In this paper, the influence of the flute profile and the core diameter on the milling process is investigated using a two-dimensional finite element (FE) model. The focus is on the analysis of chip shapes, temperature distributions and cutting forces. The model was validated through experimental cutting tests and applied to simulate slot milling in C45 steel. The results show that the investigated influences of changes in flute profile and core diameter become more significant with increasing core diameters. For core diameters exceeding 70%, significant changes in chip formation and chip flow restriction are observed. In addition, the simulations reveal that both lower temperatures and lower cutting forces can be achieved with larger core diameters and with a lower flute curvature. While temperature differences between flute profiles are substantial, variations in cutting forces remain comparatively minor. The presented FE-based approach provides a basis for optimizing chip profiles and core diameters of cutting tools.