Cutting Fluids for HSS Drill Bits — What to Use and When to Use Nothing

Cutting fluid is one of those shop topics where received wisdom and actual practice diverge constantly. Some machinists flood everything with soluble oil. Others drill dry all day and see no problems. Some swear by tapping oil for drilling steel. Some use WD-40 on aluminum and nothing else. They're all getting results — but not all of those results are optimal for drill life, hole quality, or the material being cut.

Here's what's actually happening with cutting fluid in HSS drilling and how to match the fluid to the job.

What Cutting Fluid Is Actually Doing

Cutting fluid serves three functions, and understanding which one matters for your situation clarifies which fluid is correct.

Cooling removes heat from the cutting zone. HSS starts losing hardness at around 600°F — above that temperature, the steel itself softens, the cutting edge rounds over, and the bit is permanently damaged (you'll see this as blue or black discoloration up the shank). Flood coolant and mist systems address cooling. A drop of oil applied before drilling a hole addresses it minimally or not at all.

Lubrication reduces friction between the chip and the cutting face, and between the drill body and the hole wall. Lower friction means less heat generated in the first place, better chip flow through the flutes, and less built-up edge (the phenomenon where workpiece material welds itself to the cutting edge). Lubrication is what sulfurized cutting oil provides — chemical compounds that bond to the metal surface and prevent micro-welding at the cutting interface.

Chip flushing clears chips from the hole before they pack and either bind the drill or get re-cut by the cutting edges. This is primarily a function of fluid volume, not fluid chemistry — flood coolant at high flow rate does this better than any other method. In deep holes (depth-to-diameter ratio over 3:1), chip packing is often the failure mode, not heat or edge wear.

Material-by-Material Guide

The Intermittent Application Problem

Many manual machine shops apply cutting fluid by dabbing a brush or squeeze bottle onto the drill before entering the hole. This approach is better than nothing for lubrication, but it provides almost no cooling — the fluid is consumed almost instantly at the cutting zone and can't keep up with heat generation at higher speeds or harder materials.

For production drilling in steel, flood coolant directed at the drill point is substantially better. For one-off and low-volume work, intermittent application of cutting oil is a practical compromise that works acceptably as long as feeds and speeds are conservative enough that heat isn't the limiting factor.

The worst outcome is drilling a material that needs lubrication dry at aggressive speeds. The bit heats up fast, work-hardens the material (in stainless), loses its edge rapidly, and can be permanently damaged by overtemperature if the shank turns blue. If you can only do one thing, slow down and add lubrication.

Products Worth Keeping on Hand

• Sulfurized cutting oil (Tap Magic, Relton A-9, Ridgid thread cutting oil) — for steel drilling and tapping. Dark, sulfur-smell, stains. Worth it for the lubrication chemistry.
• Soluble oil concentrate (mixed to 5–10% for general drilling) — good all-around coolant/lubricant for flood systems.
• Light machine oil or 3-in-1 — adequate for aluminum, copper, and light steel work.
• Nothing — correct for cast iron and most plastics.

WD-40 comes up constantly because everyone has it. It's an acceptable field expedient for light aluminum work but it's not a cutting fluid — it's a water displacement/rust prevention product. It evaporates fast and provides marginal lubrication at best. Use it when you have nothing better and the job is forgiving. Don't build a production process around it.