Modelling and Simulation of NC Grinding Processes for Structuring Workpiece Surfaces Taking the Tool Topography and Tool Wear into Account

Due to the process kinematics, NC grinding of machining centers is a flexible machining operation, which allows to machine free-formed workpiece surfaces. One application is the production of forming tools with hard-material coatings, which are applied by thermal spraying techniques like high-velocity-oxygen-fuel (HVOF) or arc spraying for increasing the wear resistance. Due to the high roughness and form deviations of the sprayed coatings, the coated tool cannot directly be applied in forming operations. Therefore, a subsequent finishing process, e.g. by grinding, is required.

An advantage of grinding the forming tools is the possibility to create tribologically applicable surface structures. By structuring functional surfaces in different ways, the forming process can be optimized by influencing the material flow in a suitable way. However, the surface topographies resulting from the grinding process depend on several factors, e.g., the process parameters, the tools, and especially wear of the grains and the bonding.

In order to specifically optimize the grinding process anyways, simulation systems are being developed, which allow to predict the process forces and the resulting surface topographies. In contrast to finite element or molecular dynamics-based solutions, geometric-physical approaches in combination with empirical force models allow simulating large sections of grinding processes in an efficient way. The surface topographies are strongly influenced by the distribution of the individual grains on the tool. Therefore, not only the macroscopic engagement situation is analyzed, but each individual grain is modelled separately (fig. a). The shape of CBN or diamond grains can be represented using the constructive solid geometry (CSG) modelling technique as the intersection of octahedrons, hexahedrons, and tetrahedrons of different sizes. The workpiece surface can be modelled using heightfields, which are intersected with each individual grain in each simulation step for modelling material removal. By analyzing the removed material, the undeformed chip shapes and undeformed chip areas can be calculated for each grain and, thus, process forces can be determined based on these values and empirical force models.

An important effect influencing the surface topographies is the wear of the grains. However, idealized CSG models are not suitable to model the shape of worn grains, which can become increasingly complex. Therefore, a new, more flexible simulation approach based on point clouds is developed. Based on experimental wear investigations, this flexible grain model is parameterized for different states of tool wear. This is achieved by digitizing the same grains after small amounts of process time (fig. b). Using these measurements, a wear-dependent, stochastic grain model is defined, which can be used in combination with a stochastic grain distribution to model arbitrary tool shapes without conducting further wear experiments.

Fig.: Simulation model of the cylindrical grinding tool with the resulting surface topography (a) and measurements of two exemplary grains in three different states of tool wear (b).

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