I have spent a lot of years watching machinists blame the tool, the feed rate, or the material when the real culprit is sitting right in the coolant tank. Coolant for deep hole drilling is a chemical cocktail, and each additive serves a specific purpose. Understanding what each one does and how they interact is the difference between a process that runs all shift and one that burns through drills before lunch. Here is what I have learned, the hard way.
Why Additives Matter in Deep Hole Drilling
Deep hole drilling is uniquely punishing on coolant. The drill-to-hole-diameter ratio means the cutting zone is far from the coolant inlet, pressures are high, temperatures spike, and chips have a long path to evacuate. Plain oil-water emulsion will not cut it. You need a formulated package that handles extreme pressure, prevents microbial growth, controls foam, and protects your machine tool from corrosion. Your coolant system design matters too — if you are designing a new system or retrofitting one, I covered the fundamentals in Coolant System Design for Deep Hole Drilling.
Extreme Pressure (EP) Additives
EP additives are the workhorses of deep hole drilling coolant. They chemically react with the metal surface at the high temperatures generated at the cutting edge, forming a low-shear sacrificial layer that prevents welding and seizure.
What I use them for: When I am drilling tough materials like stainless steel, Inconel, or titanium, EP additives are non-negotiable. The most common chemistries are sulfurized fats, chlorinated paraffins, and phosphorus compounds. Sulfur-based EP additives are my go-to for ferrous metals — they activate around 600–800 °C and provide excellent anti-weld protection. Chlorinated additives activate at a lower temperature (~300 °C), which can be useful in lighter cuts, but they come with disposal and corrosion concerns that I will cover later.
The trade-off: EP additives are chemically aggressive. Over-treating can stain or etch the workpiece. I have seen shops double the EP concentration thinking it will double tool life, only to end up with corroded parts and foul odor from the coolant breaking down.
Lubricity Additives
While EP additives handle extreme boundary-layer conditions, lubricity additives (sometimes called friction modifiers) reduce the overall coefficient of friction in the mixed-film and hydrodynamic regimes. Fatty acids, esters, and synthetic oils are common choices.
Where they shine: In deep hole drilling, lubricity additives are what help the drill margins glide against the bore wall without scoring. They also reduce torque, which is especially important on smaller diameter drills where torsional failure is a risk. I typically run a fatty-acid-based lubricity package in combination with EP additives for a balanced approach. The lubricity component handles the lower-temperature sliding contact; the EP component covers the high-temperature cutting edge.
Biocides
This one catches a lot of people off guard. Deep hole drilling systems are closed-loop, warm, and full of organic material — a perfect breeding ground for bacteria, fungi, and yeast. Biocides keep the microbial population in check.
Why you care: Microbial growth breaks down the emulsifier package, causes foul odors (that “rotten egg” smell is sulfate-reducing bacteria), shifts pH, and can clog fine filtration systems. I have seen a system go from perfect to unusable in a week because the biocide was neglected.
Types: Formaldehyde-releasing biocides are common and cheap but have health and regulatory baggage. Isothiazolinone-based biocides are more modern and effective at lower concentrations. I prefer a blend that includes both a fast-acting biocide for immediate knockdown and a slow-release one for residual protection. You also want to test for bacteria levels weekly — dip slides are cheap and the data will save you a lot of downtime.
Anti-Foam Agents
Foam is the enemy of deep hole drilling. When coolant foams, you lose lubricity at the cutting zone, pumps cavitate, and chip evacuation suffers. Anti-foam agents (defoamers) break surface tension and allow entrained air to escape.
The tricky part: Anti-foam agents are usually silicone-based or non-silicone (mineral oil, polyglycol). Silicone defoamers are very effective at low concentrations but can be difficult to remove from the coolant if you over-dose — they plate out on surfaces and can interfere with downstream processes like painting or parts washing. I lean toward non-silicone defoamers for deep hole drilling because the coolant is often recirculated for long periods and silicone buildup becomes a maintenance headache.
If foam keeps coming back no matter what you try, I wrote a dedicated guide on Coolant Foaming: Causes and Solutions that covers mechanical causes too — sometimes it is not the chemistry, it is a pump seal leak or a return-line design issue.
Corrosion Inhibitors
Ferrous metals rust. Aluminum corrodes. Copper alloys stain. Corrosion inhibitors form a protective barrier on metal surfaces, both on the workpiece and on the machine tool itself.
What I watch for: Amine-based inhibitors are effective but can yellow copper and brass, so if you are drilling materials with copper-rich inclusions or using bronze bushings in your spindle, you need a non-amine alternative. I also keep an eye on pH — most corrosion inhibitors work best in the 8.5–9.5 range. If pH drifts below 8.0, corrosion rates accelerate even with inhibitor present.
Emulsifiers
Emulsifiers are what keep oil and water mixed in a stable emulsion. Without them, your coolant separates into a floating oil layer and watery sump in hours instead of weeks.
My rule of thumb: Use an emulsifier package matched to your water hardness. Hard water (high calcium/magnesium) can break certain emulsifiers, causing the oil to split out. Soft water can cause excessive foaming with some emulsifier chemistries. I always test my incoming water and adjust the concentrate formulation before it ever goes in the tank. Most coolant suppliers offer formulations for different water hardness ranges — use them.
Additive Compatibility — The Gotchas
Here is where experience really matters. Additives can fight each other.
- EP additives vs. biocides: Some sulfurized EP additives can deactivate certain biocides. If you shock-treat with biocide and then add EP concentrate, you might get a microbial rebound two days later.
- Anti-foam vs. emulsifiers: Over-dosing anti-foam can break the emulsion, especially silicone types. Add anti-foam in small increments, not by dumping a quart in at once.
- Corrosion inhibitors vs. hard water: Some amine-based inhibitors form insoluble soaps with calcium and magnesium, creating a sticky deposit that clogs filters and nozzles.
- Different coolant brands: Never mix coolants from different suppliers without testing first. The additive packages are formulated as a system, and cross-mixing can cause separation, odor, or loss of performance.
Additive Quick Reference
| Additive Type | Primary Function | Typical Concentration | Effect on Tool Life |
|---|---|---|---|
| EP (extreme pressure) | Forms sacrificial layer at cutting edge | 5–15% of concentrate | +++ Major improvement in high-temp alloys |
| Lubricity (friction modifiers) | Reduces sliding friction | 3–10% of concentrate | ++ Reduces torque, scoring, and edge wear |
| Biocide | Controls microbial growth | 0.1–0.5% of sump volume | + Prevents emulsion breakdown and odor issues |
| Anti-foam | Breaks entrained air bubbles | 0.01–0.1% of sump volume | ++ Indirect: ensures consistent lubricity delivery |
| Corrosion inhibitor | Passivates metal surfaces | 2–8% of concentrate | + Prevents rust, extends machine and part life |
| Emulsifier | Stabilizes oil-in-water dispersion | 10–20% of concentrate | + Indirect: maintains consistent coolant properties |
Troubleshooting Additive Issues
| Problem | Likely Cause | Which Additive to Adjust |
|---|---|---|
| Short tool life in stainless/titanium | Insufficient EP activity | Increase EP additive concentration |
| High torque or chatter marks | Lack of lubricity | Boost lubricity additive or add fatty-acid package |
| Rotten egg smell from coolant | Bacterial bloom | Shock-treat with biocide, then maintain residual level |
| Excessive foam at return line | Over-emulsified or mechanical aeration | Reduce emulsifier or add anti-foam in small doses; also check the foaming troubleshooting guide |
| Rust on machine ways or workpiece | pH too low or inhibitor depleted | Adjust pH with buffer and top up corrosion inhibitor |
| Coolant separation in tank | Emulsion instability | Check water hardness; adjust emulsifier blend |
| Sticky deposits on filters | Hard water reacting with amine inhibitors | Switch to non-amine corrosion inhibitor |
| Yellowing of brass/bronze components | Amine-based corrosion inhibitors | Replace with non-amine or azole-based inhibitor |
Monitoring and Maintenance
I check three things weekly: concentration (refractometer), pH (meter or strips), and bacteria level (dip slides). If any one is off, I address it before it becomes a cascade problem. A log sheet on the coolant tank is cheap insurance — write down the date, concentration, pH, biocide additions, and any top-ups. When something goes wrong, that log is the first place I look.
Make-up water quality matters too. Deionized or reverse-osmosis water gives you a clean starting point and eliminates variables. Tap water can introduce chlorides, sulfates, and hardness that interact unpredictably with your additive package.
Key Takeaways
- EP additives and lubricity additives work as a pair — one handles the cutting edge, the other handles the bore wall. Do not skip either for tough materials.
- Biocide is not optional in closed-loop deep hole drilling systems. Test weekly and maintain a residual level.
- Anti-foam agents should be added in small doses. Over-dosing silicone types can cause long-term buildup and emulsion problems.
- Always match your coolant formulation to your water hardness. It solves more problems than any single additive change.
- Never mix coolants from different suppliers without testing. Additive packages are balanced systems and cross-contamination can cause sudden and expensive failures.
- Log your coolant condition weekly. The data will tell you what is drifting before the drills start breaking.