Design of Clamping Fixture for Manufacturing of Long Turbine Blades on 5 Axis Machinery

Transcription

1 Research Paper Volume 2 Issue 11 July 2015 International Journal of Informative & Futuristic Research ISSN (Online): Design of Clamping Fixture for Manufacturing of Long Turbine Blades on 5 Axis Paper ID IJIFR/ V2/ E11/ 031 Page No Mechanical Subject Area Engineering Key Words Warping, Bending, Cutting Force Analysis, Clamping System, Material Properties Received On Accepted On Published On Pavan Tejasvi 1 Dr. K. M. Purushothama 2 Dr. S. Satish 3 F. M. Lewis 4 A. Narahari 5 S. K. Chattopadhyay 6 C. R. Sujir Assistant Professor, Associate Professor, Assistant Professor, B.E. Student, B.E. Student, B.E. Student, B.E. Student, Copyright IJIFR

2 Abstract This paper deals with novel ideas for improvement in the manufacturing process of long twisted and tapered turbine blades by means of addition of a simple and adaptable clamping fixture. The attached external fixture as discussed is a possible method of preventing warping and bending while machining long twisted and tapered blades. Applying compression springs and power screws, the fixture serves the purpose of securing the blade near the machining area, in 5 axis machines without such a suitable clamping device. The fixture is so designed such that it is able to accommodate the changes in blade length and size. This fixture can be used for the cases where the turbine blade, while machining may deform or bend during subsequent stages of the machining process, the result of machining away 80% of the original rolled or annealed raw material and the residual stresses thus created. This is particularly possible for large blades, mm long, which may bend by as much as 1.5 mm. The fixture is capable of synchronizing itself with the x,y,z,a,b axes of machine movement. A profound study of machining cell configuration is conducted, the key features and designs of the clamping fixture are devised and general analyses are performed on the designs. 1. Introduction When a steel part is machined using certain methods, residual stresses are induced due to the difference in thermal dilatation between steel and enamel. Those stresses can give rise to buckling and warping. Slender designs, such as baking trays and architectural panels, are especially prone to these defects.[1] Warp is an inherent problem with heating and working steel. Everything around the blade may affect the warp. A cold counter with a warm blade set on it may make the blade warp. Uneven cooling in quench, or uneven heating in temper, etc. Stresses placed on the blade by the way it is held or suspended, etc.[11] Countermeasures affected at the granular level, such as normalizing multiple times, grain refinement, implementation of different forging techniques might be effected. A simple slotted board clamped in a vise can straighten 99% of most warped blades in two seconds as shown in figure (1) further. [10] Reworking the fixturing elements during the machining process, so that the position of the work piece in the machining centers is modified to account for the deformation, can counteract this phenomenon. 2. Experimental Details 2.1 Machining Fixture Description of Clamping Systems Machining fixture are additional parts added to the original processes which require a systematic design to clamp, hold and guide the tool during machining process. The obvious place for Fixtures is in mass production, where large quantity of output offers ample opportunity for recovery of the necessary investment. It is a special tool used for locating and firmly holding a workspace in the proper position 4150

3 during a manufacturing operation. As a general rule it is provided with devices for supporting and clamping the work piece. It is fixed to the machine bed by clamping in such a position that the work in the correct relationship to the machine tool elements.[8] The main purpose of the fixture is to locate the work quickly and accurately, Support it properly and hold it securely, thereby ensuring that all parts produced in the fixture will come out alike within the specified limits. In this way accuracy and interchangeability of the parts are provided. [9] Figure 2: Shows A 3D Representation of the Concept of the Clamp and its Skeletal Structure Description of the Concept and the Basic Fixture Drawings During the machining of cuboidal blocks to obtain a blade, it undergoes transitional errors due to vibration & heavy machining forces. This leads to loss in time & resources, in the form of correctional measures. In economical 5- axis machines, these problems are very prominent. On the other hand, the higher end ones are provided with in-built clamping systems, which reduce the aforementioned errors to a great extent. Hence the addition of a clamping system can make economical 5-axis machines much more efficient. Addition of a clamping system is done, in the form of an external fixture. There is no availability of the aforementioned function & hence can be categorized as a novel design. This design is intended for industries involved in turbine machining production, operating on economical 5- axis machines. The body is made of SAE 5 Chromium steel, & the ball bearings within the body, aiding the free movement of clamping unit is made of ANSI The clamping unit is present on the inner ring of the body, of the system & consists of 6 clamping fingers with a point contact surface attached to its tip.. It is arranged in an equiangular fashion, powered by a power screws. The power screws are accompanied by a spring to absorb unwanted vibrations. The clamping unit is capable of moving along the contours of the material & also clamps the material firmly during its machining. The fixture is placed between the head & the tail stock. It also has the freedom to move along the y-axis, over the guide rails. 3. Cutter Force Calculations and Blade Analysis The clamping force applied to this generic blade of dimensions 550mmx60mmx50mm made from a block of material x20 Cr13 should be equal or more than the cutting force. Hence cutting force 4151

4 and torque acting at various points due to various width and depth of cuts and other are calculated keeping parameters like speed and feed constant. And selecting the ones generally used for longer blades. The following formulae are utilized for cutter force calculations during the process of roughing. 3.1 Pre-process Calculations Rough machining blade profile Available Data: Material: - X20Cr13 Tool: - Φ50mm Face Mill Cutter Cutting speed: rpm (n) Cutter diameter: - 50mm Number of teeth: - 4 (z) Feed at the table: Maximum power of machine: - 20KW 1) Speed Calculation V= 2) Feed per tooth = ( = 5) 3) Feed at spindle per minute = (mm) 4) Material removal rate Q = 5) Power at spindle (N) N = UK n K r Q (KW) Unit Power = (Table 4.6 HMT Production design) Radial rake angle = -5 degree for 50mm diameter cutter Correlation coefficient (K n ) = 1.07 For radial rake angle Flank wear = 0.8mm/0.6mm/1.0mm Correction factor = 1.20/1.13/1.25 6) Tangential cutting force: P z = 6120 (N) 7) Torque of spindle T = 975 (Nm) 8) Clamping force C= FOS P z (N) 4152

5 4. Experimental Setup 4.1 Assumptions ISSN (Online): ) At any given point of time, during machining, the blade used is assumed to be a simply supported beam. 2) The material of the blade is perfectly homogeneous i.e., has the same material throughout. 3) The material of the blade is isotropic i.e., equal elastic properties in all directions. 4) The cross section has an axis of symmetry in a plane along the length of the blade. 5) The material of the blade obeys Hooke s law. 6) The transverse sections which are plane before bending remain plane even after bending. 7) Each layer of the blade is free to expand or contract, independent of the layer above or below it. 8) The Young s modulus is same in both tension and compression. 4.2 Cutting Force Analyses In case of long and slender turbine blade, during machining, these are subjected to cutting forces thereby causing it to bend irreversibly. It was observed that the long and slender turbine bend and warp during the rough cut operations. In order to avoid the bending or restrict it within the acceptable limits, there is a need to have clamping system to provide the necessary support to the turbine blade while the machining operation. Hence, in order to prove that the clamping system can turn out to be a fruitful solution, there will be two cases of FEA be performed on a generic turbine blade of dimensions, firstly, when loaded under the Cutting forces and the deflection is observed; secondly, turbine being well supported under the loading conditions using a conceptual clamping system. 4.3 Case 1 Without Clamping System Figure 3. shows the considered blade without a clamp 4153

6 4.4 Case 2: With Clamping System 1) Material Properties 2) Mesh Details ISSN (Online): Table 1:Material Properties of the Blade and Fixture Properties Name: X20 C r 13 or AISI420 Model type: Linear Elastic Isotope Default Failure Criterion: Max Von Mises Stress Yield Strength: 3.5e+008 N/m 2 Tensile Strength: 6.5e+008 N/m 2 Elastic Modulus: 2e+011 N/m 2 Poisson s Ratio: 0.27 Mass Density: 7700 Kg/m 3 Name: SS-Alloy Steel Model type: Linear Elastic Isotope Default Failure Criterion: Max Von Mises Stress Yield Strength: 6.2e+008 N/m 2 Tensile Strength: 7.2e+008 N/m 2 Elastic Modulus: 2e+011 N/m 2 Poisson s Ratio: 0.27 Mass Density: 7700 Kg/m 3 Table 2: Mesh Details of the Blade and Clamp Assembly Mesh type Mesher Used: Jacobian points Jacobian check for shell Maximum element size Minimum element size Mesh Quality Re mesh failed parts with incompatible mesh Mixed Mesh Curvature based mesh 4 Points On 5 mm 1 mm High On Figure 4: Shows a Warping and Bending Phenomenon Figure 5. Shows the Meshed Assembly 4154

7 5. Analysis & Results 5.1 Cutter Force ISSN (Online): Results of Analyses of Blade without Clamp I. Von Mises Stress: This analysis is done considering a maximum of 400 N of force being applied while machining under safe conditions. The point of application selected for the force is the midpoint of the blade. It is observed that there is a concentration of stress in the region in the inset. The maximum stress induced is 98.6 MPa, but this magnitude is well within the yield limit of the material, hence it is safe and the blade doesn t undergo any sort of failure. Figure 6: Shows the Stresses Developed In an Unclamped Blade II. Deflection: This Analysis is also done considering a maximum of 400 N of force being applied while machining under safe conditions. For the given cutting forces, it is observed that the deflection observed in the Blade is 0.47mm (almost approx. 0.5mm). Figure 7: Shows the Deflection of the Blade before Clamping 4155

8 5.1.2 Results of Analyses of Blade with Clamping Von Mises Stress: This Analysis is done considering a maximum of 400 N of force being applied while machining under safe conditions. The point of application selected for the force is close to the midpoint of the blade. The clamping forces counteract the applied forces at a safe distance from tool operation point, i.e. application of cutter force. It is observed that there is a concentration of stress in the region in the inset. The maximum stress induced in the system is MPa on the clamping fingers, but this magnitude is well within the yield limit of the material, hence it is safe. The stress experienced by the blade is now reduced to the region of around 24.1 MPa to 48.1 MPa which is well within the accepted limits of the material and is much lesser than the stress experienced in Case1. Figure 8: Shows the Stresses Developed In the Blade after Clamping Deflection: This Analysis is also done considering a maximum of 400 N of force being applied while machining under safe conditions. The clamping forces counteract the applied forces at a safe distance from tool operation point, i.e. application of cutter force. For the given cutting forces the deflection observed in the Blade is mm variable with different iterations upto about 0.08 mm. For the given cutting forces, it is observed that the deflection observed in the Blade is 0.08 mm while the deflection in the first case was nearly 0.5mm. We observe that a difference of 0.42 mm is noted in deflection and hence there is a difference of nearly 84% while deflection is brought under acceptable limits. Figure 9: Shows the Deflection Of The Blade After Clamping 4156

9 6. Conclusion Viewing from overall experimental results, the following conclusion have been drawn, i.) These results have been confined from the numerical analysis by using Ansys software. ii.) This novel fixture is designed and proposed for application has been suggested to the concerned industry. iii.) The main purpose of fixture is to locate the work accurately, support it properly and hold it securely, thereby ensuring the all parts produced in same fixture will come within specified limits. iv.) Theoretically bending is reduced by 85%. Operator conformability has prime consideration in fixture design. In this fixture design ergonomic aspects have studied carefully reducing operator fatigue to minimum. References [1] S. Cooreman, P. Gousselot, M. Leveaux, P. Pol and J. Antonissen. "Understanding thermal warping and sagging in enamelled steel parts through an integrated FE simulation." International Heat Treatment and Surface Engineering 2013; 8(2), [2] Kurokawa, Eiki. Flexible Conformable Clamps for a Machining Cell with Applications to Turbine Blade Machining, [3] BAUSCH John J Turbine Blade Fixture Design using Kinematic Methods and Genetic Algorithms, SPIE Publisher, , USA. [4] A Al-Habaibeh Proceeding of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, Sage Publications, volume 217,Number 12/2003. [5] Kailing Li Ran Liu Guiheng Bai, Development of an Intelligent Jig and Fixture Design System, Computer-Aided Industrial Design and Conceptual Design, CAIDCD'06.pp 1-5. [6] Y. Zheng, Y. Rong and Z. Hou, The Study of Fixture Stiffness Part I: A Finite Element Analysis for Stiffness of Fixture Units, The International Journal of Advanced Manufacturing Technology, Volume 36, Numbers 9-10, , DOI: /s [7] Yan Zhuang Goldberg, K. Design Rule for Tolerance Insensitive and Multi-purpose Fixtures, Advanced Robotics,1997.ICAR 97.pp [8] Frank W.Wilson, Hand Book of Fixture design, Society of Manufacturing Engineers, Tata McGraw Hill, 1997 [9] Hiram E. Grant, Jigs and Fixtures, Tata McGraw Hill, 1967, New York. [10] Apelt, Stacey E., Blade Warp, [11] [12] Tester, John T., Reducing Distortion in Simulated Injection Moulded Wind Turbine Blades 01/2004; DOI: / [13] Shigley, Joseph E., Mischke, Charles R., & Brown, Thomas Hunter, Standard Handbook of Machine Design, 3E, Tata McGraw-Hill Education,