Drill bits

[TomG]

350 is the perfect rpm for a 7/8" drill in steel. Using that rpm for smaller drills wastes time and for larger drills, would burn them up. The old standby formula for rpm based on cutting speed works on any rotating tool or workpiece. Just take the cutting speed of the material being cut (100 sfpm for steel), multiply it by 4 and divide by the diameter of the tool or rotating workpiece. A 1/4" drill in steel for instance would be 4 X 100 / .25 = 1600 rpm. Use this formula and you'll never waste time, break an end mill for going too slow, or burn up a drill for spinning it too fast.

Tom