# The Predictive Model of Surface Texture Generated by Abrasive Water Jet for Austenitic Steels
## 3 Results and Discussion  
### Evaluation of Measurement Results from the Technological Point of View  
Final evaluation of measured parameters on individual samples was realized on the basis of the analysis of the samples being cut and their subsequent processing. Such evaluation in relation to the mean roughness value _Ra_, in relation to the stroke of the cutting head above the material and in relation to the internal diameter of the abrasive nozzle was processed according to four categories (measured mean value _Ra_ represents another variable, each roughness measurement always being a new measurement):  
1. [flow, abrasive, roughness--variables; speed--constant],
2. [flow, velocity, roughness--variables; abrasive--constant],
3. [abrasive, speed, roughness--variables; flow--constant],
4. [flow, speed, abrasive, roughness--variables]. The subsequent evaluation of the samples is focused on the main cutting parameters, such as cutting speed, abrasive weight, and high-pressure water flow, in relation to the cut surface quality. Conclusions of evaluated measurement results for the combination of variable flow rate, speed, abrasive, roughness are:  
The following factors have a positive influence on the quality of the cut surface:  
* Lower cutting speed 50 and 75 mm\(\cdot\)min\({}^{-1}\),
* Lower cutting head lift above the material 3 mm,
* Abrasive nozzle, internal diameter 0.76 mm,
* Abrasive weight 150, 200 and 250 g\(\cdot\)min\({}^{-1}\),
* The high-pressure water flow rate of 1.05 and 1.65 l\(\cdot\)min\({}^{-1}\),
* The roughness of the cut surface can be achieved in the range of 6.2-15.8 \(\upmu\)m,
* The overall roughness of the cut surface can be achieved in the range of 5-19 \(\upmu\)m.  
The quality of the cut surface is negatively influenced by:  
* Higher cutting speed of 100 and 125 mm\(\cdot\)min\({}^{-1}\),
* Higher cutting head lift over material 5 mm,
* Abrasive nozzle, internal diameter 1.02 mm,
* Weight of abrasive 200-250 g\(\cdot\)min\({}^{-1}\),
* The high-pressure water flow rate of 2.37 and 3.25 l\(\cdot\)min\({}^{-1}\),
* The roughness of the cut surface can be achieved in the range of 6.5-20 \(\upmu\)m,
* The overall roughness of the cut surface can be achieved in the range of 6.5-27 \(\upmu\)m.  
The presented article is devoted to the analysis of the impact of basic factors as a separate effect; the next section addresses the impact of all input factors as a whole, in order to create a mathematical-statistical model. Experimental data related to the center of cut on the cut surface for each sample are used in the discussion, and the surface roughness is assessed based on the _Ra_ parameter. In AWJ cutting, increasing the speed of the cutting head reduces the number of abrasive particles that pass through the jet area unit. Accordingly, there are fewer impacts of cutting edges of abrasive particles per area unit, which ultimately leads to the formation of surfaces with a higher roughness value _Ra_ (Figure 4). The presented article is focused on high-pressure water flow, abrasive mass flow, and the internal diameter of the abrasive nozzle. Hydraulic parameters determine the hydraulic power of the abrasive water jet cutting and are based on the principle of fluid mechanics theory. The liquid pressure in the high-pressure water jet process is determined by the Bernoulli Equation (10) for the flow of incompressible liquid (10):  
\[v^{2}=\frac{2\cdot p}{\rho} \tag{10}\]  
where \(v\) [m\(\cdot\)s\({}^{-1}\)] is the flow rate and \(\rho\) [kg\(\cdot\)m\({}^{-3}\)] the density of the liquid medium. The pressure of the liquid is proportional to the square of the cutting speed, which is proportional to the kinetic energy of the particle of mass \(m\). As a result, the water jet pressure determines the precise kinetic energy of the particle in the water jet. At higher pressures, the average kinetic energy of the water molecules is higher and more easily break the molecular bonds in the workpiece material.