Speeds and Feeds Calculator

This speeds and feeds calculator will help you set the rotation speed and feed rate of your machine tool.

Spindle speed2,673.8 rpm350 SFM × 12 ÷ (π × 0.5") — the familiar 3.82 × SFM ÷ D shortcut is 12/π

Feed rate32.086 inches a minute2,674 rpm × 0.003" × 4 teeth

Feed per revolution0.012 inches

Chip load per tooth0.003 inchesthe number that actually governs tool life — too light rubs and work-hardens, too heavy breaks the tool

Cutting speed in metric106.68 m/min

Tool diameter in metric12.7 mm

Feed rate in metric814.98 mm/min

Material removal rate0.8021 cubic inches a minute13.14 cm³/min

Radial engagement50% of the diametera light radial cut, where chip thinning means the actual chip is thinner than the programmed load and the feed can often be raised

Rough spindle power needed32.8618 kWat about 2.5 GJ per cubic metre for steel — a very rough figure that varies severalfold with material

A 0.25" tool5,348 rpmat the same surface speed — smaller tools must spin faster

A 0.375" tool3,565 rpmat the same surface speed — smaller tools must spin faster

A 0.75" tool1,783 rpmat the same surface speed — smaller tools must spin faster

A 1" tool1,337 rpmat the same surface speed — smaller tools must spin faster

The formula

rpm = surface speed ÷ (π × diameter); feed = rpm × chip load × teeth

Surface speed is the material's limit, not the machine's

Cutting speed is a property of the material and the tool, not of the spindle. A given combination has a surface speed at which the edge lasts — and since a small tool covers less distance per revolution, it must turn faster to reach the same surface speed. That is the whole content of the rpm formula, and the familiar 3.82 × SFM ÷ D shortcut is just 12/π with the inches folded in.

Chip load per tooth is the figure that governs tool life, and it is possible to err in both directions. Too heavy and the edge chips or the tool breaks. Too light and the tool rubs rather than cuts, which generates heat, work-hardens the surface and destroys the edge faster than a heavy cut would — the common mistake of nervous feeds.

At light radial engagement, chip thinning means the actual chip is thinner than the programmed load, because the tool only sweeps a shallow arc. The feed can usually be increased substantially in that regime, which is the basis of high-efficiency milling strategies. These figures are a starting point for a rigid setup; deflection, coolant and machine stiffness all move the real answer.