How do you calculate spindle RPM from SFM?
RPM is calculated using the formula: RPM = (SFM × 3.82) / Tool Diameter (inches). The constant 3.82 is derived from 12 / π. For metric, use RPM = (SMM × 318.31) / Tool Diameter (mm), where SMM is surface meters per minute. See also our calculate Moment of Inertia Moment of Inertia Ix (about x-axis).
What is Surface Feet Per Minute (SFM)?
Surface Feet Per Minute (SFM) is the speed at which the cutting edge of a tool moves across the workpiece surface, measured in feet per minute. It is a material-dependent recommended value — harder materials require lower SFM to protect the tool edge and prevent heat buildup.
How do you calculate feed rate for milling?
Feed rate (IPM) = RPM × Number of Teeth × Chip Load per Tooth. Chip load is the thickness of material removed by each cutting edge per revolution, and it varies by tool diameter, material, and tool type. For example, a 4-flute end mill at 5,000 RPM with a 0.002" chip load yields 40 IPM.
What RPM should you mill steel?
For mild steel with a coated carbide end mill (0.5" diameter), a typical SFM is around 300–500, giving an RPM of roughly 2,300–3,800. Harder alloy steels and stainless require lower SFM (100–300), so RPM drops accordingly. Always start conservatively and increase as you observe chip color and surface finish. You might also find our calculate Frequency to Period Converter useful.
How does SFM relate to RPM for a given tool diameter?
SFM and RPM are linked by the tool's circumference. A larger diameter tool traveling at the same RPM sweeps more surface per minute — so to maintain the same SFM, a larger tool must run at a lower RPM. The relationship is: SFM = (RPM × Diameter × π) / 12.
How do you calculate feed rate for turning?
In turning, feed rate is typically expressed as inches per revolution (IPR) rather than IPM. Feed Rate (IPM) = RPM × Feed per Revolution (IPR). The chip load concept still applies — consult the insert manufacturer's recommended feed per revolution for the workpiece material and depth of cut.
Why do coated carbide tools allow higher SFM than HSS?
Coated carbide inserts and end mills are much harder and more heat-resistant than high-speed steel. The coating (e.g. TiAlN, TiN) further reduces friction and heat transfer into the tool. This allows carbide tools to run at 3–5× higher surface speeds than HSS, significantly increasing productivity.
What chip load should I use for my tool diameter?
A general rule of thumb: chip load ≈ 1% of tool diameter for most end mills in steel, and 1–2% for aluminum. For example, a 0.5" end mill in aluminum might use 0.005"–0.010" per tooth, while the same tool in stainless steel may use 0.001"–0.002" per tooth. Always check your tool manufacturer's recommendations. Check out our Torque Calculator as well.