Drill Speed Chart: Cutting Speeds & Feeds for HSS, Cobalt & Carbide

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Cutting Speed vs RPM — What's the Difference?

Before reaching for the drill speed chart, it helps to understand what "cutting speed" actually means — because it's not the same thing as spindle RPM.

Cutting speed (CS) is the speed at which the cutting edge moves through the material, expressed in metres per minute (m/min). It's a property of the cutting interface — it describes how fast the tool tip is travelling relative to the workpiece. Optimal cutting speed is determined by the tool material, workpiece material, and desired surface finish.

Spindle speed (RPM) is how fast the drill, lathe, or milling spindle rotates. It's what you actually set on the machine. RPM is calculated from cutting speed and drill diameter using the formula below.

The relationship matters because the same RPM produces very different cutting speeds for different drill diameters. A 3mm drill at 2,000 RPM has a cutting speed of 18.8 m/min. A 25mm drill at the same 2,000 RPM has a cutting speed of 157 m/min — eight times faster. Running large-diameter drills at spindle speeds appropriate for small drills is one of the most common causes of premature tool failure.

RPM Formula for Drilling and Turning

The formula for converting cutting speed to RPM is:

N (RPM) = (CS × 1000) ÷ (π × D)

Which simplifies to the practical approximation:

N ≈ (CS × 318) ÷ D

Where:

• N = spindle speed in RPM
• CS = cutting speed in m/min (from reference tables)
• D = drill or cutter diameter in mm
• 318 = 1000 ÷ π (rounded)

Worked Examples

In practice, use the calculated RPM as your starting point, then adjust based on chip colour, surface finish, vibration, and tool wear. The chart values are guidelines, not absolutes.

Cutting Speed Reference Table — by Material and Tool Type

The following table gives recommended cutting speeds in metres per minute (m/min) for drilling. These are general-purpose values for standard twist drills under typical workshop conditions with cutting fluid applied. Adjust for specific alloys, coatings, and machine rigidity as noted.

Drill Speed Chart — RPM by Diameter and Material

The following tables give directly-usable RPM values for common drill diameters and materials. Values are calculated from mid-range cutting speeds for HSS twist drills with cutting fluid. For cobalt or carbide drills, apply the multiplier from the tool-type table below.

Mild Steel (Low Carbon Steel)

Stainless Steel (304/316 Austenitic)

Aluminium Alloy

Cast Iron (Grey)

Brass (Free-Cutting)

Stainless Steel — The Work-Hardening Warning

Stainless steel deserves special attention because it behaves differently from mild steel in a way that trips up experienced machinists. Austenitic grades (304, 316) and duplex grades work-harden rapidly when cut — meaning the surface of the material becomes progressively harder as the tool rubs against it. This creates a vicious cycle: rubbing causes hardening, hardening causes more rubbing, and within seconds the work surface is significantly harder than the bulk material.

The rules for stainless steel drilling:

• Never allow the drill to dwell or rub. Constant positive feed is mandatory. If the drill stops cutting and starts rubbing, the surface hardens immediately and the drill will no longer penetrate regardless of additional pressure.
• Use cobalt HSS or carbide drills. Standard HSS drills can be used for occasional work in thin sheet, but cobalt (M35 or M42) is the correct tool for stainless. Cobalt retains its hardness at the cutting edge temperatures stainless generates.
• Flood coolant, not air blast. Stainless retains heat at the cutting interface. Cutting fluid (soluble oil or neat cutting oil) is essential to draw heat away and prevent work hardening from thermal effects.
• Pilot drill large holes. Drilling in one pass with a large drill on stainless creates excessive thrust, heat, and rubbing at the chisel edge. Pilot drill to 50–60% of final diameter, then follow with the finish drill at reduced feed.
• Reduce speed, increase feed. The instinct when a drill slows is to increase speed. With stainless, the opposite is correct: reduce speed (to limit heat) and maintain or increase feed (to ensure the cutting edge is always engaging fresh material rather than rubbing work-hardened surface).

For dedicated stainless drilling tools, see our range of cobalt drill bits — M35 (5% cobalt) for general stainless and M42 (8% cobalt) for duplex and higher-alloy grades.

Tool Type Speed Multipliers

The drill speed chart values above are for standard HSS twist drills. If you're using cobalt or carbide drills, adjust as follows:

Important: Carbide drills require rigid machine setups and accurate work holding. The brittleness of carbide means it will shatter under the deflection that HSS would tolerate in a hand drill or poorly-aligned drill press. Carbide is for CNC and rigid machining centres.

Lathe Turning Speeds

The same RPM formula applies to lathe turning. D is the diameter of the workpiece being turned, not the diameter of a tool.

Facing vs turning note: When facing (cutting across the end of a bar), the cutting speed changes continuously as the tool moves from the outer diameter toward the centre — the surface speed drops to zero at the centre. On a manual lathe, this means speed should ideally increase as the tool approaches the centre. On CNC lathes this is handled by constant surface speed (CSS) mode. On manual lathes, starting at the correct speed for the outer diameter and accepting a brief over-slow pass near the centre is standard practice.

Milling Speeds

For milling, D is the diameter of the milling cutter, not the workpiece. Feed rate in milling is specified in mm per tooth (chip load) rather than the single feed rate used for drilling.

Tapping Speeds

Tapping is significantly more sensitive to speed than drilling because the tap is driving threads into material with much less cutting clearance than a drill. Excessive speed causes tap breakage; too slow causes poor surface finish and work hardening in stainless.

Tap breakage is almost always caused by: wrong drill size (resulting in too-tight thread engagement), misalignment (tap not square to hole), incorrect cutting fluid, or excessive speed. See our Tap and Die Guide for tap drill selection charts and threading technique.

Feed Rate for Drilling

Feed rate is the distance the drill advances per revolution, expressed in mm/rev. Correct feed is as important as correct speed — a drill running at the right RPM but too-light a feed will rub rather than cut, generating heat and work-hardening the material.

Hand drilling note: Feed rate in hand drilling is controlled by feel rather than measured values. The correct feel is steady downward pressure producing a continuous chip — not intermittent grabbing. If chips are short and powdery, the drill is rubbing rather than cutting; increase feed pressure. If the drill grabs suddenly (particularly in brass), reduce feed and ensure the drill point angle is appropriate for the material.

Cutting Fluid Selection

The right cutting fluid prevents built-up edge, reduces heat, improves surface finish, and extends tool life. The wrong choice — or using none at all — can cause premature tool wear, work hardening, and poor dimensional accuracy.

For a full range of cutting fluids and metalworking lubricants, see our cutting fluids collection — including Tap Magic, Rocol, and CRC cutting and tapping compounds.

Fault-Finding: Common Drilling Problems

Deep Hole Drilling — Speed and Feed Adjustments

When hole depth exceeds three times the drill diameter (3×D), standard cutting conditions need modification. Heat and chip evacuation become the limiting factors.

Tool Selection Reference

For our full range of drilling products, including HSS and cobalt twist drills, step drill bits, and annular cutters, see our drilling collection. For cobalt-specific products, see our cobalt drill bits range.

Frequently Asked Questions — Drill Speeds and Cutting Feeds

What RPM should I use for a 10mm drill in mild steel?
Using the standard formula N ≈ (CS × 318) ÷ D, with a cutting speed of 25 m/min for mild steel with HSS: (25 × 318) ÷ 10 = 795 RPM. Set your drill press to the closest available speed — typically 800 RPM. For a cobalt drill, you can increase by 20–50% (approximately 950–1,200 RPM).

What is the difference between cutting speed (m/min) and RPM?
Cutting speed is a property of the material and tool — it describes how fast the cutting edge should move through the workpiece. RPM is what you set on the machine — it depends on both cutting speed and drill diameter. Two different diameter drills run at the same RPM will have very different cutting speeds at their tips. The formula N = (CS × 318) ÷ D converts the material/tool cutting speed recommendation into the specific RPM for your drill diameter.

What happens if I drill stainless steel too slowly?
Counterintuitively, drilling stainless steel too slowly can be worse than running at the correct speed. The key issue with stainless is work hardening — if the drill dwells, rubs, or advances too lightly, the surface hardens faster than the drill can cut it. The correct approach is: run at the recommended (relatively low) RPM for stainless, but maintain a firm, consistent downward feed pressure so the drill is always cutting fresh material. Never let the drill rub without advancing.

Why does my drill keep breaking in stainless steel?
Drill breakage in stainless steel is most commonly caused by: (1) using a standard HSS drill instead of cobalt — HSS loses its hardness at the temperatures stainless generates; (2) insufficient cutting fluid — stainless needs flood coolant or at minimum a generous application of neat cutting oil; (3) drill misalignment — even slight wobble in stainless causes loading that breaks the drill; (4) drill not perpendicular to the work surface; (5) chip packing — in blind holes, peck drilling is essential to clear chips. Cobalt (M35 or M42) drills with flood coolant and a firm, constant feed eliminate most stainless breakage issues.

How do I choose between HSS and cobalt drill bits?
Use HSS M2 for mild steel, cast iron, aluminium, brass, and general-purpose workshop drilling where temperatures are moderate. Choose cobalt (M35 or M42) for stainless steel, alloy steels, hardened materials, and any application generating significant heat. Cobalt retains its hardness at higher cutting temperatures, so it remains sharp longer under conditions that would soften HSS. The cost premium is justified whenever you are drilling more than a few holes in stainless or alloy steel. For carbide: reserved for CNC and rigid machining setups only.

What cutting fluid should I use for stainless steel?
For stainless steel drilling and tapping, use neat cutting oil (not soluble oil at low concentration). Neat cutting oil provides better lubrication at the cutting interface and more effective heat management than dilute soluble oil. For tapping stainless specifically, a sulphurised or chlorinated cutting compound gives the best results. Products like Tap Magic Stainless, Rocol RTD, or CRC TapMagic are formulated specifically for difficult ferrous materials including stainless. Do not use water-based coolant alone for stainless tapping.

What is "peck drilling" and when should I use it?
Peck drilling is a technique where the drill is repeatedly advanced a short distance (typically equal to the drill diameter) then retracted to clear chips, before advancing again. It is used for: deep holes (more than 3× drill diameter), gummy materials that produce long stringy chips (aluminium, mild steel in some alloys), blind holes where chip evacuation is restricted, and small-diameter drills where chip compaction could break the drill. On CNC machines, peck cycles are programmed with a G83 canned cycle. On manual drill presses, peck drilling is done by feel — advance, feel resistance increase as chips compact, retract to clear, advance again.

Why is my drill bit overheating?
Drill bit overheating is caused by excessive heat at the cutting interface, with insufficient heat removal. The most common causes: (1) speed too high for the material — reduce RPM; (2) insufficient cutting fluid — apply more, or switch to a more effective product; (3) dull cutting edge — the drill is rubbing rather than cutting, generating friction heat; replace the drill; (4) feed too light — a drill that is barely advancing is rubbing rather than cutting; increase feed pressure; (5) chip packing in the flutes — retract to clear and use peck drilling. A blue colour on the tip of an HSS drill indicates it has been overheated and its hardness has been tempered out — the drill should be replaced, not just sharpened.

Can I drill cast iron dry?
Yes — and for grey cast iron, dry drilling is actually preferred. Cast iron produces a fine powder chip (not a continuous chip like steel), and applying water-based coolant to grey cast iron causes thermal cracking from the temperature differential between the hot chip zone and the coolant, plus accelerated rust on any machined surfaces. Drilling grey cast iron dry with light air blasting to clear dust is standard practice. The dust produced is a respiratory hazard — use appropriate PPE and local extraction. Note that ductile (nodular) cast iron produces actual chips rather than powder and can tolerate coolant, but dry or minimum quantity lubrication remains common.

What drill speed should I use for aluminium?
Aluminium responds to high cutting speeds — much higher than steel. A typical HSS drill in aluminium alloy (6061, 7075) runs at 60–120 m/min cutting speed, which for a 10mm drill gives 1,900–3,800 RPM. The risk in aluminium is not heat from speed (aluminium dissipates heat well) but built-up edge (BUE) — where aluminium welds to the cutting edge and is then pulled out as a lump, leaving a poor surface. BUE is prevented by using flood coolant or cutting oil, keeping drills sharp, and maintaining positive feed. Kerosene is an effective and traditional cutting fluid for aluminium; mineral-based soluble oils also work well.

How do I stop a drill from wandering when starting a hole?
Drill wander on entry is caused by the drill tip not locating on the workpiece before the cutting edges engage. Solutions: (1) Always centre-punch the intended hole location — the punch indent gives the drill tip a seat to start from; (2) Use a spotting drill (a short, rigid drill with a 90° or 120° point angle) to create a precise, rigid starting indent before using the full twist drill; (3) Reduce feed pressure at the very start of the hole until the drill has committed to the location; (4) Ensure the work is flat and the drill press table is square — drilling into an angled surface will cause the drill to slide toward the low side of the surface; (5) Clamp the work — never hold a workpiece by hand when drilling.

People Also Ask — Cutting Speeds and Feeds

Q: What cutting speed should I use when drilling cast iron?

Cast iron is machined dry — no cutting fluid — because coolant can cause thermal shock cracking in cast iron and the graphite in cast iron provides its own lubrication. Cutting speeds for HSS drills in grey cast iron typically range from 20 to 30 m/min; for carbide drills this can rise to 80 to 120 m/min or higher depending on the grade and geometry. The exact speed depends on the cast iron type — grey cast iron cuts relatively freely, while hard spots in white iron or chilled cast iron may require slower speeds and carbide tooling. Cast iron produces a fine abrasive dust rather than a chip, so air blast rather than coolant is used to clear the machining zone.

Q: How do I calculate the table feed rate for a milling operation?

Table feed rate is calculated from the feed per tooth (chip load), the number of flutes on the cutter, and the spindle RPM: Feed Rate (mm/min) = chip load (mm/tooth) × number of flutes × RPM. For example, a 4-flute 10mm end mill at 3,000 RPM with a 0.02mm/tooth chip load gives: 0.02 × 4 × 3,000 = 240 mm/min. The chip load for a specific cutter and material combination is given in the cutter manufacturer’s application data. Starting at the lower end of the recommended chip load range and increasing while monitoring chip formation and surface finish is the safe approach.

Q: How deep should peck drilling intervals be?

Peck drilling — withdrawing the drill periodically to clear chips and re-introduce cutting fluid — is used for deep holes and gummy materials. The peck depth (distance between each withdrawal) depends on the drill diameter and material: for general steel, pecking every 4 diameters (4D) is a conservative starting point; for deeper holes, reducing to 3D or 2D prevents chip packing. For aluminium, longer pecks of 6 to 8D are manageable due to aluminium’s good chip evacuation. In very deep holes (beyond 8D), full-retraction peck drilling with flood coolant is needed. Inadequate pecking leads to chip packing, drill breakage, and heat damage to the hole wall.

Q: How can I tell from the chips if my cutting speed and feed are correct?

Chip formation is one of the best indicators of cutting condition. For steel, correctly formed chips should be tightly curled, silvery, and warm to the touch — not blue or brown (which indicates too much heat from excessive speed or insufficient coolant) and not long stringy ribbons (which indicate too low a feed rate). For aluminium, chips should be bright and continuous without welding back to the cutter. Powder or dust from steel suggests the feed is too low (rubbing rather than cutting). Chatter marks on the machined surface indicate vibration from excessive cutting forces, tool stick-out, or excessive speed relative to feed. Adjusting speed and feed in small increments and observing the chip change guides the optimisation.

Q: What is the axial depth of cut limit for an end mill?

The axial depth of cut (depth along the tool axis) for an end mill depends on the tool diameter, material, and machining strategy. For full-slot milling (where the cutter is engaged on all sides), axial depth is typically limited to 0.5 to 1.0 times the cutter diameter to avoid excessive deflection and vibration. For peripheral (side) milling with a small radial engagement, axial depths of 2 to 4 times the diameter are achievable. Carbide end mills in rigid setups can push these limits further. Exceeding the recommended axial depth causes cutter deflection, poor surface finish, and premature tool wear or breakage. Manufacturers publish recommended cut parameters specific to each cutter geometry and material.

AIMS Industrial stocks carbide drill bits — see the full range for trade and industrial use.