Speeds and Feeds

Tool life is influenced most by Cutting Speed, then by the Feed Rate and least by the Depth of Cut. The proper selection these are Important for time and cost.

(The proper coolant is also a factor. Some materials will work harden with the wrong coolant. This post does not cover this....you'll need to further research how to select the best lubricant/coolant.)

The phrase speeds and feeds refers to two separate velocities in machine tool practice, cutting speed and feed rate. They are often considered as a pair because of their combined effect on the cutting process. Cutting speed may be defined as the rate (or speed) that the material moves past the cutting edge of the tool. It is expressed in units of distance along the workpiece surface per time (typically surface feet per minute [sfm]).
SFM varies depending on the material grade of the cutting tool, the type of cutting tool, the type of material being removed and hardness of material being removed. The Machinery Handbook has time proven charts of recommended Cutting speeds (SFM) to use. Also, manufacturers of cutting tools and steel manufacturers have charts as well.

To determine the Spindle Speed in Revolutions per Minute (RPM): use the following formula:
(12 multiplied to the SFM) divided by (3.1414 multiplied to the Diameter)
note: in the above formula, Diameter refers to the Diameter of the mill cutter/drill or the Diameter of the work piece being turned in the Lathe.


Feed rate is the velocity at which the cutter is fed, that is, advanced against the workpiece. It is expressed in units of distance per revolution for turning and boring (typically inches per revolution [ipr] or millimeters per revolution). It can be expressed thus for milling also, but it is often expressed in units of distance per time for milling (typically inches per minute [ipm] or millimeters per minute). Cutting speed and feed rate together determine the material removal rate, which is the volume of workpiece material (metal, wood, plastic, etc.) that can be removed per time unit.
Reference the Machinery Hand Book or other charts for recommended amounts of material removal per cutting edge and "Depth of Cut"

Formula for IPM: IPM=(RPM) x (Number of cutting Edges) x (Amount of material removed per cutting Edge)

Formula for IPR: IPR=(IPM) divided by (RPM)

Formula for Tap Feed: (IPM)=(RPM) divided by (Thread Pitch)

Once you've determined the Speed and Feed, you may have to adjust it to compensate for variables such as cutter geometry, variations in the material hardness/toughness, the rigidity of the machine tool, workpiece set-up and the Horsepower of the machine. Every CNC machine center that I've ever used has Feed and Speed overrides.

Yes, there are speed and feed calculators that you can download from the internet, and master cam calculates speeds and feeds when it post a program. I feel that you should at least know what you are doing when you use feed and speed overrides on a CNC machine tool. Also, you may not always have your PC available to use the downloadable calculators. Computers and machines don't compensate for the variables discussed in the previous paragraph.

Every Machinist should have reference books and charts in their work area with formulas, cutting speeds, chip removal rates and depth of cut.