How is RPM calculated from cutting speed?

For inch tooling, RPM equals (cutting speed in SFM times 3.82) divided by the drill diameter in inches. For metric, RPM equals (cutting speed in m/min times 318.31) divided by the diameter in millimeters. Larger drills must spin slower to keep the rim speed within the material's limit.

What cutting speeds does it use?

It uses common shop values: roughly 100 SFM for mild steel, 50 SFM for stainless, 300 SFM for aluminum, and 80 SFM for cast iron with HSS drills. Carbide drills run two to three times faster. These are starting points to refine by sound, chip color, and finish.

Why does feed depend on drill diameter?

Feed per revolution scales with drill size because a bigger drill removes more material per turn without overloading. A rule of thumb is about 0.001 inch per revolution per 1/16 inch of diameter for small drills, increasing for larger ones, which this tool applies.

When should I peck drill?

Peck when the hole depth exceeds roughly three to four times the drill diameter, so chips can clear and coolant can reach the tip. The tool suggests a peck increment of about one diameter per peck for deep holes; reduce it in gummy materials like stainless.

Should I slow down for hard or thin material?

Yes. Reduce speed for hardened or work-hardening alloys to avoid burning the edge, and reduce feed when breaking through thin sheet to prevent grabbing. Always start conservative and increase once the chip and finish look right.

What is the Drill Feed & RPM per Material Calculator?

Calculates recommended spindle RPM and feed rate for twist drilling in steel, stainless, aluminum, cast iron, and plastic from drill diameter using cutting speed (SFM) and feed per revolution tables. Includes HSS and carbide recommendations and peck-depth guidance for deep holes. It runs free in your browser on Gera Tools, with nothing uploaded.

Drill Feed & RPM per Material Calculator

Name: Drill Feed & RPM per Material Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Getting drill speed and feed right means clean holes, long tool life, and no burned edges or snapped bits. This calculator converts a material’s recommended cutting speed into spindle RPM for your drill diameter, derives a feed rate from feed per revolution, and suggests a peck depth for deep holes.

How it works

The core conversion from surface cutting speed to spindle RPM keeps the drill’s rim speed within the material’s limit:

RPM (inch)   = (SFM × 3.82) / diameter_in
RPM (metric) = (m/min × 318.31) / diameter_mm
feed rate    = feed_per_rev × RPM

Feed per revolution scales with drill diameter because a bigger drill can remove more material per turn without overloading its cutting edges. Carbide tooling runs at roughly two to three times the surface speed of HSS in the same material.

Worked example

A 1/4-inch HSS drill in mild steel at 100 SFM:

RPM  = (100 × 3.82) / 0.25 = 382 / 0.25 ≈ 1,528 RPM
feed ≈ 0.004 in/rev × 1,528 = ~6.1 in/min

Round to 1,500 RPM and 6 in/min on your machine, then adjust by what you hear and see.

Starting speeds by material — HSS drill

Material	Typical SFM	Notes
Mild steel	~100	Most common starting point
Stainless steel	~40–60	Slow down; work-hardens
Aluminium	~200–300	Faster; add cutting fluid to prevent built-up edge
Cast iron	~70–90	Dry or light mist; no coolant flood
Brass	~150–200	Reduce feed or rake to prevent grabbing
Plastic	~100–200	Slow feed to prevent melting

Carbide drills: multiply the HSS SFM by roughly 2–3× for the same material.

Reading the chips

The chips coming off the drill tell you more than any table. Long, silvery, tightly curled chips mean the speed and feed are in a good range. Short, powdery chips mean too slow or too high a feed. Blue or discoloured chips mean too much heat — back off the speed. A squeal under load usually means the drill is rubbing rather than cutting; increase feed slightly.

Deep holes and peck drilling

When hole depth exceeds about three times the drill diameter, chips can pack in the flute and break the drill. Peck drilling — retracting the drill fully (or partially) on each pass — clears chips and lets coolant reach the tip. A peck increment of roughly one drill diameter per peck works in most materials; reduce to half a diameter or less in gummy materials like stainless or titanium.

Always clamp the workpiece, use cutting fluid for steel and stainless, and start with conservative settings that you increase once the first few holes look clean.