Cutting Speed Calculator
Calculate cutting speed in SFM, m/min or RPM for any tool diameter. Includes recommended SFM ranges for aluminum, steel, stainless, titanium and more.
Introduction
The Cutting Speed Calculator is a fundamental tool for CNC machinists and programmers, designed to optimize the rotational speed of a tool or workpiece relative to its diameter. In modern manufacturing, selecting the correct cutting speed (Surface Feet per Minute or SFM) is critical for managing heat generation at the cutting edge, which directly impacts tool life, surface finish, and overall process reliability. Whether you are using high-speed steel (HSS) or advanced coated carbides, understanding the relationship between spindle RPM and surface speed is the first step in any successful machining operation.
How it Works
Our calculator utilizes the mathematical relationship between the circumference of the tool (or workpiece) and its rotational speed. The primary formula used is SFM = (RPM × π × Diameter) / 12 for imperial units, or m/min = (RPM × π × Diameter) / 1000 for metric units. By inputting the tool diameter and either the desired SFM or the current RPM, the calculator instantly solves for the unknown variable. Furthermore, it provides built-in reference ranges for common materials, acting as a guide to prevent running tools too fast (leading to premature wear) or too slow (leading to built-up edge and poor efficiency).
Usage Scenarios
- Programming a 1/2 in 4-flute carbide endmill in 6061-T6 aluminum at 800 SFM: RPM = (800 × 12) / (π × 0.5) = 6,112 RPM, paired with a chip load of 0.005 in/tooth gives a feed rate of 122 IPM.
- Setting up a HSS 1/4 in drill in 1018 mild steel: HSS target SFM is 80–100, so RPM = (90 × 12) / (π × 0.25) ≈ 1,375 RPM — well below typical HSS-on-steel rubbing threshold.
- Turning a 304 stainless shaft on a manual lathe: carbide insert target SFM is 100–180; for a 1.5 in diameter, RPM = (140 × 12) / (π × 1.5) ≈ 356 RPM. Pair with 0.005 in/rev feed for chip evacuation that prevents work hardening.
FAQ
What is the difference between SFM and RPM?
SFM (Surface Feet per Minute) is the linear speed of the cutting edge — a material-and-tool property that stays constant regardless of machine. RPM is the spindle rotation rate needed to achieve that linear speed at a given diameter. A 1.0 in tool at 1,000 RPM runs at 262 SFM; a 0.25 in tool at the same 1,000 RPM only runs at 65 SFM — far below typical cutting recommendations for most materials. Every change in tool diameter requires a recalculated RPM if you want to maintain the same SFM.
What are typical SFM ranges by material and tool type?
HSS in aluminum: 200–400 SFM. HSS in mild steel: 80–120 SFM. HSS in stainless 304: 50–80 SFM. Carbide (uncoated) in aluminum: 600–1,000 SFM. Carbide in mild steel: 400–800 SFM. Carbide in stainless 304: 100–180 SFM. Carbide in titanium Grade 5: 40–80 SFM. Carbide in cast iron: 200–400 SFM. TiAlN-coated carbide can run 30–50% faster than uncoated carbide in steel and stainless. These are starting values — always cross-check tool manufacturer data.
Why does running too slow damage tools, not just waste time?
Below the minimum chip-load threshold, the cutting edge rubs the workpiece instead of shearing chips. Rubbing generates heat without removing material, work-hardens the surface (especially stainless and titanium), and produces built-up edge on the cutting flute. The next pass encounters harder material and a less-sharp tool, accelerating the cycle. The fix is to increase feed per tooth — but you cannot increase feed without first calculating the correct RPM from your target SFM.