Convert surface speed (SFM) to spindle RPM and calculate feed rate (IPM) for end mills and drills.
In machining, speeds and feeds determine how fast the spindle rotates (RPM) and how quickly the tool advances through material (feed rate in IPM). Get these wrong and you shorten tool life, damage the workpiece, or break the cutter. Get them right and you achieve optimal material removal, smooth surface finish, and maximum tool life. Our calculator converts surface speed (SFM) to spindle RPM and computes feed rate from chip load and flute count.
The constant 3.82 converts surface feet per minute to rotations per minute for a given diameter. A 0.25-inch end mill at 600 SFM runs at 9,168 RPM. The same 600 SFM on a 1-inch face mill produces only 2,292 RPM. Larger tools always run slower at the same surface speed.
Chip load is the thickness of material each tooth removes per revolution. It is the single most important variable for tool life. Too low and the tool rubs instead of cutting, generating heat and work-hardening the surface. Too high and the tool deflects, chatters, or breaks.
For a 0.25-inch 4-flute end mill in aluminum, a typical chip load is 0.003–0.005 inches per tooth. At 9,168 RPM with 4 flutes and 0.004-inch chip load, your feed rate is 146.7 IPM. If you drop to 0.001 inch per tooth, you are rubbing, not cutting. Use our chip load calculator to find the right balance for your material and tooling.
Carbide tooling runs 3–5x faster than HSS. If you are using HSS end mills, divide the SFM recommendations above by 3–4. Carbide is more expensive but pays for itself quickly through reduced cycle time and longer tool life. For production work, carbide is almost always the right choice.
Coolant serves three purposes: reducing heat, lubricating the cut, and flushing chips. Flood coolant is ideal for deep pocketing and slotting. For through-spindle coolant applications, higher pressures (500+ PSI) dramatically improve chip evacuation in deep holes. Air blast works well for aluminum and wood where liquid coolant is undesirable.
Once you have your RPM and feed rate set, verify the bolt pattern or fixture locations with our bolt circle calculator. For shop infrastructure, check wire sizing for your CNC spindle motor with the wire size calculator.
These values are starting points. Adjust based on your machine rigidity, tool holder runout, workholding stiffness, and material condition. Always run a test pass at reduced parameters before committing to full production feeds.