# Prediction Methods and Experimental Techniques for Chatter Avoidance in Turning Systems: A Review
## 3 Experimental Techniques  
### Experimental Techniques for Chatter Avoidance  
This sub-section deals with experimental techniques regarding the modification of cutting parameters during machining (see Appendix B). For instance, spindle speed variation (SSV) technique can create a time-varying delay by creating distortions on chip thickness. As a result, new more favorable  
Figure 4: Parallel turning operations [67]. (**a**) Tools mounted on different turrets; (**b**) tools mounted on the same turret. phase lags between inner and outer chip modulation reduce the chatter feedback mechanism [69; 70; 71; 72]. There are different ways to vary the rotation speed of the head, but the most successful methods introduce a sinusoidal SSV, in which the speed of the spindle sinusoidally oscillates at a convenient frequency and amplitude [73; 74]. The technique can be adapted to different cutting systems and dynamics. However, some areas in the stability lobe diagram that were previously stable can turn unstable when applying the variation. Another drawback of this technique is the high accelerations and decelerations in the spindle as well as the difficulty in tuning the frequency and amplitude of the variation. The scientific literature first proposed the SSV technique to improve stability in milling processes [75]. [PERSON] et al. [76] presented a simple criterion for computing the optimal amplitude ratio and a heuristic criterion to facilitate the stabilization of the process. [PERSON] et al. [77] proposed a criterion to obtain optimal SSV amplitude based on energy analysis of the process. Additionally, they proposed a stability increment index (SII) of SSV with respect to constant spindle speed (CSS). The research group from Ideko-IK4 developed several works on the SSV technique applied to milling and grinding processes [78; 79; 80]. This technique can be difficult to apply in some cases such as low workpiece diameters. The amplitude of SSV is often related to the reference spindle speed and this parameter is constrained by the work material and workpiece diameter. For low workpiece diameters, high spindle speeds are needed to sustain adequate cutting speeds. Furthermore, the fluctuations of spindle speed lead to higher spindle speeds. Therefore, SSV is often run using G96 to set a constant surface speed and G50 (or G92) to allow a maximum spindle speed. [PERSON] et al. [81] presented a complete formulation for the modelling of stability in turning and milling processes when using SSV for the semi-discretization method. However, some authors claim that SSV technique has a greater potential in processes with inherent lower cutting speeds such as turning rather milling. [PERSON] et al. [82] designed the stability analysis of the tool using the discrete angle approach. Using this method, the independent variable is the workpiece angular position given by C-axis. They analyzed the effects of the variable speed machining on the noncircular turning stability and employed also a stability index criterion. Then, [PERSON] and [PERSON] [83] extended the previous analysis including a closed-loop dynamic model of the noncircular turning process. They observed by experimental results some improvements in the noncircular turning stability with constant and with variable spindle speeds. [PERSON] et al. [84] showed a strong dampening and stabilization of the most unstable eigenmode in turning. [PERSON] and [PERSON] [85] presented an interesting modelling for the prediction of stability lobes with SSV technique in turning (Figure 5a). They introduced the guidelines to the implementation of this technique and stated that in turning processes it enables higher stable chip widths compared to milling processes. In addition, they proposed favorable conditions to control the maximum acceleration of the spindle speed. However, this study did not include experimental results. To overcome this, [PERSON] et al.