Surface quality of sculpture surface in case of 3D milling

Transcription

1 Surface quality of sculpture surface in case of 3D milling Balázs MIKÓ 1, Jozef BEŇO 2, Peter IŽOL 2, Ildikó MAŇKONÁ Óbuda University, Budapest, Hungary, 2 Technical University of Košice, Slovakia Abstract: The surface quality of a plastic part depends on, among other factors, the surface quality of the injection mould. The core, the cavity and other active mould parts are manufactured by CNC milling or EDM technology. In case of milling technology the cutting tool path defines the main component of the surface roughness and texture. The article presents of the different parameters of the 3D milling strategy and the connection between the parameters of the surface, the milling technology and the surface quality. Introduction The plastic parts have an increasing role in many field of everyday life. The automotive industry requires more complex and more accurate plastic parts with good quality and aesthetic surface. In the field of form part, the most important production technology is the plastic injection moulding, where the melted plastic is injected to the close cavity. During the cooling phase of the material, it will copy the shape of the cavity. The surface quality depends on the type of the plastic material, the parameters of the injection and the quality of the machined surface of the mould cavity. The active elements of a mould, which constitute the close cavity, like core and cavity insert, sliders and lifters etc., are manufactured by cutting technology, mainly CNC milling and EDM technology. The character of the surface is very different in case of these two technologies, because of the method of the material removal. In this article the milling technology is investigated. Milling is a process of generating machined surfaces by progressively removing a predetermined amount of material or stock from the workpiece witch is advanced at a relatively slow rate of movement or feed to a milling cutter rotating at a comparatively high speed (Figure 1). The characteristic feature of the milling process is that each milling cutter tooth removes its share of the stock in the form of small individual chips [1]. Figure 1 Milling technology

2 During the rough milling the aim is to remove the high amount of the material with high productivity. Then, during the finishing, the accurate size and fine surface quality will be produced. In case of mould parts the shape of the workpieces generally is very complex, so the milling process requires the use of CNC technology and CAM systems. The CNC program is generated by the CAM system, where the user (engineer) define the cutting tool geometry, the cutting strategy, and the parameters which will define the details of tool path. The harmony between the tool, the strategy and the parameter is the guarantee of the successful manufacturing process and the good surface quality. Previously the Z-level finishing strategy was investigated [2], and now the 3D surface milling. Investigation of the 3D surface milling The aim of the research was to determine the effect of the parameters of the 3D milling in order to optimize the surface quality of a sculpture surface. The size of the test part was 175x165x45 mm, which contains two cylindrical surfaces with 100 mm radii. The material of the test part was 42CrMo4 (1.7225). Before the 3D finishing a rough milling and a prefinishing were applied, the allowance for finishing was 0,2 mm. A ball end milling cutter was selected from the Fraisa catalogue (U ), the diameter (D c ) was 12 mm. The input parameters were the follows: Cutting speed (v c ): 280 m/min, 210 m=min, 140 m/min. Step over (a e ): 0,8 mm, 0,5 mm, 0,2 mm. Direction of the finishing (A): 0, 90º. the fractional factorial design of experiment method was used to generate the parameter setups in order to effective testing. The Figure 2 shows the CAD model of the test part and the 6 test zones with the specific parameters. For the better analysis 2 additional zones were define. Instead of the cutting speed the revolutions per minute (n) was programmed. The feed per tooth (f z ) was 0,085 mm, and the feed speed (v f ) were 1265 mm/min, 950 mm/min, 630 mm/min. The required CNC programs were generated based on CAD model by Pro/Engineer WF4 CAM system, and manufacturing was performed in a Mazak Nexus 410A-II manufacturing centre. Figure 2 Test zones and the simulation of a CNC program During the test the following parameters were measured or recorded: machining time (minutes),

3 cutting force components by KISTLER Type: 9257 A force measurement unit with KISTLER 5019B131 amp and DynoWare software, surface roughness (R a, R z, R t ) in parallel and perpendicular to the milling direction by Mitutoyo Surftest SJ 301. The cylindrical surfaces ensure to consider the effect of the surface gradient. Five levels were defined in the test part, where the angle to the vertical direction (centre of the tool) of the normal vectors were N i : 2,9º; 11,5º; 20,4º; 30,0º; 36,9º (i=1...5). Manufacturing time During the machining the manufacturing time was measured, and the connection between the time and the machining parameter was detected. In order to the general application a specific manufacturing time was defined, where the time was divided by the area of the milled surface. The analysis was performed by the MiniTab v14 statistical software. The main effect plots (Figure 3) shows the importance of each parameters. Main Effects Plot (data means) for T_m A ae 0,3 0,2 Mean of T_m 0,1 0 vf 90 0,2 0,5 n 0,8 0,3 0,2 0, Figure 3 Main effects plot Because of the v f is a function of n, the linear regression was build on the a e and v f : T_m = 0,619-0, vf - 0,438 ae Constant 0,6190 0,1386 4,47 0,021 vf -0, , ,80 0,170 ae -0,4383 0,1314-3,34 0,045 S = 0, R-Sq = 82,7% R-Sq(adj) = 71,2% The R-Sq(adj) is only 71,2%, the mathematical model is not so accurate. The linearization of the data is required for the better solution, we used the ln function for the linearization. ln_t_m = 4,03-0,992 ln_vf - 1,00 ln_ae Constant 4,0254 0, ,87 0,002 ln_vf -0, , ,32 0,000 ln_ae -1, , ,29 0,000 S = 0, R-Sq = 99,8% R-Sq(adj) = 99,7%

4 The R-Sq(adj) is 99,7%, so this model is very good. The Figure 4 shows the measured and the estimated time data. The differences are very small even in case of Zone 7 and 8. These two zones were excluded from the analysis, but the result match to the measured data. Figure 4 Manufacturing time Surface texture The surface texture was investigated by stereo microscope. The Figure 5 shows examples. The first two pictures show the Zone 2 in two positions: at the top of the test part, where the angle of the normal vector was 2,9º, and at middle, where the angle of the normal vector was 20,4º. The direction of the milling is parallel with the Y axis (A = 90 ). Depends on the normal vector the surface texture is very different, cause of the change of working diameter of the milling cutter. The different strips shows the up-milling and down-milling paths, and the step-over parameter (a e = 0,5 mm) is very clear in the pictures. Figure 5 Surface texture

5 The other two pictures show the Zone 5 at same position. At the shallow zone (N 1 ) the difference between the up-milling and down-milling strips very characteristic, the step-over (a e = 0,5 mm) is very clear. The effective cutting diameter is very small. When the normal vector is larger (N 3 ), the effective diameter increase, and the difference will be smaller, the texture more homogenous, the step-over parameter is invisible. Surface roughness In this section we find the relationship between the R a, R z and R t surface roughness parameters. There is no general figure to define this relationship, because it is depends on manufacturing technology [3]. Based on the measured data we try to define this connection. The simplest linear regression was applied in the MiniTab v14. The result is the next: Rz = 0, ,04 Ra Constant 0,1632 0,6100 0,27 0,791 Ra 5,0401 0, ,49 0,000 S = 1,70021 R-Sq = 93,1% R-Sq(adj) = 92,9% The R-Sq(adj) is 95,7%, so the mathematical model is correct. The Figure 6 shows the measured and estimated data. In some cases the difference looks substantial; there are several possible reasons for this, like the uncertainty of the measuring method. Figure 6 Surface texture The estimation of R t parameter is a harder work, there is no clear connection between R t and R a, the R z parameter should be added to the model: Rt = 0,629-5,38 Ra + 2,28 Rz Constant 0,6289 0,4445 1,41 0,169 Ra -5,3843 0,7182-7,50 0,000 Rz 2,2756 0, ,55 0,000 S = 1,23723 R-Sq = 98,0% R-Sq(adj) = 97,8%

6 Conclusions The plastic injection mould is the key element in the production process of a plastic part. The surface of the mould is the main component of the surface quality, so the manufacturing process of the core and cavity is very important engineering task. If the mould parts are manufactured by milling technology, the CNC programs are generated by CAM system. The CAM system create CNC programmes based on CAD model, but it is not an automatic process. The quality of CNC programs depends on the engineer, who defines the elements of the CAM process. The last step is the finishing milling, and it is defines the final look of the surface. The selection of the appropriate milling strategy, the tool and cutting parameters is the task of the engineer. The surface texture depends on the optimal set of parameters, so it is need the circumspect design of the process. The research was supported by the TéT SK-15/2008 IMPRICAM - Implementation of technological principles into CAD/CAM assisted free form machining. References [1] Schneider, George: Cutting tool applications; George Schneider [2] Peter Izol, Jozef Beno, Balázs Mikó: Precision and surface roughness when free-formsurface milling; Manufacturing Engineering 2011 Issue 1, p [3] Modern metal cutting a practical handbook; Sandvik Coromant, Contact Dr. MIKÓ Balázs PhD, Associate professor, Deputy-head of the Institute Óbuda University Bánki Donát Faculty of Manufacturing and Safety Engineering Institute of Material Science and Manufacturing Engineering Address: H-1081 Budapest Népszínáz u 8. Phone: Fax: miko.balazs@bgk.uni-obuda.hu