## 3 Experimental Results and Discussion  
The basic assumptions of the most frequently used control charts are the normal distribution of measured data. The basic assumptions of the most commonly used control charts are the normal distribution of measured data. The basic assumptions of the most commonly used control charts are the normal distribution of measured data. In practice, control charts are often used without verifying the normality of the input data. Even in such cases, the control chart is suitable for practical use. The normality of the measured input data from the drilling process was verified by descriptive statistics and histograms of technological parameters. The constructed histogram analysis was focused mainly on the centering of the histogram, which characterizes the mean value of the observed technological feature, then on the width, which characterizes the variability of values and its shape (form), which allows revealing some definable causes of variability. During the histogram analysis, the minimum and maximum values were determined. From them, the variability of the investigated technological feature, the center of the distribution, the number of peaks, the symmetry, the kurtosis, and skewness of the distribution. The following graphs show the time behaviors, histograms, and statistical characteristics calculated in the tables (see Tables 6-8). Figure 7a-c shows the original measured behavior of technological parameters during the disintegration of rocks by rotary drilling. Subsequently, histograms were constructed. Figure 8a shows a histogram of revolutions with a flat shape. These histograms are usually characterized by the data collected under variable conditions or indicating a poor process setup. It indicates a change in position during measurement. The change of position is also proved by the original measured time course of the revolutions n (rpm) (see Figure 7a). The bell-shaped histograms (see Figure 8b,c) show the normal distributions of the measurable quality characters. Histograms of the frequencies of pressure force and vibrations of acceleration shown in Figure 8b,c are histograms with partially isolated peaks, the separated spikes, respectively. It indicates the evaluation of an inhomogeneous measured quality character. The time behaviors are shown in Figure 7b,c. The histograms of the measured parameters thus obtained have a high significance. They plot values from a certain interval divided into subintervals of a certain width, which follow each other. Histograms were supplemented by statistical characteristics. The normal distribution is understood as a mathematical model of the empirical distribution of values, which can be used to analyze, explain, and implement results obtained from various measured technological parameters under comparable conditions. Deviations from the actual value are affected only by chance. A typical theoretical distribution is the Gaussian normal distribution. Its graphic representation is a bell-shaped Gaussian curve. Gaussian curves of the normal distribution were constructed and plotted in histograms for input and output technological parameters to support the normal distribution hypothesis. The references that confirm the hypothesis of a normal distribution are as follows: [54; 55; 56; 57; 58].