Cutting Speed Calculator
Spindle speed is not cutting speed.
Work out Cutting Speed. Spindle speed is not cutting speed. Answers in the units the job actually uses.
Spindle speed
3,820 rpm
Mild steel with carbide · 394 SFM
Surface speed is how fast the cutting edge moves through the material, and it is what tool life depends on. Spindle speed is not — the same 3,000 rpm is gentle on a 3 mm cutter and destroys a 20 mm one in the same steel, because the edge is travelling seven times faster. Tool life falls roughly as the fourth power of surface speed, by Taylor's equation. Running 20% fast halves the life of a cutter; running 20% slow barely extends it, because a different failure mode takes over — too slow and the edge rubs rather than cuts, work-hardening the surface. These figures are starting points for a rigid machine with adequate coolant. A flexing setup, an interrupted cut or a deep pocket all call for less, and the sound of the cut tells you more than any table does.
How the Cutting Speed Calculator works
Converts between spindle speed and surface speed for a tool diameter and material. Surface speed is what tool life depends on — the same rpm is gentle on a small cutter and destroys a large one.
Also known as: what rpm for a 10mm end mill · cutting speed for mild steel · sfm to rpm converter · spindle speed for aluminium
Frequently asked questions
How do I calculate spindle speed?
RPM = 1000 × surface speed ÷ (π × diameter). The diameter in the denominator is why one spindle setting cannot suit two different tools.
What surface speed should I use?
Around 120 m/min for carbide in mild steel and 30 for HSS. Aluminium takes two to three times as much; titanium and stainless considerably less.
What happens if I run too fast?
Tool life falls roughly as the fourth power of surface speed. Running 20% fast halves the life of a cutter, and the edge fails by heat rather than by wear.
What if I run too slow?
The edge rubs rather than cuts, generating heat and work-hardening the surface — particularly destructive in stainless. Too slow is a different failure mode, not a safer one.
Why is carbide so much faster than HSS?
It keeps its hardness at far higher temperatures. HSS softens around 600 °C where carbide holds up past 1,000, and cutting temperature is what limits speed.
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