*3.2. Tool Wear*

Tool wear generally leads to a change in process force. With increasing flank wear, friction increases, which, in turn, leads to an increasing cutting force and cutting temperatures. If the wear is mainly caused by crater wear, the change in rake angle can lead to a decrease in cutting force. Consequently, the change in cutting force is also an indicator for monitoring the type of wear that will influence the process and the workpiece quality. Tool wear is usually monitored by a static limit. Therefore, the average measured cutting force with a new tool is determined for a predefined tool path. Based on empirical values, a maximum tolerable change in force is defined as the limit for the tolerable wear. If the limit is exceeded, the monitoring system initiates a tool change before an intolerable influence on the workpiece quality occurs. In contrast to the confidence limits, the influence of the signal-to-noise ratio on the monitoring quality is significantly lower since the average value is evaluated over a defined range of the monitored signal. For a sensitive and robust monitoring of tool wear, the repeat accuracy is mostly important. In addition to the 10 good processes with a new tool, three measurements were carried out with a tool, which has a high chipping length of 465 μm. During the machining, identical process parameters of *ap*, *f*, and *vc* were used as for the previous longitudinal turning operations, depicted in Figure 5.

**Figure 5.** Cutting force characteristics for new and worn tool for di fferent depths of cut.

Tool wear results in an increasing cutting force, which varies according to the depth of cut. At a depth of cut of *ap* = 1 mm, an increase of 24% occurs with 158.5 N for *Fc,dyn*. With a reduction to *ap* = 0.5 mm, the wear lead to an average increase in the cutting force of 38.6 N, which represents a percentage change of 13%. The triple standard deviation of the measurements with the new tool is *3*σ = 9 N for *Fc,dyn*. Assuming tool wear can be detected, if this value is lower than the resulting change in cutting force, monitoring can be performed for both *ap*. The increase of *Fc,dyn* at an *ap* of 0.3 mm is, therefore, with 7.5 N too small to detect the current tool wear for the defined process parameters. For the feeling turret, a *3*σ for *Fc,SG* of 53 N is determined at a depth cut of 1 mm. Thus, the tool wear can be robustly identified for *ap* = 1 mm. For the remaining process parameters, the variation of the average cutting force is too low to realize tool wear monitoring.

As shown by the di fferent depths of cut, no general statement can be made about the e ffect of tool wear on the percentage change in cutting force. Therefore, it is not possible to make a universal prediction of the ability to monitor tool wear. The influence of the wear mechanisms on the cutting force vary significantly based on the selected process parameters. For this reason, it is more relevant to compare what change in cutting force caused by tool wear can be theoretically identified with di fferent signals. For this purpose, the cutting force was modelled according to Kienzle for a new tool and three wear-related force increases. The variation of the cutting speed has a smaller influence on the force compared to the feed and depth of cut. For this reason, the cutting speed was fixed for the research at *vc* = 300 m/min. Based on the modelled cutting forces, an increase in force was determined for a combination of process parameters. It is assumed that the change can be monitored if the di fference is higher than the triple standard deviation *3*σ of the cutting force measurement. Based on the previous investigation, *3*σ was set to *Fc,dyn,3*σ = 9 N and *Fc,SG,3*σ = 53 N. For both measuring systems, it process parameters were researched at which a force di fference of 10%, 20%, and 30% results can be detected by the individual system, which was depicted in Figure 6. This investigation was carried out for a steel (20 MnCr5) and an aluminum (EN AW-6082).

**Figure 6.** Representation of the monitorable percentage increase of the cutting force for EN AW-6082 and 20MnCr5 based on the feeling turret and dynamometer.

With the dynamometer, almost the total process parameter range considered for both materials can be monitored with regard to the three selected percentile cutting force changes. Only for aluminum, the theoretical detection is not possible for low depths of cut and low feeds. In this case, the cutting force di fference caused by a wear-related force increase of 10% is too low. The evaluation of the cutting force measured by the feeling turret shows the potential for monitoring tool wear for a wide range of process parameters. Especially for steel, a large range is covered. For aluminum, higher *ap* and *f* are required due to the di fferent material properties. For the highest expected change of 30%, this includes process parameters of *ap* = 0.5 mm and *f* = 0.17 mm. In the case of aluminum, large parts of the considered process parameter range can theoretically not be monitored, especially if the cutting force changes by only 10% due to tool wear. In general, higher *ap* and *f* are used for machining aluminum than for steel. Then tool wear can also be monitored for aluminum during roughing operations.
