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A comparison of the evolution of fr during four different drilling experiments. In three experiments the reference voltage (Ue) is unvaried, while in the other experiment Ue is varied at steps of 0.5 mm in depth after touch-in (first 0.5 mm in depth). The data points correspond to the in-line monitored values of fr during the latter experiment. The other parameters are unvaried during the four experiments. In particular, the levels of the factors are: A = +1, B = −1, C = −1.
