## 1 Introduction  
Titanium has good mechanical properties in high temperatures, good corrosion resistance and low weight rate. Generally, titanium is alloyed with elements such as Al, V, Mn, Sn, Zr, Mo, etc. [1]. In this way, many titanium alloys--Ti-6 Al-4V, Ti-5 Al-2.5 Sn, Ti-6 Al-2 Sn-4 Zr-2 Mo, etc. are manufactured for different engineering applications. Ti-6 Al-4V is used for rings, fasteners, airframes, vessels, biomedical implants, forging, and hubs; Ti-5 Al-2.5 Sn is used in gas turbine engines, aerospace structures, and chemical processing equipment; and Ti-6 Al-2 Sn-4 Zr-2 Mo is used jet engines, automotive valves, blades, and discs. Mechanical and thermal properties of Ti-6242S alloy are lower than other titanium alloys. Therefore, this alloy is preferred many industrial areas. However, there are difficulties encountered in the machining of Ti-6242S [2]. Excessive heat and tool wear due to especially low thermal properties of this alloy have occurred. Cutting forces are increased due to high mechanical properties and excessive tool wear. This also leads to an increase in vibration. In this case, the quality of the machined surface is deteriorates. Many studies of the turning, milling, and drilling of titanium alloys have been conducted in order to address this problem. However, many of the studies are related to machinability of Ti-6 Al-4V alloy. Ti-6242S alloy is the second most-used as second titanium alloys after Ti-6 Al-4V alloy. [PERSON] and [PERSON] [3] made an experimental study with milling of Ti-6242S to determine optimal cutting parameters. In the experiments, they investigated the effect of different cutting speeds and feed rates on tool wear, metal removal rate, and temperature using cemented carbide tools. They emphasized that the increase of tool wear and the decrease of tool life with increasing of cutting speed and feed rate. [PERSON] et al. [4] studied a new optimization approach based on Kriging interpolation and sequential quadratic programming algorithm for end milling of Ti-6242S using coated and uncoated Chemical Vapour Deposition (CVD) tools. They reported that the surface roughness increased with increasing depth of cut and feed rate. The lowest surface roughness was obtained from minimum cutting speed. [PERSON] et al. [5] investigated the performance of cutting tools in green machining of Ti-6242S. They researched the tool life, tool wear types, and plastic deformation of tools using uncoated and multilayer coated cemented carbide cutting tools. When considering the literature, it is known that there are many problems in the machinability of titanium and its alloys [6; 7; 8]. Because of these difficulties, machinability of these alloys was investigated using various modelling and optimization techniques. [PERSON] and [PERSON] [9] studied optimization of electro discharge manufacturing (EDM) parameters in micro-EDM of Ti-6 Al-4V. They showed that these methods are appropriate with experimental results. [PERSON] and [PERSON] [10] investigated the tool wear in micro-milling of Ti-6 Al-4V using Cubic Boron Nitride (CBN) coated tools. Then, they analyzed the experiment results using the DEFORM program. [PERSON] et al. [11] modelled the cutting forces in machining of Ti-6 Al-4V with TiAlN Physical Vapour Deposition (PVD) coated tools using response surface methodology. [PERSON] and [PERSON] [12] studied the machinability of various titanium alloys under different cutting parameters. They optimized the cutting temperatures and cutting forces by means of Taguchi Techniques. Scientists have used different optimization techniques such as Taguchi Methods (TM), Response Surface Methodology (RSM), Artificial Neural Network (ANN), etc. in machining [13; 14; 15]. Scientific studies on optimization techniques are on-going in machining. The aim of this study is to find the relationship between cutting parameters--such as cutting speeds, feed rate, and depth of cut--and optimization techniques as RSM and ANN for cutting forces, surface roughness, and tool wear in the milling of Ti-6242S alloy.