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On-line version ISSN 1806-3691
J. Braz. Soc. Mech. Sci. & Eng. vol.25 no.1 Rio de Janeiro Jan./Mar. 2003
Transverse cylindrical grinding of a eutectic alloy
E. C. BianchiI; V. L. Vargas; T. C. Magagnin; R. D. Monici; O. Vicari Filho; P. R. de AguiarII; E. Jannone da SilvaIII
IIUNESP School of Engineering at Bauru SP. Brazil
IIIUSP University of São Paulo at São Carlos SP. Brazil
We report herein on a comparison of the performance of two different grinding wheels (conventional and CBN) in the transverse cylindrical grinding of a eutectic alloy. Three cutting conditions were tested: rough, semi-finishing and finishing. The parameters of evaluation were the cutting force, roughness and wheel wear. The optimal cutting force and roughness values were obtained when grinding with the conventional wheel, due to the superior dressing operation performed under every cutting condition tested. Although the CBN wheel presented the best G ratio values, they were lower than expected owing to the inappropriate dressing operation applied. Excessive wheel corner wear was detected in both wheels, caused by the grinding kinematics (transverse grinding) employed. In terms of cutting force and roughness, the conventional wheel proved to be the better choice under the conditions tested. However, in terms of the G ratio, a cost analysis is crucial to determine whether the differences between the wheels justify the use of the CBN wheel, in which case the dressing operation requires improvement.
Keywords: Grinding, grinding wheel, performance, dressing
Grinding is one of the most widely used finishing processes in the manufacture of precision components. However, technical knowledge about this process is not as widely disseminated as other cutting processes that use tools with definite geometries (turning, drilling, milling, etc). The correct selection of cutting conditions and wheel specifications can optimize the arc length of contact, providing greater possibilities for the removal of material (Bianchi, 1992). The performance of superabrasive wheels, which are modern tools for grinding of hardened steels, is significantly superior to that of conventional wheels.
According to Hitchiner (1999), major changes in the aerospace industry have begun to occur over the last two or three years in the way grinding is employed to process components made of nickel-based superalloys (Inconel, Udimet, Rene, Waspaloy & Hastelloy). Many other industrial sectors that use the aforementioned wear and thermal resistant materials (the automotive industry, among others) seek improvements in the grinding process. This demand has led to the development of new types of cutting fluid and new application methods, improved CBN wheels, new dressing methods and more reliable monitoring systems.
The shift from conventional to CBN wheels to grind hardened materials can be a driving factor for improving grinding performance and workpiece integrity. However, this shift implies a number of requirements, including high cutting speeds, improved balancing systems, suitable machine stiffness, effective dressing systems, appropriate application of cutting fluid, reliable monitoring systems, etc. The adoption of the aforementioned requisites allow for the optimal exploitation of the mechanical and thermal proprieties of CBN grains, which are superior to those of conventional ones.
This article discusses an investigation into the performance of a superabrasive CBN wheel and of a conventional one used for the transverse cylindrical grinding of a superalloy EUTECTIC ECT-NA-4923. Three different grinding conditions (rough, semi-finishing and finishing) were employed and the performances of the two grinding wheels were compared based on the parameters of specific cutting force, roughness and wheel wear.
High Performance Deep Grinding of Inconel
According to Tawakoli and Tavakkoli (1985), Inconel is a type of material classified as difficult to cut owing to its extreme toughness, whose grinding causes hard glazing of the wheel. However, due to its thermal stability and mechanical strength, Inconel is widely used in the aeronautical, automotive and electrical turbine industries.
Because high dimensional precision and high cutting capacity are required to grind hardened materials and the conventional cutting processes (turning, milling, etc) have proved unsatisfactory, Inconel has been ground for quite some time by a process developed specifically for this purpose.
This grinding process is distinguished by continuous dressing, which causes the wheel's surface to be constantly renewed (by the dresser) during the cutting process. Despite the material's tendency to cause glazing of the wheel, this method has long allowed grinding to be used as the cutting process for Inconel. Although the process is, in principle, a costly one, the dresser wears the wheel during the cutting operation, thus keeping it sharp and free from the glazing produced by the workpiece material. Moreover, the material removal rates are very low compared with those of superabrasive wheels, which undergo intensive wear. The aeronautical and electric energy industries have therefore been investing in research on the application of superabrasives to grind Inconel.
Optimal cutting performances can be achieved through a process called high performance deep grinding (HPDG), which consists of the utilization of very deep cutting (with slow advance) and high cutting speeds to produce greater material removal rates. High cutting speeds lead to greater force and cutting power, but also generate greater heat. This heat can be reduced by using a high cutting fluid flow rate and adequate pressure, which contribute to lower the heat generated during cutting and to facilitate the removal of chips from the grinding zone.
The tests conducted by König, Yegenoglu and Stukenholz (1985) showed that the high cutting velocity of CBN wheels allow for a twentyfold increase of the material removal rate compared to that of conventional wheels, thereby representing a significant reduction of the time and costs involved in the process. The final cost of conventional grinding systems is about 30% higher than that of the system based on superabrasive CBN wheels. Because of the significant advance represented this type of abrasive tool, the manufacturing technique using CBN wheels is expected to develop apace. We believe that the grinding performance of DTG (difficult to grind) materials can be significantly improved with CBN wheels, which are more wear resistant and offer better heat dissipation than conventional aluminum oxide grinding wheels, decreasing production times without losses in quality. Optimization of the wheel's truing and dressing, which is possible only with superabrasive CBN wheels, is an important factor to match its surface characteristics to the grinding process requirements.
The wheel dressing parameter strongly affects the grinding wheel's surface characteristics and, thus, the generation of heat during the cutting process (Brinksmeier et al., 1982). Coarse dressing produces a wheel surface that is open and free cutting. In contrast, a close-grained structure results in wheel surfaces that are not free cutting, leading to greater thermal impact and higher residual tensile stresses, although the surface quality is improved in most cases. After dressing, CBN grains reveal a finely structured surface with secondary cutting edges, which ensure good chip formation and allow the cutting fluid to penetrate between the individual edges, thereby improving the conditions of lubrication and cooling. Al2O3 grains, however, often show flat grain surfaces that cause an increase of forces and, thus, friction-induced heat (Brinskmeier, 1986).
The dressing of conventional and superabrasive wheels is very dissimilar. These differences include the dressing procedures and the fracture mechanism of the grain during dressing. The active surface of the wheel after dressing and its sharpness are strongly dependent on a set of factors that include, among others, the ratio between wheel speed and dresser speed (speed ratio qd), depth of the cut per pass, total depth of the cut, transverse rate, and dresser contact width. An ineffective dressing operation can cause a drop in the wheel's grinding performance, leading to undesirable results.
Transverse Cylindrical Grinding
Considering plunge grinding, in which the wheel and the workpiece motions act on the same plane, transverse grinding involves the addition of crossfeed (transverse motion) of the workpiece relative to the grinding wheel in a direction perperdicular to the plane of wheel rotation. Figure 1 illustrates this grinding operation (Malkin, 1989).
In transverse grinding, a transverse velocity component, vt, gives a crossfeed, st, per revolution of the workpiece:
In the absence of wheel wear, the total depth of cut A (Figure 1) is made by the leading edge of the wheel over a width st. Provided that st « dw, the process is very similar to plunge grinding a width st. Owing to wheel wear, however, part of the depth is left behind to be removed during the next rotation of the workpiece by a second wheel area of width st adjacent to the first one. Wheel wear on the second area leaves behind a third width st, and so on. The portion of the wheel closest to the leading edge cuts more material and therefore wears down more rapidly. This unequal wear results in steps across the wheel's width and corner rounding on the leading edge of each width st, as depicted in Figure 1. As the wear proceeds, a chamfer is created on the active surface of the wheel, rendering the grinding process similar to a defined-cutting edge process (such as turning), in which the chamfered region is the main cutting edge (Silva, 1992). In transverse grinding, a profile is created on the wheel's surface with two distinct regions: a much sharper chamfered one, and a less sharp secondary one with worn grains (Figure 2).
Hypothetically, disregarding the corner rounding effect, the transverse grinding operation can be linked to plunge grinding with decreasing depths of cut. If the transverse direction is reversed at the end of the crossfeed path, the same process repeats itself at the opposite edge of the wheel. This leads to crowing wheel, which can cause an error in shape, especially when grinding to a shoulder (Malkin, 1989). Plunge grinding is usually preferable to transverse grinding in production, as it provides for simultaneous grinding over a wider area and is easier to control.
The tests were conducted using an external cylindrical grinder on test specimens, whose cores consisted of ABNT 1020 steel, serving as the base to support the superalloy that would be ground. A 4 mm thick layer of Eutectic ECT-NA-4923, 73 HRc was deposited on the specimen's surface using a MIG welding process, which confers a very high roughness on the welded material. The test specimen was subjected to a previous grinding operation with a conventional wheel to correct its geometrical errors. Figure 3 illustrates the workpiece ground in these tests.
A transverse grinding operation was applied in the tests. The cutting conditions were classified as rough grinding, semi-finish grinding and finish grinding. Table 1 summarizes the input parameters for grinding with both conventional and superabrasive wheels.
Two different types of grinding wheels were used: a conventional wheel (alumina), 19A100SVSB, and a resin bond superabrasive one (CBN), B125R100BK. A single-point-diamond dressing tool was used to dress the conventional wheel. Different dressing overlapping (Ud) values, defined as bd (dressing tool width) divided by sd (dressing lead), were adopted. For the rough and semi-finishing operations, the Ud was equal to 1 (maximum sharpness), while a Ud of 5 was adopted for the finishing operation. A rotary diamond disc was used to true the CBN wheel. A speed ratio, qd, of 0.7 positive was applied. The resin bond wheel was conditioned using an alumina stick. The CBN dressing parameters were the same for each cutting condition tested.
The parameters upon which the grinding wheel performance was evaluated were specific grinding force, roughness and G ratio. Table 2 lists the number of strokes between each roughness measurement.
Results and Discussion
Results of Specific Cutting Force
Figures 4 to 9 illustrate the results of specific cutting force when grinding with conventional and superabrasive wheels. Each graph shows two curves. The "cutting" curve represents the average value of the specific cutting force for the corresponding grinding stroke, while the "spark-out" curve represents the residual force during spark-out in the returning transversal movement prior to a subsequent stroke. Each point of the curve represents one grinding stroke, which is correlated to the volume of material removed during that stroke.
Figures 4 to 9 show that increasing the severity of the operation (finishing to rough) augments, as expected, the grinding forces of both wheels due to the increase of specific material removed.
These figures also show that the cutting force of the conventional wheel displayed a tendency to increase during the tests. One reason for this is the macroeffect produced by dressing. Initially, the macroeffect of the dressing is to create a sharper active surface on the wheel, which facilitates the cutting process, reducing the grinding forces. However, as the cutting operation proceeds, the sharper grains are removed from the bond, leaving behind the underlying layer grains, which are more numerous and less sharp. Because these grains are more solidly bound to the bond, the abrasive grain itself suffers the wear produced by attrition with the workpiece, increasing the force.
Another reason for the tangential cutting force behavior derives from the fact that few abrasive grains actively remove material due to the dressing macroeffect, which causes them to wear down and come loose from the wheel's cutting surface. This causes a high cutting force per grain. In the absence of this macroeffect, the dressing microeffect occurs, with many grains participating in the removal of material. Consequently, the force per grain is lower but the total force is higher owing to the dissipative losses of grains through the generation of heat, acoustic emission, attrition and scratching, among other factors.
Figures 7 to 9 depict a cutting force behavior for the superabrasive wheel that differs considerably from that of the conventional wheel. A fast increase in the cutting force is followed by a drop, a behavior that is attributed to the concept of "active surface roughness".
The surface of any grinding wheel is significantly different from its bulk during the dressing operation. This process fractures and removes abrasive particles and bond, reducing the surface concentration of both. The layer thus affected, which is termed the "active surface layer", may vary in depth from a few microns to over thirty micra (Yonekura and Yokogawa, 1983). For most medium and high stock removal applications (see Figures 8 and 7, respectively), once grinding begins, the abrasive metals chips will preferentially wear the bond and further increase the affected depth. This effect, which is accompanied by a drop in grinding forces and a rise in the surface finish, is most strikingly evident in the first few parts after dressing. Compared to a vitrified CBN wheel, in which the dressing operation creates a round wheel concentric to its axis and preferentially removes the bond from around the abrasive grits, the resin bond CBN wheel tested here has no induced porosity and requires a subsequent operation after truing to expose the abrasive grains. This operation, called "conditioning", creates the wheel's active surface roughness, which is affected by the abrasive wear caused by the chips, which may erode the bond preferentially, offering larger spaces to accommodate the chips and thereby decreasing the forces.
Practically every test showed almost periodically oscillating cutting force values. This was caused by the removal of the workpiece from the machine to measure its roughness and dimensional error. Each time the workpiece was repositioned, some eccentricity caused the force to increase in value, although it stabilized after the defect was corrected.
Results of Roughness
The roughness results demonstrate that, in the case of both the conventional and CBN wheels, the surface quality decreases as the grinding severity increases (the roughness values increase) as a result of the higher material removal rate.
Comparatively lower roughness values were obtained under every cutting condition tested when the conventional wheel was used. The wheels tested here possessed almost the same grit size and concentration. However, the dressing operation to which the conventional wheel was subjected was more suitable. The single-point diamond dresser used, combined with the appropriate dressing overlap (Ud=1 for the rough operation and Ud=5 for finishing), provided the wheel with the appropriate sharpness for the conditions tested. A different dressing condition was applied to the CBN grinding wheel. Using a rotary diamond disc, a speed ratio equal to 0.7 positive was applied, providing the CBN wheel with a higher level of the crushing effect. This fact, coupled with the low level of induced porosity, contributed to higher roughness values.
G ratio Results
Fig. 12 shows the G ratio values of each grinding wheel for the cutting conditions tested.
Figure 12 indicates that the G ratio values for the superabrasive CBN wheel are superior to those of the conventional wheel. The CBN wheel's lower wear was already expected because of the better mechanical proprieties of the CBN wheel's grains than the grains of conventional wheels (higher Knoop hardness). Nevertheless, the values found for the CBN wheel were lower than expected, i.e., only twofold higher than those of the conventional wheel. This fact may be ascribed to the poor dressing applied to the CBN wheel, with a high level of crushing effect combined with inappropriate and excessive conditioning. Improperly anchored grains can easily separate from the bond as the grinding severity increases, resulting in higher wheel wear. Excessive corner wear was detected in both wheels due to the grinding configuration (transverse grinding), which chamfers the wheel surface.
The results of this investigation allow us to draw the following conclusions:
In the grinding operation tested, the conventional grinding wheel presented lower cutting force values for the rough and semi-finishing operations and better roughness results than the CBN wheel due to the more appropriate dressing operation to which it was subjected.
Due to the better mechanical proprieties of its grains, the CBN wheel presented higher G ratio values that the conventional wheel.
The excessive wheel corner wear detected was caused by the grinding kinematics (transverse grinding).
Under the conditions tested here, the conventional grinding wheel proved to be the most appropriate one for use in the rough, semi-finishing and finishing operations, due to its better roughness and force results. In terms of the G ratio, a cost analysis was found to be fundamental to determine whether the differences between the two types of wheels justify the use of the CBN wheel. If so, the dressing operation must be improved.
The authors would like to express their gratitude and appreciation to FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo, Brazil) for its financial support, to Mr. João Teixeira Vargas for his invaluable contribution to this work, and to the company MACOEX.
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Paper accepted August, 2002. Technical Editor: Alisson Rocha Machado