Laser cleaning and dressing of vitrified grinding wheels
Introduction
Grinding wheels are used for grinding materials with poor machinability because of their longer life, high grinding efficiency, and dimensional stability [1]. The grinding wheel also gets loaded with metal chips during machining [2]. The existent grinding tools wear at a very high rate and also generate high frictional heat at the abrasive grain-metal chip contact region, which leads to surface and sub-surface damage in the ground components. Also, there is a loss of form tolerance and dimensional stability as grinding progresses over a large number of grinding passes [3].
Ideally, the abrasive particles on the surface of the grinding wheel should get automatically sharpened when worn out, by either entirely detaching from the wheel face or by fracture, thus exposing new particles with sharper cutting edges.
However, in practice both geometric and functional characteristics of the grinding wheel have to be restored periodically by dressing [2]. Dressing is a sharpening operation designed to generate a particular surface topography on the cutting face of the wheel. Dressing of the worn out grinding wheel surface is done by various means to modify its surface topography, which in turn improves grinding efficiency [4].
Dressing significantly affects the quality of the ground product, as characterized by its size and shape, surface roughness and integrity [5]. The conventional contact-type methods, like mechanical dressing using a diamond dresser, results in excessive grinding wheel material loss.
The bond fracture and abrasive grain break-off, as a result of crushing of the dresser are the material removal mechanisms for contact type processes. In fact, only 10% of the wheel by volume is removed during actual grinding, while the rest is removed during dressing operations [6]. Mechanical dressing, though effective, also induces stresses and causes deep cracks and undercuts. These factors eventually cause loosening of the chunks of grains and reduce the number of effective cutting edges [2].
The conventional processes do not produce consistent grinding results due to dresser wear which affects the wheel surface topography and its performance in grinding. A worn-out dresser cannot produce sufficient protrusion of cutting grain edges [1]. To obtain consistent grinding outputs, either the dresser geometry has to be maintained or suitable dressing conditions corresponding to modified dresser geometry have to be selected, which are impossible in contact type processes and also complicated because of dresser wear [4]. Yet in order to obtain consistent grinding results, a dressing procedure, which is reproducible in nature, is essential.
To achieve these requirements, methods have been developed to maintain an optimized grinding wheel. These methods include: slurry and steel roll application, pressurized jet dressing using abrasive slurries, and air jet abrasive dressing. In air jet dressing, the dressing rate is controlled in three ways: (a) by controlling the pressure of the mixed air, (b) by controlling the ejecting nozzle traverse speed, and (c) by controlling the number of traverse passes across the grinding wheel. Therefore, the dressing rate will control the optimum grain protrusion height.
High power lasers that are currently used as a non-contact type machining tool for various manufacturing applications such as welding, drilling, cutting, etc., can also be used as a non-contact type dressing tool. The salient features of a laser include high intensity fluence, directionality, and spatial coherence, which can be used to process hard and brittle materials efficiently. Laser induced thermal processing leads to effects such as melting, vaporization, and plasma formation on the material of the grinding wheel, which can be exploited during the dressing procedure.
When a laser beam irradiates the surface of a grinding wheel, it may be considered that the energy flows in one direction in a semi-infinite body. If there is no convection, or heat generation, the basic equation governing the flow of heat is,Where T is the temperature in the grinding wheel, z the depth from the wheel surface, α the thermal diffusivity of the grinding wheel, and t is the time after laser irradiation commences. If it is assumed that the laser power flux input into the wheel is q, with no radiant heat loss or melting, then the solution to Eq. (2) is,Where K is the thermal conductivity of the grinding wheel material, and ierfc is the integral of the complementary error function. Eq. (2) indicates that both q and t contribute to elevating the temperature of the surface of the grinding wheel. The depth of laser energy penetrated into the wheel surface is constrained by the duration of the laser pulse. Increasing irradiation time will allow the laser energy to penetrate deeper into the grinding wheel. For the purpose of laser dressing, a higher temperature is needed in order to remove metal chips and re-shape individual grinding grains.
During laser dressing, the wheel surface topography of the grinding wheel is modified by melting of the material and subsequent re-solidification of a portion of the molten layer. During the process, rapid heating and cooling induces cracks in the re-solidified layer. The microcracks help remove the re-melted layer during grinding after a few initial grinding strokes, which then exposes new cutting edges. In laser dressing, the grinding wheel is subjected to high power laser intensity, which produces craters on the surface and also induces microcracks in the re-cast molten layer. By using different laser powers during dressing, the extent of the re-cast molten layer can be controlled and controlling the dressing feed can vary the surface topography. Laser dressing removes the wheel material by ablation of the bonding material to expose sharp grains. When the heating time and energy density is selected correctly, the vitrified bond (glass phase) of the wheel is softened and even melted thus facilitating the removal of the bonding material [1].
Dressing of the grinding wheel by laser generates well-defined grooves and tracks on the surface of the wheel. Essentially, laser dressing produces microcutting edges due to the formation of microcracks on the worn-out grains. When these craters are formed on the bond, the grits are loosened and subsequently removed due to insufficient volume of load surrounding the grain [4]. Laser dressing thus has the advantage of being a non-contact type process in which selective removal of clogged material alone is possible by appropriate focusing of the laser beam on the selected portions of the wheel surface. Also the material wastage in terms of the debris produced during dressing operation, can be substantially reduced by the use of a laser as a dressing tool. Thus environmentally benign grinding operations can be made possible, which will help reduce the problem of disposing contaminated grinding debris.
There are several inherent advantages associated with the use of laser for dressing applications. Laser dressing is a very fast process and it can be easily automated. Also, selective removal of the clogged material alone is possible and desired surface structure (roughness, grain morphology and porosity) can be generated. Furthermore, consistent dressing conditions can be produced by the use of laser and this can help achieve grinding reproducibility. As the laser beam can be delivered using a fiber optic cable, remote dressing operation without discontinuation of the grinding process during laser dressing is possible. Thus the downtime in the grinding operation associated with conventional methods, can either be eliminated or substantially reduced in laser dressing.
Earlier studies [2], [4], [7] used pulsed laser to compare the laser dressing process with conventional mechanical dressing methods. The pulsed laser powers used were of the order of 1–5 × 1010 W/m2 and most of the work concentrated on comparison of the grinding performance of laser dressed wheels with that of diamond dressed ones. Laser-assisted simultaneous truing and dressing has also been attempted, to overcome the problems associated with mechanical dressing [8]. Though useful, the studies did not deal with the nature of physical changes taking place in the grinding wheel during interaction with laser energy, which happens to be a fundamental aspect to contribute towards its dressing performance.
Prior to any dressing operation, the grinding wheel tends to load with metal chips that need to be removed. Wheel loading is one of the most common problems in grinding operations, particularly for grinding aerospace materials. As grinding continues, removed chips may adhere in the spaces between abrasive grains and deteriorate the cutting ability of the grinding wheel. A common method used to prevent the wheel from loading is achieved by delivering a large amount of coolant to the grinding zone. However, this consumes huge amounts of energy in coolant delivery, especially for high speed grinding processes. Maintaining and disposing of coolant is also an environmental issue and the costs involved are substantial. Another method, which is often used, is to remove loaded chip materials by dressing the wheel periodically, to restore a sharp wheel surface. However, dressing of grinding wheels with diamond not only causes excessive wheel loss but also interrupts grinding during dressing. In addition, the dresser wears away with time due to its direct contact with the wheel surface. Frequent use of dressing wheels is also not acceptable for super-abrasive wheels whose cost is considerably more than conventional grinding wheels. Lasers have been successfully applied to material removal processes such as laser cutting, and drilling. Laser cleaning techniques have been used in removing pollution layers from valuable artifacts and without damaging the delicate patina of the substrate [9]. This suggests that a laser cleaning technique may provide a solution to prevent or minimize wheel loading and maintain a sharp wheel surface. Research on utilizing lasers to dress grinding wheel surfaces has been reported [2], [4], [10]. The results demonstrated that the use of a laser can be an option used in grinding wheel dressing. However, laser dressing did not show a great advantage over conventional dressing. The possible reason is that a high powered laser not only removes wheel bonding materials but also damages the abrasive grains which then causes higher grinding forces to occur and results in higher wheel wear. A large degree of wheel wear is not acceptable for super-abrasive grinding wheels. It should be noted that the research has also pointed out that clogged chips can be removed through evaporation caused by laser radiation [2], [4]. This suggests that a laser cleaning technique may be used to prevent, or minimize, wheel loading and maintain the sharp wheel surface created by techniques such as touch dressing. By continually irradiating the loaded wheel surface with a particular degree of laser energy, it is possible to remove clogged chips without deteriorating the wheel surface.
Section snippets
Materials
Chromium-doped alumina was selected as the abrasive grain material as it is commonly used for grinding tough engineering materials such as micro-alloyed steels, and is used in a wide variety of industrial applications such as roll grinding, camshaft grinding, crankshaft grinding, and other automotive component grinding operations [3]. The abrasive grains were bonded together in the form of a cylindrical grinding wheel by mixing a bonding system with the abrasive grains. The grains were coated
Laser cleaning
A grinding trial was carried out on a cutting tool grinding machine to obtain a loaded wheel surface for further laser cleaning experiments. A vitrified grinding wheel was used to grind samples of inconel. Fig. 2 shows inconel material (bright shaded areas) attached to the surface of the grinding wheel after a sample had been ground.
Grains of grinding wheel appear to be transparent using reflected light. A selection of control parameters is used for the laser cleaning experiments. Fig. 3 shows
Conclusion
A feasibility study has been carried out to investigate the application of laser cleaning technology to the grinding process. By irradiating a laser beam on to the surface of a loaded grinding wheel using carefully selected control parameters, it is possible to remove clogged metal chips without deteriorating the wheel's cutting surface. Both fusion and evaporation of chips are important for the laser cleaning process. Suitable laser parameters are identified and it was suggested that high
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