Why Drill Bits Fail Early
A quality HSS drill bit should produce hundreds or even thousands of holes before it needs resharpening. When bits fail after just a few dozen holes, the problem is almost never the bit itself — it is how the bit is being used.
The three factors that determine drill bit life are cutting speed (RPM), feed rate (how fast you push), and cooling. Get these right and your tooling costs drop dramatically.
Cutting Speed: The RPM Question
Cutting speed is measured in surface meters per minute (m/min) or surface feet per minute (SFM). The correct RPM depends on the drill diameter and the workpiece material.
Recommended Cutting Speeds for HSS
Mild steel — 25–30 m/min
Stainless steel (304/316) — 10–15 m/min
Aluminum — 60–100 m/min
Cast iron — 20–25 m/min
Brass and copper — 40–60 m/min
Titanium alloys — 8–12 m/min
The RPM Formula
RPM = (Cutting Speed × 1000) / (π × Drill Diameter in mm)
For example, drilling 10mm holes in mild steel at 25 m/min: RPM = (25 × 1000) / (3.14 × 10) = 796 RPM.
Common Mistakes
Running too fast — The number one killer of HSS drill bits. Excessive speed generates heat that softens the cutting edge. If you see blue discoloration on the bit, you are running too fast.
Running too slow — Less common but still a problem. Too-slow speeds cause the bit to rub instead of cut, generating friction heat and work-hardening stainless steel.
Feed Rate: Pressure Matters
Feed rate is how fast the drill advances into the workpiece, measured in mm per revolution.
General Feed Rate Guidelines
Under 3mm diameter — 0.02–0.05 mm/rev
3–10mm diameter — 0.05–0.15 mm/rev
10–20mm diameter — 0.15–0.25 mm/rev
Over 20mm diameter — 0.25–0.35 mm/rev
Signs of Incorrect Feed
Too much feed — Bit chatters, chips are thick and blue, shank may twist or break
Too little feed — Bit rubs instead of cutting, thin powdery chips, excessive heat, work-hardening in stainless
The ideal chip should come off as a continuous curl (in ductile materials) or as consistent small chips (in cast iron and brass).
Coolant: Keep the Heat Down
Heat is the enemy of HSS. Above 600°C, standard M2 HSS begins to lose hardness. Coolant serves two purposes: it removes heat from the cutting zone and lubricates the chip-to-flute interface to reduce friction.
Coolant Options
Soluble oil (emulsion) — The standard for production drilling. Mix ratios of 5–10% oil in water. Provides both cooling and lubrication.
Cutting oil (neat oil) — Best for stainless steel and difficult materials. Superior lubrication but less cooling than emulsion.
Spray mist — Acceptable for light-duty work and aluminum. Better than nothing, but not as effective as flood coolant.
Dry drilling — Acceptable for cast iron, brass, and some plastics. Not recommended for steel or stainless.
Peck Drilling for Deep Holes
When drilling holes deeper than 3× the drill diameter, use peck drilling — advance the bit 1–2 diameters, retract fully to clear chips, then advance again. This prevents chip packing, which causes heat buildup and bit breakage.
Practical Rules of Thumb
If the bit turns blue, slow down — You are generating too much heat
If chips are powdery, increase feed — The bit is rubbing, not cutting
If the bit squeals, add coolant — Friction is too high
Resharpen before the bit is completely dull — A slightly worn bit resharpens easily; a burned bit is scrap
Use the right grade for the material — M2 for general work, M35 for stainless, M42 for aerospace alloys