind milling ballscrew. Introduction Whirlwind hard milling process is a new and effective environmental protection of advanced processing method, which is based on German cutting physicists Salomon's (Carl Salomon)Theoretical basis of the high-speed cutting. It can mill high hardness (42 HRC-65 HRC) metal materials directly. The advantage of High speed whirlwind milling of ballscrew thread is that Rotary milling cutter in high speed milling in a closed environment, Small noise, Easy to concentration processing with no oil chips, Little environmental pollution, only need one processing can meet precision of tooth shape and surface quality requirements, and the Processing efficiency is 5 to 8 times of thread grinding machining. Whirlwind milling sparks were flying in the process, Chip partial presenting as citrus red, and the temperature of cutting zone up to above 800 degrees Celsius. The cutting area will produce a lot of heat. Although most of the heat take away by the chips and the high speed rotary tool, there are still part of heat conducting into workpiece, to make the workpiece temperature upper. When Screw thread milling is completed, the average surface temperature increased about 20 degrees Celsius. In the hard-milling process, the thermal deformation of workpiece caused by milling heat is one of the important error sources which led to pitch error. Effective controlling of the workpiece thermal deformation in the screw milling process is important to improve the accuracy of ballscrew. This paper is based on the heat transfer, and found the heat conduction equation of hard milling process thread on the ballscrew, and get the temperature distribution of ball screw with ANSYS simulation, it provides theory basis which for further analysis hard milling process of hot deformation law of a ballscrew and for pitch error compensation. The principle of Whirlwind hard milling thread Several molding tools are installed in the Whirlwind milling cutter head. The Rotation axis of the cutter tilt a helix lead Angle β and has an offset e relative to the workpiece axis (Fig.1). Processing of screw thread, the workpiece and the cutter rotates in the same direction. The workpiece rotates at low speed while the workpiece rotates at high speed. Workpiece rotates one turn 360 degrees, the cutter along the workpiece axial feed a lead of T, thereby milling the thread. Several forming tools are installed in the cutter, but cut piece in turn. Chipping allowance is reasonably allocated to each 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-12/05/16,16:50:12)

2 Advanced Materials Research Vols tool, and the gradual change of the chip thickness decreases the cutting force. Eccentricity e make the tool in the non-cutting area have adequate cooling time, which is helps to extend tool life and improve the surface quality [2]. Fig. 1 Whirlwind hard milling diagram Fig. 2 Ballscrew whirlwind milling diagram Introduction of the screw thread hard milling Whirlwind hard milling processing equipment is the high precision processing equipment, which request working environment temperature constant to ensure the processing precision. In the whirlwind hard milling machine, one end of the workpiece is fixed by scroll chuck, the other end is free. There is a float support every other section length to support the workpiece (as shown in Fig.2). In the processing, the cutter cut from the free end to the fixed end. The heat source that is the contact point of the tool and the workpiece is moving along the ballscrew's spiral. The temperature field and the thermal deformation of the workpiece are heterogeneous and nonlinear because of the moving heat source. Make the following assumptions in order to simplify the calculation process on the ball screw processing [3,4,5]. (1) Think the milling heat afferent the ballscrew along the ballscrew surface by spiral movement, the milling depth,the cutter rotation speed and the feed rate etc are invariant in the milling process, that is the heat inputting are equal in any period and continuous. (2) Considered theball screw as an infinite long cylinder, the diameter is the pitch diameter of the ball screw. (3) Be thinking of adiabatic at both ends of workpiece, because of the heat transfer coefficient at both ends are smaller and the heat loss is also smaller. (4) Anywhere of the ballscrew surface have the same heat transfer coefficient with outside air. (5) The material of ballscrew is uniform and isotropic, and the thermal conductivity is constant. From the above assumptions, heat conduction problem of the ballscrew hard milling process is 3-d transient heat conduction problem which belongs to Constant physical properties and no internal heat source. An internal heat conduction differential equation of ballscrew can be established according to Fourier equation under cylindrical coordinate system t t 1 t t ρ c = λ + + (1) τ r r ϕ z In the equation,τ for time variable, r for radius coordinates variables, φ for Angle coordinate variables, z for length coordinate variables, ρ for material density, c for specific heat, λ for thermal conductivity of ball screw, t is temperature difference of the unit of workpiece relative to constant ambient temperature ( 20 degrees Celsius).

3 590 Manufacturing Engineering and Automation II Constant ambient temperature ensure that the workpiece surface has the same heat exchanged coefficient with outside air, and can use the third type of boundary conditions [6], namely ( t w t ) t λ = h f (2) r In the equation, h for the convection heat transfer coefficient of workpiece and air, t w for ball screw surface temperature, t f for the ambient temperature in the processing. The finite element simulation of Ball screw temperature distribution [7] Finite element method is widely used in the project as a simulation method, it's a widely range of development and application with its unique computational advantages. The simulation calculation of the milling heat transferring into the ball screw and conduction process with Ansys is to gain the internal temperature field changes in the workpiece and analyzes the Thermal deformation law. In the temperature field simulation process of the whirlwind hard milling the ballscrew, the position of the main load changes over time and heat continues incoming ball screw, so the analysis belongs to the transient thermal analysis. The basic steps of the transient thermal analysis include the establishment of a finite element model, exert load, solution and post-processing. The establishment of a finite element model. The finite element modeling is an important step in the Finite Element Analysis; it is related to the correctness and validity of the calculation results. In the whirlwind hard ball screw thread milling process, the surface which already processing section is thread and unprocessed section is smooth cylindrical.if the model established including thread, the calculation speed is slow with dynamic meshing in the process of simulation and the result may be unpredictable error, meanwhile, the heat conduction and convection changes smaller with threads or not can be neglected, so the ball screw model can be considered as smooth surface cylinder. Meanwhile, a three-dimensional solid model used in the simulation need to determine the element type. Simulate the temperature field of screw in hard milling process, in addition to considering the spiral mobile thermal load (namely, the incoming workpiece milling heat), but also consider the heat transfer of the screw surface and air convection. Mobile thermal load is temperature load, the convective heat transfer is convection load, both of them belong to surface load, they can not be applied in the same element, so selected two kinds of element types that are SOLID70 and SURF152. SOLID70 is three-dimensional solid element and has the ability of heat conduction with 8 nodes and 6 surfaces, used to receive temperature load. SURF152 is a three-dimensional thermal surface effect element. In three-dimensional thermal analysis, generate SURF152 elements on surface of the screw, used to receive the convection load. In the hard ballscrew thread milling process, milling heat move along the spiral line of the screw. In order to simulate spiral milling heat load really, choose the cylindrical coordinate system in ANSYS, and use the ball screw end surface meshing and axial expansion method of modeling. The FEA modeling of the ballscrew entity is shown in Fig.3 The heat load and solving. Actually, in the ballscrew hard milling process, the boundary conditions are very complex and varied. In this analysis, consider only the thermal load simulating the milling heat conducting into ballscrew and the convective heat transfer load based on the previous assumptions; don t consider the conduction heat transfer of the ballscrew and support, ballscrew and chuck. It is ignored that the increase of the heat dissipation area and the influence of the thermal conductivity lead by the processed segment of the ballscrew.

4 Advanced Materials Research Vols Fig. 3 Meshing ballscrew model Fig. 4 The profile temperature field of the ballscrew During the simulation, the location and value of the surface convective loads are unchanged, so it can be loaded directly after generated the SURF152 elements in PREP7. The convection heat transfer between ballscrew surface and the air belongs to a natural convection heat transfer of a horizontal cylinder in the large space [6]. According to the experimental formula ( Nu ( ) n m = C GrPr m) widely used in engineering calculations, take the surface convection coefficient 10.5W /(m 2 K) In order to simulate the spiral mobile process of heat along the ballscrew surface, Heat source loading and the solving use DO loops of APDL in the solving module. The variable axial movement and radial movement of the heat source of each load step are defined according to milling speed and pitch. In each loop, first remove the load of the previous load step, and then select the load position of the current load step, load the temperature load to simulate the milling heat conducting into the ballscrew. Save the results of each load step in the solution process. Post-processing. For the transient thermal analysis, ANSYS provides two post processing methods, namely POST1 and POST26, The POST1 used to observe the results of a load step of the whole model in the post-processing. Enter POST1, read the temperature distribution results of the screw elevation profile after the heat source move 10 pitches along the screw surface, as shown in Fig.4 According to the simulation result, the temperature of the milling area is maximum, milling heat effect on the temperature is limited to area near the contact part of the ballscrew and milling cutter, the segment away from the contact part is less affected, at the same time, most of the milling heat is absorbed by milled segment; The heat of unmilled part is mainly from the heat conduction, temperature distribution in the direction away from the processing zone is gradually reduced, and the heat is distributed only near the processing zone. The temperature distribution of the ballscrew surface and the center are similar. It is obvious that the temperature difference between surface and center of milling area is the largest, because a lot of heat is generating on milling area instantly and has no time to conduct to ballscrew internal. The farther milling area, the smaller the temperature difference of the ballscrew surface and center. The slenderness ratio of ballscrew is generally great, so in the actual calculation each cross-section can be considered at the same temperature, and thus the temperature field of the ballscrew can be calculated as one-dimensional problem. Analysis of simulation results Milling heat and convection cooling is the main factors that affect the ballscrew temperature distribution, At the same time, the screw temperature is also affected by other factors, such as thermal conductivity between the ballscrew and support, screw and chuck, the local forced convection cooling near rapidly rotating milling cutter head in processing, etc. Modeling and simulation of the ballscrew does not consider these secondary factors, First, The above factors are

5 592 Manufacturing Engineering and Automation II effect by the workpiece material and machining parameters and are difficult to quantify; Second, consider modeling as simple as possible in order to improve computing speed, and achieve the expected results of calculation. So it is inevitably that the error compared to simulated and actual temperature distribution is existed, but the law of the temperature distribution is consistent. The main factor caused heat elongation of ballscrew is milling heat, and the heat is directly impact by the milling depth, the cutter rotation speed, and the feed rate. The diameter of screw, the ambient temperature of the milling machine, the cooling method and the initial temperature of the workpiece are also important factors to affect the ballscrew temperature distribution. Determination of thermal elongation law under different processing parameters is a very important job for compensating for thermal expansion error. The method of combining simulation and experiment to get the ballscrew temperature distribution in the processing is the important foundation work on thermal expansion law. Conclusions In the Whirlwind hard milling process of the ballscrew thread, milling heat is the main factor to cause the pitch error. In this paper, study the three-dimensional temperature field equation of the Whirlwind milling ballscrew based on Heat Transfer, and determine the boundary conditions; establish the ballscrew model Using ANSYS, load heat source along the screw helix, calculate the temperature distribution simulation results of the processing ballscrew, analyze the factors affecting the temperature distribution. It provides a basis for compensating the thermal deformation error of whirlwind milling ballscrew. Acknowledgments The authors acknowledge the financial support of national natural science foundation of China (Project # ), "High-end CNC machine tools and basic manufacturing equipment," National Science and Technology major projects (2012ZX ), Shandong Province Education Department Plan (J09LD12). References [1] Y. Li: Modular Machine Tool & Automatic Manufacturing Technique, (2011) No.6,p.107. (In Chinese) [2] B.P. Fu and M.L. Tian: Metal Processing (Cold Machining), (2010) No.6,p.25. (In Chinese) [3] P.S. Li, K.H. Yang and Z.N. Chen: Journal of Hua Zhong University of Technology, Vol. 19 (1991) No.8,p.75. (In Chinese) [4] X.C. Song and C.R Zhang: Journal of Shandong University of Technology, Vol. 30 (2000) No.2,p.160. (In Chinese) [5] Y.X. Liu and J.Y. Li: Machine Building & Automation, Vol. 36 (2007) No.3,p.44. (In Chinese) [6] S.M. Yang and W.Q. Tao: Heat Transfer (Higher Education Press of Beijing, China 2006). (In Chinese) [7] Z.H Zhang and S.K Li: ANSYS 11.0 Finite Element Analysis Theory and Engineering Applications (Electronic Industry Press of Beijing, China 2008). (In Chinese)

6 Manufacturing Engineering and Automation II / Temperature Field Simulation of Ballscrew Whirlwind Milling /