CNC Spindle Speed Calculator

How it is calculated

The tool works in metric units. Spindle speed is RPM = 1000 × Vc ÷ (π × D) with cutting speed Vc in m/min and tool diameter D in mm. Table feed is feed = RPM × fz × z with feed per tooth fz in mm and number of teeth z, giving mm/min. The feed is calculated from the unrounded RPM and both results are rounded to whole numbers. Cutting speed and diameter must be above zero; feed per tooth and teeth may be zero (the feed is then 0 mm/min), but not negative. The result is a starting point from the tool maker's chart: it does not check machine maximum RPM, depth of cut, chip thinning or the rigidity of the setup. For inch tools, convert to mm and m/min first.

Common mistakes

• Entering the radius instead of the diameter, which doubles the RPM.
• Using surface speed in feet per minute (SFM) in the cutting speed field; it expects metres per minute, and SFM is about 3.28 times larger.
• Entering the feed per revolution instead of the feed per tooth, or forgetting to count the teeth, so the feed is several times too slow or too fast.
• Using the full tool diameter for a ball nose or a chamfer cutter engaged only at its tip; the effective cutting diameter is smaller, so the real surface speed is lower than planned.
• Accepting the RPM without checking it against the spindle limit; a small cutter in aluminium can calculate to far more than the machine can turn.