Lapping machine and quality control: FEM of abrasion problem

Most of the equipment we use every day contain a mechanical component called bearing. The reason for their use lies in the possibility of drastically reducing the friction between rotating components. This is achieved by replacing the sliding friction with rolling friction. A correct geometry and a good surface finish allow to have a high-quality product. Ensuring a consistently high level of quality is the main objective for leading manufacturers of bearing components. The lapping process represents a fundamental step in the production of steel balls for ball bearings. It is the last operation with material removal of the production chain. A high relevance is given to the quality of the product at the end of this step. Quality concerns both the dimension tolerances both the geometric variations with respect to the perfect spherical shape. A high level of control is then required. This could be achieved through the automation of the process. To reach this target, some intermediate paces are needed. Improvement in the knowledge of the abrasion process is the first fundamental one. The lapping operation allows to work the balls inside the groove obtained on two discs, plate and grinding wheel. The latter is rotated and pressed against the plate. The combined effect of speed and pressure of contact allows the ball to be machined. The amount of material which is removed is minimal. It is only needed to recover on residual superficial defects or to eliminate geometric error. The factors influencing the operation are different, some are controllable, and others are not. Pressure, velocity and lubricant flow rate are controlled by the operator. Vibrations can influence the efficiency of machining. Many of these parameters can reduce the quality of the outgoing product and at same time reduce the life of the machine components. Deepen the knowledge of this machining condition is fundamental to achieve an improvement in the efficiency of production and in the quality of final products. To do that, the decomposition of a very complex problem is needed. Finite Elements Method is a powerful tool that can be used for the purpose of gaining insight into a process. The great advantage offered by Finite Element Analysis is the possibility of solving problems that otherwise could not be solved analytically or that would require excessive resources to be solved experimentally. In the specific case, this method gives the possibility of realizing simplified models where the investigated parameters can be kept under control as well as the disturbance parameters can be easily removed. As a first model, a static contact analysis has been performed together with a comparison with the theoretical results predicted by Hertz’s theory. The second model realized refers to the pin-on-disc configuration. Sliding contact gives the possibility to introduce the abrasive wear on the process. Archard wear equation is described and implemented in Ansys. The simulated results are then compared with experimental tests performed on pin-on-disc tribometer. The third step models a pure rolling contact condition. This kind of study wants to represent a starting point to improve the knowledge about the lapping process. Indeed, it give the possibility to insulate different contributions from complex machining conditions. In parallel to the Finite Element Analysis some experimental evaluations on lapping are presented focusing on wear which occurs within the machine.