## 1 Introduction  
Surface roughness is a surface irregularity with smaller intervals within the crest and bottom, unlike waviness. Roughness is a collection of true surface harshness, traditionally interpreted as a deviation of the measured form between a reference and the bounds of a length when waviness is ignored [1]. The true surface is referred to as the surface curbing the solid object configuration. It is a separate layer from the environment. The surface roughness profile is shown in Figure 1, where yi is the vertical distance from the mean line to a given data point along the profile line and l is the mean width of the profile line. The surface roughness (SR) is expressed as a value in a mean arithmetical deviation Ra (Equation (1)) of the profile from the centerline average and Rmax (also called \(\mathrm{Rt}\) or \(\mathrm{Rtotal}\)), which is the height from the lowest point to the highest peak. It is expressed mathematically as follows:  
\[\mathrm{Ra}=\frac{1}{\mathrm{L}}\int_{0}^{\mathrm{L}}|\mathrm{y}(\mathrm{x})| \mathrm{d}\mathrm{x} \tag{1}\]where y represents the absolute values of distances between profile points and the centerline along a length L of the measured surface. The surface roughness is at the center of the interaction between an object and elements in its proximity. For this reason, its value is of major concern in the manufacturing process. The surface finish in contact with other components significantly affects friction, wear, and component life. When manufacturing a workpiece, caution is required to set the surface roughness within the recommended range in terms of Ra and Rt. Two common methods are used to measure the surface roughness, namely the contact and non-contact methods. The contact method includes a stylus profilometer and a white light interferometer [2], while the non-contact method includes the use of a Focus Variation Microscope [3]. In the present study, the experimental results were measured using a Mitutoyo surface roughness tester and Press o-firm [4]. As for milling machining, several factors may affect the surface roughness. These include, but are not limited to, the following:  
* Cutting parameters: feed rate and spindle speed;
* Depth of cut: radial and axial depths;
* Tool configuration: insert geometry, insert materials, tool length;
* Machining condition: method of lubrication;
* The rigidity of the machine, tool, and workpiece;
* and the workpiece's raw material. Researchers have employed one or a combination of the above factors to study their influence on the surface roughness. [PERSON] et al. [5] considered the effect of the cutting parameters, tool geometry, and tool interference, as well as the tool-workpiece relative movement, on surface generation to devise a 3D surface topography simulation model for ultra-precision raster milling processes. In order to achieve an optimum or at least more acceptable machining result that meets a work requirement, several approaches have been proposed to predict the surface roughness from selected variables that influence the surface texture of a workpiece in the end-milling machining process. Trial-and-error, empirical, and analytical methods were the earliest methodologies to predict surface roughness. Researchers used empirical methods to test a set of findings that were supported by experimental evidence. Analytical approaches are frequently used to solve equations for specific parameter circumstances [6]. However, empirical models are frequently developed by regression analysis of practical experiments [7]. [PERSON] and [PERSON] [4] explored the effects of four cutting parameters on the machined surface of Al 6061 alloy under minimum quantity lubricant (MQL) conditions. The study used a numerical statistical method. The surface roughness was predicted with accuracy of 89.5%. Furthermore, their study reveals a link between input factors (spindle rotational speed, axial depth of cut, radial depth of cut, and feed rate) and output variables (surface roughness obtained by CNC milling an alloy Al6061).