Cutting Fluid for HSS Drill Bits — What to Use by Material

Cutting fluid does two things for HSS drill bits: it cools the cutting zone and lubricates the interface between the drill margin and the hole wall. Get it right and your bits stay sharp longer. Get it wrong and you're either wasting money on fluid that does nothing or, worse, actively damaging your work.

STEEL (MILD AND LOW-ALLOY)

Mild steel is the baseline case. Soluble oil (also called water-miscible coolant) at 5–10% concentration works well for production work. For manual drill press operations, a sulfurized cutting oil applied from a brush or squirt bottle is simpler and effective. The sulfur additives provide extreme-pressure lubrication that soluble oil at low concentrations can miss.

Avoid: dry drilling mild steel at high production rates. Heat builds fast, burns the cutting edge, and sends bits in for resharpening earlier than necessary.

STAINLESS STEEL

Stainless work-hardens under the tool. If you dwell, peck too slowly, or run without fluid, the zone ahead of the drill gets harder than what you started with. Use a high-sulfur cutting oil or a dedicated stainless cutting fluid. Keep feed rates up — dwelling is worse than moving too fast. Flood coolant if you have it; otherwise, apply cutting oil continuously through the operation.

ALUMINUM

Aluminum wants lubrication, not coolant. The material cuts easily but tends to weld to the tool face — called built-up edge (BUE). This is what causes the sudden grab and galling you see when drilling aluminum dry.

Use a light cutting oil, WD-40, or even isopropyl alcohol for thin aluminum. Kerosene works well for production drilling on aluminum. Avoid sulfurized cutting oils — some react with aluminum alloys and cause staining. Water-miscible coolants at proper concentration also work but are overkill for manual work.

CAST IRON

Drill cast iron dry. This is the exception to almost everything else. Cast iron produces a graphite powder chip rather than a curling chip. Coolant turns that powder into an abrasive slurry that accelerates wear on both the drill and the hole surface. Use air blast or vacuum instead to clear chips. Reduce speed versus mild steel (cast iron is abrasive) and use shorter peck cycles.

PLASTICS

Most plastics drill best with sharp bits and no fluid. Coolant can crack some thermoplastics, especially acrylic. Compressed air to clear chips is the better approach. If the material is melting rather than cutting, slow down the speed — plastics have low thermal conductivity and build heat fast. A slower RPM with a sharp bit almost always beats a faster one with coolant on plastics.

THE SHORT VERSION

• Mild steel: soluble oil or sulfurized cutting oil
• Stainless steel: high-EP sulfurized oil, continuous application, don't dwell
• Aluminum: light oil, kerosene, or isopropyl — not sulfurized, not dry at speed
• Cast iron: dry — air blast only
• Plastics: dry — reduce speed if melting

The fluid choice is secondary to sharp geometry and correct feeds/speeds. A sharp bit with the wrong fluid usually outperforms a dull bit with the perfect one. Keep your bits sharp and the fluid decision gets easier.

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