3.1.1 Introduction
Knowledge of the basic principles of a process is a prerequisite for its effective improvement and optimization. During grinding, surface formation is one of the basic mechanisms. In the case of cutting with geometrically defined cutting edges, a singular engagement of the cutting edge defines the removal mechanism. The consequent removal mechanisms can be directly observed by means of modern investigation methods.
In the case of grinding, the investigation of removal mechanisms is complicated due to many different factors. The first problem is posed by the specification of the tool. The abrasive grains are three-dimensional and statistically distributed in the volume of the grinding wheel. The geometry of the single cutting edges is complex. Moreover, there is a partially simultaneous engagement of the cutting edges involved in the process. The surface formation is the sum of these interdependent cutting edge engagements, which are distributed stochastically. Furthermore, the chip formation during grinding takes place within a range of a few microns. The small chip sizes make the observation even more difficult.
3.1.2 Defining Basic Behavior
In spite of the complexity, some statements can be made on the removal mechanisms, surface formation, and the wear behavior during grinding. Analogy tests and theoretical considerations on the basis of the results of physical and chemical investigations are used for this purpose. In the past few years, chip and surface formation have been modeled with the help of high-performance computers and enhanced simulation processes.
• Indentation tests—In analogy tests, the engagement of the cutting edge in the material surface is investigated first. The advantage of this method is that single cutting edges can be investigated before and after the process, and their geometry is known. With the help of so-called indentation tests with singular cutting edges, the material behavior to a static stress can be observed without the influence of the movement components typical for grinding. On the basis of these indentation tests, elastic and plastic behavior as well as crack formation can be observed in the case of brittle-hard materials at the moment the cutting edge penetrates the material.
• Scratch tests—A further method is the investigation of the removal mechanisms during scratching with single cutting edges, which allows the accurate examination of the geometry and the wear of the cutting edges. Contrary to the indentation tests, there is chip formation during this test method. Furthermore, the influence of different cooling lubricants can be investigated.
• Cutting edge geometry—A further prerequisite of a comprehensive understanding of the material removal during grinding is the geometrical specification of the single cutting edges. This mainly takes place in analogy to the geometrical relations at geometrically defined cutting edges.
• Thermal and mechanical properties—The thermal and mechanical characteristics of the active partners of the grinding process also have a significant influence. Heat is generated in the working zone through friction. This contact zone temperature influences the mechanical characteristics of the workpiece as well as of the tool.
• Surface modification—As a result of the removal mechanisms, the subsurface of the machined workpiece is influenced by the grinding wheel due to the mechanical stress. Residual stresses develop depending on the specification of the machined workpiece material. These stresses can have a positive effect on component characteristics; hence, they are in some cases specifically induced. Due to the mechanical stresses, cracks or structural and phase changes may occur on the subsurface that have a negative effect on the component characteristics.
This shows the complexity of the mechanisms of surface formation during grinding. The better the surface formation is known, the more specifically and accurately the process parameters, the tool specification, and the choice of an eligible cooling lubricant can be optimized.