CUTTING FORCES AND CONSECUTIVE DEFORMATIONS AT MILLING PARTS WITH THIN WALLS

Transcription

1 Proeedings of the International Conferene on Manufaturing Systems ICMaS Vol. 4, 2009, ISSN University POLITEHNICA of Buharest, Mahine and Manufaturing Systems Department Buharest, Romania CUTTING FORCES AND CONSECUTIVE DEFORMATIONS AT MILLING PARTS WITH THIN WALLS Ioan TĂNASE, Ionuţ GHIONEA Abstrat: Some results of experimental researhes and CAD-CAM simulations at milling plane surfaes of thin walls parts are presented in the paper. The utting fore measurement was done using a multiomponent quartz dynamometer with a speial data aquisition system for the utting fores. The elasti deformation and the stress values in the mahined part were determined by experimental tests and simulated using the FEM analysis. Depending on data that geometrially define the part and the utting tool, their materials and the utting parameters, are the values of the utting fore and power. There is presented a omparison of values obtained by measuring during the proess with those established by applying FEM. This study results lead to some remarks and useful reommendations for determining the proess parameters and the requirements for the tehnologial system in the mahining of parts having thin walls, with minimum deformations. Key words: thin wall milling, utting onditions, fore measuring, deformation and stress analysis. 1. INTRODUCTION Modern CAD-CAM tehniques applied in the parts manufaturing proesses have a large use in all the mehanial industry domains. In the literature [2] are presented many results of FEM appliations in order to determine the elasti deformations values and the stresses indued in parts at the ontat with the tools during the mahining proesses. The influenes of the tehnologial system (mahine-tool, tools, fixture devies) determine dimensional variations and irregularities of the proessed surfaes, [5] dynamial behavior of the mahine-tool in the utting proess and also the premature wear of the utting edges [7]. The results of the surfaes generation by simulation, with the aid of CAM tehniques offer a large number of data: mahining times, surfae auray, behavior of the mahine-tool in working onditions (utting fores and power onsumption). Also, by FEM simulations there are determined the stress and deformation values reated by the utting fores in the thin walls of the analyzed parts [1, 5]. For the optimization of the tehnologial proess there are onsidered and applied various riteria, software environments, tables of data. 2. THE CAM SIMULATION Preparing the part for proessing on a mahine-tool with numerial ontrol involves the generation of ommand information, all data is then stored in a preset order within a storage devie. Programs an be generated diretly on the mahine, the operator writes the neessary instrutions using the interfae or by using a CAD-CAM program and a virtual model of the piee [3]. Defining the piee in a CAD environment is used as the entry data to generate the program with one of the omplex existing programming languages. Thus, the simulation is justified to optimize the proess beause CAM programs elaborate the NC mahine ode. For the study, it is onsidered a part having its 3D model made in CATIA Part Design module and presented in Fig. 1. The overall dimensions of the stok part are mm. This part has a side thin wall and a avity. The mahined surfae of this thin wall is marked with S1 and the wall has 1 mm thikness. The stok part has two side pokets for diret lamping on the mahinetool table. The CAM simulation involves multiple passes in onditions of roughing and finishing millings with speifi tehnologial parameters. The mahining proess of the part was simulated using a 3-axis CNC vertial milling mahine tool with the following main harateristis: spindle speed: rpm with infinite variable speed range (diret drive spindle), the power of the main spindle motor: P = 15 kw (ontinuous rating), working strokes on axes X = 550 mm, Y = 450 mm, Z = 500 mm, mahining feed rate max: 8000 mm/min and rapid feed rate: mm/min; the mahine tool has a CNC Sinumerik ontroller. Fig. 1. 3D model of the analyzed part.

2 226 Fig. 2. Finish milling of S1 surfae. The tools used in manufaturing simulation are hosen from a ompany atalogue [9]. Also, the tool holders are in orrespondene with the spindle nose and the holding system of the mahine-tool. Only two steps of the CAM simulation (surfae S1) in the ase of a steel (OLC45) work piee, 190 HB are presented below (figure and parameters): a. Slot mill, three passes, D = 12 mm - tool diameter, h m = 0.06 mm average hip thikness, v = 280 m/min - utting speed, n = 7400 rpm - spindle speed, v f = 2900 mm/min - feed speed, P = 13 kw - utting power for removal of hips, M = 17 Nm - utting torque, Q = 235 m 3 /min - metal removal rate, f z = 0.1 mm feed per utting edge, a p = 6.6 mm - utting depth, z = 4 number of teeth, t m = 6 s mahining time, t t = 9 s total time. b. Side wall finishing end mill (Fig. 2), two passes, D = 10 mm, h m = 0.01 mm, v = 300 m/min, n = 9500 rpm, v f = 1900 mm/min, P = 1.7 kw, M = 1.7 Nm, Q = 19 m 3 /min, f z = 0.05 mm, a p = 20 mm, a e = 0.5 mm working engagement, z = 4. Figure 3 presents a fragment of the NC ode obtained after the CAM simulation [3] in the ase of the finishing milling. Fig. 4. The dimensions and positions of the utting spots. In order to establish these spots where the radial medium utting fore is applied it was neessary to determine the ontat length of eah helial edge with the part. The dimensions and the positions of the spots depend on the ontat angle φ between the part and the utting edge and the helial pith of the utting teeth, determined using the equations (1), (2) and (3). ae ϕ = 2 = [rad], (1) D L ϕ L = sh = = 2 [mm], (2) 2π π b 0.25 D (0.5 D a ) = 2.18 [mm], (3) = e where: L length of the utting spot, b width of the utting spot and L sh - helial pith of the utting edge, a e =0.5 mm working engagement. The medium utting fore ating on the ontat spots is determined using the values for utting power P depending on the utting parameters [4]: tm = [N], (4) v F P Fig. 3. Fragment of the NC ode. 3. DEFORMATION AND STRESS ANALYSIS BY FEM SIMULATION In this FEM simulation it is onsidered a finishing end mill with the diameter D = 10 mm, number of utting edges z = 4, pith of the helial utting tooth L sh = 28 mm, helix angle ω = 50 0 [6]. On the ontat line between the milling edge and the part there are reated three utting spots plaed on the height of the helial pith, shown in Fig. 4. and it has the values: 287 N (roughing), respetively, 203 N (finishing). The radial medium utting fore (whih is pratially applied on the three spots) is F rm = (0.5 1) F tm, onsidered F rm = 0.9 F tm = 183 N. As follows, there are presented some results of FEM simulations and analysis in the ases of the part being mahined of steel OLC45. Thus, the radial medium utting fore is F rm1 = 61 N on eah of the three utting spots. Figure 5 presents the deformations of the thin wall in the ase that the utting fore is applied in the middle area. The simulation results show that the maximum deformation of mahined wall in the middle area is mm, very great value, ompared to 1 mm the mahined wall thikness.

3 227 Fig. 5. Deformations of the thin wall in the middle area. Fig. 6. Deformations of the thin wall at one of the ends. Fig. 7. Von Mises stresses of the thin wall in the middle area. Fig. 8. Von Mises stresses of the thin wall at one of the ends. Also, Fig. 6 shows the deformations when the utting fore is applied on one of the thin wall s ends. The maximum deformation at the free end of mahined wall is mm, an inadmissible value, due to signifiant variation of the wall thikness whih results. Using the CATIA software FEM analysis apabilities, there is simulated the distribution of the stresses in both situations. Figure 7 shows the Von Mises Stresses alulated in the ase of the fore applied in the middle area of the thin wall. For a fore of 61 N (on eah utting spot) the results are: maximum stress = N/m 2 and max. elasti deformation x = mm (Fig. 5) with an error of 27.7 %. Figure 8 shows the Von Mises Stresses alulated in the ase the fore applied in one of the ends of the thin wall. For a fore of 61 N (on eah utting spot) the results are: maximum stress = N/m 2 and maximum elasti deformation x = mm (Fig. 6) with an error of 31.2 %. The onsidered steel has the yield strength of N/m 2. In the FEM pratie, an error of 20% 35% is aeptable and it is onsidered lose to the real ase [2]. Anyway, in both situations for the flat surfae S1, the maximum stresses are lower than the materials yield strengths, so the thin wall is deformed only in the elasti domain. 4. EXPERIMENTAL RESEARCHES 4.1. Experimental setup The purpose of the experimental researhes was to determine the elasti deformations of the thin wall for the onsidered part, in real onditions of milling. In the proess it was used an universal milling mahine, type FN- 32. Also, for the utting tool it was hosen a solid arbide end mill CoroMill Plura (GC 1620), with diameter D = 10 mm, tool helix angle = 50 0, helial pith of the utting teeth = 28 mm, total length = 100 mm, length of the ative part = 26.5 mm. The utting onditions were: utting speed v = 63 m/min, feed per tooth f z = mm, utting depth a p = 20 mm, working engagement a e = mm. The milling proess was done without ooling, down-milling method. It is reommended to avoid up-milling proedure, beause of strong hatter or high frequeny vibrations ourring, related to low feed per tooth. The experimental arrangement for the determination of utting fores and for the measurements of elasti deformations was omposed of the next main omponents (Fig. 9 and Fig. 10): work piee (1), Quartz 3 omponent Dynamometer Type 9257 B Kistler (2) fixed on the mahine table (5); Multi-Channel Charge Amplifier for Multiomponent Fore Measurement Type 5070 A (6); Data aquisition board Type PCIM-DAS1602/16 (7); DynoWare Type 2825A data aquisitions and manipulation software [8]; PC with Windows XP. The milling utter (3) was fixed in the main spindle of the tilting milling head (4), through a tool holder.

4 228 Fig. 9. View of experimental setup The omponents of the utting fore, F x, F y and F z were measured with the dynamometer on the diretions of the onsidered referene system (Fig. 9). The omponent F x is normal oriented on the surfae of the proessed wall and ause its deformation. The omponent F y is oriented on the diretion of feed movement, and the F z omponent is on the diretion of tool s axis. The measuring of the elasti deformation of the mahined thin wall S1 (Fig. 11) was done with a dial gauge C with a division of mm whose detetor was loated at the middle of the wall length of the work piee W, at its maximum height. The tool T is presented at the end of the finishing milling pass, done by down milling. In the utting zone there are hips Ch resulted from mahining proess Results of the experimental determinations The utting fores were measured in onditions of roughing and finishing milling, with orresponding utting speeds. 12 measurements were done in order to determine the utting fores and the maximum deformations of the thin wall, mahined from the thikness of 3.5 mm to 1 mm. Within eah measurement it was aomplished a file with proessed data and the measurements results of 8 omponents using the Kistler dynamometer signals. Every data file ontains a page with omments (Fig. 12) whih allows the user to annotate the onditions for eah measurement. By proessing the aquired data files with Dyno- Ware type 2825 A, there are obtained variations diagrams of the utting fores omponents previously seleted by the user, and also the minimum, medium and maximum values on the hosen intervals of time, as needed (e. g. for a tool rotation). The numerial values of eah fore omponent may be determined for eah moment or interval of the determination using options from the program toolbar. Some results for a few of the 12 experimental determinations are shown in Table 1. The values of the measured fores are determined diretly by the utting depth a e and by the feed per utting edge f z, all the other parameters being maintained onstant. Fig. 10. Experimental setup. Fig. 11. Mahining area. Fig. 12. Comments for the utting onditions.

5 229 Fig. 13. Cutting fores for the determination D10-1. Fig. 14. Detail of utting fores F x for the determination D10-1. Fig. 16. Cutting fores for the determination D10-9. Fig. 15. Cutting fores for the determination D10-6. The omponents F x, F y and F z are determined at the first tool s pass, noted with D10-1, on a time interval of 0.5 s and they are presented in the Figure 13. The graphis appear overlapped and they ould be distinguished by different olors. In the studied ase it presents a great interest the F x fore omponent (Fig. 14) oriented normal to the surfae of the mahined thin wall. During the milling pass for a wall thikness of 2 mm, test noted with D10-6, there were obtained the omponents of the utting fore represented in Fig. 15. The utting onditions in this test were the same as for the test D10-1 and the measured values of fore omponents are losely related. Also, the general aspets of utting fores variations presented in Figs. 13 and 15 are quite similar. For another test D10-9, the utting fore omponents have the variations represented in the Fig. 16, for a wall thikness of 1.5 mm. The feed peer tooth was greater (see Table 1) and the utting fore omponents present a sensible inrease, ompared to D10-6 test. The general aspet of fores diagrams is modified: F x omponent reahes greatest absolute value and as a onsequene the defletion of the mahined wall x has inreased. The last milling pass, test D10-12 (see Table 1) was aomplished with the same utting onditions as for the test D10-9. The utting fore omponents measured values present a little derease, ompared to D10-9 test. This is due to elasti deformation of mahined thin wall and, as a onsequene the derease of atual working engagement a e ompared to settled value. Also, for the determination of the utting fore omponent F x it was applied a alulus algorithm using some data from the Coromant atalogue, or a speialized software CoroGuide for the proess parameters. For the rough milling regime, orresponding to the test D10-9, the average tangential fore, using CoroGuide data results: F tmed9 = 287 N. Experimental results at fores measurements Table 1 Det. F x [N] F y [N] F z [N] f z a e Thikness Deformation No. med max med max med max [mm/tooth] [mm] wall [mm] x[mm] D D D D

6 230 The average radial fore is: F rmed9 = (0.5 1) F tmed 9 = 229 N. The measured value for F x omponent in the D10-9 determination was F xmed = 248 N and the orresponding deformation was x = mm. For the finish milling regime, orresponding to the test D10-6, the alulated values for these two fore omponents are: F tmed6 = 203 N and F rmed6 = 162 N. The measured value for F x omponent in the D10-6 determination was F xmed = 149 N, for the alulated radial fore F rmed6. Making a omparison between the results obtained by measurement of the omponent F x with a dynamometer and the alulated values of the average radial fore is found that the differenes are below 10%, thus proving out the validity of alulations and measurements made in the experimental tests. The diagrams of F x, F y and F z omponents of the utting fore shows relatively large ylial variations with the orresponding period of time of a tool rotation. Thus, in the atual working onditions, when the spindle speed is n = 2000 rpm, the orresponding time to a tool rotation is 0.03 s. This fore variation is due to inherent radial run-out of the edge related to the tool rotation axis, and also to a low feed per tooth. 5. CONCLUSIONS The determination of the utting fores that the utting edges reat on the mahined surfae was made using an experimental arrangement based on multiomponent Quartz dynamometer. The value of the thin wall s maximum elasti deformation, measured during the finish milling is very high ompared with the wall thikness. Thus the mahined wall thikness results uneven. This shows that the utting regime (the parameters a e and f z ) has to be dereased if the permissible deviations (preision required during real mahining by milling) are smaller than the resulted maximum elasti deformation. The elasti deformations x are higher at both ends of the wall (0.176 mm) ompared to the deformation at the middle of the wall (0.108 mm). The result is an unexpeted great variation of the wall thikness in both diretions, length and height. The measured values of the radial fores and elasti deformation x are in a good orrespondene with the alulated values. Also, these deformations orresponding to the onsidered working regime seem to be unaeptable influening the auray of the mahined surfaes. If the aim is a higher preision, the parameters of the mahining regime should be dereased and the simulation proess resumed. REFERENCES [1] Budak, E., Altintas, Y. (1994). Peripheral milling onditions for improved dimensional auray, International Journal of Mahine Tools and Manufature. 34 (7), ISSN , pp [2] Constantinesu, N. I., Sorohan, Şt., Pastramă, Şt. (2006). The pratie of finite element modeling and analysis, Edit. Printeh, Buharest, ISBN [3] Ghionea, I. (2009). CATIA v5. Apliaţii în ingineria meaniă (CATIA v5. Appliations in mehanial engineering). Edit. Bren, ISBN , Buharest. [4] Ghionea, I., Ghionea, A., Tănase, I. (2009). Appliation of CAM-FEM tehniques in the establishement of milling onditions of the parts with thin walls surfaes, 4th International Conferene Optimization of the Robots and Manipulators, OPTIROB, May, pp , Edit. Bren, Buharest, ISSN [5] Rao, S. V., Rao, M. V. P. (2006). Tool defletion ompensation in peripheral milling of urved geometries, International Journal of Mahine Tools and Manufature. Design, Researh and Appliation 46 pp [6] Tănase, I, (2009), Sule aşhietoare (Cutting Tools), Edit. BREN, ISBN , Buharest. [7] Trent, E., Wrigt, P. (2000). Metal utting, Butterworth Hinemann, Oxford, Boston. [8] *** (2008). Operating Instrutions KISTLER Instrumente AG, Switzerland. [9] *** (2008). Metalworking produts CoroGuide 08.2, SANDVIK Coromant. [10] *** (2005). Metalutting Tehnial guide, SANDVIK Coromant, C-2900:3 ENG/01, Printed in Sweden by Elanders. Authors: PhD, Eng, Ioan TĂNASE, Professor, University "Politehnia" of Buharest, Mahine and Prodution Systems Department, Buharest, Romania. tanase.ioan@yahoo.om Eng, Ionuţ GHIONEA, Leturer, University "Politehnia" of Buharest, Department of Mahine Manufaturing Tehnology, Buharest, Romania. ionut76@hotmail.om