## 1 Introduction  
The cutting process represents a consistent removal of a machined material's layers. The deformation is always accompanied by heat generation. The heat generation, together with the resulting friction forces, increases the temperature of a cutting tool at the point of its contact with a workpiece [1]. The high temperature, in turn, negatively affects the properties of a utilized tool material [2], As a consequence, it provokes wear, which reduces the tool life. The most common way to counteract the above negative factors is the utilization of coolants, often pure mineral oils or oil-in-water emulsions [3]. When turning, milling, etc., the external coolant supply reduces the temperature in the cutting zone along with the friction forces [4], thus positively affecting the tool life. Traditionally, the external supply has been organized by flooding when coolant is delivered to the cutting tool in large volumes and at low pressure. Mechanically, the supply is delivered through special hoses, plastic nozzles, metal tubes, or directly through the spindle to the rake and flank faces of a cutting tool or to the cutting zone as a whole. Continuous improvements in the machining field have led to the introduction of other external supply methods. For example, coolant supply under high pressure (2.0 to 7.0 MPa) and the MQL (minimum quantity lubrication, coolant mixed with compressed air and supplied in small volumes) technique can be cited. The main advantage of those, in comparison with flooding, is better penetration into the high-temperature region with often a smaller coolant volume used [5]. Nevertheless, the main problem of the external supply is the inability of the coolant to directly reach the contact point between the tool and the workpiece (Figure 1a). In addition, externally supplied coolants tend to lose effectiveness as cutting speed increases (Figure 1b). Simultaneously, mineral oil-based coolants used for the external supply are often hazardous to the environment and operators' health and require disposal [6]. To overcome the external supply disadvantages, researchers have proposed alternative ways to deliver coolants to the cutting zone. One of those includes an internal supply, where coolant is delivered exclusively inside the tool through a closed circuit [8; 9; 10]. Yet another option is combined supply, where coolant is simultaneously fed inside the tool and to its contact point with the workpiece via separate internal and external circuits [11]. In addition to the alternative supply options, alternative coolants have been proposed to solve the problems of mineral oils and oil-in-water emulsions. These include solutions of vegetable origin (coconut oil, sunflower oil, etc.) [12] and gases supplied at low (\(\sim\)\(-\)20 \({}^{\circ}\)C, air [13]) and cryogenic temperatures (\(\sim\)\(-\)190 \({}^{\circ}\)C, nitrogen [14]). According to the studies' results for turning, the utilization of alternative supply options together with alternative coolants provides better temperature, surface quality, tool life, and environmental performance compared to flooding and dry cutting. This, combined with recent advances in tool design (additive manufacturing [15]), has led to a significant increase in the number of relevant publications since 2010 (Figure 2). However, for milling, there are fewer works addressing the same topic. One possible reason is the complexity of organizing the delivery of the alternative coolant to the rotating tool. At the same time, standard supply systems of milling machine tools (designed to work with mineral oils and their emulsions) cannot be used with alternative coolants. This is due to the risk of damage to the systems' elements because of low temperatures, corrosion, etc. (especially, when the system supports the spindle-through supply). As a result, milling lags behind turning both in variations of alternative supply and in the introduction of alternative coolants.