At present, in the processes of manufacturing and assembling multi-layer (sandwich) industrial complexes with different materials, such as mold or fixture production, firstly the parts must be assembled and then drilled simultaneously; otherwise, the desired dimensional or geometrical accuracy will not be achieved. To make these holes, conventional drilling processes are mainly used, which due to the geometric shape and zero cutting speed at the drill tip, cause severe plastic deformation in the cutting zone. The use of conventional drilling process may reduce the dimensional and geometrical accuracy as well as the surface integrity of the inner wall of the holes. To reduce the mentioned problems, one of the relatively new methods proposed for drilling is the helical milling technique. In the present study, in order to compare the capabilities, advantages and disadvantages of these two different drilling methods, a total of 18 drilling tests using conventional drilling and helical milling processes and considering 2 variable parameters of cutting speed and feed rate (each in 3 levels) on metal multi-layer (sandwich) workpiece with 3 different layers of steel including MO40, 304L and CK45 was performed. Based on the obtained results, it can be stated that in order to achieve the desired dimensional (hole diameter) and geometrical (hole roundness) tolerances, the conventional drilling process has a better performance than the helical milling method, but in terms of achieving less surface roughness and better surface integrity (less surface imperfections), helical milling method has better performance than conventional drilling