In the process of micro-milling, the appearance of the edge-size-effect of micro-milling tools cannot be ignored when the cutting parameters are smaller than the cutting edge arc radius (r 0 ) of the micro-milling tool or close to it, and it could easily lead to low cutting efficiency and poor surface quality of the micro-slot. Through micro-milling experiments on Al7075-T6 materials, the change of milling force in the plough zone and shear zone during micro-milling was studied, and the minimum cutting thickness (h min ) range was determined to be 0.2r 0 -0.4r 0 based on r 0 of the micro-milling tool. Subsequently, the effect of f z /r 0 (f z denotes feed rate per tooth) on the top burr formation of the micro-slot, the surface roughness (R a ) of the micro-slot bottom, and the milling force was studied, and a size-effect band of micro milling was established to determine the strong size-effect zone, transition size-effect zone, and the weak size-effect zone. Finally, two different f z /r 0 in the strong size-effect zone and the weak size-effect zone are compared, which proves that the main purpose of the cutting parameters optimization of micro-milling is to avoid cutting parameters locating in the strong edge-size-effect zone. The above conclusions provide a theoretical basis for the selection of micro-milling cutting parameters, and an important reference in improving the surface quality of micro-milling.Micromachines 2020, 11, 197 2 of 15 Kiswanto et al. (2014) [11] studied the size-effect in micro-milling, and found f z and r 0 had the greatest impact on the surface quality of micro machining. Kang et al. (2008) [12] considered that r 0 has a great influence on the micro-milling force in all directions in the micro-milling process; the relationship between the cutting edge arc radius (r 0 ) and the minimum cutting chip thickness (h min ) decides whether chips are generated in the micro-milling process. In the process of micro-milling, when h min is equal to r 0 or close to it, the positive rake angle α of the cutting edge changes to the negative rake angle α e [13], and elastic deformation occurs between the workpiece and the tool; however, no chips are formed, the tool mainly produces compression and friction force on the workpiece surface, and the ploughing phenomenon occurs. Therefore, in order to form chips continuously and stably in the cutting process, it is necessary to increase the cutting thickness (h 2 ) (Figure 1), so that h min may experience from elastic plastic deformation to shear deformation [14,15]. Takacs & Vero (2007) [16] found that h min has a high correlation with f z . Other researchers [4,17,18] reported that h min can be determined by analyzing the cutting force without considering the workpiece material, and h min varies from 1/4 r 0 to 1/3 r 0 .In summary, most researchers in the research field of micro-milling firstly determine the h min range by experiment or simulation, and then use the qualitative analysis method to study the influence of micro-milling surface quality and the ...