Milling machines are used to machine solid materials. Milling machines are often classed in two basic forms: horizontal and vertical, which refers to the orientation of their main spindle. Unlike a drill press, which holds the workpiece stationary as the drill moves axially to penetrate the material, milling machines also move the workpiece radially against the rotating milling cutter, which cuts on its sides as well as its tip. Workpiece and cutter movement are precisely controlled to less than 0.001 in (0.025 mm), usually by means of precision ground slides and lead-screws. Milling machines may be manually operated, mechanically automated, or digitally automated via computer numerical control.