Machinery's Handbook - 4: Cutting Speeds and Feeds WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor. Now the machining must begin. What RPM should be used for a 3" diameter work piece? What should be the feed rate? A table in the Machinery's handbook gives RPM and Feed rate for 4130 steel with 380 BHN: Feed rate is shown to be .028 inches per revolution and speed is 240 feet per minute. Note: Cutting Speed in surface speed per minute is often simply abbreviated as CS. So the CS of this metal is 240. To find the correct RPM the following formula can be applied: RPM = (CSx4)/Dia. or (240 x 4)/3 = 320 Feed rate in inches per minute (instead of inches per revolution) can be found with this formula: Feed=RPM x IPR where IPR is inches per revolution Feed = 320 x .028 - 8.96 inches per minute.