Machining of axisymmetric forms and helical profiles on cylindrical workpiece using wire cut EDM. Harshal G. Dhake and G.L.

Transcription

1 252 Int. J. Machining and Machinability of Materials, Vol. 12, No. 3, 2012 Machining of axisymmetric forms and helical profiles on cylindrical workpiece using wire cut EDM Harshal G. Dhake and G.L. Samuel* Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai , India *Corresponding author Abstract: The unique feature of wire electrical discharge machining (EDM) is using thermal energy to machine electrically conductive parts; this distinctive advantage has been utilised in the manufacture of moulds and dies, automotive, aerospace and surgical components. In this paper, the application of wire EDM for machining axisymmetric form and helical profiles on cylindrical workpieces with different diameters is presented. The required rotary and linear motions of the workpiece are obtained by using a special rotary attachment and a motorised linear stage. The rotary motion of the spindle, linear motion of the motorised linear set-up and the wire EDM table movement were controlled to obtain a desired form of the workpiece. The workpieces after machining were measured and evaluated for dimensional accuracies. The dimensional accuracy of the axisymmetric components machined in the present work is found to be about 3.3%. The accuracy of the helical profiles is found to be high, with a maximum error mm in average and a standard deviation of mm. The present work can be extended to optimise machining parameters for further improving the quality of the machined parts. Keywords: wire electrical discharge machining; EDM; axisymmetric and helical profile; cylindrical workpiece. Reference to this paper should be made as follows: Dhake, H.G. and Samuel, G.L. (2012) Machining of axisymmetric forms and helical profiles on cylindrical workpiece using wire cut EDM, Int. J. Machining and Machinability of Materials, Vol. 12, No. 3, pp Biographical notes: Harshal G. Dhake obtained his Masters in Mechanical Engineering from Indian Institute of Technology Madras, Chennai in His areas of interest include computer aided manufacturing, non-conventional machining processes and metrology. G.L. Samuel is currently working as an Assistant Professor at the Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai. He obtained his BE in Mechanical Engineering from Mysore University in 1991, MTech in Production Engineering and Systems Technology from Kuvempu University in 1994 and PhD in Mechanical Engineering from Indian Institute of Technology Madras in He has been a Postdoctoral Fellow at School of Mechanical Engineering, Kyungpook National University, South Korea. His active areas of research are: conventional and non-conventional machining processes, measurements and inspection of freeform surfaces, micromachining and laser vision systems. Copyright 2012 Inderscience Enterprises Ltd.

2 Machining of axisymmetric forms and helical profiles Introduction With the recent developments in material processing technology, as well as increasing in demand for more complex and economic machining processes, conventional methods of machining are unable to cater to the need of the industries. Non-traditional machining has started to play an important role in processing of advanced materials. In recent years, there is a considerable interest in the developments of special materials such as high strength and tough alloys, ceramics and composites for certain applications in aerospace, automotive and nuclear industries. Technical and economical difficulties in processing of such materials with conventional machining processes led to the development of several non-traditional machining processes. Some of the popularly employed non-traditional machining techniques are electro discharge machining (EDM), electrochemical machining, ultrasonic machining, abrasive water jet machining, electron beam machining, laser beam machining and plasma arc machining, etc. Among various non-traditional machining techniques, EDM technology has gradually evolved and become an important manufacturing process. EDM is a thermoelectric process that erodes workpiece materials by a series of discrete electrical sparks between the workpiece and electrode flushed by or immersed in a dielectric fluid. Unlike traditional cutting and grinding processes which rely on a much harder tool or abrasive material to remove softer workpiece materials, EDM utilises electrical sparks or thermal energy to erode the unwanted workpiece material and generate the desired shape. The material removal rate (MRR) of EDM processes mainly depends on the electrical conductivity and melting temperature of the workpiece material. A workpiece with a higher electrical conductivity and a lower melting temperature can be EDM-ed more efficiently. The hardness and strength of the workpiece material are no longer the dominating factors that affect the tool wear and hinder the process. This makes EDM particularly suitable for machining hard, difficult-to-machine materials. EDM processes have the ability to machine precise, complex, and irregular shapes with a CNC control system. In addition, the cutting force in EDM processes is small, which makes it ideal for fabricating parts with miniature features. EDM is commonly used in mould and die-making industry and in manufacturing automotive, aerospace and surgical components. Since there is no mechanical contact between the tool and the workpiece, thin and fragile components can be machined without the risk of part damages. In wire EDM, the mechanism of material removal is similar to that of EDM. Wire EDM is a specified thermal machining process capable of accurately machining parts with varying hardness and complex shapes. The wire is used as an electrode, generally made of copper or brass and the gap between work and wire must always be filled with a dielectric fluid. Generally, de-ionised water is used as a dielectric fluid, which flushes away the eroded particles from the work zone. Wire EDM technique can be used for machining cylindrical workpieces by adding a rotary axis to wire EDM. In literature, studies have been conducted for modelling, visualisation and influence of process parameters to the quality of wire EDM turned surfaces. Ho et al. (2004) presented a review about the research work carried out in wire EDM processes highlighting some of its applications. Tarng et al. (1995) used a feed forward neural network to construct a wire EDM process model to determine the optimal cutting parameters in WEDM. Rajurkar and Wang (1993) developed a wire EDM sparking frequency monitor to detect thermal loads for online control to prevent the wire from rupture. The wire rupture phenomena were also analysed with a thermal model. An

3 254 H.G. Dhake and G.L. Samuel extensive experimental investigation has been carried out to determine the process performances such as MRR and surface finish with overall control parameters of EDM. The relationship between the MRR and surface finish under optimal machine settings have been determined by means of a multi-objective model. Hewidy et al. (2005) used the response surface methodology to model the machining parameter effect of wire EDM of Inconel 601. The experiments were conducted to observe the effect of machining parameters on the output response. Attempts made by several researchers on wire EDM turning include turning of long and small-diameter parts. A rotary axis was added to a conventional wire EDM machine to enable the generation of a cylindrical form. The initial shape of the workpiece need not be cylindrical according to Masuzawa s research group at University of Tokyo (Masuzawa and Tonshoff, 1997). These research activities were aimed to manufacture small-diameter pins and shafts. Cylindrical pins as small as 5 μm in diameter were machined and small-diameter pins were used as tools for 3D micro-edm applications. Haddad and Tehrani (2008a) developed the cylindrical wire electrical discharge turning (CWEDT) process to generate precise cylindrical forms on complicate, hard and difficult-to-machine materials. Mohammadi et al. (2008) developed a mathematical relation between machining parameters on the MRR using regression analysis and reported that the power, the voltage and the servo voltage are most significant parameters affecting the MRR. The authors also mentioned that the rotational speed and the servo voltage have an inverse effect on the MRR. The wire speed exhibits no effect on the MRR. Haddad and Tehrani (2008b) modelled the variation in surface roughness and roundness with machining parameters using the response surface methodology for AISI D3 steel. Rhoney et al. (2002) used the cylindrical wire EDM method for truing a metal bond diamond wheel for precision grinding of ceramics. The same grinding wheel was trued by the wire EDM and the single point diamond methods. Janardhan and Samuel (2009) developed a rotary setup to carry out cylindrical turning and investigated the characteristics of the wire electrical discharge turning (WEDT) process. Matoorian et al. (2008) presented the application of a Taguchi robust design method to optimise the precision and accuracy of an EDM process for machining of precise cylindrical forms on hard, difficult-to-machine materials. This paper presents the development of control strategies for effective machining of axisymmetric and helical profiles on cylindrical parts using wire EDM. In the present work, a rotary setup developed by previous researchers was used for achieving the required rotational motion of the workpiece (Janardhan and Samuel, 2010). A motorised linear stage was developed in this work for achieving linear motion of the workpiece for machining axisymmetric forms and helical profiles. Different axisymmetric forms and helical profiles were machined using the setup and the dimensions of the features were evaluated. 2 Details of experimental setup Generally, wire EDM is designed to cut various profiles in 2D and 3D components. However, by adding a precise rotary spindle, cylindrical workpieces can be turned using wire EDM. Also, in wire EDM, the linear cutting speed of the machine is controlled by the servo mechanism. In order to maintain the constant spark gap between the wire and the workpiece, as soon as the material is removed by the spark, the workpiece or the wire

4 Machining of axisymmetric forms and helical profiles 255 will be moved closer. In wire EDM, the wire will be continuously replenished so that the fresh electrode wire surface will be available for sparking. If the linear feed increases, and if sparks occur before the fresh wire is fed, the wire will break. Figure 1 shows the basic motions required for machining axisymmetric forms and helical profiles on cylindrical components. In the present work, a precise rotary spindle set-up developed by Janardhan and Samuel (2010) was used to provide rotary motion to the workpiece. A motorised linear stage was developed for providing the linear motion along the axis of the cylindrical workpiece apart from the servo controlled motion of the work table of the EDM machine. The rotary and linear motions are controlled carefully to avoid wire breakage and to achieve the required form on the cylindrical parts. Relevant details of the experimental setup are discussed in the following sections. Figure 1 Schematic diagram of proposed set up 2.1 Rotary spindle setup The rotary spindle system comprises of a speed control drive and the spindle. The speed control drive includes a DC power supply, a motor and a gear. The motor used in the present work is a DC wiper motor. Speed of the motor is controlled with a regulated DC power supply. Worm gear is made of plastic to provide insulation to the motor shaft. The motor shaft has a double start helical thread on it and acts as a worm for transmitting rotation to the spindle shaft through the gear. With this gear arrangement, the speed can be varied from 4 to 100 rpm by modifying the input voltage to the DC motor. The speed range can be further increased by reducing the size of the gear with the same module. The components of the spindle are the spindle shaft, bearings, housing and work holding device. The spindle shaft and housing are connected by bearings. The work holding device is mounted on the spindle shaft. The speed control components and spindle are mounted on the fixture and the assembly is enclosed with the outer casing. The fixture is an L-section and outer casing is made of acrylic. The fixture is powder coated to make it corrosion resistant. Care has been taken during machining and assembly of various components of the spindle set-up to satisfy the requirement of run out (less than

5 256 H.G. Dhake and G.L. Samuel 10 micrometers). The model of the rotary set-up is shown in Figure 2. The specifications of the rotary set-up are given in Table 1. Figure 2 Rotary spindle system (see online version for colours) Table 1 Specifications of the rotary spindle and the motorised linear stage Sl. no. Set-up Specifications 1 Rotary spindle Motor type: DC wiper motor set-up Power: 30 W Maximum voltage capability: 0 to 24 V Maximum current capability: 0 to 1.25 A 2 Motorised linear set-up Speed range with gear: Travel range: Table size: Lead screw: Motor type: Torque: Minimum linear speed: Maximum linear speed: Microcontroller: 4 to 100 rpm 300 mm mm Square threads, 20 mm diameter, 2 mm pitch Permanent magnet, bifilar wound stepper motor 0.7 Nm 0.3 mm/s 0.83 mm/s 89C51(P89C51RD2BN) 2.2 Motorised linear set-up The motorised linear set-up is developed for obtaining the linear motion of the rotating workpiece. The proposed motorised linear set-up is shown in Figure 3. It consists of a stepper motor, micro controller, lead screw and nut assembly for achieving the different

6 Machining of axisymmetric forms and helical profiles 257 linear speeds. The linear motion of the stage is controlled by using the microcontroller. The linear speed of the workpiece can be varied by adjusting the controls of the microcontroller. A stepper motor shaft is coupled to a lead screw and nut assembly. Presently, a lead screw and nut assembly with 2 mm pitch square thread is used for converting the rotary motion of the motor to the linear motion. A nut is connected to the base plate of the linear stage on which the rotary assembly system is mounted on it. As the forces in the wire EDM process are very small, the load on the bearing is very less series bearings are selected for the application as those types of bearings have a good radial load capacity. Aluminium alloy 1050 is used for fabricating the base and supporting plates of the set up so to have minimum possible weight of the linear set up. The detailed specifications of the linear set-up are also given in Table 1. Figure 3 Photograph of the motorised linear set-up (see online version for colours) 3 Machining helical profiles and axisymmetric forms An ELECTRA ECOCUT CNC WIRE EDM machine was used for conducting the experiments. The rotary spindle assembly was mounted on the base plate of the linear stage and the complete assembly was mounted on the wire EDM machine table as shown in Figure 4. Helical forms and axisymmetric profiles were produced on cylindrical workpieces using the proposed experimental arrangement. The workpiece used for the experiments is machined to the required shape from a brass rod of 12.7 mm diameter. The dimensional sketch of a specimen is shown in Figure 5(a) and the photograph of the specimen is shown in Figure 5(b). 3.1 Machining of axisymmetric forms To produce the axisymmetric form on cylindrical workpieces, the required table path was generated on the NC control of the EDM machine using ELAPT software supplied by the manufacturer of the machine. The NC tool path of the wire EDM along the axis of the rotating workpiece was shown in Figure 6 for two sample profiles. Similar experiments were conducted for different specimens with various dimensional features. Various ranges of parameters of the machine and the parameters selected for machining the axisymmetric forms are in the Table 2. Different axisymmetric forms machined in the

7 258 H.G. Dhake and G.L. Samuel present work are shown in Figure 7. The critical dimensions measured and evaluated are indicated with letters a to g. Figure 4 Experimental arrangement with rotary setup and linear setup on wire EDM, (a) experimental set-up (b) close-up view of rotary and linear set-ups (see online version for colours) (a) (b)

8 Machining of axisymmetric forms and helical profiles 259 Figure 5 Cylindrical specimen used in the present work, (a) dimensions of specimen (b) photograph specimen for wire EDM (see online version for colours) (a) (b) Figure 6 Axisymmetric form and helical groove specified for machining, (a) axisymmetric form (b) helical groove (see online version for colours) (a) (b) Table 2 Machining parameters used during EDM Pulse off time(μs) T-on (knob setting) T-off (knob setting) Wire feed (knob setting) Spark gap level Machining speed (knob setting) Linear feed (mm/s) Rotational speed (rpm) Range 10 to 50 1 to 10 1 to 10 1 to 10 1 to 10 1 to to to 100 Parameter selected

9 260 H.G. Dhake and G.L. Samuel Figure 7 Axisymmetric forms machined on different specimens (see online version for colours) (a) (b) (c) (d) (e) 3.2 Machining of helical profiles To obtain a helical profile on a cylindrical workpiece, the workpiece is rotated using the rotary set-up and the linear stage is moved with a required speed. The rotary speed and the linear motion are selected based on the diameter of the workpiece and the pitch required. The rotational speed of the workpiece is varied by changing the input voltage to the motor of the rotary setup. The linear speed of the workpiece is varied by adjusting the controls of the microcontroller. Various helical profiles machined on different cylindrical workpieces are shown in Figure 8.

10 Machining of axisymmetric forms and helical profiles 261 Figure 8 Helical profiles machined on different workpieces, (a) specimen 1: 10 mm diameter, mm pitch (b) specimen 2: 2 mm diameter, mm pitch (c) specimen 3: 3 mm diameter, mm pitch (d) specimen 4: 1 mm diameter, mm pitch (see online version for colours) (a) (b) (c) (d) 4 Results and discussion The images of the workpieces with axisymmetric forms and helical profiles were measured using an optical microscope. Different features measured on the axisymmetric forms are shown in Figure 7 and indicated with letters from a to g. The pitch of the helical forms is measured at the indicated points as shown in Figure 8. The results observed from these measurements have been discussed below with particular accent on dimensional evaluations. 4.1 Dimensional evaluations of axisymmetric forms The measured dimensions of axisymmetric forms denoted with letters a to g are given in Table 3. Along with these, the percentage deviation of the dimension from the design specification is also given in Table 3. The designed dimension for features a, e, and f is 1 mm. The measured dimensions of these features are 1.012, 1.002, and mm, respectively. The maximum percentage error is in dimension f and it is 2.20%. The minimum dimensional error is at feature e and it is 0.20%. The designed dimension of features b and d is 2 mm. The percentage error on dimension b is less compared to that of feature d. In case of 4 mm features (c and g), the maximum percentage error is observed at g. Another important observation is that among all the features, the maximum percentage error is in feature g, which is 3.10% and the minimum percentage error is

11 262 H.G. Dhake and G.L. Samuel associated with the dimension b, 0.10% error. Such a high percentage error might have occurred due to the circular interpolation during machining of feature g. Surface roughness values were measured using a Veeco 3D optical surface profiler. The surface roughness is characterised using the arithmetic mean (R a ) and the roughness profiles of components before and after machining. The 3D topography images of the surfaces before and after machining are given in Figures 9 and 10, respectively. The regular tool marks are observed on the surface of the specimen prepared by turning. It is noticed that the surfaces after machining with EDM have pits indicating the mechanism of material removal by electrical discharges. Table 3 Feature Dimensional error and surface roughness of the axisymmetric forms machined by WEDT process Design dimension Dimensional accuracy Measured dimension % error Surface roughness, R a (µm) a b c d e f g Figure 9 The surface topography of the specimen before WEDT (see online version for colours)

12 Machining of axisymmetric forms and helical profiles 263 Figure 10 The topography of a typical WEDT machined surface (see online version for colours) Table 4 Variations in pitch of helical profiles Theoretical pitch Position Measured pitch Pitch of helical profile Specimen number Error Measured pitch Error Measured pitch Error Measured pitch Error 0 to to to to to to to to Mean Standard deviation

13 264 H.G. Dhake and G.L. Samuel 4.2 Dimensional evaluations of helical profiles Table 4 shows the measured pitch values for helical forms on different specimens shown in Figure 8. The locations on the specimens where the pitch is measured are listed in Table 4 and shown in Figure 8. The error, mean and standard deviation of the pitch values for each specimen are also presented in Table 4. The maximum variation of the average pitch is mm. The maximum standard deviation is mm. It can be observed from Figure 8(c), Figure 8(d) and Figure 8(e), that there is a slight taper in the size of the workpiece. The taper could probably be due to the set-up errors and eccentricity in the rotational axis. This can be overcome by mounting the workpiece more precisely. 5 Conclusions Cylindrical wire EDM has the capability to establish itself as a critical technological process for machining a wide range of forms and profiles. The present work demonstrates the process capabilities of wire EDM to machine axisymmetric forms and helical forms on cylindrical workpieces. In the dimensions of axisymmetric forms, the maximum percentage error is 3.10% and the minimum percentage error is in one of critical dimensions [Figure 6(b)], which is 0.10%. The maximum standard deviation is mm associated with the dimensions of the helical pitch. The process parameters are kept constant during the experiments. The present work can be extended to optimise the parameters to achieve better dimensional accuracies and good surface finish. The micro components with integrated parts such as gears and shafts, threaded bolts, etc., can also be machined using the experimental set-up developed in the present work. Acknowledgements The authors sincerely thank Prof. M.S. Shunmugam, Department of Mechanical Engineering for providing the facility for measurement of surface topography. We also like to thank the reviewers for providing valuable suggestions and encouraging comments. References Haddad, M.J. and Tehrani, A.F. (2008a) Material removal rate (MRR) study in the cylindrical wire electrical discharge turning (CWEDT) process, Journal of Materials Processing Technology, Vol. 199, Nos. 1 3, pp Haddad, M.J. and Tehrani, A.F. (2008b) Investigation of cylindrical wire electrical discharge turning (CWEDT) of AISI D3 tool steel based statistical analysis, Journal of Material Processing Technology, Vol. 198, Nos. 1 3, pp Hewidy, M.S., EI-Taweel, T.A. and EI-Safty, M.F. (2005) Modelling the machining parameters of wire electrical discharge machining of Inconel 601 using RSM, Journal of Material Processing Technology, Vol. 169, No. 2, pp Ho, K.H., Newman, S.T., Rahimifard, S. and Allen, R.D. (2004) State of art in wire electrical discharge machining (WEDM), International Journal of Machine Tools & Manufacture, Vol. 44, Nos , pp

14 Machining of axisymmetric forms and helical profiles 265 Janardhan, V. and Samuel, G.L. (2009) Investigations into turning of cylindrical components using wire electrical discharge machine, Proc. of the International Conference on Advances in Mechanical Engineering, pp Janardhan, V. and Samuel, G.L. (2010) Pulse train data analysis to investigate the effect of machining parameters on the performance of wire electrical discharge turning (WEDT) process, International Journal of Machine Tools and Manufacture, Vol. 50, No. 9, pp Masuzawa, T. and Tonshoff, H.K. (1997) Three-dimensional micromachining by machine tools, Annals of CIRP, Vol. 46, No. 2, pp Matoorian, P., Sulaiman, S. and Ahmad, M.M.H.M. (2008) An experimental study for optimization of (EDT) process, Journal of Material Processing Technology, Vol. 204, Nos. 1 3, pp Mohammadi, A., Tehrani, A.F., Emanian, E. and Karimi, D. (2008) Statistical analysis of wire electrical discharge turning on material removal rate, Journal of Material Processing Technology, Vol. 205, Nos. 1 3, pp Rajurkar, K.P. and Wang, W.M. (1993) Thermal modeling and on-line monitoring of wire EDM, Journal of Material Processing Technology, Vol. 38, Nos. 1 2, pp Rhoney, B.K., Shih, A.J., Scattergood, R.O., Akemon, J.L., Gust, D.J. and Grant, M.B. (2002) Wire electrical discharge machining of metal bond diamond wheels for ceramic grinding, International Journal of Machine Tools & Manufacture, Vol. 42, Nos. 1 2, pp Tarng, Y.S., Ma, S.C. and Chung, L.K. (1995) Determination of optimal cutting parameters in wire electrical discharge machining, International Journal of Machine Tools and Manufacture, Vol. 35, No. 12, pp