Appearance
A deep hole drilling shop operating six BTA and four gun drilling machines uses a neat oil coolant system with a 12,000‑litre central reservoir. Over 18 months, the shop experiences increasing tool wear, bore surface finish deterioration, and coolant system fouling. Analysis reveals that the active sulfur EP additive concentration has depleted from the initial 1.8% to 0.6% due to continuous filtration and chip carry‑off, while water ingress from coolant leaks has increased the acid number from 0.15 to 0.85 mg KOH/g, accelerating corrosion on guide pads and spindle components. The shop implements a coolant additive maintenance program including monthly sulfur concentration testing (ASTM D1662), quarterly acid number testing (ASTM D664), biocide dosing for water‑miscible coolant on two aluminium‑dedicated machines, and a scheduled additive replenishment protocol. After program implementation, tool life recovers to baseline levels, guide pad wear reduces by 40%, coolant service life extends from 8 months to 24 months, and annual coolant‑related costs decrease by $38,000.
Coolant Types for Deep Hole Drilling
| Coolant Type | Base Oil Viscosity | Typical Use | Additive Requirements | Cooling | Lubrication | Chip Flushing |
|---|---|---|---|---|---|---|
| Neat cutting oil (low viscosity) | 7–15 cSt at 40°C | Gun drilling, small BTA | EP (S/P/Cl), corrosion inhibitor, anti-mist | Moderate | Excellent | Excellent |
| Neat cutting oil (medium viscosity) | 15–32 cSt at 40°C | BTA drilling, large diameters | EP (high sulfur 15–35%), wetting agents | Moderate | Excellent | Good |
| Water-miscible semi-synthetic | Emulsion | Aluminium, non-ferrous, moderate depths | EP, biocide, corrosion inhibitor, emulsifier | Excellent | Good | Moderate |
| Water-miscible synthetic | Solution | High-speed machining, cooling-critical | Corrosion inhibitor, biocide, wetting agent | Excellent | Moderate | Moderate |
| High-EP heavy-duty oil | 30–50 cSt at 40°C | Large BTA, trepanning, difficult materials | High sulfur (20–40%), chlorinated or replacement | Low | Excellent | Low |
EP Additive Types
| Additive Type | Active Element | Typical Concentration | Max Operating Temperature | Yellow Metal Safe | Environmental Concerns |
|---|---|---|---|---|---|
| Sulfurized fatty oil | Sulfur 8–20% | 5–15% in concentrate | 300°C | Yes (with inhibitor) | Low — biodegradable |
| Sulfurized hydrocarbon | Sulfur 15–40% | 3–10% in concentrate | 400°C+ | No (stains copper) | Moderate |
| Chlorinated paraffin | Chlorine 40–70% | 5–20% in concentrate | 400°C+ | Yes | High — disposal restrictions |
| Phosphate ester | Phosphorus 5–15% | 2–8% in concentrate | 250°C | Yes | Low |
| Overbased calcium sulfonate | Calcium, sulfur | 3–15% in concentrate | 350°C | Yes | Low |
| Nano-MoS₂ / WS₂ | Molybdenum, tungsten | 0.5–3% in concentrate | 600°C+ | Yes | Very low — solid lubricant |
| Borate ester | Boron | 1–5% in concentrate | 250°C | Yes | Low |
Active Sulfur EP Additives Comparison
| Product | Total Sulfur | Active Sulfur | Viscosity at 40°C | Copper Corrosion (ASTM D130) | Best For |
|---|---|---|---|---|---|
| Additin RC 2541 | 40% | 35% | Low | 1a–1b (non-corrosive) | Deep hole drilling, high reactivity |
| Additin RC 2526 | 26% | 15% | 80–120 cSt | 2–3 (moderate) | Heavy-duty cutting, moderate pressure |
| Additin RC 2418 | 18% | 9% | 55 cSt | 2 (moderate) | General drilling, broaching |
| Additin RC 2317 | 17% | 8% | 55 cSt | 1b–2 (low) | Deep hole drilling, honing |
| Sulfurized ester (custom) | 10–15% | 5–10% | Variable | 1a–1b | Environmentally sensitive applications |
TIP
Active sulfur content is the key performance parameter for deep hole drilling EP additives — not total sulfur. Active sulfur is the fraction that reacts with the metal surface at cutting temperatures to form the iron sulfide tribofilm that prevents welding and reduces friction. A finished BTA drilling oil should contain 1.5–2.5% active sulfur. If your tool life has degraded and sulfur analysis shows active sulfur below 1.0%, replenish the additive package to restore performance. Using oil analysis is more reliable than calendar-based replacement.
Corrosion Inhibitors for Deep Hole Drilling
| Inhibitor Type | Mechanism | Concentration | Temperature Limit | Best For |
|---|---|---|---|---|
| Calcium sulfonate | Forms protective film on metal surface, neutralises acids | 2–10% | 350°C | Guide pads, spindle bearings, coolant lines |
| Amine carboxylate | Adsorbs on metal surface, pH buffering | 0.5–3% | 200°C | Cast iron, steel workpieces |
| Benzotriazole (BTA) | Forms protective layer on copper alloys | 0.05–0.5% | 250°C | Yellow metal protection (bushings, seals) |
| Sodium nitrite (restricted) | Oxidises metal surface to passive layer | 0.5–2% | 150°C | Steel and cast iron (banned in many regions) |
| Molybdate | Forms iron molybdate passive layer | 0.5–2% | 300°C | Non-toxic alternative to nitrite |
| Phosphate ester | Adsorbs and forms iron phosphate film | 1–5% | 250°C | Combined EP + corrosion inhibition |
Biocide Selection for Water-Miscible Coolants
| Biocide Type | Active Ingredient | Effective pH Range | Target Organisms | Service Life | Restrictions |
|---|---|---|---|---|---|
| Triazine | 1,3,5-tris(2-hydroxyethyl)-hexahydrotriazine | 7–10 | Bacteria | 2–4 weeks | Formaldehyde releaser |
| Isothiazolinone | CMIT/MIT blend | 6–9 | Bacteria, fungi | 4–8 weeks | Skin sensitiser |
| Phenolic | o-phenylphenol | 7–10 | Fungi, yeast | 4–6 weeks | Environmental concerns |
| Bronopol | 2-bromo-2-nitropropane-1,3-diol | 6–8 | Bacteria | 2–4 weeks | Nitrosamine risk |
| Glutaraldehyde | 1,5-pentanedial | 7–9 | Bacteria, biofilm | 2–4 weeks | Respiratory sensitiser |
| Grotan OX (Vink) | Oxazolidine-based | 7–10 | Bacteria, fungi, SRB | 4–8 weeks | Low toxicity profile |
WARNING
Biocides must be selected based on the specific coolant formulation and operating conditions. Adding an incompatible biocide can cause coolant splitting, emulsion breaking, or additive precipitation. Always perform a compatibility test with a small coolant sample before full-system dosing. Over-dosing biocide does not improve microbial control and can accelerate coolant degradation. For neat oil systems (the standard for BTA drilling), biocides are not required — water-miscible coolants only.
Additive Depletion Monitoring
| Parameter | Test Method | New Coolant Baseline | Action Limit | Replenishment Action |
|---|---|---|---|---|
| Active sulfur | ASTM D1662 | 1.5–2.5% | < 1.0% | Add sulfurized EP additive to restore to target |
| Total sulfur | ASTM D1552 | 2.5–4.0% | < 2.0% | Verify with active sulfur test |
| Acid number | ASTM D664 | 0.1–0.3 mg KOH/g | > 0.5 mg KOH/g | Check water content, consider coolant reclamation |
| Viscosity at 40°C | ASTM D445 | 12–18 cSt | ±20% from baseline | Verify additive depletion or tramp oil contamination |
| Copper corrosion | ASTM D130 | 1a–1b (light tarnish) | > 2c (moderate tarnish) | Replenish corrosion inhibitor package |
| Water content | ASTM D6304 | < 0.1% | > 0.5% | Find and repair coolant leak sources |
| Particle count | ISO 4406 | < 18/16/13 | > 22/20/17 | Improve filtration or replace coolant |
| Iron content | ICP/OES | < 10 ppm | > 50 ppm | Investigate abnormal tool or guide pad wear |
| pH (water-miscible) | pH meter | 9.0–9.5 | < 8.5 or > 10.0 | Adjust with pH buffer or replace coolant |
| Refractive index (emulsion) | Refractometer | Per manufacturer | ±0.5% from target | Adjust concentration |
Coolant Change Criteria
| Parameter | Neat Oil (BTA/Gun Drilling) | Water-Miscible (General) |
|---|---|---|
| Typical service life | 12–24 months | 3–12 months |
| Change trigger — primary | Acid number > 0.8 mg KOH/g | pH < 8.5 or bacterial count > 10⁶ CFU/mL |
| Change trigger — secondary | Viscosity change > 20% | Concentration drift > 2% |
| Change trigger — tertiary | Particle count > 22/20/17 | Tramp oil > 5% |
| Reclamation possible | Yes (filtration, additive replenishment) | Limited (emulsion breaking) |
| Disposal method | Licensed waste oil collector | Wastewater treatment or licensed collector |
| Cost per litre (disposal) | $0.50–1.50 | $1.00–3.00 |
Coolant Maintenance Schedule
| Task | Frequency | Method | Responsibility |
|---|---|---|---|
| Visual inspection | Daily | Check coolant clarity, odour, tramp oil layer | Machine operator |
| Concentration check (emulsion) | Weekly | Refractometer reading | Machine operator |
| pH check (emulsion) | Weekly | pH meter or test strips | Machine operator |
| Active sulfur test (neat oil) | Monthly | ASTM D1662 — wet chemistry | Coolant lab or supplier |
| Acid number | Monthly | ASTM D664 — titration | Coolant lab |
| Viscosity | Monthly | ASTM D445 — viscometer | Coolant lab |
| Copper corrosion test | Monthly | ASTM D130 — copper strip | Coolant lab |
| Water content | Monthly | ASTM D6304 — Karl Fischer | Coolant lab |
| Particle count | Quarterly | ISO 11500 — laser particle counter | Coolant lab |
| Microbiological plate count (emulsion) | Monthly | Dip slides or plate count | Maintenance team |
| Biocide dosing (emulsion) | As needed (per test results) | Calculate based on system volume | Maintenance team |
| EP additive replenishment | As needed (per sulfur test) | Calculate based on system volume | Coolant supplier |
| Coolant reclamation | Annually | Centrifuge or filter to remove fines | Coolant supplier |
| System clean and recharge | Per change criteria | Drain, clean, biocide wash, refill | Maintenance team |
Coolant-Related Problem Diagnosis
| Symptom | Likely Additive-Related Cause | Diagnostic Test | Corrective Action |
|---|---|---|---|
| Increased tool wear | EP additive depleted (sulfur < 1.0%) | Active sulfur test (ASTM D1662) | Replenish EP additive to restore 1.5–2.5% active sulfur |
| Poor surface finish | Lubricity loss from EP depletion or wrong viscosity | Viscosity test, EP concentration | Adjust viscosity, replenish EP package |
| Guide pad scoring | Corrosion from acid number > 0.5 or water ingress | Acid number, water content, copper corrosion | Neutralise acids, repair coolant leaks, improve corrosion inhibitor |
| Coolant foaming | Tramp oil contamination or wrong additive balance | Visual foam test, tramp oil measurement | Install skimmer, adjust defoamer dosage |
| Bacterial odour (emulsion) | Biocide depletion, low pH, tramp oil food source | Dip slide, pH test | Shock dose biocide, adjust pH, remove tramp oil |
| Skin irritation | Incorrect concentration, wrong biocide type | Concentration check, biocide review | Adjust concentration, change biocide type |
| Corrosion on machine ways | Corrosion inhibitor depleted or pH too low | Copper corrosion test, pH | Replenish corrosion inhibitor, adjust pH |
| Coolant separation (emulsion) | Incompatible additives, incorrect dilution | Visual emulsion stability test | Stop additive addition, contact supplier |
| Chip sticking to tool | Insufficient EP activity | Active sulfur test | Increase EP additive concentration |
| Filter media clogging | Additive degradation byproducts or excessive fines | Particle count, acid number | Improve pre-filtration, consider coolant reclamation |
FAQ
What EP additive is best for deep hole drilling of steel?
Active sulfur EP additives provide the best performance for deep hole drilling of ferrous materials. At the cutting zone temperatures of 200–400°C in BTA drilling, active sulfur reacts with the iron surface to form an iron sulfide (FeS) tribofilm with a low shear strength, reducing friction and preventing adhesive wear. For most steel drilling, a finished oil with 1.5–2.5% active sulfur is optimal. For difficult materials (Inconel, stainless steel, titanium), active sulfur content up to 3.5% combined with phosphate ester or nano-MoS₂ provides additional extreme pressure capability.
How often should coolant additives be tested?
Active sulfur concentration and acid number should be tested monthly for neat oil coolant systems. Viscosity and water content should be tested monthly as well. For water-miscible coolants, pH, concentration, and microbiological counts should be tested weekly, with full additive analysis quarterly. More frequent testing is warranted during periods of high production, after coolant top-ups, or when processing difficult materials.
Why does the acid number increase in neat oil coolants?
Acid number (AN) increases primarily from water ingress, thermal oxidation of the base oil, and additive depletion byproducts. Water reacts with sulfurized EP additives to form sulfuric and sulfurous acids, which corrode guide pads, spindle components, and workpiece surfaces. Thermal oxidation from continuous high-pressure pumping generates organic acids. Maintaining AN below 0.5 mg KOH/g is critical for corrosion control. AN above 0.8 mg KOH/g typically requires coolant reclamation or replacement.
Can I mix different brands of coolant additives?
Mixing coolant additive brands is not recommended unless compatibility has been verified by the supplier. Different additive chemistries can react — for example, mixing sulfurized and chlorinated EP additives from different manufacturers can form corrosive byproducts, and mixing biocide types can cause emulsion splitting in water-miscible coolants. Always consult the coolant supplier before adding any product to an existing system.
Is chlorinated paraffin still used in deep hole drilling coolants?
Chlorinated paraffin was historically the standard EP additive for deep hole drilling but is increasingly restricted due to environmental and health regulations (REACH, RoHS). Many regions now limit or ban medium- and long-chain chlorinated paraffins. High-sulfur EP additives (Additin RC 2541 at 40% total sulfur) and phosphate esters provide effective alternatives for most deep hole drilling applications without the regulatory burden of chlorinated products.
What biocide is best for water-miscible coolants in deep hole drilling?
Isothiazolinone-based biocides (CMIT/MIT) provide the broadest spectrum of bacterial and fungal control for water-miscible coolants in deep hole drilling. For systems with persistent fungal or mold problems, a phenolic or oxazolidine-based biocide (such as grotan OX) provides better fungal control. The biocide should be selected based on the coolant formulation and confirmed by compatibility testing. For biocide-free formulations, biostable chemistry (such as SARCOOL ECO 4648) provides microbial resistance through formulation design rather than biocidal additives.
How is coolant additive concentration maintained?
Concentration is maintained through regular testing and targeted replenishment. For neat oil systems, active sulfur concentration (ASTM D1662) is the primary control parameter — when it falls below 1.0%, sulfurized EP additive concentrate is added to restore the target level. For water-miscible emulsions, refractive index (Brix) is the primary control, with additive concentrate added when concentration drifts below the target range. Automated dosing systems with conductivity or refractive index sensors are available for large central systems.
What coolant is used for aluminium deep hole drilling?
Aluminium deep hole drilling requires a coolant that provides adequate lubrication without staining the workpiece. Chlorine-free, sulfur-free synthetic or semi-synthetic water-miscible coolants with low EP activity are preferred. The coolant must have excellent aluminium corrosion inhibition (staining is a common problem with high-sulfur oils). Viscosity should be low (7–12 cSt at 40°C) for chip flushing. Coolant pH should be maintained at 8.5–9.0 for aluminium to avoid etching.
How does water contamination affect deep hole drilling coolant?
Water contamination in neat oil coolants causes multiple problems: (1) hydrolysis of sulfurized EP additives creates acidic byproducts that increase the acid number and cause corrosion; (2) water reduces the oil's viscosity and lubricity, increasing tool wear; (3) water promotes bacterial growth, creating foul odours and potential health hazards; (4) water accelerates filter media degradation. Water content should be kept below 0.1% and tested monthly.
What is the economic case for coolant additive maintenance?
For the 10-machine shop in this article's scenario, the annual savings from implementing coolant additive maintenance total $38,000. These savings come from: extended coolant service life (reduced from an 8-month to 24-month change interval saves $18,000/year in coolant purchases and disposal costs); reduced tool wear from maintained EP levels (12% improvement saves $12,000/year); reduced guide pad wear from corrosion control (40% reduction saves $5,000/year); and reduced downtime from coolant-related problems ($3,000/year). The cost of monthly testing and periodic additive replenishment is approximately $5,000/year, yielding a net saving of $33,000/year.
Summary
Coolant additive management for deep hole drilling centres on three additive categories: EP additives (primarily active sulfur compounds) that form tribofilms at the cutting interface to reduce tool wear and prevent welding; corrosion inhibitors that protect guide pads, spindle components, and the workpiece from acid attack and rust; and biocides that control microbial growth in water-miscible coolants used for non-ferrous drilling. Active sulfur concentration is the most critical performance parameter for neat oil systems — maintaining 1.5–2.5% active sulfur through monthly testing (ASTM D1662) and targeted replenishment is essential for consistent drilling performance. The acid number (ASTM D664) and water content (ASTM D6304) are the primary indicators of coolant health, with rising values signalling contamination or additive degradation that requires correction. The economic case for additive maintenance is strong: a 10-machine shop implementing systematic testing and replenishment saves $33,000/year net through extended coolant life, reduced tool wear, and lower maintenance costs. Proper additive management transforms coolant from a passive consumable into an active performance enabler that directly affects tool life, hole quality, and production uptime.