Quickly determine the quality of CBN grinding wheels

When using CBN diamond grinding wheels for grinding operations, in addition to the very important service life of the grinding wheel, it is also crucial to understand the quality factors of the grinding wheel in order to meet the high requirements for the grinding process and processing accuracy.

The grinding process has always played a very important role in the processing of tools and components. Compared with other types of cutting technologies, the advantages of the grinding process are its good machinability for hard materials, high geometric tolerance accuracy (IT5~6) and small ripples and roughness of the grinding surface (Rz=1~3μm).

Since the basic shape of the electroplated grinding wheel is highly plastic and easy to cut, this process is particularly suitable for processing occasions with higher requirements. The characteristic of the electroplating layer of the grinding wheel is that on the one hand, the wear limit range requirements need to be met through high-quality grinding particles, and on the other hand, space for chip discharge needs to be left.

Overall observation of the grinding process

As the processing technology becomes more and more complex, the grinding process must be observed as a whole. This means that specific requirements should be put forward for the configuration of the grinding wheel used according to various different applications. This often involves the problem of surface shape (i.e. the effective area of ​​the cutter head). In order to be able to characterize and evaluate the relevant surfaces in detail, modern measuring technologies and methods must be used. This allows an objective assessment of the results and quality level of the tool process with particles at any point in time. This is also a prerequisite for ensuring stable machining quality.

During electroplating bonding, the grinding wheel grains are deposited on the conductive disc by means of electrolyte and nickel or chromium precipitation (Figure 1). This process has a stochastic character, which affects the detected quality characteristics. A large number of different detection characteristics are used to characterize the grinding wheel lining. The important characteristics obtained by detection are bond hardness, grain size and grain density. In addition, data such as the static number of blades, the dynamic number of blades and the dynamic distance between blades are also used for the evaluation of the tool surface. The latter two parameters already include the dynamic nature of the process and are more expensive with today's advanced measurement technology.

Material load ratio is an important detection feature

Another important detection feature for the evaluation of grinding wheel quality is the material load ratio and the material ratio curve (Abbott curve), as shown in Figure 2. The material load ratio reflects the percentage of the grinding wheel surface below this value in the 3D section at a certain value. This parameter is very important for the evaluation of the relationship between the use of the grinding wheel and the specific processing task. As part of a research project, a non-contact measuring technique for determining the material load ratio was developed in cooperation between the two companies FGW Remscheid eV and GFE Schmalkalden eV with the participation of a coordination committee. In order to validate this measuring technique, a correlation study was carried out between the measurement results obtained with electroplated grinding wheels and the machining results.

The first step was to develop image processing methods and image acquisition strategies as well as test bench planning, design and manufacturing in order to automatically and objectively determine the material load ratio on the grinding wheel using a combination of stereo and confocal microscopy. Given the definition of the material load ratio, this detection feature was first measured contactlessly with the aid of a 3D optoelectronic measurement technique. The tests were carried out on a new test bench (Figure 3). With this test bench, 3D image data can be generated in the microscopic range by stereo microscopy and by varying the working distance. The data generated formed the basis for the development of the logarithmic method required for determining the material load ratio.

Determination of the load ratio of electroplated grinding wheels using a non-contact method

The second step is to conduct a correlation study of the grinding wheel processing results. For this purpose, realistic wear states are generated using reproducible parameter sets and a series of grinding tests are carried out. Compensation is then achieved by electronic raster and microphotographic measurement of tool and workpiece characteristics, based on novel optoelectronic structures and using the results of newly developed image processing technology. This repetitive process can be used to verify the correlation between the measured material load ratio and the actual grinding wheel processing result, and the function and usability of the image collection method can also be verified with the help of the instrument.

As a result, a non-contact technical process for determining the material load ratio of electroplated grinding wheels was developed within the research project. The instrumental basis for this work is a test bench that combines stereomicroscopy and confocal microscopy.

The economic advantage of this method for companies lies in the rapid characterization of the surface state. The test bench allows the material ratio curve of the tool to be determined quickly, simply, reproducibly and documented. The knowledge gained helps users to continue to develop their products. (end)