Phenomenological aspects of a modified fragmentation of the ground material

Transcription

1 Phenomenological aspects of a modified fragmentation of the ground material Lucjan Dabrowski, Mieczyslaw Marciniak Warsaw University of Technology, Warsaw, Poland Summary: The main point of this paper is to show that treating the study of the grinding process in a phenomenological manner gives an opportunity to analyze the friction effects in this process. It indicates the importance of having a mechanism for approaching surface irregularities of the tribological couple elements which displace mutually during the grinding. This creates an indirect link between this concept and the elementary tribological criteria [1], as to the relation of an average immersion depth h [µm] of the top of surface roughness into the counter surface to the radius r [µm] of this top. It is consequently related to the maximum or average height parameters of the cutting wheel surface roughness (Rt, Rz), the average grain size a [µm] and the average distance between grains L av in the grinding wheel respectively. It has been concluded in the experimental research that the grinding forces F t, F n and of the energetic intensity material removal K E [mm 3 /N] are dependent on cutting wheel surface roughness parameters. 1. INTRODUCTION Analysis of the authority-based literature on the grinding process compared to the results obtained in the tests indicates a wide divergence. Tests conducted in similar conditions, but with different groups of factors accompanying the decohesion of the grinding material layers do not often give unequivocal or comparable results. This appears especially in the case of tests determining the efficiency, forces and energy of the grinding, when different models of grain shapes, their changes during the grinding, geometrical contact with the ground material and dynamic load conditions have been applied. It is assumed that the energy dissipation in the grinding process may be examined in a similar way as in the machining process with the application of a geometrically defined tool point. This is confirmed by numerous studies applying single grain interacting with machined material in an exactly specified way and the, not hitherto, existing theoretical model of grinding based widely on the achievements of tribology. In grinding metals three distinct processes take place at the interface of the abrasive grain and the workpiece: 1. rubbing where the grain rubs on the work causing elastic and/or plastic deformation in the work material with essentially no material removal. 2. ploughing where the grain causes plastic flow of the work material in the direction of sliding, extruded material being thrown up and broken off along the sides of the groove, resulting in low rates of stock removal. 3. cutting where a fracture takes place in the plastically stressed zone just ahead of the rubbing grain, causing the formation of a chip and resulting in fairly high stock removal rates. It is confirmed in the fundamental research on microcutting with the use of

2 these three processes it could make an important contribution to the explanation of the physical processes that occur during grinding [2]. In order to approach the kinematics conditions in single grit scratching as closely as possible to grinding, the testing device should enable cutting and workpiece speeds comparable to those in grinding (of the shallow depth of cut in scratching 5 30 µm). In fiqure 1 the scratched boundary layer of the workpiece C 105 W1 62 HRC has been presented. The high degree of hardness of this steel leads to high scratch forces, due to which (in comparison to more ductile materials e.g. C 105 W1 annealed and C 105 W1 normalized) more heat is conducted into the workpiece. The temperature becomes so high that recast and drawing zones arise. The longitudinal cut in figure 1 shows clearly a broken off grit tip, along which liquid work material has flowed. Fig. 1. Single grit scratching on a converted surface grinder: examination of structure in C 105 W1 62 HRC in scratching [1] During the scratching of hardened steel the cutting edge becomes thickly coated; this is due to the mechanical clamping of the first chip onto the grit surface area followed by the build-up of a thick layer of chips onto the first one. The hardened steel has the highest degree of heat

3 stress on the cutting edge compared to normalized steel (Fig. 2). In the cutting of hardened steel the splintering of large parts of the grit can be observed. Fig. 2. Lifetime of the grit cutting edge in the scratching of C 105 W1 normalized [1] Fig. 3. Effects of dressing on metal removal parameter. For a given lead, increasing depth of dress a d increases Q w [2] Changes in the scratched cross section during the lifetime of the grit indicate a grit wear. But all operations that mark the grinding process and determine the result of grinding are the consequence of the interaction of the grinding wheel, workpiece and coolant in the contact area. Many of the phenomena that can be observed in grinding, can only be understood when the different influences on the elementary physical processes on the cutting edge are known. It applies particularly to the change of stereometric characteristics of the cutting wheel surface. The roughness studies [3] have determined that variations in the wheel-dressing parameters could influence Q w. Assuming a perfectly-sharp 90-degree pointed diamond, figure 3 shows the effect of increasing the depth of dress with a constant lead. When the depth of dress equals one-half the dress-lead, a theoretical sharp thread is dressed onto each grain; when the a d /f d ratio is 0.05, the grains have very shallow grooves machined into them. Using this physical model, we would expect that a wheel whose grains were dressed with a a d /f d value of 1.0 would act "sharper" and so "cut" better than a wheel dressed at a d /f d = 0.05 (here "a d " is the dressing compensation or depth-of-dress measured radially). Fig. 3 shows that dressing does play a significant role in grinding performance. In each graph, increasing the dressing compensation, a d increased Q w thus, "sharper" dressed wheels do cut faster. Note also that at the same a d /f d ratio, increasing the lead increases Q w.

4 2. EXPERIMENTAL EVALUATION OF DECOHESION IN THE AREA OF CONTACT BETWEEN A CUTTING WHEEL SURFACE AND GROUND MATERIAL The cutting surface of the grinding wheel creates a set of protrusions, whose influence on the machined material depends also on the real conditions of the grinding having an effect on the continuous changes of penetration into the ground material. Tests completed on sections of chips and friction products obtained during grinding of ŁH15 steel (Polish standard) and optical glass, have indicated that decohesion is a result of ductile breaking of both plastic and brittle materials. This means, that dispersion of ground material by working wheel surface occurs due to local plasticizing stresses σ pl, generating relative deformations ε, incurring decohesion of material (Fig. 4a). It was confirmed in our research by using unique method of control for ŁH15 steel dispersion degree in the real machining time, shown in Fig. 4b. Products of micro grinding and friction were collected on the transparent sticky tape forwarded in front of the grinding wheel with velocity v t. Fig. 4. Model of rigid-ideally-plastic material (a) and its dispersion during grinding (b) into particles of various sizes (c). Following sequences of the tape 1, 2, 3 and 4 (Fig. 4c) illustrate degree and intensity of material dispersion after grinding time t 1 = 15 s, t 2 = 45 s, t 3 = 105 s and t 4 = 150 s. Analysis of microscopic geometry of dispersed material particles indicates that in each listed phase of grinding majority of particles are of 1 µm size. Share of typical chips of 5 10 µm size generated in the first phase of grinding estimated with microscopic research (40 times) doesn t exceed 10% and 3% in its second phase. Total efficiency of material decohesion is illustrated by blackening degree of the sticky tape in Fig. 4c. On sequences 3 and 4 of the tape, meaningful reduction of dispersed material volume in form of typical friction products was recorded. Specific pressure and conditions of friction generate condition of complex stresses in the ground material. To a valuate intensity of decohesion of the ground material,

5 based on average plasticizing stresses, the well-known Huber-Mises formula might be used, where main stresses σ 1 and σ 2 depend on component forces F n and F t (Fig. 4b): σ pl = Ft FF t n + Fn = σ k µ µ + 1 (1) Ar( h) where: A r(h) - is a real area of working wheel surface contact, σ k = F n /A r(h), µ = F t /F n. In our research of coefficient of friction during grinding of ŁH15 steel with CrA, SiC and CBN grinding wheels, it was confirmed that µ = ( ). Fig. 5. Conditions of material plasticizing in the area of contact between grinding wheel and a workpiece, described with formula (1); µ = F t /F n coefficient of friction [3] For the established range of the coefficient of friction µ = 0,4 0,9, the relation of plasticizing stresses σ k was described according to formula (1). Graph shown in Fig. 5 indicates that for the mentioned range of coefficient of friction µ, plasticizing stresses in the ground material σ pl are about 10% lower than contact stresses σ k. These data indicate for similarities between grinding and high energizing tribology processes. The test stand shown in Fig. 6 enables changing in condition of contact between cutting wheel surface and machined material with the same grinding parameters. Grinding velocity v c = 23 m/s results from rpm and diameter of the workpiece 1. In this case workpiece 1 has been machined with grinding segment 2. Grinding segment 2 Fig. 6. Stand to test conditions of friction in the grinding area rotates slowly around its axis. Depending on rotational velocity of the grinding segment 2 when standard characteristics (hardness, structure, grain size) are set, conditions of friction, and microcuting in the area of machining are changing.

6 This occurs because an additional motion of grinding wheel in relation to the machined workpiece varies distances between grains Lav in the main direction of motion. The SEM pictures of typical chip sections show differences in fragmentation of material ground in the conventional (Fig. 7a) and in the modified kinematic system (Fig. 7b). b) a) Fig. 7. Fragmentation of material ground in the conventional (a) and modified (b) systems For evaluation of these changes in the tribological categories, the formula of energetic wear intensity was used [5] KE = Vw Ft L é mm3 ù ê J ú ë û (2) where: Vw volume of the material removed within the displacement L. L is a multiplication factor between an average grain distance Lav and a tangential force Ft which depends on an average angular tilt of the roughness of cutting wheel surface expressed with a ratio Rt/Lav. Based on the large number of force Ft samples, results shown in Fig. 8 were recorded variations of force Ft for CrA grinding wheel is similar to that shown in Fig. 8a. Considering time t needed for making the displacement L in the area of contacting tribological elements, formula (2) is Q* KE = w (3) Ft v where Qw* tribological wear [mm3/s], v relative velocity of tribological pair [m/s]. For a grinding velocity v = const., changes of the energetic intensity of wear depend only on the varying efficiency Qw and tangential grinding force Ft Q KE = C w (4) Ft

7 where C constant parameter expressing technical condition of machining. As indicated in the performed research [6], tangential force F t and grinding efficiency Q w depend on the roughness height Rt of the working wheel surface and characteristics of roughness profile described by the relation Rt/L av. In view of above results, formula (4) might by used as a base for characterizing grinding energy absorption e s as follows: K E 1 c1 c2 = C Lav Rt es = (5) where: R t the measured height roughness for of cutting surface wheel correlated with structural composition of the grinding wheel, 1/ 3 74 Lav = a - depends on the characteristic grain size a and the structure number N. 62 2N Fig. 8. Effect of surface roughness parameters of grinding wheel (R t, R t /L av ) on value of force F t for SiC (a) and CBN (b) Fig. 9 shows relation (5) for SiC (a) and CBN (b) grinding wheels. Results for CrA grinding wheel are similar to those shown in Fig. 9a. The increase of the K E ratio for CBN grinding wheels is caused by lower grain concentration, equal to 100, contributing to the enlargement of the area of contact between the matrix and the ground material.

8 a) b) Fig. 9. Effect of surface roughness parameters of wheel (R t, R t /L av ) on the energetic intensity of material removal with SiC (a) and CBN (b). 3. CONCLUSIONS Presented in the paper results of grinding efficiency tests, verified statistically indicate a right connection between the parameters of surface roughness of the cutting wheel, their changes during grinding, and the intensity of fragmentation of the ground material. These results of grinding forces and energy consumption tests are strictly connected with the cutting wheel surface stereometry in the machining, either with or without cooling-lubricating fluids. This new approach to grinding process analysis in aspect of friction in the macro-scale enables complex descriptions of the phenomena related to the decohesion of the material during machining. This method of research based on the high-speed material deformation theory enables easier development of computer simulation programs for grinding process as well as new designs of grinding wheels and new kinematic solutions for machining systems. 4. REFERENCES [1] Czichos H.: Tribology. A system approach to the science and technology of friction, lubrication and wear, Elsevier Scientific Publishing Company, New York. [2] Konig W., Steffens K., Ludewig T.: Single grit tests to reveal the fundamental mechanizm in grinding, Milton G. Shaw Grinding Symposium, PED, Vol. 16, Florida, USA. [3] Hahn R. S., Lindsay R. P.: Principles of grinding, Machinery, July November, (A series made up of 5 parts), New York, USA. [4] Moor D. F.: Principles and applications of tribology, Pergamon Press, Oxford. [5] Dabrowski L., Marciniak M.: Efficiency of Special Segmental Grinding Wheel, Journal of Materials Processing Technology, Elsevier, Vol. 109, No. 3. [6] Marciniak M.: Investigation into grinding process in phenomenological aspect, of Warsaw University of Technology, Works Scientific, Mechanics, No. 178, (in Polish).