How do I convert surface speed to RPM?

Multiply the surface speed by 12 (to turn feet into inches), then divide by π times the tool diameter in inches. A cutter running 100 SFM at half an inch across is (100 × 12) ÷ (π × 0.5) ≈ 764 RPM. In metric it is 1000 times the speed in m/min divided by π times the diameter in millimetres.

What surface speed should I use?

It depends on the tool material and what you are cutting — that is why the surface speed is a field you enter, from your tooling chart or the manufacturer, rather than a number this tool guesses. Soft aluminium runs fast, tough stainless runs slow, and a carbide tool takes more speed than high-speed steel.

Does the same formula work for a lathe?

Yes, with the diameter being the work rather than the tool. On a drill or mill you enter the cutter diameter; on a lathe you enter the diameter of the part where the tool is cutting. The arithmetic — surface speed over circumference — is identical either way.

Why does a bigger bit need fewer RPM?

Because the rim of a big bit travels further per turn. Surface speed is what the cutting edge actually feels, and a larger circumference covers that distance in fewer revolutions. Halve the diameter and, for the same surface speed, the RPM doubles.

Drill speed calculator

This is the spindle speed for the surface speed and diameter you enter. It is a starting point, not a guarantee — feed, coolant, tool condition and how the work is held all matter. Ease in, watch the chip and the finish, and adjust to what the cut tells you.

Enter the surface speed your cutter wants and its diameter, and the tool gives you the spindle RPM to set. It is the same calculation whether you are drilling, milling or turning — surface speed divided by the distance the edge travels each revolution.

Exemple détaillé

A ½-inch high-speed-steel drill in mild steel might run around 100 SFM. That is (100 × 12) ÷ (π × 0.5) ≈ 764 RPM. The same cut in metric — a 12.7 mm bit at 30.5 m/min — is (1,000 × 30.5) ÷ (π × 12.7) ≈ 764 RPM. Swap to a ¼-inch bit at the same speed and it jumps to about 1,528 RPM — half the diameter, double the RPM.

Sur le chantier

Picking the pulley on the drill press. The formula says 764 RPM for the ½ in bit; the press offers 620 and 1,100. Take 620. Under the number the bit cuts a little slower and lasts; over it the bit burns and the hole work-hardens. When in doubt, the next speed down.

Why hole saws smoke. A 4 in hole saw at the same 100 SFM wants (100 × 12) ÷ (π × 4) = 95 RPM (in metric: a 102 mm saw at 30 m/min is (1,000 × 30) ÷ (π × 102) ≈ 94 RPM). A hand drill's slow gear is several hundred — three or four times too fast for steel — which is why the teeth go blue and the cut stalls. Bigger diameter, slower speed; the rim is what matters, not the chuck.

Metric tooling. A 10 mm carbide-tipped bit at 30 m/min from the tooling chart: (1,000 × 30) ÷ (π × 10) = 955 RPM. The chart is the tool maker's for the material; the page just turns the surface speed into a spindle speed.

Exercez-vous

Quick referencefor spindle speed

RPM from SFM: SFM × 3.82 ÷ diameter (in)
RPM from m/min: m/min × 318 ÷ diameter (mm)
At 100 SFM: ⅛ in 3 056 · ¼ 1 528 · ⅜ 1 019 · ½ 764 · ¾ 509 · 1 in 382 · 2 in 191 · 4 in 95
At 30 m/min: 3 mm 3 183 · 6 mm 1 592 · 10 mm 955 · 12 mm 796 · 20 mm 477 · 25 mm 382 · 50 mm 191 · 100 mm 95
Speed units: 1 m/min = 3.281 SFM · 100 SFM = 30.5 m/min · 1 in = 25.4 mm
Scaling: halve the diameter, double the RPM · double the SFM, double the RPM
The SFM: is the tooling chart's for the material and the cutter — not this page's

Mistakes to catchbefore you trust the number

Using the cutting speed for the bit material when the number is for the workpiece — the chart is by what you cut, and steel wants a slower feed than aluminium at the same bit.
Entering a hole-saw diameter as the pilot bit — the RPM belongs to the biggest cutting edge, and a 4 in saw at the pilot's speed burns out.
Typing the diameter in mm while the speed is in SFM; pick one system with the toggle.

La formule

RPM = (SFM × 12) ÷ (π × diameter_in) · RPM = (1000 × m/min) ÷ (π × diameter_mm)

Surface speed is a length per minute; the circumference (π × diameter) is the length the edge covers in one turn. Dividing one by the other gives turns per minute. The 12 and the 1000 just line the length units up — feet to inches, metres to millimetres.

Questions

How do I convert surface speed to RPM?: Multiply the surface speed by 12 (to turn feet into inches), then divide by π times the tool diameter in inches. A cutter running 100 SFM at half an inch across is (100 × 12) ÷ (π × 0.5) ≈ 764 RPM. In metric it is 1000 times the speed in m/min divided by π times the diameter in millimetres.
What surface speed should I use?: It depends on the tool material and what you are cutting — that is why the surface speed is a field you enter, from your tooling chart or the manufacturer, rather than a number this tool guesses. Soft aluminium runs fast, tough stainless runs slow, and a carbide tool takes more speed than high-speed steel.
Does the same formula work for a lathe?: Yes, with the diameter being the work rather than the tool. On a drill or mill you enter the cutter diameter; on a lathe you enter the diameter of the part where the tool is cutting. The arithmetic — surface speed over circumference — is identical either way.
Why does a bigger bit need fewer RPM?: Because the rim of a big bit travels further per turn. Surface speed is what the cutting edge actually feels, and a larger circumference covers that distance in fewer revolutions. Halve the diameter and, for the same surface speed, the RPM doubles.

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