COMPARISON OF FLATNESS AND SURFACE ROUGHNESS PARAMETERS WHEN FACE MILLING AND TURNING

Transcription

1 1 COMPARISON OF FLATNESS AND SURFACE ROUGHNESS PARAMETERS WHEN FACE MILLING AND TURNING Mikó B.; Farkas G. Institute of Material and Manufacturing Science, Óbuda University, Budapest Hungary Abstract The requirements of the accuracy in case of machine parts can be defined in different ways and in different levels. Beside the size tolerancing, the geometrical tolerancing (GD&T) has increasing importance in the industrial practice. The micro geometry of the machined surfaces can be described by several parameters too, like 2D and 3D surface roughness parameters. The aim of the actual research is to study the nature of the GD&T parameters in function of machining processes. In this article the flatness parameter is investigated and compared in case of face milling and face turning, and the surface roughness parameters are measured too. The question is how can we describe an inhomogeneous surface by micro and macro parameters, how can it be classified by the different error parameters. Keywords: geometric tolerances, flatness, surface roughness, plane surfaces 1. Introduction - deviations of a machine part During the machining process there are several deviations on a machine part. The origin of these deviations can be found in the uncertainty of the parameters of the manufacturing process. The main factors are related to the material, machine tool, measuring technology, manufacturing environment and the man, who controls and does the process. The error factors can be reduced by provident process planning, more accurate equipments and automated control, but cannot be eliminated. In order to successful work of the product and economical manufacturing process, tolerances must be defined. The tolerance is the allowed deviation. The tolerancing, the definition process of the tolerances, is very complicated, because the designer should consider many viewpoints and circumstances. The first aim is the right and accurate work of the product, but the capability

2 2 of the manufacturing and assembly system and the production cost are important aspects too. Figure 1 The system of tolerancing The appearance of the deviation can be a different, so a different type of tolerances can be defined. The dimensional or size tolerances can be defined in an indirect way by standards [1], or the deviation can be defined by tolerance values, or IT standards [2] can be used. Roughness measuring features can be used to characterize the quality of the surface quality in the technical practice which can be 2D, they are micro geometric parameters and 3D so called micro topographic characteristics. The surface micro geometric measuring and characterizing are mostly standardized so it is suitable to compare different types of surfaces. The surface quality can be controlled by different surface roughness parameters, like Ra, Rz, Ry, Rsk, Rku etc. [3, 4]. Wide-ranging set of parameters is available to characterize the surface texture but the industrial practice uses only 2-3 measuring characteristics (e.g. Ra, Rz, Rmax). There are several ways to predict of surface roughness. Felhő and Kundrák [5] define the surface roughness of a face milled plain surface based on 3D CAD model. Based on cutting experiment data Horvath and Drégelyi-Kiss [6] presents a phenomenological model, Markopolous et al. [7] presents a neural network based on estimation method. The more complex requirements can be defined as geometrical tolerances [1]. The standard defines four groups of tolerances. In case of form tolerances the accuracy of geometric element should be defined, in that case there is no need to use datum features. The four tolerances are the straightness, flatness, roundness, cylindricity, profile any line and profile any surface. The other three groups need datum features. In case of orientation (parallelism, perpendicularity and angularity) the angular accuracy of two geometric

3 3 elements are defined. The location tolerances (position, concentricity, coaxiality and symmetry) control the position of the element based on datum feature. The run-out tolerances (circular run-out and total run-out) define a complex tolerance Figure 2 Classification of geometric tolerances The increasing importance of the geometric tolerances point out lot of questions related to the mathematical method of the evaluation, the point sampling strategies in CMM, and the manufacturing possibilities and constrains. Calvo et al. [8] presents the application of minimum zone method in case of flatness and straightness. The developed vector method ensures the optimization of point sampling strategy. Hadžistević at al. [9] presents flatness error in case of five different specimens, which were machined by turning, milling and grinding. Different number of points was processed, which has significant effect on the flatness value. Two methods were used during evaluation, least square and minimum zone, and except grinding, where the flatness error is very small, the evaluation methods resulted different values. Radlovački et al. [10] analyses different evaluation methods, and points out the importance of them. The random sampling method was used, and the different algorithms ensure different results. Seth and George [11] shows the effect of the cutting parameters in case of face milling on the surface roughness and flatness. The two parameters changed nearby parallelly. The article presents the result of a preliminary investigation, where the evaluation of the flatness deviation is studied. Four plains were manufactured by different cutting technologies, and flatness and surface roughness parameters were measured. The question is, what is the relationship between

4 4 the manufacturing conditions and the flatness deviation, how can we control not only the surface roughness, the dimensional errors, but the geometrical deviations. 2. Methods and equipments Four test parts were machined by different cutting technologies. The material of the test part is 42CrMo4 (1.7225). The overall dimension of the part is 175x155 mm. 2-2 test surfaces were machined by milling and face turning. The machining conditions were the next (Table 1). A conventional milling machine (1), a CNC machining centre (2) and a conventional lathe engine (3, 4) were used. Table 1 Machining conditions No Machine tool Tool Parameters 1 Milling UF-231 milling machine 2 Milling Mazak A410-II CNC machining centre 3 Face turning 4 Face turning D=80, z=7 ISCAR HeliDo SOF 45 8/16-D R D=50, z=4, SECO R A / XOEX R-M07, F30M E-400 lathe ISCAR PCLNR 2020K08/ CNMM HTW IC8150 E-400 lathe ISCAR PCLNR 2020K08/ CNMM HTW IC8150 v c = 60 m/min f z = mm n = 2401/min v f = 78 mm/min a p = 1 mm a e = 40 mm v c = 60 m/min f z = mm n = 3821/min v f = 70 mm/min a p = 1 mm a e = 25 mm n = 1901/min a = 0.5 mm f = 0.6 mm/rev n = 190 1/min a = 0.5 mm f = 0.2 mm/rev The surface deviation was measured by Mitutoyo Crysta-Plus 544 coordinate measuring machine points were measured in 30x34 pattern and the flatness values were calculated by Kotem Smart Profile 4.0. The different dimension, shape and position tolerances can be defined based on shop drawing s requirements (Figure 3) and evaluate, visualise and report based on different standards.

5 5 Figure 3 Smart Profile s user interface The 2D roughness parameters of machined surfaces were measured by Mahr Perthometer-Concept type instrument with Mahr MFW-250:1 type feeler in the measuring technique laboratory of the institute. Several profiles and parameters can be measured, like raw (P) profile; filtered roughness (R) profile; 9 roughness parameters: Ra, Rmax, Rz, Rq, Rp, Rt, RSm, RSk, RKu; 5 waviness parameters: Wt, Wa, WSm, WS, Wdq; 6 raw parameters: Pt, Pa, PSm, PSk, PKu, Pdq. Evaluation of surfaces was done according to ISO 4288:1996 standard: evaluation length, lm = 4 mm; the prescribed filter, lc = 0,8 mm. The surfaces were divided into 4x4 areas, and the surface roughness was measured in X and Y directions. 3. Results Flatness The flatness parameters are very different, of course, but only the values cannot characterise the nature of the surfaces. In case of milled surfaces (#1 and #2), the origin of the difference is the machine tool, the conventional milling machine is not as rigid and accurate as the CNC milling centre. The face turned surfaces (#3 and #4) has similar flatness error values (Figure 4). The topology of the surfaces, which was created by SmartProfile, shows larger differences. In case of milled surfaces, the lanes of down-milling and up-

6 6 milling are well recognised (Figure 5/a). In case of #2 test part these lanes are smaller thanks to the smaller flatness deviation (Figure 5/b). In case of surfaces, which were machined by face turning, the topologies show concentric nature (Figure 5 c and d). Figure 4 The values of the flatness error a) b) c) d) Figure 5 Flatness evaluation in Smart Profile

7 7 The characteristic nature of the topologies harmonizes with the look of the surfaces (Figure 6), the parallel and the concentric scratches of the cutting tool are well observed. Surface roughness Figure 6 Picture of the surface #1 and #4 Based on 32 measured surface roughness parameters, in case of each surfaces the average parameters and standard deviation can be determined (Figure 7). In case of CNC machining centre (#2) the surface roughness and the deviation are smaller than in case of conventional milling machine (#1). In case of turning the smaller feed causes the better surface quality (#3 and #4). Figure7 The values of the Ra and Rz parameters The standard deviation indicates that the surface quality is not so homogenous, as it can be followed from the geometric model or the look of the surfaces. As Figure 8 shows, X and Y direction show different characteristics in case of milling, because of the down- and up-milling lanes and the random

8 8 position of the measuring in the actual area. In case of turned surface there are not big differences, so this surface is more homogenous. Figure 8 Ra surface roughness in X and Y direction in case of # 1 and #4 surfaces Figure 9 Ra values along the centre line of #4 surface

9 9 In order to discover the changing of the surface roughness, the Ra parameter was measured along a symmetry axis. The Ra parameters were measured in every 5 mm, with the same length and filter parameters. As the diagram shows (Figure 9) the Ra parameter is smaller at the sides, and increases to the centre of the part. But near to the centre it decreases again. This diagram shows a little bit different look as the Figure 8, where the lower centre values cannot be visible. The reason of it is that the 16 zones (4x4) of surface roughness measuring are too big (about 35x35 mm) for show these differences. 4. Conclusions In case of machine part, not only the dimensional parameters have importance, but the geometrical accuracy and surface quality too. In the article the study of flatness deviation and the surface roughness were studied in case of milled and turned plain surfaces. The different machining technologies cause different topology, and have influence on the flatness error. The inhomogeneous surface structure requires that the selection of measured points cover whole surface and they can represent the nature of the surface. The increasing number of measured points helps to enhance the accuracy of flatness evaluation, but increases the time of the measuring. The surface roughness values are influenced by this inhomogeneity too. In order to have accurate surface roughness measuring, the number of the sampling zones must be increased and size of the zones must be decreased. The aim of the further research is to develop a method, which ensures accurate and fast measuring process of flatness and considers the nature of the surface through the surface roughness parameters. 5. Acknowledgement The authors would like to thank the support and help of Mitutoyo Hungary LTD and KOTEM LTD. 6. References [1] ISO Geometrical product specifications (GPS) - Geometrical tolerancing - Tolerances of form, orientation, location and run-out [2] ISO Geometrical product specifications (GPS) - ISO code system for tolerances on linear sizes Part 1: Basis of tolerances, deviations and fits [3] ISO Geometrical Product Specifications (GPS) - Indication of surface texture in technical product documentation

10 10 [4] ISO Geometrical Product Specifications (GPS) - Surface texture: Profile method - Terms, definitions and surface texture parameters [5] FELHŐ, CS., KUNDRÁK, J. (2014). CAD-based modelling of surface roughness in face milling. International Journal of Mechanical, Aerospace, Industrial and Mechatronics Engineering, 8(5), [6] HORVÁTH R., DRÉGELYI-KISS Á. (2015) Analysis of surface roughness of aluminium alloys fine turned: united phenomenological models and multiperformance optimization. Measurement 65: [7] MARKOPOULOS A.P., GEORGIOPOULOS S., MANOLAKOS D.E.: On the use of back propagation and radial basis function neural networks in surface roughness prediction. Journal of Industrial Engineering International 12.3 (2016): [8] CALVO R., GÓMEZ R., DOMINGO R. (2014) Vectorial method of minimum zone tolerance for flatness, straightness, and their uncertainty estimation. International Journal of Precision Engineering and Manufacturing 15.1: [9] HADŽISTEVIĆ, M., ET AL. (2015) Factors of estimating flatness error as a surface requirement of exploitation. Metalurgija 54.1: [10] RADLOVAČKI, V., ET AL. (2016) Evaluating minimum zone flatness error using new method Bundle of plains through one point. Precision Engineering 43: [11] SHETH, S., GEORGE P.M. (2016) Experimental Investigation and Prediction of Flatness and Surface Roughness During Face Milling Operation of WCB Material. Procedia Technology 23: