Analysis of Grinding Wheels in the Grinding of Industrial Rolls

The diploma thesis presents the production process of manufacturing rolls for the metallurgical industry with emphasis on their final machining, i.e. grinding. Based on the analysis of test results of grinding wheels from three different manufacturers in grinding the working surface of a roll made of material designation CCr-N-77, it can be concluded that the grindability of this material is influenced by chemical composition, microstructure and consequently primarily the hardness of the material due to numerous hard chromium carbides present in the microstructure. Based on the calculated G factor, two grinding wheels are suitable: Tyrolit and Swaty.

Solution and Method

The diploma thesis presents the production process of manufacturing rolls for the metallurgical industry with emphasis on their final machining, i.e. grinding. The grinding tool, i.e. grinding wheels, and working conditions in the process of grinding industrial rolls are presented. Knowledge of conditions during grinding greatly contributes to the optimisation of the grinding process as well as to the development of new grinding wheels and optimisation of their manufacturing process.

The procedure for testing and analysing grinding wheels in grinding difficult-to-machine materials with designations CCr-N-77 and CGHSS-NF-7 is presented. By monitoring the machining process, grinding wheels from three different manufacturers (Tyrolit, Swaty and Theleico) were analysed under identical grinding conditions. The analysis primarily focused on the values of grinding factor G, where it is desirable for the value to be as high as possible.

Results

Based on the analysis of test results of grinding wheels from three different manufacturers in grinding the working surface of a roll made of material designation CCr-N-77, it can be concluded that the grindability of this material is influenced by chemical composition, microstructure and consequently primarily the hardness of the material due to numerous hard chromium carbides present in the microstructure. Based on the calculated G factor, two grinding wheels are suitable: Tyrolit and Swaty. For both, the G factor was greater than 8, which is also the baseline parameter when grinding this material.

Due to the slightly harder structure of the Swaty grinding wheel, it would be advisable to carry out further testing with a change or reduction in the peripheral speed of the grinding wheel and to limit infeed to a maximum of 35% motor load. It is interesting that when grinding material with designation CGHSS-NF-7, the results for all three grinding wheels differ considerably from the results of grinding material CCr-N-77. The wear of grinding wheels was greater, and the effect in terms of volume of ground roll surface per time unit is also greater, which is partly due to greater infeed of the grinding wheel to the workpiece.

The motor load under the given conditions for grinding material CGHSS-NF-7 was lower, which indicates lower forces required for material removal. However, the calculated value of factor G, which reaches optimal value when using Tyrolit and Swaty grinding wheels for material CCr-N-77, is completely unsuitable for material CGHSS-NF-77. Based on the calculated G factor, it can be concluded that productivity when grinding material CGHSS-NF-7 with the best grinding wheel Tyrolit, which is currently available in the company, is up to twice lower than the value obtained when grinding material CCr-N-77, which also affects the economic efficiency of machining and thus increases costs due to tool wear.

From the obtained test results of grinding wheels, it was established that it is indeed sensible to continue using the Tyrolit grinding wheel for grinding working layers of rolls made of materials CCr-N-77 and CGHSS-NF-77. The Swaty grinding wheel would be usable for grinding material CCr-N-77, whilst for grinding material CGHSS-NF-7 it is not suitable primarily due to excessive wear of the grinding wheel, which increases costs in terms of tool wear. The Theleico grinding wheel, due to excessive wear and low G factor value, would not be suitable for grinding either of the materials used for this testing.

It is established that when selecting the type of grinding wheel, in addition to its basic characteristics such as composition, structure and hardness of the grinding wheel, the chemical composition, microstructure and hardness of the workpiece are also extremely important. Based on the results when grinding material CGHSS-NF-7, which is still under development, cooperation with the grinding wheel manufacturer is necessary in developing a new grinding wheel. The aim would be to exceed baseline parameters and optimise grinding, primarily from an economic perspective, due to excessive wear of grinding wheels when grinding this material.

The analysis showed that the wear of the best grinding wheel (Tyrolit) when grinding material CGHSS-NF-7 is up to twice greater than when grinding material CCr-N-77. Using the obtained results, a table with recommendations for the use of grinding wheels (which were tested and their usability established) for rough grinding of working surfaces of rolls made of materials CCr-N-77 and CGHSS-NF-7 on the Hercules grinding machine was produced. Table 18: Recommendation for the use of grinding wheels Recommendation for the use of Tyrolit grinding wheel when grinding materials CCr-N-77 and CGHSS-NF-7 Material Peripheral speed of workpiece m/s Peripheral speed of grinding wheel m/s Longitudinal feed Z axis [mm/min] Continuous infeed X axis [mm/min] Recommended motor load of grinding wheel axis [%] CCr-N-77 40 40 2500 0.02 45 CGHSS-NF-7 45 40 2000 0.03 50 Recommendation for the use of Swaty grinding wheel when grinding material CCr-N-77 CCr-N-77 40 35 2500 0.01 35 Source: (Own research, 2016) Based on the analysis of grinding wheel test results, conclusions were derived which will greatly assist the author in future work. ** Knowledge of the cutting properties of grinding wheels during grinding is of key importance if we wish to improve machining quality.** It is sensible to use grinding wheels that ensure lower grinding costs, high material removal per time unit and a high value of grinding factor, which in this particular case must be greater than 8.

Due to ever-increasing customer demands for more durable rolls in terms of wear resistance, we can expect ever-greater challenges in the future regarding correct selection of tools and machining of these materials for machining to be economically justified. Due to time constraints, it was not possible to test materials in terms of the effects of cutting parameters such as peripheral speed of the grinding wheel, grinding wheel feed and peripheral speed of the workpiece for individual grinding wheels. However, this does not mean that the research is concluded.

The author is convinced that this diploma thesis has provided useful information for the company and also for the grinding wheel manufacturer, with whom the company intends to cooperate in advance in the development and testing of new grinding wheels that will contribute to the productivity of this machining process in the company under consideration.