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Resource-efficient use of pCBN tools
The Institute of Production Engineering and Machine Tools (IFW) at Leibniz University Hannover is working on a current research project to develop an optimized regrinding process for ultra-hard indexable inserts. The focus is not only on resource conservation but also on the economic efficiency and process reliability when using reground tools. This is achieved by considering not only the visible tool wear for determining regrinding parameters but also internal damage.
___by Berend Denkena, Benjamin Bergmann, Lars Luthe
Ultra-hard cutting materials, such as pCBN or ceramics, stand out compared to carbide tools due to their higher hardness, greater wear resistance, and better thermal and chemical resistance. Compared to carbide tools, higher cutting speeds and consequently higher material removal rates can be achieved when machining materials like nickel-based alloys and hardened steel. However, these advantages are offset by the significantly higher acquisition costs of these tools. Due to the small cutting depths when using pCBN tools, only a minor volume fraction of the cutting material is utilized during machining. This limits the resource-efficient use of ultra-hard tools. Achieving a higher utilization of the cutting material is fundamental for cost- and resource-efficient use. One approach to fully utilize the cutting material is through tool regrinding. However, the current challenge is that reground tools often do not achieve the same tool life as new tools. This is because there is a lack of knowledge about reliable regrinding parameters.
Issue with Regrinding Ultra-Hard Cutting Materials
One reason why reground indexable inserts do not achieve the same tool life as new tools is that only the visibly detectable wear is considered for regrinding. Internal damage within the tool is not taken into account, but it can exceed the visually measurable wear and therefore is not completely removed by regrinding.
As a result, the reground tool has pre-existing damage, which leads to a premature end of its tool life upon reuse. Figure 1 shows the current state of the regrinding process. The goal is to develop an optimized regrinding process that allows reground indexable inserts to achieve comparable performance to new tools. To achieve similar tool life with reground inserts as with new tools, a thorough analysis of both the visibly measurable wear and internal damage is required. From this analysis, adjusted parameters for the regrinding process can be derived.
The challenge here is that different materials are used in machining, and the process parameters are varied. This variation leads to different stresses on the tools, resulting in various types of wear, particularly internal damage, which must be considered when determining the regrinding parameters.
Wear Behavior in Hard Machining
In initial experiments, pCBN indexable inserts with a CBN content of 50% and a ceramic TiC binder phase were used for external longitudinal turning of hardened 100Cr6. Here, the cutting speed (vc) and feed rate (f) were varied to generate different wear levels after different cutting times (tc). Figure 2 depicts three different wear states at different cutting times and process parameters. Due to the shallow cutting depth in the process, wear primarily occurs in the area of the corner radius and the flank face.
It is observed that in these test series, crater wear dominates on the rake face. This is due to the high temperatures during hard machining. Furthermore, an increase in process parameters leads to an increase in thermal load, resulting in higher wear rates. As a result, the wear mark width increases to VBmax = 189µm and crater wear width to 260µm.
Damage Analysis
Subsequently, the tool with the most wear underwent an investigation of internal damages. For this purpose, micrographs and detailed images were taken using scanning electron microscopy (SEM) (see Fig. 3). Microcracks are visible in the micrographs of this tool. However, it is evident that these damages are localized and occur in the same areas as visible fractures. Additionally, SEM images show cracks on the flank face and further small fractures along the cutting edge.
In conclusion, internal damages in the cutting material were identified. These identified cracks within the tool propagate locally in the region of the cutting edge and do not exceed the flank wear during these investigations. Based on these findings, regrinding of the tools indicates that crater wear on the rake face is significant for the feed rate during flank-side regrinding. However, there is still no information available regarding internal damages in the region of the rake face.
Further investigations will be conducted in the future to analyze the propagation of cracks in this tool region. A portion of the rake face will be removed by grinding, followed by imaging of the tool substrate. The goal is to analyze damages in the cutting material that cannot be removed by regrinding processes based on the current investigations, thereby reducing the performance of reground indexable inserts. Based on these damage analyses, further regrinding experiments will be conducted to remove any wear inside the tool. In final experimental turning tests, the performance of the reground indexable inserts will be evaluated.
Source: IFW Hannover