New Kinematic in Dressing of Grinding Wheels

Transcription

1 Proceedings of the ASME 2013 International Mechanical Engineering Congress & Exposition IMECE2013 November 15-21, 2013, San Diego, California, USA IMECE New Kinematic in Dressing of Grinding Wheels T. Tawakoli Institute of Grinding and Precision Technology, KSF, Furtwangen University, Villingen-Schwenningen 78054, Germany A. Daneshi Institute of Grinding and Precision Technology, KSF, Furtwangen University, Villingen-Schwenningen 78054, Germany Abstract Grinding is the most important process in manufacturing of many precision components. The grinding wheel topography that is generated through dressing operation can directly influence the grinding forces and material removal mechanism. A new dressing concept is addressed in this investigation in order to reach the optimum chip formation condition. The innovative dressing profile roller, T Dress, creates a new structure on the wheel owing to which remarkable reduction in grinding forces occurs. The experiments prove about 40% reduction in grinding forces with no great difference in the ground surface roughness values when dressing with the T Dress. Dressing costs reduction owing to less diamond grits used on the roller body as well as about 25% lower dressing force are other advantages which were achieved with utilizing the T-Dress. Keywords: Grinding, T-Dress, Wheel Structuring INTRODUCTION The efficiency of grinding process is largely affected by conditioning and dressing of the grinding wheels owing to the topography which is created on the grinding wheel surface. Hence, moving towards optimum grinding operation by changing the grinding wheel topography would be of great interest in this regard [1]. Generation of new structures on grinding wheels to reach a process with optimum chip formation mechanism has lately become a subject of interest for researchers. The motivation for this is performing a grinding operation with relatively lower grinding forces and consequently lower heat generation in the grinding contact zone. This is contributed to the less number of active grains participating in material removal process when grinding with the structured wheels [2, 3]. Special attentions from different points of view were paid to the use of structured grinding wheels. Number of researchers [4-6] used a grinding wheel with premanufactured circumferential slots and reported remarkable reductions in the grinding forces and consumed energy Okuyama et al. [7] studied the effect of axially grooved grinding wheels on the surface grinding process. They examined the use of 4, 12, and 36 grooves around the periphery of the grinding wheel and reported that the maximum heat transfer coefficient increases with the number of grooves. Rabiey and Tawakoli [8, 9] used the structured wheels for dry grinding in both surface and cylindrical grinding applications. In their investigation, a pre-manufactured laser structured wheel was used for surface grinding. For cylindrical grinding, circumferential grooves were made on the wheel during the process. They showed that structuring of grinding wheels leads to remarkable reductions in grinding forces and specific energy, while the surface roughness values increase The single diamond was used by some researchers to generate circumferential grooves on the wheel surface [4-6, 2, 9]. The authors generated number of circumferential grooves on the grinding wheel by a diamond form roller (Fig.1). In this technique, the diamond form roller is moved 1

2 axially so that the dressing overlap ratio, U d, is smaller than 1. Although this method can be employed during the grinding process, the helical grooves cannot be made on the grinding wheel by this method. Fig.2.The two dressing profile rollers used in this research: a) conventional electroplated diamond profile roller D427 which is fully covered by diamond grains, b) CVD T-Dress profile roller which is partially covered by CVD diamond segments. Fig.1. Generation of circumferential grooves on the grinding wheel surface by means of a form roller. Although few researches were conducted on applying the structured wheels in recent years, no method has been introduced to create helical grooves with different geometries on the wheel surface during grinding so far. A new method of dressing through which the desired structures can be produced on the grinding wheel surface during the grinding process is presented in this article. STRUCTURING BY T-DRESS The design of T-Dress is based on a metallic dressing body, on which one or more CVD (chemical vapor deposition) diamond segments with a definite form and geometry may be set. With the T-Dress, the contact between the dressing tool and grinding wheel, in contrast to the conventional profile rollers, not a line but in theory only takes place at a point. The dressing forces and therefore deformation and thermal developments are therefore at T- Dress rollers lower than those in conventional profile rollers. Furthermore, the manufacturing cost of this tool is significantly lower than the cost of manufacture of a full profile roller due to the much less diamonds used in T- Dress. Fig.2 illustrates the conventional diamond profile roller and T-Dress profile roller which were used in this investigation. The conventional diamond roller is fully covered by diamond grains, while the T-Dress is partially covered by the diamond segments. This makes interruptions during removing from the grinding wheel. Therefore, the structures are created on the grinding wheel. With this new dressing method, macro structures are created on the grinding wheel surface. A schematic of structuring with the T-Dress profile roller is illustrated in Fig.3. Fig.3. Schematic of Structuring of a grinding wheel by the T- Dress. The structures are generated by the T-Dress CVD segments through interrupted removing from the grinding wheel by therm. w d : the width of the diamond segments on T-Dress roller, L s : the dressed width on the wheel surface, α: the angle between the diamond segments on the T-Dress roller and the roller axis, β: the angle between the dressed width on the grinding wheel and the grinding wheel axis, λ: the angular shift of the T-Dress diamond segments on the grinding wheel. The CVD segments set on the T-Dress roller remove from the wheel surface the width (L s ) with the angle (β) shown in Fig.3. However, due to the angular difference between the profile abrasive segments and dresser roller axis (α) as well as the speed ratio (q d ) between the grinding wheel speed and T- Dress roller speed, the L s and β are different from those on the dresser roller, i.e. w d and α. This makes the T-Dress enable to create various structures with different dimensions and angles on the grinding wheel surface. GRINDING WITH STRUCTURED WHEELS The topography of the grinding wheel surface plays an important role in the bi-lateral interaction of the wheel and 2

3 workpiece in terms of chip formation, heat generation, workpiece surface quality and wheel wear. The cutting process in grinding is the sum of the singular microscopic cutting process whose temporal and local super position leads to macroscopic material removal [10]. The chip formation for metals occurs when the attack angle, the angle through which the grain penetrates the material, is more than a critical value (about 40 degree), otherwise the grains just rub and plough the material and there is no cutting action. For many grains which come to contact with the workpiece the attack angle is so small that no chip formation occurs. The ratio of kinematic grains to the total number of grains on the wheel is very low even less than 1% [1, 11]. At a definite set of grinding parameters, the rest of the grains are divided into two groups. The first group is the grains which have no contact with the workpiece during the process. The second group is the grains which have contact with workpiece in form of sliding (rubbing) and ploughing without chip formation. These grains are active as they are in contact with the workpiece and have a great role in the generation of heat, energy and also grinding forces [12]. The number of cutting edges which take part in real cutting depends on the static distribution of cutting edges on the wheel as well as kinematic of the grinding process. The static number of cutting edges, which is the total number of cutting edges on the wheel is determined by the initial wheel structure and characteristics, as well as by the wheel dressing operation. The high efficiency of the grinding performance may be achieved by optimization of the grinding wheel surface topography and chip formation [13]. Any topography change has a significant influence on the total grinding process [1]. With structuring of the wheel by special conditioning, it is possible to change its micro and macro-topography so that the grits which do not apply real cutting but instead cause ploughing and rubbing action are out of contact as much as possible. The reduction of static cutting edges by definite conditioning of the grinding wheel produces a specific macro-structure on the wheel surface, giving more chance to each cutting edge to perform real cutting. With this new structure, the maximum chip thickness will be increased. This maximum chip thickness has a key role in grinding process. The larger maximum chip thickness, the lower specific energy of grinding. EXPERIMENTAL INVESTIGATION The experiments were carried out on a high efficiency surface grinding machine with a continuous dressing (CD) unit. The grinding forces are measured by the piezoelectric dynamometer Kistler 9257B which is mounted beneath the workpiece and on the machine table. The roughness values of the ground surfaces were measured by a portable roughness measurement system Hommel Tester T8000. Two different dressing rollers, a conventional profile roller which is fully covered by diamond abrasive grits and a CVD T- Dress roller with similar profiles were used as shown in Fig.2. Fig. 4 shows the structures in different dimensions which were created by the CVD T-Dress. The grinding forces and surface roughness were measured when grinding with the wheels dressed by the T-Dress profile roller and conventional profile roller. The results of grinding forces and surface roughness when using Al 2 O 3 wheels are shown in figures 5 and 6. Fig.4. Various structures with different dimensions may be made on the grinding wheel surface by the T-Dress As it is seen, the normal grinding forces were higher than tangential grinding forces as expected and both normal and tangential forces were reduced considerably (33-46% reduction in tangential and 25-35% reduction in Normal forces) when the grinding wheel was structured by the CVD T-Dress profile roller (Fig.5). This is due to the fact that the number of active grains in the wheel-workpiece contact zone is reduced. The similar ground surface roughness (less than 6% difference) when dressing by the two different rollers is the interesting result which is seen in figure 6.This shows that using CVD T- Dress instead of the conventional dresser, the grinding process can be performed with much lower forces and almost the same surface roughness values. After every test with the structured wheel, the wheel was flattened by removing the structures by the T-Dress rollers. Afterwards, the new structures were generated by the T-Dress for the next experiment. Thus, the grains which come in contact during grinding with structured wheels have been dressed by the CVD T-Dress during the flattening process. In other words, the macro-topography of the grinding wheels is determined by structuring, while its microtopography is created by the CVD T-Dress during flattening. This micro-topography that governs the material removal mechanism can be quite different from that generated by conventional profile roller. Hence, despite the macro-structures the surface roughness remains in the same range as shown in figure 6. 3

4 Fig.5. Tangential and normal grinding forces are reduced 30-40% and 25-35% when using T-Dress compared to the case dressing with conventional dressing roller Fig.7. Dressing radial force is reduced with T-Dress compared to the conventional dressing roller. However, the wear rate is higher for T-Dress. Nevertheless, the 90% lower diamonds on the T- Dress rationalizes economically the application of T-Dress The measured radial wear of the dressing roller (Δr R ) shows that the T-Dress has about 30% higher wear rate. This is due to the fact that during dressing with T-Dress, each single diamond grain removes higher volume of the grinding wheel grits compared to the diamond grains on the conventional profile roller. Nevertheless, the 90% less diamonds consumed on the T-Dress than those on the conventional dresser rationalizes economically the application of T-Dress instead of the conventional dressing roller. SAMMARY This investigation is concluded as follows: Fig.6. Although significant reductions in grinding forces occurs when using T-Dress, no significant changes in ground workpiece surface roughness is seen. During dressing with conventional diamond roller, the contact between the diamond roller and grinding wheel is linear. However, the diamond segments on the T-Dress make point contact with the grinding wheel grits and therefore the dressing forces decrease. The experimental results shown in Fig.7 demonstrate that the dressing radial force (F rd ) is reduced about 25% using the T-Dress profile roller as expected. This is due to the less number of the diamond grains contacting the grinding wheel grits through the point contact as described. Due to the lower dressing force, deflection and bending during dressing process is reduced and therefore, the precision of dressing operation increase. This can be important when dressing of precision profiles on the grinding wheel. Two types of diamond profile rollers including a CVD T-Dress and a conventional profile roller which is fully covered by diamond grits were used in this research. The experiments with corundum wheels with the material removal rates between 2 and 8 mm 3 /mm s proved that the grinding forces decrease in the range of 25 45% when the grinding wheel is dressed by the T-Dress instead of the conventional profile roller. Similar ground surface roughness obtained by the CVD T-Dress and conventional roller is the interesting result. A 25% reduction in dressing radial forces occurred when using T-Dress instead of normal dresser. Hence, the deflection and bending during dressing operation is reduced when using the T-Dress roller. Although the radial wear of the T-dress is more than conventional dresser, the much less diamond consumed on the T-Dress, as well as, the lower dressing force rationalize the use of T-Dress. 4

5 REFERENCES [1] Tawakoli, T.; Rabiey, M., 2006, Trockenschleifen, Möglichkeiten und Grenzen, 6. Seminar Moderne Schleiftechnologie und Feinstbearbeitung, Stuttgart. [2] Tawakoli, T.; Westkämper, E.; Rabiey, M, 2007, Dry grinding by special conditioning, International Journal of Advanced Manufacturing Technology, 33: [4] Nakayama, K., Takagi, J., Abe, T., 1977, Grinding wheel with helical grooves an attempt to improve the grinding performance. Manufacturing Technology, 27th General Assembly of CIRP 25 (1), [5] Verkerk, J., 1979, Slotted wheels to avoid cracks in precision grinding. In: Proc. Annu. Abrasive Engineering Society Conference/Exhibition, Pittsburgh, Pennsylvania May 14 16, pp [6] Al-Mokhtar, O. M., Bauer, R., Warkentin. A, 2013, Application of shallow circumferential grooved wheels to creepfeed grinding. Journal of Materials Processing Technology 213: [7] Okuyama, S., Nakamura, Y., Kawamura, S., 1993, Cooling action of grinding fluid in shallow grinding. International Journal of Machine Tools and Manufacture 33, [8] Rabiey, M., 2010, Dry Grinding with CBN Wheels, The Effect of Structuring, Dissertation. [9] Tawakoli, T., Heisel, U., Lee, D. H., Daneshi, A., 2012, An Experimental Investigation on the Characteristics of Cylindrical Plunge Dry Grinding with Structured CBN Wheels, 5Th CIRP Conference on High Performance Cutting, Procedia CIRP [10] Marinescu, I.D.; Hitchinger, M.; Uhlmann, E.; Rowe, W.B.; Inasak, I.: Handbook of machining with grinding wheels, CRC Press, (2007). [11] Yegenoglu, K., 1986, Berechnung von Topographiekenngrößen, Dissertation, RWTH Aachen. [12] Tönshoff, H.K.; Peters, J., 1992, Inasaki, T.; Paul, T.: Modelling and Simulation of Grinding Processes, Annals of the CIRP, Vol. 41/2, pp [13] Tawakoli, T., Rabiey, M., 2008, An innovative concept and its effects on wheel surface topography in dry grinding by resin and vitrified bond CBN wheel, Machining Science and Technology, 12/4: