 Welcome to Calculating Cutting Speeds for Twist Drills. Cutting speeds determine how much material should pass a cutting edge in one minute. The formula for determining revolutions per minute, or RPM, is the cutting speed multiplied by 4 divided by the diameter. Below is a list of common metals and their cutting speeds, which we used in the formula. These cutting speeds are for use with high speed cutting tools only. The number 4 used in the formula is a constant. The diameter in the formula refers to the diameter of the drill bit. Here is an example. We need to drill a 1 ½ inch hole in mild steel. What is the proper RPM? Watch how the formula is filled out. Coming up, we have a few quiz questions. Please pause the video if you need extra time to solve them. Using a calculator, determine the RPM required to drill a 1 ¼ inch hole in a low carbon steel plate. The final answer is 1600. Using a calculator, determine the RPM required to drill a 1 ⅛ inch hole in aluminum. The final answer is 6400. Using a calculator, determine the RPM required to drill a 1 ½ inch hole in a stainless steel plate. The final answer is 640. Congratulations! You have completed calculating cutting speeds for twist drills.