Udrills are indexable insert drill commonly used in drilling operations. It is estimated that the market share among drilling tools is around 53%. They are produced in the same nominal diameter but in different lengths. There are usually two cooling holes, one behind the central insert and the other behind the peripheral insert. However, in some Udrills, an extra third cooling hole is drilled by the manufacturer. In this study, the effects of different length/diameter ratios, extra coolant hole and drilling parameters on thrust force, torque and spindle load were investigated in order to determine the effect of differences in U drills on hole drilling. AA 2024-T351 was drilled to a depth of 40mm using Udrills with a body diameter of 20mm. A total of 4 Udrills (3D, 4D, 4De, 5D) were used. Three feed rates (0.06, 0.09, 0.12mm/rev) and three cutting speeds (200, 250, 300m/min) were used in the experiments. According to the Anova analysis, it was concluded that while the length/diameter ratio of Udrills had an effect of 56.97% on thrust force and 21.46% on spindle load, it was not effective for torque. The 4De Udrill with extra coolant hole gave 34.2% higher thrust force, 0.3% higher torque value and 26.53% lower spindle load compared to the 4D Udrill without extra coolant hole.

Inconel 600 is widely used in the manufacturing industry due to its high-temperature resistance, excellent mechanical strength, and corrosion resistance. However, these same properties make it difficult to machine, resulting in high cutting forces, elevated temperatures, and poor surface quality. Optimizing cutting parameters alone is not sufficient; sustainable cooling strategies must also be considered. While previous studies have explored individual cooling techniques, there is a lack of comparative research on hybrid methods—particularly those combining high-pressure air and minimum quantity lubrication (MQL). This study examines the turning of Inconel 600 under eco-friendly cooling conditions: dry machining, high-pressure air (7 and 10 bar), MQL, and a hybrid method integrating compressed air and MQL. Machining performance was assessed based on cutting force, surface roughness, and temperature. A structured multi-criteria decision-making (MCDM) framework—employing TOPSIS, MOORA, PROMETHEE II, and COCOSO with three weighting schemes (equal, entropy, and critic)—was used to determine the optimal strategy. Final rankings were consolidated using Copeland and Borda count methods. Results indicate the hybrid method is the most effective, significantly reducing main cutting force (20.18%), feed force (31.79%), surface roughness (53.58%), and temperature (35.84%) compared to dry machining, demonstrating its potential for sustainable and high-performance machining.