where SFPM is the surface feet per minute required for cutting, D is the diameter in inches, and RPM is the spindle speed. For cutting hard steel with carbide, the surface speed should probably be in the 500 to 800 surface feet per minute range. Thus, for drilling a 0.25 mm hole, that's about 0.04*(1/4) or 10 mils in diameter. Using 500 ft/minute for SFPM gives 200,000 rpm. Using 800 for SFPM gives 320,000 rpm. Few people have drill presses capable of those speeds. Some folks substitute Dremel tools to get around 20,000 rpm.