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Application of variable speed strategies for the optimization of the grinding process

The main objective of this research, in which the present doctoral thesis is contextualized, is the development of strategies for the optimization of the set-up and the analysis of grinding processes, focusing on infeed configurations of cylindrical and centerless grinding and throughfeed and traverse configurations of centerless and cylindrical grinding respectively. Those strategies are based on the application of commanded variable speeds over the main parameters that govern the process: the feed rate, the workpiece speed (or the regulating wheel speed for centerless grinding) and the grinding wheel speed.

 

According to previous studies of the workteam and based on the current state of the art, it has been noticed that the application of a variable workpiece speed is an efficient technology in order to suppress or reduce the dynamic instabilities of the process generated by self-excited vibrations or chatter. To put into practice this technology, it is necessary to know the optimal parameters of the variation (amplitude and frequency for a sinusoidal variation), task that is done so far by methods of trial and error or by using simulation models in the time-domain.

 

As a first work within this thesis, a previous work by the research group comprised of IK4-Ideko and Mondragon Unibertsitatea presented by David Barrenetxea has been completed, in which the variable workpiece speed was applied successfully for suppression of chatter in infeed centerless grinding. To that end and applying a similar methodology, the efficiency of this technology has been validated in the throughfeed configuration of the centerless grinding process, also analyzing the influence in the process behavior (grinding forces, roughness,…) due to the direct relation between the regulating wheel speed and the traverse feed rate.

 

It has been noticed that using a time-domain modeling for the stability analysis with variable parameters supposes a high computing cost. Therefore, the next step of this thesis has focused on the development of a methodology for the stability analysis with variable parameters by means of semidiscretization method. By using this technique a better efficiency related to computing cost is achieved without being detrimental to the results accuracy. This method is based on the analysis of the eigenvalues of the transition matrix that relates the profile defects of the workpiece between two consecutive revolutions and it has been applied and validated in different grinding process configurations. By means of the development of a software based on this methodology, a tool for the achievement of the optimal combinations of workpiece speed variation parameters (amplitude and frequency) to avoid chatter is available. These optimal combinations correspond to frequencies below one hertz and amplitudes higher than a ten per cent of the nominal speed in the analyzed study cases.

 

On the other hand and taking advantage of the developed technology for the continuous variation of the axis controlled by the machine CNC, a software has been implemented with which an ensemble variation of the main parameters of the grinding process is carried out: feed rate, workpiece speed and grinding wheel speed. With this software, infeed cycles can be defined for centerless and cylindrical grinding processes. Prior to the implementation of the software in a grinding machine, a simulation environment has been developed in which different variation strategies can be analyzed and defined in order to achieve optimal grinding cycles according to a theoretical analysis of the influence of the variations in the process behavior, taking into account factors such as surface finish, dimensional tolerances, grinding forces and power, thermal damage or the process stability.

Once the combined variation of the three proposed parameters has been defined, these variation strategies have been included in a CNC software and experimental trials have been carried out in which variable cycles and conventional ones (with different constant values of feed rate and workpiece speed) equivalent in cycle time and stock removal have been compared. In these trials, improvements in the final quality of the part have been noticed and the possibility of reducing the cycle time (increasing the productivity) has been analyzed, achieving the same workpiece quality. Besides, it is also remarkable the simplicity of using this software, since it is not necessary to define every stage of the conventional cycle, related to the feed rates, the stocks and the workpiece speeds.

 

Next, the references of the published papers that are part of this thesis by compendium of articles are presented chronologically, in which the developments of the proposed research are exposed. Later in this thesis, the non-published additional work, where defined objectives are completed, is described. It is also noticeable the patent in process to protect the idea of grinding cycle definition with continuous variation of main process parameters.

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