End mills can be classified into two categories based on their cutting direction: up-cut or down-cut. Up-cut end mills eject chips upwards, away from the workpiece. This helps with chip evacuation and reduces the likelihood of chip recutting. We use up-cut end mills when a good finish on the top surface is essential. Down-cut end mills, conversely, push chips downward into the material. This results in cleaner top surfaces but can cause chip packing in slots. Down-cut end mills are ideal for thin materials to prevent lifting or distortion.