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World Academy of Science, Engineering and Technology 70 2010 CNC Wire-Cut Parameter Optimized Determination of the Stair Shape Workpiece Chana Raksiri, Pornchai Chatchaikulsiri Abstract—The objective of this research is parameters optimized of the stair shape workpiece which is cut by CNC Wire-Cut EDM (WEDW). The experiment material is SKD-11 steel of stair-shaped with variable height workpiece 10, 20, 30 and 40 mm. with the same 10 mm. thickness are cut by Sodick's CNC Wire-Cut EDM model AD32
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     Abstract — The objective of this research is parameters optimizedof the stair shape workpiece which is cut by CNC Wire-Cut EDM(WEDW). The experiment material is SKD-11 steel of stair-shapedwith variable height workpiece 10, 20, 30 and 40 mm. with the same10 mm. thickness are cut by Sodick's CNC Wire-Cut EDM modelAD325L.The experiments are designed by 3 k  full factorial experimentaldesign at 3 level 2 factors and 9 experiments with 2 replicate. Theselected two factor are servo voltage (SV) and servo feed rate (SF)and the response is cutting thickness error. The experiment is dividedin two experiments. The first experiment determines the significanteffective factor at confidential interval 95%. The SV factor is thesignificant effective factor from first result. In order to result smallestcutting thickness error of workpieces is 17 micron with the SV valueis 46 volt. Also show that the lower SV value, the smaller differentthickness error of workpiece. Then the second experiment is done toreduce different cutting thickness error of workpiece as small as possible by lower SV. The second experiment result show thesignificant effective factor at confidential interval 95% is the SVfactor and the smallest cutting thickness error of workpieces reduceto 11 micron with the experiment SV value is 36 volt.  Keywords — CNC Wire-Cut, Variable Thickness Workpiece,Design of Experiments, Full Factorial Design I.   I  NTRODUCTION  HE mechanism of Wire-cut Electric Discharge Machine  ,WEDM  , uses electricity to cut the metal with a brasswire conductors resulting in the discharge as shown in Fig.1The work of WEDM is like cutting wood using a fret saw.WEDM are cutting as such, but uses electricity instead of sawcutting. WEDM will cut the material that has electricalconductivity. Harden steel and carbide are cut by WEDM. Theadvantages of WEDM over CNC conventional machine is thevery hard material can be cut.   The current cutting conditionthat controls WEDM to cutting different height as stair shapeshow in Fig. 2 cannot be controlled the accuracy of workpiececutting thickness. The stair height is 10, 20, 30 and 40 mm inthe same piece as show in Fig. 2 normally, the cutting parameter are designed to cut with constant workpiece height. Chana Raksiri, Department of Industrial Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand,Tel: 662-942-8566-70; fax 662-942-8571; e-mail: fengcnr@ku.ac.thPornchai Chatchaikulsiri,   Engineering Management Program, Departmentof Industrial Engineering, Faculty of Engineering, Kasetsart University,Bangkok 10900, Thailand, e-mail: pornchai1981@yahoo.com Conducttivity piece Wire feed(from top to bottom)carrying electricity NozzleDischarge sparkWorkpieceThicknessChipWire guideWire electrodeWire guideConductivity pieceWire tank   Fig. 1 Mechanism of Wire-cut EDM [1] In this case, the operators cannot select an optimal cuttingcondition to cut stair shape workpiece for more accuracythickness. For the result example in Table I, three pass cuttingworkpiece stair shape with height 10, 20, 30 and 40 mm in thesame piece was cut by selected cutting condition for 20 mmconstant height. Cutting directioZ   +X+ Y-   Cutting directioZ   +X+ Y-   Fig. 2 The cutting of variable thickness of workpiece with WEDM The accept thickness error of WEDM for 1, 2 and 3 passcutting is 20, 15 and 10 microns, respectively. From the resultin table I, the thickness errors were randomly and out of specification. CNC Wire-Cut Parameter OptimizedDetermination of the Stair Shape Workpiece Chana Raksiri, Pornchai Chatchaikulsiri T World Academy of Science, Engineering and Technology 70 2010101   The objective of this research was parameters optimized bystatistical design of experiments for the stair shape which wascut by CNC Wire-Cut EDM (WEDW). The experimentmaterial was SKD-11 steel of stair-shaped with variableheight workpiece 10, 20, 30 and 40 mm. with same 10 mm.thickness, were cut by Sodick's CNC Wire-Cut EDM modelAD325L. The experiments were designed by 3 k  full factorialexperimental design at 3 level 2 factors and 9 experimentswith 2 replicate. The selected two factor were servo voltage(SV) and servo feed rate (SF). Fig.3 Stair shape workpiece cutting byWire-Cut Electrical Discharge Machine TABLE   IC UTTING S IZE E RROR IN THICKNESS 10,   20,   30 AND 40 MM  Cutting passMeasure positionSize of workpiece in eachworkpiece thickness (mm)AverageCuttingthickness error (Micrometer)10 20 30 40Top4.995 4.989 4.980 4.9831 Center 5.000 4.997 4.989 4.98120Bottom4.998 5.000 4.996 4.990Top5.004 5.010 5.006 5.0032 Center 5.004 5.007 5.006 5.0058Bottom5.005 5.011 5.010 5.007Top5.003 4.997 4.983 4.9753 Center 5.003 4.997 4.984 4.97034Bottom5.004 4.999 4.989 4.982 II.   L ITERATURE REVIEW  Machine Wire Cut (Wire-Cut Electric Discharge Machine:WEDM) first used in 1960, which evolved from EDM in1974. Dulebohn be applied to fiber optic control systems toautomate the WEDM Later in 1975, its popularity is rapidlyincreasing as people know and understand many processes inindustry. Later in the late 1970's solutions are set computer control (CNC) to use in WEDM are consequences. Has beenused extensively to cut holes in the surpassed the wire that passes part to cut jobs at the WEDM in manufacturing willwork press forming work dies RP Job aircraft parts, medicalequipment and head Cut (Ho. et al., 2004)[2].WEDM cutting process based on principles of electric heatthat can be accurately cutting parts with complex shapes aredifficult to cut. Part of edge cut is difficult to cut easily byWEDM technology WEDM based on taking advantage of thespark so that material come off where the spark is not thecontact between the wire and WEDM is made the parts for theinstruments used to head for a bite and make parts that aresmall and require high precision and good surface. Cuttingmachine that relied on discharge is not a traditional process.Electrical heating process. The erosion of material off theworkpiece by spark between workpiece and wire by wire in adielectric fluid. Electricity distribution will be made partmelting and evaporation, and blown to wash out by thedielectric fluid, WEDM generate spark the wire (acting aselectrodes) and part by water fine ions act as an intermediaryfor the electrical resistance and cutting the path defined by the program. The main goal of WEDM manufacturers and userswill achieve stability and productivity by making better.Because new materials are being develop. Body parts aremore complex. To overcome restrictions on use and WEDMin manufacturing growth increased continuously and rapidly.WEDM manufacturers and users to focus on success in theability to manufacture with precision and surface interest.However, because of high variability, even workers with goodskills can not be made to optimize. Effective way to solve this problem is to find the relationship between performancemeasurement process to fill the cutting condition used in thecontrol (Mahapatra et al., 2003) [3].WEDM process is a measure of variety and consistency of  performance. Determine the best parameters with different performance properties is different. One possible way to solvesuch problems, problems with many variables to consider for the WEDM process, the first control and influence their level best corresponds to each variable and then it will resolve theconflict with relevant experience and knowledge engineering.However, the decision of men validity of results simply cannot be assured. Beyond that different results can be found bydifferent engineers. This means that the uncertainty in thecontrol factor increases (Gauri et al., 2009) [4].The problem when cutting with WEDM are wire break andinstability cutting. These are major factors resulting in reduced performance for cutting WEDM. Especially when you look atcutting variable thickness workpiece. Traditional methods todetermine the appropriate value for the cut is to select a valuethat is used for cutting the thickness of the smallest parts. Thismethod will allow to reduce the possibility of the wire break and the lack of stability in cutting. However, the cutting speedis reduced dramatically (Liao et al., 2002) [5].Ho et al. (2004)   [2] found that the ON time wave, thefrequency of discharge, the voltage open circuit, the servovoltage, capacity charge, speed tables and the intensity of current in the discharge ions affect the ability to cut jobs, suchas surface roughness and cutting speed while the wire speed,wire tension and rate of flow of the medium with electricalresistance minimal impact.Tarng et al. (1995)[6] The servo reference voltage (SV): World Academy of Science, Engineering and Technology 70 2010102   The value of the servo reference voltage high, resulting in atime of waiting for the release of energy also high. However,the cutting speed will gradually slow down. Leads to theaverage of the difference of the release energy, so theconditions of large energy release is more stable but thenumber of cycles of energy is released within less time for energy Because of the stable cutting surface accuracy of  better.Servo speed: the speed setting in motion when there is noload or the voltage higher than average the servo referencevoltage (SV). General differences in the release of energy isless when the speed of movement of the work table is highand the result is a speed of cutting increased compliance withspeed the movement of work table, but the accuracy of thesurface is not good because the cutting speed increased.   Liao, YS and YP Yu. (2004)[7] showed that ON time andthe servo voltage is a significant factor in the discharge.Period of time will make more power efficient dischargereduction. Energy used in cutting depends ON time to supplya very long time more energy will be a lot of which imply the power of cutting the high percentage of the performanceenergy is paid to the pieces come off and the material isreduced significantly. The impact of the servo voltagedischarge at this value even more will be made effectivedischarge decreased reason is because the values that willaffect the phase will result in discharging the valuable lot ismade. more distance parts of the energy is less than thedischarge near the work piece.Kanlayasiri and Boonmung (2007)[8] the experimentaldesign using the design strategy full factorial (2 k  ) where k isthe number of control variables in the experiment. The valuein this experiment is power (ON), the time to stop paying power (OFF), the current maximum (Ip) and the tension wire(Wt) and the experimental use confidence interval at 95% andusing ANOVA tests various hypotheses. Before that can beused ANOVA, will test the residual is normal distribution or not, are independent of each other or not and a constantvariation does.Surface roughness describe by Fig.4 (Banleng and Prasert,1981) [9]The average roughness Ra is equal to the height of therectangle. The long-term lm rectangle is equal to the area astotal area of the rough on the southern and central lines.The depth of roughness Rz that allows for the calculation of serial arithmetic mean of the depth of roughness (Z1 ... Z5) 5values that are similar area.Maximum roughness depth Rmax is a maximum depth of the groove roughness during lm.From the literature review it is show the relationship between the performance of cutting parameters, experimentaldesign, testing to find the cutting parameter that caused the Fig. 4 Explain meaning of Ra, Rz and Rmax cutting performance of the best. The cutting with the cutting parameters of WEDM machine different. Cutting performancewill get the different result.   Therefore, cutting parameter accuracy allows the performance of cutting up.   One of the problems encountered when cutting with WEDM occurs whencutting variable thickness workpiece.   The problem is duringthe cutting WEDM cutting areas in which change thethickness is wire break. In addition, the sizes of the specimenthickness in each are different. By finding the appropriatevalue to solve these problems. Require statistical methods toassist in the experiment. This research will lead to a fullfactorial experimental design used.   Because the method iseasy to understand and not complex.   III.   E XPERIMENTAL M ETHODS    A.   scope of this experiment  1) Cutting by hard brass wire diameter 0.2 mm2) Workpiece was steel type SKD-113) Cutting with dielectric fluid resistivity 55,000 to 65,000(Ohms.cm)4) Thickness Cutting error less than 20 micrometer 5) Cutting with stair shape workpiece that have height 10,20, 30 and 40 mm as show in Fig.56) Three pass cutting.7) Responses of this experiment was difference cuttingthickness error.8) Not consider other factor such as temperature, dielectricflow rate.  B.    Experimental Sequence 1) Consideration factor for experimentThe consideration factor of this experiment were servovoltage (SV) and servo feed rate (SF). The gap voltage andgap between workpiece and wire were controlled by servovoltage (SV) and the response of wire speed changed by gapvoltage was controlled by servo feederate (SF). Duringcutting, gap voltage was controlled follow with servo voltage(SV) by speed adjustment and the response of speed changewas controlled by servo feederate (SF). The response of thisexperiment is cutting thickness error of workpiece.2) Design of experimentalThe full factorial 3 k  design at 3 level 2 factors, 9 Mean line   Roughness   World Academy of Science, Engineering and Technology 70 2010103   experiments, 2 replicate was designed for this experiment asshow in table II. TABLE   IIF ACTOR AND LEVEL IN 1 ST SECTION EXPERIMENT  Factor LevelHigh Middle LowSV (Volt) 56 51 46SF (mm/min/volt) 0.63 0.48 0.33 3) ExperimentThe workpieces were cut into dimension as shown in Fig.5The ten cutting thickness measuring points were measured asshow in Fig. 6 by 0.001 mm resolution micrometer. Fig. 5 Cutting workpiece dimension (unit: mm)Fig. 6 Measurement position TABLE   IIIT ABLE FOR RECORD SIZE DATA  Measure positionHeight of stair shape (mm)10 20 30 40Top Point 1 Point 4 Point 7 Point 10Center Point 2 Point 5 Point 8 Point 11Bottom Point 3 Point 6 Point 9 Point 12Cutting thickness error = (Maximum thickness size – Minimum thicknesssize) for each height of stair shape IV.   R  ESULT AND DISCUSSION    A.   First experiment result  The Minitab software was used to sort the first experimental by randomly. The first experiment results are shown in TableIV.The general full factorial design with confidential interval95% ( α = 0.05) analyze the experiment result by Minitabsoftware with General Linear Model. The residual plots for cutting size error (Diff) show in Fig.8, that show the residual plots with a normal distribution, mean of error equals zero,constant variance and errors are independent.From the analysis in Fig.9 consider the confidential intervalof 95 percent ( α = 0.05) are the main factors affectingsignificantly the factor SV (which is determined by the P-value less than 0.05)   and with no interaction effectsignificantly. This is determined by the P-value greater than0.05 and graphs with no potential impact on the Fig.10. TABLE   IVF IRST EXPERIMENT RESULT  Order experimentLevelCutting thickness error (micrometer)SV(volt)SF (mm/min/volt)1 st  replicate2 nd  replicate1 46 0.33 18 162 46 0.48 16 173 46 0.63 18 164 51 0.33 20 265 51 0.48 23 276 51 0.63 27 257 56 0.33 34 358 56 0.48 37 399 56 0.63 34 38 Residual    P  e  r  c  e  n   t 420-2-4 999050101 Fitted Value    R  e  s   i   d  u  a   l 4035302520 3.01.50.0-1.5-3.0 Residual    F  r  e  q  u  e  n  c  y 3210-1-2-3 6.04.53.01.50.0 Observation Order    R  e  s   i   d  u  a   l 18161412108642 3.01.50.0-1.5-3.0 Normal Probability Plot of the ResidualsResiduals Versus the Fitted ValuesHistogram of the ResidualsResiduals Versus the Order of the Data Residual Plots for Diff Fig. 8 Residual plot for cutting thickness error (Diff)in 1 st experiment World Academy of Science, Engineering and Technology 70 2010104
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