Abstract Micro slot milling micro channels in a single pass vs two step upmilling has the benefit of higher productivity. This problem is approached by showing the theoretical benefits of more than two cutting edges. The acceptable feed per tooth and cutting depth at the maximum undeformed chip thickness for ductile-brittle transition and the benefit of upmilling versus downmilling and other parameters (tool wear, tool trajectory, tool deflection, air pressure/cutting oil) have been assessed by micro and macro-geometrical analyses versus surface finish/transparency, form error of the cross section shape in the micro scale (aspect ratio, tolerances, parallelism, flatness, straightness), chipping and productivity. Quantitative correlation between transparency and roughness is also proposed to estimate the acceptable amount of brittle mode.