KOMISJA BUDOWY MASZYN PAN ODDZIAŁ W POZNANIU Vol. 26 nr 2 Archiwum Technologii Maszyn i Automatyzacji 2006 ANDRZEJ WERNER *, MAŁGORZATA PONIATOWSKA * DETERMINING ERRORS IN COMPLEX SURFACES MACHINING WITH THE USE OF CNC MACHINE TOOLS The present paper concerns the question of determining errors in the process of producing parts with the use of CNC machine tools in order to introduce intermediary (off-line) correction of the processing programme. The elements were processed with a CNC FYS 16N milling machine with a TNC 360 Heidenhain numerical control system. The methodology of determining the errors was presented on the example of an object made from an aluminium alloy described with a free surface panel. Both the geometrical model and the processing programme were created in the Mastercam system. The production accuracy inspection were conducted on a MISTRAL Brown & Sharpe coordinate measuring machine. The obtained information relating to the values and distribution of processing errors was used to correct the CNC machine tool control programmes. Key words: processing errors, complex surface, coordinate measuring 1. INTRODUCTION Processing with the use of numerically controlled machine tools is inevitably connected with errors in the machining method, fixing the processed object, thermal expansion of the material as well as the processing system stiffness. There are two methods of error correction in machining. One of them is the online method, applied during the processing. The other is the off-line method involving intermediary correction of a processing programme after analysing the machining accuracy [3], for which it is essential to collect information on the geometry of the produced object. In the case of free shape surfaces, converting a three-dimensional object into a discreet description of its geometry requires applying a surface scanning method called digitalisation. This process aims at machining a geometrical model of an object in the computer memory with the use of a measuring points cloud. Both coordinate measuring machines and CNC machine tools equipped with suitable measuring probes can be used for * Ph.D. Eng. Faculty of Mechanical Engineering, Bialystok Technical University.
212 A. Werner, M. Poniatowska measuring [1]. There are a few methods of surface digitalisation on coordinate measuring machines (CMMs). The choice of the method depends mostly on the type of a CMM, of the measuring probe head as well as of software. The process of 3D surface digitalisation is most often carried out with a contact probe, taking into account the correction of the measuring ball radius in the direction normal to the surface at a particular measuring point. CMMs may be also equipped with non-contact (optical) measuring probe systems. The distance between two measuring points may be constant or may change depending on the shape of the measured surface. The points obtained as a result of the measurement may be distributed according to an regular or irregular network, consecutive sequential lines, or even selectively. The coordinate values of measuring points in a digital form may be further processed in order to determine the values of processing errors. The article presents a method of determining summary errors in processing surfaces of complex spatial shapes with numerically controlled machine tools with the use of a coordinate measuring machine and the CAD/CAM systems. 2. RESEARCH STAND The experiment was conducted on a research stand including the following elements (Fig. 1): a PC computer 3 integrated with a numerically controlled machine tool and with a coordinate measuring machine; with the CAD/CAM (Mastercam) and the PC-DMIS measuring software installed, a FYS 16N machine tool 1 with Heidenhain TNC 360 numerical control, a DEA (Brown & Sharpe) Mistral 070705 coordinate measuring machine 2. Mastercam is an integrated CAD/CAM system which creates geometry, prepares details and finished drawings, visualizes the graphic tool path and generates the NC programme. Thanks to its possibilities it is an ideal tool for preparing technological processes for any numerically controlled machine tools. Mastercam allows the user to change the processing parameters or tool at any time during the process, to modify the tool path, and later to visualise the machining again. Before applying the programme directlyon a machine tool, it is possible for a programmer to check the effect of his or her work on the screen. In addition to creating geometry, the programme user can analyse it, control the tool path, and see on the screen the machining effect, i.e. the real appearance of the worked-on detail including the tool tracks formed in the machining process. The PC-DMIS programme is the software of a Mistral Standard CNC coordinate measuring machine. PC-DMIS for Windows is a full geometrical measurements package. It translates high level commands required in measuring
Determining errors in complex surfaces machining 213 Fig. 1. Research stand scheme: 1 numerically controlled milling machine, 2 coordinate measuring machine, 3 PC computer Rys. 1. Schemat stanowiska badawczego: 1 frezarka sterowana numerycznie, 2 współrzędnościowa maszyna pomiarowa, 3 komputer klasy PC parts into detailed steps necessary to control coordinate measuring machines (CMMs). PC-DMIS for Windows uses the Microsoft Windows interface both for creating and executing programmes. It is easy for a user to start the measuring process with the use of a drop-down menu, dialog boxes and icons. The PC- DMIS interface versatility assures an easy way of adapting the software to the user's individual needs. 3. RESEARCH INTO THE FREE SURFACE MACHINING ACCURACY The method of determining errors in the process of machining a 3D object is shown on the example of an object made from aluminium and described with a free surface panel. The geometrical description of the object was created in the Mastercam programme (Fig. 2). Fig. 2. 3D free surface a view of the surface generated in the Mastercam programme Rys. 2. Powierzchnia swobodna 3D widok powierzchni wygenerowanej w programie Mastercam
214 A. Werner, M. Poniatowska 3.1. Programming and machining The programme controlling the object machining was created with the use of the Mastercam system 3D milling module. In order to generate the tool path (Fig. 3), the procedure of parallel passages was used (two-side machining without return lost motion). The applied tool was an end mill φ10 with a ball end, made of high-speed steel (DIN 1889 BA 10 K-H-HSS-E). Fig. 3. Tool path Rys. 3. Ścieżka przejścia narzędzia Conditions of machining: feed 200 mm/min, rotational speed of spindle 1600 rpm, parrarel passages distance 0.3 mm, machining tolerances 0.01 mm. The machine tool controlling programme was generated with the use of a postprocessor adapted to control a Heidenhain TNC 360. After appropriate equipping and setting of the machine tool, a cubicoid aluminium alloy workpiece was fixed in the device. Machining of the object was realized in the DNC mode (the volume of the controlling programme exceeded the memory capacity of the numerical control system). 3.2. Evaluation of machining surface accuracy Control measurements were taken with a Mistral 070705 coordinate measuring machine. A TP200 measuring probe equipped with a stylus with a ball tip of 2 mm in diameter was used. After having defined the co-ordinate system of the measured object in the manual control mode, the researchers
Determining errors in complex surfaces machining 215 transferred the geometrical model of the object, prepared in the Mastercam system to the PC- DMIS system controlling the measuring machine performance [4]. In order to transfer the geometrical data, the researchers applied the IGES format. With the surface model of the object, the surface scanning procedure was automatically started (Fig. 4). Fig. 4. The imported surface with the system of measuring points, the number of which is established by the user Rys. 4. Zaimportowana powierzchnia wraz z układem punktów pomiarowych, których liczbę użytkownik The area of scanning was defined through setting the boundary values of the u and v parameters (the parameterization directions used in a surface objects group in CAD systems). The number of points for different scanning directions was also established (10 and 15 respectively). As a result, 150 measuring points were generated on the surface panel (Fig. 5). Fig. 5. Surface panel with the 150 measuring points marked Rys. 5. Płat powierzchni z naniesionymi 150 punktami pomiarowymi After accomplishing the measurement phase, the co-ordinate values of the measuring points were obtained. The next step was to determine the Δ total spatial error of the surface machining [2]. In order to determine the values of this error for every measuring point, the obtained co-ordinate values were substituted to the following formula:
216 A. Werner, M. Poniatowska Δ = ( ) 2 ( ) 2 ( ) 2 X T X A + Y T Y A + Z T Z A where: Δ total spatial error of machining, X T, YT, ZT co-ordinates of nominal points, X A, Y A, Z A co-ordinates of measuring points. The calculated results are illustrated in Fig. 6. It results from the graph that the deviation range amounts to 0 0.4 mm. The main reason of occurred machining errors was size (length, diameter, nose radius) and form errors of the mill. Control measurements of it revealed thad the nose radius was 0.2 mm less then nominal. processing error [mm] 0,4 0,35 0,3 0,25 0,2 0,15 0,1 0,05 0 1 13 25 37 49 61 73 85 97 109 121 133 145 measuring point number Fig. 6. Graph illustrating a error of surface machining before correction Rys. 6. Wykres przedstawiający błąd obróbki powierzchni przed korekcją Information relating to the values and distribution of the machining errors was used in correcting the CNC machine tool control programme. The influence of the correction is presented in Fig. 7. 0,4 processing error [mm] 0,35 0,3 0,25 0,2 0,15 0,1 0,05 0 1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145 measuring point number before correction after correction Fig. 7. Graph representing the influence of the correction Rys. 7. Wykres przedstawiający wpływ korekcji
Determining errors in complex surfaces machining 217 4. CONCLUSIONS The coordinate measuring method is a quick and effective tool for determining errors in machining objects with numerically controlled machine tools. It is particularly significant in the case of machining objects of complex spatial shapes. The obtained information on the values and distribution of machining errors may be used to correct machine tools controlling programmes. In consequence, significant increase in the production accuracy is obtained. This fact is very important in the aspect of applying modern production systems aiming at achieving high product quality. REFERENCES [1] Ainsworth M., Ristic M., Brujic D., Cad-Based Measurement Path Planning for Free-Form Shapes Using Contact Probes, Advanced Manufacturing Technology, 2000, 16, p. 23 31. [2] Ikua B. W.,Tanaka H., Obata F., Sakamoto S., Prediction of cutting forces and machining error in ball end milling of curved surfaces I theoretical analysis, Precision Engineering, Journal of the International Societies for Precision Engineering and Nanotechnology, 2001, vol. 25, p. 266 273. [3] Lechniak Z., Werner A., Skalski K., Kędzior K., Methodology of the Off-line Software Compensation for Errors in the Machining Process on the CNC Machine Tool, Journal of Material Processing Technology, 1998, 73, p. 42 48. [4] Min-Yang Y., Jin-ho P., Analysis of setting errors in precision ballscrew machining and the automatic adjustable center, International Journal of Machine Tools & Manufacture, 1998, 38, p. 965 979. Praca wpłynęła do redakcji 24.03.2006 Recenzent: dr hab. inż. Roman Staniek WYZNACZANIE BŁĘDÓW WYKONANIA PRZEDMIOTÓW NA OBRABIARKACH CNC S t r e s z c z e n i e Artykuł dotyczy zagadnienia wyznaczania błędów wykonania części na obrabiarkach CNC, co pozwoli na wprowadzenie pośredniej korekcji (off-line) programu obróbkowego. Elementy obrobiono na frezarce CNC FYS 16N z numerycznym sterowaniem TNC 360 Heidenhain. Dokładność wykonania kontrolowano na współrzędnościowej maszynie pomiarowej MISTRAL Brown&Sharpe. Wyniki pomiarów posłużyły do wyznaczenia błędów obróbki. Informacje dotyczące wartości i rozkładu błędów obróbkowych wykorzystane zostały do skorygowania programów obróbkowych. Słowa kluczowe: błędy obróbki, powierzchnia złożona, pomiary współrzędnościowe