## 4 Discussion  
Machining of non-circular contours is possible with the use of lathes, but the tool must be additionally driven and controlled while processing in order to obtain the assumed shape. Conventional tool feed drives do not provide sufficient dynamics due to the high weight of the entire assembly. This niche is filled by FTS used commonly in this type of application, but they are strictly specialized solutions. They are usually built on the basis of a stacked piezoelectric drive with a backlash-free transmission to which the cutting tool is directly mounted. Such a solution provides high dynamic parameters but limits the possibility of changing the tool type, and due to the dedicated structure, it can be characterized by a high price. The use of general-purpose components (piezoelectric drive, linear guides, and universal tool holders) allows building a device for manufacturing non-round shapes, but fulfilling its task for limited cutting speeds. An additional aim of the research was to check whether it is possible to estimate the deviation of the part contour based on registering the position of the tool during machining. In this study, it was found that positioning accuracy \(A\) of the drive after its compensation (including the analog control circuit) does not exceed 2.533 \(\upmu\)m, which is a value close to the linearity of the drive characteristic declared by the manufacturer. The results of the contour deviation do not exceed 40 \(\upmu\)m.  
The results obtained during the verification tests allow for the conclusion that it is possible to equip a conventional turning lathe with the FTS to produce parts of non-circular  
Figure 14: Correlation of the profile deviation to ideal geometry measured offline and online during the turning process: (a) deviation of profile shape; (b) standard deviation of profile shape. contours. The tested configuration allowed for maintaining the profile deviation in relation to the nominal shape of \(e=39.8\)\(\upmu\)m with the standard deviation of \(s_{\text{e}}=9.9\)\(\upmu\)m for the drive operating frequency lower than the limit frequency. After exceeding the limit frequency, the drive travel range \(\Delta x\) contributed to an increased deviation from the ideal geometry. The values provided in the context of using a conventional turning lathe should be considered satisfactory. In the analyzed cases, the deviations tested with a laser sensor demonstrated high convergence with the results obtained when conducting the tests with a coordinate measuring machine--the determination coefficient \(R^{2}\) for the linear regression between the errors calculated for those two methods was 0.987. This fact enables the assumption that such a measurement can quickly and inexpensively allow estimation of as-machined geometry of contour without additional off-line measurements. The main limitation of this study was the use of a conventional lathe. The construction of the spindle drive does not allow continuous control of its speed. The use of an asynchronous motor as the main drive additionally influences the actual rotational speed, which deviates from the setting. Maintaining the assumed nominal diameter of the shape has also proved problematic due to the manual control of the lathe. Future research will focus on eliminating the most problematic components of the test stand by replacing the conventional lathe with a modern numerical machine tool in order to increase the possibility of controlling the spindle speed, using a faster control system, and extending the research plan with surface texturing. In addition, the plan includes testing with the use of a device intended for measuring shape offsets, i.e., a form tester. All things considered, it is possible to manufacture non-circular contours using a conventional lathe and an FTS drive, but the accuracy of the parts obtained in this way is limited by the operating frequency of the piezoelectric stack.