Investigation on precise measurement of cutting tool edges using coordinate measuring machines

Transcription

1 Investigation on precise measurement of cutting tool edges using coordinate measuring machines Adam Woźniak 1), René Mayer 2), Marek Bałaziński 2), Mélissa Côté 2) 1) Institute of Metrology and Measuring Systems, Warsaw University of Technology, Św. A. Boboli 8 Street, Warsaw, Poland Wozniaka@mchtr.pw.edu.pl 2) Département de génie mécanique, École Polytechnique, C.P. 6079, succ. Centre-ville, Montreal, Canada [Rene.Mayer, Marek.Balazinski, Melissa.Cote]@polymtl.ca Abstract: In this paper, the probe radius compensation problem during the measurement of tool cutting edges using a coordinate measuring machine (CMM) has been addressed. We first show that information about the real shape of the measured edges can be distorted using the CMM machine built-in data processing software for probe tip radius compensation. In order to accurately measure cutting tool edges which include discontinuous regions we propose to apply a new algorithm for the compensation of the probe radius in a CMM scanning process. The comparison of the measured cutting tool edge profile using the indigenous CMM software and the proposed method of probe radius compensation shows that our proposed method has better accuracy. Keywords: cutting tool edge, coordinate measuring machine, precise measurement, stylus tip radius compensation. 1. INTRODUCTION Advanced machining systems require high accuracy geometric features on cutting tools. For instance, the complex 3D geometries of tools and also the cutting tool edges are found to have a significant influence on machining performance and tool life. Many papers like [Fang et al., 2005] or [Thiele et al., 1999] present experimental and theoretical study on the effects of tool edge geometry in machining, such as cutting forces, cutting temperature, tool wear, tool life, chip formation and machined surface quality, but there is little investigation on the development of new methods of edge geometry measurement. Tool inspection using a contour projector or microscope gives a more qualitative then quantitative evaluation due to accuracy. Nowadays, the new generation high performance coordinate measuring machines (CMM) equipped with scanning probes offers new and effective possibilities for shape measurements. The accuracy of these high end CMM scanning probes is in the submicrometer range and thus could be useful for the measurement of precise geometric features of cutting tools. However, CMM data processing software is often designed

2 and tested on rather large features relative to the stylus tip. As a result maintaining this accuracy for freeform surface measurement is hard to obtain for features such as tool edges because of the CMM built in algorithms for probe radius compensation. The stylus tip radius correction is an offset vector of norm equal to the effective stylus tip radius R which is added to the indicated measured point O i (the measured stylus tip centre point) to estimate the actual contact point P i (the stylus tip contact point on the real surface), i.e. the corrected measured point on the surface like in Figure 1. The nature of the contact between a sphere and surface results in the offset vector being normal to the surface at the point of contact so the primary task for correction is to estimate this vector at each data points. However in case of freeform surfaces (like a cutting edge) the normal vectors have to be calculated taking into consideration the set of stylus tip centre points. Thus, because of inherent measuring machine inaccuracy, small deviations of centre point coordinates can cause big deviation in the direction of the normal vector calculated by the CMM software. As a result we observe incoherent connected measured point patterns as shown in Figure 1. O i-1 O i O i+1 O i+2 R P i-1 P i P i+1 P i+2 Figure 1; The deviation of corrected measured points position involved by wrong calculation of normal vector during CMM compensation of stylus tip radius. A number of algorithms have been proposed for solving this problem. Most of these use B-splines (NURBS) or cubic splines [Zongwei et al, 2003] to obtain a best fit curve to the centre points and its normal vectors in order to compensate as a postmeasurement operation. Most of the CMM manufacturers use this technique for data processing. Unfortunately in case of measurement of very sharp edges it is not adequate. According to some initial trials of scanning measurement on such discontinuous surfaces and using the stylus tip radius correction built-in the CMM software, it was found, as will be shown later, that the indigenous CMM software do not compensate well the stylus tip radius. Additionally for internal discontinuities of the measuring surface we can observe a phenomenon of smoothing of the real shape of the surface. As a result the information about the real shape of the measured edges can be distorted. In order to accurately measure cutting tool edges we propose a new algorithm for the compensation of the stylus tip radius in a CMM scanning process. Our algorithm is dedicated to high definition measurement. Advantages of the algorithm are that we do

3 not calculate the normal vector and we do not use a NURBS for smoothing (filtrating) of measuring shape. Since we are considering the possibility of patent application the principle of this algorithm will be presented in a future paper. The planned publications of this work are currently on hold until patent protection is granted. In this paper we will show the results of measurements of the profiles of cutting tool edges using the indigenous CMM software with comparison to the new proposed method of stylus tip radius compensation. 2. TOOL EDGE GEOMETRY A sharp tool is a theoretical tool that possesses an edge radius equal to zero, as shown in Figure 2a. Of course a zero radius does not exist in practice, reached up tenth of micrometer. To reduce edge related problems during machining, such as breakage, cracks or edge chipping, tool manufacturers provide various type of modification to tool edge geometry. At least three types of tool edge geometry are now customary such as: hone (Figure 2b), chamfer (Figure 2c), and chamfer plus hone (Figure 2d) [Fang et al., 2005]. Moreover there exist other type of tool edge geometry, for example double chamfers included in the tool insert identification system specified by Sandvik Coromant [Sandvik Coromant Company, 2004] and American National Standards Institute [Cemented Carbide Products Association ANSI, 2002]. a) honed b) radius c) chamfer width d) chamfer plus honed radius chamfer angle Figure 2; Typical tool edge geometry on commercial tool inserts: a) sharp edge, b) honed edge, c) chamfer, d) chamfer plus honed radius. For the blunt tool inserts the honed edge radius varies between 5 and 150 micrometers. The chamfer angle varies between 20 and 40 degrees, and chamfer width between 0.1 and 2 mm. This special tool edge preparation eliminates the sharp edge and reduces edge related problems but it affects the machining process. In order to investigate the influence of tool edge geometry in machining, the precision of the methods of tools measurement must be improved.

4 Three different cutting tools have been used to demonstrate the feasibility of the proposed method of stylus tip radius compensation for cutting edge measurement. The first tool is a Minicut 1-1/4 inch diameter end mill. Its four flutes have a 35 helix angle. This tool is intended to machine titanium alloys. The second tool is a DoAll high speed steel cutting end mill with a 1 inch diameter, 4 flutes and a 30 helix angle. The last tool is a carbide cutting tool designed at Pratt & Whitney Canada especially for evaluation of cutting edge measurement methods. It has a 0 helix angle and is honed in order to form a larger edge radius. 3. CMM MACHINE SET-UP AND MEASUREMENT CONDITION Tests were carried out on a fixed bridge, moving-table Mitutoyo LEGEX 910 CMM equipped with a MPP-300 scanning probe. The length measuring system of the LEGEX, which is the reference scale on each axis, employs a linear encoder system with a resolution as high as 0.01 μm. Ambient temperature in the laboratory is maintained in the range 20.0±0.1 o C. The machine maximum permissible error of indication for size measurement, MPE E = 0.48+L/1000 m (with L in meters) as per standard ISO However, in case of measurement of small features like cutting edges in scanning mode the maximum permissible scanning probing error, MPE Tij = 0.32 m is a more appropriate parameter to describe the expected precision. In order to minimize stylus elastic deflection the stems were 10 mm long, 0.4 mm in diameter and were made of tungsten carbide. Furthermore in order to reduce geometric filtering by the stylus tip, a small tip made of a 0.5 mm sapphire ball was used. Because the measuring velocity could influence probe tip vibration, scanning velocity was limited to the value of 0.2 mm/s which is the smallest value possible in this application. Figure 3 shows the measurement setup. The sampling step was set to the smallest possible value of 11 m. Figure 3; View of the edge measurement of cutting tool on CMM Legex.

5 4. MEASURING RESULTS For each measuring section of a cutting edge two sets of data are stored in computer files. The first set is stylus tip centre points (i.e. uncompensated data set), while the second set contains the CMM compensated data set. Figure 4 shows the two sets of compensated points for measured results of the plane section of Minicut tool edge. The dashed line and square points represent the CMM built-in software compensated data. In this case we can observe large deviations of the points. The character of this deviation is similar to that presented in Figure 1. The continuous line and circles in Figure 4 shows the result of the same data but using the prepared new radius compensation scheme. In order facilitate visualization, a 5 m offset have been applied to separate the two data sets. We can see that the new algorithm of compensation give a better estimate of the edge radius. In Figure 4 the tool radius is approximately 1 m. Thus it is a sharp tool edge. Combining a few such sections of the measured edge, i.e. without a deviation caused by CMM built-in processing it is possible re-construct a 3D computer model of the whole cutting edge. Such a model of a Minicut tool edge is presented in Figure MINICUT tool edge New correction CMM correction (5um offset) Figure 4;The results of Minicut cutting edge measurement in a plane section.

6 Figure 5; 3D visualization of scanning measurement of Minicut cutting edge using new algorithm for probe tip radius compensation. Similar scanning measurements have been carried out for DoAll high speed steel end mill cutting tool. The results presented in Figure 6 (using the same scale as for Figure 4) show a less sharp edge with an edge radius equal to 4 m. In this case the incoherence of the CMM compensation is not as evident as for a sharp edge. However if we look at a magnified area we can notice similar distortions as for sharp edges. DoAll tool edge New correction CMM correction (5um offset) DoAll tool edge Figure 6; The results of DoAll cutting edge measurement in a plane section.

7 The third test tool is a honed carbide cutting tool. The results of measurement of a section of this tool are presented in Figure 7. The measured radius of the honed cutting edge is approximately 14 m. Although the CMM compensated points are fairly well located on the honed edge, it can nevertheless be observed that their order along that edge is incoherent with the way in which the scanning process naturally occurred as can be deducted from the raw centre point data. On the other hand the points processed with the proposed method are both on the expected edge profile and in the anticipated order. Additionally, a few repetition of measurement on the same path shows that the repeatability of the proposed method is approximately 0.1 μm for features with external as well as internal curvature. To verify the accuracy of the proposed method we measured micro-feature reference artifacts. Deviations not exceeding 0.3 μm were obtained. This is of a level similar to the CMM machine repeatability itself. Honed tool were designed at P&W New correction CMM correction (5um offset) Honed P&W Figure 7; The results of DoAll cutting edge measurement in a plane section.

8 5. CONCLUSION The following main conclusions are drawn: i) current probe radius compensation algorithms do not perform satisfactorily during the measurements of tool cutting edges using coordinate measuring machine; ii) a new compensation method for high definition measurements, especially for sharp edges and internal discontinuous surfaces has been proposed; iii) the results of cutting edge measurement of three different types of tool edge have been presented. The inclination of the probe was fixed but several inclinations of probe tip were used. Because of the geometric filtering by the stylus tip, a small tip (0.5 mm) gave the best results. However, some measurements of the step artifact have been carried out with another probe. The obtained results have confirmed the usefulness of the algorithm and iv) The performance of the proposed method has been compared with built-in CMM machine software compensation data processing. It has been shown that the proposed method has better performance in term of the closeness of the compensated points to the actual surface as well as the coherence of those points. ACKNOWLEDGEMENTS We are grateful to Mr. Engin Serafettin from Pratt & Whitney Canada for providing some of the cutting tools for our measurements. This work was funded by NSERC Discovery Grant RGPIN with equipment funded by the Canadian Foundation for Innovation grant FCI Adam Woźniak is holder of the Foreign Postdoc Fellowships of Foundation for Polish Science for one year visit to École Polytechnique de Montréal. REFERENCES [Cemented Carbide Products Association ANSI, 2002] Cemented Carbide Producers Association, ANSI B : Indexable Inserts Identification Systems, Cemented Carbide Producers Association, Cleveland, OH, 2002 [Fang et al., 2005] Fang, N.; Wu, Q.; "The effects of chamfered and honed tool edge geometry in machining of three aluminum alloys"; In: International Journal of Machine Tools & Manufacture, Vol. 45; pp ; 2005 [Sandvik Coromant Company, 2004] Sandvik Coromant Company, Turning Tools and Inserts, Sandvik Coromant Company, Fair Lawn, NJ, 2004 [Thiele et al., 1999] Thiele, J.D.; Melkote, S.N.; Peascoe, R.A.; Watkins, T.R.; " Effect of cutting-edge geometry and workpiece hardness on surface residual stresses in finish hard turning of AISI steel"; In: ASME Journal of Manufacturing Science and Engineering, Vol. 122; pp ; 1999

9 [Zongwei et al., 2003] Zongwei, Y.; Yuping Z.; Shouwei J.; Methodology of NURBS surface fitting based on off-line software compensation of errors of a CMM ; In: Precision Engineering, Vol. 27; pp ; 2003